|Title of Invention||
A METHOD FOR PREPARING PHASE CHANGE-TYPE HEAT SINK MATERIAL
|Abstract||A method for preparing phase change-type heat sink material for a portable, single-use, refrigerator that cools by evaporation of a refrigerant and does not release gas or vapor to the atmosphere, the method comprising: (A) providing a quantity of solid phase change heat sink material that undergoes a volume increase upon changing from a solid phase to a liquid phase; (B) encapsulating said phase change-type heat sink material in, gas-impermeable capsules essentially free of air or other gases, said capsules containing void volume when the material is a solid, said void volume being sufficient to accommodate the volume increase upon melting without rupturing said capsules; and (C) placing the encapsulated phase change-type heat sink material in thermal contact with a sorbent material, wherein the sorbent material receives refrigerant vapor during operation of said refrigerator, subsequently evolving heat.|
|Full Text||FORM 2
THE PATENTS ACT 1970 [39 OF 1970]
[See Section 10]
“A METHOD FOR PREPARING PHASE CHANGNE TYPE”
TEMPRA TECHNOLOGY INC., of 6140 15th Street East, Bradenton, Florida 34203, United States of America
The following specification particularly describes the nature of this invention and the manner in which it is to be performed :-
The present invention relates to a method for preparing phase change-type heat sink material.
The invention relates to self-refrigerating devices employing evaporation/condensation processes. Specifically the invention relates to methods for the preparation of heat sink materials useful in such devices, and the heat sink materials resulting from these preparation methods.
Self-refrigerating devices are known in the art. These devices are designed to provide cooling without resort to external sources of cooling such as electricity, ice and the like. These devices can also be designed to be highly portable. Conveniently, they are designed to deliver cooling on a single-use basis, and are therefore disposable.
Many products, including liquid products, have more favorable properties when cold than when at ambient temperatures. Thus, cooling of these products to temperatures of between about 0°C and 20°C is desirable. Generally, such cooling is carried out by electrically-powered refrigeration units, or by means of a phase change material such as ice. The use of these units to cool such foods and beverages is not always practical because refrigerators generally require a source of electricity, they are not usually portable, and they do not cool the food or beverage quickly.
An alternate method for providing a cooled material on demand is to use portable insulated containers. However, these containers function merely to maintain the previous temperature of the food or beverage placed inside them, or they require the use of ice cubes to provide the desired cooling effect. When used in conjunction with ice, insulated containers are much more bulky and heavy than the food or beverage. Moreover, in many locations, ice may not be readily available when the cooling action is required.
Ice cubes have also been used independently to cool food or beverages rapidly. However, use of ice independently for cooling is often undesirable because ice may be stored only for limited periods above 0°C. Moreover, ice may not be available when the cooling action is desired
In addition to food and beverage cooling, there are a number of other applications for which a portable cooling device is extremely desirable. These include medical applications, including cooling of tissues or organs; preparation of cold compresses and cryogenic destruction of tissues as part of surgical procedures; industrial applications, including production of cold water or other liquids upon demand; preservation of biological specimens; cooling of protective clothing; and cosmetic applications. A portable cooling apparatus could have widespread utility in all these areas.
Most attempts to build as self-contained miniaturized cooling device have depended on the use of a refrigerant liquid stored at a pressure above atmospheric pressure, so that the refrigerant vapor could be released directly to the atmosphere. Unfortunately, many available refrigerant liquids for such a system are either flammable, toxic, harmful to the environment, or exist in liquid form at such high pressures that they represent an explosion hazard in quantities suitable for the intended purpose. Conversely, other available refrigerant liquids acceptable for discharge into the atmosphere (such as carbon dioxide) have relatively low heat capacities and latent heats of vaporization. As a result, some cooling devices which release carbon dioxide are more bulky than is commercially acceptable for a portable device.
An alternate procedure for providing a cooling effect in a portable device is to absorb or adsorb the refrigerant vapor in a chamber separate from the chamber in which the evaporation takes place. In such a system, the refrigerant liquid boils under reduced pressure in a sealed chamber and absorbs heat from its surroundings. The vapor generated from the boiling liquid is continuously removed from the first chamber and discharged into a second chamber containing a desiccant or sorbent that absorbs the vapor.
