Final Report on

Electric Clothes Dryers and Lint Ignition Characteristics

May 2003

Abstract

In FY 2002, U.S. Consumer Product Safety Commission staff completed a test program

to evaluate residential electric clothes dryers under various test conditions. The test

program included measurements of temperature and airflow characteristics for different

electric clothes dryer designs under normal operating conditions and conditions of

partially-blocked and fully-blocked exhaust ducting. The ignition characteristics of lint in

relation to electric clothes dryer operation were also evaluated. The test program

produced data on different dryer operating and design characteristics that can be used

to help reduce the risk of lint ignition in a clothes dryer and help prevent fires.

The report and appendices are provided in portable document format (pdf). Only the

report is available on CPSC's web site.

The appendices and video clips (as well as the report) are available on compact-disk

read-only memory (CD-ROM) after June 29, 2003. 

To request the CD-ROM “Final Report on Electric Clothes Dryers and Lint Ignition

Characteristics, May 2003”, write to:

U.S. Consumer Product Safety Commission

Office of Information and Public Affairs

Washington, D.C. 20207

 Or send an e-mail to info@cpsc.gov.

When requesting the CD-ROM, please include the name of the report, the number of

copies you want (limit 3 copies per request), and the name and address where the report

should be sent.
IMPORTANT

All airflow measurements are reported in Standard Feet per

Minute (SFPM). These measurements are referenced to 25oC

and 29.92 inches Hg. Figures and text references listing the

SFPM velocities have been converted to actual velocities

(FPM). This data will be posted as an addendum on the CPSC

web site (www.cpsc.gov) after September 2, 2003. The

addendum will be titled,

“Addendum to Final Report on Electric Clothes Dryers and Lint

Ignition Characteristics, May 2003 – September 2003
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
i

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

E

S

Directorate for

ngineering

ciences
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May 2003

ii

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.
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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.
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♦ 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.
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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.
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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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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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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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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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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
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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
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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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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 1⁄4-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

124248372496620744868992

111612401364148816121736186019842108223223562480260427282852297631003224334834723596372038443968

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

4182

123164205246287328369410451492533574615656697738779820861902943984

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

205246

287.001

328

369.001

410451492533

574.001

615

656.001

697

738.001

779820861902943984

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

4080

120160200240280320360400440480520560600640680720760800840880920960100

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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21

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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22

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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23

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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24

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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25

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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26

Dryer Design A

Bypassed Primary Thermostst

Dry Load

December 10, 2001

0

50

100

150

200

250

300

350

400

450

0

53

106159212265318371424477530583636689742795848901954

1007106011131166121912721325137814311484153715901643169617491802185519081961

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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27

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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28

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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30

Dryer Design B

Wet Load, No Blockage

January 30, 2002

0

50

100

150

200

250

300

0

110220330440550660770880990

110012101320143015401650176018701980209022002310242025302640275028602970308031903300341035203630374038503960

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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31

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

159318477636795954

1113127214311590174919082067222623852544270328623021318033393498365738163975

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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32

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

161322483644805966

112712881449161017711932209322542415257627372898305932203381354237033864

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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33

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

160320480640800960

1120128014401600176019202080224024002560272028803040320033603520368038404000

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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34

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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35

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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37

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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38

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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39

Dryer Design B

Bypassed Primary Thermostst

Wet Load

February 11, 2002

0

50

100

150

200

250

300

350

400

0

163326489652815978

114113041467163017931956211922822445260827712934309732603423358637493912407542384401456447274890505352165379554257055868

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

T1 T5

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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40

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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41

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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43

Dryer Design C

High Heat, Wet Load

December 11, 2001

0

50

100

150

200

250

0

109218327436545654763872981

109011991308141715261635174418531962207121802289239825072616272528342943305231613270337934883597370638153924

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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44

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

161322483644805966

112712881449161017711932209322542415257627372898305932203381354237033864

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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45

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

163326489652815978

114113041467163017931956211922822445260827712934309732603423358637493912

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

158316474632790948

1106126414221580173818962054221223702528268628443002316033183476363437923950

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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48

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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50

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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51

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

127254381508635762889

101611431270139715241651177819052032215922862413254026672794292130483175330234293556368338103937

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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53

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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54

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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55

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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56

Dryer Design D

High Heat, Wet Load 

January 03, 2002

0

50

100

150

200

250

300

0

106212318424530636742848954

1060116612721378148415901696180219082014212022262332243825442650275628622968307431803286339234983604371038163922

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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57

Dryer Design D

High Heat, Wet Load, with Analog (T2)

