Thermal Insulation with Aluminum Foil RALPH B. MASON, Aluminum Company of America, New Kensington, Pa.
HE principles governing heat flow by conduction, con- vection, and radiation have long been known, but T their application in the production of aluminum foil..
air cell insulation is a relatively recent development. By combining the low thermal conductivity of air with the low emissivity or radiating power of bright aluminum foil in a structure designed to minimize air convection currents, there has been introduced an insulation called Alfol, that l i a ~ met with commercial success (9, If).
Quiet air is one of the hest availilable mediums for prevent- ing heat transfer, and the practical problem resolves itself into a determination of the best structure for confining the air in cells so as to eliminate heat transfer by convection and radia- tion. The most practical structure for any purpose may he a compromise het,ween maximum thermal efficiency and cost.
on aluminum foi la i r eell insulation introduce a variety of experiments1 complications. After a careful survey of avail- able methods, the so-called guarded hot plate and the cold box methods were adapted to the problem a t hand. In the guarded hot plate method, a measured quantity of heat, developed electrically, is forced to flow through the insuls- tion under test. By maintaining a guard ring around the hot plate and at the same temperature, the lateral flow of the measured heat outsidc of the test area is prevented.
The guarded hot plate apparatus was constructed along the lines suggested by Van Dusen (fd) and the National Research Council (7). The faces of the hot and cold plates were made of heavy aluminum sheet. The hot plate, meas- uring 24 X 24 inches (61.0 X 61.0 em.) was so constructed that an inner square on each face, 16 X 16 inches (408 X
The p r e s e n t investigation has determined some of the funda- mental information necessary for the commercial design and utili- zation of this type of insulation.
There are two general nieth- ods of using aluminum foil for insulation. One is to provide a f r a m e w o r k which supports the aluminum foil and forms a series of air cells between the br ight foil s u r f a c e s ; in the other, the foil is first crumpled and then partially s t r e t c h e d so that the resulting wrinkles in the foil separate the sheets when they arc la id a g a i n s t each other and p r o v i d e t h e necessary separation and air cells.
METHong FOR DETERMINING THERMAL CONDUCTIYITY
T h e a c c u r a t e measurement of t h e r m a l conductivity is a difficult determination at hest, and thermal m e a s u r e m e n t s
Thermal insidation with a metal is made possible by laking advantage of the low thermal emissivity of aluminum foil and the low thmnal conductivity of air. The various factors deter- mining the qDiciency of aluminum foil-air cell insulation are analyzed, and the insulation values of a wriety of structwes determined experi- menlally in the guarded hot plate apparatus or the cold bos upparatus. I t is possible with this type of insulation practically to eliminate heal transfer by radiation and convection, and to up- prmch the insulating value of still air. Thp aluminum foil-air cell insulations of the plain air-cell type are found to be better than struc- tures with corrugated separators or crumpled foil. The best results are obtained when the distance between foils is from 0.25 to 0.33 inch (0.64 100.84cm.). The light weight ofaluminum
foil-air cell insulation and its excellent insuh- tion properties make it especially suitable for use in the transportation industry.
40.6 cm.), was isolated from the outer or guard portion by a '/e inch (0.32-cm.) slot. Alternat ing current was used to heat the hot plate, and the energy input WPS determined by means of a Weston wattmeter. The tem- p e r a t u r e s were measured by means of copper-constantan t h e r m o c o u p l e s . A Leeds & Northrup type K potentiometer a n d a sensitive galvanometer were used io conjunction with the tliermocouples.
The test panels were placed in the hot plate apparatus and the wliole assembly well insulated with alumina slag wool and cork board. The conductance was determined at three mean tem- peratures. About 24 hours were usually necessary for the ap- paratus to reach equilibrium for each p o i n t determined. The temperatures were determined by means of the thermocouples and the energy input read from
246 I 1\; D U S T R I A L A N D E N G I N E E R I Iu G C H E JI I S T R Y Vol. 23, No. 3
the wattmeter. All measurements made in the guarded hot plate apparatus and reported here were made upon panels placed in a vertical position in the apparatus.
A I R SPACE WIDTH- INCHES.
CONDUCTAKCE AND WIDTH OF SINGLE ALUMINUM FOIL-AIR CELL INSULA- TIONS AT Two MEAN TEMPERATURES
FIGURE 1. RELATIONSHIP BETWEEN
SPACIKG OF FOIL IN PANELS One of the objects of this investigation was to determine
the optimum spacing for air cells bounded by bright alumi- num foil which would give the maximum heat insulation for a given thickness.
