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REPORT ON ELECTRICAL CLOTHES DRYERS AND LINT IGNITION CHARACTERISTICS
FINAL REPORT ON
ELECTRIC CLOTHES DRYERS
AND LINT IGNITION CHARACTERISTICS
May 2003
U.S. CONSUMER PRODUCT SAFETY COMMISSION
4330 EAST WEST HIGHWAY, BETHESDA, MARYLAND 20814
U.S. CONSUMER PRODUCT SAFETY COMMISSION
DIRECTORATE FOR ENGINEERING SCIENCES
FINAL REPORT ON
ELECTRIC CLOTHES DRYERS
AND LINT IGNITION CHARACTERISTICS
May 2003
Arthur Lee
Electrical Engineer
Division of
Electrical Engineering
ES
Directorate for
Engineering
Sciences
US CONSUMER PRODUCT SAFETY COMMISSION
FINREPORT ON ELECTRIC AL CLOTHES DRYERS AND LINT IGNITION CHARACTERISTICS
May 2003
ACKNOWLEDGMENTS
This project was completed with the help of CPSC staff in the Directorate for
Laboratory
Sciences; Directorate for Engineering Sciences; Directorate for Economics Analysis; and
Directorate for Epidemiology.
Frank Dunmore, Ph.D., Directorate for Laboratory Sciences, provided technical expertise
in all aspects of the project, and specifically in coordinating and implementing Task 3 -
Monitor
Lint Distribution. He also authored the section of this report that discusses Task 3.
Timothy Smith, Directorate for Engineering Sciences - Division of Human Factors,
conducted an analysis of consumer behavior with regard to clothes dryers, particularly related
to
abnormal operating conditions.
Sheela Kadambi, Directorate for Engineering Sciences, Division of Electrical
Engineering, conducted the initial work on clothes dryers in 1998, which set the foundation
for
this project.
Terrance R. Karels, Directorate for Economic Analysis, provided information on the
clothes dryer market over the past 10 years.
Mark Levenson, Ph.D., Directorate for Epidemiology – Division of Hazard Analysis,
developed estimates of clothes dryer fire losses for a 10-year period and analyzed in-depth
investigations of fire incidents associated with clothes dryers.
US CONSUMER PRODUCT SAFETY COMMISSION
FINAL REPORT ON ELECTRIC CLOTHES DRYERS AND LINT IGNITION CHARACTERISTICS
May 2003
EXECUTIVE SUMMARY
The U.S. Consumer Product Safety Commission (CPSC) initiated a project in
Fiscal
Year 2002 to investigate possible conditions that may lead to dryer fires and to develop
recommendations for revisions and/or additions to the voluntary standards to address
potential
hazards. In 1998, CPSC estimates that there were approximately 15,600 clothes dryer fires
resulting in 20 deaths, 370 injuries and $75.4 million in property damage1.
CPSC staff tested clothes dryers to evaluate the effects of lint accumulation and abovenormal
operating temperatures and determine whether such conditions may result in lint ignition
and/or dryer fires. The data was used to help determine if dryer fires result from a single
event
or a combination of events.
The basic approach was to conduct several tasks that could link the cause of lint
accumulation to possible dryer fires and/or lint ignition. The tasks included:
· Task 1. Inspect and Record Dryer Design
· Task 2. Document Dryer Operating Characteristics
· Task 3. Monitor Lint Distribution
· Task 4. Determine Characteristics Required for Lint Ignition
Although selected dryer designs were used to document the variety of temperature and
airflow patterns in a dryer, the conclusions are based on and can be applied to general dryer
designs.
Tests included examining the effects of restricted and unrestricted airflow on dryer
operation. Airflow restriction was created by placing an iris – which created blockages of 25,
50
and 75 percent – in the exhaust duct. A blast plate covering the exhaust opening was used to
create a fully blocked exhaust vent (100 percent blockage). Hot wire anemometers were used
to measure the airflow entering and exiting the dryers. Thermocouples were placed at the
heater intake, heater housing, heater exhaust, tumbler intake, blower intake, and dryer
exhaust.
The results of the CPSC staff tests showed that lint that accumulates inside the dryer
can ignite if the lint contacts certain areas of the heater housing, if the lint is in proximity
to the
heater, or if the lint is ingested by the heater box. Observations made for each task during
testing include:
Task 1. Inspect and Record Dryer Design
¨ All four dryer designs used the same method and order (heater, tumbler, lint
screen,
blower, and exhaust duct) for moving the air through the dryer.
1 Mah, J., “Table 1, Estimated Residential Structure Fires Selected Equipment
1998,”1998 Residential Fire Loss
Estimates, Directorate for Epidemiology, US Consumer Product Safety Commission,
1998.
US CONSUMER PRODUCT SAFETY COMMISSION
FINAL REPORT ON ELECTRIC CLOTHES DRYERS AND LINT IGNITION CHARACTERISTICS
May 2003
iv
¨ The length of the dryer’s exhaust duct extending out of the dryer may not allow
the
house duct to slide far enough onto the dryer’s exhaust duct to provide a secure
pressure fit.
¨ Using rigid external ducting does not allow for a secure pressure fit around
the
dryer’s male duct.
Task 2. Document Dryer Operating Characteristics
¨ The temperatures within a dryer, under both normal and abnormal conditions,
were
similar for the four different dryer designs tested, with only slight variations due to
dryer internal configurations.
¨ The temperatures measured inside the heater box, heater intake, and intake into
the
tumbler increased when the exhaust vent was partially blocked or fully blocked. The
temperatures inside the tumbler, blower and exhaust vent decreased when the
exhaust vent was partially blocked or fully blocked.
¨ When the exhaust vent was blocked up to 50 percent, the temperatures inside
the
dryer were similar to those measured when there was no blockage of the exhaust
vent. When the exhaust vent was 75 percent or 100 percent blocked, temperatures
in certain areas inside the dryer increased significantly.
¨ Under normal operation, the airflow inside the exhaust vent decreased
dramatically
as the lint screen became blocked with lint particles.
¨ In general, the dryers only cycled on the high-limit thermostat when the exhaust
vent
was 75 or 100 percent blocked, which caused the temperatures near the heater to
increase significantly.
¨ When the primary thermostat was bypassed (simulating a thermostat failure),
the
dryer operated at higher than normal temperatures – temperatures similar to those
measured when the exhaust vent was blocked 50 to 75 percent. In general (3 of the
4 dryer designs tested), a failed-closed primary thermostat did not cause the dryer to
cycle on the high limit thermostat for the unblocked exhaust vent condition.
Task 3. Monitor Lint Distribution
¨ Lint begins to accumulate inside a dryer chassis upon first use. Lint accumulates
on
the dryer’s components, including the heater and the dryer floor. This accumulation
occurs even when the dryer’s lint screen has been cleaned after each usage, and the
dryer is properly exhausted.
¨ Seals in the dryer’s interior exhaust venting may not be adequate to prevent linty
air
from escaping into the dryer’s interior.
US CONSUMER PRODUCT SAFETY COMMISSION
FINAL REPORT ON ELECTRIC CLOTHES DRYERS AND LINT IGNITION CHARACTERISTICS
May 2003
v
Task 4. Determine Characteristics Required for Lint Ignition
¨ Lint that accumulates on the heater housing can easily ignite under conditions of
a
failed high-limit thermostat and a blocked exhaust vent.
¨ Lint accumulating near the heater intake can ignite before the high-limit
thermostat
switches the heater element off.
¨ Lint ingested by the heater and embers expelled from the heater outlet can
easily
ignite additional lint or fabric in the air stream, resulting in additional embers in the
dryer system and exhaust vent.
The CPSC staff noted the following during testing and analysis:
¨ The high-limit thermostat may prematurely fail when subjected to high ambient
temperatures.
US CONSUMER PRODUCT SAFETY COMMISSION
FINAL REPORT ON ELECTRIC CLOTHES DRYERS AND LINT IGNITION CHARACTERISTICS
May 2003
vi
TABLE OF CONTENTS
ACKNOWLEDGMENTS
..................................................................................................ii
EXECUTIVE SUMMARY
................................................................................................iii
1.0 INTRODUCTION
...................................................................................................... 1
1.1 General
...............................................................................................................
1
1.2 Project
Objective................................................................................................. 1
1.3 Focus Objective
.................................................................................................. 1
1.4 Technical Approach
............................................................................................ 2
1.4.1 Task 1: Inspect and Record Dryer Design ..................................................
2
1.4.2 Task 2: Document Dryer Operating Characteristics.................................... 2
1.4.3 Task 3: Monitor Lint Distribution
................................................................. 2
1.4.4 Task 4: Determine Characteristics Required for Lint Ignition ...................... 3
1.4.4.1 Ignition of Lint on and near the Heater Housing .................................
3
1.4.4.2 Ignition of Lint Ingested into the Heater..............................................
3
1.5 Organization of the Report
.................................................................................. 3
1.6 Statement of Test
Methodology........................................................................... 3
1.7 Global Terminology
............................................................................................. 4
2.0 DESCRIPTION OF TESTS AND TEST RESULTS ...................................................
6
2.1 Task 1: Inspect and Record Dryer
Design........................................................... 6
2.1.1 Airflow Pattern
............................................................................................ 6
2.1.2 Heater Location and Configuration
............................................................. 7
2.1.3 Lint Screen
Location................................................................................... 7
2.1.4 Tumbler
Design.......................................................................................... 7
2.1.5 Blower (Fan)
Design................................................................................... 8
2.1.6 Operating Features
.................................................................................... 8
2.1.7 Safety Device Locations
............................................................................. 8
2.2 Task 2: Document Dryer Operating Characteristics.............................................
8
2.2.1 Exhaust Vent
Setup.................................................................................... 8
2.2.2 Instrumentation Setup
.............................................................................. 10
2.2.3 Global Test Procedure and Setup
............................................................ 13
2.2.4 Dryer Design
A......................................................................................... 14
2.2.4.1 Dryer Design A –Temperature and Airflow Characteristics, Blocked
and Unblocked Exhaust Vent Conditions ............................................... 14
2.2.4.2 Dryer Design A – Airflow..................................................................
20
2.2.4.3 Dryer Design A – Primary Thermostat Bypassed.............................
25
2.2.5 Dryer Design
B.........................................................................................
27
2.2.5.1 Dryer Design B –Temperature and Airflow Characteristics, Blocked
and
Unblocked....................................................................................... 27
2.2.5.2 Dryer Design B – Airflow..................................................................
34
2.2.5.3 Dryer Design B – Primary Thermostat Bypassed.............................
38
2.2.6 Dryer Design
C.........................................................................................
40
2.2.6.1 Dryer Design C – Temperature and Airflow Characteristics, Blocked
and
Unblocked....................................................................................... 40
2.2.6.2 Dryer Design C – Airflow..................................................................
47
2.2.6.3 Dryer Design C – Primary Thermostat Bypassed.............................
51
2.2.7 Dryer Design
D.........................................................................................
53
2.2.7.1 Dryer Design D –Temperature and Airflow Characteristics, Blocked
and
Unblocked....................................................................................... 53
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May 2003
vii
2.2.7.2 Dryer Design D – Airflow..................................................................
60
2.2.7.3 Dryer Design D – Primary Thermostat Bypassed.............................
64
2.3 Task 3: Monitor Lint Distribution
........................................................................ 66
2.3.1 Test
Setup................................................................................................ 66
2.3.2 Test Description
....................................................................................... 67
2.3.2.1 Method 1 – Weight Loss of Load .....................................................
67
2.3.2.2 Method 2 – Visual Examination........................................................
68
2.3.3 Examination of Dryer Design
A................................................................. 69
2.4 Task 4: Determine Characteristics Required for Lint Ignition .............................
71
2.4.1 Test
Setup................................................................................................ 71
2.4.2 Instrumentation Setup
.............................................................................. 71
2.4.3 Phase I – Ignition Characteristics of Lint On and Near the Heater Box ..... 75
2.4.3.1 Top of Heater Housing.....................................................................
75
2.4.3.2 Side of the Heater Housing..............................................................
79
2.4.3.3 Lint Samples at the Heater Intake....................................................
83
2.4.4 Phase II – Ignition Characteristics of Lint Ingested into the Heater Box ....
89
2.4.4.1 Part 1 – Lint Samples Ingested by the Heater Box...........................
89
2.4.4.2 Part 2 – Ignition of Target Materials Downstream from the Heater
Exhaust
................................................................................................. 90
3.0 DISCUSSION
.........................................................................................................
95
3.1 Task 1: Inspect and Record Dryer
Design......................................................... 95
3.2 Task 2: Document Dryer Operating Characteristics...........................................
95
3.2.1 Normal Operation (Unblocked Exhaust Vent) ...........................................
95
3.2.2 Partially-Blocked and 100%-Blocked Conditions ...................................... 99
3.3 Task 3: Monitor Lint Distribution
...................................................................... 108
3.3.1 Dryer Design A at Positive Pressure.......................................................
108
3.3.2 Other Dryer Designs at Positive Pressure ..............................................
109
3.4 Task 4: Determine Characteristics Required for Lint Ignition ...........................
111
3.4.1 Lint on the Heater
Housing..................................................................... 111
3.4.2 Lint Samples at the Heater Intake
.......................................................... 111
3.4.2.1 High-Limit Thermostat Bypassed and No Airflow ...........................
111
3.4.2.2 High-Limit Connected in Series and No Airflow..............................
116
3.4.2.3 High-Limit Connected in Series and Airflow ...................................
119
3.4.3 Lint Samples Ingested into the Heater....................................................
121
3.4.4 Ignition of Target Material Downstream from the Heater Exhaust........... 122
3.4.5 High-Limit Thermostat Analysis
.............................................................. 122
3.4.5.1 High-Limit Thermostat 1 (HLT1) Analysis ......................................
123
3.4.5.2 High-Limit Thermostat 2 (HLT2) Analysis ......................................
127
4.0 SUMMARY AND
CONCLUSIONS........................................................................ 133
Appendices A to
J........................................................................................................A-J
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FINAL REPORT ON ELECTRIC CLOTHES DRYERS AND LINT IGNITION CHARACTERISTICS
May 2003
viii
LIST OF FIGURES
Figure 1. Dryer Design Characteristics
......................................................................................6
Figure 2. Heating Element in a Heater Box Configuration
...........................................................7
Figure 3. Circular Heating Element in Dryer Design D
................................................................7
Figure 4. Exhaust Vent Setup
.....................................................................................................9
Figure 5. Thermocouple Response Before Task 2 Testing
.......................................................11
Figure 6. Thermocouple Response After Task 2 Testing
..........................................................11
Figure 7. Average Responses per Oven Set Temperatures and
Day........................................12
Figure 8. Dryer Design A Intake and Exhaust
...........................................................................14
Figure 9. Dryer Design A – Thermocouple and Airflow Measurements with a Wet Load...........16
Figure 10. Dryer Design A – T1 Heater Intake Comparison
......................................................17
Figure 11. Dryer Design A – T3 Heater Housing
Comparison...................................................18
Figure 12. Dryer Design A – T2 Heater Exhaust Comparison
...................................................19
Figure 13. Dryer Design A – Intake Air Velocity
........................................................................21
Figure 14. Dryer Design A – Exhaust Air Velocity
.....................................................................22
Figure 15. Dryer Design A – Average Airflow Velocities (High Heat and Dry
Load)...................23
Figure 16. Dryer Design A – Average Airflow Velocities (No Heat and No Load)
......................24
Figure 17. Dryer Design A – Primary Thermostat Bypassed
.....................................................26
Figure 18. Dryer Design B – Air Flow Through the
Tumbler......................................................27
Figure 19. Dryer Design B – Thermocouple and Airflow Measurements (Wet
Load).................30
Figure 20. Dryer Design B – T1 Heater Intake Comparison
......................................................31
Figure 21. Dryer Design B – T3 Heater Housing
Comparison...................................................32
Figure 22. Dryer Design B – T5 Intake into the Blower
Comparison..........................................33
Figure 23. Dryer Design B – Intake Air Velocity
........................................................................35
Figure 24. Dryer Design B – Exhaust Vent Air Velocity
.............................................................36
Figure 25. Dryer Design B – Average Airflow Velocities (High Heat and Wet
Load)..................37
Figure 26. Dryer Design B – Primary Thermostat Bypassed
.....................................................39
Figure 27. Dryer Design C – Airflow through the Tumbler
.........................................................40
Figure 28. Dryer Design C – Thermocouple and Airflow Measurements (Wet
Load).................43
Figure 29. Dryer Design C – T1 Heater Intake Comparison
......................................................44
Figure 30. Dryer Design C – T3 Heater Housing
Comparison...................................................45
Figure 31. Dryer Design C – T5 Intake into the Blower Comparison
.........................................46
Figure 32. Dryer Design C – Intake Air Velocity
........................................................................48
Figure 33. Dryer Design C – Exhaust Vent Air
Velocity.............................................................49
Figure 34. Dryer Design C – Average Airflow Velocities (High Heat and Wet
Load)..................50
Figure 35. Dryer Design C – Primary Thermostat
Bypassed.....................................................52
Figure 36. Dryer Design D – Airflow Through the
Tumbler........................................................53
Figure 37. Setup with a Relay and Battery for T2
Analog..........................................................54
Figure 38. Dryer Design D – Thermocouple and Airflow Measurements (Wet
Load).................56
Figure 39. Thermocouple, Airflow and Relay (T2 Analog)
Measurements.................................57
Figure 40. Dryer Design D – T1 Heater Intake Comparison
......................................................58
Figure 41. Dryer Design D – T3 Heater Housing Temperature
Comparison..............................59
Figure 42. Dryer Design D – Intake Air Velocity
........................................................................61
Figure 43. Dryer Design D – Exhaust Vent Air
Velocity.............................................................62
Figure 44. Dryer Design D –Average Airflow Velocities (High Heat and No Load)
....................63
Figure 45. Dryer Design D – Primary Thermostat
Bypassed.....................................................65
Figure 46. Dryer Design A Used in Task 3 Testing
...................................................................66
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May 2003
ix
Figure 47. Lint Accumulated at Bottom of Dryer After 100
cycles..............................................68
Figure 48. Lint Accumulated on Interior of Dryer After 48 Cycles
..............................................69
Figure 49. Lint Accumulated on Interior of Dryer After 100 Cycles
............................................69
Figure 50. Lint Build-up at Blower Housing Seal and Dryer
Floor..............................................70
Figure 51. Test Setup for Lint Ignition
.......................................................................................73
Figure 52. Test Setup for Lint Ignition
.......................................................................................74
Figure 53. Location of Lint Samples on the Housing
.................................................................75
Figure 54. Lint Sample on Heater Housing at Location
A..........................................................76
Figure 55. Sample Smoldering at Location A
............................................................................77
Figure 56. Sample s5-8 at Location B
.......................................................................................78
Figure 57. Thermocouple Data for Sample s5-8
.......................................................................78
Figure 58. Sample s2-7 on the Side of the Heater Housing at Location
C.................................80
Figure 59. Sample s2-7 Thermocouple Traces
.........................................................................80
Figure 60. Sample s2-2 Tested at Location D
...........................................................................81
Figure 61. Sample s2-6 Tested at Location E
...........................................................................82
Figure 62. Thermocouple Traces for Sample s2-6 at Location
E...............................................83
Figure 63. Lint Sample at Heater
Intake....................................................................................84
Figure 64. Infrared Camera Setup
............................................................................................84
Figure 65. Sample s3-10, Video Camera and Infrared
Camera.................................................87
Figure 66. Sample s3-10 – Thermocouple and Airflow Data
.....................................................88
Figure 67. Setup with Glass
Tube.............................................................................................89
Figure 68. Setup with Target Material in the Glass
Tube...........................................................91
Figure 69. Target Sample Igniting During Test
1.......................................................................93
Figure 70. Test 1 – Thermocouple and Airflow Measurements Task 4- Test
1..........................94
Figure 71. Dryer Design Comparison for T1, Heater Intake
......................................................97
Figure 72. Dryer Design Comparison for T3, Heater Housing
...................................................97
Figure 73. Dryer Design Comparison for T5, Blower Intake
......................................................98
Figure 74. Dryer Design Comparison for T4, Exhaust
Vent.......................................................98
Figure 75. Maximum Temperatures Measured at Location
.......................................................99
Figure 76. Intake into the
Heater.............................................................................................101
Figure 77. Heater Exhaust
Temperature.................................................................................101
Figure 78. Heater Housing
Temperature.................................................................................102
Figure 79. Exhaust Vent Temperature
....................................................................................102
Figure 80. Intake into the Blower Temperature
.......................................................................103
Figure 81. Intake into the Tumbler Temperature
.....................................................................103
Figure 82. Maximum Temperatures for All Dryers at Each Location and
Condition.................105
Figure 83. Comparison of Exhaust Air Velocity for all Dryer Designs
......................................107
Figure 84. Calculated Average Exhaust Air Velocity
...............................................................107
Figures 85 (a) and (b). Dryer Design A – Blower Assembly
....................................................108
Figure 86. An Illustration of Airflow through the Dryer and Blower
..........................................109
Figure 87. Junction Between the Blower and Exhaust Vent
....................................................110
Figure 88. Lint Samples Tested 3 Inches from the Heater Housing Edge
...............................112
Figure 89. T1 Only - 3 Inches from the Heater Housing
Edge.................................................113
Figure 90. T2 Only - 3 Inches from the Heater Housing
Edge.................................................113
Figure 91. T3 Only - 3 Inches from the Heater Housing
Edge.................................................114
Figure 92. Lint Samples Tested 4 Inches from the Heater Housing Edge
...............................115
Figure 93. Ignition Time for 2 and 3 Inches from Heater Edge
................................................116
Figure 94. Lint Samples s3-16 and s3-17 Tested 1 Inch from the Heater Edge
......................117
Figure 95. Lint Samples s3-14 and s3-18 Tested 2 Inches from the Heater
Edge...................118
Figure 96. High Limit Activation Time Comparison at Sample 2 and 3
inches.........................119
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x
Figure 97. Lint Samples Tested 1 Inch from the Heater Edge with Airflow
..............................120
Figure 98. Samples Tested at 1 Inch, With and Without High-Limit
Thermostat......................121
Figure 99. Sample s3-13 Test with the High Limit Thermostat in Series
.................................123
Figure 100. Failed Thermostat compared to a Replacement
Thermostat................................124
Figure 101. X-ray Images of High-Limit Thermostat 1
(HLT1).................................................125
Figure 102. Contacts for High-Limit Thermostat 1
(HLT1).......................................................125
Figure 103. Contact Pads for High-Limit Thermostat 1
(HLT1)................................................126
Figure 104. Sample s5-14 with High Limit Thermostat in Series
.............................................127
Figure 105. High-Limit Thermostat 2 (HLT2) before and after Failure
.....................................128
Figure 106. X-ray Images of High-Limit Thermostat 2
(HLT2).................................................129
Figure 107. Internal View for High-Limit Thermostat 2
............................................................129
Figure 108. Setup for Testing High-Limit Thermostats
............................................................130
Figure 109. New High Limit
Thermostat..................................................................................130
Figure 110. Thermocouple Data for the New High-Limit
Thermostat.......................................131
Figure 111. Second Failed High Limit Thermostat,
HLT2........................................................131
Figure 112. Comparison between New and Failed High Limit Thermostats
............................132
LIST OF TABLES
Table 1. Unrestricted and Restricted Duct
Configurations...........................................................9
Table 2. Equivalent Linear Duct Length
....................................................................................10
Table 3. General Locations of Thermocouples and
Anemometers............................................12
Table 3-continued. General Locations of Thermocouples and Anemometers
...........................13
Table 4. Dryer Design A Measurement Data (°C and sfpm)......................................................15
Table 5. Dryer Design B Statistics (°C and
sfpm)......................................................................28
Table 6. Dryer Design C – Statistics (°C and
sfpm)...................................................................41
Table 6. Thermocouple Location and
Setup..............................................................................71
Table 7. Testing on Top of the Heater Housing at Location A
...................................................76
Table 8. Testing on Top of the Heater Housing at Location B.