January 07, 2002

0

50

100

150

200

250

300

0

109218327436545654763872981

109011991308141715261635174418531962207121802289239825072616272528342943305231613270337934883597370638153924

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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58

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

4182

123164205246287328369410451492533574615656697738779820861902943984

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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59

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

4182

123164

204.999

246287328369410451492533574

614.999

656697

737.999

779820861

901.999

943984

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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60

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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63

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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65

Dryer Design D

Wet Load (6 towels) 

January 16, 2002

0

50

100

150

200

250

300

0

123246369492615738861984

11071230135314761599172218451968209122142337246025832706282929523075319833213444356736903813

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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66

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

Inlet Heater

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.

Dir

A

e

irflow

ction

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 1⁄4 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 ƒ

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 1⁄4 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

07

14212835424956637077849198

105112119126133140147154161168175182189196

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 1⁄4 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 ƒ

0@400s

40 bypassed ignited

C s5-2 0.32 instant 0 40 bypassed smolder

C s5-3 0.32 instant 650 ƒ

0@400s

40 bypassed smolder

C s5-4 0.30 instant 650 ƒ

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

300310320330340350360370380390400410420430440450460470480490500510520530540550560570580590600

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 ƒ

0@400s

43 bypassed smolder

E s2-5 0.38 instant 650 ƒ

0@400s

43 bypassed ignited

E s2-6 0.39 instant 650 ƒ

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

300310320330340350360370380390400410420430440450460470480490500510520530540550560570580590600

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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87

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

0

100

200

300

400

500

600

700

0

102030405060708090

100110120130140150160170180190200210220230240250260270280290300

Time (seconds)

Temperature (C)

0

200

400

600

800

1000

1200

1400

1600

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 1⁄2 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 1⁄2 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 1⁄2 x 1⁄2 to 5.08

2 Lint Sheet 7 x 5 1⁄2 x 1⁄2 x 3.04

3 Lint Sheet 7 1⁄2 x 6 1⁄4 to 2.08

4 Cotton Towel 7 1⁄2 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

0

50

100

150

200

250

300

350

400

0

102030405060708090

100110120130140150160170180190200210220230240250260270280290300

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

159318477636795954

1113127214311590174919082067222623852544270328623021318033393498365738163975

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

161322483644805966

112712881449161017711932209322542415257627372898305932203381354237033864

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

160320480640800960

1120128014401600176019202080224024002560272028803040320033603520368038404000

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

158316474632790948

1106126414221580173818962054221223702528268628443002316033183476363437923950

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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99

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

Not Temperature A B C D

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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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

10192837465564738291

100109118127136145154163172181190199208

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

102030405060708090

100110120130140150160170180190200

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

112131415161718191

101111121131141151161171181191201

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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T3 Only - Lint Sample Intake at 3 inches

High Limit Bypassed, No Airflow

2002

0

100

200

300

400

500

600

700

1

112131415161718191

101111121131141151161171181191201

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

10192837465564738291

100109118127136145154163172181190199

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 8190 107 364773

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

09

18.001

2736455463

72.001

81

90.001

99

108117126135144153162171180189198

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

09

18.001

2736455463

72.001

81

90.001

99

108117126135144153162171180189198

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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119

Ignition Tim e s for Lint Samples 2 and 3 inche s from Intake 8190 107 364773

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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120

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

09

18273645546372819099

108117126135144153162171180189198

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

09

18273645546372819099

108117126135144153162171180189198

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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122

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

048

12162024283236404448525660646872768084889296

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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124

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

212835

41.999

4956

62.99969.999

7784

90.99997.999

105112119126133140147154161168175182189196

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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126

Fixed Contact

Moving Contact

Figure 103. Contact Pads for High-Limit Thermostat 1 (HLT1)
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3.4.5.2 High-Limit Thermostat 2 (HLT2) Analysis

The second high-limit thermostat failed during tests in July 2002 during Task 4:

Determine Characteristics Required for Lint Ignition. This high-limit thermostat was used in

Task 3 Monitor Lint Distribution testing and as a replacement thermostat for Task 4:

Characteristics for Lint Ignition tests. The high-limit thermostat was in series with the heating

element when it was discovered that it had failed to open. Figure 104 shows the thermocouple

data for test Sample s5-14 when the high-limit failed to operate. The figure shows the T2

temperature rose to almost 600°C. (The T1 thermocouple and V1 anemometer were not in the

heater intake air stream during this test.)