A series of panels was prepared, each approximately 2 inches (5.08 cm.) in thickness, which contained two, three, four, five, six, seven, and eight air cells, respectively, bounded by aluminum foil. The panels were made 24 X 24 inches in area to fit the hot late apparatus, and were similar in construction except for t&e number of sheets of aluminum foil dividing the 2 inches of thickness into air spaces. The sheets of foil were separated and supported by strips of Masonite of the requisite thickness to give the desired number of air cells per 2 inches. The con-
ri . I5
'I ; .14 2 2 .'3 m
L L .I2
II 0 z 0 u .IO
60 80 100 120 I40 IN 180 200 MEAN TEMPT.
FIGURE 2. CONDUCTANCE VALUES OF ALUMI- NUM FOIL-AIR CELL INSULATIONS 2 INCHES
struction of the Masonite separator frames was somewhat com- plicated because of the fact that the guarded hot plate had a measuring area of 16 X 16 inches. The outer shell of each separator frame was made from strips of Masonite 1 inch (2.54 cm.) wide. Crosspieces of the same separator material I/? inch (1.27 cm.) wide were used to form an inner square 16 X 16 inches, which coincides with the hot plate in size and position and is the area of insulation actually under test. This central s uare was divided into four smaller cells, each of which was 7 3 7 4 X 7 3 / 4 inches (19.7 X 19.7 cm.). The purpose of the crosspieces in the center area was to support the aluminum foil and insure good contact with the hot and cold plates in the test apparatus. The aluminum foil was fastened to the separator frames with a rubber resin cement. The Masonite used in the inner square
(16 X 16 inches) or measuring area occupied about 6.15 per cent of that area. Only one height of air cell was measured in these experiments-namely, 7 3 / d inches.
Most measurements of air cell insulation reported in the literature have been made on single air cells. It is much easier to construct panels of this type, but the thermal insulation of narrow air cells is low, and the energy input to the hot plate is small for temperature differences of 10' or 20" F. (5" or 10" C.) between the hot and cold plates. It was also considered best to make the measurements with multiple air cells, since single cells are seldom used.
The first series of measurements with the 2-inch (5.08- cm.) panels but with varying numbers of air cells is shown in Table I. The thermal conductance of each panel was de- termined a t three different mean temperatures. The tem- peratures of the hot and cold plates are reported in the table, as a knowledge of these factors is essential in air cell insula- tion.
In thermal measurements of this type, the common prac- tice is to hold the cold plate constant and increase the tem- perature of the hot plate. It proved to be impractical to hold the cold plate a t the same definite temperature for all the experiments. It was also difficult to construct panels exactly 2 inches in thickness, and because of slight variations
3 Air Spaces
3 4 5 6 7 8 A I R SPACE W I D T H - INCHES
FIGURE 3. RELATIONSHIP BETWEEN CONDUCTANCE OF 2 INCHES OF ALU- MINUM FOIL-AIR CELL INSULATION AND WIDTH OF INDIVIDUAL AIR
in thickness the data are not directly comparable. To make comparisons easy, the data of Table I were first plotted for a fixed cold-plate temperature of 40" F. (4.44' C.). To do this, small corrections were applied to the measured con- ductances in order to adjust the data for a cold-plate tem- perature of 40' F. These corrections were small, being less than the probable error in thermal measurements of this type, except in the case of the insulations containing three air spaces per 2 inches, where there is considerable convection and where differences in temperature play a more important role. From these corrected values the conductances of single air spaces were calculated for mean temperatures of 100' F. (37.8" C.) and 150" F. (65.6" C.) as shown in Figure 1. For example, the average width of each air space in the in- sulation containing six air spaces per 21/8 inches (5.4 cm.) was 0.354 inch (0.899 cm.) (allowance was made for the aluminum foil). The conductance of this insulation a t a mean temperature of 100" F. was 0.100 B. t. u. (0.488 kg.- cal.), and the conductance of a single air cell would be 0.600 B. t. u. (2.929 kg.-cal.) a t the same mean temperature. The relationship between conductance and width of single alumi- num foil-air cell insulations a t mean temperatures of 100' and 150" F. is shown in Figure 1. In drawing smooth curves through the points, a minimum conductance is indicated as
I (2.54) 1 (2.64)
2J/n ( 5 . 3 2 )
I > & / , * (4.82)
. . 85.4 41.2 44.2 03.3 (17.4) 112.3 40.6 71.7 76.5 (24.71 132.3 40.3 92.0 86.3 (30.2)
. . . , , . , . 86.0 130.01
ciccurring fur air spaces betwen 0.6 tmd 0.7 inch (152 and 1.78 cm.) in t,hickncss. Tising approximately the same height air cell, Dickinson and Van Dusen (S) fonnd a mini- mum condiictance at ahout 0.03 inch (1.6 cm.).