..................................................77
Table 9. Testing on the Side of the Heater Housing at Location C
............................................79
Table 10. Testing on Top of the Heater Housing at Location D
.................................................81
Table 11. Testing on the Side of the Heater Housing at Location E
..........................................82
Table 12. Lint Samples at the Heater Intake
.............................................................................86
Table 13. Lint Samples for Task 4 - Phase II
Testing................................................................90
Table 14. Target Materials for Task 4 - Phase II
Testing...........................................................91
Table 15. Change in Minimum Temperature
...........................................................................106
Table 16. Change in Average Temperature
............................................................................106
Table 17. Change in Maximum Temperature
..........................................................................106
Table 18. Ignition Times at Heater Intake
...............................................................................115
Table 19. Approximate High Limit Thermostat Activation Times
.............................................118
Table 20. Life History of Failed High-Limit Thermostats
..........................................................123
US CONSUMER PRODUCT SAFETY COMMISSION
FINAL REPORT ON ELECTRIC CLOTHES DRYERS AND LINT IGNITION CHARACTERISTICS
May 2003
1
1.0 INTRODUCTION
1.1 General
Over the years, the number of safety devices included in clothes dryers to reduce
the
incidence of fire has increased. The increased safety of dryers has likely contributed to the
slowed growth rate in dryer fires. However, there were an estimated 15,600 clothes dryer
fires
resulting in 20 deaths, 370 injuries and $75.4 million in property damage in 1998 2.
In 1998, 56.2% of all U.S. households had an electric clothes dryer, and 17.9% had a
gas dryer3. In total, over 74% of all U.S. households had at least one clothes dryer in
1998.
There are just over 100 million US households; thus, there are over 74 million clothes dryers
in
use in the U.S. Over the period 1990 through 1998, shipments (gas, electric, and compact
dryers) increased 41% overall, from almost 4.6 million units to almost 6.5 million
units3. In 2001,
the number of dryer shipments was over 6.7 million units4. There was a consistent growth in the
sales of clothes dryers in the U.S. from 1990 to 2001.
The U.S. Consumer Product Safety Commission (CPSC) initiated a project in Fiscal
Year (FY) 2002 to investigate possible conditions that may lead to dryer fires and to assess
the
adequacy of the applicable voluntary standards in addressing potential hazards.
Four years earlier, in FY 1998, CPSC staff began an initial evaluation of clothes dryers.
The results of the tests conducted during that evaluation showed that, when the dryer exhaust
was blocked, some areas of the dryer would run cooler than normal and other areas would run
hotter. CPSC staff believed that long-term operation of a dryer under conditions of
restricted
airflow, such as that caused by lint accumulation, could eventually lead to premature failure
of
components that may result in a fire.
1.2 Project Objective
The CPSC clothes dryer project was initiated to determine the cause(s) for clothes
dryer
fires and to develop recommendations for revisions and/or additions to the voluntary
standards
to address those causes and help prevent dryer fires.
1.3 Focus Objective
The focus objective of this project was to evaluate the effects of lint accumulation
and
above-normal operating temperatures in electric clothes dryers and determine whether such
conditions may result in dryer fires and/or lint ignition.
2 Mah, J., ”Table 1, Estimated Residential Structure Fires Selected Equipment
1998,”1998 Residential Fire Loss
Estimates, Directorate for Epidemiology, US Consumer Product Safety Commission,
1998.
3 Appliance Magazine, Statistically Review, A Dana Chase Publication, May 1999
4 Appliance Magazine, U.S. Shipment Statistics, A Dana Chase Publication, March 2002
US CONSUMER PRODUCT SAFETY COMMISSION
FINAL REPORT ON ELECTRIC CLOTHES DRYERS AND LINT IGNITION CHARACTERISTICS
May 2003
2
1.4 Technical Approach
The overall goal was to determine whether lint accumulation could result in clothes
dryer
fires and/or lint ignition. To accomplish this large task, smaller tasks were designed to link
the
cause of lint accumulation to possible dryer fires and/or lint ignition. The tasks were set up
to
eliminate as many dryer design variables as possible.
Tests were conducted with sample clothes dryers at both normal and above-normal
operating temperatures; with dryers operating in the high-limit cycle mode (caused by either
a
fully or partially blocked exhaust vent); with bypassed temperature limiting devices
(simulating
component failure); and without any safety temperature limiting devices.
The following four smaller tasks were defined to accomplish the main objective:
1.4.1 Task 1: Inspect and Record Dryer Design
The objective of this task was to record the types and locations of the internal
components and layout for each sample dryer and to photograph the internal configuration of
each dryer. The airflow path throughout each dryer was illustrated and the locations of the
heater, blower, and lint screen were documented. Based on the results of the examination of
the dryers, the location of instrumentation was determined for Task 2 - Document Dryer
Operating Characteristics.
1.4.2 Task 2: Document Dryer Operating Characteristics
The objective of this task was to characterize the airflow patterns and temperatures
inside each dryer design during normal (unblocked) and restricted (blocked) airflow through
the
exhaust vent. The dryers were characterized by operating according to the manufacturer’s
instructions with a (wet and/or dry) clothes load and without a load (a dryer may have been
fully
characterized with a wet load if the test data developed for a dry load and wet load, with no
exhaust blockage, were not comparable.) The data in this task was used to set the test
variables in Task 4 - Determine the Characteristics for Lint Ignition.
1.4.3 Task 3: Monitor Lint Distribution
The objective of this task was to monitor lint distribution and accumulation in areas within
a clothes dryer during operation with normal airflow. The load for these tests consisted of
10
wet (washed and spun dry) bath towels. A dryer was operated according to the manufacturer’s
instructions and subjected to normal use drying cycles. The dryer was operated for a total of
100 cycles. After 48 cycles, the accumulation of lint was recorded near the heater housing,
internal exhaust duct, and internal dryer floor. The dryer was examined at the end of 100
cycles.
Lint accumulation was characterized in one dryer. An analysis of potential causes for lint
accumulation in other dryer designs was conducted.
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1.4.4 Task 4: Determine Characteristics Required for Lint Ignition
This task had two objectives: Evaluate the ignition characteristics of lint samples on and
near the heater box, and evaluate the ignition characteristics of lint when samples were
ingested into the heater box. The lint samples used in these tests were 100% cotton and were
taken from the lint that accumulated during tests conducted in Task 3, unless otherwise
noted.
1.4.4.1 Ignition of Lint on and near the Heater Housing
The heat output and airflow through a heater were varied. Lint samples were placed
at
various locations on and near the heater box. Power to the heating element was introduced
instantaneously or was stepped up to observe the different effects on the lint samples. Some
tests were conducted with the high limit thermostat in series with the heating element, and
some
were conducted with the thermostat bypassed. The test results were categorized as one of the
following: no ignition, charred only, smoldered, or ignition (flames).
1.4.4.2 Ignition of Lint Ingested into the Heater
Lint was introduced into the airflow of the heater intake. The lint samples were placed
in
front of the heater opening and restrained until ready for release. The system (heater,
temperature, and air velocity) was stabilized for 5 minutes before proceeding. When the
system
had stabilized, the lint samples were released. The results were observed for 15 minutes, or
less if the samples were consumed.
1.5 Organization of the Report
This report is presented in two parts. The first part discusses the overall testing
program
and includes pertinent test descriptions and resultant data, analysis, findings and
conclusions.
The organization is such that each task is headed as a major test phase.
The second part contains appendices that present expanded test data to support the
findings and the conclusions. The appendices are also contained on the compact disk (CD),
which can be accessed through a sub menu.
1.6 Statement of Test Methodology
The test program was designed to identify and eliminate as many dryer design
variables
as possible for Task 4 testing. During the tests, temperature and airflow characteristics of
each
dryer design were recorded. Observations regarding dryer designs that may have caused
variances in the test data are noted in the report.
A large amount of test data was collected during this test program. Only the pertinent
data for each task are presented. All data collected are noted in the report but may not be
presented if the data are not pertinent to the discussion or conclusions.
The clothes dryers used in this test program were selected by design, cost and features
available. Different dryer designs were selected to demonstrate the variety of temperature
and
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4
airflow patterns in a dryer. The dryers selected were in the price range of $300 to $400 and
included similar selectable dryer settings and features.
Although only electric dryers were tested in this program, many of the conclusions may
be applicable to gas-fueled clothes dryers.
1.7 Global Terminology
Lint 100% cotton fibers that were expelled from a clothes load of cotton terry
towels
during the drying process. The lint material may refer to the material collected
from the lint screen or the material that accumulated inside the cabinet (housing)
of the dryer.
Units Unless otherwise specified, all temperatures reported are in degrees Celsius
(°C), all airflow measurements are in standard feet per minute (sfpm), and all
weight measurements are in grams (g), and length in inches (in) and feet (ft).
Top The side of the dryer that is viewed from above.
Front The side of the dryer containing the door.
Rear The side of the dryer containing the dryer’s exhaust vent and power cord.
Floor The internal side of the bottom of the dryer that rests on the building floor.
Inside The interior of the dryer containing the motor, drum, blower, and heater.
Smolder May contain one or more of the following: smoke, embers, or charring with no
evidence of flames.
Ignition Flames are visible.
Test Load The load of towels used in the dryer, either dry or wet load.
Wet Load Ten 100% cotton terry towels that were washed and spun dry in a washing
machine, unless otherwise specified. No detergent was used.
Dry Load Ten 100% cotton terry towels that were dry to the touch, unless otherwise
specified.
Damp Load Ten 100% cotton terry towels that were wet to the touch, but weighed
substantially less then a wet load, unless otherwise specified. No detergent was
used.
High Limit Thermostat
The thermostat located near the heating element of the dryer (High-Limit Switch).
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Primary Thermostat
The thermostat located between the lint screen and the blower (Operating
Switch, Operating Thermostat).
Thermostat Opened
The thermostat reached its upper set point temperature and separated the
contacts.
Thermostat Closed
The thermostat reached its lower set point temperature and closed the contacts.
Abnormal Operating Condition
The dryer operating or cycling on the high-limit thermostat, or tests conducted
with all devices bypassed.
Normal Operating Condition
The dryer operating or cycling on the primary thermostat.
High-Limit Cycling
The dryer operating or cycling on the High-Limit Thermostat.
Exhaust Vent
The venting from the dryer to the outside of the building. All venting material was
4-inch rigid metal duct. All joints were sealed with foil tape except for the
connection to the dryer; the exhaust vent was secured to the dryer using a 4-inch
duct (hose) clamp.
Fully Blocked Exhaust
The exhaust vent was completely obstructed with a blast plate (100% blocked).
Partially Blocked Exhaust
The airflow in the exhaust vent was restricted by an iris opening less than 4
inches in diameter (25%, 50%, and 75% blocked).
Unblocked Exhaust
The exhaust vent contained no obstruction.
Heater The mechanism to warm the air flowing through the dryer (the heating element).
Heater Box A rectangular shaped housing containing the heating element.
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2.0 DESCRIPTION OF TESTS AND TEST RESULTS
2.1 Task 1: Inspect and Record Dryer Design
To form the basis for the analysis to characterize lint ignition, the
operating
characteristics of four different dryer designs were examined. The pattern of airflow; the
locations of the heater, safety devices, and lint screen; and the tumbler and fan design were
examined for each dryer design.
2.1.1 Airflow Pattern
All four dryer designs had the same basic path for airflow. Air is pulled into the dryer
through any gaps in the dryer housing, particularly through rear vents. Air is drawn over the
heater, which warms the air, and then enters the tumbler. The air exits the tumbler and is
directed through the lint screen. It then passes through a duct and into the fan. The fan
forces
the air through an exiting duct to the rear of the dryer. Figure 1 shows the airflow patterns
for
each of the dryer designs tested.
Figure 1. Dryer Design Characteristics
(Not to scale)
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2.1.2 Heater Location and Configuration
In all four clothes dryers, the heaters were located at the rear of the dryer. In three of
the dryer designs, the heating element was contained in a rectangular metal enclosure, or
“heater box” (Dryer Designs A, B and C). The location of the heater box varied among these
three dryers. In two dryers, the heater boxes were oriented with the air intake directed
towards
the floor of the dryers (Dryer Designs A and B); and in the other dryer, the heater box was
located near the top and the air intake directed towards the top of the dryer (Dryer Design
C).
The heating elements contained either one or two rows inside the heater box, as shown in
Figure 2. The fourth dryer (Dryer Design D) had a circular heating element configuration as
shown in Figure 3. The circular heating element was located behind the tumbler.
Figure 2. Heating Element in a Heater Box Configuration
(Does not represent any specific dryer design)
Figure 3. Circular Heating Element in Dryer Design D
(Not to Scale)
2.1.3 Lint Screen Location
In three of the four dryers, the lint screen was located at the front of the dryer and was
accessible at the bottom of the door opening (Dryer Designs B, C and D). The lint screen of
the
fourth dryer was located at the rear of the dryer and was accessible from the top of the
dryer
(Dryer Design A).
2.1.4 Tumbler Design
The tumbler designs and sizes were similar for all four dryers with several noted
differences. All the tumblers contained gaskets at the front and rear of the tumbler. Three
of
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the tumbler designs contained three baffles each that were spaced evenly. The other tumbler
contained two baffles and a hump, evenly spaced.
2.1.5 Blower (Fan) Design
All four dryers used a centrifugal-type fan. The centrifugal fan drew air through the
center and forced it outward. All the fan “blades” were constructed of plastic or plastic
reinforced with fibers.
2.1.6 Operating Features
All dryer designs offered a choice of drying cycles: Timed Dry, Air Dry and Auto Dry. To
eliminate any possible design variance in drying times during testing, only the Timed Dry
cycle
was used.
2.1.7 Safety Device Locations
The locations of the safety devices on the dryers were similar. All four dryers had a
minimum of two safety devices. One temperature switch (the primary or operating thermostat)
was located after the lint screen and before the blower air intake. The second temperature
switch (the high-limit thermostat) was located near the heater air intake.
2.2 Task 2: Document Dryer Operating Characteristics
The main objective of this task was to record any similar or varying characteristics of
a
clothes dryer during normal and abnormal operations. The normal operation test was
conducted with unrestricted airflow in the exhaust vent. Abnormal operation was defined as
the
dryer not cycling on the primary thermostat or cycling on the high-limit thermostat. The
effects
of a restricted exhaust vent were examined. The effects of a bypassed primary thermostat (to
simulate thermostat failure) were also characterized.
2.2.1 Exhaust Vent Setup
The same length and configuration of exhaust duct was used to vent each dryer design
during testing. All venting material was 4-inch rigid metal duct. All joints were sealed with
foil
tape except the connection from the ducting to the dryer, which was secured using a 4-inch
duct
(hose) clamp. The exhaust vent was connected to a 4” angled wall cap. The wall cap
contained a rodent screen with a grid of ¼-inch square openings. Figure 4 illustrates the
setup
used to vent the dryers.
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Figure 4. Exhaust Vent Setup
For tests in which airflow was restricted, an iris – placed in series with the exhaust duct
between the first and second 90-degree elbows – was used to create a partially-blocked
exhaust vent condition. Blockages of 25%, 50% and 75% were created using the iris. The
percentage of blockage was calculated from that part of the cross section of the 4-inch duct
that
remained unblocked, as shown in Table 1.
A blast plate covering 100% of the opening was used to create a fully-blocked exhaust
vent (100% blockage). The blast plate was placed in the same location as the iris in the
exhaust vent.
Table 1. Unrestricted and Restricted Duct Configurations
d (inches) Blockage (%)
4 0
unrestricted
3.46 25
partially
2.83 50
partially
2 75
partially
0 100
blocked
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Table 2 is a guide to determine the equivalent linear length of exhaust duct used in the
testing. The table references were taken from Engineering and Design – Clothes Dryer Exhaust
Venting published by the Department of the Army, Army Corps of Engineers, 23 March
1998,
No. 1110-3-483. For every 90-degree elbow, the document specifies an equivalent length of 8
or 10 linear feet. Based on the calculated equivalent linear feet shown below, the exhaust
vent
configuration used in the test set up was within dryer manufacturer recommendations for
installation for all dryers tested.
Table 2. Equivalent Linear Duct Length
Equivalent Linear Duct Length (feet)
8’ equivalent for elbows 10’ equivalent for elbows
Elbow 1 8 10
Straight 1 5 5
Elbow 2 8 10
Straight 2 5.75 5.75
Wall Cap* 6 6
TOTAL 32.75 36.75
* Conversion taken from dryer installation manuals
2.2.2 Instrumentation Setup
Each dryer was instrumented with six thermocouples and two hot wire anemometers.
One additional thermocouple was used to record the ambient room temperature. All seven
thermocouples were 24 gauge, K type. The 24 gauge thermocouple is relatively stiff and
resulted in good stability in positioning the thermocouples. In addition, with a sampling rate
of
one sample per second, the response time of the thermocouples was adequate.
The thermocouples were calibrated using a thermocouple oven before each dryer design
was tested. The sampling rate during calibration was one sample every 1/10 second. Figure 5
shows the responses of the thermocouples versus the set oven temperature, recorded on
November 14, 2001, before Task 2 (Document Dryer Operating Characteristics) testing began.
