US1914207A - Heat insulator - Google Patents

Description

June 13 1933.

J. L. KNIGHT 1,914,207

HEAT INSULATOR Filed March 9, 1931 Invent or:

1m gniiht;

His Abtown e g.

Patented June 13, 1933 "um'rao srA'ras PATENT OFFICE I name 1.. KNIGHT, or 1mm, rmmmvam, assrenon *ro GENERAL ELECTRIC column, a coarona'r on or NEW YORK Application filed Karen 9,

My invention relates to heat insulators, more particularly to heat insulating structure of the cellular type. K

' In the insulation of cold surfaces, as in refrigeration, the best available heat insulating materials generally used for this purose may be divided into two classes, 1. e., oose fibrous materials, as mineral wool for example, and comparatively rigid materials, as pressed wood pulp or fibre board. In con-- nection with the loose fibrous materials, it should be noted that although the insulating efliciency is good when initially installed, the fibre tends to settle and pack, thereby reducing the insulating efliciency and leaving an air space above the fibre causin further lowering of the insulating efi'iciency y reason of convection currents within the air space.

This material is likewise comparativelyex, 7 pensive and retains its best insulating efliciency only when loosely packed.

Inthe class of comparatively rigid insulating materials formed as boards or blocks, an example of a well known material is a composition of mineral wool and asphalt. Since mineral wool, which is made by steam blastin blast furnace slag or fusible rocks, is britt e like spun glass and tends tosettle to a great extent, it"is mixed with a stiffening I binder, as asphalt, so that the composite insulating material may be formed into a block. The asphalt, however, decreases the insulating efficiency of the material, the conductivity constant K being approximately .35 3.5 B. t. u. per square foot per inch per degree F. per hour as compared with aproximately .30 for mineral wool. Another well known insulator in this class consists of a pressed wood pulp or fibre board which is. comparatively expensive as is the mineral wool and asphalt composition above referred to.

" Insulating blocks of cellular structure genorally in the form of built-up layers of air cells are known in the art and are comparatively inexpensive and easily constructed, but have, heretofore, involved the disadvantage that the insulating value was considerably less than in insulating materials having high enou h insulating efliciency to be considered satis actory for use in'refrigerator cabinets.

1981. Serial No. 521,184.

In the manufacture of refrigerator cabinets, it has been found that the greatest sulating materials. This is due to the fact that more efli cient insulators provide the necessary insulation for the cabinet while maintaining a minimum thickness of wall thereb reducing the cost of manufacture of the re rigerator cabinet.

A principal object of my invention is the provlsion of an improved and comparatively inexpensive heat insulator of the cellular type for cold surfaces which shall have an insulating efiiciency comparable to that of the best available surface heat insulators.

My invention will be more fully set forth in the following description referring to' the accompanying drawing, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to andforming a part of the specification.

Referring to the drawing, Fig. 1 is a perspeetive view, partly in section, of an insulator unit embodying my invention; Fig. 2 is an enlarged fragmentary View of the cellular structure of the insulator unit in Fig. 1, and Fig. 3 is a graph representing conductivity. Y

In the specific embodiment of my invention illustrated in Fig. 1,;the cellular heat insulating structure 1 comprises built-up layers of air cells 2 formed by stacking corrugated paper sheets 3 spaced by. fiat spacer or liner paper sheets 4. It will be apparent that the liner sheets may be dispensed with by arranging the corrugations transversely in alternate layers. This last named arrangement, however, involves increased cost of assembly ofthe insulating structure and does not constitute a preferred construction. It will likewise be noted in connection with the assembly that the corrugations of the stacked sheets are staggered for the purpose of preventing nesting or settling of the insulating structure into less space. In other words, referring to Fig. 2, theridge of a corrugation of one sheet is'arranged substantially to abut the ridge of a corresponding corrugation in an opposing sheet so that there isno tendency of the corru'gationsto nest.

The method of building up theiheatinsulating structure may comprise alternately stacking the liner andcorrugated.sheetswithin a suitable carton 5 to form a unitary insulatingblock, the sheets being either looselystacked within the carton or glued together with an adhesive, as silicate of soda or the like, for the purpose of obtaining greater rigidity. Y Another-method comprises aflixing.

a singleliner sheet to a corrugatedsheet and loosely ,stacking the cell layer structure so formed within the carton. For the purpose 7 of preventing. condensation and; accumulationof moisture within'the'insulating material causing a reduction of insulating efficiency due to the fact that a cold surface insulator is at a temperature lowerth-an that of the surrounding atmosphere, the carton. enclosing the cellular insulating structure is hermetically sealed and waterproofed in any suitable manner.

In a cellular structure of the type above described the transfer of heat therethrough at low temperatures takes place chiefly in two ways; first, by air convection currents in the air cells carrying the heat from one layer to the next, and second, by conduction through the solid material forming the cell Walls of the structure. Since stagnantair is 5 a heat insulator of high efficiency, the transfer of heat due to convection currents may be substantially eliminated by decreasing the cross section of the air cells so that the resulting high surface friction greatly restricts convection currents. The reduction .of heat transfer, however, has'not heretoforebeen accomplished to such a degree that this type of insulator favorably compares-withxother types of high grade cold surface insulators.

