NATIONAL BUREAU OF STANDARDS REPORT

5880

PROGRESS REPORT FOR PERIOD JULY 1, 1957

THROUGH DECEMBER 31, 1$57

STANDARDS FOR MECHANICALLY- REFRIGERATED ENCLOSURES

HEAT TRANSFER AND OTHER MEASUREMENTS DURING LABORATORY

AND ROAD TESTS OF REFRIGERATED SEMI-TRAILERS

by

C. W. Phillips

P, R. Achenbach

R. W. Penney

Report to

Transportation and Facilities Branch

Agricultural Marketing Service

U. S. Department of Agriculture

Washington, D. C.

and

Quartermaster Research and Engineering Command

Department of the Army

Natick, Massachusetts

ON

U. S. 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 by 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,

devices, and structures; advisory services to Government Agencies on scientific and technical

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

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. \ mayor portion of the Bureau’s work is performed

for other Government Agencies, particular!) the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the. back cover.

Reports and Publications

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

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Washington 25, D. C.

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

NBS PROJECT MBS REPORT

1003-12-4884 PROGRESS REPORT FOR PERIOD 588O

JULY 1, 1957 THROUGH DECEMBER 31, 1957

ON

STANDARDS FOR MECHANICALLY- REFRIGERATED

ENCLOSURES

HEAT TRANSFER AND OTHER MEASUREMENTS

DURING LABORATORY AND ROAD TESTS OF

REFRIGERATED SEMI-TRAILERS

by

C. W. Phillips and. P. R. Achenbach

Air Conditioning, Heating and Refrigeration Section

Building Technology Division, National Bureau of Standards

and

R 0 W. Penney

Transportation and Facilities Branch

Agricultural Marketing Service

U. S. Department of Agriculture

to

Transportation and Facilities Branch

Agricultural Marketing Service

U. S. Department of Agriculture

Washington, D. C.

and

Quartermaster Research and Engineering Command

Department of the Army

Natick, Massachusetts

IMPORTANT NOTICE

NATIONAL BUREAU OF STAN

Mended for use within the Go

to additional evaluation and revi listing of this Report, either in 1 the Office of the Director, Natlo however, by the Government ag

to reproduce additional copies l

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9. 201 5.

gross accounting documents

ally published It is subjected

production, or open-literature

in is obtained in writing from

uch permission is not needed,

•epared if that agency wishes

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Introduction

Culminating more than two years of planning and preliminary investigation of a satisfactory method for rating the

heat transfer of refrigerated semi-trailers * several industry

organizations and government agencies jointly sponsored a

study by the Air Conditioning, Heating and Refrigeration

Section of the National Bureau of Standards, The objective

of this study is the development of a standard rating method

for determining the heat transfer of these vehicles, by means

of laboratory tests, so that the performance under road conditions can be predicted.

The Truck-Trailer Manufacturers Association, acting as

coordinator for the industry sponsors, entered into a contract

with the Transportation and Facilities Branch, Agricultural

Marketing Service, U, S. Department of Agriculture, which, in

turn, sponsored the necessary research and development work by

the Air Conditioning, Heating, and Refrigeration Section at the

National Bureau of Standards, The initial phase of the study

was scheduled for completion by June 30, 1958., and commenced

about the first of July, 1957*

Among the industry sponsors were:

Truck-Trailer Manufacturers Association

Truck Body and Equipment Association

American Trucking Associations, Inc,, Irregular Route

Common Carrier Conference

American Trucking Associations, Inc., Equipment

Committee

Individual trailer manufacturers

Individual refrigerating unit manufacturers

Individual component manufacturers.

Cooperating Government Agencies:

U, S. Department of Agriculture, Transportation and

Facilities Branch, Agricultural Marketing Service

U. S. Army, Quartermaster Research and Engineering

Command
* .V
- 2-

To advise the NBS in the conduct of the study a Steering

Committee was formed of the following members:

Paul R. Achenbach

National Bureau of Standards

H. Co Brown,, Jr»

American Society of Refrigerating Engineers

Technical Committee 1 0 7

J 0 Co Cahill

Bureau of Yards and Docks

R 0 Jo Campbell

Quartermaster Research & Engineering Command

T. Ho Christensen

Transportation Research & Engineering Command

Harry Eyler

Truck-Trailer Manufacturers Association

C. P„ Hoffman., Jr 0 American Trucking Associations., Inc 0

J. Bo Hulse

Truck-Trailer Manufacturers Association

H. D„ Johnson

U. So Department of Agriculture

Co Wo Phillips

National Bureau of Standards

F 0 J. Reed

Air Conditioning and Refrigeration Institute

Ho G. Strong

American Society of Refrigerating Engineers

Technical Committee 4.3
,
-3-

J. Co Winter

U. So Department of Agriculture

Wo H 0 Redit, Consultant to Steering Committee

Uo S„ Department of Agriculture

Paul R. Achenbach, Chief., Air Conditioning, Heating., and

Refrigeration Section., NBS was named Chairman, and C. W. Phillips,

of the same Section., was named Secretary. Two meetings of the

Steering Committee have been held during the period covered by

this report. Both were held at NBS, the first on July 26 and

the second on September 30^ 1957

•

The principal objectives of this study, as outlined in

the proposal originally presented to the Truck-Trailer Manufacturers Association, were fourfolds

1„ Complete the development of the comparison type

heat sink method for rating the heat transfer of refrigerated

trucks and trailers.

Notes The National Bureau of Standards had

previously designed and constructed a prototype ’’metering or comparison heat sink

apparatus” for measuring heat transmission of

trailers under normal direction of temperature

gradient and had shown that the reverse heat

flow method is not adequate when the outer

skin of a trailer permits air and moisture

infiltration,

2o Compare cooling load of several trailers in the

laboratory using the metering heat sink method and on the road

using a modified metering heat sink method.

3. Correlate and revise laboratory method in light

of field results to adequately account for wind factor and

solar radiation.
.

.
- 4~

4, Provide information on air leakage and moisture

transfer processes in refrigerated trailers looking toward

improved insulating and vapor-sealing methods.

The principal ta^ks necessary to attain these objectives

were outlined as follows:

I. Construct prototype metering heat sink to be

used as a pattern for industry standard apparatus including

instrumentation,

II. Prepare sketches, drawings, diagrams and operating instructions for building standard apparatus,

III. Prepare instruction manual on instrumenting

specimen trailers and making performance tests.

IV. Prepare a test facility to permit testing of

35-ft- trailers or trucks in ambient conditions of 100F dry

bulb and 50 percent relative humidity.

V. Develop a method for simulating the road infiltration of air and moisture in a laboratory test,

VI. Measure cooling load of three trailers in

laboratory using metering heat sink method under standard

ambient and interior conditions,

VII, Modify a trailer refrigerating unit for measuring

cooling load of trailers during road tests.

VIII. Measure cooling load of same three trailers

under summer conditions on the road at road speeds of 30 mph

and 50 mph.

IX. Correlate road test data and laboratory cooling

load data.

X. Modify laboratory tests to adequately simulate

solar load and air infiltration existing under road conditions.
•

-

**

'
-5-

The subsequent sections of this report cover the various

major aspects of the tasks carried out during the reporting

period^ and are further identified as follows:

1. Preparation of laboratory.

2. Preparation of equipment for road tests.

3« Description of trailers tested.

4. Laboratory test procedures and test results.

5. Road test procedures and test results.

6. Comparison of laboratory and road test results.

7. Air infiltration.

8. Discussion and conclusions.

1 . Preparation of Laboratory

A suitable structure in which to conduct laboratory tests

of 35-foot refrigerated semi-trailers was assigned to the project by the National Bureau of Standards. The test structure

on the Bureau grounds in Washington; D„ C„; is a lean-to type

addition to a permanent concrete block building and its exterior

surfaces are composed of corrugated metal sheathing. See Fig. l s which shows; beside the building; some of the vehicles involved

in the reported tests. The floor of the structure is concrete.

To insulate the roof three-inch batts of fibrous insulation

were placed between the rafters and held in place with corrugated aluminum sheets. After the roof insulation was installed;

the inside height of the test space was approximately 15 feet

on the low side of the sloping roof and 17 feet on the high

side. Fig. 2 shows the interior of the structure during modification. The outer walls of the chamber consist of corrugated

metal sheathing attached to each side of two-inch by six-inch

wood studding; and the space between is filled with blown rock

wool insulation. A sliding; overhead type door 12 feet wide

and 14 feet high was installed at one end of the test room to

provide a means for moving semi-trailers into the space. At

the opposite end of the chamber an adjoining room approximately 11 feet square with a window and an outside door; was

utilized as a control room and was equipped with suitable controls and instruments for conducting the laboratory tests.