The figure shows a close match to the oven temperature except near the upper temperatures.
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Thermocouple Calibration
November 14, 2001
0
20
40
60
80
100
120
140
160
180
200
0 50 100 150 200
Oven Temperature Data Points
Temperature (C)
Oven temperature
T1
T2
T3
T4
T5
T6
T7
Figure 5. Thermocouple Response Before Task 2 Testing
At the end of this task, the thermocouples were re-checked, as shown in Figure 6.
Figure 6 shows the high temperature closely follows the oven set temperature. The
calibration/check tests taken in December 2001 and early January 2002 also have similar
responses.
Thermocouple Calibration
January 22, 2002
0
50
100
150
200
250
300
0 50 100 150 200 250 300
Oven Temperature Data Points
Temperature (C)
Oven temperature
T1
T2
T3
T4
T5
T6
T7
Figure 6. Thermocouple Response After Task 2 Testing
Figure 7 shows the average temperatures for all thermocouples compared to the oven
temperatures for the days they were calibrated. The responses of the thermocouples show they
are slightly below the oven set temperatures. The data presented in this document do not
incorporate any correction factors in the thermocouple readings.
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Thermocouple Calibration Comparison
0
50
100
150
200
250
300
0 50 100 150 200 250 300
Oven Tempertuare Data Points
Temperature (C)
Oven temperature November 14 2001 December 10 2001 January 2 2002 January 22
2002
Figure 7. Average Responses per Oven Set Temperatures and Day
Table 3 lists the general locations of the thermocouples and anemometers in the dryers.
Depending on the dryer design, the actual location may have varied slightly.
Two hot wire anemometers were used to measure the airflow entering and exiting each
dryer. One anemometer was placed at the intake into the heater, and the second anemometer
was placed in the exhaust vent. Because of instrumentation limits, the sampling rate for the
anemometers was one sample every 2 seconds. However, this was adequate since the
analysis only needed to show trends in airflow.
Table 3. General Locations of Thermocouples and Anemometers
Thermocouple
Number
Location
T1 Heater Intake
T2 Heater Exhaust
T3 Heater Housing
T4 Vent Exhaust
T5 Intake into Blower
T6 Intake into the Tumbler
T7 Ambient Room
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Table 3-continued. General Locations of Thermocouples and Anemometers
Anemometer
Number
Location
V1 Heater Intake
V2 Dryer Exhaust
2.2.3 Global Test Procedure and Setup
All dryers were tested at the high heat settings with no test load, with a dry load and with
a wet load, unless otherwise specified. All dryers were tested with 25%, 50%, 75%, and 100%
blockage in the exhaust vent with no test load and with a dry load, unless otherwise
specified.
Some of the clothes dryer instruction manuals specified a maximum load. The smallest
of those was chosen as the test load. The standard test load was 10 bath towels of white 100%
cotton terry, 45” long by 25.5” to 27” wide, each weighing approximately 570g (1.25 lbs.).
For
wet loads, the loads were washed and spun dry in a standard size washing machine set for
Hot/Cold (hot wash/cold rinse) and Regular 10 [minutes]. No detergent was used in the wash.
The clothes dryers were set for drying times of 60 minutes for wet loads and 15 minutes
for dry loads using the Timed Dry feature.
To accelerate the test process, the data for dry loads with no exhaust vent blockage was
compared with data for wet loads with no exhaust vent blockage. If the thermocouple and
airflow data were similar, additional tests with fully-blocked or partially-blocked exhaust
vents
were conducted using dry loads only (unless otherwise noted).
The rodent screen was checked and cleaned after each set of dryer design tests.
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2.2.4 Dryer Design A
Dryer Design A was configured with the heater at the rear bottom of the dryer and the
lint screen at the rear top of the dryer. The heated air entered one opening at the rear of
the
tumbler, and the moist air exited through a separate opening at the rear of the tumbler, as
shown in Figure 8.
Inside the Tumbler
Back of the Tumbler
(rear cover removed)
Warm Moist Air
exits the tumbler
Exhaust Dryer
Duct Intake Dryer
Duct
Heated air into
the tumbler
Figure 8. Dryer Design A Intake and Exhaust
2.2.4.1 Dryer Design A – Temperature and Airflow Characteristics, Blocked and
Unblocked Exhaust Vent Conditions
The dryer was tested at the high heat setting with no test load, with a dry load and with
a
wet load. Figure 9 (at the end of this section) shows a graph of the thermocouple and airflow
measurement data for Dryer Design A with a wet load. (Similar graphs of data for the tests
with
no load and with a dry load are contained in Appendix A.) As can be seen in the graph, the
dryer was started approximately 120 seconds after data collection began. The primary
thermostat disconnected power to the heater approximately 3183 seconds into the test.
It was observed that the exhaust vent airflow (V2) steadily decreased as the dryer
operated. This was caused by the lint screen progressively becoming blocked with lint.
Table 4 below lists the average, minimum, and maximum temperatures recorded during
the main drying phase. The data listed in the table cover the period between 120 seconds
after
the dryer was started to 120 seconds before the primary thermostat removed power from the
heating element (to eliminate any unstabilized readings from energizing and de-energizing the
heating element).
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Table 4. Dryer Design A Measurement Data (°C and sfpm)
Temp Min Average Max Flow Min Average Max
T1 43 62 73 V1 718 825 952
T2 249 285 323 V2 871 1034 1234
T3 81 109 127
T4 33 56 70
T5 37 58 84
T6 198 236 262
T7 21 23 24
The dryer was tested with a partially-blocked exhaust vent (25%, 50%, 75% blockage),
and a 100% blocked exhaust vent. For these tests, dry loads were used, since the temperature
data from the thermocouples were similar for both dry and wet loads for the unblocked
condition. (Appendix A shows the comparison between the dry and wet towel loads with no
blockage in the exhaust vent.)
Figures 10 through 12 (at the end of this section) show a comparison of temperature
measurement data for thermocouples T1, T3 and T2 for unblocked, partially-blocked, and
fullyblocked
vent conditions. (Appendix A contains graphs of the temperature data for the remaining
thermocouples, T4, T5, T6 and T7.)
In Figure 10, it can be seen that the dryer began to operate on the high limit thermostat
only when the exhaust duct was fully (100%) blocked. For the 25% and 50% blocked exhaust
vent conditions, the temperatures measured were similar to those measured for the unblocked
exhaust vent condition.
With a 75% blocked exhaust vent, the dryer still operated on the primary thermostat but
at elevated temperatures; Figure 11 shows the heater box reached up to 150°C. In the
unblocked condition or with exhaust vent blockages of 25% or 50%, the temperature of the
heater box reached slightly over 100°C.
At exhaust vent blockages of 100% and 75%, the peak heater exhaust temperatures
were similar – around 325°C to 375°C – as shown in Figure 12. However, the period during
which the heater exhaust operated near the peak temperature differed for the two conditions.
At
100% blockage, the temperature was not maintained at the peak temperature very long
because the high limit thermostat cycled the heating element more rapidly. At 75% blockage,
the duration the heater exhaust stayed near the peak temperature was approximately 100 to
300 seconds – until the primary thermostat switched the heating element off
.
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Dryer Design A
Wet Load, High Heat Setting
November 19, 2001
0
50
100
150
200
250
300
350
0
124
248
372
496
620
744
868
992
1116
1240
1364
1488
1612
1736
1860
1984
2108
2232
2356
2480
2604
2728
2852
2976
3100
3224
3348
3472
3596
3720
3844
3968
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Heater Housing
T4 Exhaust Vent T5 Intake into Blower T6 Tumbler Intake
T7 Room Ambient V1 Airflow into Heater V2 Airflow Exhaust Vent
T6
T4
T2
T3
T7
T1
T5
V2
V1
Figure 9. Dryer Design A – Thermocouple and Airflow Measurements with a Wet
Load
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Dryer Design A
T1 Heater Intake Temperature
0%, 25%, 50%, 75%, and 100% Blocked
Dry Load, December 2001
0
20
40
60
80
100
120
140
160
0
41
82
123
164
205
246
287
328
369
410
451
492
533
574
615
656
697
738
779
820
861
902
943
984
Time (seconds) Temperature (
C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 10. Dryer Design A – T1 Heater Intake Comparison
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Dryer Design A
T3 Heater Housing Temperature
0%, 25%, 50%, 75%, and 100% Blocked
Dry Load, December 2001
0
50
100
150
200
250
0
41.001
82
123
164.001
205
246
287.001
328
369.001
410
451
492
533
574.001
615
656.001
697
738.001
779
820
861
902
943
984
Time (seconds)
Temperature (C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 11. Dryer Design A – T3 Heater Housing Comparison
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Dryer Design A
T2 Heater Exhaust Temperature
0%, 25%, 50%, 75%, and 100% Blocked
Dry Load, December 2001
0
100
200
300
400
500
600
0
40
80
120
160
200
240
280
320
360
400
440
480
520
560
600
640
680
720
760
800
840
880
920
960
100
Time (seconds)
Temperature (C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 12. Dryer Design A – T2 Heater Exhaust Comparison
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2.2.4.2 Dryer Design A – Airflow
Airflow was measured using hot wire anemometers placed at the intake of the
heater
box and in the exhaust vent. Both anemometers were placed in the center of the cross-section
of the air streams. The anemometer placed in the exhaust vent was positioned 36” after the
first
elbow to avoid circular turbulence effects from the dryer blower. As mentioned earlier, the
iris
and blast plate for the partially-blocked and 100% blocked conditions, respectively, were
placed
before the hot wire anemometer located in the exhaust vent.
Figure 13 shows comparisons of intake air velocities when the exhaust vent was
unblocked, partially blocked, and fully blocked, and the dryer contained a dry load. The
intake
velocities were similar for the unblocked and 25% and 50% blocked conditions. There was a
slight drop in the air velocity for the 75% blocked condition, but it was not significant enough
to
cause the dryer to operate in the high-limit cycling mode (as can be observed from the
thermocouple data in Figure 12). With a 100% blocked exhaust vent, the air intake velocity
dropped to about 200 sfpm.
Figure 14 is a graph comparing exhaust air velocities when the exhaust vent was
unblocked, partially blocked, and fully blocked, and the dryer contained a dry load. The
graph
shows a slight overshoot in air velocity at initial startup of the dryer. The size of the
overshoot
decreased as the blockage increased. This was expected, since the anemometer was
positioned after the blockage, which created a dampening effect. There was a slight increase
in
the velocity around 400 seconds, which was when the primary thermostat de-energized the
heater. (This was also seen in Figure 9 – Thermocouple and Airflow with a Wet Load, when the
dryer began cycling.) This was caused by the hot wire anemometer not responding fast enough
to the change in temperature.
Figure 15 shows the average intake and exhaust air velocities for the different conditions
of exhaust blockage. The data from 120 seconds to 1000 seconds were averaged. This
excluded the initial overshoot when the blower was first powered. The intake air velocity
stayed
fairly constant for the unblocked, 25% and 50% blocked conditions; however, it began to
decrease more rapidly when the exhaust vent was 50% and 75% blocked. At 100% blockage,
the exhaust air velocity was near zero, as expected, and the average intake velocity was 179
sfpm.
Figure 16 is a graph similar to Figure 15 except that the dryer was operated with no test
load and with no heat. The graph shows increased intake and exhaust air velocities when the
tumbler was not filled as expected. Compared to Figure 15, there was a slightly more dramatic
effect in the decrease of the air velocities as the exhaust vent was progressively blocked.
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Dryer Design A
Velocity Intake Comparison
High Heat, Dry Load
0
200
400
600
800
1000
1200
1400
0 22 44 66 88 110 132 154 176 198 220 242 264 286 308 330 352 374 396 418 440 462 484
Time (seconds)
Velocity (sfpm)
No Block Intake 100% Blocked Intake 75% Blocked Intake
50% Blocked Intake 25% Blocked Intake
Figure 13. Dryer Design A – Intake Air Velocity
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Dryer Design A
Velocity Exhaust Comparison
High Heat, Dry Load
0
200
400
600
800
1000
1200
1400
0 22 44 66 88 110 132 154 176 198 220 242 264 286 308 330 352 374 396 418 440 462 484
Time (seconds)
Velocity (sfpm)
No Block Exhaust 100% Blocked Exhaust 75% Blocked Exhaust
50% Blocked Exhaust 25% Blocked Exhaust
Figure 14. Dryer Design A – Exhaust Air Velocity
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Dryer Design A
Intake and Exhaust Velocity Average
High Heat, Dry Load
December 2001
0
200
400
600
800
1000
1200
0 (No Block) 25 50 75 100 (Fully
Blocked)
Blockage (percentage)
Average Velocity (sfpm)
Intake
Exhaust
Figure 15. Dryer Design A – Average Airflow Velocities (High Heat and Dry Load)
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Dryer Design A
Intake and Exhaust Velocity Average
No Heat, No Load
December 2001
0
200
400
600
800
1000
1200
1400
1600
1800
0 (No Block) 25 50 75 100 (Fully
Blocked)
Blockage (percentage)
Average Velocity (sfpm)
Intake
Exhaust
Figure 16. Dryer Design A – Average Airflow Velocities (No Heat and No Load)
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2.2.4.3 Dryer Design A – Primary Thermostat Bypassed
The primary thermostat was bypassed to simulate a failure mode in which it
failed
closed. The thermostat was removed from the intake blower housing, and the hole it left was
covered with foil tape. The thermostat was placed in a cool part of the dryer to prevent it
from
switching open. The dryer was operated with full airflow (no exhaust vent blockage) and a dry
load.
Figure 17 shows the temperature and airflow data for Dryer Design A with a bypassed
thermostat. The dryer was switched to Air Dry at approximately 1500 seconds into the test.
The temperature at the heater intake appeared to level out at 95° C and would not have
triggered the High-Limit Thermostat.
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Dryer Design A
Bypassed Primary Thermostst
Dry Load
December 10, 2001
0
50
100
150
200
250
300
350
400
450
0
53
106
159
212
265
318
371
424
477
530
583
636
689
742
795
848
901
954
1007
1060
1113
1166
1219
1272
1325
1378
1431
1484
1537
1590
1643
1696
1749
1802
1855
1908
1961
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Heater Housing T4 Exhaust Vent T5 Intake into
Blower
T6 Tumbler Intake T7 Room Ambient V1 Airflow into Heater V2 Airflow Exhaust Vent
T6
T4
T2
T3
T7
T1
T5
V2
V1
Heater Element turned Off
Switched to Air Dry @ 1500s
Figure 17. Dryer Design A – Primary Thermostat Bypassed
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2.2.5 Dryer Design B
Dryer Design B was configured with the heating element at the rear bottom of the dryer
and the lint screen at the front of the dryer. The heated air entered the rear of the tumbler,
and
the moist air exited through an opening at the top of the lint screen, as shown in Figure 18.
Warm Moist Air
exits the tumbler
Heated air into
the tumbler
Lint Screen
Figure 18. Dryer Design B – Air Flow Through the Tumbler
2.2.5.1 Dryer Design B – Temperature and Airflow Characteristics, Blocked and
Unblocked
The clothes dryer was tested at the high heat setting with no test load, with a dry
load
and with a wet load. Figure 19 (at the end of this section) shows a graph of the thermocouple
and airflow measurement data for Dryer Design B with a wet load. (Graphs of data for similar
tests with no load and with a dry load are contained in Appendix B.) As can be seen in the
graph, the clothes dryer was started approximately 30 seconds after data collection began.
The
primary thermostat disconnected power to the heater approximately 3721 seconds into the test.
It can be observed that the exhaust vent (V2) airflow significantly decreased over time as
the dryer was operating. This was caused by the lint screen becoming progressively blocked
with lint. This graph shows a larger drop in exhaust airflow than previously observed for
Dryer
Design A. In Dryer Design A, the exhaust air velocity dropped from approximately 1300 to 950
sfpm, or a delta of 350 sfpm. In Dryer Design B, the exhaust air velocity dropped from
approximately 1400 sfpm to 800 sfpm, or a delta of 600 sfpm.
Table 5 below lists the average, minimum, and maximum temperatures recorded during
the main drying phase. The data listed in the table cover the period between 120 seconds
after
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the dryer was started to 120 seconds before the primary thermostat removed power from the
heating element (to eliminate any unstabilized readings during energizing and de-energizing
of
the heating element).
Table 5. Dryer Design B Statistics (°C and sfpm)
Temp Min Average Max Flow Min Average Max
T1 35 48 57 V1 307 375 483
T2 214 252 282 V2 796 1181 1432
T3 61 81 92
T4 31 50 67
T5 31 51 73
T6 140 167 183
T7 22 24 26
The dryer was tested with a partially-blocked exhaust vent (25%, 50%, 75% blockage)
and 100% blocked exhaust vent. For these tests, wet towels (washed and spun dry) were used,
since there was enough variation in the temperature data between dry and wet loads.
(Appendix B shows the comparison between the dry and wet towel loads with no blockage in
the exhaust vent.)
Figures 20 to 22 (at the end of this section) show comparisons of temperature data for
thermocouples T1, T3, and T5 for unblocked, partially-blocked, and fully-blocked vent
conditions. (Appendix B contains graphs of the temperature data for the remaining
thermocouples, T2, T4, T6, and T7.)
In Figure 20, it can be seen that the dryer began to operate on the high limit thermostat
only when the exhaust duct was either 75% or 100% blocked. For the 25% and 50% blocked
exhaust vent conditions, the temperatures measured were similar to those measured for the
unblocked exhaust vent condition.
With a 75% and 100% blocked exhaust vent, the dryer operated on the high-limit
thermostat. However, in the 75% blocked vent condition, the dryer operated at a higher
temperature than that seen for the 100% blocked condition. Figure 21 shows the heater box
reached up to 150°C for the 75% blocked condition whereas, for the 100% blocked condition,
the high-limit thermostat periodically switched the temperature at approximately
125°C.
The period at which the high-limit thermostat switched on and off also differed for the
75% and 100% blocked exhaust vent conditions. This was most likely caused by the difference
in airflow for the two conditions. With the 75% blockage, the additional airflow resulted in
an
increase in the time required for the high-limit thermostat to reach its set point for opening.
In
addition, the periodic rate at which the thermostat switched on and off changed during the
course of the test; the high limit thermostat began switching at a faster rate around 2850
seconds. This was most likely caused by the lint screen becoming progressively blocked, which
further reduced airflow through the dryer.
Figure 20 also shows that the temperatures measured for the unblocked, 25% blocked
and 50% blocked exhaust vent conditions are similar – both in value and signature – until
2331
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29
seconds. The graph for the 50% blocked condition shows the heating element began switching
rapidly on the primary thermostat, which is explained in the next paragraph.
Figure 21 shows that the temperature at which the operating thermostat switches on and
off is nearly equal for the unblocked, 25% blocked and the 50% blocked conditions. The rapid
switching of the primary thermostat was caused by the load no longer tumbling, although the
drum was still rotating. (More on this phenomenon is discussed under Dryer Design D.) This
effect was more evident for the 50% blocked exhaust condition, as shown in Figure 22.
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Dryer Design B
Wet Load, No Blockage
January 30, 2002
0
50
100
150
200
250
300
0
110
220
330
440
550
660
770
880
990
1100
1210
1320
1430
1540
1650
1760
1870
1980
2090
2200
2310
2420
2530
2640
2750
2860
2970
3080
3190
3300
3410
3520
3630
3740
3850
3960
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
1800
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Heater Housing T4 Exhaust Vent T5 Intake into
Blower
T6 Heater Exhaust T7 Room Ambient V1 Airflow into Heater V2 Airflow Exhaust Vent
T6
T4
T2
T3
T7
T1 T5
V2
V1
Figure 19. Dryer Design B – Thermocouple and Airflow Measurements (Wet Load)
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Dryer Design B
T1 Heater Intake Temperature
0%, 25%, 50%, 75%, and 100% Blocked
Wet Load
January 2002
0
10
20
30
40
50
60
70
80
90
0
159
318
477
636
795
954
1113
1272
1431
1590
1749
1908
2067
2226
2385
2544
2703
2862
3021
3180
3339
3498
3657
3816
3975
Time (seconds) Temperature (
C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 20. Dryer Design B – T1 Heater Intake Comparison
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Dryer Design B
T3 Heater Housing Temperature
0%, 25%, 50%, 75%, and 100% Blocked
Wet Load
January 2002
0
50
100
150
200
250
0
161
322
483
644
805
966
1127
1288
1449
1610
1771
1932
2093
2254
2415
2576
2737
2898
3059
3220
3381
3542
3703
3864
Time (seconds)
Temperature (C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 21. Dryer Design B – T3 Heater Housing Comparison
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Dryer Design B
T5 Intake into Blower Temperature
0%, 25%, 50%, 75%, and 100% Blocked
Wet Load
January 2002
0
10
20
30
40
50
60
70
80
90
100
0
160
320
480
640
800
960
1120
1280
1440
1600
1760
1920
2080
2240
2400
2560
2720
2880
3040
3200
3360
3520
3680
3840
4000
Time (seconds)
Temperature (C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 22. Dryer Design B – T5 Intake into the Blower Comparison
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2.2.5.2 Dryer Design B – Airflow
The airflow was measured using hot wire anemometers placed at the air intake into
the
heater box and in the exhaust vent. Both anemometers were placed in the center of the
crosssection
of the air streams. The anemometer placed in the exhaust vent was positioned 36” after
the first elbow to avoid circular turbulence effects from the dryer blower. As mentioned
earlier,
the iris and blast plate for the partially and 100% blocked conditions, respectively, were
placed
before the hot wire anemometer.
