NATIONAL BUREAU OF STANDARDS REPORT

5352

A STUDY OP CERTAIN PROCEDURES FOR MEASURING

THE HEAT TRANSFER IN REFRIGERATED TRAILERS

by

C. W. Phillips

J. W. Grimes

P. R„ Achenbach

Report to

Transportation and Facilities Branch

Marketing Research Division

Agricultural Marketing Service

U. S. Department of Agriculture

Washington 25., D. C.

U. $. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended h\ Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

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problems; invention and development of devices to serve special needs of the Government; and the

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The results of -the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project or program. Published papers appear either in the Bureau’s own series of publications or in the

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Inquiries regarding the Bureau’s reports should be addressed to the Office of Technical Information, National Bureau of Standards, Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-4830 July 1, 1957

A STUDY OF CERTAIN PROCEDURES FOR MEASURING

THE HEAT TRANSFER IN REFRIGERATED TRAILERS

by

C. W. Phillips

J. W. Grimes

P. R. Achenbach

Air Conditioning, Heating, and Refrigeration Section

Building Technology Division

to

Transportation and Facilities Branch

Marketing Research Division

Agricultural Marketing Service

U. S, Department of Agriculture

Washington 25s D. C.

IMPORTANT NOTICE

NATIONAL BUREAU OF STA

intended for use within the G

to additional evaluation and re listing of this Report, either in the Office of the Director, Nat

however, by the Government a to reproduce additional copies

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

ogress accounting documents

nally published it is subjected

eproduction, or open-literature on is obtained in writing from

such permission is not needed,

>repared if that agency wishes

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

5352

A STUDY OF CERTAIN PROCEDURES FOR MEASURING

THE HEAT TRANSFER IN REFRIGERATED TRAILERS

by

C. W„ Phillips, J. Wo Grimes, and P„ R. Achenbach

Abstract

At the request of the Transportation and Facilities

Branch, Marketing Research Division, Agricultural

Marketing Service, of the Department of Agriculture,

studies were made to determine the time required to

attain steady-state heat transfer In a typical

refrigerated semi-trailer under two laboratory

procedures, and to develop and describe an accurate

method for measuring brine temperatures during

rating tests of trailers. Representatives of the

National Bureau of Standards were requested to

present the principles of the metering heat sink

method for rating refrigerated structures to the

truck and trailer manufacturing industries using

models and graphical materials developed for the

purpose. The tests of a typical 35-foot semitrailer with 6-inches of glass fiber insulation

in walls, ceiling, and floor showed that steadystate heat transfer was approached only after

subjecting it to the desired temperature condiifc'ions

for a minimum of 24 hours. The lag in attaining

steady-state heat transfer was approximately the

same whether a fixed quantity of heat was supplied

inside the trailer or whether a fixed temperature

difference between inside and outside the trailer

was established as soon as possible after starting

the test. Recommended methods of temperature

measurement are described in the report as well

as the procedures used for disseminating information on the metering heat sink method for rating

refrigerated structures.

- 2 -

INTRODUCTION

In recent years the increased use of frozen food products

has created a need for the transportation of these frozen

products. This need for transportation has resulted in a

tremendous increase in the use of refrigerated semi-trailers.

The Transportation and Facilities Branch, Agricultural

Marketing Service, U. S. Department of Agriculture, has been

interested in and worked on some of the technical problems

of frozen food transportation for several years. The

Air Conditioning, Heating, and Refrigeration Section of the

National Bureau of Standards has made a number of measurements of the heat transfer of refrigerated semi-trailers

in connection with their studies of the performance of such

vehicles for the Quartermaster Research and Engineering

Command, Department of the Army.

