NATIONAL BUREAU OF STANDARDS REPORT

I4276

COMPARISON OF THREE INSULATED FOOD CONTAINERS

by

Minoru Fuji!

C. W. Phillips

to

HEADQUARTERS, QUARTERMASTER RESEARCH & DEVELOPMENT COMMAND

NATICK, MASS.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

ms PROJECT NBS REPORT

IOO3 -IO-I4832 August 9, 1955 U276

COMPARISON OF THREE INSULATED FOOD CONTAINERS

by

Minoru Fujii

C. W. Phillips

To

HEADQUARTERS, QUARTERMASTER RESEARCH & DEVELOPMENT COMMAND

NATTOK MASS.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication,

unless permissioi

25, D, C, Such p

cal ly prepared II

or in part, is prohibited Approved for public release by the

standards , Washington Director of the National Institute of re p 0rt has been specifi- Standards and Technology (NIST) report for its own use, on October 9,2015. —

COMPARISON OF THREE INSULATED FOOD CONTAINERS

by

Minoru Fujii and C. W. Phillips

ABSTRACT

Tests were made to determine the cooling rate

of water in three similar food containers insulated with block insulation, shredded insulation, and an air space, respectively. The

lengths of time required for water to cool

from an initial temperature of about 200°F to

final temperatures of 130°F and 32°F were determined for each container during simultaneous

tests in an ambient temperature of about

The cooling times to reach a water temperature

of 32 °F were: 29 hours for the specimen with

block insulation, 26.5 hours for the specimen

with shredded insulation, and 17„5 hours for

the specimen with an air space for insulation.

For the same final temperature of the water

the heat transmission coefficients in Btu/hr

( °F temp diff) ( sq ft of exterior surface) were

0.17, 0.19, and 0.28 for the containers with

block, shredded and air insulation, respectively.

1.

INTRODUCTION

At the request of the Headquarters Quartermaster Research

and Development Command, Quartermaster Research and Development Center, United States Army, by letter dated August 16,

1954, observations were made on the insulating characteristics

of three types of Insulated Food Containers. Comparative

outward heat flow through three types of insulated wall construction was determined in terms of the length of time required for water at an initial temperature of about 200 °F

in the container to be cooled to 130°F and to 32°F in an

ambient temperature of about minus i40 °F.

2.

TEST SPECIMENS

The attached photograph, Fig. 1, shows the three specimen Insulated Food Containers. The three specimens were

identical in dimension, configuration, and appearance. The

materials used were the same except for the insulation which

was identified only by the stamped description on the bottom

panel of each container as block, shredded, and air, respectively. The specimens were not cut open to identify the insulating materials.
The insulated containers were manufactured by Ekco

Products Company. The inside and outside linings were made

of aluminum sheets, embossed with ridges and grooves. The

handles and latches were also made of aluminum. A cellular

rubber gasket was used for sealing the lid of the container.

Each insulated container enclosed three aluminum cans

manufactured by the Knapp -Mon arch Company. The three cans

with aluminum lids fitted fairly tight in the insulated

container. A rubber gasket was used for the seal between

the can and the lid.

The overall dimensions of the insulated container,

including the handles and latches, were, length 21", height

15 l/2H

, and width 10 l/2". The overall dimensions of an

insert can were, 5 l/2"x6 l/l|"xl2 n

.

K The total weights of the three insulated containers

with block, shredded and air insulation were 16 l/2 pounds,

16 3/4 pounds and 15 3/I4 pounds, respectively including

the three cans. Each insert can weighed one pound I4.7

ounces

.

3. TEST PROCEDURE

Prior to the test, the insulated containers and the

cans had been kept in an ambient room temperature of approximately 70°F. The cans were filled to within one inch from

the top or 95$ of The can volume with 10.9 lbs of water at

a temperature of about 200°F. The cans were inserted in

the insulated containers as soon as they were filled with

hot water. The insulated containers were then put into a cold chamber where the average temperature was controlled

at approximately minus III 0

?.

Two thermocouples were placed along the vertical

center line of each can; one an inch above the bottom of the

can and the other one inch below the water surface. Readings

of the temperatures were taken every 20 or 30 minutes until

the water temperature of each insulated container reached

32°F.

li. TEST RESULTS

Fig. 2 shows the cooling curves of the water in the

insulated food containers for three types of insulating

materials when exposed to an ambient temperature averaging
- 3

-

-Ijl^ . The temperature of the water plotted in Fig. 2 is

the average of six thermocouple measurements taken one inch

below the water surface and one inch above the can bottom

in the three cans in each food container. It took 7.3 hours

and 29 hours to cool the water from 203. 5°F to 130°F and 32°F,

respec tively^for the food container with block insulation.

