NBS

PUBLICATIONS

A 111 DM 02223E

NBSIR 78-1419

Life Test Development for

Clothes Dryers

Julius Cohen

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Interim Report

October 1 976 - September 1 977

*QC """ U.S. DEPARTMENT OF COMMERCE

100

,056

78-1419

CL- ^

NATIONAL BUREAU OF STANDARDS

DEC 1 2 1979

NBSIR 78-1419 QCJ.ce>

LIFE TEST DEVELOPMENT FOR

CLOTHES DRYERS C v '2—

Julius Cohen

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

Interim Report

October 1 976 - September 1 977

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
*•
Table of Contents

Page

1. Introduction 1

2. Test Planning: Constraints & Strategy 1 2.1 Restatement of objective 1 2.2 Sample selection 2 2.3 Practical considerations 2 2.4 Type of testing 2 2.5 Data sought 3 2.6 Performance criteria 3 2.7 Use conditions 3 2.8 Energy efficiency 4 2.9 Field testing ..... 5 2.10 Recording data 5

3. Instrumentation 5

4. Experimental Procedure 6

4.1 Performance 6

4.1.1 Normal-weight load 7 4.1.2 Overload 7 4.1.3 Abrasive/ impact load 8

4.1.4 Monitoring 8

4.2 Energy efficiency 8

4.3 Field testing 8

5. Results 9

5.1 Performance 9

5.2 Energy efficiency 9

5.3 Field testing 9

6. Discussion 9

Appendices

A. Comparison of components for various make dryers .... 11

B. Forms for reporting data 12

LCP-1 Failure report 13

LCP-2 Failure analysis report 14

LCP-3 Field installation report 15

LCP-4 Energy test for clothes dryer 16

Acknowledgment 17
List of Figures

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Circuit diagram of control box for automatically recycling

dryers

Dryer control panel equipped for automatic recycling — make-A.

Close-up view of timer-advancer.

Dryer control panel equipped for automatic recycling — make-B

Make-A dryer equipped for squirting water onto loads.

(Normally the door is closed; it is open here for illustration.)

Make-B dryer equipped for squirting water onto loads.

(Normally the door is closed; it is open here for illustration.)
1. Introduction

The objective of this ongoing work is to demonstrate the

feasibility of developing standardized laboratory test methods for

estimating useful life* and associated performance-based

characteristics—e.g., failure modes** and reliabilities—of major

appliances. Other performance testing work generally is made on components for an arbitrary duration of time, and a complete system may

be tested for only a fraction of its expected life.

The electric clothes dryer has been chosen for initial work

because of its relative electromechanical simplicity. The program is

threefold: 1) laboratory test development; 2) energy efficiency

testing; 3) controlled field testing. The emphasis of the work is on

1); 2) is an ancillary undertaking in which the time dependence of

energy efficiency is being investigated; 3) is quite limited in

extent, and is being done to elucidate field testing problems and

methodology; it is not expected to yield definitive results. This

publication reports on the progress of the program to date.

2. Test Planning: Constraints & Strategy

The test planning, like all other aspects of the testing, is

guided closely by a methodology formulated in anticipation of carrying

out work such as this: product life testing. The highlights only of

the test design are given in this section; for a detailed listing of

procedural steps followed, see NBSIR 76-1157 (Chapter 12).

2.1 Restatement of objective

Different make clothes dryers have many superficial

similarities, yet they are sufficiently different in design,

materials, and construction to render a mixed sampling

heterogeneous; (see, for example, Appendix A: Comparison of

typical components for various make dryers). Thus, the primary

objective of the present work can be restated specifically as the

development of a standardized test method for estimating useful

life and associated performance-based characteristics of a heterogeneous sampling of electric clothes dryers.

Useful life is defined as the period of satisfactory reliability measured

from the time the product is first put into service (for a discussion of

this concept see NBSIR 76-1157). Admittedly it is subjective, but so is

the concept of satisfactory performance, which must and is being dealt with.

