US2037592A - Refrigeration - Google Patents

Description

E, RICE, JR

April 14, 1936.

REFRIGERATION Original Filed July 7; 1955 Patented Apr. 14, 1936 UNITED STATES PATENT OFFICE REFRIGERATION Edward Rice, Jr., Croton-on-Hndson, N. Y., as

signer to International Carbonic, Inc., Wilmington, Del., a corporation of Delaware frigerant such as dry ice which is incorporated The metallic conductor.

in the same package. consists of a series of separated parallel pieces of a ductile good heat-conducting metal such as copper or aluminum of the maximum possible heat-conducting cross section and heat-absorbing surface-preferably in the form of round wires. The wires are embedded in a suitable carrying structure which can be easily bent to conform to the inside shape of the refrigerated package, for instance a double faced corrugated paper board which, with the embedded heat-conducting wires, can be used as a conductor liner for the refrigerated package. package is made up with the conductor liner suitably placed between the insulation of the package casing and the refrigerated contents. The dry ice is placed in conductive contact with a comparatively small section of the conductor liner, but preferably covering a portion of all the heat-conducting wires. The remaining portions of the wires absorb heat which leaks in through the package casing and conduct this heat along to the portions coveredby the dry ice, which portions in turn transmit the heat to the dry ice, thus preventing such heat from being absorbed by the refrigerated materials. This novel construction of the metallic heat conductor makes it possible to set up small inexpensive refrigerated shipping packages ofice cream and frozen foods in which the temperature differentials are reduced to a minimum, the product not being frozen too hard where near the refrigerant, and at the same time not being too soft in other locations. It also permits the use of dry ice in similar packages for unfrozen products like fresh meat and fish, without the risk of freezing part of the contents and spoiling the balance with too much heat. This novel construction also permits a certain amount of regulation of the temperature of the refrigerated products by the use of a conductor resistance to the heat flow between the conducting wires and the dry ice, a

In operation, the

regulation which has been heretofore unkno in packages similarly refrigerated.

An object of the invention is to provide a means for transferring heat to a solid refrigerant at a higher rate than provided in previously known methods, the refrigerant being used in the usual and ordinary processes of refrigerated storage 'and transportation of perishable foodstuffs.

Another object is to provide a means for transferring heat to small quantities of a solid refrigerant at the same rate as to comparatively large quantities, the refrigerant being usedin the usual and ordinary processes of refrigerated storage and transportation of perishable foodstuffs with the result that approximately, constant refrigerating temperatures can be maintained substantially as long as any of the refrigerant remains.

Another object is to provide a means for main taining substantially constant temperatures in the refrigeration of perishable, foodstuffs within a range of approximately F. to 50 F. when using solid refrigerants, such as solid C0: of a considerably lower temperature than that required in the refrigerating process.

More specifically an object is to provide a solid metallic heat conductor between the area or mass to be refrigerated and the solid refrigerant with sufiicient surface in a uniform or controlled heat exchange relation with the refrigerant, with sufficient surface in heat exchange relation with the refrigerated area or mass, and with suflicient heat transfer capacity or cross section to pick up and transfer to the refrigerant the maximum amount of heat as required to effect the desired refrigeration.

Figure 1 is a perspective plan view of a heatconducting liner constructed according to my invention; while Figs. 2 and 3 are vertical sections of refrigerated packages in which the conductor liner of Fig. l is incorporated.

During the past two summers I have designed and had constructed and commercially used a variety of improved forms of refrigerating apparatus for the storage and transportation of perishable foods, ice cream etc. which employ as the source of refrigeration solid refrigerants, such as solid carbon dioxide, in combination with an extended solid metallic heat conductor which is chilled either by direct contact with the solid refrigerant or by some other uniform or controlled heat exchange relationship with the refrigerant.

The characteristics of this metallic heat conductor must be such that (a) it forms a principal I path of heat transfer between the refrigerated area or mass and the solid refrigerant-4b) it has a surface area. in suitable heat exchange relationship with the space or mass refrigerated, this surface area being sufiiciently extended to provide the number of square feet of heat-absorbing surface required to absorb the heat removed from the refrigerated area, at the temperature differential ranges used in any given apparatus between the conductor surface and the refrigerated area(c) that it has a minimum cross sectional mass at any particular point sufficient to conduct in the direction of the solid refrigerant the heat absorbed by the surface extended beyond that point, at the temperature differential ranges used in any given apparatus between the heat-absorbing conductor surfaces and the conductor surfaces exposed to the solid refrigerant-(d) it has a surface area in suitable heat exchange relationship with the solid refrigerant, this surface always being smaller than the surface exposed to and absorbing heat from the refrigerated area.

This general method of using solid refrigerants is fully disclosed and claimed in my pending application Serial Number 467,999, filed July 14, 1930, and various forms of apparatus for carrying out'the method are there described. In this application I wish to disclose more specifically the principles of this method of refrigeration, to describe in more detail the characteristics of the solid metallic heat conductor which is the essence and concrete embodiment of the invention, and to claim improved forms of the conductor and ways of using it.

