NATIONAL BUREAU OF STANDARDS REPORT

U875

SOME OBSERVED HEAT TRANSFER CHARACTERISTICS OF SEVEN

TYPICAL REFRIGERATED SEMI-TRAILERS

Draft of September 28, 19^6

by

C. Wo Phillips

R. S. Dill

H. D. Johnson

W, H, Redit

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Asl in. Director THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1003-20-1011} September 28, 1956

NBS REPORT

4875

SOME OBSERVED HEAT TRANSFER CHARACTERISTICS OF SEVEN

TYPICAL REFRIGERATED SEMI-TRAILERS

by

C. W. Phillips

R. S. Dill

H. D. Johnson^

W. H. Redit ^

-‘-Transportation and Facilities Branch, Agricultural Marketing

Service, U. S. D. A.

^Biological Sciences Branch, Agricultural Marketing Service,

U. S. D. A..

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication

unless permissi c

25, D.C. Such

cal ly prepared 1

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

or in part, is prohibited

Standards, Washington

i report has been specifir report for its own use,

SOME OBSERVED HEAT TRANSFER CHARACTERISTICS OF SEVEN

TYPICAL REFRIGERATED SEMI-TRAILERS

by

Co W u Phillips, Ro Si Dill, H« D 0 Johnson, & W. H. Redlt

Draft of September 2b, 1956

For discussion only; not for publication or referencing prior

to approval by the National Bureau of Standards

Abstract

The performance of seven refrigerated semi-trailers of

nominal 35 -foot length was observed to obtain general information on the range of such characteristics as heat

transmission, solar heat gain, temperature pull-down rate

warm-up rate and refrigerating unit capacity for equipmen

now on the raarketo Considerable differences were found

between the specimens; I4 O percent in heat transmission

(75 bo 107 Btu per hour ( °F ) )

,

3 L1 O percent in solar heat

gain (230 to 1020 Btu per hour), 55 percent in warm-up

time from OF to 70F (9.6 to II4.7 hours), I46O percent pulldown time from 9 IF to OF (2.7 to 15.1 hours), and 150

percent in refrigerating unit capacity ( 6,1400 to 15,900

Btu per hour. These differences emphasise the need for

prompt standardization of purchase requirements for

trailers and trailer refrigerating units, including

standardization of test methods and test apparatus.

1. BACKGROUND

Transportation of perishable foods in motor trucks and

trailers has increased tremendously in recent years in both

volume and importance, and among the several problems

C+'-i
accompanying this rapid growth is the need for standard method

for testing and rating truck and trailer bodies for heat trans

mission characteristics, and refrigerating units for capacity.

Formulation of such standards is a necessary first step to

permit purchasers and suppliers of such vehicles and their

[components to reach an understanding concerning their mutual

\ problems

.

The problem is not simple. In addition to the basic

methods of measurement which must be developed or adapted

for the purpose, factors to be considered include solar heat,

effect of moisture accumulation in the insulation, rain and

increased air infiltration due to wind, motion and vibration.

Recognizing this, many organizations have been studying

various aspects of the problem. Among them were the Committee 031 Transportation of Perishables by ho tor Truck,

Regular Common Carrier ’ Conference and Irregular Carrier

Conference, all of the American Trucking Associations, Inc.,

The Truck-Trailer Manufacturers Association, Truck Body and

i' ;• Equipment Association, Inc., truck refrigerating unit manufacturers, the American Society of Refrigerating Engineers,

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

of standards.
New truck bodies are usually free from excess moisture in

the insulation and because of this can be tested for heat flow

characteristics with a simple heating test. After they have

been used for some time they may accumulate significant amounts

of moisture in the insulation. To determine the heat gain

(or loss) of these vehicles is more difficult. No satisfactory

method for making the determination is in general use.

Apparatus for making this determination needs to be developed.

A program to develop a standard method (and apparatus)

for determining heat gain and air infiltration of refrigerated

trailers, both new and used, in the laboratory and on the

road, has been proposed by the Bureau of Standards and the

Department of Agriculture to the Committee on Transportation

of Perishables by Motor Truck, of the American Trucking

Associations, Inc. A major part of this program would be

the development of simple apparatus to establish heat gain

ratings of trailers in the field. This proposal is currently

under consideration by the A. T. A. committee.

The Regular Common Carrier Conference of the American

Trucking Associations, Inc., an attempt to focus attention

on the necessity of adoption of standard ratings for trailers

and limits, requested members of the Truck-Trailer Manufacturers Association to submit insulated refrigerated
-14-

semi -trail cr s for comparative tests to be conducted by the

Department of Agriculture and the national Bureau of Standards.

The tests proposed were to show the range in relative heat

transmission, pull-down rates and warm-up rates of the

trailers, and were to be held at the Bdgewater Gulf Hotel,

JEdgewater Park, Mississippi, May 9-16, 1956 . Seven trailers were submitted and tests were conducted

at Bdgewater Park as scheduled. This report deals with these

tests.

2. THST SPBCIMBhS

Refrigerated, insulated,

length were submitted by the

Name of Trailer

Dorsey

Fruehauf

semi-trailers of 35-foot nominal

following seven manufacturers;

Manufacturer

Dorsey Trailers Inc., Bib a, Ala.