Fig. 3 shows the instrument panel. A door with a glass panel
*

.
Figure



Figure

- 6 TM

in the upper half connecting the two spaces was used to admit

personnel into the test chamber during tests and was also used

for observation,. Near the north end of the outside wall of

the test space^ a pair of sliding doors approximately 12 feet

wide and 8 feet high provided an opening for moving smaller

equipment into the test room.

Additional electrical circuits were installed to provide

the required electric power for lights,, instruments, refrigerating equipment, etc., and steam lines were extended to serve

two thermostatically-controlled space heaters for heating the

ambient air, and both manual and automatic valves to introduce

live steam into the area for humidification,

A prototype comparison heat sink apparatus consisting of

sectionalized, semi-portable, refrigerating equipment which

had been assembled for, and used in conjunction with, previous

tests of smaller refrigerated semi-trailers in another test

laboratory at NBS was installed outside the test chamber in a

space approximately 23 feet, 8 inches long by 4 feet, 8 inches

wide. The outer side of this space was enclosed with wooden

framing and corrugated metal sheathing. The ends were left

open and overhead protection was provided by the overhang of

the test chamber roof.

This refrigeration equipment consisted of three electrically driven, two-speed compressors with water cooled condensers,

two brine chillers, a brine pump, and a metered heat comparator.

Fig. 4 shows the primary refrigerant and brine circuits of this

equipment in schematic form. An enclosed self-contained cooling

tower was installed to cool and recirculate the condenser water.

Refrigerants 12 and 22 were used as primary refrigerants, and

methylene chloride was used as the brine or secondary

refrigerant

.

For the laboratory tests a refrigeration coil with an

integral blower (see Fig. 5) was placed in the trailer with

the blower at approximately the same height above the floor

as the blower of typical trailer-refrigerating units. A

shielded electric space heater with built-in fan was placed on

the housing of the cooling coil to assist in maintaining the

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WATER

Figure

Figure

- 7

-

desired test temperature within the semi-trailer. The semitrailer was backed into the test chamber and placed on three

calibrated heavy-duty platform scales to determine the gain of

weight caused by condensation of infiltrated moisture during

the tests. Fig. 6 shows one of the three scales in place.

Copper-constantan thermocouples were installed inside and

outside of each trailer as needed for control and measurement

of temperatures. The inside temperature was determined by

taking the average of twelve thermocouples positioned as follows: one in each corner of the front and rear of the trailer

and one in each corner of a section midlength of the trailer.

All of these thermocouples were in air about six inches from

each adjacent surface. The test room temperature was determined by taking the average of eight thermocouples, one at

each exterior corner of the trailer,, installed in air not less

than six inches from any adjacent surface. Numerous other

thermocouples were installed for various test observations.

The methylene chloride brine lines were heavily insulated

between the metered heat comparator and the refrigerating coil

inside the trailer. The brine lines, the electric circuits to

the fans and heaters, and the thermocouple extensions entered

the trailer through a specially-designed, insulated wooden plug

installed in the refrigeration unit opening in the front of the

trailer.

Controls and equipment were set and adjusted to maintain

the standard test conditions of OF temperature in the trailer,

100F ambient air temperature, and 50 percent relative ambient

humidity. Six pedestal-type, 30-inch electric fans were used

to circulate the ambient air around the trailer during the

tests to promote uniformity of temperature and humidity. All

temperature readings throughout the tests were made by means

of thermocouples and either galvanometer or electronic potentiometers .

2 o Preparation of Equipment for Road Tests

For road tests of the refrigerated semi-trailers, a

tractor was provided, at a nominal rental, by the White

Motor Company of Cleveland, Ohio. This vehicle was a Model

-8 -

3024-PLT, tilt-cab over engine type of tractor and had a 127

l/2-inch wheel base,. Fig. 7 shows the tractor before any

modifications were made.

To equip the tractor for the road tests., it was necessary

to mount various instruments within the cab and to install a

gasoline-engine-driven generator and a two-piece, two-stage

gasoline-engine-driven refrigerating unit immediately behind

the cab.

To provide space for mounting the necessary test equipment the fifth wheel assembly on the tractor was moved backwards

on the frame to a position directly over the single rear axle,

and the twin 50 -gaH°n cylindrical gasoline tank assembly, with

platform, was moved approximately 40 inches toward the rear of

the frame. The air brake hose column was also moved back. To

support the gasoline-engine-driven generator and the two-stage

gasoline-engine-driven refrigeration units, supports were

fabricated and attached to the tractor frame in the space where

the gasoline tanks and platform had been. These supports consisted of two 80-inch lengths of 4-in. x 1 1/2-in. steel

channel sections each welded edgewise to a pair of 3-in. x 10-

in. x 3/8-in. steel bearing plates, so spaced as to rest upon

the upper part of the channels comprising the sides of the

tractor frame. To prevent side motion, two 11-inch lengths of

2-in. x 2-in. steel angle sections were welded to the outer

edge of each bearing plate and to the channels so that they

would extend downward outside the frame of the tractor. These

supports were placed laterally across the tractor frame and

spaced on 30-inch centers. They were secured to the tractor

frame by means of bolts through the angle irons and through

the frame. Existing bolt holes in the frame were utilized

wherever possible to prevent weakening the tractor frame by

drilling unnecessary holes in it. No holes were drilled in

other than the tractor frame web. Adequate bracing was provided for the overhanging portion of the channels by welding

diagonal lengths of angle iron to the channel and to the

angle irons attached to the outside of the tractor frame. Two

steel tool boxes were mounted, one on each side, under the

overhanging frame supports.

Figure

-9-

A gasoline engine generator capable of continuous loading

at 2„3KVA was placed on the left side of this supporting frame,

and the two sections of the gc£pline-engine -driven refrigerating unit were placed one above the other on the right side.

Each of the three auxiliary gasoline engines driving these

units received its gasoline supply from the tractor tanks.

These units supplied the electric power and the primary refrigeration used during the road tests.

Pig. 8 is a view of the left side of the tractor showing

the engine generator, tool box and gas tank, and Fig. 9 shows

the two-piece refrigerating unit, tool box, and gas tank on the

right side. Note the grouping of all of the cables, thermocouples, and flexible refrigerant lines in a single flexible

connection between the tractor and trailer. Air brake hoses

and trailer lighting cables were not included in this grouping

but were connected as in normal service.

m The refrigerating unit was specially designed for this

application by Tru Kooler, Inc., of Oelwein, Iowa, and consisted

of a refrigerant-12 high stage and refrigerant-22 low stage

section. Each stage was powered by a Wisconsin Model VH4D aircooled engine. This unit was used to cool a secondary brine

by means of a chiller which was mounted inside of each trailer

under test. Pig. 10 shows schematically the arrangement of

both the primary and secondary refrigerant circuits of the

mobile equipment. Methylene chloride was used as the secondary

refrigerant or brine. Pig. 11 shows the physical location of

the refrigerating components as installed for the road tests.

To facilitate the installation of the brine chiller in

each trailer for the road tests, it was placed inside a

reinforced 3/4-inch plywood box 12 feet long, 28 inches wide

and 24 inches high. It was securely positioned inside the

box together with the necessary brine lines, primary refrigerant

liquid lines, a heat exchanger, pressure equalizing tube,

thermal expansion valve sensing bulbs and capillaries. The

brine lines and the primary refrigerant liquid and suction lines

were extended through the front end of the box and provisions

made for the necessary external connections, A separate small

Figure

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compartment was provided in the upper left-hand corner of the

rear of the box to house the two thermal expansion valves.

This valve compartment had an external plywood cover so that

adjustments could be made to the valves without taking the lid

off the entire box. After the necessary equipment was placed

in the box,, the space around the components was carefully

packed with glass wool insulation. After the insulation was

installed,, the lid was placed on the box and secured.

A framework of 2-in. x 6-in. lumber was attached to the

front portion of the chiller box and extended above it to

support the air cooling coil for the trailer. This coil was

mounted at such a height that the discharge from its built-in

blower would be at about the same height above the floor as

that from a typical trailer refrigerating unit. This cooling

coil was rated at 17,500 Btu/hr at 0°F entering air, -10°F

leaving air, 1750 cfm at l/2-in. static pressure, -23 °F

entering brine (methylene chloride), -13 °F leaving brine. It

had a six-row, two-circuit brine coil with a pressure drop of

1.4 psi at 5.22 gpm. The galvanized casing of the coil

measured 44 1/2 in. high, 52 1/4 in. long, and 28 5/8 in. deep.