The use of this metallic conductor conforms to the usual and well known principles of heat absorption and emission by, and transfer through, a solid homogeneous material. For instance a certain number of heat units per hour are to be absorbed to maintain the required temperature in a household refrigerator, say equal to the meltage of one pound of water ice per hour. In using water ice for this purpose which melts at 32 F. a metallic conductor of copper or aluminum can be chilled to an average temperature say of 36 to 44 F. by direct contact with a surface of the ice. This conductor can in turn chill the air of the refrigerator to say an average of 47 F. in an 80 room. To accomplish this approximately 144 B. t. us. per hour must be transferred from the air of the refrigerator to the ice, or sufficient to melt one pound of ice per hour. Where air circulates by natural convection over a cooling surface under these conditions approximately 1 B. t. us. per hour per degree F. diiferential between the air and the cooling surface, per square foot of cooling surface can be absorbed. Therefore about fifteen square feet of chilled conductor surface is exposed to the air at an average temperature of 40 F., thus absorbing approximately 15 sq. ft. x 1 B. t. us. per hour 7 F. difierential-IS? B. t. us. per hour-this meets condition (b) above. Next a surface area of the conductor must be in direct contact (in the present instance) with the ice. This surface need be only a fraction of the surface absorbing heat from the air; and in this fact lies the great practical advantage in this method of refrigeration, as well as one of the principal objects of the invention, as it permits satisfactory refrigeration with small amounts of solid refrigerant. There is no exact physical data covering this heat transfer from the metal toa. solid refrigerant but experimental observation shows that it is from fifty to several hundred times as much per unit area per degree difference in temperature as from the air to the con ductor surface. For instance with a sufficiently thick conductor, say of aluminum, fifteen square inches of water ice contact with the conductor can absorb the heat that is picked up from the air by fifteen square feet of conductor surface exposed to convection currents in the form of fins-this corresponds to condition ((1) above.

With a given surface exposed to the air and a certain amount of ice available for minimum contact with the conductor then condition (0) above becomes an absolutely essential consideration in constructing a conductor for use in this method of refrigeration. After means are provided to get the heat into the metal conductor, by providing a necessary area of chilled surface, and to get the heat out of the conductor to the solid refrigerant, by providing an area of the conductor in uniform or controlled heat exchange relationship with the refrigerant, it is necessary to make the conductor sufliciently thick to transfer the maximum amount of heat to be taken out of any given refrigerated space or mass to effect the desired refrigeration under the most adverse conditions of actual use. This thickness is determined for the minimum temperature differential that it is feasible to use between the points where the heat enters and leaves the conductor, or between any two points along the conductor path. The rate per hour for this form of heat transfer is established for all the available metals and can be easily ascertained in the usual physics handbooks. Briefly for any given metal it varies directly as the cross section and temperature differential used. For instance, other features being equal, approximately twice as much heat will be transferred per hour by an aluminum plate thick as by one A;" thick for the same temperature differential; or approximately the same amount of heat will be transferred per hour by conductor plates, other factors being equal, made of A, copper, V aluminum, or iron. It is to be expected that these rules can not be very reliable for overall temperature differentials of less than 2 or 3 F. or more than 20 to 30; or for excessively long distances, say over 6 or 8 feet; or for excessively thin or thick conductors, say a conductive capacity of less than of aluminum or over of copper. But the conductor can always be constructed with enough surface and sufficiently thick to take care of extreme conditions of use, and once constructed its heat transfer properties do not change. And experimental observation and wide commercial use show thatfor the commonly available solid refrigerants such as water ice, brine ice and solid CO2; and for the cheaply available metals such as aluminum and copper, and to a lesser extent iron; and for ordinary commercial purposes in which it is common to use solid refrigerants, such as the transportation'and storage of perishable foodstuifs it is possible to construct refrigerating apparatus on the principles described above that is a great improvement over anything previously known in the art for usewith solid refrigerants.

In eflfect such apparatus provides a new means of transferring energy or power in the form of heat, and greatly enlarges the entire field of use for solid refrigerants as a class, as well as providing a means of temperature control in the use of solid CO2, often called dry ice, the newly developed solid refrigerant which is now coming into such wide commercial use.

In constructing refrigerating apparatus for solid refrigerants using this solid metallic heat conductor it is not necessary that all the heat taken from the refrigerated space or mass be passed through the conductor, but only as stated in condition (a) above that it provide a principal path of heat transfer; although in usual practice most of the heat is thus transferred. It is sufficient that the fixed heat absorbing surface and transfer capacity provided by the conductorbe large enough to take up the minimum amount of heat necessary to maintain a useful temperature when the volume of the refrigerant is considerably reduced-say to 5% or 10% of the bunker capacity. There will always be some heat get into the bunker space, and to the ice surface exposed there, through the walls of the bunker no matter how well insulated. I