Pruehauf Trailer Co., Detroit, Mich

by

Great Dane

Hi ghway

Lufkin

'

' f •

•

Miller

f'r ailmobile

/

* Table I lists descriptive

respective manufacturers, of

Great Dane Trailers, Savannah, Ga.

Highway Trailer Co., Ldgerton, Wise

I Lufkin Poundary and Machine Co.,

Lufkin, Texas

Miller Trailers Inc., Bradenton , Fla

Tr ailmobile, Inc., Cincinnati, Ohio

characteristics, as furnished

each trailer. Figures 1 and 2

are aerial views of the trailers in position as tested. In
TABLE I

Trailer Specifications

Trailer

. Mod . , Ser . No

.

Dorsey

KCT-lu

Fruenauf

i-iL h/Id

Kefrig. Unit

Mod., Ser. Mo.

Thermo -King

KL-30

Thermo -King

KL-30 , 930: Inside Dimensions

Length

Wi dth

H e i gilt

Volume, cu. ft.

33.7"

6 1 9

11 6 •S' 1 1500

34 ' 0

M 7 « o"

7

1 1”

1700

Int. Surface 990 1055

Weight (incl. refrig,

unit

)

124b0 12520

Insulation

Koof--type, thickness

Floor-- type , thickness

Walls --type, thickness

Doors-- type, thickness

Ultr alite ,

b

u hub at ex, 4”

Ultr alite, 6”

U^-trali te, 6"

ot yrof oam, 4

Styrofoam, 5

' Styrofoam ,

4

Styrofoam ,

4

Surface

Interior

Exterior

Alum, ceiling, Plastic Walls

Aluminum

Plasti

c

Alumihum

lixtruded Alum. Grooved

Floor Yes Yes

Int. Wall Air Channels Yes Yes

Door Air Channels Yes No

Meat Kails No No

Floor Drains No Yes

Other Features Side door, air delivery

duct
TABL I (Cont. )

':'r ai 1 or dp o c I i c c*. C J. Odl ‘h trailer Gr e at D an e hi ! huav Lufkin

.Mod., ber. No. AAr'h2 12916 TL3SW‘-

3'^ (OP), #17 i-iL V

Hefrig. tJnit Thermo -King Tru-Kooler Tran si -Cold

Mod., oer . No. -L- 30 r G-L'00ic,#1919 161 , ,/-lu 06 ( hutane

Inside Dimensions

Length. 33* 5" 33' 0

" 33' 4

" Width b'io" CIO" 6 ' 6

" Height CIO" 6 'll" 6

6

" Volume, cu. ft. 1550 1600 1500

Int. burface 1005 1020 9o0

Weight (incl. refrig.

uni t

)

11970 13680 13140

Insulation

Roof --t;/pe , thickness Mi crolite , 6’* Hi crolit$6" Fiberglass, 6"

Floor-- type , thi ckness Rub at ex, 4“ Styrofoam, 6" btyrqf08271 , 5

" Walls--type , tnickness Microli te, 6" Mi crolite , 6" Fiberglass, b"

Door s-- type, thickness Mi crolite, 6“ Mi crolite ,6" Fiber glass,

6

M

Surface

Interior Aluminum Plywood Plywood

Exterior Alurainum Aluminum Aluminum

Extruded Alum. Grooved

, Floor Yes Yes Yes

Int. Wall Air Channels Yes Yes No

Door Air Channels Yes Yes No

Meat Rails No No Yes

Floor Drains

Other Features

Yes No Yes

bide door, front

and rear ice

hatch doors,

temp,

recordercontroller
'JjA.b.L/.l'j I ( G Oi l l , )

I'r ail or specifi c a cion

3

Trailer Miller Tr ailinobile

V - o d « j oor • .0 . PML-22-3

:

1 hi -cube 0-0222, pi -17125

t Full necfer, . oil

7

nefrig. unit o 0 1 ih'. . 0 o i 1

G

Thermo -King

hod • , Our

.

Ho

.

2 klec. Units, oasoline i 1 Cj —

3

0 , v 4 ^4 0

t.i e n or a c or

Inside Dimens i on s

1 Lengtn 33 *6" * 33 1 9

U Width 5’ 9” 6 'll"

Height 7'c" and 7

1 6"“'* o' 9"

Volume, cu. ft

.

1700 loOO

Int. surface 10 oO 1015

Weight (incl. refrig. 14300 13220

uni t

)

Insulati on

Hoof --type

,

thi ckness dltralite, 6" Ultralite, 6"

Floor--type , thickness .styrofoam, o

11 Ultralite, 6"

Walls--type , thickness Ultralite, 6*' Ultralite, 6"

Door s--type , thickness Ultralite, 0

" Ultralite, 6"

Surface

Interior Plastic w al 1 s , Alurni nuin Plywood

ceiling

Exterior Aluminum Aluminum

Extruded Alum . Grooved

i''loor Yes Yes

Int. Wall 'Air Channels Yes no

Door Air Channels Yes Ho

Meat Hails 'Yes Yes

Floor Drains Ye s Ye s

Other Features

G"-Dtepdown floor; nose to

step-- 1 ;

1 , step to

rear door--24 1 b"

.