Figure 23 shows the intake air velocity comparisons for the unblocked, partially blocked,
and fully blocked exhaust vent conditions. The intake air velocities were similar for the
unblocked, 25% and 50% blocked conditions. There was a slight drop in the velocity for the
75% blocked condition that was significant enough to cause the dryer to operate in the
high-limit
cycling mode, as seen in the thermocouple data (shown previously in Figure 21). When the
exhaust vent was 100% blocked, the intake air velocity fluctuated between near zero to about
200 sfpm. The fluctuation to 200 sfpm coincided with the heating element being de-energized
by the high-limit thermostat. This was caused by the anemometer not responding fast enough
to the change in temperature at the heater intake.
Figure 24 shows the exhaust air velocity comparisons for the unblocked, partially
blocked, and fully blocked exhaust vent conditions. The graph shows a slight overshoot at
initial
startup of the dryer, but it was not as prominent as that seen in Dryer Design A. The size of
the
overshoot decreased as the blockage increased, as expected, since the second anemometer, in
the exhaust vent, was located after the iris.
Figure 25 shows the average intake and exhaust air velocities for the different blockage
conditions. The data from 120 seconds to 1000 seconds were averaged. This avoided the initial
overshoot when the blower was first powered. The intake velocity stayed steady until the
exhaust vent was 25% blocked, and it began to decrease more rapidly between 25% and 50%
blockage. When the exhaust vent was 100% blocked, the exhaust velocity was near zero, as
expected, and the average intake air velocity was 90 sfpm.
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Dryer Design B
Velocity Intake Comparison
High Heat, Wet Load
0
200
400
600
800
1000
1200
1400
0 22 44 66 88 110 132 154 176 198 220 242 264 286 308 330 352 374 396 418 440 462 484
Time (seconds)
Velocity (sfpm)
No Block Intake 100% Blocked Intake 75% Blocked Intake
50% Blocked Intake 25% Blocked Intake
Figure 23. Dryer Design B – Intake Air Velocity
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Dryer Design B
Velocity Exhaust Comparison
High Heat, Wet Load
0
200
400
600
800
1000
1200
1400
1600
1800
0 22 44 66 88 110 132 154 176 198 220 242 264 286 308 330 352 374 396 418 440 462 484
Time (seconds)
Velocity (sfpm)
No Block Exhaust 100% Blocked Exhaust 75% Blocked Exhaust
50% Blocked Exhaust 25% Blocked Exhaust
Figure 24. Dryer Design B – Exhaust Vent Air Velocity
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Dryer Design B
Intake and Exhaust Velocity Average
High Heat, Wet Load
January 2002
0
200
400
600
800
1000
1200
1400
1600
0 (No Block) 25 50 75 100 (Fully
Blocked)
Blockage (percentage)
Average Velocity (sfpm)
Intake
Exhaust
Figure 25. Dryer Design B – Average Airflow Velocities (High Heat and Wet Load)
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2.2.5.3 Dryer Design B – Primary Thermostat Bypassed
The primary thermostat was bypassed to simulate a failure mode in which it had
failed
closed. The thermostat was removed from the intake blower housing, and the hole it left was
covered with foil tape. The thermostat was placed in a cool part of the dryer to prevent it
from
switching open. The dryer was operated with full airflow and with a wet load.
Figure 26 shows the thermocouple and airflow measurement data for Dryer Design B
when the thermostat was bypassed. The dryer’s Timed Dry feature shut off at approximately
4000 seconds. The dryer was restarted for an additional 30 minutes with the Timed Dry
feature.
The temperature at the heater box intake appeared to level out at approximately 65° C and
would not have triggered the High-Limit Thermostat.
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Dryer Design B
Bypassed Primary Thermostst
Wet Load
February 11, 2002
0
50
100
150
200
250
300
350
400
0
163
326
489
652
815
978
1141
1304
1467
1630
1793
1956
2119
2282
2445
2608
2771
2934
3097
3260
3423
3586
3749
3912
4075
4238
4401
4564
4727
4890
5053
5216
5379
5542
5705
5868
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Heater Housing T4 Exhaust Vent T5 Intake into
Blower
T6 Heater Exhaust T7 Room Ambient V1 Airflow into Heater V2 Airflow Exhaust Vent
T6
T4
T2
T3
T7
T5 T1
V2
V1
Dryer Timed Setting
Switched Off
Reset Timed Setting
for 30 minutes
Switched Off
Figure 26. Dryer Design B – Primary Thermostat Bypassed
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2.2.6 Dryer Design C
Dryer Design C was configured with the heating element at the rear-top of the dryer and
the lint screen at the front of the dryer. The heated air entered the rear of the tumbler, and
the
moist air exited through openings inside the front cover that led to the lint screen, as shown
in
Figure 27.
Warm Moist Air
exits the tumbler
Heated air into
the tumbler
Lint Screen
Inside View of
Front Cover
Figure 27. Dryer Design C – Airflow through the Tumbler
2.2.6.1 Dryer Design C – Temperature and Airflow Characteristics, Blocked and
Unblocked
The dryer was tested at the high heat setting with no load, with a dry load and with a
wet
load. Figure 28 shows the thermocouple and airflow measurement data for Dryer Design C with
a wet load. (Graphs of data obtained for the tests with no load and with a dry load are
contained in Appendix C.) The dryer was started approximately 60 seconds after data
collection
began. The primary thermostat disconnected power to the heating element at approximately
3664 seconds into the test.
This graph shows a significant decrease in the exhaust air velocity as the dryer
continued to operate. As seen in Dryer Designs A and B, this was caused by the lint screen
becoming progressively blocked with lint. In Dryer Design C, the exhaust air velocity dropped
from approximately 1400 sfpm to 700 sfpm, a delta of 700 sfpm.
Table 6 below lists the average, minimum, and maximum temperatures recorded during
the main drying phase. The data listed in the table cover the period from 120 seconds after
the
dryer was started to 120 seconds before the primary thermostat removed power from the
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heating element (to eliminate any instability in the readings from energizing and
de-energizing
the heating element).
Table 6. Dryer Design C – Statistics (°C and sfpm)
Temp Min Average Max Flow Min Average Max
T1 36 54 72 V1 111 197 296
T2 106 140 177 V2 654 1023 1450
T3 34 50 67
T4 31 48 61
T5 31 48 63
T6 146 185 228
T7 20 21 22
The dryer was tested with a partially-blocked exhaust vent (25%, 50% and 75%
blockage) and a 100% blocked exhaust vent. For these tests, wet towels (washed and spun
dry) were used. (A comparison of temperature and airflow data obtained for the dry and wet
towel loads with no blockage in the exhaust vent are contained in Appendix C.)
Figures 29 through 31 (at the end of this section) show a comparison of the temperature
measurement data for thermocouples T1, T2, and T5 for unblocked, partially-blocked, and
fullyblocked
vent conditions. (Appendix C contains graphs of the temperature measurement data
for unblocked, partially-blocked, and fully-blocked vent conditions for the remaining
thermocouples, T3, T4, T6, and T7.)
Figure 29 shows that the dryer operated on the high-limit thermostat only when the
exhaust duct was 75% or 100% blocked. For the 25% and 50% blocked exhaust vent
conditions, the temperatures measured were similar to those measured when the exhaust vent
was unblocked.
Figure 29 also shows that, with 75% and 100% exhaust vent blockages, there appeared
to be a difference in the temperature at which the high-limit thermostat operated. At 75%
blockage, upper high-limit switching occurred around 195°C; however, for a 100% blocked
exhaust vent, upper high limit switching appeared to have occurred at 225°C. This discrepancy
in the data was caused by the location of the T1 thermocouple and the orientation of the
heater
box. As mentioned previously, Dryer Design C drew intake air into the heater box from the top
and directed it downward toward – and eventually into – the tumbler. When the exhaust vent
was 75% blocked, there was slight airflow over the heating element to direct some heat away
from the T1 thermocouple and the high-limit thermostat. With 100% blockage, convective heat
caused the T1 thermocouple to read a higher value – a higher temperature than was actually
present at the high-limit thermostat when it opened.
Figure 30 shows that when the exhaust vent was either partially or fully blocked, the
heater housing temperature was only slightly above the maximum temperature for the
unblocked vent condition. The operation of the high-limit thermostat prevented the heater box
temperature from increasing much higher than 80°C.
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Figure 31 shows the air temperature at the intake to the blower reached around 50°C in
the unblocked and partially-blocked conditions, until around 2100 seconds. For the unblocked,
25% and 50% blocked conditions, the temperature continued to increase to over 60°C. With the
exhaust vent 75% blocked, switching of the high-limit thermostat prevented the temperature of
the air entering the blower from increasing to the set point at which the primary thermostat
would open.
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Dryer Design C
High Heat, Wet Load
December 11, 2001
0
50
100
150
200
250
0
109
218
327
436
545
654
763
872
981
1090
1199
1308
1417
1526
1635
1744
1853
1962
2071
2180
2289
2398
2507
2616
2725
2834
2943
3052
3161
3270
3379
3488
3597
3706
3815
3924
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
1800
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Heater Housing T4 Exhaust Vent T5 Intake into
Blower
T6 Heater Exhaust T7 Room Ambient V1 Airflow into Heater V2 Airflow Exhaust Vent
T6
T4
T2
T3
T7
T1
T5
V2
V1
Figure 28. Dryer Design C – Thermocouple and Airflow Measurements (Wet Load)
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Dryer Design C
T1 Heater Intake Temperature
0%, 25%, 50%, 75%, and 100% Blocked
High Heat, Wet Load
December 2001
0
20
40
60
80
100
120
140
0
161
322
483
644
805
966
1127
1288
1449
1610
1771
1932
2093
2254
2415
2576
2737
2898
3059
3220
3381
3542
3703
3864
Time (seconds)
Temperature (C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 29. Dryer Design C – T1 Heater Intake Comparison
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Dryer Design C
T3 Heater Housing Temperature
0%, 25%, 50%, 75%, and 100% Blocked
High Heat, Wet Load
December 2001
0
10
20
30
40
50
60
70
80
90
100
0
163
326
489
652
815
978
1141
1304
1467
1630
1793
1956
2119
2282
2445
2608
2771
2934
3097
3260
3423
3586
3749
3912
Time (seconds) Temperature (
C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 30. Dryer Design C – T3 Heater Housing Comparison
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46
Dryer Design C
T5 Intake into Blower Temperature
0%, 25%, 50%, 75%, and 100% Blocked
High Heat, Wet Load
December 2001
0
10
20
30
40
50
60
70
0
158
316
474
632
790
948
1106
1264
1422
1580
1738
1896
2054
2212
2370
2528
2686
2844
3002
3160
3318
3476
3634
3792
3950
Time (seconds) Temperature (
C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 31. Dryer Design C – T5 Intake into the Blower Comparison
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2.2.6.2 Dryer Design C – Airflow
The airflow was measured using hot wire anemometers placed at the intake airflow
into
the heater box and in the exhaust vent. Both anemometers were placed in the center of the
cross-section of the air streams. The anemometer placed in the exhaust vent was positioned
36” after the first elbow to avoid circular turbulence effects from the dryer blower. As
mentioned
earlier, the iris and blast plate for the partially- and fully-blocked conditions, respectively,
were
placed before the hot wire anemometer located in the exhaust vent.
Figure 32 shows the intake air velocity comparisons for unblocked, partially-blocked, and
fully-blocked exhaust vent conditions. The intake air velocity appeared to have very low
readings; this was caused by the presence of side louvers in the heater box. The anemometer
did not measure the air intake from the side louvers. As seen previously when the exhaust
vent
was 100% blocked, the intake air velocity fluctuated between near zero to about 200 sfpm. The
fluctuation to 200 sfpm coincided with the heating element being de-energized by the
high-limit
thermostat. This was caused by the anemometer not responding fast enough to the
temperature change when the heater was de-energized.
Figure 33 shows the exhaust air velocity comparisons for unblocked, partially-blocked,
and fully-blocked exhaust vent conditions. The graph shows a slight overshoot at initial
startup
of the dryer, but it is not as prominent as that seen for Dryer Design A. The size of the
overshoot decreased as the blockage increased, as expected, since the second anemometer
was located after the exhaust vent blockage. The exhaust air velocity decreased when the
exhaust vent was 25% and 50% blocked, and it decreased further when the exhaust vent was
75% blocked.
Figure 34 shows the average intake and exhaust air velocities for the different blockage
conditions. The data from 120 seconds to 1000 seconds were averaged. This avoided the
initial overshoot when the blower was first powered.
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Dryer Design C
Velocity Intake Comparison
High Heat, Wet Load
0
200
400
600
800
1000
1200
1400
0 22 44 66 88 110 132 154 176 198 220 242 264 286 308 330 352 374 396 418 440 462
484
Time (seconds)
Velocity (sfpm)
No Block Intake 100% Blocked Intake 75% Blocked Intake
50% Blocked Intake 25% Blocked Intake
Figure 32. Dryer Design C – Intake Air Velocity
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49
Dryer Design C
Velocity Exhaust Comparison
High Heat, Wet Load
0
200
400
600
800
1000
1200
1400
1600
1800
0 22 44 66 88 110 132 154 176 198 220 242 264 286 308 330 352 374 396 418 440 462
484
Time (seconds)
Velocity (sfpm)
No Block Exhaust 100% Blocked Exhaust 75% Blocked Exhaust
50% Blocked Exhaust 25% Blocked Exhaust
Figure 33. Dryer Design C – Exhaust Vent Air Velocity
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Dryer Design C
Intake and Exhaust Velocity Average
High Heat, Wet Load
December 2001
0
200
400
600
800
1000
1200
1400
1600
0 (No Block) 25 50 75 100 (Fully
Blocked)
Blockage (percentage)
Average Velocity (sfpm)
Intake
Exhaust
Figure 34. Dryer Design C – Average Airflow Velocities (High Heat and Wet Load)
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2.2.6.3 Dryer Design C – Primary Thermostat Bypassed
The primary thermostat was bypassed to simulate a failure mode in which it
failed
closed. The thermostat was removed from the intake blower housing, and the hole it left was
covered with foil tape. The thermostat was placed in a cool part of the dryer to prevent it
from
switching open. The dryer was operated with full airflow (no exhaust blockage) and with a dry
load.
Figure 35 shows the thermocouple and airflow measurement data for Dryer Design C
with a bypassed primary thermostat. The graph shows that the dryer began operating on the
high-limit thermostat. Since evaluations of Dryer Designs A and B did not result in
high-limit
cycling when the primary thermostat was bypassed, inspection for a blocked exhaust vent was
conducted. It was determined that lint on the rodent screen at the vent hood reduced airflow
sufficiently to cause the dryer to operate in the high-limit cycling mode. The lint
(approximately
0.69 grams and distributed evenly) was removed at approximately 2000 seconds into the test.
For the remainder of the test, the dryer no longer cycled on the high-limit thermostat.
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Dryer Design C
Primary Thermostat Bypassed
Dry Load
January 02, 2002
0
50
100
150
200
250
0
127
254
381
508
635
762
889
1016
1143
1270
1397
1524
1651
1778
1905
2032
2159
2286
2413
2540
2667
2794
2921
3048
3175
3302
3429
3556
3683
3810
3937
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
Air Flow (sfpm)
T1 Heater Intake T2 heater Exhaust (3.5) T3 Heater Housing
T4 Exhaust Vent T5 Intake into Blower T6 Heater Exhaust (5.5)
T7 Room Ambient V1 Airflow into Heater V2 Airflow Exhaust Vent
T6
T4
T2
T3
T7
T1
T5
V2
V1
Switching on the
High-Limit Thermostat
Cleaned Blocked
Exhaust
Figure 35. Dryer Design C – Primary Thermostat Bypassed
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2.2.7 Dryer Design D
Dryer Design D was configured with the heating element behind the tumbler and the lint
screen at the front of the dryer. The heated air entered the rear of the tumbler, and the moist
air
exited through openings at the rear of the front cover leading to the lint screen, as shown
in
Figure 36.
Warm Moist Air
exits the tumbler
Heated air into
the tumbler
Lint Screen
Inside View of
Front Cover
Figure 36. Dryer Design D – Airflow Through the Tumbler
2.2.7.1 Dryer Design D – Temperature and Airflow Characteristics, Blocked and
Unblocked
Limited testing of this clothes dryer at the high heat settings with no load, with a dry
load
and with a wet load was conducted. At the end of the wet load testing (60 minutes on Timed
Dry), the load was still very damp and had two different cycling periods as shown later.
To determine why the heater had two different cycling periods and the load was still
damp at the end of the test, a relay and a 9-volt battery were placed in parallel with the
heating
element and the high-limit thermostat, as shown in Figure 37. The T2 thermocouple was
replaced with the signal from the 9-volt battery. The relay monitored the voltage between the
primary and high-limit thermostats. If both thermostats were in their normally-closed
positions,
the relay would apply 9-volts (actually ~ 7 volts) to the T2 input of the data acquisition system. If
the primary thermostat opened, the relay would close and the signal to the data acquisition
system would be near zero. If the high-limit thermostat opened, the relay would still be
powered
and a 9-volt signal would be recorded on the data acquisition system – and the thermocouples
would indicate that the heating element was cooling down.
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120 VAC
120 VAC
240 VAC
Primary
Thermostat
High-Limit
Thermostat
Heater
Element
9-volt
battery
Relay
To Data
Acquisition (T2)
Figure 37. Setup with a Relay and Battery for T2 Analog
Figure 38 shows the thermocouple and airflow measurement data for Dryer Design D
with a wet load before setup, as shown in Figure 37. The primary thermostat first
disconnected
power to the heater at approximately 779 seconds into the test. The heater cycled
approximately every 374 seconds, until 3133 seconds into the test. After 3133 seconds, it
began to cycle approximately every 190 seconds. At the end of the test – 60 minutes on Timed
Dry – the load was still very damp (Lists of the tests and additional figures for Dryer Design
D
are located in Appendix D.)
The standard load of 10 towels was tested again, but with the T2 analog signal/relay.
Figure 39 shows the thermocouple, airflow, and T2 analog data for a wet load. The graph
shows that the dryer behaved the same as it had in the previous test; i.e., it exhibited the
phenomenon of two periodic cycles for the thermostats. In this case, however, it could be
determined from the T2 signal that the first periodic cycling was caused by the high-limit
thermostat, and the second periodic cycling was caused by the primary thermostat. As before,
the towels were still very damp at the end of the 60-minute (Timed Dry) drying cycle.
The most likely cause for the dryer to begin operating on the high-limit thermostat in the
unblocked exhaust vent condition was inadequate airflow through the tumbler. The dryer
appeared to be responding to an overload condition in which the load restricted the airflow
through the tumbler.
To determine why the primary thermostat began cycling later in the test, the dryer door
was removed and replaced with a sheet of rigid clear plastic. It was observed that, after a
period, the load stopped tumbling and began to ride along the sides of the rotating drum.
This
explained why the dryer began cycling on the primary thermostat. When the load was riding on
the rotating drum, this allowed the heated air to pass freely through the tumbler and into the
lint
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screen/blower. The primary thermostat was able to detect the increase in temperature, causing
it to open or cycle the heater off. When the tumbler temperature cooled, the primary
thermostat
closed or cycled the heater on. This also explained why the load was still damp at the end of
the 60-minute drying cycle. This same phenomenon occurred when the dryer was tested with
wet or damp loads of 10, 7, or 5 towels, as shown in Appendix D. (A video clip demonstrating
this phenomenon is included in Appendix J.) This phenomenon also occurred with Dryer
Design B, but it was not as repeatable as it was with Dryer Design D.
The thermocouple and airflow measurement data for Dryer Design D are not listed here.
