Armored 
Medical Research Laboratory 
Fort Knox, Kentucky 
PROJECT NO. 1 - COLD WEATHER OPERATIONS 
Second Partial Report 
On 
Sub-Project No. 1-20, Heat Retaining Capacities of Insulated Jugs 
Action cop-i.es have been forwarded to Require- 
ments Section, AGP for approval and execution. 
Project No. 1-20 
18 February 1944 ARtdORED MEDICAL RESEARCH LABORATORY 
Fort Knox, Kentucky 
Project No® 1-20 
430o3 GNOML 
18 February 1944 
lo PROJECT: No, 1 - Cold Weather Operations® Second partial report 
on: Sub-Project No0 1-20, Heat Retaining Capacities of Insulated Jugs® 
a0 Authority - Letter Commanding General, Headquarters Armored 
Force, Fort Knox, Kentucky, 400o112,/6 GNOHD, dated September 24, 1942® 
b0 Purpose - To determine the heat-retaining capacity of an insula- 
ted food container constructed with Santocel, a new insulating material, 
instead of the cork employed in the containers previously tested® 
20 DISCUSSION: 
According to continued reports from combat areas there is need for 
special facilities for furnishing front line troops and casualties with hot 
food, since, in many cases, kitchen units have not been able to catch up with 
advance troops until late at night, if at all® This need applied especially 
to winter operations but exists also when only moderately low temperatures are 
encounteredo The most feasible method for motorized troops is to carry the 
hot food vdth them in insulated containers. Vacuum jugs, while extremely 
effective, are too fragile for practical use. Other means of insulation are 
available, however, which will provide sturdy containers for the purpose. An 
18-quart jug similar to one previously described and recommended* but with a 
new improved insulating material has been made available for test. The results 
of tests on this jug are to be found in the Appendix, 
3o CONCLUSIONS; 
a® Insulated jugs are effective in keeping food hot and provide an 
adequate means for the transportation of foods to front line areas® 
b0 The cooling rate of water in the subject container was found to be 
approximately 1,36 Btu/°F/hr® At this rate, even under extreme environmental 
conditions (-40°F and 12 mph), the "Santocel" insulated jug will keep food hot 
for periods up to eight (8) hours and warm for longer periods. 
c. The shape and size of the subject food container are not suitable 
for use in armored vehicles and an improved design is suggested. 
* See AiiRL Report, Project No® 1-20 "Study of the Heat Retaining Capacities 
of Insulated Jugs," dated December 9, 1942® 
1 ko RECOMMENDATIONS: 
ac That food containers with "Santocel" insulation and improved 
design be subjected to field tests0 
Submitted by: 
Steven M. Horvath, Captain, SnC 
Howard Golden, T/4 
APPROVED: 
WILLARD MACHLE, 
Colonel, Medical Corps, 
Commanding 0 
3 Incls. 
Incl. #1 - Appendix. 
Incl, if2 - Tables 1 & 2. 
Incl. #3 - Figures 1 & 2. 
2 APPENDIX 
The heat-retaining capacity of an 1B02 quart hot food container of im= 
proved construction from the standpoint of heat insulation* was measured 
under various combinations of environmental temperature and air movemento 
Before each test the container was pre-heated with boiling water for one 
half hour, at the end of which time the water was poured out and the con- 
tainer refilled to capacity with hot water for the test0 The drop in tempera 
ture of the water was then determined over periods of from seven to twelve 
hours, the temperature being measured by means of three thermocouples placed 
respectively near the bottom, center and top of the jug. Air and floor 
temperatures in the immediate vicinity of the food container were also deter- 
mined at intervalso Floor temperatures required some time before reaching 
equilibrium with air temperatures (See Table II), In certain experiments 
immediately after temperature readings were obtained the jug was shaken 
thoroughly and another set of measurements taken. No appreciable differences 
in temperature were observed0 j 
In Table I and II are summarized the pertinent data obtained from obser- 
vations made with environmental temperatures ranging from *92°F to -40°F, and 
with different degrees of air movement. In one pair of experiments, at an 
environmental temperature of approximately 90°, the rate of heat loss with 
the jug exposed to solar radiation was compared with the rate in the absence 
of sunshineo 
Theoretical 
For a given environmental condition, the rate of heat loss from an 
insulated food container varies with the resistance to heat flow through the 
insulated wall and with the heat capacity of the contents0 The drop in tem- 
perature with time may be expressed by the exponential equations- 
— -AlL-° t 
ws 
T « T0e 
Where: 
T = temperature of contents above environmental temperature 
at anytime, t0 
= initial temperature of contents above environmental 
temperature0 
t s time of observation in hours from initial observation0 
AH 35 overall rate of heat flow through food container, Btu/0F/hro 
W = weight of contents in pounds» 
S “ specific heat of contents0 
* Employing "Santocel" insulation; manufactured by Landers, Frary & Clarko 
Incl. $1 
1 This equation plots as a straight line on semi-logarithmic paper and when 
the ratio, T/T0, is plotted against time, the observations become independent 
of the initial temperature of the contents, and of the environmental tempera- 
ture, Hence for a given jug, with constant A H, all observations should fall 
on a single straight line, the slope of which is a function of the insulating 
value of the container and of the heat capacity of the contents, as indicated 
in the equation above„ 
The temperature measurements obtained during several tests plotted as 
straight lines on semi-logarithmic paper as shown for two tests in Figure I, 
The rates of cooling per unit temperature difference, obtained from these 
lines were not constant, as shown in Table I, but did not, however, vary 
systematically with wind velocity or environmental temperature,, The influence 
of floor temperature is much more important. Table II0 The partial elimina- 
tion of an avenue of heat loss by having floor temperatures higher than air 
temperatures results in a false impression of the insulative efficiency of 
the container. Experiments in which the jug was raised four inches off the 
floor so that temperatures were identical on all sides of the jug gave 
duplicate results on close agreement with the average of our series„ 
It is interesting to note that the rate of heat loss was not signifi- 
cantly changed at a given environmental temperature by increasing the wind 
velocity. This is a reflection of the high quality of insulation provided in 
the container. The overall resistance to heat flow from the inside of the 
jug to the surrounding atmosphere is a function of both the thermal resist- 
ance of the wall and of the surrounding air film, as indicated by the 
following equation 
A H = Ak. 