The blower motor was 1/3 hp 115/230 V, 1 phase, 60 cycle. An

electric heater of about 600 watts capacity with a built-in

fan was installed to assist in maintaining the desired trailer

temperature. Fig. 12 shows the chiller, coil and heater

assembly being placed in a trailer, and Fig. 13 is a view of

the assembly after installation. It can be seen from these

two figures that the brine chiller, the air cooling coil and

the control heaters were assembled and installed in each test

trailer as a unit. Included in the brine piping between the

chiller and coil was the sensing element of an electronic

flowmeter for measuring the flow,, of methylene chloride . brine

during the tests. This sensing element was connected to

instruments mounted in the tractor cab. After the chiller

assembly was installed in the trailer, brine line connections

to and from the brine pump were made up, and the primary

refrigerant liquid and suction lines were connected.
'
Figure

12

Figure

13

11 -

An aluminum faced, insulated wooden plug approximately

five inches thick was placed in the refrigerating unit opening

provided in the front of each test trailer. This plug was

reinforced near its outer edges with angle iron and was provided with a steel shelf on its exterior surface on which was

mounted an electrically-driven brine pump connected to brine

lines extending a short distance beyond the inner face of the

plug. A ten-gallon brine storage tank, for make-up and

expansion, was also attached to the front face of this plug,

above the pump and motor and was connected into the brine

circuit at the pump inlet. The circulating pump, the storage

tank, and the brine lines were insulated. On the right-hand

edge of the plug a primary refrigerant heat exchanger was

mounted to which the flexible primary refrigerant liquid and

suction lines to the tractor-mounted refrigerating unit were

connected. Provisions were also made in the plug for introducing thermocouple extensions, electric power and control

circuits, and tubes for measuring air pressure differences

into the inside of the trailer. Pig. 14 shows the arrangement

of equipment on the plug.

Instruments mounted in the tractor cab can be seen in

Fig. 15, and these instruments are identified in the sketch of

Pig, 16. In addition to the normal instruments provided for

operation of the tractor, the following instruments were provided in the cab:

1. Electronic indicating potentiometer, for readings

of temperature, temperature difference, relative humidity,

relative incident radiation, and brine flow rate.

2. Electronic recording potentiometer, for permanent

record of each of the variables listed in item 1.

3. Recording speedometer, for permanent record of

rate of speed and time of operation.

4. Clock for time,

5. Electric clock frequency meter, for observing

performance of engine generator.

Figure

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POTENTIOMETER

SELECTER

SWITCHES

12

6. Ammeter and voltmeter, for observing electric

generator output

.

7. Watthour meter, for integration of power input

to trailer.

8. Brine flow totalizer and frequency converter,

for integrated flow and flow rate observation.

9. Electric hygrometer, for indication of ambient

relative humidity.

10. Pressure gages, for measurement of impact and

static air pressures.

11. Various switches, constant voltage transformer,

hot and cold reference baths, etc.

Pyrheliometer (for measurement of relative incident

radiation), Pitot tube (for measurement of impact and static

air pressures during motion), and protective tube for the

electric hygrometer elements were mounted on the cab roof, and

may be seen in Fig. 17°

Figs. 18 and 19 show, respectively, right- and left-hand

views of the tractor-trailer combination connected for

operation.

3 . Description of the Test Vehicles

Six trailers have been involved in the tests referenced

in this report. Of these, three were commercial trailers and

three were military vehicles. The three commercial vehicles,

each described in more detail later in this section, were:

1. Fruehauf, new, 33 feet 6 inches long, loaned

by the Fruehauf Trailer Company.

2. Trailmobile, new, 35 feet long, loaned by

Trailmobile, Inc.

3. Fruehauf, used, 35 feet long, loaned by the

Emery Transportation Company.

Figure

17

!

Figure

l8

Figure

19

-13

The three military vehicles were 7 l/2-ton, 21-foot, singleaxle trailers manufactured to U. S„ Army specifications, and

were studied for heat transfer and moisture transfer characteristics by the National Bureau of Standards for the Quartermaster Research and Engineering Command prior to the tests of

the commercial vehicles. Although the three vehicles differed

slightly. Pigs. 20 , 21 , and 22 show, respectively, the front,

rear and interior of one which is typical. The comparison

heat sink apparatus later modified for the laboratory tests of

the three commercial vehicles was first developed by the National

Bureau of Standards in order to study the effect of accumulation

of ice or water in the insulation of one of these military

trailers . One of these vehicles was used in the study of air

pressures on the surfaces and in the insulation and cargo spaces

during operation on the road at fifty miles per hour. They

were also used for infiltration measurements prior to the tests

of the commercial vehicles. None was used for road tests of

heat transfer. The military vehicles had aluminum exterior

and interior surfaces and had various forms of extruded or

formed aluminum floors. Glass fiber insulation was used in

the walls and roof, and expanded polystyrene was used in the

floors, with six-inch insulation thickness in most cases.

The empty weight was about 8,000 pounds.

The three commercial insulated semi-trailers were loaned

for these tests by organizations interested in the development

of a standard rating method and are described as follows:

I. Pruehauf - New, from Fruehauf Trailer Company

The Fruehauf refrigerated semi-trailer furnished for

laboratory and road tests was new and was loaned for the tests

by the Pruehauf Trailer Company. The Model Number was

SSWR533-6 and the Serial Number was FW108753* It was a

single-axle trailer as shown in Pig. 23 . The overall length

was 34 feet, 1 inch, the width 8 feet, and the height 12 feet.

The body length of the trailer was 33 feet, 4 inches, the

width 7 feet, 10 inches and the height 8 feet, all exterior

dimensions. The inside length was 32 feet, 4 inches, the width

6 feet, 6 inches and the height 6 feet, 7 inches. The empty

weight was 9*722 pounds.
I ' I
are

Figure 21



gure

-14-

The exterior skin of the trailer was stainless steel of

spot-welded construction and the side walls were corrugated in

the form of vertical channels with trapezoidal cross-section.

These channels were 1 inch wide across the outer face., 1 3/4

inches across the base and the flat areas between the bases

were 4 1/2 inches wide. The sides and ceiling were insulated

with 6 inches of glass fiber insulation, and the floor with

5 inches of expanded polystyrene. The interior lining was

plywood, and the floor was extruded, interlocking channel

type aluminum alloy with integral flashing. There did not

appear to be any vapor barrier in the walls or ceiling other

than the exterior skin. There was a curbside door 4 feet,

4 inches wide at a distance 12 feet, 9 inches from the front

of the body 0 There were no meat rails or bows in this trailer.

II. Trailmobile - New, from Trailmobile, Inc

The Trailmobile refrigerated semi-trailer was furnished

for laboratory and road tests by Trailmobile, Inc., and was

apparently in new condition. The Model Number was L-8225<>

and the Serial Number 6=01772- (35 )

Fig. 24 is a view of this trailer.

The overall length was 35 feet, 5 inches, the width 8

feet, and the height approximately 12 feet. The outside body

length was approximately 35 feet, the width 7 feet, 11 inches,

and the height 8 feet, 5 inches. The inside body length was

34 feet, the width 6 feet, 11 inches, and the height 7 feet,

1 inch. The empty weight was 12,563 pounds.

The exterior skin was of riveted aluminum sheets and the

interior lining was treated plywood. The walls and roof were

insulated with six inches of glass fiber insulation; the

floor with six inches of expanded polystyrene. There was no

evidence of a vapor barrier other than the exterior skin, but

the specifications indicated that all interior joints of the

shell were to be sprayed with an undercoat. There were approximately 72 holes, l/4-in. in diameter, in the upper lip of the

side channels at the base of the walls. These holes communicated with the insulation space in the walls. The floor was
"

.
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15-

extruded, interlocking,, channel-type, aluminum alloy with

integral flashing. There were cross rails,, or bows, for meat

rails but no meat rails were provided. This was a two-axle

trailer with the axle assembly designed for shifting forward

or backward to adjust for loading and road conditions,

III. Fruehauf - Used, from Emery Transportation Company

The semi-trailer loaned to the National Bureau of Standards.,

for laboratory and road tests., by the Emery Transportation

Company was built by the Fruehauf Trailer Company to Emery

Transportation Company specifications, and it had been in commercial use before being delivered for tests. The Model

Number was WSRRR5535SR, and the Serial Number 28314. The hub

odometer mileage was about 60,000 miles. Fig. 25 is a view of

this trailer.