The data would not be meaningful because of the operating characteristics of the dryer; i.e.,
immediate high-limit cycling due to overloading and the phenomenon of the load riding along
the
sides of the rotating drum.
To obtain steady and consistent data, the dryer was tested with no load with a
partiallyblocked
exhaust vent (25%, 50%, 75% blockage), and a 100% blocked exhaust vent.
Figures 40 and 41 show comparisons of temperature measurement data for unblocked,
partially-blocked, and fully- blocked exhaust vent conditions for T1 and T3 thermocouples.
(Appendix D contains graphs of the temperature data for the remaining thermocouples, T2, T4,
T5, T6, and T7.)
Figure 40 shows that the dryer began to operate on the high-limit thermostat when the
exhaust duct was 75% or 100% blocked. For the 25% and 50% blocked exhaust vent
conditions, the temperatures measured were similar to those measured when the exhaust vent
was unblocked. For the 75% and 100% exhaust vent blockage conditions, the graph shows two
different periodic rates at which the high-limit thermostat cycled. With the exhaust vent 75%
blocked, the lower set point temperature for the high-limit thermostat was 50° C; when the
exhaust vent was 100% blocked, it appeared to be switching at 100° C. It could not be
determined why the high-limit thermostat was cycling at two different lower set points for
the
75% and 100% blocked conditions.
Figure 41 shows comparisons of temperature data at the heater box T3 thermocouple,
for unblocked, partially-blocked, and fully blocked exhaust vent conditions. The housing
temperature increased to approximately 50°C for the unblocked, 25% and 50% blocked vent
conditions. At blockages of 75% and 100%, the housing temperature increased to
approximately 150°C.
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Dryer Design D
High Heat, Wet Load
January 03, 2002
0
50
100
150
200
250
300
0
106
212
318
424
530
636
742
848
954
1060
1166
1272
1378
1484
1590
1696
1802
1908
2014
2120
2226
2332
2438
2544
2650
2756
2862
2968
3074
3180
3286
3392
3498
3604
3710
3816
3922
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
1800
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Heater Housing T4 Exhaust Vent T5 Intake into
Blower
T6 Tumbler Intake T7 Room Ambient V1 Airflow into Heater V2 Airflow Exhaust Vent
T6
T4
T2
T3
T7
T1
T5
V2
V1
Figure 38. Dryer Design D – Thermocouple and Airflow Measurements (Wet Load)
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Dryer Design D
High Heat, Wet Load, with Analog (T2)
January 07, 2002
0
50
100
150
200
250
300
0
109
218
327
436
545
654
763
872
981
1090
1199
1308
1417
1526
1635
1744
1853
1962
2071
2180
2289
2398
2507
2616
2725
2834
2943
3052
3161
3270
3379
3488
3597
3706
3815
3924
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
1800
Air Flow (sfpm)
T1 Heater Intake Analog T3 Heater Housing T4 Exhaust Vent T5 Intake into
Blower
T6 Tumbler Intake T7 Room Ambient V1 Airflow into Heater V2 Airflow Exhaust Vent
T6
T4
T2 analog
T3
T7
T1
T5
V2
V1
Figure 39. Thermocouple, Airflow and Relay (T2 Analog) Measurements
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Dryer Design D
T1 Heater Intake Temperature
0%, 25%, 50%, 75%, and 100% Blocked
High Heat, No Load
January 2002
0
50
100
150
200
250
300
350
400
0
41
82
123
164
205
246
287
328
369
410
451
492
533
574
615
656
697
738
779
820
861
902
943
984
Time (seconds) Temperature (
C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 40. Dryer Design D – T1 Heater Intake Comparison
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Dryer Design D
T3 Heater Housing Temperature
0%, 25%, 50%, 75%, and 100% Blocked
High Heat, No Load
January 2002
0
50
100
150
200
250
0
41
82
123
164
204.999
246
287
328
369
410
451
492
533
574
614.999
656
697
737.999
779
820
861
901.999
943
984
Time (seconds)
Temperature (C)
No Block 75% Blocked 50% Blocked 25% Blocked 100% Blocked
Figure 41. Dryer Design D – T3 Heater Housing Temperature Comparison
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2.2.7.2 Dryer Design D – Airflow
The airflow was measured using hot wire anemometers placed at the intake airflow
into
the heater and in the exhaust vent. The anemometer at the exhaust vent was placed in the
center of the cross-section of the air stream. The other anemometer was placed at top-center
of
the circular heater intake. The anemometer placed in the exhaust vent was positioned 36”
after
the first elbow to avoid circular turbulence effects from the dryer blower. As mentioned
earlier,
the iris and blast plate for the partially-blocked and fully-blocked conditions, respectively,
were
placed before the hot wire anemometer located in the exhaust vent.
Figure 42 shows the intake air velocity comparisons for the unblocked, partially-blocked,
and fully-blocked exhaust vent conditions with no load. Due to the configuration of the
heating
element, the intake air velocity measurement data were not as accurate as the exhaust air
velocity data. The air intake was configured in a 360° ring behind the tumbler, with a long
narrow slot. The shape and surface area of the heater intake caused the airflow velocity to
be
below the resolution of the anemometer instrumentation.
Figure 43 shows the exhaust air velocity comparisons for the unblocked, partiallyblocked,
and fully-blocked exhaust vent conditions with no load. The graph shows a slight
overshoot at initial startup of the dryer, which was caused by the blower being energized, as
seen with the other dryer designs. There were periodic increases in exhaust airflow, which
corresponded to the heating element cycling off when the primary thermostat opened. The
increase in exhaust airflow was caused by the anemometer responding slowly to the sudden
temperature changes.
Figure 44 shows the average intake and exhaust air velocities for the different blockage
conditions. The data from 120 seconds to 1000 seconds were averaged. This would avoid the
initial overshoot when the blower was first powered. The 25% and 50% blocked exhaust
conditions displayed a more pronounced, higher exhaust airflow velocity than did the
unblocked
condition; this was not seen during examination of the other dryer designs.
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Dryer Design D
Velocity Intake Comparison
High Heat, No Load
0
200
400
600
800
1000
1200
1400
0 22 44 66 88 110 132 154 176 198 220 242 264 286 308 330 352 374 396 418 440 462
484
Time (seconds)
Velocity (sfpm) No Block Intake 100% Blocked Intake 75% Blocked Intake
50% Blocked Intake 25% Blocked Intake
Figure 42. Dryer Design D – Intake Air Velocity
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Dryer Design D
Velocity Exhaust Comparison
High Heat, No Load
0
200
400
600
800
1000
1200
1400
1600
1800
0 22 44 66 88 110 132 154 176 198 220 242 264 286 308 330 352 374 396 418 440 462
484
Time (seconds)
Velocity (sfpm)
No Block Exhaust 100% Blocked Exhaust 75% Blocked Exhaust
50% Blocked Exhaust 25% Blocked Exhaust
Figure 43. Dryer Design D – Exhaust Vent Air Velocity
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Dryer Design D
Intake and Exhaust Velocity Average
High Heat, No Load
January 2002
0
200
400
600
800
1000
1200
1400
1600
0 (No Block) 25 50 75 100 (Fully
Blocked)
Blockage (percentage)
Average Velocity (sfpm)
Intake
Exhaust
Figure 44. Dryer Design D –Average Airflow Velocities (High Heat and No Load)
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2.2.7.3 Dryer Design D – Primary Thermostat Bypassed
The primary thermostat was bypassed to simulate a failed closed primary
thermostat.
The thermostat was removed from the intake blower housing, and the hole was covered with foil
tape. The thermostat was placed in a cool part of the dryer to prevent it from switching
open.
The dryer was operated with full airflow (no exhaust blockage) and a wet (washed and spun
dry)
load. To avoid the phenomenon where the load stopped tumbling after a period, the dryer load
was reduced to six cotton towels. Five of the six towels were of the same size as used
before.
The sixth towel was a different size, approximately 25% larger, to help prevent the load from
riding the tumbler. The T2 analog signal was used to determine if either the primary or the
highlimit
thermostat opened.
Figure 45 shows the thermocouple and airflow measurement data for Dryer Design D
with a bypassed primary thermostat. The load began to ride the sides of the tumbler at
approximately 1900 seconds after the dryer was started. Approximately 3000 seconds after the
dryer was started, the high-limit thermostat opened, as shown by the graph of the T2 analog
signal. For unknown reasons, the T1 thermocouple became very erratic just before the
highlimit
switch opened. The test was stopped at approximately 3250 seconds after the dryer was
started.
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Dryer Design D
Wet Load (6 towels)
January 16, 2002
0
50
100
150
200
250
300
0
123
246
369
492
615
738
861
984
1107
1230
1353
1476
1599
1722
1845
1968
2091
2214
2337
2460
2583
2706
2829
2952
3075
3198
3321
3444
3567
3690
3813
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
1800
Air Flow (sfpm)
T1 Heater Intake T2 Analog T3 Heater Housing
T4 Exhaust Vent T5 Intake into Blower T6 Heater Exhaust
T7 Room Ambient V1 Airflow into Heater V2 Airflow Exhaust Vent
T6
T4
T2
T3
T7
T1
T5
V2
V1
The Load stops
Tumbling
High-Limit Switch Opens
T3
Power Turned Off
Switch Opens
Figure 45. Dryer Design D – Primary Thermostat Bypassed
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2.3 Task 3: Monitor Lint Distribution
The objective of this task was to monitor lint distribution and accumulation in areas
within
a clothes dryer during operation with normal airflow. Lint accumulation was characterized in
one dryer, and an analysis of potential causes for lint accumulation in other dryer designs
was
conducted.
2.3.1 Test Setup
Dryer Design A was selected for testing. It provided the most convenient access to the
interior of the dryer without removing or disturbing any interior dryer components.
Two view ports of clear plastic were installed on opposite sides of the dryer chassis (one
is shown in Figure 46(a)). One view port measured 3.5 inches by 19.5 inches; the other
measured 3.5 inches by 23 inches. A silicone-type caulk was used to seal the edges of the
view ports. During testing, the back panel of the dryer was installed; however, it was
temporarily removed, as shown in Figure 46(b), to inspect the lint accumulation before,
during,
and after testing.
(a) (b)
Figure 46. Dryer Design A Used in Task 3 Testing
The dryer was vented to the outside of the building. All venting material was 4-inch rigid
metal duct. All joints were sealed with foil tape except for the connection to the dryer; the
exhaust vent was secured to the dryer using a 4-inch duct (hose) clamp.
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2.3.2 Test Description
For this task, loads of towels were alternately washed in detergent-free hot water
and
spun dry, then dried for approximately 60 minutes in the dryer using the Timed Dry
feature (one
washing followed by drying equals one cycle). A total of 100 cycles was completed.
After 48
and 100 cycles, the interior of the dryer was inspected and the presence of any
lint
accumulation was documented.
Four loads of towels were used in Task 3 testing; a load consisted of 10 white
cotton
towels. During the first 48 cycles, two of the four loads were used for the test.
For cycles 49
through 100, the remaining two loads were used.
Before each cycle, a load was conditioned overnight (for approximately twelve
hours) in
a conditioning chamber with a controlled relative humidity (RH) of 50
± 5% and a temperature
of
23° to 25° C.
Two test methods to record the amount of lint accumulation in the dryer were
attempted.
Method 1 was to record the weight loss of a load resulting from the washing and
drying cycles.
Method 2 was to visually inspect the interior of the dryer for lint
accumulation.
2.3.2.1 Method 1 –
Weight Loss of Load
Method 1 was used to estimate the amount of
towel material lost (as lint) into the interior
of the test dryer by recording the weight loss of a load resulting from the
washing and drying
cycles. The weight of a conditioned load was recorded before the start of a cycle.
The amount
of towel material lost during washing, the amount that accumulated on the lint
screen during
drying, and the amount lost through the dryer vent were recorded. The remainder of
any lost
material was assumed to have escaped into the interior of the dryer
chassis.
This method proved to be unsuitable for two reasons. The first was the fluctuation
in the
humidity level in the conditioning chamber, which varied by ± 5% RH. The second was the
rate
at which the towels lost moisture outside the conditioning chamber, depending on
the humidity.
To determine the weight variation in the conditioned load due to the fluctuation
in
relatively humidity of the conditioning chamber, a load of towels was stored
overnight in a room
with approximately 25% RH. The load was then transferred to the conditioning
chamber. After
conditioning for a minimum of 14 hours, the towel load was weighed. The consequent
weight
gain was approximately 100g, or 4g per 1% RH. With the conditioning chamber having
a
variation of ± 5% RH, the weight of a 5000g load could vary by ± 20g.
The difference in humidity between the conditioning chamber and the test room
caused
the weight of the towels to vary more significantly than the amount of lint lost.
The towels were
weighed in the test room after they were removed from the conditioning chamber.
During the
time that the towels were weighed, moisture in the towels could evaporate. The
relative
humidity levels in the test room and the conditioning chamber were 20-30% RH and
50 ± 5%
RH, respectively.
The average weights of lint samples collected during one cycle were 0.6g (after
wash),
0.4g (in the lint screen), and 0.2g (in the dryer vent), or a combined weight of
1.2g. The
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uncertainty in the weight of a load ( ± 20g) due to changes in humidity
was much greater than
measurable lint loss due to the washing and drying cycle. It can also be assumed
that the
uncertainty in the weight of a load would be even greater than the measurable lint
lost in the
dryer chassis interior per cycle.
The unsuitability of this test method was further verified by calculating the
standard
deviation of the load weight before wash. The standard deviation was
± 100g, and the
standard
deviation of the mean was ± 20g. In order to detect a loss of
lint during the drying process, it
would be necessary to resolve a weight differential of less than 1.0g and a
relative humidity of
1%. This was not possible with the available test conditions (load weight varying
by ± 20 g and
fluctuation of ± 5% RH in the conditioning
chamber).
2.3.2.2 Method 2 – Visual Examination
Method 2 was to visually observe any accumulation of lint inside the dryer
chassis
before, during, and after testing. After 48 cycles (using the first two loads of towels), a
preliminary count of accumulated lint and dust particles was performed. Some lint
accumulation, as well as some towel degradation, was observed. The lint accumulation test
continued for another 52 cycles using the second two loads of new towels. A total of 100
cycles
was completed.
After 100 cycles, visible evidence of lint accumulation was detected (as shown in Figure
47). A lint ball approximately one inch in diameter was observed inside the chassis. The lint
was white and, therefore, it was inferred that it was from the towel material. There appeared
to
be no significant admixture of room dust, which was not likely to have been white in color.
Figure 47. Lint Accumulated at Bottom of Dryer After 100 cycles
Figure 48 shows lint that accumulated on the interior dryer vent after 48 cycles. Figure
49 shows the same area after 100 cycles. An increase in the number of lint particles was
noticeable.
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Figure 48. Lint Accumulated on Interior of Dryer After 48 Cycles
Figure 49. Lint Accumulated on Interior of Dryer After 100 Cycles
(same location as shown in Figure 48)
The number of lint particles in an approximately half-inch square shown in Figures 48
and 49 was estimated. Approximately 83 particles were counted after 48 cycles; there were
approximately 250 particles after 100 cycles. Some areas of the dryer did not show an
observable gain in lint coverage, while other areas had significant gains.
2.3.3 Examination of Dryer Design A
Figure 50 shows an area of the blower fan housing after 100 cycles. A significant
accumulation of lint was observed at a sealed joint for the blower housing (shown by white
arrow), and a lint ball was observed on the dryer floor (shown by black arrow). This
accumulation was not apparent at the start of Task 3 testing or after 48 cycles.
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Figure 50. Lint Build-up at Blower Housing Seal and Dryer Floor
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2.4 Task 4: Determine Characteristics Required for Lint
Ignition
The main objective of this task was to record the ignition characteristics of lint on,
near,
and ingested into the heater. The test variables used in the Task 4 testing were developed
from
data collected in Task 2. Document Dryer Operating Characteristics. The testing was divided
into two phases. Phase I consisted of examining the ignition characteristics of lint on and
near
the heater. Phase II consisted of examining the ignition characteristics of lint that was
ingested
into the heater.
2.4.1 Test Setup
A test apparatus was constructed to house the heater box/element (from Dryer Design
A) and venting material for the heater exhaust, as shown in Figures 51 and 52. The test setup
allowed precise control of the test conditions. Two fans and irises were installed to
independently control the airflow external and through the heater box. The heater was wired
to
a variable transformer to control the heat output, if needed. All venting material was 4-inch
rigid
metal duct. All joints were sealed with foil tape. Appendix F illustrates the dimensions of
the
test setup used in Task 4.
Although a heater from one of the dryer designs was used, the test setup removed many
of the design variables found in any particular dryer design, and staff believes the test
results
may be applied to the other dryer designs.
2.4.2 Instrumentation Setup
The test setup was instrumented with six thermocouples and two hot wire anemometers.
One additional thermocouple was used to record the ambient room temperature. All seven
thermocouples were 24 gauge, K type. The same thermocouples used in the Task 2 testing
were used in this phase of testing.
Table 6 lists the locations of the thermocouples. The T3 thermocouple was cemented to
the heater housing using a ceramic based cement.
Table 6. Thermocouple Location and Setup
Thermocouple Location
T1 Heater Intake
T2 Heater Exhaust
T3 Heater Housing or Lint Sample*
T4 Intake into Blower/Heater Exhaust
T5 Intake into Blower/Outside Heater
T6 Chamber Temperature
T7 Ambient Room
* Location may differ, as specified in the report
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Two anemometers were used in this phase of testing. One anemometer was placed at
the heater box intake. The second anemometer was placed near the top of the heater box to
measure the airflow over the heater box housing.
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Heater
View Ports
Heater Inlet
Exhaust
Figure 51. Test Setup for Lint Ignition
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Anemometer
Heater Box
Airflow Direction
Figure 52. Test Setup for Lint Ignition
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2.4.3 Phase I – Ignition Characteristics of Lint On and Near the Heater Box
In this phase of testing, lint samples were placed on top of the heater housing, at one
side of the heater housing, and at the heater intake. All lint samples were conditioned for a
minimum of 24 hours at 23°C to 25°C and 50% to 55% relative humidity.
The lint samples were tested at five different locations on the heater housing as shown
in Figure 53. Lint samples were tested in two locations on top of the heater housing, locations
A
and B. Lint samples were tested at three locations on one side of the housing, locations C, D
and E.
Airflow
Direction
Figure 53. Location of Lint Samples on the Housing
2.4.3.1 Top of Heater Housing
The first series of tests was conducted with lint samples placed on top of the
heater
housing at location A, as shown in Figure 54. The high-limit thermostat was bypassed for this
series of tests.
The lint samples measured approximately 2 x 2 x ¼ inches and weighed approximately
0.30 grams. The samples were 100% cotton lint collected from the lint screen during Task 3
testing.
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T3 Thermocouple
Lint Sample
Figure 54. Lint Sample on Heater Housing at Location A
Table 7 provides a list of the lint ignition tests that were conducted at location A on top
of
the heater housing. The table includes the weights of the lint samples tested, test
conditions,
and the observed results.
Table 7. Testing on Top of the Heater Housing at Location A
Location Number Weight
(grams)
Power Intake Airflow
(sfpm)
Outside
Airflow
(sfpm)
High-Limit
Thermostat
Result
A s1-1 0.32 step 0 0 bypassed smolder
A s1-2 0.33 step 0 0 bypassed smolder
A s1-3 0.33 step 0 40 bypassed smolder
A s1-4 0.34 instant 0 40 bypassed smolder
A s1-5a 0.32 instant 0 40 bypassed smolder
A s2-8 0.30 instant 0 40 bypassed smolder
A s5-1 0.32 instant 650 Ä
0@400s
40 bypassed smolder
The samples either smoldered or charred during the tests, as shown in Figure 55. In the
first three tests (Sample Numbers s1-1, s1-2, and s1-3), the power to the heater was
incrementally increased. In the next three tests (s1-4, s1-5a, s2-8), power was applied
instantaneously to the heating element. In all these tests, the samples smoldered but did not
ignite. In test s5-1, the airflow through the heater was initially 650 sfpm until 400 seconds
when
it was decreased to zero sfpm. The sample charred and smoldered.
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Sample 1-5a Sample 1-5a
Before Smoldering
Figure 55. Sample Smoldering at Location A
Table 8 provides a list of the lint ignition tests that were conducted at location B on top
of
the heater housing. The lint samples measured approximately 2 x 2 x ¼ inches and weighed
approximately 0.30 grams. The samples were 100% cotton lint collected from the lint screen
during Task 3 tests. The table includes the weights of the lint samples tested, test
conditions,
and the observed results.
Table 8. Testing on Top of the Heater Housing at Location B.