s5-14

July 12, 2002

0

100

200

300

400

500

600

700

0

102030405060708090

100110120130140150160170180190200210220230240250260270280290300

Time (seconds)

Temperature (C)

0

200

400

600

800

1000

1200

1400

1600

Airflow (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 Lint Sample

T2 Heater Exhaust

Power to the heater

manually turned off

T4 Heater Exhaust

Figure 104. Sample s5-14 with High Limit Thermostat in Series

Figure 105 shows a comparison of temperature data from thermocouples T2 and T3 for

the high-limit thermostat before it failed (HLT2 Before) and after it failed (HLT2 After). The

thermocouples were placed at the heater exhaust and 2 inches from the heater edge for T2 and

T3, respectively. (The T1 thermocouple and V1 anemometer were not in the heater intake air

stream during the HL2 After test.)

The figure shows T1 temperature for HL2 Before rose to approximately 150°C before the

high-limit thermostat opened and disconnected power to the heating element. The sample lint

in the HLT2 After test ignited unexpectedly approximately 55 seconds after powering the heater.
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The temperatures for HLT2 After continued to rise until power was manually turned off at

approximately 170 seconds.

High Limit Thermostat 2 Before Failure (HLT2 Before) 

and After Failure (HLT2 After)

0

100

200

300

400

500

600

0

6.999

142128354249

55.99962.999

7077849198

105112119126133140147154161168175182189196

Time (seconds)

Temperature (C)

T1 HLT1 after T2 HLT1 after T3 HLT1 after T1 HLT2 before T2 HLT2 before T3 HLT2 before

T1 HLT2 after

T1 HLT2 (before)

Intake Heater

T2 HLT2 (after)

Heater Exhaust

T3 HLT2 (before)

 2" from Heater Intake

T3 HLT2 (after)

 2" from Heater Intake

T2 HLT2 (before)

Heater Exhaust

High-Limit Opens

(before)

Power to the heater

manually turned off

Sample Ignites

Figure 105. High-Limit Thermostat 2 (HLT2) before and after Failure

The high-limit thermostat was removed and x-rayed, as shown in Figure 106. The theory

of operation for the second failed thermostat, HLT2, is the same as that for the first failed

thermostat, HLT1. The x-rays did not reveal any abnormalities.
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Rod

Contacts

Bi-Metal Disc

Figure 106. X-ray Images of High-Limit Thermostat 2 (HLT2)

A portion of the thermostat housing was removed to reveal the contacts and the rod, as

shown in Figure 107. From visual examination, it could not be determined if the contacts were

welded together or not. This sample was not disassembled, to maintain the operational integrity

of the thermostat; and an inspection of the contact pad surfaces was not performed.

Moving Contact

Moving Contact

Rod

Fixed Contact

Fixed Contact

Figure 107. Internal View for High-Limit Thermostat 2

Before conducting tests of the failed thermostat HLT2, tests were first conducted on a

new high limit thermostat of the same make and model. The setup included a heat gun (heat

source), a microscope, and dual channel logging thermocouple temperature meter, as shown in

Figure 108. Thermocouple 1 was placed at the face of the enclosure, near the bi-metal disc.

Thermocouple 2 was used as an indicator of when the contacts opened and closed.
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High-Limit Thermostat

Microscope

T2 Thermocouple

T1 Thermocouple

Heat Source

Figure 108. Setup for Testing High-Limit Thermostats

A small grinder was used to produce a small hole to reveal the internal conponents

around the contacts of the new thermostat. Figure 109 shows a “before” picture (contacts in the

normally-closed position) and an “after” picture (contacts opened). The after picture shows the

rod had pushed on the arm and caused the contacts to separate.

Rod

Contacts are

Separated (opened)

Bi-Metal Disc

“Normal”

Bi-Metal Disc

“Flipped”

Figure 109. New High Limit Thermostat

Figure 110 shows the temperature data for T1 thermocouple at the bi-metal disc and T2

at the contacts for the new thermostat. As mentioned previously, T2 was used as an idicator of

when the contacts opened and closed. The temperature values for T2 were translated to 0 and
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1, where 0 indicates a closed contact and 1 indicates an open contact. The contacts opened

when T1 was at 113.4°C and closed when T1 was at 49.6°C.