FIGURE 2.

figure 2 , the trailers are arranged as follows: front row,

1 left to right, Mfller,. Great Dane, Tr ailmobile and Fruehauf;

back row, left to right, Dorsey, Highway and Lufkin. Figure 3

shows, left to right, Hiller, Great Dane, Trailmobile and

Fruehauf trailers. Figure I4 shows in the seme order Dorsey,

Highway and Lufkin. Figure 5 i s a schematic drawing of the

trailer test layout.

The trailers were all of nominal 35-foot length and

ranged in interior volume from 1,500 cubic feet for the Dorsey

and Lufkin trailers to 1^700 cubic feet for the Fruehauf and

Miller trailers. Weight ranged from 11,970 pounds for the

Great Dane trailer to 14,300 pounds for the Miller trailer.

Four of the trailers, Fruehauf, Irailmofcile, Great Dane,

and Dorsey, were equipped with Model RL30 Thermo Ming ’gasoline-engine driven refrigerating units. The Miller Trailer

was equipped with a Coldmobile electric unit-gasoline-enginedriven generator combination, and the Highway and Lufkin

Trailers were, respectively, equipped with a Model G-500-R

Tru Kooler, gasoline-engine-driven refrigerating unit and

a Model lol Transicold butane engine-driven refrigerating

unit

.

The Miller, Lufkin, and Trailmobile Trailers were

equipped with meat rails. All trailers had tandem wheel

assemblies.
-oVarious ,ai crialu wor c cm. do./vd as in tori or wall and

ceiliir surfaces in tx & variouo tra.lers a a shown in table 1.

All of the trailers were covered v;itn bri Lt aluminum on the

exterior

.

3« J. f x I xx! jJL i x ,t\ i. 0

0

j . D Ux iM

Near transfer chair ac' teri sties oi' all seven trailers were

observed as follow’s : (1) uppr omimate stead,, state heat loss;

(2) Pull-down time ; and (3) V/ arm -up time.

The trailers were tested in an open lot in the rear of

the hdgewater Gulf hotel in .h, ewater Park, mississippi, in

May, 19pb. They were exposed to the weather and were

arranged as shown in figure 5»

Mach trailer was equipped with five thermocouples (for

remote measurement of temperature), an electric heater,, and

t a 16-inch electric fan. These items were positioned as

shown in figure 6. The . eater s 'were all of the same model

and were rated at 1^0 watts at 115 volts and each was equipped

with a small internal fan. The lo-inch electric fans were

selected for comparable air delivery characteristics. An

actual installation of the fan and heater is shown in

figure 7.

The thermocouples were made of calibrated do.. 3^ copper

and No. 30 cons tan tan wire and temperature readings were

taken witxi an electronic constant-balance poten liomoter
FIGURE 4

O

Q

PARKING LAYOUT OF TEST TRAILERS

USDA-NBS-TTMA - ATA

EDG EWATER GULF HOTEL

MAY 7-20,1956

0 10 20 30 40 FEET Flfl. 5

POSITION

OF

FAN

usin> hue null -flow principle which o uua'll a-cd the effect , of

i \

'

I thermocouple length. . In oacu trailer l.u-ce cqual-len, txr

thermo coupler used for me as ur in; interior air temperature

were par all el -connected ana read a a a single aver a, ,e temperature. a thermocouple was attached to the roof surface of

two of the trailers apd the instrument truck for inculcation

of coinparati ve day and nifi.it thermal raaiation effects. All

ambient, condenser inlet, and interior air thermocouples

were checked for proper performance after installation and

again before the pull-down test. figure u shows the

group couple being checked in one of the trailers by

immersion in crushed ice. file performance of the potentiometer was monitored at tiie time of each obser- vation by

reading the temperature of both high and low temper a

1 tore

reference baths. To reduce likelihoou of accidental damage,

all thermocouple leads were carried overhead between trucks.

The electric input for heating, 'each trailer was measured

by a calibrated watt -hour meter. All to at t -hour meters used

had a correction or less them ±1,0 . Performance of cacn of

the watt -hour meters was monitored by volt ammeter readings

throU: hout • tiie tests. The power leads to each truck were

the s afre ' length and of suen size (;, 12-2 wire At)-.} that

resi stahW loss differences were negligible. The resistance

©T each power lead, including attachment fittings was checked
- a after installati tj. Power for the to eta was • supplied by

transformers installed for the purpose and was 3 phase 3

wire nominal 23 ^ volu service with one center tap provided

for lip volt lighting use. Voltage to each trailer was

adjustable to per,. it balancing heat input between vehicles.

All equipment for the test was transported to the test

^ * site from Washington, D. b. in a one-ton panel truck equipped

with an instrument panel, as shown in figure 9, constructed

for this test. To facilitate reading of instruments, ,a seat was equipped with casters permitting the operator tomove easily along the u-foot length of tn, panel. Figure 9

shows the physical lay-out of major items in the instrument

truck. In figure 10 can be seen tne two fans, one mounted

In each window of Lhe instrument, true 1 : cab, which provided

ventilation of the vehicle.

Installation of all instrumentation at the test site

and all observations were made by 0. *W. Phillips, jIBS,

W. IT. liedit and Harold D. Johnson, J. B. D. A., shown in

figure 11.

Data from all tests was coded to prevent accidental

or* Improve!'- release of competitive information.