Ways for controlling the heat transfer from the conductor to the refrigerant form probably the best method of controlling the temperature of the desirable large heat-absorbing conductor surface chilled only a few degrees below the refrigerated area, when using low temperature solid refrigerants like solid carbon dioxide. This method operates by controlling the spaced relation of the conductor and the solid refrigerant. When the ice is hard down on the bare conductor, a maximum heat flow results, and consequently the lowest refrigerating temperatures for a given apparatus. If the ice is held more than from the conductor, there is not much effective heat transfer. However, from the point of contact to about 2" away from the conductor provides a region where the surface of the solid CO: can beheld in a controlled heat exchange relationship with the conductor by which the temperature of the chilled conductor, and, there- I fore, the refrigerating temperatures, can be varied at will. The first break in the direct contact of the ice with'the conductor will result in a marked resistance to the heat flow; and every change of a fraction of an inch toward or away from the conductor will speed up or retard the heat fiow correspondingly. This action is in accordance with the ordinary physical laws of heat flow already referred to. Heat is accumulated by the conductor and brought by it to that part of its surface opposing the ice surface. The amount of this heat taken up by the ice varies directly as the temperature differential between the surface of the ice and the conductor surface opposed to it; this differential in turn flcient means for regulating the refrigerating temperatures. The first factor, or temperature differential, changes as a result of the variation of the other two factors and so need only be regarded as a corollary indication of what has taken place.

Thus it is the nature and amount of the substance between the ice and the conductor that governs in this method of temperature control. The rate of heat transfer varies with the substance. Also the rate will vary with the thickness or distance between the ice and the conductor-the further apart the less heat transmitted, and vice versa. The vsubstance can be of any convenient character, preferably a solid such as sheets of paper or cardboard.

Figures 1, 2 and 3 'show a form of conductor for use with dry ice which can be incorporated in a cheap throw-away transportation package made up of corrugated cardboard. The metal heat conductor forms part of a conductor liner which is illustrated in Fig. 1. It is essentially a single ply corrugated sheet faced on both sides with heavy paper. Inside running crossways and embedded 'in the corrugations, are a series of approximately ,4 aluminum or copper wires 'spaced'about 1" apart. The corrugations are cut and the wires laid in mechanically when the, liner is being manufactured and before the top facing of paper is put on. The drawing shows the paper face 54 turned back to show the wires 3 which. constitute the conductor. The liner is used inside a built up corrugated box as shown in Figures 2 and 3. Fig. 2 shows an arrangement for holding perishable foodstufis at above freezing temperatures. An outer cardboard casing 55 surrounds a series of corrugated boards I constituting the insulation. The corrugated-liner 3 is folded to surround the refrigerated space 2 on four sides as shown. Dry ice 4 is placed on top of the bottom conductor liner wall, the paper facing of the liner serving to reduce the rate of heat transfer between the ice and the metal conductor wires.

' If a further reduction is wanted additional sheets of paper or cardboard are placed between the ice and the conductor liner. Over the ice is an insulating shelf'56 formed of corrugated board. This shelf also supports the lading. A principal amount of the heat is picked up by the wires and conducted by them to the bottom surface of the ice. The refrigerating temperatures will therefore remain practically constant until the ice is all melted down. Fig. 3 shows a similar box arranged for temperatures below freezing. The conductor liner here need only form the ,top and two side walls of the compartment and the ice is placed on top of the conductor liner as shown. Several thousand of these boxes have been used with success commercially.

I claim:

1. In a refrigerated package for perishable foodstuffs, the combination with an insulating casing for said package, of a heat-conducting liner disposed around the foodstuffs within said casing, and a quantity of solid carbon dioxide in contact with a portion of said liner, the said liner being formed by a flexible carrying structure within which are embedded a series of parallel strips or wires of a good heat-conducting metal.

2. In a storage and shipping container, the combination of a paper-board container having a space therein to be refrigerated, a cake of carbon dioxide at one end of the container, and a series of spaced wires lying in proximity to the space to be refrigerated and' having portions thereof arranged to support the cake of carbon dioxide.

3. In a 'storage and shipping container, the

a combination of a paper-board container having a space therein to be refrigerated, a cake of carbon dioxide at one end of the container, and a series of spaced wires lying in proximity to the space to be refrigerated, said wires at least partially surrounding said space and being in heatconduct-ing relation with the cake of carbon dioxide.

4.In a storage and shipping container, the

combination of a paper-board container having a space therein to be refrigerated, a cake of dry ice in said container, and a series of spaced metallic conductors lying in proximity to the space to be refrigerated and having portions thereof arranged in heat-conductive relation with the cake of dry ice.

5. A storage and shipping container for perishable material, comprising an insualting casing, having a space to be refrigerated, a flexible liner having a metal of good heat conductivity embedded therein disposed around said space, and a quantity of solid refrigerant in heat-conductive relation with a portion of the metal of the metal of said liner.

6. A storage and shipping container for perishable material, comprising an insulating casing having an space to be refrigerated, a solid refrigerant in said casing and a plurality of spaced flexible wires of good heat conductivity disposed around said space and having parts thereof located between the refrigerant and the material to be refrigerated and inheat-conductive rela- 10 tion with the solid refrigerant.

EDWARD RICE, JR.

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