**Failure modes here means the prevailing, or most frequent failures.
Owing to the heterogeneity, it is not possible to test the

different makes identically, and this complicates test

development. However, identicalness is not required.

Standarized, or uniform, testing requires that different makes

receive equivalent, unbiased treatment, with due regard to the

prevailing use conditions of each.

2.2 Sample selection

The laboratory facilities available for this work limited

testing to a maximum of 16 dryers. Rather than test one or two

dryers of many makes — the results would be of doubtful

significance — 8 dryers each of the two most popular makes,

representing over 50% of the home market, were chosen. Although

lot sampling was used where feasible, the present sampling cannot

strictly be considered random. Nevertheless, it may still be

representative of the population at large.

Implementation of the adjunctive controlled field testing

required that the dryers be limited to eight in number. All were chosen from a single make from the same lots which were to undergo

laboratory testing. Half were placed in very high use situations,

the other half in average use situations, for control.

2.3 Practical considerations

Clothes dryers are long-lived machines: their life expectancy

is greater than 14 yrs. Obviously, laboratory testing must be

completed within a small fraction of that (real) time.

The average-size household of four uses a dryer about 400 times

(cycles) a year for ~30 minutes per cycle. In the laboratory, the

test cycles need to be run continually (a test cycle consists of a dryer on period, followed by an off period). Also, labor

considerations generally require unattended operation for the

major amount of time; i.e., other than the normal work hours.

Intensive testing as outlined here can be implemented only if

operation is substantially automated.

In summary, testing must be intensive (around-the-clock, if

possible); machines must be automated for restarting and must be

capable of running unattended for long durations.

Notwithstanding, dryers are supposed to be maintained by frequent

removal of lint from the filters. Accordingly this latter

condition presents a challenge, as will be discussed subsequently.

2.4 Type of testing

Because no historical data regarding failure modes and times to

failure were available, it was decided to simulate, as far as

2
practicable in the laboratory, use in the home. An attempt was made to keep the principal stresses normal so that failure modes

and total use times would also be normal, thus providing a benchmark.

2.5 Data sought

Data sought for presentation at the conclusion of this project

are:

o Test method specifications for (electric) clothes dryer life.

o Identification of factors which under - or overstress

components.

o Years of equivalent life simulated in the laboratory,

o Description of maintenance actions.

o Numbers and types of failure; failure modes; times to failure;

reliability; estimated useful life; repair actions.

o Time dependence of energy efficiency.

o Life of test load.

2.6 Performance criteria

The primary performance criterion is drying ability; i.e., the

ability to remove moisture from a specified load within a specified time, satisfactorily.

A secondary criterion is that the load not be abnormally

degraded, as determined by visual inspection, by the dryer per se e.g., burn spots or abrasion.

Failure occurs if either criterion is not met.

2.7 Use conditions

Formulation of use conditions (including load) was aided by

availability of extensive private-sector market research data.

Three types of load are specified here:

1) a normal, or average-weight load;

2) a customarily-expected overload;

3) an abrasive/impact load. Table 1, below, lists load

compositions together with specified drying times and use

3
expendable* frequencies in percent —> loads are * and costing was

a factor in selection.

Table 1. Load compositions, drying times, & use frequencies

Type of load

normal-weight

customarilyexpected

overload

Composition Drying time

1 doz. towels, make-Ac 0.5 x 1 m ("20 x 40 in)

2.5 + 0.1 kg (5 1/2 + 1/4 lb.)

86% cotton

+2

40-0 min,

+2

14% polyester

double the normal

weight load, above

make-B .45-0 min.

70 + 2 min,

Use

frequency

(%)

'

90

abrasive/ impact 1 doz. metal buttons

riveted to 0.33kg/rn (10 oz./sq.yd)

cotton denim, ~ 0.6 x 0.75m (24 x 30 in) 5

30+2 min. (after folding in half and

hemming)

a The drying times for the two makes are required to be different.

Additionally, the last 10 minutes of the drying cycle of make-A

is run without heat, while for make-B, the last 5 minutes of

the cycle is run without heat.