Because of the lack of a standard method or standard

conditions for rating the heat transfer of refrigerated

semi-trailers, and because they recognized that such standards

would be desirable, the American Trucking Associations, Inc.,

in October, 1955; requested the National Bureau of Standards

to submit a project proposal for developing standard

rating conditions and a simple rating method and apparatus

to determine the heat transfer characteristics of such

vehicles. Although this program was not initiated, the

National Bureau of Standards and the U. S. Department of

Agriculture conducted a comparative study of seven typical

refrigerated semi-trailers in Edgewater Park, Mississippi,

in May, 1956. The trailers were submitted by members of

the Truck-Trailer Manufacturers Association at the request

of the American Trucking Associations, Inc. The results

of this test were reported in NBS Report No. 4875^ entitled

"A Comparison of the Heat Transfer Characteristics of Seven

Typical Refrigerated Semi-Trailers" in October, 1956. The

wide variations in the various heat transfer characteristics

of these seven vehicles and in the performance of their

refrigerating units further emphasized the need for standard

rating techniques. Concurrently with this activity, the

National Bureau of Standards was developing in cooperation

with the Quartermaster Research and Engineering Command

Department of the Army, a prototype metering heat sink

apparatus for measuring the heat transfer of a refrigerated

structure (such as a trailer) while the structure was

refrigerated under conditions normal to intended service.

Figure 1

-3-

The Transportation and Facilities Branch of the U. S.

Department of Agriculture, in December,, 1956, requested the

National Bureau of Standards to make certain studies designed

to further the development and acceptance of standard rating

conditions and test methods. The requested work was described

as a part of a continuing study of various methods of measuring

heat transfer of refrigerated vehicles lending to selection of

the most accurate and practicable method, which may be adopted

as a standard for the measurement of trailer performance by

the trucking and truck manufacturing industries, since previous

cooperative studies by the National Bureau of Standards and

the U. S. Department of Agriculture had indicated that a method

which refrigerate's the interior of the vehicle under test

is desirable.

The request incorporated the following four tasks:

1. Make laboratory tests of a typical 35-foot refrigerated

semi-trailer to determine the length of time required for

steady-state heat transfer under controlled heat loss conditions

2. Determine the best means for the measurement of tempera

ture difference in secondary refrigerant lines. Select best

means of instrumentation.

3. Prepare charts and other apparatus which may be used

to demonstrate and explain the basic recommended method to the

truck-trailer manufacturing industry and other interested

groups

.

4. After this has been done, the results of the research

program up to that point will be presented to the truck-trailer

manufacturing industry, at which time determination will be

made jointly by the National Bureau of Standards, the U. S.

Department of Agriculture, and the Truck-Trailer Manufacturers

Association as to further work to be done, to be supported

primarily by funds contributed by that industry.

This report describes the work done to complete these

four tasks.

Task 1

"Make laboratory tests of a typical 35-foot refrigerated

semi-trailer to determine the length of time required for

steady-state heat transfer under controlled heat loss conditions

-4-

A secondary purpose of this test was to evaluate the

adequacy of short heat loss tests, some reported as short as

ten to twelve hours, to determine the heat transfer rate of

a refrigerated trailer.

A 35-foot Trailmobile refrigerated semi-trailer. Model

No. T/M C-8222, Serial No. 1-17126, manufactured by

Trailmobile, Inc., was used for the three tests made. This

was the same trailer furnished by Trailmobile, Inc., for

the May, 1956 tests at Edgewater Park, Mississippi. Figure

1 is a photograph of the interior of the specimen looking

through the trailer doors. The fan and heater shown in the

figure were not used for these tests.

The trailer was equipped with a Thermo King refrigerating

unit. Model RL-30, which remained in place during the tests

but was not operated. The trailer had six meat rails, had an

aluminum exterior skin and a plywood interior skin^ and was

insulated with s3k- inches of Ultralite in the walls, floor,

and ceiling. It was equipped with a tandem wheel assembly

(two axles, four double wheels) and weighed 13^092 lbs,

according to the manufacturer's delivery acknowledgement.

For the tests, the trailer was placed in an enclosure

in which the temperature could be controlled. An electric

heater and two 16-in. desk fans were placed in the trailer

and means were provided to record the amount of power

supplied to them. Thermocouples were installed on the

interior and exterior of the trailer to measure the respective temperatures of the air inside and out. All exterior

thermocouples, four in number, were mounted six inches from,

the exterior surfaces, and one set of twelve interior^ thermocouples was mounted six inches from the interior surfaces.