Location Number Weight
(grams)
Power Intake
Airflow
(sfpm)
Outside
Airflow
(sfpm)
High-Limit
Thermostat
Result
B s5-8 0.34 instant 0 40 bypassed ignited
B s5-9 0.28 instant 0 40 bypassed ignited
B s5-10 0.34 instant 0 40 bypassed ignited
B s5-11 0.32 instant 0 40 series smolder
B s5-12 0.34 instant 300 40 series smolder
B s5-13 0.34 instant 200 40 series smolder
In three of the six tests conducted at location B, the high-limit thermostat was bypassed;
in the other three, the high-limit thermostat was connected in series with the heating
element.
The airflow through the heater was set at either 200 or 300 sfpm in the last two tests. In the
first
three tests at location B (s5-8, s5-9, and s5-10), the samples ignited. Figure 56 shows
sample
s5-8 igniting during the test. Figure 57 shows a graph of the thermocouple data for sample
s5-
8. The sample ignited approximately 30 seconds after power was applied to the heating
element.
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Sample 5-8
Before Smoldering Ignition
Figure 56. Sample s5-8 at Location B
Sample s5-8
0.34 grams
July 12, 2002
0
100
200
300
400
500
600
700
0
7
14
21
28
35
42
49
56
63
70
77
84
91
98
105
112
119
126
133
140
147
154
161
168
175
182
189
196
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Liint Sample
T4 Exhaust Vent Heater T5 Exhaust Vent Outside Heater T6 Chamber
T7 Room Ambient V1 Airflow into Heater V2 Airflow External Heater
Lint Sample
Ignites
Power to
Heater Removed
T2
T3
T1
T4
Figure 57. Thermocouple Data for Sample s5-8
In the last three tests, the high-limit thermostat was connected in series with the heating
element. In the test for sample s5-11, the power was applied instantaneously to the heating
element, and the sample smoldered until the high-limit thermostat opened and disconnected
power to the heating element. For samples s5-12 and s5-13, the intake airflow into the heater
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was 300 and 200 sfpm, respectively. The samples smoldered until the high-limit thermostat
opened and disconnected power to the heating element.
2.4.3.2 Side of the Heater Housing
Twenty-two tests were conducted with samples placed at three different locations (C,
D,
and E) on one side of the heater housing. All the samples were held in place with a finger
probe. Three of the tests were conducted using lint samples obtained from a consumer’s dryer
and are labeled as “hl” in Tables 10 and 11 below.
Table 9 provides a list of the lint ignition tests that were conducted at location C on the
side of the heater housing. The high-limit thermostat was bypassed in all tests conducted at
location C. The lint samples used measured approximately 2 x 2 x ¼ inches and weighed
approximately 0.30 grams. The samples were 100% cotton lint taken from the lint screen
collected during Task 3 testing. The table includes the weights of the lint samples tested,
test
conditions, and the observed results.
Table 9. Testing on the Side of the Heater Housing at Location C
Location Number Weight
(grams)
Power Intake
Airflow
(sfpm)
Outside
Airflow
(sfpm)
High-Limit
Thermostat
Result
C s2-7 0.32 instant 650 Ä
0@400s
40 bypassed ignited
C s5-2 0.32 instant 0 40 bypassed smolder
C s5-3 0.32 instant 650 Ä
0@400s
40 bypassed smolder
C s5-4 0.30 instant 650 Ä
0@400s
40 bypassed smolder
Four samples were tested at location C. Three of the samples (s5-2, s5-3 and s5-4)
smoldered, and one (s2-7) ignited.
Lint sample s2-7 was tested at location C, as shown in Figure 58. The T3 thermocouple
was placed above the sample as an indicator of ignition. Figure 59 graphs the temperatures
and airflow obtained during the test. Sample s2-7 was tested with 650 sfpm airflow (V1)
initially
through the heater box. At 400 seconds, the blower for the V1 airflow was stopped, and the
temperature inside the heater rose rapidly, as expected. The sample ignited approximately 90
seconds after the blower was stopped.
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High-Limit
Thermostat
Sample 2-7
T3 Thermocouple
Finger Probe
Figure 58. Sample s2-7 on the Side of the Heater Housing at Location C
Sample 2-7
300s to 600s
May 21, 2002
0
100
200
300
400
500
600
700
800
300
310
320
330
340
350
360
370
380
390
400
410
420
430
440
450
460
470
480
490
500
510
520
530
540
550
560
570
580
590
600
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Liint Sample
T4 Exhaust Vent Heater T5 Exhaust Vent Outside Heater T6 Chmaber
T7 Room Ambient V1 Airflow into Heater V2 Airflow External Heater
T2
T3
Lint Sample
Ignites
Power to
Heater Removed
V1 reduced to zero
sfpm @ 400s
Figure 59. Sample s2-7 Thermocouple Traces
Table 10 provides a list of the lint ignition tests that were conducted at location D, which
was adjacent to the high-limit thermostat. The table includes the weights of the lint samples
tested, test conditions, and the observed results.
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Table 10. Testing on Top of the Heater Housing at Location D
Location Number Weight
(grams)
Power Intake
Airflow
(sfpm)
Outside
Airflow
(sfpm)
High-Limit
Thermostat
Result
D s1-5 0.30 instant 0 43 bypassed ignited
D s2-1 0.35 instant 650,
0@500s
43 series charred
D s2-1f 0.35 instant 0 43 bypassed smolder
D s2-2 0.35 instant 0 43 bypassed smolder
D hl1-1* 0.36 instant 0 43 bypassed smolder
D s5-5 0.31 instant 0 40 bypassed smolder
* Consumer lint
Six lint samples were tested at Location D, as shown in Figure 60. The samples
were
held in place using a single finger probe. The T3 thermocouple was placed on the lint sample
as an indicator of lint ignition. Four of the samples smoldered, one charred, and one
ignited.
Sample
T3 Thermocouple
Finger Probe
High-Limit
Thermostat
Figure 60. Sample s2-2 Tested at Location D
In five of the tests, including one using lint material from a consumer’s home, the highlimit
thermostat was bypassed, and there was no airflow through the heater box. Four of the
tests resulted in smoldering of the lint sample (s2-1f, s2-2, hl1-1, and s5-5), and in one test,
the
sample ignited (s1-5).
In a sixth test with the high limit thermostat in series with the heater, the lint sample
charred (s2-1). The initial airflow was at 650 sfpm; at 400 seconds, the airflow was dropped to
0
sfpm. The sample charred until the high-limit thermostat opened and disconnected power to the
heating element.
Table 11 below provides a list of the lint ignition tests that were conducted at location E,
which was at a tab opening (punch-out) used to hold the heating element. Twelve tests were
conducted at this location. The table includes the weights of the lint samples tested, test
conditions, and the observed results.
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Table 11. Testing on the Side of the Heater Housing at Location E
Location Number Weight
(grams)
Power Intake
Airflow
(sfpm)
Outside
Airflow
(sfpm)
High-Limit
Thermostat
Result
E s1-6 0.35 instant 0 43 bypassed ignited
E s1-7 0.31 instant 800 40 bypassed charred
E s1-7f 0.31 instant step
down
40 bypassed smolder
E s2-3 0.36 instant 0 43 bypassed ignited
E hl1-2* 0.26 instant 0 43 bypassed ignited
E s2-4 0.37 instant 0 43 bypassed ignited
E hl1-3* 0.40 instant 650 Ä
0@400s
43 bypassed smolder
E s2-5 0.38 instant 650 Ä
0@400s
43 bypassed ignited
E s2-6 0.39 instant 650 Ä
0@400s
43 bypassed ignited
E s3-1 0.35 instant 0 43 bypassed ignited
E s5-6 0.29 instant 0 40 bypassed ignited
E s5-7 0.39 instant 0 40 bypassed ignited
* Consumer lint
In seven of the tests, including one using consumer lint material, the airflow into
the
heater was 0 sfpm, and power to the heating element was applied instantaneously. All seven
samples ignited (s1-6, s2-3, hl1-2, s2-4, s3-1, s5-6 and s5-7).
Three tests were conducted with the airflow initially set at 650 sfpm; and at 400 seconds,
airflow was dropped to 0 sfpm. Two samples (s2-5, s2-6) ignited, and the consumer lint sample
(hl1-3) smoldered.
Figure 61 shows sample s2-6 before and after it ignited. Figure 62 shows the
thermocouple traces for sample s2-6. The T3 thermocouple shows when the lint sample ignited
by the rapid increase in temperature. The sample ignited approximately 60 seconds after the
blower was turned off.
Sample 2-6 Sample2-6
Before Ignition
Figure 61. Sample s2-6 Tested at Location E
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Sample 2-6
0.39 grams, Location E
May 21, 2002
0
100
200
300
400
500
600
700
800
300
310
320
330
340
350
360
370
380
390
400
410
420
430
440
450
460
470
480
490
500
510
520
530
540
550
560
570
580
590
600
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Liint Sample
T4 Exhaust Vent Heater T5 Exhaust Vent Outside Heater T6 Chmaber
T7 Room Ambient V1 Airflow into Heater V2 Airflow External Heater
T2
T3
Lint Sample
Ignites
Power to
Heater
Removed
V1 reduced to zero
sfpm @ 400s
Figure 62. Thermocouple Traces for Sample s2-6 at Location E
A single test was conducted with the airflow set at 800 sfpm. The sample (s1-7) charred
slightly. The last test was conducted with the voltage to the heating element stepped down
incrementally; the sample (s1-7f) smoldered (Appendix F shows the temperature and airflow
data for sample s1-7f).
2.4.3.3 Lint Samples at the Heater Intake
Lint samples were placed at various distances from the heater intake, as shown
in
Figure 63. The samples were approximately 1” x 1” x 1/4” and weighed approximately 0.20
grams. The samples were held in place by a finger probe. The T3 thermocouple was placed
above the sample as an indicator of ignition (Appendix G illustrates the setup).
The lint samples were placed at 1-inch increments from the front edge of the heater
housing. The distance from the heating element to the edge of the heater housing was 2
inches.
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Lint Sample
T3 Thermocouple
Finger Probe
Figure 63. Lint Sample at Heater Intake
An infrared camera was positioned to view the lint samples, as shown in Figure 64, and
was used to capture the temperature of the lint during the test. Infrared data was not
captured
for all of the lint samples tested.
IR Camera
Video
Heater Box
Cooling Fan
Figure 64. Infrared Camera Setup
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For the following test series, the airflow outside the heater boxes for the different dryer
designs measured between 0 and 43 sfpm depending on whether the exhaust vent was blocked
or unblocked. The airflow over the heater box/housing was set at approximately 43 sfpm to
simulate a slight airflow caused by the drum rotating and intake into the heater.
The first series of tests was conducted with no airflow through the heater box and the
high-limit thermostat bypassed. In this series of tests, the lint samples ignited at a
distance
ranging from 3 to 4 inches from the housing edge.
A second series of tests was conducted similar to the first set, except that the high-limit
thermostat was connected in series with the heating element. The samples ignited at a
distance
between 1 to 2 inches from the housing edge.
A third series of tests was conducted similar to the second series but with airflow through
the heater box. The lint samples were fixed at a distance of one inch from the heater edge.
The samples ignited when the airflow at the heater intake was between 300 and 400 sfpm.
A single test was conducted with a lint sample placed 1-inch from the edge of the heater
housing, the high-limit thermostat bypassed, and the airflow through the heater box set at
400
sfpm. The lint sample ignited.
Table 12 lists the tests conducted with lint samples at the heater intake. The list is
presented in chronological order (the order in which the tests were conducted). The distance
is
the number of inches from the heater housing edge to the front face of the sample. The column
labeled High-Limit Thermostat indicates whether the thermostat was connected in series with
the heating element or not (“yes” means the thermostat was connected in series; “no” means
the thermostat was bypassed.)
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Table 12. Lint Samples at the Heater Intake
Lint
Sample #
Weight
(grams)
Distance
(inches)
High-Limit
Thermostat
Airflow
(sfpm)
Result
s3-2 0.19 2 no 0 ignition
s3-3 0.20 3 no 0 ignited
s3-4 0.18 4 no 0 no ignition
s3-5 0.19 2 no 0 ignited
s3-6 0.18 2 no 0 ignited
s3-7 0.19 2 no 0 ignited
s3-8 0.18 2 no 0 ignited
s3-9 0.19 3 no 0 ignited
s3-10 0.16 3 no 0 ignited
s3-11 0.21 4 no 0 no ignition
s3-12 0.16 4 no 0 no ignition
s3-13* 0.19 2 yes 0 ignited
s3-14 0.16 2 yes 0 no ignition
s3-15 0.19 0 yes 0 ignited
s3-16 0.18 1 yes 0 ignited
s3-17 0.17 1 yes 0 ignited
s3-18 0.18 2 yes 0 no ignition
s3-19 0.19 1 yes 200 ignited
s3-20 0.21 1 yes 400 no ignition
s1-8 0.21 1 yes 300 ignited
s1-9 0.20 0 yes 400 no ignition
s1-10 0.21 1 no 400 ignited
* The high-limit failed; s3-13 is later discussed in the Discussion Section, Task
4
Figure 65 shows a sequence of images from the infrared camera and a video camera
for
Sample s3-10. The figure shows the lint sample began to ignite at approximately 452° C.
Figure 66 shows the thermocouple data for sample s3-10. The sample ignited 1 minute 11
seconds into the test. The sample ignited approximately 40 seconds after the heater was
energized.
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Smoldering
Flames
00:50
01:11
01:13
Sample 3-10 Sample 3-10
Sample 3-10 Sample 3-10
Ignition
Sample 3-10 Sample 3-10
Figure 65. Sample s3-10, Video Camera and Infrared Camera
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Sample 3-10
weight 0.16g
sample 3" from edge
June 4, 2002
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700
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Time (seconds)
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Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Lint Sample
T4 Exhaust Vent Heater T5 Exhaust Vent Outside Heater T6 Chamber
T7 Room Ambient V1 Airflow into Heater V2 Airflow External Heater
Ignition
Power to Heater turned
off
T2
T3
T4
T1
Figure 66. Sample s3-10 – Thermocouple and Airflow Data
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2.4.4 Phase II – Ignition Characteristics of Lint Ingested into the Heater Box
This phase of testing was divided into two parts. Part 1 of the tests involved observation
of the heater exhaust as the lint samples of varying weights and sizes were ingested into the
heater box. Part 2 was to observe whether target materials downstream of the heater exhaust
could be ignited by embers as they exited the heater exhaust.
The test setup was designed to allow the airflow into the heater intake to carry lint
samples past the heating element and exhaust any embers through the ducts. To observe the
heater exhaust, a 28-inch section of the metal duct from Phase I tests was replaced with a 4”
diameter high-temperature glass tube, as shown in Figure 67. (The actual dimensions for the
test setup can be found in Appendix H.)
Heater
Glass Tube
Airflow Direction
View Ports
Figure 67. Setup with Glass Tube
2.4.4.1 Part 1 – Lint Samples Ingested by the Heater Box
For these tests, different weights and sizes of lint were drawn into the heater box.
Six
series of tests were completed:
1. The first series included 5 samples weighing between 0.24g and 0.77g.
2. The second series consisted of 8 samples weighing between 0.04g and 0.18g.
3. The third series consisted of 5 samples weighing between 0.14g and 0.48g.
4. The fourth series consisted of 10 samples at 0.10 ±0.10 grams.
5. The fifth series consisted of 10 samples at 0.20 ±0.10 grams.
6. The sixth series consisted of 10 samples at 0.30 ±0.10 grams.
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Table 13 below lists the six series of tests and the lint sample weights used in each.
Table 13. Lint Samples for Task 4 - Phase II Testing
(Observation of Samples Ingested into Heater Box)
Series
Number
Sample Number Weight
(grams)
1 0.24
2 0.37
3 0.38
4 0.64
1
5 0.77
1 0.18
2 0.18
3 0.04
4 0.10
5 0.07
6 0.06
7 0.11
2
8 0.08
1 0.48
2 0.43
3 0.27
4 0.25
3
5 0.14
4 1 to 10 0.10
5 1 to 10 0.20
6 1 to 10 0.30
For all tests conducted, embers were visible exiting the heater box. The embers varied
in size and number. It was observed that larger lint samples produced more and/or larger
embers than smaller lint samples did, but two lint samples of the same size did not produce
the
same ember size and/or number of embers. The number and size of the embers in the heater
exhaust were dependent on how and where the lint samples contacted the heating element.
For example, if the lint sample became trapped on the heating element, smaller embers were
produced in the exhaust as the sample burned. However, if the sample contacted the heating
element, ignited, and continued through the heater box, larger embers were observed exiting
the heater exhaust.
2.4.4.2 Part 2 – Ignition of Target Materials Downstream from the Heater Exhaust
Four series of tests were conducted to observe whether target materials downstream
of
the heater exhaust could be ignited by embers as they exited the heater exhaust. Target
materials were placed inside the glass tube, as shown in Figure 68. After the target material
was placed in the glass tube, lint samples were drawn into the heater box approximately every
3
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to 4 seconds. The airflow was set at 800 sfpm at the heater intake. This airflow velocity
simulated the airflow inside a dryer near the end of the drying cycle (when the dryer would
have
a full lint screen). For each of the four tests, the lint samples drawn into the heater box
consisted
of ten samples of 0.10g each, then 10 samples of 0.20g each and, finally, ten samples of
0.30g.
Target Material
Glass Tube
Clips to secure
material
Airflow Direction
Test 1
Figure 68. Setup with Target Material in the Glass Tube
Table 14 outlines the types, sizes and weights of the target materials used for each test
series. The target samples were either sheets of 100% cotton lint or cotton terry towel
material.
The lint target samples weighed between 2 and 5 grams and measured approximately 6 x 10
inches. The lint target material varied in thickness from 1/8 to ½ inch. The lint target
materials
were collected during Task 3 testing. The cotton terry towel material was a section cut from
a
towel used in Task 3 testing; the section measured 7 ½ x 10 inches and weighed 35.2 grams.
Table 14. Target Materials for Task 4 - Phase II Testing
(Ignition of Target Materials by Exhaust Embers)
Test Number Target Material Size (inches) Weight (grams)
1 Lint Sheet 10 x 6 ½ x ½ to 5.08
2 Lint Sheet 7 x 5 ½ x ½ x 3.04
3 Lint Sheet 7 ½ x 6 ¼ to 2.08
4 Cotton Towel 7 ½ x 10 35.20
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In all four tests, the target material ignited:
1. In Test 1, the target material ignited on the fifth lint sample of 0.10 g.
2. In Test 2, the target material ignited on the sixth sample of 0.20 g.
3. In Test 3, the target sample ignited on the tenth sample of 0.10 g.
4. In Test 4, the target material ignited on the eighth sample of 0.30 g.
Figure 69 shows the target sample igniting during Test 1. The sample was consumed in
less than 6 seconds and produced additional embers downstream. The exhaust temperature,
T4 thermocouple, increased to over 260° C in less than 5 seconds, as shown in Figure 70.
(Test data for Tests 2, 3, and 4 are contained in Appendix H. Video clips of the lint sample
testing are included in Appendix J.)
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Embers
00:00 Ignition
-00:01
00:01
00:02
00:03
00:04
Figure 69. Target Sample Igniting During Test 1
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Test 1
Target Material - Lint - weight 5 grams
June 14, 2002
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400
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200
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220
230
240
250
260
270
280
290
300
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Glass Tube Internal
T4 Exhaust Vent Heater T5 Exhaust Vent Outside Heater T6 Chamber
T7 Room Ambient V1 Airflow into Heater V2 Airflow External Heater
Ignition
Heater Turned Off
T4
Figure 70. Test 1 – Thermocouple and Airflow Measurements Task 4- Test 1
(Ignition of Target Materials by Exhaust Embers)
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3.0 DISCUSSION
This section of the report presents a discussion of the tests and the test results reported
in
Section 2.0 Description of Test and Test Results for each of the four project tasks. Each
discussion section is supported by its corresponding section in Section 2.0, unless otherwise
specified.
3.1 Task 1: Inspect and Record Dryer Design
Four clothes dryers (Dryer Designs A, B, C and D) were used for testing. As part
of
each dryer’s exhaust system, there was a 4-inch diameter male duct, a short length of which
protruded outside the dryer housing. This short length of duct was slightly tapered and was
intended for connection to external venting (the external vent slid over the tapered male
duct).
The external vent material used for the clothes dryers was 4-inch diameter rigid metal
ducting,
as recommended in the manufacturers’ installation instructions.