No data were available regarding environmental conditions of

dryers in the field. In the laboratory, dryers were tested at

whatever temperatures and relative humidities prevailed.

2.8 Energy efficiency

The time dependence of energy efficiency is to be investigated

by making measurements on dryers as new, and periodically

thereafter as they age. The Department of Energy Test** is being

used because it is state-of-the-art.

*The survival time of towels against degradation by drying has been found

to be ~600 hours.

ft# "Clothes Dryer Test Procedure", Federal Register, dated September 14, 1977,

Vol . 42, No. 178, pg. 46145.

4
2.9 Field testing

Controlled-field testing will be used to monitor both

performance and energy efficiency. In case of failure, repairs

will be made by qualified (NBS) personnel. Energy efficiency will

be measured shortly after installation and periodically thereafter

at six month intervals.

2.10 Recording data

Forms were devised, preparatory to the experimental work, for

the systematic and comprehensive recording of data. These are:

1) failure report, 2) failure analysis, 3) field installation, 4)

energy test; and examples are given in Appendix B. 1) in

particular is operational in the sense that it initiates a systematic course of action from the initial allegation of

failure, to inspection, and finally to disposition or corrective

action, if required; 2) is used for dryer failures other than

early, where the cause is not readily discernable; 3) is used to

identify the recipient, and to record safety test data; 4) is

used for energy efficiency tests in the laboratory and in the

field.

3. Instrumentation

Each dryer is instrumented with a cycle counter and an elapsed

time meter to register total operating time. Dryers for the field have

an additional elapsed time meter installed across the heater element

for estimation of energy consumption.

In the laboratory, each dryer is equipped with a control box — of

which a circuit diagram is shown in figure 1 — used in automatizing

the following: 1) recycling of dryers; 2) rewetting of loads at the

start of each drying cycle; and 3) discontinuation of recycling in the

event of water leaks (associated with dryer failure).

1) Figure 2 is a photograph of the control panel of dryer make-A,

as equipped for automatic recycling. A motor mounted above the timer

knob (left) advances the timer to the desired setting and a solenoid

above the start button pushes down. Figure 3 is a more detailed view

of the mechanism used to advance the timer. When activated by the

control box, a one rpm electric motor turns the dryer timer knob

through a mechanical coupling. The angular displacement, or time

setting, is determined by means of an adjustable cam which actuates a microswitch to stop the motor. For the starting operation, a solenoid

is used to depress a plunger which rests on the start push button.

Figure 4 is a photograph of the control panel of dryer make-B,

as equipped for automatic recycling; the timer is at the right, and the

start switch at the left. The automation instrumentation is similar to

5
that used for make-A, except for starting, the solenoid is mechanically

coupled to pull the start switch instead of pushing down.

2) Immediately after a dryer is started, a metered amount of

water is squirted into the drum of the closed dryer, and onto the

tumbling load of towels; thus, the load is fairly uniformly wetted.

The water-squirting system consists in sequence of 1) an on-off rotary

valve at the source; 2) filter; 3) main line solenoid valve (normally

energized to remain open); 4) pressure regulator; 5) pressure gauge;

and 6) water pipe to the dryers. Coming off the water pipe at each

dryer station is a solenoid valve coupled to a flexible length of

plastic tubing, connected in turn at the dryer, to a small copper

tubing (nominal 1/8" o.d., 0.030" wall thickness). The amount of water

dispensed is a function of the water pressure, pipe dimensions, and

time. The pressure is held constant by means of the regulator so that

the quantity of water is adjusted by regulating the time.

Figures 5 and 6 show respectively, makes A and B equipped for

squirting. (Normally doors are closed; they are opened here for

illustration.) For make-A, the tubing enters at the bottom (for ease of installation); approaches, but does not extend to the rim of the

drum; and is directed to squirt upwards and to the right into the

tumbling load. The copper tubing is terminated with neoprene tubing to

prevent possible load damage. For make-B, on the other hand, the

tubing enters at the top and is directed to squirt downward and to the

right into the drum. (A p^ior arrangement similiar to that used for

make-A was found to be unsatisfactory because water from the squirt

tube would be deflected by the tumbling load into the lint filter and

blower assembly, as well as out the door.)