Another set of three thermocouples was installed in the

trailer interior, one in the exact center of the vehicle,

the other two at midheight on the longitudinal center line,

1/6 of the interior length from either end. These two sets

were installed for comparative purposes, to determine whether

the set of three would be adequate for a proposed standard

test procedure.

For the first test, the trailer was left open and

allowed to come to thermal equilibrium in an ambient

temperature of 70°^ during a period of more than 24 hours.

The doors were then closed, the interior fans started, and

sufficient electrical heating supplied to bring the interior

temperature of the trailer to about 150°F in approximately
s

.


FIGURE 2

TIME REQUIRED TO REACH STEADY STATE HEAT TRANSFER

. JpQMPUTED :at transfer RATE

TRAILER TEMPI R4TURE

B1ENT TEMPERATURE

ELAPSED. TIME, HOURS

Rnn.

J\)

3<

u

O 200-

II

r IP n

1

. X- X 1 r — rv

V

CL

!

IJ

\J

J

12 n

in

1 0

n

9(K_

50^

4 Un

3

oU

J

o

r\ C U

QC If1 V

<1

ll

9C)

\

Ll

OV./

H

7< )

CA

l- |-hl

vy

-5-

2 1/2 hours. A thermostat in the trailer then regulated the

heating to hold the interior temperature at 150°F. The

power required was observed at regular intervals for more than

45 hours.

For the second test, the trailer was again permitted to

come to thermal equilibrium at 70 °F. The doors were then

closed, the interior fans started, and the amount of electric

heating required in the first test to hold the interior at

150°F was supplied. The interior temperature of the trailer,

therefore, rose much more slowly than in the first test.

This test was also continued for more than 45 hours.

The third test consisted of reducing the ambient

temperature from 70°F to 60°F at the conclusion of the second

test, with all other conditions unchanged, to determine the

length of time required for the heat transfer rate to again

become constant.

During the three tests, the trailer decreased approximately 40 lbs. in weight. Water began to drip from

approximately 26 points around the bottom edge of the exterior

skin soon after the interior temperature reached 150°F,

and continuing for about four hours in Test 1. Again in

Test 3 ) when the ambient temperature was lowered from 70°F

to 60°F, water dripped for about three hours from most of

the same places. No volumetric measurement was made of the

amount of water which dripped from the trailer, but it is

believed that the weight reduction can be attributed to

the water driven from the vehicle during the tests.

The rates at which the apparent heat transfer and

interior temperature approached constant values in tests

1, 2, and 3 are shown in Figure 2. It is significant to

note in comparing test 1 with test 2 that about the same

length of time was required for the heat transfer to come

to a steady-state regardless of the initial rate of increase

of the interior temperature, and that at least 24 hours

of "conditioning” time was required before a steady-state

heat loss test could be started. The average of the

temperatures indicated by the set of three thermocouples

along the center line of the trailer was less than one degree

different from the average of the set of twelve thermocouples

located six inches from the interior skin, indicating that

the air circulation rate was adequate to provide essentially

a uniform interior temperature.

- 6 -

In test 3, which was in reality a continuation of test

2, except that the ambient temperature around the trailer

was reduced from 70°^ to 60°F, it was observed that more

than ten hours elapsed before the heat transfer rate again

approached a steady-state condition.

DISCUSSION AND CONCLUSIONS - Task 1

Two significant conclusions are drawn from the data

of tests 1 and 2, shown in Figure 2; first, that a

’’conditioning” period of at least 24 hours is required

before a steady-state heat loss test can be initiated of a

refrigerated semi-trailer of the type, weight, and degree

of insulation typified by the Trailmobile vehicle used

for the test, and second; the time required for the vehicle

to come to a steady-state heat loss rate is not essentially

different whether the interior trailer temperature is changed

rapidly at first, as in test 1, or more slowly, as in test 2.