The first test conducted was to determine relative heat

•leakage Af the sever aI trailers when the stunc aiAbunt of

he bt was added lo each. -ill fans and neater s Wire checked
FIGURE g

FIGURE 10



Tor sail sf actor , t rforiiiancc cl hi hour a bcfoi-e the start of

1 the teat at 9:00 n.h. 1 . a;- 11. The door a wore closed at

*9:00 A.ii. and the fane and heater a wore turned on at 9:30 A. 11. One trailer was late in arrivin' and was atarted on teat at

3:00 P.K. the same day. Trie teat was concluded at o;00 A.ii.

on! May 1 [4 , an elapsed time of 69 hours. lower and temperature

* observations were made at 30-minute intervals throughout the

test. Lffect of daytime, sun and nighttime conditions were

observed. No rain fell burin; any of the tests. Relative

solar heat gain obsei vations were made as a part of this

test.

The second test conducted was to determine the relative

temperature pull-down rate of each trailer- and* the rela/tive

capacity of the refrigerating units. Irior to tne staift of

this test the doors of all trailers were opened and the interiors

ventilated by fan for tlirce and one-quarter hours. The trailer

door's were closed 30 minutes before the refrigerating units

\

1 were started, and sufficient observations of interior temperatures

were made to adequately determine the initial temperature for

this test. After starting the refriger ating units, temperature observations were made at 15-minute intervals until each
trailer was at or below OF for at least four hours, The

pull-down test was started at 9:45 a.L. , hay 14 , a time, . when the tracks were closed to stron,- sunlight. Industry

personnel in attendance were permitted to make any desired

adjustments to their respective units during the first 30

minutes of the test, and to remain at the site for the

entire test. All thermostats were set well below OF to

assure continuous operation of the units during pull-doVm.

Tests for refrigerating unit capacity began after each

trailer reached -1 **'. As each trailer reached -IF, thd

heater in that trailer was energized. As each trailer

reached -3F, the manufacturer's representative was req/uested

to set the thermostat for cyclic operation at OF. Because

of variation in reserve refrigerating, capacity in fou/t- cases

after the trailers reached -IF, the refrigerating units

cycled on thermostat with the heaters energized continuously,

and in two cases the refrigerating units operated continuously and the heaters were cycled by test personnel.

In one case the heater was riot energized. All units were

operated at about OF for at least four hours at the end

of the pull-down portion of the test. hefriger aiding unit

capacity 'V/ds commuted from data recorded during this
-11

Tour -Lour period. observations were made of. the area and ,of

the extent of condensation or sweating on the trailer exteriors. Automatic or manual defrosting of unit evaporators

was permitted.

The final test conducted was a

u warm-up test.” After

jeach trailer had been o{peratin<; at Oh for four hours or

\ longer, the unit, the heater, and the fan were turned off

and the trailer allowed to warm up with temperature

measurements made at ten-minute intervals during the first

hour, 15 -minute intervals during the second hour, and

30-minute intervals for the remainder of the test. The

warm-up test was started for all trailers between midnight

and 3:00 A.M. , Hay 15 and was concluded at 5:00 P. M. the

same day.

]i . TlibT RKSULIR

Test results reported, in this section are presented

in the following order;

1. Heat transmission

2. Relative solar heat gain

3 . Warm-up rate

I4. (Comparison of heat transmission, solar heat gain,

and warm-up rates

5 » Temperature pull-down rate

6 . Refrigerating unit capacity

7 , Sweating of trailer e:. teriors
All phases of the test series were conducted with the

y t trailers exposed to the weatner. ho rain fell during the

tests and a slight on-shore breeze blew almost constantly.

For the most part, days and nights were clear, and bright

I sunlight gave excellent opportunity to observe solar effects.

r Ambient dry bulb temperatures were relatively constant,

ranging from a low of 75F to a high of B6F.

Heat Transmission

The observed heat transmission rates ranged from

75 Btu per hour (°F) to 107 Btu per hour ( °F ) , a variation

of I4O percent. In this test heat was added to the interior

of each of the seven trailers and the heat loss coefficients

were determined by observing the resultant -diff erential

4 between ambient and interior temperatures . The values

reported, shown in figure 12, were computed by the equation:

Hmn = heat Input

X 1

i 2' Fq. 1

when HTC = Heat transmission coefficient, Btu/hr ( °F

Temperature difference between ambient

and trailer)

heat Input = heat equivalent of electric input

for each trailer, Btu/hr

Tp = Average trailer temperature, C F

T'2 - Average ambient temperature,
RELATIVE

HEAT

TRANSMISSION

9

to.

Btu

per

hr(°F)

-13-

All values are avefa i'or Lhoao observed* froin 3:00 A. 14. to 6:00 A.M. on Hay 12 and 13 , when trailer temper at urea were

at a nl grit time minimum for a significant portion of the time

most nearly free of sun effects. The average hoot input to

all trailers was 14535 Btu per hour with individual averages

raging from [41466 to 45^2 Btu per hour. The outdoor ambient

temperature averaged about 76 . 6F and the temperature rise of

the trailers above ambient ranged from 41*8 to 60. 3 degrees F.

The actual temperature in the trailers ranged from about

118F to 137F.