3) Under each dryer, at a strategic location for early

warning, is fastened a small pan containing a water sensor. In case of

leaks due to dryer failure, the sensor will signal the control box not

to restart the affected dryer (the dryer cycle in progress, however,

will run to completion).

In the event of leaks due to solenoid valve failures at the

dryers, additional sensors positioned near the front of each dryer will

activate an independent control box, thus closing the main solenoid

valve to the water supply. The dryers will continue to recycle, but

with unwetted loads.

4. Experimental Procedure

4.1 Performance

Compositions of the various type loads, together with their

drying-time settings and use frequencies have been given in Sect.

2.7. An abridged procedure for running these loads is given

6
below; all tests are made with dryer temperature set at HIGH, or NORMAL

4.1.1 Normal-weight load

For attended operation normal-weight loads are placed

in each of 16 dryers; clean lint filters are installed, and

the on-off switches of the individual dryer control boxes

and the master timer are turned ON. Groups of four dryers

each will be activated sequentially and automatically at 15

minute intervals, and will be recycled every 60 minutes

thereafter. At activation, first the dryer timer is

rotated to the prescribed setting, then the machine is

started and simultaneously a metered amount of water, 75-

80% by weight relative to the nominal weight of the dry

load, is ejected for approximately two minutes onto the

tumbling load for uniform wetting. At the completion of

the drying cycle the machine turns off for 15 or 20

minutes, depending on make, until being automatically

restarted. The attendant frequently cleans the lint

filters.

Owing to design differences it is found necessary to use different methods for running the different makes

unattended for long times. For rnake-A, (clean) filters may

be left in the machine overnight; for weekend runs, the

filters are removed and the lint collected in a filter

external to the machine. For make-B, a special auxiliary

filter, developed for this work, is installed in the normal

air stream to extend the filtering capacity sufficiently to run the dryers overnight — it is essential, however, that

the towels not be new or old, as these are very prolific

generators of lint. At present it is not feasible to run make-B dryers weekends — attempts to run with the filter

removed were unsuccessful — and development work to meet

such an objective is continuing.

4.1.2 Overload

Running the customarily-expected overloads requires

operator action. Two dozen towels, or double the normalweight load, are placed in the dryers, and the amount of

water to be dispensed may be doubled also by resetting the

timer on the individual control boxes to twice that used

for the normal-weight load. As before, control boxes and

master timer are switched ON; when machines start

automatically, the operator advances the timers to 70

minutes (this obviates cool-down). Recycling will occur every 60 minutes.

7
4.1.3 Abrasive/ impact load

The special test cloths are thoroughly wetted in a clothes washer (new cloths are first washed twice with

detergent and thoroughly rinsed, to remove sizing), and

spin dried to completion of the spin cycle. One test cloth

is placed in each dryer, and the control box switches are turned OFF to disengage the automatic restart system. The

operator sets the dryer timer and pushes the start button.

4.1.4 Monitoring

Several times each day, dryness of the loads is

monitored tactually*, soon after the drying cycles are completed. In case of failure, repairs are carried out,

repair data are recorded, and the unit is restored to test.

Drum linings are visually inspected after running

impact/abrasive loads, for signs of degradation.

4.2 Energy efficiency

For measurements of energy efficiency the test procedure

previously cited, is being used. Thirty-three pieces of special

test cloth of 50% cotton - 50% polyester composition are spin

dried in a washer, then tumble dried in a dryer, both to

prescribed tolerances of moisture content. For procedural details

see the test procedure cited; for details of the quantities

measured, as well as the method for calculating energy efficiency,

see Form LCP-4, Energy Test, in the Appendix to this report. The

tests are being carried out periodically in the laboratory with

different small samplings each time, and in the field.