Although these tests were made with the interior

temperature higher than the ambient temperature around the

trailer, and corollary studies show that determination of

heat loss of a refrigerated vehicle by refrigerating the

interior is preferable; the information gaiiied by tests

1 and 2 is important because it shows that sufficient

refrigerating capacity to reduce the interior temperature

rapidly is not necessary. Taking advantage of this finding

will reduce the size and cost of the refrigerating apparatus

required for such a test.

Tests 1 and 2 also point out the desirability of

observing and recording temperature and heating or cooling

loads during the ’’conditioning” period in such a manner as

to indicate when the apparant heat transfer rate does not

change more than, say, one percent for two consecutive

six-hour periods. If the data is so recorded it can be

used to indicate when the steady-state heat transfer test

can be started. For example, applying this rule to test

1, it would indicate that a six-hour steady-state test

could have been started at about the 28 th hour and for test

2 at about the 34th hour.

Although tests 1 and 2 were not intended as ’’rating”

tests it was interesting to note that under the existing

conditions the heat loss of the trailer was 98.3 Btu per
.* .

'
HEAT TRANSFER CALORIMETER

FOR

REFRIGERATED SEMI-TRAILERS *

SYMBOLS: ATI TEMPERATURE DIFFERENCE OF BRINE ACROSS TRAILER

AT2 " " " M “ CALORIMETER

AT 3

”

" BETWEEN AIR IN TRAILER

AND TEST ROOM

tel, tc2,tc3, etc. TEMPERATURE MEASUREMENT POINTS

etc. ELECTRIC HEATERS RATED IN BTU/HOUR

EQUATION FOR HEAT GAIN OF 'TRAILERS :

( H2) - (HI)

BTU PER HOUR(°F)= _AJ2

A T3

* "DEVELOPED BY NAT'L BUREAU OF STANDARDS"

FIGURE 3

LOCATION OF THERMOCOUPLE

WELLS IN SECONDARY REFRIGERANT CIRCUIT

THERMOCOUPLE CIRCUIT FOR

MEASUREMENT OF TEMPERATURE DIFFERENCE

INLET THERMOCOUPLE WELL OUTLET THERMOCOUPLE WELL

FIGURE 4B

TO

POTENTIOMETER



TYPICAL THERMOCOUPLE WELL

IN 3/4" FITTING

FIGURE G

-7-

hour (°F) at the end of test 1 and 96.9 Btu per hour (°F)

at the end of test 2.

Task 2

"Determine the best means for the measurement of temperature difference in secondary refrigerant lines. Select best

means of instrumentation.”

Concurrent studies of methods for determining the

heat transfer rate of refrigerated structures for the

Quartermaster Research and Engineering Command, Department

of the Army, has indicated that a "metering heat sink"

such as shown diagrammatically in Figure 3 has several

advantages over the heat loss method. The most important

advantage of the metering heat sink method, which refrigerates

the interior of the structure under test to the temperature

at which it is designed to operate, measures the heat

transfer of the walls, roof, and ceiling at the same mean

temperature, and with the same direction of heat flow as in

normal service. Further, it provides for normal movement

of water vapor into or through the walls of the structure

under test, since the ambient conditions of temperature and

humidity are also maintained at normal service levels.

Figure 4 (A) shows the design of the thermocouple wells

and piping adjacent to them which have been found satisfactory for the accurate determination of the temperature

difference of the secondary refrigerant or brine entering

and leaving the trailer under test, and entering and leaving

the calorimeter or calibrated heater (see H2 in Figure 3).

Five copper-constantan thermocouples are series-connected

with the "cold" junctionsin one of the two wells and

the "hot" junctions in the other. Since the |iF per device

produced by a single pair of copper-constantan junctions

is about 20 microvolts, the five thermocouples in series

produce a measuring signal of about 100 microvolts per

degree of temperature difference of the two wells. Figure

4 (B) shows the wiring circuit for such a differential

thermocouple. Figure 5 is a view of a calibrated heater,

showing the piping arrangement and thermocouple wells.

Figure 6 shows the construction of a typical thermocouple

well. Conventional commercial electronic indicating or

recording potentiometers have been found satisfactory.