The interface of the male duct and the external venting was sealed and/or secured with
a 4-inch hose clamp. The hose clamp nut was tightened by hand with a nut driver until it
could
not be tightened any further. The amount of torque applied to the hose clamp nut had little
effect on actually tightening the connection between the male duct and the external vent. Two
factors contributed to the poor seal:
1. The length of the dryer’s male duct (which varied slightly among the designs) was
insufficient to allow the external vent to slide far enough onto the duct to provide a
secure pressure fit.
2. Both mating pieces were rigid; thus, the external venting could not compress around
the dryer’s male duct when the hose clamp was tightened.
(Additional comments regarding dryer designs are included in the Discussion section of
Task 3: Monitor Lint Distribution.)
3.2 Task 2: Document Dryer Operating Characteristics
Testing showed that each dryer design reacts slightly differently when the exhaust
vent
is unblocked, partially blocked, or 100% blocked. The data for temperature and airflow varied
slightly with each dryer, but certain characteristics were similar for all the dryer designs
tested.
3.2.1 Normal Operation (Unblocked Exhaust Vent)
When the dryers were tested with an unblocked exhaust vent, similar temperatures – the
intake air temperature into the heater, the heater housing temperature, the intake air
temperature into the blower, and the exhaust vent temperature – were measured among all
dryer designs. Figures 71 through 75 show comparisons of temperatures measured for each of
the dryer designs when tested with a wet load. Dryer Design D used only 7 towels so that the
dryer would not operate in the high-limit cycling mode. (Additional graphs demonstrating
dryer
design comparisons can be found in Appendix I).
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Figure 71 shows that the heater intake temperature (T1) was higher for Dryer Design C
than for other dryer designs, and it reached a maximum of 75°C. Dryer Designs A and C
exhibited similar maximum temperatures before the primary thermostat opened. Dryer Design
B had a slightly lower maximum temperature. Since the load in Dryer Design D began riding the
drum after approximately 1900 seconds, the airflow through the tumbler was less restricted
causing the primary thermostat to reach its opening set point sooner.
Figure 72 shows the heater housing temperatures did not extend much higher than
120°C for all the dryer designs tested. The housing temperatures for Dryer Designs A and
D
were similar – until the load in Dryer Design D stopped tumbling after approximately 1900
seconds. The heater housing in Dryer Design B had a significant number of punch-outs on one
side, which were used to mount the heating element inside the heater housing. This appeared
to allow the heater housing to operate at a slightly cooler temperature than either Dryer
Design
A or D. Dryer Design C had louvers on one side of the heater housing; this allowed the heater
housing to operate at an even lower temperature – approximately 66°C, as shown in the graph.
Figure 73 shows the intake temperature into the blower for all the dryer designs tested.
The temperatures are very similar for all the dryers, as shown in the graph. The intake
temperature into the blower did not extend much higher than 100°C for all the dryer designs
tested. Dryer Design A and D had a slightly higher operating curve than the other dryers. The
load in Dryer Design D began to stop tumbling at approximately 1900 seconds after the dryer
was started, as shown in the graph.
Figure 74 shows the exhaust vent temperatures for all the dryer designs tested. The
temperatures are very similar for all the dryers, as shown in the graph. The exhaust venting
temperature did not extend much higher than 70°C for all the dryer designs tested. The exhaust
vent temperatures for Dryer Design A were slightly higher than those for the other dryers.
Figure 75 shows the maximum temperatures measured at each thermocouple location
for each dryer design. All the dryers had similar temperatures at each thermocouple location,
except at the heater exhaust and tumbler intake. Design characteristics had a contributing
factor on temperatures at these locations. Dryer Design C had an increase in temperature from
the heater exhaust to the tumbler intake, which was also observed during testing in Task 2.
The
louvers on the side of the heater housing may have contributed to the higher temperature
readings at T6, but it has not yet been determined why this actually occurred.
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T1 Heater Intake Temperature
Dryer Design Comparison
Wet Load
0
10
20
30
40
50
60
70
80
0
159
318
477
636
795
954
1113
1272
1431
1590
1749
1908
2067
2226
2385
2544
2703
2862
3021
3180
3339
3498
3657
3816
3975
Time (seconds)
Temperature (C)
Design A Design B Design C Design D
A
B
C
D
Figure 71. Dryer Design Comparison for T1, Heater Intake
T3 Heater Housing Temperature
Dryer Design Comparison
Wet Load
0
20
40
60
80
100
120
140
160
0
161
322
483
644
805
966
1127
1288
1449
1610
1771
1932
2093
2254
2415
2576
2737
2898
3059
3220
3381
3542
3703
3864
Time (seconds) Temperature (
C)
Design A Design B Design C Design D
A
B
C
D
Figure 72. Dryer Design Comparison for T3, Heater Housing
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T5 Intake into Blower Temperature
Dryer Design Comparison
Wet Load
0
10
20
30
40
50
60
70
80
90
0
160
320
480
640
800
960
1120
1280
1440
1600
1760
1920
2080
2240
2400
2560
2720
2880
3040
3200
3360
3520
3680
3840
4000
Time (seconds)
Temperature (C)
Design A Design B Design C Design D
A
B
C
D
Dryer Design D
Load Stops Tumbling
Figure 73. Dryer Design Comparison for T5, Blower Intake
T4 Exhaust Vent Temperature
Dryer Design Comparison
Wet Load
0
10
20
30
40
50
60
70
80
0
158
316
474
632
790
948
1106
1264
1422
1580
1738
1896
2054
2212
2370
2528
2686
2844
3002
3160
3318
3476
3634
3792
3950
Time (seconds)
Temperature (C)
Design A Design B Design C Design D
A
B
C
D
Figure 74. Dryer Design Comparison for T4, Exhaust Vent
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Dryer Design Comparison
Maximum Temperature at Test Locations
Wet Load
0
50
100
150
200
250
300
350
Heater Intake Heater
Exhaust
Heater
Housing
Dryer Vent Intake to
Blower
Intake into
the Tumbler
Room
Temperature
Location
Temperature (C)
Design A Design B Design C Design D
Analog Signal
A B C D Not Temperature
Figure 75. Maximum Temperatures Measured at Location
3.2.2 Partially-Blocked and 100%-Blocked Conditions
Each dryer design reacted slightly differently when the exhaust vent was partially
blocked and/or 100% blocked. The responses for each dryer design were discussed in Task 2
of the Testing section. The discussion below examines the general characteristics of a dryer
by
analyzing the data without compensating for variances in dryer designs. The data presented in
this section are in general terms and do not represent a specific dryer tested.
The data was generated from Dryer Design A/dry load, Dryer Design B/wet load, Dryer
Design C/wet load, and Dryer Design D/no load. Dryer Design A/dry load was used since the
operating temperatures for a dry load were similar to a wet load, although the time scale was
different. Dryer Design D/no load was used to avoid using data in which the dryer quickly
began
operating in the high-limit cycle mode for a full load or in which operation was unpredictable
(the
load stopped tumbling).
The maximum temperatures measured for each dryer for each test condition (blocked
exhaust vent, partially blocked vent, or unblocked vent) were extracted from each data set.
The
temperatures were then categorized as highest, lowest, and average for each set. Figures 76
through 81 show comparisons of maximum temperatures measured at each test location under
unblocked conditions, the range of blocked conditions, and with the primary thermostat
bypassed.
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Figure 76 shows the range of highest to lowest maximum intake air temperatures at the
heater for each test condition. The temperatures did not change significantly up to a 50%
blockage of the exhaust vent. At 75% blockage, the temperatures (highest, average, and
lowest) increased significantly, with the highest temperature measuring over 350°C. The
average maximum temperature was slightly over 150°C with a 75% blockage. The maximum
temperatures when the exhaust vent was 100% blocked were similar to the 75% blocked
condition. The maximum temperatures with the primary thermostat bypassed were similar to
the temperatures when the exhaust vent was 50% blocked.
Figure 77 shows the range of highest to lowest maximum heater exhaust temperatures
for each test condition. The temperatures did not change significantly from the unblocked
exhaust vent condition to a 25% blocked exhaust vent. As the exhaust vent was blocked
beyond 25%, there was a slow increase in temperatures. The highest maximum temperature
measured nearly 600°C for the 100% blocked condition. The maximum temperatures with the
primary thermostat bypassed were similar to the temperatures when the exhaust vent was 75%
blocked.
Figure 78 shows the range of highest to lowest maximum heater housing temperatures
for each test condition. The temperatures generally showed a slight increase from the
unblocked exhaust vent condition to a 50% blocked exhaust vent. When the vent was 75%
blocked, the maximum temperatures increased more rapidly. When the exhaust vent was 100%
blocked, the maximum temperatures were similar to the temperatures reached for a 75%
blocked vent. The highest maximum temperature measured between 200°C and 250°C for the
75% and 100% blocked conditions. The maximum temperatures with the primary thermostat
bypassed were similar to those for a 75% blocked exhaust vent.
Figure 79 shows the range of highest to lowest maximum dryer exhaust vent
temperatures for each test condition. The temperatures did not change significantly from the
unblocked exhaust vent condition to a 50% blocked exhaust vent. Beyond a 50% blockage, the
maximum temperatures measured began to drop rapidly. The highest maximum temperature
measured dropped from approximately 70°C to 30°C. The maximum temperatures measured
when the primary thermostat was bypassed were much higher than those measured for any of
the blocked or unblocked exhaust vent conditions. The highest maximum temperature
measured was slightly over 100°C.
Figure 80 shows the range of highest to lowest maximum temperatures at the intake into
the blower for each test condition. The temperature curves shown in this graph are similar to
those for the dryer exhaust vent (Figure 79), although the changes were not as dramatic.
Another similarity between Figures 80 and 79 is that, when the primary thermostat was
bypassed, maximum temperatures were much higher than those measured for any of the
blocked or unblocked exhaust vent conditions. The highest maximum temperature measured
was slightly over 120°C.
Figure 81 shows the range of highest to lowest maximum temperatures at the intake into
the tumbler for each test condition. The temperature curves shown in this graph are similar
to
those for the heater exhaust (Figure 77), except that the maximum temperature measured when
the exhaust vent was fully blocked was lower – approximately 400°C. Another similarity
between Figures 81 and 77 is that, when the primary thermostat was bypassed, maximum
temperatures were similar to those for the 75% blocked condition.
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All Dryers
Intake into the Heater
0
50
100
150
200
250
300
350
400
0%
25%
50%
75%
100%
Bypassed
Percent Blocked
Temperature C
maximum
average
minimum
Figure 76. Intake into the Heater
All Dryers
Heater Exhaust
0
100
200
300
400
500
600
700
0%
25%
50%
75%
100%
Bypassed
Percent Blocked
Temperature C
maximum
average
minimum
Figure 77. Heater Exhaust Temperature
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All Dryers
Heater Housing Temperature
0
50
100
150
200
250
0%
25%
50%
75%
100%
Bypassed
Percent Blocked
Temperature C
maximum
average
minimum
Figure 78. Heater Housing Temperature
All Dryers
Exhaust Vent Temperature
0
20
40
60
80
100
120
0%
25%
50%
75%
100%
Bypassed
Percent Blocked
Temperature C
maximum
average
minimum
Figure 79. Exhaust Vent Temperature
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All Dryers
Intake into the Blower
0
20
40
60
80
100
120
140
0%
25%
50%
75%
100%
Bypassed
Percent Blocked
Temperature C
maximum
average
minimum
Figure 80. Intake into the Blower Temperature
All Dryers
Heated Air into the Tumbler
0
50
100
150
200
250
300
350
400
450
0%
25%
50%
75%
100%
Bypassed
Percent Blocked
Temperature C
maximum
average
minimum
Figure 81. Intake into the Tumbler Temperature
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Figure 82 shows the data from Figures 76 through 81 in an illustrated dryer
configuration. The dryers’ main components are illustrated in a linear representation. The
figure shows the maximum temperatures for all the dryer designs at each location for the
range
of unblocked and blocked exhaust vent conditions, as well as for when the primary thermostat
was bypassed.
The figure shows that there was a significant increase in maximum temperatures at the
intake into the tumbler when the vent was 75% and 100% blocked. The air temperatures exiting
the tumbler and entering the exhaust vent decreased, as expected. For the unblocked, 25%
blocked and 50% blocked exhaust vent conditions, the maximum temperatures did not show a
significant change. When the primary thermostat was bypassed, there was little change in
maximum temperatures at T1 (heater intake) and T3 (heater housing). There was an increase
in the maximum temperatures after the heater exhaust and into the exhaust vent.
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T1 Heater
Intake
T3Heater
Housing
T2 Heater
Exhaust
T6 Tumbler
Intake
T5 Blower
Intake
T4 Exhaust
Vent
Blower
Lint
Screen
Tumbler
Heater
Element
Intake
Air
Exhaust
Air
Figure 82. Maximum Temperatures for All Dryers at Each Location and Condition
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Tables 15 through 17 show the percentage of change in minimum, average and
maximum temperatures for the different percentages of exhaust vent blockage conditions (and a
bypassed primary thermostat) as compared to the unblocked condition for all dryer designs.
There was an increase in temperature from the heater intake to the tumbler for the partially
and
fully blocked conditions. As shown in Table 17, there was as increase in temperature ranging
from 69% to 351% from the unblocked to 100% blocked conditions, depending on the location.
After the tumbler, the temperature increased between 13% to 55% from the unblocked to 100%
blocked conditions, depending on the location, as expected. When the primary thermostat was
bypassed, the temperature after the tumbler was higher by 38% to 48% when compared to the
unblocked condition.
Table 15. Change in Minimum Temperature
Percent Percent Change in Minimum Temperature from Unblocked Condition
Blocked T1
Heater Intake
T3
Heater Housing
T2
Heater Exhaust
T6
Tumbler Intake
T5
Blower Intake
T4
Exhaust Vent
25% 1% 0% 2% -1% -2% -2%
50% -4% 20% 32% 37% -2% -3%
75% 34% 29% 56% 53% -21% -20%
100% 39% 41% 30% 26% -37% -56%
Bypassed 1% -10% 24% 41% 41% 35%
Table 16. Change in Average Temperature
Percent Percent Change in Average Temperature from Unblocked Condition
Blocked T1
Heater Intake
T3
Heater Housing
T2
Heater Exhaust
T6
Tumbler Intake
T5
Blower Intake
T4
Exhaust Vent
25% 0% 1% 0% -1% 0% -1%
50% 3% 11% 8% 7% 4% 1%
75% 137% 81% 37% 24% -2% -9%
100% 162% 100% 78% 39% -30% -55%
Bypassed 39% 20% 21% 18% 34% 43%
Table 17. Change in Maximum Temperature
Percent Percent Change in Maximum Temperature from Unblocked Condition
Blocked T1
Heater Intake
T3
Heater Housing
T2
Heater Exhaust
T6
Tumbler Intake
T5
Blower Intake
T4
Exhaust Vent
25% 1% -7% -1% -1% 0% 1%
50% 6% 6% 3% 3% 0% 5%
75% 386% 93% 33% 23% 6% -1%
100% 351% 92% 89% 69% -13% -55%
Bypassed 43% 15% 23% 17% 36% 48%
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107
Figure 83 shows the exhaust air velocities with an unblocked vent for the different dryer
designs. The graph shows that the air velocities for Dryer Designs A and D were similar
throughout the 60-minute drying cycles. The graph shows that the air velocities for Dryer
Designs B and C were also similar. In addition, Dryer Designs B and C showed a steady
decline in exhaust air velocities as the drying cycle progressed.
Velocity Exhaust Comparison
All Dryer Designs
Wet Load
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 214 428 642 856 1070 1284 1498 1712 1926 2140 2354 2568 2782 2996 3210 3424 3638 3852
Time (seconds)
Velocity (sfpm)
Design A Design B Design C Design D
Figure 83. Comparison of Exhaust Air Velocity for all Dryer Designs
Figure 84 shows the average exhaust air velocity with an unblocked vent for all the dryer
designs. The graph shows a steady decline in the exhaust velocity during the drying cycle.
The
average velocity drops from 1300 sfpm to 900 sfpm.
Average Velocity Exhaust
Wet Load
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 214 428 642 856 1070 1284 1498 1712 1926 2140 2354 2568 2782 2996 3210 3424 3638 3852
Time (seconds)
Velocity (sfpm)
Average
Figure 84. Calculated Average Exhaust Air Velocity
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3.3 Task 3: Monitor Lint Distribution
Based on the test data and observations presented in Section 2.3, even when the
lint
filter was cleaned after each use of the dryer, there was visible lint accumulation inside
the
chassis. This section further evaluates the mechanisms within dryers that may allow lint to
leak
into the dryer structure. The areas of positive pressure in the airflow were the most likely
causes for lint leaking into the dryer chassis.
3.3.1 Dryer Design A at Positive Pressure
Examination of Dryer Design A showed that lint/air can leak through a sealed joint in the
blower housing. To examine more closely the nature of the origin of the lint/air leak, the
blower
fan compartment was opened on a blower from another dryer of Design A. Figures 85(a) and
(b) show the blower housing assembled and opened, respectively.
(a) Blower removed from the dryer
(b) Blower housing separated
Figures 85 (a) and (b). Dryer Design A – Blower Assembly
The blower is a centrifugal type fan. The blower operates by sucking air through the
center hole at negative pressure and forcing the air outward at positive pressure as shown in
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Figure 86. It is noteworthy that a foam gasket was used to seal the outer portion where there
is
positive pressure.
Figure 86. An Illustration of Airflow through the Dryer and Blower
3.3.2 Other Dryer Designs at Positive Pressure
The other dryer designs (B, C, and D) did not contain the same configuration that
resulted in positive pressure after the blower. As shown in Dryer Design A, the blower was at
the rear of the dryer chassis. This configuration allowed the blower housing and connection
to
the external dryer vent to be the same component. In the other dryer designs, the blower was
located toward the front of the dryer chassis, where the lint screen was also located.
These dryers required a short section of duct between the rear of the dryer and the
blower housing. The section of the dryer vent/ducting after the blower and inside the chassis
was an area of positive pressure. Most importantly, the connection between the short section
of
duct and the blower was at positive pressure. Figure 87 shows the junction was at positive
pressure (white arrow). Though the fitting of the metal duct to the plastic nozzle of the fan
was
snug, it was not airtight. In some case, there was no seal between the blower housing and the
short section of exhaust vent.
DRUM
HEATER
COIL
FAN
DRYER CHASSIS
EXTERIOR
DRYER CHASSIS
INTERIOR
LINT
SCR
ATMOSPHERIC
PRESSURE
BELOW
ATMOSPHERIC
PRESSURE
ABOVE
ATMOSPHERIC
PRESSURE
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Dryer Design B
Figure 87. Junction Between the Blower and Exhaust Vent
Under the test conditions, which included rigid exhaust ducting, two 90° elbows, and no
blockage in the ducting, there was lint accumulation in the chassis interior. The setup
produced
minimal backpressure in the dryer exhaust vent (See Appendix E). In the event that a serious
blockage did occur, lint leakage could be expected to increase as the backpressure in the
exhaust vent increased. The extent to which leakage would be made worse would depend on
the location of the leakage. Close to the blower impeller – on the side opposite of the
exhaust
port yet inside impeller housing – the positive pressure is greatest.
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3.4 Task 4: Determine Characteristics Required for Lint
Ignition
Based on the test data and observations presented in Section 2.4, ignition
characteristics of lint were observed on and near the heater housing (heater box) and when
lint
was ingested into the heater box. This section further assesses the ignition times and
temperatures under the various conditions tested. This section also includes an examination
of
high limit devices that had failed during the course of testing.
3.4.1 Lint on the Heater Housing
Twenty-two lint samples were tested at five different locations on the heater housing.
The samples at either locations B or E were more likely to ignite than samples placed at
locations A, C, or D on the heater housing. The samples located at B and E ignited more
frequently under conditions of no airflow and when the high-limit thermostat was bypassed.
The
samples would smolder but generally not ignite when placed at the other locations and/or when
tested under other conditions. Instead, the lint material would be consumed by pyrolysis or
thermal decomposition.
3.4.2 Lint Samples at the Heater Intake
This phase of testing was divided into three test series. The first series of tests was
conducted under conditions of no airflow through the heater box, and the high-limit
thermostat
bypassed. This simulated a condition in which the exhaust vent was blocked and the high-limit
thermostat had failed to open. The second test series was conducted with no airflow through
the heater box, but the high-limit thermostat was connected in series with the heating
element.
This simulated a condition where the exhaust vent was fully blocked, but the high-limit
thermostat was operational. The third series of tests was conducted with various airflow
velocities through the heater box, and the high-limit thermostat connected in series with the
heating element. This simulated a partially blocked exhaust vent with an operational
high-limit
thermostat.