Summary of the Invention
The invention provides methods for the preparation of heat sink materials used in
evaporation/condensation-type self-refrigerating devices, and heat sink materials which "are produced with these methods. The invention is born out of the requirement for high efficiency heat transfer from sorbents found in such devices to the heat sink material. The refrigerant devices require economical, mass production of vacuum-impermeable, encapsulated phase change material of similar size, shape and density to the sorbent material used in such devices. The invention provides a method for a large scale manufacturing of a heat sink material within the needs and constraints of refrigeration units substantially as previously described in U. S. Patent Application Ser. No. PCT/USOO/04639 (publication number WO 00/50827) entitled "Dispersion of Refrigerant Material", and U. S. Patent Application Ser. No. PCT/US00/04634 (publication number WO 00/50823), entitled "Preparation of Refrigerant Materials".
In one aspect, the invention provides a method for preparing heat sink material for a
portable, single-use, non-releasing evaporation-type refrigerator that produces a
refrigerant vapor during evaporative heating. The method includes providing a quantity
of phase change heat sink material (which can under go a phase change at a temperature
of, for example, between about 50°C and about 75°C having a solid density, and which
undergoes a volume change (for example, between 10 and 25%, or between 12 and 20%)
upon it phase change, adjusting the density of the heat sink material to a new density
which is less than the solid density by a percentage, which is within 2% of the percentage
volume change experienced by the phase change material when it melts. The method
further includes encapsulating the heat sink material at the new density, for example in a
metallic layer, for example, between 0.0005 and 0.002 inches thick, or a polymeric layer,
for example, carried out by a radiation-induced polymerization, such as a UV-light
induced polymerization, for example, with light having an energy in the range of from
about 280 nm to about 420 nm. This type of
encapsulation can be carried out using UV-curable epoxy resin-based monomers. The encapsulation can be accomplished by placing the phase change material in a substantially gas-and fluid-tight capsule. The encapsulated heat sink material is then placed in thermal contact with a sorbent material, which receives refrigerant vapor during operation of the refrigerator, subsequently evolving heat. The phase change
material can be, for example, paraffin hydrocarbons, sodium acetate trihydrate, sodium thiosulfate pentahydrate, and disodium phosphate dodecahydrate.
In another aspect, the invention provides a method for preparing heat sink material for a portable, single-use, non-releasing evaporation-type refrigerator that produces a refrigerant vapor during evaporative heating. The method includes providing falling drops of a phase change-type heat sink material from a drop tower apparatus, passing these falling drops through a volume of IN-curable monomer, providing UV-light of energy and intensity sufficient to cure the UV-curable monomers, so as to produce a polymer-encapsulated falling drop of heat sink material, breaking the fall of the polymer-encapsulated heat sink material, placing the polymer-encapsulated heat sink material in thermal contact with a sorbent material. The sorbent material is designed to receive refrigerant vapor during operation of the refrigerator, subsequently evolving heat.
In another aspect, the invention provides a method of cooling a product with a portable, single-use, non-releasing evaporation-type refrigerator that produces refrigerant vapor during evaporative heating. The method includes providing a portable, single-use, non-releasing evaporation-type refrigerator including an evaporator chamber in thermal contact with a product to be cooled, wherein the evaporator chamber comprises a refrigerant, dispersed in intimate contact with a refrigerant dispersant, an evacuated sorber in thermal contact with a solid phase change-type heat sink material, wherein the sorber comprises a sorbent, wherein the solid phase change type heat sink material is encapsulated at a density which is a percentage less than its solid density, wherein the percentage is within 2% of the percentage change experienced by the phase change material upon phase change, a means for preventing refrigerant vapor flow between the evaporator chamber and the sorber, until operation of the device; operating the means for preventing refrigerant vapor flow, thereby permitting the flow, whereby the pressure in the evaporator chamber is reduced, causing the refrigerant to vaporize and form a refrigerant vapor, the vapor collected by the sorbent material in the sorber, and heat is generated in the sorbent, removing the vapor from the evaporator chamber by collecting the vapor until an equilibrium is reached, wherein the sorbent is substantially saturated or
substantially all the refrigerant has been collected in the sorbent material, and containing the heat generated in the sorbent within the sorber by means of the phase change-type heat sink material.