- 8 -

With a brine temperature difference of eight degrees,

the five thermocouples in series produce a signal of about

800 microvolts and since these potentiometers depending on

type and range, can indicate or record with an accuracy of

from three to ten microvolts, the temperature difference

can readily be measured within two percent. It should be

noted that corrections must be made for the change in emf

per degree produced by the differential thermocouples

with change in the absolute temperature of the thermocouple.

Task 3

"Prepare charts and other apparatus which may be used

to demonstrate and explain the basic recommended method to the

truck-trailer manufacturing industry and other interested

groups .

A working model of a metering heat sink apparatus

capable of measuring the heat leakage of a toy trailer was

constructed for demonstration to the trailer industry.

Figure 7 shows the circuit of this demonstration apparatus

schematically which used ice for a cooling means and water

as the secondary refrigerant or "brine". Figures 8 and 9

show two views of the demonstrator in operation. In Figure

9 the operator is measuring a temperature or temperature

difference on the potentiometer. The calibrated heater

can be seen under the inverted Dewar flask. Enlarged

graphs showing the results of tests 1, 2, and 3 under Task

1 were prepared and used in conjunction with the working

model of the metering heat sink.

Another working model of the metering heat sink, using

a mechanical refrigerating unit, and a heated enclosure

over the model trailer is shown in Figure 10. This model

was constructed prior to the Tasks outlined in this report,

but was modified for demonstration at several advisory

group meetings of the Department of Agriculture.

Task 4

"After this has been done, the results of the research

program up to that point will be presented to the truck-trailer

manufacturing industry, at which time determination will be

made jointly by the National Bureau of Standards, the U. S.

SCHEMATIC CIRCUITS FOR

MODEL OF METERING HEAT SINK

A. C.

FIGURE 7

Figure S

Fj gure 9

Figure 10

-9-

Department of Agriculture, and the Truck-Trailer Manufacturers

Association as to further work to be done, to be supported

primarily by funds contributed by that industry."

The working model of the metering heat sink shown in

Figures 8 and 9 was demonstrated at a three day annual

meeting of the Truck-Trailer Manufacturers Association in

January, 1957* at the Coronado Hotel, Coronado, California.

A discussion of the steps required to develop a standard

method and conditions for rating refrigerated trailers was

presented by representatives of the National Bureau of

Standards and the Department of Agriculture at a meeting

of the Refrigeration Committee of the Truck-Trailer

Manufacturers Association during this same three day

meeting. The results of the tests described under Task 1

were presented using charts and graphs and the working

model of the metering heat sink described under Task 3.

Following this demonstration and discussion of the

possibility of establishing such a standard rating method,

the Board of Directors of the Truck-Trailer Manufacturers

Association, at their final meeting at Coronado, voted to

request the continued cooperation of both the U. S.

Department of Agriculture and the National Bureau of

Standards in this endeavor and further requested that

these agencies prepare a proposal covering the necessary

laboratory and field studies to develop a standard rating

method for presentation at their next meeting which was

scheduled for April, 1957* in Washington, D. C.
. *

'
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V, Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its headquarters in Washington,

D. C, and its major field laboratories in Boulder, Colorado, is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section carries out spe- cialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the

inside front cover of this report.

WASHINGTON, D. C.

Electricity and Electronics. Resistance and Reactance. Electron Tubes. Electrical Instruments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation.

Electrochemistry,

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

Heat and Power. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology

and Lubrication. Engine Fuels.

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Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications, Polymer Structure Organic Plastics. Dental Research.

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Metal Physics.

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Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings, Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital

Circuitry. Digital Systems. Analogue Systems. Application Engineering.

• Office of Basic Instrumentation • Office of Weights and Measures

BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials.

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Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular

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Radio Propagation Engineering, Data Reduction Instrumentation. Modulation Systems.

Navigation Systems Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Radio

Systems Application Engineering.

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Standards. Radio Broadcast Service. High Frequency Impedance Standards. Calibration

Center Microwave Physics. Microwave Circuit Standards.