4.3 Field testing

Dryers are checked for safety 1) in the laboratory, after being

instrumentated and, 2) in the field, after being installed; for

details see Form LCP-3, Field Installation Report, in the

Appendix. Subsequently, qualified laboratory personnel carry out

periodic energy efficiency tests, and make repairs when necessary.

#

For better reproducibility , it is planned to replace this subjective

determination by one based on weighing.

8
5. Results

5.1 Performance

Laboratory testing began, for all practical purposes, some six

months ago, and since then, make-A dryers have logged six years of

equivalent life* and make-B, four years of equivalent life. There

have been three bona-fide dryer failures: one early, and two

random, all with make-A. Failed components were a timer, a timer

knob, and a heater element.

5.2 Energy efficiency

Thus far, energy efficiencies measured in the laboratory show

little or no change with simulated age of dryers. The value is

roughly 1 kg(H~0)/kWh, with make-B appearing slightly more

efficient.

5.3 Field testing

Dryers were placed in the field about five months ago, and

there have been no reports of failure. Energy efficiencies

measured shortly after installation were similar to those measured

in the laboratory before installation.

6. Discussion

The reliability of the dryers so far has been high, as was expected for an established and slowly evolving product. The dryers

are now in their random failure period of life where reliability is

maximum, and appreciable failures are not expected until well into

life, when parts wear out.

On the other hand, the automation system, new and untried,

required considerable debugging. Most problems appear to have been

corrected with the exception of the water injection system: leaks or flooding occurs frequently owing to malfunctioning solenoid valves.

The primary cause of failures, however, appears to be the water supply

itself, or specifically, the mineral salts dissolved therein. These

precipitate in the pipe, then migrate as particles to the solenoid

valves and prevent complete closure. Corrective action will be

attempted: it is planned to install a water softener, and to use another kind of valve which may be less sensitive to debris.

An important finding has been made already in field testing. At one site the initial energy efficiency measured was about 20% lower

than expected. Investigation showed the existent ductwork to have been

improperly installed; this led eventually to substantial accumulation

of trapped lint, and as a consequence, appreciable decrease in air

flow. Cleaning the lint from inside the ductwork restored energy

Based on 416 cycles per year. In the laboratory work the average duration

of a cycle is 40 min., in the field it is a 30 min., and the difference is

due to the types of load.

9
efficiency to a normal value . Improperly installed ductwork can lead

to decreased energy efficiency and higher operating costs, as well as failure to dry.

A reminder: This is a test development project being made from

scratch and modifications in procedure and instrumentation may be

expected before its completion.

10
Comparison

of

typical

components

for

various

make

dryers

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11
Appendix B

Forms for reporting data

LCP-1 Failure report

LCP-2 Failure analysis report

LCP-3 Field installation report

LCP-; t Energy test for clothes dryer

12
ACTION

INSPECTION

NOTIFICATION

LCP-1

FAILURE REPORT 1. No

2. System 3. Ident. 4. Environmental

5. Unit 6. Unit oper. time A. Lab

A. Inoperative

B. Intermittent

A. Hr. elapsed B. Field

B. Cycle no.

C. Other

7. Symptoms

8. Originator Ext

Date

10. Tests performed, results, comments

11. Confirmation

A.

B.

Yes

NO

12. Causes

A

_

B _

C.

Defective part D.

Testing error E.

Primary

Secondary

Other

] 3 . Analysis required

A.

B.

Yes

No

15. Cognizant engineer

14. Part operating time to failure

Hr

Cy c 1 e

;Ext. 16. Date

16. Describe corrective action taken (list parts replaced, part nos.)

17. Active repairs time hr 38. Repairman 19. Date

20. Dale returned to test 21. Associated failure reports

13
FAILURE ANALYSIS REPORT

LCP-2
FIELD

INSTALLATION

SAFETY

TESTS

(LAB)

'

IDENTIFICATION

LCP-3

FIELD INSTALLATION REPORT Code No.

1. System clothes dryer 2. Make & Serial No.

3.

Location of site

4.