Using the curves of Figure 1, a derived series of curves (Figure 2) w ~ s plotted for exactly 2 inches of insulation, and for the condition where the mold face of the insulation is st 40' F. For example, the. conductance of a single air space
inch in width a t 100' F. may be taken from the curve in Figure 1. The conductance of four air spaces (2 inches in total thickness) will be onefourth the conductance of a single air cell. The dotted portion of the curve for the insulation containing three air spaces is uncertain, since the conduo- tance measurements in this region were erratic, as a result, no doubt, of the increased effect of convection. As the num- ber of air spaces in 2 inches of insulation is increased from four to eight, a decrease in conductance is noted. The differ- ence bet&een the conduct- ance of the insulation con- taining seven air spaces and the one containing eight air spaces is small; the one ex- tra foil adds to the insula- tion value by decreasing slightly the small fraction of h e a t t ransmiss ion by radiation.
The optimum spacing for the air cells with a given number of aluminum foils would be about 0.6inch (1.5 em.), as is shown in Figure 1. However, where maxi- mum insulation must be obtained in any givenspace, t h e m i n i m u m conduct- ance is obtained by using more foils with closer spac- ing. Figure3, whichis from the data of Figure 2, shows the relationship between the conductance, at a mean temperature of 60" F , of 2 inches of aluminum foil-air
B . t. zi. (K#.-cd.) 0.142 (0.693) 0.144 (0.703) 0.145 (0.708) 0.302 (1.4741 0.419 (2.046) 0.600 (a.W211) 0.050 (3.173) 0.123 (O.RO0) 0.145 (0.70s) 0.15s (0.771) 0.112 (0.547) 0.117 (0.671) 0.130 (0.035) 0.110 (0.537) 0.1111 (O.SS1) 0.126 (0.615) 0.101 (0.493) 0.10S (0.527) 0.116 (0.566) 0.106 ( 0 . 5 1 7 ) 0.115 (0.561) 0.119 (0.5811 0.103 (0.503) 0.110 (0.537) 0.117 (0.571)
cell insulation and the width oS iridividual air cells. The in- sulation containing eight air cells per 2 inches gave the lowest value in this series OS measurements. If the conductance of 2 inches of insulation containing ten air spaces is calculated from Lhe curves of Figure 1, it will be found to have ap- proxirnatcly t.he same value as the one containing eight. air cells. Ilovever, no measurements were made in this region.
The curve shown in Figure 3 is similar to that obtained by Cregg (4), but the conductance values reported by him Sor aluminum foil-air cell insulation are slightly higher than those given here. However, the material used for the separator Srames in this investigation has a lower conductance than the wooden separator frames used by Gregg, and, if allowance is niade for this difference, the results check in a very satissactory manner. Most of the oonductance values reported by Queer (8) are somewhat hialter than the results
COLD Box APPARATUS
given here. The curves of Figures 2
and 3 show that the in- sulation with the '/Xnch (0.63-em.) spacing gave the lowest conductance of the in su la t ions measured. After allowing for the small amount of heat transferred by radiation, it was found that the ratio of heat trans- ferred to the conductance of still air was practically constant for 2 inches of in- sulation c o n t a i n i n g s ix , seven, and eight air spaces, which mould indicate that the effect of convection was constant and, as will he shown, probably negligible. The experiments of Creregg (4) have shown that the conductances of air cells inch (1.27 em.) in width do not vary, when measured in the vertical position and in a horizontal position. with the
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 25, No. 3
TABLE 11. THERMAL COXDUCTIVITY ASD CONDUCTANCE OF ALUMINUM FOIL AND PAPER INSULATIONS (Plain air-cell type)
TEMPERATURE -. Av. WIDTH TOT.AL THICK-
OF EACH A-ESS OF High Low Dif- PANEL DESCRIPTION OF INSULATION AIR SPACE INSULATION aide side ference Mean CONDUCTIVITY4 CONDUCTANCE
Inch (cm.) Inch (em.) F . O F . O F. O F . (" C.) Center baffled Masonite frames + black 0 . 3 4 (0 .86) 2 .09 (5.31) 105 .1 7 8 . 0 2 7 . 1 9 1 . 6 (33.1)
paper (0.0095-inch or 0.0241-cm.) : 6 1 6 6 . 8 7 8 . 2 8 8 . 6 122 .5 (50.3) air spaces 214 .4 7 8 . 8 135.6 146.6 (63.7)
C. B. Masoniteframes + aluminum- 0 . 3 4 (0 .86) 2 . 0 6 (5 .23) 120 .8 7 2 . 3 4 8 . 5 9 6 . 5 (35 .8) painted paper; 6 air spaces 181 .3 7 3 . 2 108 .1 127.2 (52 .9)
2 5 1 . 4 7 3 . 0 178 .4 162.2 (72 .3) C. B. Masonite frames + 0.0006-inch 0 . 3 4 (0 .86) 2 . 0 6 (5 .23) 133.2 7 6 . 7 5 6 . 5 104.9 (40.5)
(0.0015-c,m.) lacquered foil (thin coat- 198 .3 7 6 . 2 122.1 137 .3 (58.5) ing) : 6 air spaces 276 .5 7 6 . 1 2 0 0 . 4...