The invention provides a self-contained and disposable refrigeration device. The device according to the invention does not vent a gas or vapor of any kind. There are no hazardous or toxic materials or components included in the device, and recycling of the materials of the device is facilitated. There are no pressurized gases present in the device and no enviromentally objectionable materials such as unstable refrigerants. The device does not explode, even when consumed by fire, and is not flammable.
As used in the specification, the term "solid density" refers to the density a single crystal of a solid material would have, if the material is crystalline. In other instances, the solid density refers to the density an ideally compacted sample of the solid would have. As used in the specification, the terms "polymeric", "polymers" and "polymerization" include the corresponding materials and methods using block and random copolymers.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the present invention, suitable methods and materials are described below. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the
following detailed description, and from the claims.
Brief Description of the Accompanying Drawings
Fig.l is a schematic diagram of a refrigeration device useful in certain
embodiments of the invention.
Fig. 2 is a schematic diagram of evaporation and cooling processes occurring at the evaporation chamber during operation of a particular embodiment of the refrigeration device.
Fig. 3 is a perspective view of a circular arrangement of evaporator fingers which can be used in particular embodiments of the invention.
Fig. 4 is a perspective view of a concentric circular arrangement of evaporator fingers which can be used in particular embodiments of the invention.
Fig. 5 is a perspective view of a cruciform arrangement of evaporator fingers which can be used in particular embodiments of the invention.
Fig. 6 is an overhead view of a particular embodiment of a sorber employing a rib and arch, self-supporting structure.
Detailed Description of the Invention
The self-refrigerating device used in the present invention includes three basic sections: an evaporator chamber containing a refrigerant, an evacuated sorbent chamber containing a sorbent and a heat sink material, and a means to prevent the flow of refrigerant vapor between the evaporator chamber and the sorbent chamber. This flow-preventing means is also adapted to allow the flow of refrigerant vapor between the evaporator and sorbent chambers, such as when the device is in operation. The functional relationships between these sections in a particular refrigeration device have been roughly described in U. S. Patent Nos. 5,197,302 and 5,048,301. The inventive devices are generally utilized in conjunction with a product to be cooled. These products and associated uses will be detailed after discussion of the device itself, which follows directly below.
Regarding Fig.l, a particular embodiment of refrigeration device I according to the general principles of the invention is displayed. This view shows product 5, which is to
be cooled, in contact with evaporator 10. Evaporator 10 comprises a chamber within which evaporation of a refrigerant takes place. This generally involves desorption of refrigerant from a surface during the operation of the device. Before the device is activated, the refrigerant is present in the evaporator, both in liquid and vaporous states. In devices such as the present invention, this desorption is driven by a pressure differential which is manifested when a flow-preventing means 44 is operated. Thus, activation of the device amounts to allowing refrigerant vapor flow. Adsorption takes place from inner surface 12 of evaporator chamber 10, the outer surface 14 becomes cold. This in turn is able to cool product 5 in thermal contact with outer evaporator surface 14. This is represented in Fig. 2, showing the desorption of refrigerant (H2O) proceeding in direction 18 leading toward lower pressure. This lower pressure is exposed to the refrigerant upon operation of the refrigeration device, as explained herein.
A wide variety of refrigerants are operative in the device. The general requirements are that the refrigerants be vaporizable and condensable at pressures which can be relatively easily attained in chambers. The refrigerant must also be compatible with the sorbent, that is, it must be capable of being absorbed or adsorbed by the sorbent. Suitable choices for refrigerants must also be those which are able to produce a useful change in temperature in a short time, meet government safety standards, and be relatively compact. The refrigerants used in the devices of the present invention preferably have a high vapor pressure at ambient temperature, so that a reduction of pressure will result in a high vapor production rate. The vapor pressure of the refrigerant at 20°C is preferably at least about 9 mmHg. Moreover, for some applications (such as cooling of food products), the refrigerant should conform to applicable government standards in case any discharge into the surroundings, accidental or otherwise, occurs. Refrigerants with suitable characteristics for various uses of the invention include: various alcohols, such as methyl alcohol and ethyl alcohol; ketones or aldehydes, such as acetone andacetaldehyde; ammonia; water; short chain hydrocarbons and short chain halo-hydrocarbons; and freons, such as freon C318, 114, 21, 11, 114B2, 113 and 112. A preferred refrigerant is water.