Responsible person Tel.

FIELD INSTALLATION REPORT Code No.

System clothes dryer

5. Dielectric strength Acceptable Not acceptable

6. Leakage currents (worst case)

A. Appliance cold (before running) ma

B. Appliance hot (after running) ma

Acceptable Not acceptable. Explain

7. Tested by Ext ._ 8. Date

8. Safety tests

A. Visual inspection of site wiring Adequate Inadequate

B. Leakage currents on site 1) appliance cold ma

2) appliance hot ma

Acceptable Not acceptable • Explain

9. Initial cycles 10. Initial elapsed time

A. Heater hr

B. Motor hr

10. Date installed 11, By Ext

15
LCP-4

ENERGY TEST FOR CLOTHES DRYER

1 . Site: A. Lab

.

Bldg. Identification

B. Field Code No.

2. Environmental (for field use only)

A. Location (kitchen, laundry room, basement, other)

B. Conditioning (heated, air conditioned, neither)

C. Type of dwelling (private res., apt. bldg., other)

3. History

A. Elapsed time: Heater hr; Motor hr. B. Cycles

4. A. Load B. Rinse water temp.

5. Weights 6. Energy 7. Ambient

A. Bone dry kg A. Initial kWh A. Dry bulb °F

B. Spin dry kg B. After min. kWh B. Wet bulb °F

C. Heat dry kg C. After min. kWh C. %RH

( min .

)

D. Heat dry kg'

(add i t ional min

.

)

8. Ob server ; Ext

.

9 ., Date

10. Computation

A. DoE

1) Moisture removed = 50 - 5C = kg

2) Energy used = 6B - 6A = kWh

3) Efficiency = Moisture removed

Energy used

Mod i f ied

1) Moisture removed = 5B - 5D = kg

2) Energy used = 6C - 6A = kWh

3) Efficiency = Moisture removed

Energy used

k£

kWh

16
Acknowledgment

Thanks are due Owen Laug for designing the automating instrumentation.

1

.

17

Figure

1.

Circuit

diagraa

of

control

bo*

for

automatically

recycling

dryers.

control

panel

equipped

for

automatic

recycling

—

make-.

Figure

3

.

Close-up

view

of

timer-advancer

Figure

4.

Dryer

control

panel

equipped

for

automatic

recycling

—

make-

Figure

5.

Make-A

dryer

equipped

for

squirting

water

onto

loads.

(Normally

the

door

is

closed;

it

is

open

here

for

illustration).

Figure

6.

Make-B

dryer

equipped

for

squirting

water

onto

loads.

(Normally

the

door

is

closed;

it

is

open

here

for

illustration.
1 !
N8S-I 14A <h E v /• / t>

U S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC A 1 ION OK RF.POK 1 NO. NBSIR 78-1419

2. C »ov 't A etc ss ion No.

3. Krt i pu nt

’ s Accession No.

4. Tin. i: and scptulf:

Life Test Development For Clothes Dryers

5. Pnhl ii ai ion 1 Lire January 1978

6. Performing Organization < ode

7. AUTHORISE

Julius Cohen

8o Performing Organ. Report No.

9. PL K FOR MING ORGANIZATION NAMF AND ADDRFSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task /Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as No. 9

13. Type of Report & Period Covered T . Interim

Oct. 1976-Sept. 1977

14. Sponsoring Agency Code

15. SUPFLFMFNTARY NOILS

16. ABSTRACT (A 200-word or less (actual summary of most significant in formation. If document includes a significant

bibliography or literature survey, mention it here.)

This interim report deals mainly with development of a life test for

electric clothes dryers. The testing is being guided by a previously

formulated methodology, and highlights of the test planning process are given. Testing is intensive, and up to seven years of life have been

simulated in the laboratory. As expected for a long-lived product, there

have been only a few dryer failures so far. Additionally, the temporal

dependence of energy efficiency is being investigated.

17. KF.Y WORDS (.six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Clothes dryer; energy efficiency; life test; performance; reliability

engineering

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