This was a two-axle semi-trailer and the overall length

was 35 feet, the width 8 feet, and the height approximately

12 feet. The outer skin was of riveted aluminum construction.

The inside length was approximately 34 feet, the width 7 feet,

3 inches and the height above the drainage level of the floor

6 feet, 11 inches. The empty weight was 13,510 pounds.

According to specifications furnished by the Emery Transportation Company the floor of the trailer consisted of a 3/8-

inch marine plywood subfloor, coated on both sides with a

water emulsion type undercoating. Over the subfloor wpre 4

inches of expanded polystyrene with all joints sealed with

the same type of emulsion, and over the insulation there was

an aluminum-alloy interlocking channel type flooring. This

flooring was supported on wood fillers over each frame cross

member. The depth of the channel grooves in the flooring was

1 1/8 inches. The side walls contained a four-inch thickness

of 3/4-pound glass fiber insulation with a paper vapor

barrier in the center of the insulation thickness except the

lower ten inches of the walls, which were insulated with

expanded polystyrene. The inner ; wall surface of the trailer

consisted of 5/16-inch plasticized plywood except for the

aluminum flashing at the floor. The roof was insulated with

six inches of 3/4-pound glass fiber insulation with a paper

Figure

25

- 16 -

vapor barrier in the center of the Insulation thickness., and

the ceiling surface was 5/l6-inch plasticized plywood. The

construction of the rear door was the same as the side walls

except that the flashing on the lower portion of the door

was stainless steel. The trailer was equipped with seven

2 -inch pipe meat rails, each supported by 13 -hangers attached

to wooden cross beams. The three center rails were shorter

than the four outside rails leaving a 45-inch space at the

front end for the refrigeration unit.

4 0 Laboratory Test Procedures and Results

The laboratory tests of each trailer were made to determine the heat transfer rate and simultaneous gain in weight

due to accumulation of water or ice under specified laboratory

conditions for comparison with the heat transfer rates under

actual road operation.

Conditions of 100F dry bulb temperature and 50 percent

relative humidity were chosen for the ambient space around

the trailer with OF temperature in the cargo space of the

trailer as typifying moderately severe operating conditions.

The weight gain during the test was measured with three platform scales on which the trailer was supported. The heat

transfer rate was determined by use of the prototype heat

sink apparatus. Fig. 26 is a schematic diagram of the heat

sink apparatus. See Fig. 4 for schematic arrangement of the

refrigerant circuits.

The principle of operation of the heat sink is as

follows

The temperature rise of the brine in the coil inside the

trailer under test and the temperature rise across a heater

in series with this coil are measured., (See Fig. 25.) The

heat input to the heater is measured. Since the brine flows

through the coil in the trailer and the heater chamber at the

same rate, the ratio of the temperature rise of the brine in

the coil to the temperature rise of the brine in the heater is

equal to the ratio of the amount of heat absorbed by the brine
'
COMPARISON TYPE HEAT SINK CALORIMETER

FOR REFRIGERATED TRAILERS

DEVELOPED BY NATIONAL BUREAU OF STANDARDS

1

SYMBOLS: AT | TEMPERATURE DIFFERENCE OF BRINE ACROSS TRAILER

Aj 2

" " " " " BRINE HEATER

At 3

"

" BETWEEN AIR IN TRAILER

AND TEST ROOM

tcl,tc2, etc. TEMPERATURE MEASUREMENT POINTS

©,© ELECTRIC HEATERS RATED IN BTU/HOUR

FIGURE 26

COMPARISON TYPE HEAT SINK CALORIMETER

FOR REFRIGERATED TRAILERS

DEVELOPED BY NATIONAL BUREAU OF STANDARDS

SYMBOLS: AT | TEMPERATURE DIFFERENCE OF BRINE ACROSS TRAILER

A T 2

" " " " " BRINE HEATER

At 3

"

" BETWEEN AIR IN TRAILER

AND TEST ROOM

tc I, tc2, etc. TEMPERATURE MEASUREMENT POINTS

©,© ELECTRIC HEATERS RATED IN BTU/HOUR

FIGURE 26

- 17 -

in the coil to the amount of heat absorbed by the brine in

the heater. The equality of these two ratios would be as

follows

AT]_ _ Heat Gain of Trailer + Hi

~ H2

' ~~

where Hp is the sum of all of the items of auxiliary

heat added to the trailer interior for

purpose of test* such as fans, blower(s),

controlling heat, etc.

H

2 = heat release in the heater in series with

the brine coil.

ATp = temperature rise of the brine in the coil

inside the trailer.

AT2 = temperature rise of the brine from jnlet

to outlet of the series heater.

The heat gain of the trailer would then be:

Heat Gain (Btu/hr)

=

^ x H2 ^

- Hp

To determine the heat gain of the trailer per unit temperature

difference between interior and exterior the total heat gain

in Btu/hr can be divided by this temperature difference, AT/.

Heat Gain, Btu/hr(°P)

AT3

Hi

The slight change in specific heat of the brine between

the coil and the heater, and the slight heat leakage of the

insulated brine heater, were found to be negligible for these

determinations 0

-18-

To measure the net heat leakage and weight gain of each

test trailer, the following variables had to be controlled

and/or measured:

1. Temperature (lOOF) and relative humidity (50/)

of the ambient space around the trailer,,

2. Temperature (OP) inside the trailer.

3. Temperatures at various points in the brine circuit :

a. inlet to coil in trailer

b. outlet from coil in trailer

c. difference across coil in trailer

d. inlet to series heater

e. outlet from series heater

f. difference across series heater.

4. Electric power introduced into trailer during test

for fans, etc.

5. Weight of the trailer.

6. Time (relative).

All temperatures were measured with calibrated copperconstantan thermocouples. Temperature differences in the brine

circuit were measured with five- junction thermopiles in wells

to increase the accuracy of these readings. The ambient temperature was taken as the average of the air temperatures six

inches from each of the eight corners of the test vehicle.

The interior temperature was taken as the average of the air

temperatures at 12 points, one each six inches from the adjacent

surfaces in each of the eight interior corners, and four at

similar positions at the center (midlength) of the trailer. In

addition, other temperature measurements of exterior surfaces

etc., were made for comparison with conditions observed during

road tests.

All electric power used to operate fans, lights, and the

heater which was used to obtain final temperature control in

the trailer was measured by means of watthour meters.

-19-

The total weight gain of the trailer was determined by

periodic observation of the three platform scales on which

the test vehicle was supported,, The portion of the weight

gain due to accumulation of frost or ice on the cooling coil

was determined by weighing the defrost water at the end of

each test. The scales used to weigh the trailer were read

to the nearest one-half pound and had previously been calibrated through the range of weights involved. Fig. 27 shows

a trailer in position for test.

All readings were taken at regular time intervals for a

sufficient period after steady state operation was attained

to establish weight gain and heat transfer rates.

The measured cooling loads of three commercial trailers

under laboratory conditions are shown in Fig. 28 (also in

Table II ) and the corresponding rates of weight gain are

shown in Table I.

Table I

Weight Gain of Three Commercial Trailers

under Laboratory Conditions

(Ambient 100F, ^ 0% R. H, Interior OF)

Trailer

A

B

C

Avg. Weight Gain Rate,

lbs per hour

0,32

0,98

0.54

In Fig. 28 and in Table II trailers A, B, and C are

shown to have total cooling loads of 10^300^ 9520_, and 7500

Btu/hour, respectively. The portion of the total heat gain

due to air leakage effects was computed to be 7^0^ 2270^ and

1250 Btu/hour^ respectively. For this computat ion, it was

assumed that the weight gain was due entirely to accumulation

of water or ice and that the water vapor entered the insulation

and cargo space as a result of movement of the moist ambient

air through the trailer exterior. It was further assumed
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- 21 -

It is probable that the air leaving the trailer was not

cooled to OF and possible that it was not saturated. In either

event., the amount of water vapor surrendered per pound of air

would have been less than was assumed and the corresponding

air leakage would have been greater. For example., if the

leakage air were assumed to leave the trailer saturated at

32°F instead of 0°F, the amount of moisture surrendered would

be 119.5 grains per lb of air instead of 140.5 grains. In

this case, the computed air leakage would have been 273^. 837,

and 46 1 cu ft of ambient air per hour and the sensible heat

gain due to air leakage would have been 307 * 939^ and 517

Btu/hr for trailers A, B, and C respectively; the latent heat

gain would remain essentially the same for either case since

it is directly related to the average gain in weight of the

trailer. It is probable that the leakage air actually left

the trailer at a dew point somewhere between OF and 32F since

most of the moisture was deposited as frost in the walls where

the temperatures were above OF.