In addition, the refrigerant may be mixed with an effective quantity of a miscible nucleating agent having a greater vapor pressure than the refrigerant to
promote ebullition so that the refrigerant evaporates even more quickly and smoothly, and so that supercooling of the refrigerant does not occur. Suitable nucleating agents include ethyl alcohol, acetone, methyl alcohol, propyl alcohol and isobutyl alcohol, all of which are miscible with water. For example, a combination of a nucleating agent with a compatible refrigerant might be a combination of 5% ethyl alcohol in water. The nucleating agent preferably has a vapor pressure at 25 C of at least about 25 mm Hg. Alternatively, solid nucleating agents may be used, such as the conventional boiling stones used in chemical laboratory applications.
The desorption processes taking place in the evaporator chamber are most efficiently carried out if the layer of refrigerant is as thin as possible, to the limit of a monolayer of refrigerant spread over as much of the inner desorption chamber surface as possible. These thin films maximize the area for surface evaporation. Multiple layers of refrigerant cause heat transfer through layered refrigerant molecules to a refrigerant molecule which is disposed at the innermost surface of the evaporator. This type of refrigerant overloading results in a temperature difference across the refrigerant layer that is larger than would exist if the layer were thinner. Thus, overloading decreases heat conduction, reducing the efficiency of evaporation. In preferred embodiments- with thin layers of refrigerant, the layer thickness is reduced as the refrigeration device operates, decreasing the temperature difference across the layer, improving heat conduction processes as the refrigerator operates. If a refrigerant dispersant is employed, this is also desirably layered as thinly as possible across as much of the internal evaporator chamber surface as possible.
Fig. 2 shows the desorption of refrigerant (H2O) proceeding in direction 18 leading toward lower pressure. This lower pressure is exposed to the refrigerant upon operation of the refrigeration device, as explained herein. The particular embodiment illustrated in Fig. 2 uses water as the refrigerant, but the principles discussed will be applicable to vaporizable refrigerants in general.
The refrigeration devices according to the invention contain a fixed amount of non-circulating refrigerant. If the amount of product to be cooled and the amount of cooling desired are known, the amount of heat to be removed is easily calculated. The amount of heat to be removed specifies precisely the amount of refrigerant which
must be evaporated from the evaporator chamber. For example, if 8 fluid ounces (236 mL) of an aqueous liquid is to be cooled by 22°C, about 8.9 grams of water refrigerant is needed as a theoretical minimum. If heat leaks back into the system, more refrigerant will be required.
As mentioned above, the refrigerant desirably forms a layer on inner surface 12 of evaporator 10. This layer of refrigerant is preferably substantially evenly distributed over as much of surface 12 as possible. In certain embodiments of the invention, such as the one shown in Fig. 2, this will be accomplished with the aid of refrigerant dispersant 16, which is preferably deposited in a layer on inner evaporator chamber surface 12, and covers as much of this surface as possible. The layer of dispersant is adapted to allow refngerant to be absorbed into and/or adsorbed onto it. A variety of materials are available as refrigerant dispersants, as detailed in U. S. Patent Application Ser. No. (PCT/US00/04639 (publication number WO 00/50827) entitled" Dispersion of Refrigerant Materials". In such an arrangement, heat flows from the product across the wall of the evacuator chamber, across a layer of refrigerant dispersant, and then vaporizes the surface refrigerant molecules from the dispersant.
In selecting the refrigerant dispersant, any of a number of materials may be chosen, depending upon the requirements of the system and the particular refrigerant being used. The refrigerant dispersant may be something as simple as cloth or fabric having an affinity for the refrigerant and a substantial wicking ability. Thus, for example, when the refrigerant is water, the refrigerant dispersant may be cloth, sheets, felt or flocking material which may be comprised of cotton, filter material, natural cellulose, regenerated cellulose, cellulose derivatives, blotting paper or any other suitable material. It is important that the refngerant dispersant be able to be applied to a surface which is highly thermally conductive, such as a metal-containing surface.