In these tests, the primary purpose of the T3 thermocouple, which was placed on the lint
sample, was to indicate when the sample ignited. The T3 thermocouple temperature data
should not be used as an analysis tool to compare samples, since the placement of the
thermocouple on the samples may not have been identical. The thermocouple was placed
above the sample, to achieve the maximum change in temperature should the sample ignite.
3.4.2.1 High-Limit Thermostat Bypassed and No Airflow
Eleven tests were conducted in the first series of tests. Lint samples were placed
at
distances ranging from 2 to 4 inches from the edge of the heater box. (This would equate to 4
to 6 inches from the heater element.) The data shows the threshold for ignition and no
ignition
was approximately 3 inches and 4 inches (from the edge of the heater housing), respectively.
(Appendix I shows data for the lint sample tested 2 inches from the heater intake.)
Figure 88 below shows the temperatures at the T1 Heater Intake, T2 Heater Exhaust
and T3 Lint Sample thermocouples for samples s3-3, s3-9, and s3-10, which were tested 3
inches from the heater housing edge. (The heating element was energized approximately 35
seconds into the test.)
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Samples s3-9 and s3-10 show similar curves for the T1 thermocouple as shown in
Figure 89. For tests with sample s3-3, the T1 thermocouple showed slightly lower
temperatures, which coincides with the later ignition time shown by T3 thermocouple. The
temperature curves for the T2 thermocouple were similar for all three samples tested as shown
in Figure 90.
The times to ignition are similar for samples s3-9 and s3-10 – approximately 83 and 81
seconds after powering the heating element, respectively. Sample s3-3 had a slightly longer
time to ignition – approximately 107 seconds after powering the heating element as shown in
Figure 91.
Lint Sample Intake at 3 inches
High Limit Bypassed, No Air flow
2002
0
100
200
300
400
500
600
700
1
10
19
28
37
46
55
64
73
82
91
100
109
118
127
136
145
154
163
172
181
190
199
208
Time (seconds)
Temperature (C)
s3-3 T1
s3-3 T2
s3-3 T3
s3-9 T1
s3-9 T2
s3-9 T3
s3-10 T1
s3-10 T2
s3-10 T3
s3-10 T1
s3-9 T1
s3-9 T3
s3-9 T2
s3-3 T3
s3-10 T3
s3-3 T2
s3-10 T2
s3-3 T1
Figure 88. Lint Samples Tested 3 Inches from the Heater Housing Edge
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T1 Only - Lint Sample Intake at 3 inches
High Limit Bypassed, No Airflow
2002
0
50
100
150
200
250
300
350
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
Time (seconds)
Temperature (C)
3-3 T1
3-9 T1
3-10 T1
average T1
Figure 89. T1 Only - 3 Inches from the Heater Housing Edge
T2 Only - Lint Sample Intake at 3 inches
High Limit Bypassed, No Airflow
2002
0
100
200
300
400
500
600
1
11
21
31
41
51
61
71
81
91
101
111
121
131
141
151
161
171
181
191
201
Time (seconds) Temperature (
C)
3-3 T2
3-9 T2
3-10 T2
average T2
Figure 90. T2 Only - 3 Inches from the Heater Housing Edge
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114
T3 Only - Lint Sample Intake at 3 inches
High Limit Bypassed, No Airflow
2002
0
100
200
300
400
500
600
700
1
11
21
31
41
51
61
71
81
91
101
111
121
131
141
151
161
171
181
191
201
Time (seconds)
Temperature (C)
3-3 T3
3-9 T3
3-10 T3
average T3
Figure 91. T3 Only - 3 Inches from the Heater Housing Edge
Figure 92 shows the temperatures at the T1, T2 and T3 thermocouples for samples s3-
4, s3-11, and s3-12, which were tested 4 inches from the heater housing edge. (The heating
element was energized approximately 35 seconds into the test.) All three samples smoldered,
but no flames were present. The T1, T2 and T3 thermocouple traces were similar for all three
samples tested.
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Lint Sample Intake at 4 inches
High Limit Bypassed, No Airflow
2002
0
100
200
300
400
500
600
700
1
10
19
28
37
46
55
64
73
82
91
100
109
118
127
136
145
154
163
172
181
190
199
Time (seconds)
Temperature (C)
s3-4 T1
s3-4 T2
s3-4 T3
s3-11 T1
s3-11 T2
s3-11 T3
s3-12 T1
s3-12 T2
s3-12 T3
s3-11 T1 s3-12 T1
s3-4 T3
s3-4 T2
s3-11 T3
s3-12 T3
s3-11 T2
s3-12 T2
s3-4 T1
Figure 92. Lint Samples Tested 4 Inches from the Heater Housing Edge
Table 18 lists the ignition times for samples tested at 2 and 3 inches from the heater
intake edge. Figure 93 plots the minimum, maximum, and average ignition times for samples
tested at 2 and 3 inches from the heater edge.
Table 18. Ignition Times at Heater Intake
3 inches from Heater Intake 2 inches from Heater Intake
Sample
Number
Data
Time
(sec)
Heater
Energized
(seconds)
Actual
Time
(sec)
Sample
Number
Data
Time
(sec)
Heater
Energized
(seconds)
Actual
Time
(sec)
s3-3 143 36 107 s3-2 108 35 73
s3-9 118 35 83 s3-8 83 37 46
s3-10 115 34 81 s3-5 75 37 38
max 107 s3-6 80 39 41
average 90 s3-7 76 40 36
min 81 max 73
average 47
min 36
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Ignition Times for Lint Samples 2 and 3 inches from Intake
81
90
107
36
47
73
1
2
3
4
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Time (seconds)
Sample Distance
Figure 93. Ignition Time for 2 and 3 Inches from Heater Edge
3.4.2.2 High-Limit Connected in Series and No Airflow
Six tests were conducted in this series of tests in which the high-limit thermostat
was
connected in series with the heating element and there was no airflow through the heater box.
Lint samples were placed at distances ranging from 0 to 2 inches from the edge of the heater
box (2 to 4 inches from the heating element, respectively). The data show a threshold of
between 1 and 2 inches from the heater housing edge for ignition and no ignition,
respectively.
In the first test, sample s3-13 ignited, and the high-limit thermostat did not open and
disconnect power to the heating element. It was determined that the high-limit thermostat had
failed. (An investigation of the cause of failure for the high-limit thermostat was conducted,
and
the results are reported at the end of this Discussion Section.) The remainder of the
discussion
in this subsection, therefore, will not include data from sample s3-13.
The high-limit thermostat was replaced with a similar high-limit thermostat taken from a
dryer of the same make and model. Several baseline tests were conducted to confirm that the
replacement thermostat operated properly when the temperature inside the heater housing
reached a set temperature.
Figure 94 shows the T1, T2, and T3 thermocouple data for samples s3-16 and s3-17,
tested at a distance of 1 inch from the heater edge. The times to ignition for samples s3-16
and
s3-17 were approximately 55 and 65 seconds after powering the dryer, respectively. The steep
increases observed in the data for thermocouple T3 indicate ignition of the sample. The
profiles
of the data obtained from thermocouples T1 and T2 were similar for both samples tested.
This figure shows that the samples ignited before the high-limit thermostat opened and
disconnected power to the heating element. For both samples, the sample ignited
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117
approximately 5 to 8 seconds before the high-limit thermostat opened, which is shown by the
sudden decrease in temperature for the T1 thermocouple.
Lint Sample Intake 1 inch
High Limit, No Airflow
2002
0
50
100
150
200
250
300
350
400
450
500
0
9
18.001
27
36
45
54
63
72.001
81
90.001
99
108
117
126
135
144
153
162
171
180
189
198
Time (seconds)
Temperature (C)
s3-16 T1
s3-16 T2
s3-16 T3
s3-17 T1
s3-17 T2
s3-17 T3
s3-16
s3-17 T3
s3-16 T2
s3-17 T2
s3-16 T3
s3-17 T1
High-Lmit
Opens
Figure 94. Lint Samples s3-16 and s3-17 Tested 1 Inch from the Heater Edge
Figure 95 shows the T1, T2, and T3 thermocouple data for samples s3-14 and s3-18,
tested at a distance of 2 inches from the heater edge. Both samples smoldered, and no flames
were present before the high-limit thermostat opened and disconnected power to the heating
element. The profiles of the data obtained from thermocouples T1, T2 and T3 were similar for
both samples tested.
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Lint Sample Intake at 2 inches
High Limit, No Air flow
2002
0
50
100
150
200
250
300
0
9
18.001
27
36
45
54
63
72.001
81
90.001
99
108
117
126
135
144
153
162
171
180
189
198
Time (seconds)
Temperature (C)
s3-14 T1
s3-14 T2
s3-14 T3
s3-18 T1
s3-18 T2
s3-18 T3
s3-18
s3-14 T2
s3-14 T3
s3-18 T3
s3-18 T2
s3-14 T1
Figure 95. Lint Samples s3-14 and s3-18 Tested 2 Inches from the Heater Edge
In previous tests in which the high-limit thermostat was bypassed, a lint sample placed 2
inches from the heater edge ignited. Table 19 lists the high-limit thermostat activation
times.
The average activation time was 39 seconds. Figure 96 shows a similar figure as in Figure 93
but with the average high-limit thermostat activation time at 93 seconds. The figure
indicates
the high-limit thermostat opens 8 seconds before the average sample at 2 inches from the
heater edge would ignite. In some cases, a lint sample 2 inches from the heater edge may
ignite before the high limit activates as shown in the figure.
Table 19. Approximate High Limit Thermostat Activation Times
High Limit Activation
2 inches from Heater Intake with High Limit
Sample
Number
Data
Time
(seconds)
Heater
Energized
(seconds)
Actual
Time
(seconds)
s3-18 74 36 38
s3-14 75 35 40
average 39
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Ignition Times for Lint Samples 2 and 3 inches from Intake
81
90
107
36
47
73
1 39
2
3
4
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Time (seconds)
Sample Distance
High Limit Activation
Figure 96. High Limit Activation Time Comparison at Sample 2 and 3 inches
3.4.2.3 High-Limit Connected in Series and Airflow
Four tests were conducted in this last series of tests, in which the high-limit
thermostat
was connected in series with the heating element and airflow through the heater box was
varied. Lint samples were placed at distances either 0 or 1 inch from the edge of the heater
box
(either 2 or 3 inches from the heating element). Airflow velocity, as measured at the intake,
was
varied from 200 to 400 sfpm. For a sample placed 1 inch from the heater housing edge, the
thermocouple and anemometer data showed a threshold between 300 and 400 sfpm for ignition
and no ignition, respectively.
Figure 97 shows T1, T2, and T3 thermocouple data for samples s3-19, s3-20, and s1-8,
which were tested at a distance of 1 inch from the heater edge with different airflow
velocities.
The graph shows that samples tested with airflow below 300 sfpm ignited. With an airflow of
200 sfpm, the sample ignited approximately 60 seconds after powering the heating element.
With an airflow of 300 sfpm, the sample ignited approximately 80 seconds after powering the
heating element.
The graph also shows that samples tested with airflow below 300 sfpm ignited before the
high-limit thermostat opened and disconnected power to the heating element. For both samples
tested at 200 and 300 sfpm airflow, the samples ignited approximately 10 seconds before the
high-limit thermostat opened (as shown by the decrease in temperature measured at
thermocouple T2). Since thermocouple T1 was in close proximity to the lint sample, the
thermocouple data for T1 and T3 showed similar responses. T2 thermocouple was used to
approximate the time at which the high-limit thermostat opened and disconnected power to the
heating element.
Sample s1-9 was placed 1 inch from the heater housing edge; the airflow was set at 400
sfpm, and the high-limit thermostat was connected in series with the heating element. Sample
s1-9 did not ignite. With an intake airflow of 400 sfpm and the high-limit thermostat
connected
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in series with the heating element, samples at 0 or 1 inch from the edge of the heater
housing
did not ignite.
Lint Sample Intake
High Limit, Air flow
2002
0
100
200
300
400
500
600
700
0
9
18
27
36
45
54
63
72
81
90
99
108
117
126
135
144
153
162
171
180
189
198
Time (seconds)
Temperature (C)
s3-19 T1
s3-19 T2
s3-19 T3
s3-20 T1
s3-20 T2
s3-20 T3
s1-8 T1
s1-8 T2
s1-8 T3
s1-9 T1
s1-9 T2
s1-9 T3
s3-19
s3-19 T2
s3-20 T2
s1-8 T2
s1-9 T2
s3-19 T1
High-Lmit
Opens
s1-8 T3
s1-8 T1
Figure 97. Lint Samples Tested 1 Inch from the Heater Edge with Airflow
Sample s1-10 was placed 1 inch from the heater housing edge; the airflow was set at
400 sfpm and the high-limit thermostat was bypassed. The sample ignited. Figure 98 shows a
comparison between samples s1-9 and s1-10. The figure shows that sample s1-10 ignited
approximately 25 seconds before the high-limit thermostat operated for sample s1-9.
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Lint Sample Intake at 1 inch
with and without High Limit, Air flow 400 sfpm
2002
0
100
200
300
400
500
600
700
0
9
18
27
36
45
54
63
72
81
90
99
108
117
126
135
144
153
162
171
180
189
198 Time (
seconds)
Temperature (C)
s1-9 T1
s1-9 T2
s1-9 T3
s1-10 T1
s1-10 T2
s1-10 T3
s1-10 T2
s1-9 T2
High-Lmit
Opens
s1-9 T1
s1-9 T3 s1-10 T1
s1-10 T3
sample 1-9 ignites
power turned off
Figure 98. Samples Tested at 1 Inch, With and Without High-Limit Thermostat
3.4.3 Lint Samples Ingested into the Heater
The ignition characteristics of lint were evaluated by observing the heater exhaust as lint
samples of varying weights and sizes were ingested into the heater box. The test setup was
designed to allow the airflow into the heater intake to carry lint samples past the heating
element and to exhaust any embers through a 4” diameter glass tube.
In these tests, lint samples ranging from 0.04 g to 0.48 g were observed igniting, and
embers traveled through the exhaust vent. The following observations were made (small
embers are defined as particles approximately the size of a pinhead; large embers are defined
as particles close to the size of a thumb tack head):
1. Sample contacted the center heating element coil and ignited.
Result – Small embers exited the heater box.
2. Sample contacted the edge of the heating coil and ignited.
Result – Small to large embers exited the heater box. In some instances, small
portions of the sample that were either flaming or contained large embers broke away.
3. Sample passed between the heating element and the heater housing.
Result – The sample ignited and exited the heater box.
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4. Sample became lodged between the two layers of heating coils, on the insulator.
Result – Small embers exited the heater housing, and unburned lint stayed lodged
on the insulator.
3.4.4 Ignition of Target Material Downstream from the Heater Exhaust
Four tests were conducted to determine whether embers that exited the heater could
ignite target materials placed downstream of the heater exhaust. The target material was
either
100% cotton lint or 100% cotton terry towel.
In all four tests conducted, the target material ignited. The test results showed that
fewer ingested lint samples were required to ignite the 100% lint target material than the
100%
cotton terry target material. The target material of 100% cotton lint ignited on the fifth sample
of
0.10 g, the sixth sample of 0.20 g, and the tenth sample of 0.10 g ingested into the heater
box.
The target material of 100% cotton terry ignited on the eighth sample of 0.30 g.
When the target material ignited, the target material would produce additional embers
that were carried through the exhaust vent. The 100% cotton terry towel burned substantially
longer than the lint target material. The terry towel also produced significantly more
embers,
which were carried further into the exhaust venting.
3.4.5 High-Limit Thermostat Analysis
Two high-limit thermostats failed during the course of testing. The life histories of both
thermostats were compiled and are presented in Table 20:
– Column 1 (High Limit Thermostat) labels the thermostats (HLT1 represents the
highlimit
thermostat that failed first, and HLT2 represents the second).
– Column 2 (Life Below 100°C) lists the approximate number of hours to which the
high-limit thermostat was exposed to the following conditions: The dryer operated on
the primary thermostat or it operated while the temperature at T1 (intake air into the
heater box) was below 100°C. This would include testing (Task 2) in which the dryer
operated with an unblocked exhaust vent or with a partially-blocked exhaust vent,
and it would include testing (Task 4) with just the heater box in the unblocked and
partially blocked conditions.
– Column 3 (Life Above 100°C) lists the approximate number of hours to which the
high-limit thermostat was exposed to the following conditions: The high-limit
thermostat was bypassed or the temperature at T1 (intake air into the heater box)
was above 100°C. This would only include testing in which the heater box was
installed in the test apparatus.
– Column 4 (Number of Cycles) lists the number of times the high-limit
thermostat
cycled; one cycle represents opening and closing of the high-limit thermostat. The
high-limit cycling duration was not included in either of the total times listed in column
2 and 3.
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Table 20. Life History of Failed High-Limit Thermostats
High Limit
Thermostat
Life Below
100° C
Life Above
100° C
Number of Cycles
HLT1 9.3 hours 0.79 hours 47
HLT2 97.6 hours 0.20 hours 26
3.4.5.1 High-Limit Thermostat 1 (HLT1) Analysis
The first high-limit thermostat failed in June 2002 during Task 4 testing. This
high-limit
thermostat was used in Task 2: Dryer Design A – Document Dryer Operating Characteristics,
and Task 4: Determine Characteristics Required for Lint Ignition. The high-limit thermostat
was
connected in series with the heating element when it was discovered that it had failed to
open.
Figure 99 shows the thermocouple data for test Sample s3-13 when the high-limit first failed
to
operate. The figure shows the sample ignited, which was unexpected, and the T1 temperature
rose to almost 200°C.
Sample s3-13
High Limit in Series
0.19g, 2" from heater box edge, ignition
June 2002
0
100
200
300
400
500
600
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
64
68
72
76
80
84
88
92
96
100
Time (seconds)
Temperature (C)
0
200
400
600
800
1000
1200
1400
1600
Air Flow (sfpm)
T1 Heater Intake T2 Heater Exhaust T3 Liint Sample
T4 Exhaust Vent Heater T5 Exhaust Vent Outside Heater T6 Chamber
T7 Room Ambient V1 Airflow into Heater V2 Airflow External Heater
T3 Sample Ignites
T2
T1
T4
Figure 99. Sample s3-13 Test with the High Limit Thermostat in Series
Figure 100 shows a comparison for thermocouples T1, T2, and T3 between the failed
high-limit thermostat (HLT1) and a replacement high-limit thermostat (HLT2). The
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thermocouples were placed at the heater intake, the heater exhaust, and 2 inches from the
heater edge for T1, T2 and T3, respectively. The figure shows T1 for HLT2 rose to
approximately 150° C before the high-limit thermostat opened and disconnected power to the
heating element. The temperatures for HLT1 continued to rise until power was manually turned
off at approximately 170 seconds.
High Limit Thermostat 1 (HLT1) and Replacement High Limit Thermostat 2
(HLT2)
June 2002
0
100
200
300
400
500
600
0
6.999
13.999
21
28
35
41.999
49
56
62.999
69.999
77
84
90.999
97.999
105
112
119
126
133
140
147
154
161
168
175
182
189
196
Time (seconds)
Temperature (C)
T1 HLT1 T2 HLT1 T3 HLT1 T1 HLT2 T2 HLT2 T3 HLT2
T1 HLT1 Intake
Heater
T1 HLT2 Inatke
Heater
T2 HL2 Heater Exhaust
T3 HLT2 2" from Heater
Intake
T3 HLT1 2" from Heater
Intake
T2 HLT1 Heater Exhaust
High-Limit Opens
Power to the heater
manually turned off
Figure 100. Failed Thermostat compared to a Replacement Thermostat
The high-limit thermostat was removed and x-rayed as shown in Figure 101. The highlimit
thermostat uses a bi-metal disc that “pops” at a certain set temperature. When the
concave disc pops and becomes convex, it pushes on a rod that sequentially opens the
contacts. The x-rays did not reveal any abnormalities.
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Contacts
Rod
Bi-Metal Disc
Figure 101. X-ray Images of High-Limit Thermostat 1 (HLT1)
The high-limit thermostat was opened and examined under a microscope, as shown in
Figure 102. It appeared that the contacts were welded together, but this could not be
confirmed.
Moving Contact Fixed Contact
Moving Contact
Fixed Contact
Figure 102. Contacts for High-Limit Thermostat 1 (HLT1)
Figure 103 shows the surface of the contact pads. Pitting and scorch marks are visible
and indicate there was a concentration of high heat. Figures 102 and 103 also show that
approximately 50% to 60% of the contact pad surfaces were making contact when the
thermostat was in the closed position.
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