Relative Solar Heat Gain

Solar heat gain observations were made during the heat

transmission test when trailer temperatures were at a daytime

maximum due to sun effects. The relative heating effects due

to the sun, as shown in figure 13 , ranged from 233 to 1015 Btu

per hour and were computed by the equation:

HG S = (T^-T^) HTC - Heat' Input Eq. a

when HG a — Relative solar heat gain, Btu/hr

T^ = Average daytime maximum trailer t einper ature , °F

T

4 = Average ambient temperature, °F

HTC = Heat transmission coefficient

Btu/(hr)(°F Temperature difference),

equation 1

Heat Input = Electrical Lnput for each trailer,

Btu/hr
-lLiAll values in the above equation, except HTC, are

average for those observed from 3:60 P.H, to 6:00 P.h.

on hay 12 and 13 . HTC, from equation 1, was computed from

observations made from 3:60 A.h. to 6; 00 A. II. on Hay 13

and ll[ . The time periods 3; 00 A.H. to 6:00 A.H. and 3:60 P.H,

to 6j00 P.H. were selected because the lowest and highest

temperatures, respectively, in the trailers were observed

at those times.

The average heat input to each trailer during the periods

3:00 P.H. to 6:00 P.H. on May 12 and 13 was 4537 Btu per hour,

with individual averages ranging from 4566 to 4579 Btu per

hour. The average ambient temperature during these periods

was 00. 4F. The maximum temperature observed in each trailer

ranged from 151.6F to I32.6F as shown in table II.

Table II

Maximum Trailer Interior Air Temperature

Trailer Temp . A 145. IF

B 151.6

c 141.0

D 137.3

K 136.9

F I30.6

G 132.6

(These capital letter designations refer to trailers

similarly referenced in figures' 12 , 13 , and 14' and

in tables III, IV, V, VII, and VIII.

)
RELATIVE

SOLAR

HEAT

GAIN

(Observed

During

Heat*

Transmission

Test)

-l l - For the periods 3: u(j I' * i'i - to 6:00 i .h. on hay XT and 13*

the additional t emper aturo rise of the trailers above ambient

caused by the solar heatin c, effect over and above that due 'to

the electrical input, is shown In table III.

Table III

Effect} ox Oolar Energy on

Internal Heating of Trailers

Trailer

Temperature rise

above ambient

due to electrical

heat input

Electrical

heat Input

Btu/hr

Additional

temperature

rise due to

solar heat

Btu/hr

equivalent o. solar heat

A o0 . l\ 1)539 3.1 233

B 59.2 45'n • m

i

—

1

i

—

1

67 o

C 51.5 4506 8.5 744

D 51.0 U575 5,3 572

h 1*9 .5 4514 7.6 696

F 1)9.1 4535 7 .

6

702

G 1)2.5 4579 9.5 1015

The surface temperature of the roof of one of the trailers,

(roofs of all the trailers were of bright aluminum), was 145E

during expo sum: to the sun. This was approximately 60 degrees F

above ambient temper at ire , At night surface temperatures on

| the roof t u inuch as 10 degrees F below ambient were observed.
-loWarm -up rate

Figure li; shows ijpe lime required for each empty trofier

I to warm up to I4 OF and then to ?0F, with no internal heating,

after being held at 01' for at least four hours.

The time required, for the trailers to warm up from OF to

t fUF under the test conditions ranged from nine and one-half

to more than ll;f hours.

Although- the warm-up test of one of the trailers (other

than A) was started '

3

hours later than the other six, this

is purposely not indicated in figure lit to prevent identification. Test data recorded beyond 7 OF trailer temperature

is not shown in figure II4 because one of the trailers was

inadvertently opened after it had reached 70F. Its temperature position, relative to the other trailers, did’ not

change, however, because of the opening. The warm-up test

was continued for 17 hours for six trailers and 1 J4 hours for

one trailer. The temperatures in theutr ailer s and the

ambient temperature at the conclusion of the test at 5:00 P.M.

on May 15 are given in table IV.
TIME

REQUIRED

for

TRAILERS

to

WARM

UP

from 0

°F

to70

®F

Elapsed

time —

hoars

-“17 -

'l'uble IV

Tr ai 1 or*' I

1 cmper uti.r e s and Aiubi ent

Temperature at Conclusion oi' the Warm-up Test

T r_ai ler Temperature

A 7 iii'' b

' c

d6

04

• D

E

87

b6

P 02

G 00

Ambient 03.4

Comparison of Heat Transmission bolar Gain, and Warm-up Kate

A comparison of the observed values of heat transmi «si on

,

solar heat gain, and warm-up rate is given in table V', These

results are shown separately in figures 12, 13 , and llu In

table V, trailer A, which had the lowest heat transmission

and the least solar heat gain is assigned a value of 1.00

and all others are shown in relative order. In figures 12,

13, arid II4 and tables II, III, IV, V, VII, and VIII, trailers

are crosp-identifled by capital letter designation^. In

other figures arid tables they are not so identified.
Pull-down Hate

The time required for the trailers to be reduced in, temperature by their respective refrigerating units from about

91F to 32F and then to -IF is shown in figure 13'. The time

required to lower the temperature to -IF ranged from less

tjqan three hours to more than fifteen. The air temperatures

in the trailers at the start of the pull-down test ranged

from 88F to 94F. These temperatures were observed before

the refrigerating units were started and after the doors had

been closed for 30 minutes, during which time the 16'1 fan in

each trailer was in operation. Neglecting sun load, the

approximate mean wall temperature of the trailers ranged

from 8 I4F to 8?F at the start of the test, and was approximately 38F for all trailers at the end of the test. Tlie

electrical input to the 16° fan in each truck ranged from

190 Btu per hour to 392 Btu per hour during the pull-down,,

The time required for reduction of trailer temperature

from initial temperature to -IF, from initial temperature

to 321 from 60F to I4 OF, and from 30F to 10F is shown in

table VI. Trailers are similarly identified by number in

f i gur e 15 'add'- table VI.