The most preferred refrigerant dispersant would be highly hydrophilic, such as gel-forming polymers which would be capable of coating the interior surface of the evaporation chamber. Such materials are recited, and methods for their preparation are given in U. S. Patent Application Ser. No., (PCT/US00/04639 (publication number WO 00/50827) entitled "Dispersion of Refrigerant Materials", filed contemporaneously
The refrigerant dispersant may be sprayed, flocked, or otherwise, coated or applied onto the interior surface of the evaporator chamber. In a preferred embodiment, the refrigerant dispersant electrostatically deposited onto that surface. In another embodiment, the refrigerant dispersant is mixed with a suitable solvent, such as a non-aqueous solvent, and then the solution is applied to the interior surface of the evaporator chamber.
In another preferred embodiment, the refrigerant dispersant is able to control any violent boiling of the evaporator and thus reduces any liquid entrainment in the vapor phase. In such an embodiment, the refrigerant dispersant is a polymer forming a porous space-filing or sponge-like structure, and it may fill all or part of the evaporator chamber.
In the particular embodiment shown in Fig.l, evaporator 10 has fins 20 and a central passage 22, although a wide variety of shapes and configurations of the evaporator are possible. If fins are used, they can be of a large variety of configurations, and the central passage maybe omitted or substantially shortened. In other embodiments, evaporator 10 takes the form of a number of hollow finger-like elements (fingers 24) which do not branch from a central passage as do fins 20, but pass into finger base 26, shown in Figs. 4-6. Base 26 can contain short passages (not shown) to connect the interior of hollow fingers 24 together to form a short central passage. Alternatively, base 26 can be substantially hollow, with a central outlet leading to the means for preventing/allowing vapor flow to the sorbent chamber. Fingers 24 can be arranged in a circle (eight fingers are shown in this arrangement in Fig. 3, but any number could be so arranged), a number of concentric circles (shown in Fig. 4), in a cruciform arrangement (shown in Fig. 5), or a more random arrangement. The general aim is to provide for efficient heat transfer from the bulk medium to inner evaporator 12, by maximizing the area of this surface. The evaporator is desirably also reasonably simple to manufacture and assemble. Additionally, refrigerant vapor flow paths inside the evaporator chamber are desirably adequate to prevent excessive pressure drops in the low density vapor flows.
Normal boiling processes (ebullition), which are initiated by streams of tiny bubbles rising from discrete and easily visible spots on surfaces, require nucleation sites consisting of reentrant cavities containing non-condensable gases such as air. The evaporator chamber in refrigerators according to the present invention is subjected to partial evacuation, effectively removing nucleation sites from the internal surfaces of the evaporator chamber, and degasses the refrigerant as well. Thus, refrigerant molecules subjected to the evacuator chamber preparation methods (as detailed in U. S. Patent Application Ser. No. (PCT/US00/04639 (publication number WO 00/50827), entitled" Dispersion of Refrigerant Materials"), which can also be used in refrigerator devices of the present invention, when exposed to the reduced pressure present in a properly prepared sorbent chamber (as discussed below) evaporate from the surface of a quiescent pool of refrigerant. Heat transfer in such a pool is subject to the same limitations of conduction and convection as in bulk fluids.
The refrigerant vapor pressure within the evaporator chamber at the beginning and end of the cooling process can be determined from the equilibrium vapor pressure temperature function for water, based on the expected beverage temperatures and temperature differences required for heat transfer.
It is desirable to carry out an evacuation of the refrigerant-loaded evaporator chamber
prior to assembly. The evacuation should be limited to pressures above or equal to the
vapor pressure of water at the temperature at which the evacuation is carried out. For
example, at room temperature with water as the refrigerant, the
evacuation of the refrigerant-loaded evaporator should be carried out to pressures of
about 20 Torr. This evacuation serves to sweep contaminants such as air, wash
solvents and the like from the evaporator chamber.