During the laboratory tests of each trailer, simultaneous

measurements of air leakage into the cargo space were made

using helium as a tracer gas. These results are presented in

Section 7 on Ain Infiltration.

5 . Road Test Procedures and Results

The road tests of the three commercial trailers were

made primarily to determine the heat transfer rates of each

trailer while being operated at a road speed of 50 miles per

hour for comparison with the laboratory results when the

vehicles were stationary. The trailers were empty except for

test equipment. As outlined in Section 2, provisions were

made for observing a number of variables, such as skin temperatures, static and impact air pressures on various parts

of the trailer, relative solar radiation incidence, temperature and relative humidity of the ambient air, road speeds,

brine flow, etc. It will be noted in comparing Fig. 10

(refrigerant circuits for the mobile equipment) with Fig. 4

(refrigerant circuits for the laboratory tests) that two

variations of the heat sink method were used for the road and

laboratory tests. The refrigerating effect at the coil in
.

\
22 -

the trailer for the road tests was determined by means of a

flowmeter in the brine circuit and the temperature difference

of the brine entering and leaving the coil,, whereas in the

laboratory tests the heat gain in the trailer was determined

by comparing the temperature rises produced in the same

brine circuit across a measured electrical source and the

cooling coil in the trailer. This modification was employed

to reduce the electric power and refrigerating capacity

demand for the road tests. The heat gain rate per unit temperature difference across the walls for the trailer during

each test run was computed using the following equations

Heat Gain Rate,, Btu/hour (°F) = ( M x C P x ^1 ) - H-|

at2

where M = flow of methylene chloride brine,,

pounds per hour

Cp = specific heat of brine., Btu per

pound (°F)

ATjl = temperature rise of brine through coil

in trailer., °F

AT2 = average temperature difference between

air in trailer and ambient air, °F

Hj = heat equivalent of electric power in

trailer for fans and heater., Btu per

hour

All road tests were made on the Ohio Turnpike during

September and October, 1957- This location was selected

because it was the nearest to Washington, D. C„, which most

nearly suited the conditions necessary for continuous operation at 50 miles per hour. The tests were conducted from

Milan, Ohio, which is located at about the center of the

Turnpike. Fig. 29 shows one of the test trailers at the

Milan entrance to the Turnpike

„

Figure

-23=

A typical test run was of approximately 5 hours and 30

minutes duration for each driver and observer team including

turn-around and refueling,, The distance traveled was 119

miles each way from Milan to the western terminus of the

Turnpike and return, a total of 238 miles, and the 50 mile

per hour road speed was maintained except for the time

required to turn around at the far end of the run and to

refuel at Milan „ The time interval between runs was determined by the time required to change drivers, refuel and

check oil, brakes, lights, etc, for the tractor-trailer and

the three gas engines used to drive the generator and

refrigerating unit. A retail gasoline station, used as a

change point was located less than 1/4 mile from the Milan

entrance to the Turnpike and, usually, the time off the

highway for refueling did not exceed 20 minutes. On the

basis of 10 minutes for the turn-around and a total of 30

minutes for leaving and reentering the highway, and refueling,

about 87 percent of the elapsed time for each test run of

238 miles represented operation at 50 miles per hour. For

a majority of the tests, the drivers were able to control

the speed to within ±2 miles per hour during the 50-mile-perhour portions of the run. As far as possible the test runs

were continuous and deviated from this schedule only when

repairs or maintenance were required between runs.

About 72 gallons of fuel (regular grade gasoline) were

required for a typical test run for the tractor and the three

other engines driving the generator and refrigerating unit,

an average of 13 .1 gallons per hour for the total run.

All test runs were made with the trailer interior temperature controlled as near as possible to OF, regardless of

variation in ambient temperature, relative humidity and

incident radiation.

During the

measurements of

complete set of

tract or- trailer

end of the road

involved in the

15, 000.

test runs the observer read and recorded

all variables at 30-minute intervals, and a

readings required about 20 minutes. The

combination was weighed at the beginning and

tests of each trailer. The total mileage

road tests of the three vehicles was about
) -

J

'
-24-

An attempt was made to measure the air infiltration of

one of the test trailers using a modified form of the helium

tracer-gas technique, but the results were not considered

dependable because of possible malfunctioning of the sensing

instrument

In general, the heat transfer values reported are the

average for not less than 24 hours of continuous test

operation for a period when the initial and final ambient

temperatures and incident solar radiation were not far different. It must be understood that ambient temperature and

humidity and solar radiation varied from one test run to

another, so that "steady-state” operation was not attained,

even though the interior temperature was held essentially

constant

Pig. 30 shows the heat gain rate of the three test

vehicles. Because the ambient conditions were not identical,

the heat transfer rates, to be comparable, are expressed in

Btu per hour per degree P air temperature difference between

interior and exterior of the trailer. From Fig. 30 it can

be seen that trailers A, B and C had heat gain rates of

about 117* 113 and 88 Btu per hour (°P) respectively.

Pigs. 31 and 32 show the corresponding ambient humidities

and heat gain rates for the several test runs on trailers A

and C. In Pig. 31 the ambient absolute humidity was of the

same order of magnitude for all test runs and there was no

trend toward a rising or falling heat gain rate for the

series of runs.

In Fig. 32 the ambient absolute humidity decreased

significantly during the 19 test runs, and the measured heat

gain rate also showed a significant corresponding decrease.

Fig. 33 shows the heat gain rates of the three test

trailers extrapolated from the comparative results of the

laboratory and road tests for assumed operating conditions

of 100F ambient temperature at 50 percent relative humidity,

an interior temperature of OF, and a road speed of 50 miles

per hour.
.

.
HEAT

GAIN

RATE

OF

THREE

COMMERICAL

TRAILERS

DURING

ROAD

TESTS

*

(do) UH/nia

FIGURE

30

AMBIENT HUMIDITY VS. HEAT GAIN RATE

DURING ROAD TESTS

FIGURE 31

UIV

JO

SNIVU9

AMBIENT

HUMIDITY

VS.

HEAT

GAIN

RATE

DURING

ROAD

TESTS

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-25-

The extrapolated values were obtained by the following

steps

1 „ The heat transmission rate (exclusive of air

leakage) was assumed to be the same for road operation as

for laboratory operation at the same temperature difference,

and was so determined for the temperature difference

observed during the road tests

„

2. The remainder of the heat gain rate observed

on the road was assumed to be caused by air leakage at 50

mph road speed.

3. The air leakage rates required under the

ambient conditions observed on the road to cause the increment of heat gain in excess of heat transmission (item 2

above) were computed assuming that the leakage air left the

trailer saturated at OF. The air leakage rates thus determined for trailers A, B, and C were 66. 5^ 109. 3, and 59»1

pounds per hour, respectively, corresponding to about 970 , 1590, and 860 cu ft/hr at conditions of 100F and 50 percent

R. H.

4. The heat gain rate due to air leakage at

ambient conditions of 100F and 50 percent R. H. and 50 mph

road speed was computed on the assumption that the leakage

rate at a given road speed was independent of ambient temperature and humidity.

5. This heat gain rate due to air leakage was

added to the transmission heat gain rate determined in the

laboratory for 100 degrees temperature difference between

inside and outside the trailer to determine the total

anticipated heat gain rate under road operation at 50 mph

in ambient conditions of 100F and 50 percent R. H.

It is recognized that the transmission heat gain for a

given air temperature difference was probably greater during

the road tests because of solar radiation, heating of the

under surface of the floor, and an improved heat transfer

coefficient on all surfaces due to higher air velocity, but

computation shows that these effects are small as compared to

the increase due to air leakage.

-26-

The extrapolated heat gain rates, as shown in Fig, 32,

are 12,630, 12,280, and 8980 Btu per hour for trailers A, B

and C respectively.

All of the test vehicles gained weight during the series

of week-long test runs, the increase ranging from 130 to 230

lbs. Since operating and ambient conditions varied considerably for the three trailers and because the precision of

the weighing process is uncertain, no direct relation of rates

of moisture gain can be established from the weight values,

other than to confirm that a significant weight gain, due

primarily to accumulation of ice and/or water, does occur.

For two of the test trailers which were refrigerated below

20F for the entire period between initial and final weighing

in Washington, D. C., an average time of eight days, the coil

in the trailer was not defrosted until after the final

weighing and the amount of ice on the coil did not exceed

15 pounds in either case. The doors on these two trailers

were not opened between weighings.