The elapsed time to cool the water in the food container with

shredded insulation from 202. 0°F to 130°F and 32°F was 6.6

hours and 26.5 hours, respectively. For the food container

with air insulation, the time required to cool from 199 .l4 °F

to 130°F and 32°F was I4.3 hours and 17«5 hours, respectively.

These results are summarized in Table 1 . The effectiveness of the insulation in the three specimens can be compared more accurately by computation of the

heat transmission rate for the entire box per unit temperature

difference between water and ambient air or by evaluating the

heat transmission coefficient in terms of the exterior surface

area of the specimens. Such factors eliminate the effect of

differences in the initial temperature of the water at the

beginning of the test.

The heat transmission rates for the three specimens

were evaluated for each food container for two different

periods, from the beginning of the test until the water

reached a temperature of 130°F, and from the beginning of

the test until the water reached a temperature of 32°F. The

results are summarized in Table 2 . The specimen with block

insulation had the lowest heat loss: 0.17 Btu/hr ( °F temp

diff)(sq ft of exterior surface area) for a final water temperature of 32°F. The heat loss rate of the specimen with

shredded insulation was about 10$ greater and the specimen

with air insulation about 62$ greater than for the specimen

with block insulation. The heat transmission rate from the

beginning of the test to a final water temperature of 130°F

was about 18 percent higher than the value computed from the

beginning of the test to a final water temperature of 32°F

in each case. This is accounted for in part at least by the

higher mean temperature of the insulation in the former case.

The heat transmission coefficients summarized in Table

2 were obtained with the food containers in a test chamber

in which the air velocities were moderately high. Somewhat

lower coefficients would be expected if the containers were

surrounded by still air due to lower exterior film conductance.
- 4 -

Fig. 3 shows the cooling curves of the water in the

insulated food container with shredded insulation. The

water temperatures plotted in Fig, 3 are the average of

three thermocouple measurements one inch below the water

surface (one in each can) and the average of three thermocouple measurements one inch above the can bottom (one in

each can). Figs. I4 and 5 show similar curves for the food

containers with air insulation and block insulation, respectively. At water temperatures above the differences

between the water temperatures measured at one inch below the

surface and one inch above the can bottom averaged for

the food container with block insulation, 2.7°F with the

shredded insulation and 10.6°F with the air insulation. The

crossing of two curves in each figure at a water temperature

of about 39°F indicates a reversal in the convection currents

inside each can when the density of water reached the maximum value at 39.2°F. Downward movement of water at the exterior walls of the individual cans and upward movement in

the center of the cans would be expected for water temperatures above l40 °F. A resersal of this direction of flow would

be expected when the water temperature dropped below 39 °F.

The observed temperatures shown in Fig. 3 to 5 indicate that

this reversal did occur.

5. DISCUSSION AND CONCLUSION

The test results showed that the heat loss of the food

container with air insulation was much larger than the food

containers with block and shredded insulation in an ambient

temperature of minus l4 l°F.

It is obvious from the test results that the food containers with block and shredded insulation would keep food

warm longer and keep water from freezing longer than the one

using an air space as insulation.

The water temperature spread between the top and bottom

of the can was more pronounced in the case of the food container with air insulation than for the other two specimens.

It is probable that the temperature difference between top

and bottom of the container would be different for fluids of

other viscosities than that observed for water in these tests.
TABLE 1

COMPARISON OF WATER COOLING RATES IN THREE

INSULATED FOOD CONTAINERS

Type of Insulation

Block Shredded Air

Initial water temperature, °F 203.5 202.0 199.4

Initial temp, difference compared

to specimen with block insulation,

°F 0 1.5 4.1

Time required for water to cool

to I30 °F, hr 7.3 6.6 4.3

Time required for water to cool

to 130°F, compensated for initial

temp, diff., hr 7o3 6.8 4 .

6

Time required for water to cool

to 32 ^F, hr 29 26.5 17.5

Average water temperature differenc

between top and bottom of cans, °F

e 4-4 2.7 10.6
TABLE 2

HEAT TRANSMISSION RATES FOR THREE INSULATED

FOOD CONTAINERS

Type of Insulation

Block Shredded Air

Final Water Temp. , 130°F

Change in enthalpy of water, Btu 21407 2359 2271

Log

to

mean temp, difference of

ambient air, °F

water

206.2 205.1 204.1

Heat transmission, Btu/hr (°F) 1.60 1 . 74 2o 59

Heat transmission coefficient,

(°F)( sq.ft, exterior surface)

Btu/hr

0.20 0.22 0.32

Final Water Temp. 32°F

Change in enthalpy of water, Btu %6l6 5562 54 60

Log mean temp, difference of water

to ambient air, °F 142.4 141.9 141.0

Heat transmission, Btu/hr (°F) 1.36 I.I48 2.22

Heat transmission coefficient.

( °F) ( sq ft exterior surface)

Btu/hr

0.17 0.19 0.28
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