In prior types of built-up cellular; insu1ation,-, rigidityhas been considered i an essentialrequisite, the sheet material forming thecell .walls being fairly rigid and of such. thick ness as to decrease its insulating value considerably below that of the class of goodin-, sulators which [is required for economical construction of the refrigerator cabinet.

My invention comprises, amongother features, cell walls, particularly thoseextending in the direction of heat transfer, having comparatively hi h resistance to heat flow thereby greatly re ucing the amount of heat transferred by conduction. To this end'the cross sectional area of the cell wallsis limited to such adegree that the conductivity is com-' paratively small. This result is due to the factthat the cross sectional area-of the cell walls bears an inverse relation to the resistanceto heat flow longitudinally of the walls. Referring again to Fig. 2, itwill be noted that heat transferred by conduction transversely of the insulator blockmust flow edgewise through the cell walls formed by the corrugations, in order to pass from one liner sheet or cell layer to the next. Accordin ly, the resistance of the cell walls to heat ow determinesto a great extent the amountof heat transferred by conduction through the insulator. n

I have found that an order of thickness not exceeding 3.5 to 4 mils (.0035" to .OO L") of paper forming the air-cell walls results in a greatly increased insulating efliciency.

The lower limit of thickness is determined generally'fby the stock available having the mechanical requisites for this type of insulation. The type of paper used may be selected according to cost, an example being known in the trade askraft or sulphate, which is inexpensive but not too fragile in thin stock. A sulphate paper approximately. "2.5 mils in thicknessmay be readily obtained and has thefurther advantage that it does not deteriorate with age to an appreciable extent. I

The spacing of the liner sheets determining the cross-sectional area of the air cells likewise is a factor in obtaining high insulating efliciency. In this connection, reference is had to the graph, Fig. ,3 showing a relation cated at X on the graph, limits the convection currents so that a low conductivity constant is obtained. Itwill also be noted in comes appreciably greater than the'spacing at X and similarly that the conductivity also increases rapidly when the spacing decreases below a value giving approximately the same conductivity constant as that at X. A spaccognection with the graph that the conduc I 'tyincreases rapidly as the spacing being not xceeding .18 inches furthermore does not involve diflicult'ies in manufacture and assembly.

I have'incorporated in a refrigerator cabinet a cellular insulating structure of the type 'above' referred to having thin 'paperwalls approximately 2.5 mils in thickness and a liner spacingof about .14 inches, and found the conductivity constantof the insulation to be approximately .28, an increase of approxi-. mately 20 to 25 per cent in 'efliciency which was heretofore believed impossible in this 125.

type of insulation. Furthermore, this insulating 'value is equal to or better than those of well known high grade insulators.

It should be understood that my invention is not'limited to specific details of eonstr'uc tion and arrangement thereof herein illustrated, and that changes and modifications may occur to one skilled in the art without departing from the spirit of my. invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1: A heat insulator comprising a cellular structure forming multiple layers of air cells limited in cross section'substantially to prevent convection currents therein, the walls of said air'cells being composed of thin paper of the order of thickness not exceeding 4 mils. r

2. A heat insulator comprising a built-up 1 cellular structure forming layers of small air cells, the walls of said air cells extending substantially in the direction of heattransfer being composed of thin paper of the order of thickness not exceeding 4 mils.

3. A heat insulating cellular structure comprising a plurality of stacked corrugated sheets forming air spaces, said corrugated sheets being composed of thin paper of the order of thickness not exceeding 4 mils, the height of the corrugations in said corrugated sheets being not more than .18 inches and being not less than the height at which the conductivity'of the insulation is approximately the same as that having corrugations .18v inches in height.

4. A heat insulating cellular structure comprising built-up layers of liner and corrugated sheets alternately arranged, said liner and corrugated sheets being composed of thin paper of the order of thickness not exceeding 4 mils, the spacing of said liner sheets be- 1 ing not more than .18 inches and being not less than the spacing at which the conductivityof said structure is approximately the .same as that having said liner sheets spaced apart .18 inches. L

5. A heat insulating cellular structure comprising built-uplayers of liner and corrugated sheets of thin paper alternately arranged, said corrugated sheets being approximately 2.5 mils in thickness, the spacing of said lmers being approximately .14 inches.

6. A heat insulator comprising a cellular structure forming multiple layers of air cells limited in cross section substantially to prevent convection currents therein, the walls of said air cells being composed of thin paper 'within the range of thickness between 2.5

and 4'mils. p 7. A heat insulator comprising a cellular structure forming'multiple layers of air cells limited in cross section substantially to prevent convection currents therein, the walls of said air cells being composed of thin paper of approximately 2.5 mils in thickness. h Irawitness whereof I have hereunto set my an a v JAMES L. KNIGHT.

Images