Although the tests were not intended to include detailed

studies of skin temperatures, the temperature at one point

each on the exterior surface of the two sides, top and bottom,

was observed. Fig. 3^ shows observations made during one of

the return trips from the Ohio Turnpike when it was convenient

to stop just before noon (Eastern Standard Time) with a clear

sky and bright sun. Although the sky clouded over somewhat

between noon and 2:00 P. M. , certain comparisons can be made

from Fig. 3^* The right hand scale shows relative incident

radiation measured with a pyrheliometer, and is expressed in

microvolts. Bright sunlight gave a reading of nearly 3000

microvolts, no sunlight gave a reading near zero. Note the

decrease in the temperature of the top and right side surfaces

and the simultaneous increase in the temperature of the bottom

surface when travel at 50 miles per hour commenced about 10:35

and the reverse when the vehicle was stationary during the

times before and after this period. The observations during

the period when the vehicle was again in motion after 1:10

P„ M„ are not as conclusive because of intermittent clouds

and change of speed between 20 and 50 miles per hour, including the two stops involved in leaving the Ohio Turnpike and

FIGURE

34

"27

entering the Pennsylvania Turnpike between 2:30 and 2:40 P. M,

While Pig. 34 shows data from only one of the test vehicles,

it was noted that all of the vehicles showed similar

characteristics. The bottom surface temperature was from 5

to 15 degrees F warmer than the ambient air temperature

whenever the vehicle was in motion, day or nighty in rainy

or dry weather. These skin effects will be discussed further

in Section 6 . During the road tests,, the velocity pressure of the air

above and slightly ahead of the leading edge of the tractor

was measured with a Pitot tube. In addition, the differences

between the static pressure at the Pitot tube and (l) the

static pressure in the trailer interior and ( 2 ) the static

pressure in the trailer wall were measured. The velocity

pressure of standard air ( 7 OF and dry at 29.92 in„ Hg) moving

at 50 miles per hour (4400 feet per minute) is very nearly

1.21 inches of water pressure. During the test runs at 50

miles per hour, velocity pressures observed ranged from

0.6 in. W.G„ to 2„0 in. W.G. depending primarily on the

velocity and direction of the wind. At the time that the

2.0 in. W.G. velocity pressure was observed, the driver

reported that, at full throttle on level ground, he could

maintain only 49 miles per hour. On the return leg of that

same trip, the driver stated that only light throttle was

required to maintain 50 miles per hour and the observed

velocity pressure was much lower. In all cases where wind

direction or gusts did not seriously affect the performance

of the Pitot tube, observed velocity pressures at 50 miles

per hour were from 1.1 to 1.25 in. W.G.

The observed difference between the static pressure at

the Pitot tube and the static pressure in the cargo space of

the vehicle ranged from 0.2 to 0.4 in, W.G. with lower

pressure In the cargo space for all of the test vehicles at

50 miles per hour. The difference observed between the static

pressure at the Pitot tube and the static pressure at one

point in the trailer wall was essentially the same as that

for the trailer cargo space.

-28-

6 . Comparison of Laboratory and Hoad Test Results

As described In Sections 1 and 2, two variations of the

heat sink method were used to measure the total heat gain of

the test trailers during laboratory and road tests , A test

was made to determine the agreement between these different

and completely independent measuring systems,, (l) the stationary

laboratory equipment using the comparison principle, and

(2) the mobile equipment using the flowmeter principle. This

was accomplished by comparing the observed heat gain rate

measured with the laboratory equipment with the observed heat

gain rate measured with the mobile equipment with the same

trailer exposed to identical conditions^ (Ambient 100F, 50%

R. H., Interior Temp. OF) in the laboratory in each case. To

test the trailer in the laboratory with the mobile equipment

the trailer was placed in the test chamber with the front end

near the overhead sliding doors. The tractor was uncoupled

and moved forward as far as the electrical and refrigeration

lines would permit, and the overhead doors lowered as far as

possible. The open spaces below and around the door were

closed off so that the desired ambient conditions could be

maintained in the test chamber. By placing the tractor outside the test area, the heat produced by the gasoline engines

of the electric generator and of the two refrigeration machines

could be dissipated to the outside atmosphere without affecting

the conditions in the test chamber.

All test data recorded we re taken from measurements and

observations made with the road test instruments in the tractor

cab. All of the electrical current and refrigeration required

for the test was produced by the equipment mounted on the

tractor, just as had been done during the road tests. Only

the temperature and humidity conditions of the test chamber

were controlled by laboratory equipment.

The test room ambient temperature and relative humidity

were controlled at 100F and 50 percent, respectively. The

trailer interior temperature was controlled at OF.

-29

The heat transfer rate of the trailer as determined by this

test agreed with the rate determined by the laboratory test,

described in Section 4, within one percent.

This agreement, which is less than the expected testing

error, indicated that the two measuring systems, laboratory

and mobile, were suitable for comparison of laboratory and

road results.

Prom Section 4, the observed heat gain rates of three

trailers were 10,300, 9,520 and 7,500 Btu per hour at laboratory conditions of 100F ambient temperature at 50 percent

relative humidity and a trailer temperature of OF,

From Section 5 , the observed heat gain rate and average

ambient humidity for each trailer during the road tests was

as shown in Table III, and the trailer temperature was at or

near OF in each case.

Table III

Observed Heat Gain Rates of Three Commercial

Trailers in Laboratory and Road Tests

Laboratory Road

(Amb. 100F, 50^ R.H 0 (Trailer Interior

Trailer Interior OF) _ Temp, OF)

Btu/hr Btu/hr ( °F ) Avg, Conditions Btu/hr Btu/hr

A 10,300 103.0

B 9,520 95.2

C 7,500 75.0

53 *7B, 51# R.H a 67.4f,59^ R.H.

7 2 . OF, 80/ R.H.

6,260 116.7

7,610 113.0

6,360 88,3

The different conditions of ambient humidity and temperature during the road tests, as well as changing solar radiation,

etc., undoubtedly affected somewhat the absolute value of the

coefficients determined by dividing the observed heat gain

rates by the difference in temperature inside and outside of

the test vehicles. The coefficient so determined, however,

is perhaps the most suitable way to compare the overall performance of vehicles when ambient conditions vary between

-30-

tests, and from Table III It can be seen that the increases

in heat transfer coefficient during road tests for trailers A,

B and C were, respectively, 13.3 percent, 17 .6 percent and

17.7 percent as compared to the laboratory results. The

increase in heat gain of a trailer during road operation is

due primarily to greater air leakage when the vehicle is

moving, but some of the increase is the results of (l) solar

radiation and ( 2 ) heating of the under surface of the vehicle

due mainly to waste heat from the tractor engine and partly

from radiation off the road surface, and ( 3 ) an improved heat

transfer coefficient at all exterior surfaces because of

higher air velocity. Other than the effect of air leakage,

the increase in heat gain rate due to the several skin effects

will probably not account for more than five or six percent

increase for a 24-hour period with bright sunlight during the

day. Pig. 34 in Section 5* a graph of the effect of motion

on skin temperatures for one of the test vehicles, shows that

when the trailer is in motion, the under side of the vehicle

is the greatest source of increased heat transfer due to skin

effects, and, in the case of these tests, was affected by

location of the engine exhaust «rde^*~the tractor rear axle

and the waste heat from the v generator and refrigerating unit

engines. 7V*cri*)

When a trailer Is stationary in bright sunlight. Fig. 34

shows that the top and one side can be heated by solar

radiation at least 25 degrees F higher than the ambient air

temperature. The top and one side of a typical 35-foot

trailer Is about 45 percent of the total surface. The effect

of increasing the temperature of these surfaces 25 degrees is

equivalent to increasing the ambient temperature around the

entire vehicle by about 12 degrees or 12 percent at conditions

of 100F ambient temperature and OF trailer temperature. This

is offset some by the decrease in the temperature of the under

surface when the vehicle is stationary, and it must be

remembered that heavy solar loading will not exist more than

about six hours per day.

In Section 5 5 the method used to extrapolate the heat gain

rates of the three test vehicles at conditions of 100F ambient

-31-

temperature, 50 percent relative humidity, OF trailer temperature and a road speed of 50 miles per hour was outlined and

the extrapolated values are shown in Fig, 33, Fig, 35 is a

comparison of the observed laboratory and extrapolated road

heat gain rates at these conditions, and is based on the

assumption that air leaving the trailer is at OF and saturated.