New High Limit Thermostat

Test New 1-3

0

20

40

60

80

100

120

140

18

15222936435057647178859299

106113120127134141148

Time (seconds)

Temperature (C)

0

0.2

0.4

0.6

0.8

1

Contacts (0 closed, 1 

open)

T1 Contacts

113.4

49.6

Figure 110. Thermocouple Data for the New High-Limit Thermostat

The second failed high limit thermostat (HLT2) was tested in the setup. An audible click

could be heard when the bi-metal disc “popped.” When the bi-metal disc popped, it pushed on

the rod, which should have separated the contacts. However, the T2 signal did not indicate a

separation of the contacts. The resistance was measured at 0.0 to 0.1 ohms throughout the

test.

Figure 111 shows a picture before the bi-metal disc popped and an after picture. The

before picture (Bi-Metal Disc “Normal”) shows a small gap between the rod and the arm for the

moving contact. The after figure shows the rod had moved and contacted the arm which should

separate the contacts. However, the contacts remained closed.

Rod

Contacts remain

Closed

Bi-Metal Disc

“Normal”

Bi-Metal Disc

“Flipped”

Figure 111. Second Failed High Limit Thermostat, HLT2
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Figure 112 shows a comparison between a new (exemplar) high limit thermostat and the

second failed high limit thermostat (HLT2). When the HLT2 thermostat was tested, an audible

click could be heard at approximately 113°C and 35.5°C, indicating that the opening and closing

temperatures for the failed HLT2 thermostat. The new thermostat opening and closing

temperatures were similar.

New and Old (HLT2) High Limit Thermostat

Test New 1-3 and Old Test 10

0

20

40

60

80

100

120

140

1

14274053667992

105118131144157170183196209222235248261274287

Time (seconds)

Temperature (C)

0

0.22113

0.44226

0.66339

0.88452

1.10565

Contacts (0 closed, 1 open)

T1 New T1 Old (HLT2) Old Disc Flipped

Old Disc Flipped Contacts New Contacts Old (HLT2)

New

Opened

113.4

New

Closed

49.6

Old Disc Flipped 113 degrees

Old Disc Flipped 35.5 degrees

Figure 112. Comparison between New and Failed High Limit Thermostats
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4.0 SUMMARY AND CONCLUSIONS

The overall goal of the project was to determine whether lint accumulation could result in

clothes dryer fires. To accomplish this large goal, separate tasks were designed to link the

cause of lint accumulation to possible dryer fires and/or lint ignition. The smaller tasks

completed were:

Task 1. Inspect and Record Dryer Designs

Task 2. Document Dryer Operating Characteristics

Task 3. Monitor Lint Distribution, and

Task 4. Determine Characteristics Required for Lint Ignition.

The objective of this project was to evaluate the effects of lint accumulation and abovenormal operating temperatures in electric clothes dryers and determine whether such conditions

may result in dryer fires and/or lint ignition. The project demonstrated the following,

♦ Lint accumulates inside a dryer with properly vented exhaust ducting and with

properly maintained lint screen (as discussed in Section 3.3 Task 3, Monitor Lint

Distribution)

♦ Lint on the heater housing and in proximity to the heater intake can ignite (as

discussed in Section 3.4 Task 4, Determine Characteristics Required for Lint

Ignition), and

♦ Material downstream of the heater can be ignited by lint ingested by the heater (as

discussed in Section 3.4 Task 4, Determine Characteristics Required for Lint

Ignition).

The following provides a summary for each task presented in Section 3.0 Discussion:

Task 1. Inspect and Record Dryer Design

♦ All four dryer designs used the same method and order for moving the air through

the dryer.

• Air was pulled into the body of the dryer through any gaps in the dryer, but

typically through rear vents. It was drawn through the heater, which warmed the

air, and then entered the tumbler. The air exited the tumbler and was directed

through the lint screen. It passed through a duct in the dryer and into the fan.

The fan forced the air through an exiting duct to the back of the dryer, where it

was exhausted.

♦ The dryer designs had variations in the location and configuration of the heater, the

location the air exited out of the tumbler, and location and size of the lint screen.

♦ 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.
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♦ Using rigid external ducting does not allow for a secure pressure fit around the

dryer’s exhaust duct.

Task 2. Document Dryer Operating Characteristics

♦ The temperatures within the 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 duct 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.

û For one dryer design, the heater intake temperature reached temperatures

over 200° C for the 100% blocked exhaust condition, a 92% increase over

the unblocked condition. For all dryer designs, the average maximum

temperature was near 325° C for the 100% blocked exhaust condition, a

351% increase over the unblocked condition.

û For one dryer design, the heater exhaust temperature reached nearly 600° C,

an 89% increase over the unblocked condition. For all dryer designs, the

average maximum temperature was near 600° C for the 100% blocked

exhaust condition, an 89% increase over the unblocked condition.