Pive^of ' the seven trailer units were defrosted^, automatically or manually, once during the temper ature,, pull -dowr

to dip 0 No defrosts ,'occurred during the other temperature
LxJ crID

\~<

q:

LjJ £Ls

Ld

H*

CC

Id

Sj_ < «

cr

U- U_ 1 c

UJ Oil o3o

LdcrQ

Ld

cr

IDO

Ldcr

Ld co c

•*- 36© 3 'A

Ol

C: — enC

C)

<u n:

><O

>_Ql

Q. <eo

-c\ass

03

o>'9-

CVII

0) <\J C\J tvi

K

Sf 03

fr> fr> C\| n-> I I I I n o>

IN If) 9- lo (o

V.3

u_ u. u. ou. U. o ® <3^

CD o —

O

O ff) 60 'J- CD

V.0) 03 O) 03 03 03 03 00

Q. 1 1 1 1 1 1 1 6

Q>

— 09 ff) 4 5 0)

K

"J:

N.O

"D

•: X2

no

OJ ro LO CO rElapsed

time—

hours

Table V

Relative Heat Transfer

Characteristics of Seven Trailers

Trailer

A

B

Heat Transmission

(1.00=75.1 Btu/hr ( °P )

)

Ree fig. 12

1.00

1.01

Solar Heat Gain

(1. 00^233 :.Btu/hr)

See fig. 13

1.00

3-76

c 1.17 3.19

D 1.19 2.03

E 1.22 2.99

P 1.23

| 3.01

G 1.42 4.36

Warm-up Rates

Trailer

0-70F O-I4 OP

( 1 . 00=14 . 8 °F /hr ) ( 1.00=5. b °F/hr)

See fig. Ill

40-70F

( 1 . 00=3.8

A 1.00 1.00 1.00

B 1.44 1.64 1.30

C 1.31 1.1)7- 1 . 2C

D 1 . 1|0 1.77- 30 1

—

i

•

11—

E 1.27 1.33 1.22

P 1.53 1.77 + 1.37

G 1.40 1.1)7 + if. 35
Time

Required

for

Temperature

Pull-down

ftO

rH

G

•H oft ft) o 3ft oft 3ft 3ft

O 3 PT ION

O Oft Oft

O

ft 54 o ft! oft

O

OO rH ftj ftj 3ft

ftO • -=r q

•H o 3 p •• 0

ft 54 O w

vO

O' 04 1

04 3ft 1 1 3ft Pft rH 04 04 C\l oft Oft PT o O O O O O O 0 • aJ P ft q

•H oj (\J *H

-P 54 ns •H 0 q ft o w H g P 54 0 fEj EH

Oft 0ft

O Oft vO CO 3ft pft 3ft 3ft O o Pf rH O O O rH rH rH 04 0 H P « d P ft q

•H erf rH *H O' CO -=t

O 3ft 3ft 3ft P 54 1 3 Oft o rH rH -CT PT O

•H 0 q ft o 0 04 Oft Oft PT CO QD 3ft H S P 54 0 W

EH

rH 0U

*1 3p

•H ctf -P•H q ft 0 o q ft H e0

Eh

rHO'

O

O' o

O' on

O'

oft O' PT

O'

3D3D

540

rH3

54 Eh

04m pt 3ft vO o-
-19-

ranges listed in table VI* Possible malfunctioning of one or

more units during the pull-down test was reported by industry

personnel in attendance, and control failure of one unit was

remedied promptly at the start of the test. The question was

raised whether interior temperatures experienced during the

heat loss test might hajve damaged components of the refrigerating

unit. This is not considered likely since the maximum single

air temperature observed in the trailers, as shown in table II,

ranged from I32.6F to 151.6F. All panels, doors, etc., attached

to the refrigerating unit enclosures during the test were

closed or adjusted as they would be in normal service.

No determination was made as a part of this test whether

the units furnished were similarly rated, or whether accessory equipment, such as protective guards, controllers, etc.,

was typical to that normally furnished by respective suppliers

for commercial application. All units operated on inherent

engine power throughout the entire test.

Refrigeratinp Unit Capacity

The comparative net refrigerating capacity of the units

varied from 6,400 to 15,900 Btu per hour, es 3hown in figure 16,

and was observed during the final four hours of operation in

the pull-down test, when each trailer was at about OF

interior temperature. The ambient temperature averaged 77. 9F
“ 20 =

for 3ix of the trailers and 75»7P for one of the trailers,

& 7id average interior temperatures ranged from -lalpF to 0«0p 9 Some units were cycling on thermostat during this period;

others ran continuously and internal heat loading was dif~

ferent for all trailers.

The net refrigerating capacities reported were computed

as follows:

K(HTC)(T2

- T

1

) + H-jwhere Q, = net refrigerating capacity, Btu/hr

HTC f heat transmission coefficient, Btu/hr(°F)

( see fig. 12 )

.