From Fig. 35 it can be seen that the expected increase in

heat rates between similar laboratory and road conditions is

22. 5, 29.0 and 19.7 percent for trailers A, B and C, respectively,

7 . Air Infiltration

Air infiltration or air leakage is a major cause of

increased heat gain into refrigerated trailers, as the data

In Sections 4, 5 and 6 indicate.

In tests prior to the start of the work with the commercial trailers, one of the military trailers ( c

) accumulated

about 85 O lbs of ice in tie insulated space when refrigerated

to an interior temperature of OF for 75 days under laboratory

conditions of 110F ambient temperature and 60 percent relative

humidity. This significant accumulation led to an investigation

of the mechanism of moisture transfer into the trailer construction. Computation indicated that a negligible amount of

the total moisture could have entered the insulated space by

diffusion so it was concluded that air leakage served as the

mechanism for bringing in the moisture. Several approaches

were made to the problem using two military and three commercial

vehicles as test specimens. These investigations were comprised of the following main subdivisions?

1. Construction of an air infiltration meter using

the tracer gas principle.

2o Air leakage measurements into the cargo space of

four trailers, two military and two commercial, under st^ionary

conditions using the air infiltration meter.

3. Effect of sealing the door and refrigerating unit

plug on air leakage under stationary conditions.

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-32-

4. Computation of air leakage required to deposit

the observed quantities of ice in the insulation.

5. Study of air flow patterns inside the insulated

space

.

6. Relation of forced ventilation air flow and

pressure difference between inside and outside of a trailer.

7. Air pressure pattern on the exterior and interior

skins of a trailer moving at 50 rnph.

Based on the experience obtained in the design of an

infiltration meter for buildings, an apparatus for measuring

the air leakage of refrigerated trailers was designed and

constructed. This new instrument is practical for use in

both laboratory and road tests.

The sensing element is an aluminum cylinder 3 1/2 in.

long and 1 3/4 in. in diameter with two cavities in each of

which a thermistor is installed. One cavity is hermetically

sealed and the other one is ventilated with the ambient air

through two small holes at the top and two small holes at the

bottom of the cavity. This sensing element., or probe,, is

installed in the trailer and connected by means of a threeconductor cable to the instrument in the tractor. This instrument contains the fixed and balancing resistors which form a, Wheatstone bridge circuit with the two thermistors. It also

contains an 18-volt dry-cell battery for the bridge power

supply. The bridge current,, as well as the voltage across

the bridge, can be measured with a microammeter without altering

their values during the test. This was possible by using

high series resistances with the microammeter and a DPDT

switch to change over from voltage to current readings,

bridge is balanced with a galvanometer. Another DPDT switch

disconnects the leads of the galvanometer from the balancing

circuit and connects the leads of a recording potentiometer or

a recorder in their place to determine the unbalance of the

bridge during the test. A shunt and a series resistor are

provided to compensate for the amplification of the different

electronic meters.

- 33 -

When the thermistor bridge circuit is balanced,, helium,

in the amount of approximately one half of one percent of the

volume of the trailer, is introduced and mixed witii the air in

the trailer by the cooling unit blower. The thermal conductivity of helium is about six times as high as that of air

and the helium-contaminated air causes a slight cooling of the

thermistor in the ventilated cavity of the probe. This cooling

in turn increases the resistance of this thermistor, which can

he observed as an unbalance of the Wheatstone bridge circuit.

The unbalance was found to be linearly proportional to the

concentration of helium tracer gas. As air leaks into the

trailer and an equal amount leaks out, the helium concentration

will decay. By basic mathematic computation, it can be shown

that the following relation exists between the concentration of

the tracer gas and the air changes if the possibility of a

build up in helium concentration in the insulation space is

neglected.

A = In 21

c 2

where A = air changes during the time interval 1-2

In = natural logarithm

cp = tracer gas concentration at time 1

C 2 = tracer gas concentration at time 2

As only the knowledge of the ratios of the tracer gas

concentrations is required for determining the air change, the

absolute tracer gas concentrations need not be determined.

The EMF values of the observed temperatures on the

recorder or indicating potentiometer are determined for about

one hour or longer in five- to ten-minute intervals and the

natural logarithm of the ratio of each two EMF values gives

the air change of the trailer for that time interval. In

order to compensate for errors in individual readings and

fluctuations in the infiltration due to wind or other effects,

the average hourly infiltration rate is then determined as the

arithmetical mean. The air leakage then is the product of the

infiltration rate and the inside trailer volumes

-34-

L = V x A

where L = air leakage,, CFH

V = trailer cargo space volume, cu ft

A = air changes per hour

The following table shows the air leakage values from

the cargo space observed of four trailers in the laboratory

Table IV

Cargo Space Cargo Space

Cargo Space Temp. Air Air

Volume, Gradient, Changes Exchange,

Trailer cu ft °F hr"l CFH

Military

(a)*

Military

677 89 0.075

0.28

51

M 750 109 211

Comm ? 1

.

(A) rKj 1700 100 0.049 83

Comm* 1.

(c) ^1700 100 0.036 61

*Observation made during reverse heat flow conditions.

The infiltration rates shown in the above table were

determined by feeding helium gas into the interior of the

trailer at a steady rate until the concentration therein

became constant. This procedure tends to eliminate the error

that is introduced if there is a two-way exchange of air

between the insulation space and the interior of the trailer.

A comparison of Table IV with computed values of air

leakage in Table II (Section 4) which are based on the

observed rate of ice accumulation indicates that a greater

amount of air is exchanged between the ambient and insulation

space than between the ambient and cargo space. Further study

of this relationship is needed to be certain of the air

exchange pattern.

-35-

Initial observations of the helium trace apparatus indicate that further development is needed to improve the accuracy

when the probe is exposed to low temperatures.

Because the air leakage of the military trailer (b) in

Table IV was relatively high, a second test was made with the

door and the joint around the panel of the cooling unit doubly

sealed. The latter was caulked with a mastic., the door was

sealed with masking tape,, and another air leakage test under

otherwise similar conditions was made. This second test

indicated a decrease of the specific air leakage by abou t one

percent when the door and refrigerator plug were sealed, which

shows hat the leakage caused by these unsealed joints had

not appeciably affected the values observed during the first

test

.

A few measurements were made to study the air flow pattern

inside the insulated space of the military trailer (b) by the

tracer gas method.

Gas samples were taken alternately from (l) the inside of

the trailer, (2) near the inside wall of the insulated space

and (3) near the outside wall of the insulated space adjacent

to (2). The inside temperature of the trailer averaged 5P and

the outside temperature averaged 112P. By introducing 0.61

cu ft/hr of helium steadily into the trailer, a concentration

of approximately 0.3 percent was maintained at steady state

conditions. In order not to disturb the air flow pattern in

the insulated space significantly the air samples were taken

at a rate of 0.6 ml/min (0.0013 cu ft/hr) and were drawn

through an infiltration probe until a steady state was reached.

Several samples were taken from each of the three stations.

The averages of the helium concentrations determined for each

station were:

1. Inside trailer 0.297 percent

2. Near inside wall 0.180 percent

3. Near outside wall 0.275 percent.

These data show that more of the fresh air from the space

surrounding the trailer moved through the insulation in a vertical wall near the inside skin than near the outside skin.

-36

Natural convection forces would cause the fresh air to enter

leaks near the top of the trailer and leave through openings

near the floor. At the same time* if there was natural convection within the insulated space, air would move downward

near the inside skin, which is colder, and upward near the

warmer outside skin. Thus the warm and humid fresh air would

be drawn toward the inside skin and deposit its moisture on

the colder surface. The frost pattern observed in the military trailer (c) and the helium concentrations cited above

both corroborate this pattern of air movement.

Other tests were made in which the military trailer (b)

was both pressurized and exhausted by means of a blower.

Three different air flow rates were used and the inside pressure was compared with the outside pressure. The pressure

gradient across the inside skin was also determined by

installing a tube through the outside skin and insulating

material terminating close to the inside skin. Table V shows

the pressure differences observed.

Table V

Effect of Pressurizing and Exhausting of

Military Trailer (b)

Air Flow Rate,, CFM

Pressurized rl * 5

“ JS'

Inside to outside pressure, in. W.G.

Pressure across inside skin, in. W.G.

0.037

0.024

0.065

o.o4o

0.101

0.060

Exhausted

Inside to outside pressure, in. W.G,

Pressure across inside skin, in. W.G.