Returning to Fig. 1, there is also shown sorber 30. This section of the refrigeration device includes sorbent 32, which is disposed throughout the interior of sorbent chamber 34. Also included in sorber 30 is heat sink 40. Refrigerant vapor which is formed upon operation of the refrigeration device moves from the evaporator chamber into sorbent chamber 34, carrying heat. This heat is deposited into finite capacity sorbent 32, and further deposited into finite capacity heat sink 40.
The sorbent receives heat not only from the latent heat of vaporization resulting from condensation of the refrigerant vapor, but also from the chemical reaction heat released when refrigerant is combined with the sorbent. Sorbent 32 is in thermal contact with heat sink 40, via internal surface 36 and external surface 38 of sorbent chamber 34. This thermal contact desirably results in highly efficient heat transfer from sorbent 32 to heat sink 40. This heat must be stored in the heat sink in such a manner that it does not leak back into the product during the time that cold product is required.
Gas molecules tend to adhere to surfaces. Sorbent materials can have porous structures with a very large surface area per unit volume. The volume of noncondensable materials becomes significant in systems requiring final pressures below 220 to 500milliTorr. As an example, a container filled with molecular sieve (a typical sorbent) can be evacuated at room temperature to a pressure of from about 1 to 5milliTorr day after day, but will rise in pressure over a few hours to as much as 500milliTorr between serial evacuations. This rise is attributable to the gradual desorption of sorbed gas molecules. It is unlikely that an economical high production rate refrigeration device could incorporate such a process in its manufacture. Since the sorption process in the sorbent acts as a pump to draw vapor from the evaporator during operation of the device, the refrigerant vapor pressure over the sorbent must at all times be well below the equilibrium saturation pressure of refrigerant in the evaporator. Essential to the usefulness of sorbents in the refrigeration devices discussed herein is the removal of non-condensable gases from the refrigeration system. The presence of non-condensable gases must be avoided anywhere in the system, for such gases are carried by the flowing refrigerant vapor into the sorbent, or could be already present in the sorbent. The presence of non-condensable gases forms a barrier through which refrigerant vapor must diffuse before it can condense. If such gases are present, the refrigeration device will operate at a rate which is limited by the diffusion barrier.
In a similar way, the sorbent must be made as free of condensable gases as possible before the device is operated. The volume of the sorbent is desirably minimized for some preferred embodiments of the invention. Thus, competition
between refrigerant and a condensable gas already present in the sorbent will also limit the operation of the refrigeration device to levels below optimum performance. Methods for preparing various sorbents for use in refrigeration devices are detailed in U. S. Patent Application Ser. No. (PCT/US00/04634 (publication number WO 00/50823), entitled "Preparation of Refrigerant Materials", filed contemporaneously herewith.
The sorbent chamber into which the sorbent is to be loaded also includes a heat sink material. The function of the heat sink material is to absorb heat released by the sorbent, and to prevent leakage of this heat back to the product which is to be cooled by the refrigeration device. Thus, it is critical to maximize the thermal contact between the sorbent and the heat sink material.
Materials which are suitable as sorbents are those which have aggressive refrigerant vapor-binding properties, low chemical reaction heats, and are not explosive, flammable or toxic. These materials can be available in a variety of forms, including flakes, powders, granules, as well as supported on inert shapes or bound with clay. It is desirable that the material have sufficient vapor flow passages through it that refrigeration performance is not limited by the passage of refrigerant vapor through the sorbent. Additionally, the sorbent must be able to transfer heat to the heat sink material, and thus be in good thermal contact with the inner surface of the sorbent chamber. Preferred sorbents for use in the present refrigeration device include flaked sorbent or clay-supported sorbent. The latter is available in a wide variety of shapes, including spheres, chips, rectangular solids.
The sorbent chamber into which the sorbent is to be loaded also includes a heat sink material. The function of the heat sink material is to absorb heat released by the sorbent, and to prevent leakage of this heat back to the product which is to be cooled by the refrigeration device. Thus, it is critical to maximize the thermal contact between the sorbent and the heat sink material. This can be accomplished by ensuring that sorbent is in good physical contact with the inner surface of the sorbent chamber.
Synthetic zeolite materials comprising metallic alumino silicates can be used in the present refrigeration devices. These materials include a water absorbing material
supported by a porous insert clay. Such materials must be heated to drive absorbed and absorbed water from them. The amount of sorbent required to absorb refrigerant vapor depends on the sorption capability of the sorbent for the refrigerant vapor. This is generally a function of temperature.