T^ = average ambient temper at ure , °F

= average trailer temperature, °F

Hj = internal electric heat load, Btu/hh

T

u

= running time of refrigerating unit, percent

K = 1-0.002 ( °P change)

= factor for approximate change in HTC with

change in mean wall temperature between

heat transmission test (in which ffTC was

determined) and refrigerating unit

test.

For all trailers the final four- hours of the pull-down

test began well sifter sundown and solar effects wehe neglected

in the above computation.
REFRIGERATING

UNIT

CAPAC

ITY /

000 Btu/hr

-21 -

Sweating of Trailer Interiors

During the even in ; Hours when all trailers were being

held at interior temperatures of about OF, some u sweating11

was observed on all of the test vehicles. In was generally

predominant around doors and door frames. Some condensation

was observed on roofs and sidewalls and appeared to be forming

at regularly-spaced intervals, indicating frame member

locations. Since no observation was made of relative humidity

during this period, test results are qualitative only.

• GONGL JSI Oil S ilHij D1SG UhSIGN

1. The principal conclusion indicated by this work is

that commercial insulated, refrigerated semi-trailers, as

typified by the seven units tested, differ to such extent

in heat transfer characteristics that prediction of performance is difficult or impossible. Findings which support

this conclusion and indicate differences between trailers

are discussed later.

2. Adequate procurement speciiic ations are riocucu

which include refrigeration or heat transfer requirements.

3* Adequate test methods and apparatus are needed to

permit determination by both operator and manufacturer of

heat transfer performance of not only new trailers (and

their refrigeratin' units) but of these same trailers and

units after tney have neer in service
- 22 -

Ije The "warm-up" teat suggested as a simple test to

determine heat transmission characteristics of refrigerated

trailers is completely unreliable for this purpose. This is

discussed later under "warm-up test."

It should be no tec that the series of tests covered by

this report dealt with tew, presumably dry trailers and did

not encompass heat transfer problems arising from motion,,

wind, rain, vibration, loading, and other variables which

should be considered in a comprehensive study of these

vehicles.

The findings which illustrate the differences in performance of different trailers were:

a. Heat Transmission; The heat transmission of hie

vehicles varied sharply, ranging from 75>«1 to 106.8 Btu/hr

for each °F of temperature difference between inside ar.d

outside.

At hypothetical conditions of 110F ambient temperature

and OF trailer temperature, the heat load due to transmission

(correcting for change in mean wall temperature) would be

7100 Btu/hr for the trailer with the lower rate and 10,100

Btu/hr for the one with the higher rate of 106.8 Btu/hr

(

°F)

.

The difference between trailers of 3000 Btu/hr at hypothetica

conditions 'of 110F and OB1 is nearly half the total capacity
“23-

observed. for one of the refrigerating units tested in this

study at the more favorable conditions of 80F and OF. If

the unit with the lowest capacity shown in figure 16 had

been used to refrigerate the trailer with the highest heat

transmission shown in figure 12, it would not have been

able to reduce the trail|er temperature to OF in the pulldown test.

b. Solar heat load; The heating effect of the sun

differed sharply, nearly 5 to 1, between the several trailers.

Bearing in mind that the temperature rise in the trailers

due to sun load was determined during the heat loss test when

trailer temperatures were already above ambient, it can be

3afely assumed that the heat gains due to the sun would be

still greater when the trailer temperature is lower than

ambient, hven so, as shown in table III, heat gain due

to sun load ranged from 230 3tu/hr to more than 1000 Bfcu/hr

for the several trailers. As a further- indication of the

difference between trailers it is interesting to note., from

figures 12 and 13 that trailer A, which had the lowest

transmission rate, also had the least solar heating effect,

while trailer G, which had the highest transmission rate,

also had the highest solar heating effect. This wo^ld be
-2i|-

expected. However, trailer B, which had a transmission rate

almost as low as trailer A, had a solar heating effect of

876 Btu/hr, approaching that of the highest, trailer G,

1015 Btu/hr. In table VII the lack of relationship is more

clearly demonstrated.

Table VII

Relative Magnitude of Heat

Transmission and Solan* Heat Effect

Trailer

Transmission

Goeffi ci ent

Btu/hr ( °F)

Solar

Heat Effect

Order Btu/hr

A

B

CD

E

F

G

75.1 1 1 233

76.2 2 6 876

87.5 3 5 744

89.0 4 2 472

91.6 5 3 696

92.4 6 4 702

106. 8 7 7 1015

Even though the trailers were at different interior

temperatures during this test and therefore some comparative

value of the preceding is lost, it is significant to note

that trailers A and B with the greatest variation in relative

order between transmission rate and solar heat effect had

similar interior temperatures during the 3:60 A.M. to 6:00 A,M*

periods used to determine basic heat transmission rate.
-25-

c. Vi arm-up Test: If this test was suitable for

.determining heat transmission of a refrigerated trailer,

the warm-up rate would be a function of the heat transmission

rate. In table VIII, the data shows the extreme lack of

relationship between heat transmission rate and warm-up rate

from OF-ljOF. (The OF-I4 OF range is used for example because

all trailers passed through this range before sunrise and

solar heating is, therefore^ not a factor).