0.038

0.026

0.067

o.o4i

0.108

0.068

This table shows that the pressure gradients were slightly

higher when the trailer was exhausted than when it was pressurized at the same air flow rate. Several of these tests

were repeated after the door and the refrigerator plug were

sealed. The pressure gradients., then observed, did not differ

-37-

from the ones shown in Table Vs which substantiates the previous finding that the leakage of the trailer was not caused

by the door or refrigerator plug seals.

To study the possible effects of air pressure semip as

a result of .motion., military trailer (c) was used to determine

the pressure patterns around a moving trailer, A Pitot tube

was mounted in front of a military M~52 tractor and 44 small

holes at selected places on the outside and inside skin of the

trailer were connected to a manifold so that the pressure at

each could be compared with the inside pressure of the trailer

and also with the ambient outside pressure as represented by

the static line of the Pitot tube. Stations for pressure

observation were established as follows;

1 on the inside of the trailer

2 on the nose

6 on the one side

2 on the rear

2 on the second side

7 on the roof

2 on the floor

Except for the station inside the trailer,, openings were pro=

vided at each station which permitted measuring the total

pressures on the exterior skin of the trailer and in the insulation space. Pressures were measured at each station while te

the trailer was pulled down the highway at a nearly-constant

road speed of 50 mph. The following conclusions were derived

from the data:

a. The pressure on the nose of the trailer was

closely related to the impact pressure in the Pitot tube

except where the front of the trailer was shielded by the

tractor. This pressure approximated 1 in. W.G. at 50 mph.

b. The interior trailer pressure was approximately

1/4 in. W.G. lower than the static pressure at the Pitot tube

in front of the tractor.

-38

c. The insulation space acted essentially as a

constant-pressure plenum., ranging from 0 to . 06 in. W.G. below

interior trailer pressure.

d. The pressure difference between exterior skin

and insulation space was such as to drive air into the insulation space on the nose and rear of the trailer and on the

rear five stations on the roof. At the other stations on the

sides,, floor,, and roof the exterior skin pressure ranged from

.07 in. W.G. below to . 05 in. W.G. above that in the insulation

space. The pressure difference at some of these latter groups

of stations fluctuated from positive to negative during the

test

.

These preliminary investigations of air pressure distribution and exchange patterns within an insulated trailer showed

the need for further work in these areas. Improvement of the

helium trace apparatus and additional laboratory tests of the

commercial trailers are currently in progress.

If a trailer did not leak air into the insulated space or

cargo space, there would be but little increase in heat gain at

50 miles per hour as compared with stationary performance under

similar ambient conditions. Since, at present, these vehicles

appear to have considerable leakage. Pig. 36 was prepared to

show the additional heat load above the transmission heat gain

due to air leakage at various ambient conditions, when the

trailer interior is at OF. For this graph it was assumed that

all air entering the trailer would leave saturated at OF.

HEAT

LOAD

DUE

TO

AIR

LEAKAGE,

TRAILER

TEMPERATURE

0

°F

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TRANSMISSION

LOAD) (unoH) yiv jo ujj oiano oooi /ni8

0 0 0 O

0 0 0 O

0 0 O

in ro CM

(ynoH) div jo soNnod 001/ me

I

ATMOSPHERIC

TEMP
.
39-

8. Discussion and Conclusions

The tests made of three commercial trailers in the laboratory and on the road and the air leakage and pressure distribution studies made on three military trailers indicate the

following conclusions.

1. The heat gain of a refrigerated trailer is significantly greater during road operation at 50 mph than under

stationary laboratory conditions for the same conditions of

ambient temperature and humidity. This increase ranged from

about 20 to 30 percent for the three commercial trailers tested

at ambient conditions of 100F dry bulb temperature and 50 percent relative humidity. The extrapolated heat gain values for

these conditions ranged from 9000 Btu/hr to 12., 600 Btu/hr for

a road speed of 50 mph.

2. The increase in heat gain on the road is due

principally to air leakage into the trailer construction under

the impact pressure of the ram air although small increases are

caused by solar radiation and the movement of engine heat

under the floor of the trailer.

3. The minimum air leakage rates for the three commercial trailers at a road speed of 50 mph were 970.? 1590 ,, and

860 cu ft/hr and may have been somewhat greater than these

amounts. The corresponding computed heat gains due to air

leakage at ambient conditions of 100F and 50 percent R.H.

would be 3070., 5030^ and 2730 Btu/hr respectively assuming that

the leakage air left the trailer saturated at 0F o This latter

assumption should be further studied as a part of a larger

analysis of air and moisture movement in insulated structures.

On this basis., the heat gain due to air leakage ranged from 32

percent to 69 percent of the heat transmission loss at a road

speed of 50 mph under design test conditions. These percentages

indicate the significant reductions possible in heat gain by

eliminating air leakage in trailer bodies. The air leakage and

ice accumulation in the trailers were not negligible under

stationary conditions. The air leakage amounted to 235.? 715^.

and 395? cu ft/hr and the ice accumulation rates averaged 0.32.,

0 . 98 , and 0.5^ lb/hr respectively during the laboratory tests.

4. The effects of solar radiation on a trailer

are largely nullified by the rapid air motion over the vehicle

at a road speed of 50 mph. In a typical test in bright sunshine..

-40-

incident solar radiation raised the surface temperature of the

roof and one side of the trailer about 7->5°R above ambient

air temperature at this road speed. On a weighted average

basis this corresponded to approximately three degrees rise in

temperature for the entire exterior surface. Under stationary

conditions the roof temperature could be increased 25 degrees

or more by solar radiation.

5.

The under side of the trailer was heated as much

as 15 degrees above ambient temperature in some cases during

road operation, principally by waste heat from the tractor

engine. On a weighted average basis this may not represent

more than three degrees rise in temperature for the entire

trailer surface. This would not cause a very large increase in

overall heat gain of the trailer, but could significantly

affect the preservation of food on the interior floor of the

trailer, if chilled air were not circulated around and under

the load.

6. Ram air pressures up to 1.25 in. W.G. will occur

on the nose of a trailer at a road speed of 50 mph on the

portions unshielded by the tractor, although the average ram

pressure is probably considerably below this value. The

static pressures in the cargo space, in the insulation space,

and over most of the exterior skin surface (excluding the

nose) were about equal and ranged from 0.2 to 0.4 in. W.G.

below atmospheric pressure for road speeds of 50 mph. The

leakage air probably enters the trailer body primarily on the

nose of the trailer and leaves the body over the remainder of

the surface, leaving most of the moisture in the ambient air

deposited as ice in the insulation space.

7. The exterior skins of the commercial trailers

tested leaked considerably. Under laboratory test conditions

the stack effect tending to cause leakage was only about

0.030 in. W.G., but the air leakage rates ranged from 235

to 715 cu ft/hr as indicated by the rate of weight gain of

the trailers. There was probably several times as much air

exchange between the insulation space and the outdoors as

between the cargo space and outdoors.
.

>
-41-

8. The results of these studies show that a laboratory rating method for refrigerated trailers needs to incorporate some means for taking into account the air leakage that

will occur on the road with present types of body construction.

9. The results further suggest that smaller

refrigerating units could be used if air leakage could be

eliminated, or alternately, less insulation might be required

if air leakage were significantly reduced. It is also probable

that the deterioration of trailer bodies would proceed more

slowly if moisture could be kept out of the insulation spaces.

U. S. DEPARTMENT OF COMMERCE

Sinrliiir W eeks. Secretary

NATIONAL IIUHEAII 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 laboratories in Boulder, Colo., is suggested in the following listing of the

divisions and sections engaged in technical work. In general, each section carries out specialized

research, development, and engineering in the field indicated by its title. A brief description of

the activities, and of the resultant publications, appears on the inside front cover. WASHINGTON, D. C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instru- ments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation, Electrochemistry.

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

Technology. Length. Engineering Metrology.

II eat. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology. Engine

Fuels. Free Radicals Research.

Atomic and Radiation Physics, Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. Atomic Physics. Neutron Physics. Nuclear Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic Chemistry. Electrodeposition. Molecular Structure and Properties of Gases. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

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

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Metal Physios.

Mineral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstrueture.

Building Technology. Structural Engineering. Fire Protection. Air Conditioning, Heating,

and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat

Transfer.

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

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

Circuitry. Digital Systems. Analog Systems. Application Engineering.

* Office of Basic Instrumentation. * Office of Weights and Measures.

BOULDER, COLORADO

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

Gas Liquefaction.

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

Propagation Services. Sun-Earth Relationships. VHF Research.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Systems.

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

Radio Systems Application Engineering. Radio Meteorology.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High

Frequency Impedance Standards. Calibration Center. Microwave Physics. Microwa\e Circuit

Standards.