û For one dryer design, the heater housing temperature reached over 200° C, a

92% increase over the unblocked condition. For all dryer designs, the

average maximum temperature was over 200° C for the 100% blocked

exhaust condition, a 92% increase over the unblocked condition.

• Under normal operation, the airflow inside the exhaust vent decreased

dramatically as the lint screen became blocked with lint particles. For all dryer

designs, the average exhaust air velocity with an unblocked exhaust vent

dropped from 1300 sfpm (standard feet per minute) to 900 sfpm.

• In general, the dryers only cycled on the high-limit thermostat when the exhaust

vent was 75 or 100% blocked, which caused the temperatures near the heater to

increase significantly.
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• 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%. 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.

♦ After 100 cycles, there was evidence of lint settling on dryer components and inside

the dryer floor. Leaks in the fan housing and exhaust vent allowed lint-containing air

inside the dryer chassis.

♦ Seals in the dryer’s interior exhaust venting may not be adequate to prevent linty air

from escaping into the dryer’s interior.

Task 4. Determine Characteristics Required for Lint Ignition

♦ Lint that accumulates on the heater housing can ignite under conditions of a failed

high-limit thermostat and a blocked exhaust vent.

• Lint on the heater housing ignited under certain conditions.

û Lint ignited on the heater housing when the airflow through the heater was

100% blocked and the high-limit thermostat had been bypassed (simulating a

thermostat failing to open).

û Only some areas of the heater housing were susceptible to lint igniting, under

certain conditions.

û In most conditions, lint tended to be consumed by pyrolysis or thermal

decomposition.

♦ Lint accumulating near the heater intake can ignite before the high-limit thermostat

switches the heater element off.

• Lint placed at the heater intake ignited under certain conditions. Under some

conditions of reduced airflow, lint near the heater ignited even with a functional

high-limit thermostat.

û Lint ignited up to 5 inches from the heater element with no airflow into the

heater and the high-limit thermostat bypassed.
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û Lint ignited up to 3 inches from the heater element with no airflow through the

heater and with a functional high-limit thermostat (i.e., thermostat connected

in series with the heater element).

û Lint placed 3 inches from the heater element ignited with airflow into the

heater reduced to 300 sfpm. The high-limit thermostat was connected in

series with the heater element.

♦ Lint ingested by the heater and embers expelled from the heater exhaust can easily

ignite additional lint or fabric in the air stream, resulting in additional embers in the

dryer system and exhaust vent.

• Lint ingested into the heater ignited material downstream.

û In all three tests conducted, lint samples ingested into the heater ignited

additional lint material downstream. The lint samples ingested by the heater

weighed only 0.10 gram.

û Lint samples ingested into the heater ignited a sample of 100% cotton towel

material in the air stream. The lint samples ingested by the heater weighed

0.30 gram.

The CPSC staff noted the following during testing and analysis of the data:

♦ The high-limit thermostat may prematurely fail when subjected to high ambient

temperatures.

• Two high-limit thermostats failed during the test program.

û The first high-limit thermostat failed after approximately 10 hours and 47

cycles (includes total estimated time the thermostat was exposed to

temperatures above and below the upper set point).

û The second high-limit thermostat failed after approximately 98 hours and 26

cycles (includes total estimated time the thermostat was exposed to

temperatures above and below the upper set point).
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A-J

Appendices A to J

Running the CD

Insert CD in the drive. An autorun display will appear with choices of Main report and Appendix

Menu.

Select MAIN Report to view report.

Select APPENDIX MENU to view appendix menu

Select the appendix to view.

Starting the CD

Starting the Appendix CD manually

– Click the START Button, scroll up and select RUN

–At the RUN menu, select BROWSE

– Navigate to the drive where the CD has been inserted. Double click on Autorun.exe

– Click OK

Select MAIN REPORT to view the report

Select APPENDIX MENU to view the appendix menu

In the APPENDIX MENU, select the appendix to view.

Appendices

Appendix A - Additional test data on Dryer Design A – Task 2

Appendix B - Additional test data on Dryer Design B – Task 2

Appendix C - Additional test data on Dryer Design C – Task 2

Appendix D - Additional test data on Dryer Design D – Task 2

Appendix E - Additional data for Task 3

Appendix F - Additional test data for Task 4 (Lint on the Heater Housing)

Appendix G - Additional test data for Task 4 (Lint at the Heater Intake)

Appendix H - Additional test data for Task 4 (Lint Ingested into the Heater)

Appendix I - Supporting test data for Discussion

Appendix J - Video clips of Task 4 testing