Table VIII

Relative Magnitude of

Heat Transmission and Warm-up Rate

Trailer

Heat

Transmission

Coefficient

Btu/hr ( °F

)

Order

Warm-up

Rate

0F-40F,

°F /hr

A 75.1 1 1 5.8

B 76.2 2 c' 9.5

C 87.5 3 3 8.5

D 89.

0

4 6 IO .3

91.6 5 2 7.7

F 92.1| 6 7 10.3 +

G 106 . b 7 4 9.5+

Note that trailer A with the lowest heat transmission

rate had the lowest warm-up rate, as might be expected,

iiowever, trailer ]3, with a transmission rate nearly as low
as trailer A, had a warm-up rate almost twice as great. Note

also trailers C and G, with almost identical warm-up rates,

while trailer C had a transmission rate of 07*5 Btu/hr(°F)

compared to 106.8 Btu/hr(°F) for trailer G.

d. Pull-down test: The time required to precool a

trailer from ambient temperature to below OF is of major

significance to the owner or operator. A strict interpretation

of test results (figure 15 and table VI) shows excessive

differences in pull-down time (from approximately 92F to

-IF) ranging from less than three hours to over 15 hours.

Since the excessive time required by trailer 7 can be

attributed to probably malfunctioning during at least a

part of the pull-down, it may be disregarded for comparative purposes.

Comparing, instead, the average pull-down time for

trailers 1 and 2, 2 hr, 54 min., and trailers 5 and. 6,

0 hrs, 45 min., shows that variations of more than 3 to 1

can be expected unless purchase specifications and tests

to determine this important performance requirement are

included in the procurement of trailers. It must, of

course, be clearly recognized that pull-down time is not

a function solely of trailer mass or construction, or

3 olely of 'refrigerating unit performance, but is, instead,

a function of all of these variables.
-27-

e. Refrigerating unit capacity test: The net

refrigerating effect of the units in the several trailers

differed greatly. Observed under conditions of OF trailei*

temperature and 80F ambient temperature, the average of the

two highest, 15,370 Btu/hr, is nearly twice the average of

the two lowest, 7,780 Dtu/hr. Assuming these units were

competitively rated, this points out the need for adequate

specifications and performance tests to assure proper matching

of the trailer and its refrigerating unit.

It follows then that the logical first step in matching

a refrigerating unit to a refrigerated trailer is the

determination of all pertinent thermal characteristics of the

trailer. Then and then only can an applicable set of

requirements for refrigerating unit performance be set out.

To establish the necessary thermal characteristics for

refrigerated trailers, detailed laboratory and over-theroad studies are needed to e stablish requirements in

keeping with the best commercial practice^ and standard

test methods and apparatus must be devised for evaluating

performance of trailers both at the time of procurement and

at intervals throughout their service life.
- 28-

(b • bbilid'iAjb DIoGu oblON

The manufacturers who voluntarily submitted trailers

and units for these observations have rendered a significant

service, not only to their part of the industry but also to

the operators who buy and use their products. The tests,

perhaps for the first time, have demonstrated certain ranges

of .performance which exist among reputable products competitively and normally furnished for similar service.

Naturally, every reasonable precaution has been t aken to

prevent the presenting of test results so as to aid or

otherwise affect any one manufacturer-participant in comparison with others. bince the work was conducted without

restricting observers from the area, some conclusions will

probably be expressed which may or may not correctly

indicate relative performance. The essential temperature

data for, all phases of the test was coded at the time of

observation in such a manner that specific performance

j values necessary for accurate reporting of any test for a particular trailer are not known to others than the authors.

/The necessity for this fanner of reporting is obvious;

i the purpose of the project was to d et ermine ranges of

performance of typical new refrigerated, 3^-foot semitrailer s

} not to show which trailer was the best or pooi^est

in any category.
“ 29 -

In order to assist the engineering departments of the

. various participating manufacturers information pertaining

to the general (not specific) relative position of their

respective trailer has been made available to each participant who has agreed to refrain from using the information

in advertising or sales promotion in any manner.

Although these tests yielded reliable relative data,

they are only preliminary in nature to the important comparative work yet to be done. Only new, supposedly dry,

trailers were used; no interpretation was made of air

infiltration; no comparison was made between still tests

and o ver- the -road conditions; and steady-state conditions

could not be achieved because trailers were exposed to

changing wind, sun, and ambient temperatures. A major

unknown variable In connection with the heat load of

refrigerated trailer s--the effect of moisture gain in

the insulated panels--was not considered.

Prompt coordinated industry and government action in

conducting further study of these problems can result in

better equipment, rated under standard conditions, permitting

optimum economic utilization of these products by the

/ truoking industry.
- 30 -

It is not contemplated that comparative performance

data on competing products shall ever be published, but the

authors recommend that standard rating conditions, test

methods and apparatus be adapted or developed by cooperation

of all parties to the rapidly-expanding industry of trans-

• • ' ( portation of perishables by motor truck. By means of such

standards, manufacturers can give assurance to purchasers

about the performance of equipment; spoilage in transit can

be minimized and foods can be delivered to consumers in good

condition with greater ease.
THE NATIONAL BUREAU OE 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. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly 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 front 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

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Information on the Bureau’s publications can be found in NBS Circular 460, Publications ol the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of Standards, Washington 25, D. C.
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