JL 4 JL 
C f 
Where j 
A = surface area of container, sq. ft0 
C - thermal conductance of wall, Btu/°F/sq0ft./hro 
f = surface air film conductance, Btu/°F/sq,ft,/hr 
k = constant 
The value of f varies from 1,65 for still air to 6,0 for moving air at 15 mph, 
approximately a fourfold increase. The value of C is, for the subject food 
container, approximately 0„3* Entering this value of C and the two values of 
f in the equation results in the following values forA-H:- 
Still Air, AH = 0,256 x Ak 
Moving Air, 15 mph, AH = 0.285 x Ak 
Thus, the effect of increasing the air movement to 15 mph is to increase the 
rate of cooling over the rate in still air approximately 7 percent. This 
change could not be detected by the methods of measurement employed in these 
tests. 
Incl, #1 
2 The average cooling rate for the subject food container was 1„36 Btu/°F/hr. 
At this rate it is capable of keeping foods above the minimum desired tempera- 
ture for periods of eight hours or more under extreme conditions of outside 
temperature and wind velocity. For, example, with an outside temperature of 
and 12 mph wind velocity and an initial temperature of the contents of 
the container of the temperature at the end of seven hours will be 138°F, 
which is the maximum desirable temperature for coffee. The temperature will 
not drop below 120°F until approximately 12 hours. In Fig. 2, the rate of 
cooling of the insulated container’s contents has been plotted for three 
environmental temperatures. It may be concluded from the tests that the heat- 
retaining capacity of this food container meets the practical requirements for 
field use by the Armored Command, The shape and overall design of the con- 
tainer, however, are not entirely acceptable, A food container which will be 
more suitable from the standpoint of conserving space and for general utility 
in armored vehicles have been developed by the Armored Medical Research Labora- 
tory and the plans are available for consultation and use by interested 
parties. An important feature of the new design is that multiple containers 
are included which will permit separation of rations for five men. The design 
of the cover has also been improved to insure greater efficiency of insulation. 
APPENDIX 
Inclo #1 
3 TABLE I 
Summary of Tests on Heat-Retaining Capacity 
of 
"Santocel" Insulated Food Container (1802 Qt) 
TEST CONDITIONS 
TEMP. OF CONTENTS 
A H 
Btu/°F/hr„ 
Air Tempo 
°F 
Wind 
Velocity 
Initial 
°F 
After 10 hrso 
492 
Sunshine 
Irregular 
178 
158oO 
lo060 
+90 
Cloudy 
Irregular 
194 
160 o 5 
1o568 
481 
Laboratory 
None 
200 
161,5 
1.568 
471 
Laboratory 
None 
192 o 5 
158oO 
1.356 
434 
None 
188 
147 
1.236 
- 7 
12 mph 
1/12 of time 
195 
137 
lo380 
- 8 
12 mph 
1/12 of time 
195 
131 
1.548 
- 9 
None 
171 
124 
1*244 
-18 
12 mph 
Continuously 
212 
143 
lo428 
-23 
12 mph 
1/3 time 
191 
125 
1.496 
-29 
12 mph 
Intermittent 
202 
149 
lo060 
-42 
12 mph 
Continuously 
199 
130 
1.356 
Average 
1o360 
♦ 
Inclo #2 
TABLE I TABLE II 
Summary of Tests on Heat-Retaining Capacity 
of 
nSantocelM insulated Food Container (18,2 Qt) 
TEST CONDITIONS 
TEMPo OF CONTENTS 
1 A H 
Btu/°F/hr. 
Day of 
Tost 
Air Tempo 
Op 
A °F 
(Air-Floor) 
Wind 
Velocity 
Initial 
°F 
After 10 Hrso 
°F 
1 
- 8 
21 
12 mph 
Intermittent 
194 
138 
Io220 
2 
-14 
1 
12 mph 
Intermittent 
194 
133 
lo375 
3 
-14 
2 
12 mph 
Intermittent 
194 
132 
lo400 
5 
-18 
9 , 
12 mph 
Intermittent 
194 
133 
lc2?2 
6 
-18 
0 
12 mph 
Continuous 
194 
130 
lo403 
8 
-19 
0 
5 mph 
Continuous 
192 
131 
1.435 
9 
-16 
0 
12 mph 
Continuous 
189 
128 
1.315 
Inclo #2 
TABLE II FIG. 1 
COOLING RATE OF INSULATED FOOD CONTAINER 
TYPICAL TEST RESULTS 
Tr = -19° F., CONTINUOUS WIND, 5 M.P.H. ' 
AH = 1.435 
Tr = -42° F, CONTINUOUS WIND, 12 M.P.H. 
AH = 1.356 
Inclo #3 
FIG. I FIG. 2 
RATE OF COOLING OF CONTENTS OF JUG 
AT DIFFERENT AIR TEMPERATURES 
(INITIAL TEMPERATURE OF CONTENTS 190° F) 
MAXIMUM COFFEE DRINKING TEMPERATURE 
Incl. #3 
FIG. 2