The refrigeration device of the present invention also includes a heat sink located in the sorber. The heat sink is in thermal contact with the outer surface of the sorbert chamber, and thus is in thermal contact with the sorbent.
The heat-removing material may be one of three types: (1) a material that undergoes a change of phase when heat is applied; (2) a material that has a heat capacity greater than the sorbent; or (3) a material that undergoes anendothermic reaction when brought in contact with the liquid refrigerant.
Suitable phase change materials for particular applications may be selected from
paraffin, naphthalene, sulphur, hydrated calcium chloride, bromocamphor, cetyl alcohol,
cyanimede, eleudic acid, laurie acid, hydrated sodium silicate, sodium thiosulfate
pentahydrate, disodium phosphate, hydrated sodium carbonate, hydrated calcium nitrate,
Glauber's salt, potassium, sodium and magnesium acetate, including hydrated forms of
these materials, such as sodium acetate trihydrate, and disodium phosphate
dodecahydrate. The phase change materials remove some of the heat from the sorbent
material simply through storage of sensible heat. In other words, they heat up as the
sorbent heats up, removing heat from the sorbent. However, the most effective function
of the phase change material is in the phase change itself. An extremely large quantity of
heat can be absorbed by a suitable phase change material in connection with the phase
change (i. e., change from a solid phase to a liquid phase, or change from a liquid phase
to a vapor phase). There is typically no change in the temperature of the phase change
material during the phase change, despite the relatively substantial amount of heat
required to effect the change; which heat is
absorbed during the change. Phase change materials which change from a solid to a liquid, absorbing from the sorbent their latent heat of fusion, are the most practical in a closed system. However, a phase change material changing from a liquid to a vapor is also feasible. Thus, an environmentally-safe liquid could be provided in a separate container (not shown) in contact with the sorbent material (to absorb heat therefrom)
but vented in such a way that the boiling phase change material carries heat away from the sorbent material and entirely out of the system.
Another requirement of any of the phase change materials is that they change phase at a temperature greater than the expected ambient temperature of the material to be cooled, but less than the temperature achieved by the sorbent material upon absorption of a substantial fraction (i. e., one-third or one-quarter) of the refrigerant liquid. Thus, for example, in most devices according to the present invention which are intended for use in cooling material such as food or beverage, the phase change material could change phase at a temperature above about 30°C, preferably above 35°C, but preferably below about 70°C, and most preferably below 60°C. Of course, in some applications, substantially higher or lower phase change temperatures may be desirable. Indeed, many phase change materials with phase change temperatures as high as 90°C, or 110°C may be appropriate in certain systems.
Materials that have.a heat capacity greater than that of the sorbent simply provide a thermal mass in contact with the sorbent that does not effect the total amount of heat in the system, but reduces the temperature differential between the material being cooled and the sorber, with two results.
When heat is added to a material which doesn't melt or evaporate as a result of that heat addition, the heat can be sensed by an increase in temperature. By contrast, if the material undergoes a phase change, from solid to liquid for example, the material can absorb heat without a sensible temperature change. The heat energy instead goes into the phase change of the material. The hidden heat is referred to as latent heat. Heat sink materials useful in the present refrigeration device are all melting materials, they absorb significant latent heat, and are able to keep the sorbent at a more even temperature. The cooler the sorbent, the more vapor it can condense, so it is the combined volume of heat sink and sorbent that is of direct interest. A low density material and a high density material may, in principle, has equal total heat capacity, but a refrigeration device utilizing the low density material will require more volume. This increased volume can be undesirable in certain critical applications.
Some of the heat sink materials considered useful in the present invention will undergo a volume change upon phase transition. For example, a solid phase change type heat sink material can change in volume by anywhere from 1% to about 30%.
For most materials this change in volume is an increase experienced upon phase change. This can present a problem if encapsulation of such materials is desired. The capsules must either be flexible enough to maintain their structural integrity, or a dead volume must be left in place so that expansion will not rupture a more rigid capsule. One way to overcome this problem is to change the density of a solid phase change type heat sink material by an amount which is roughly equal to the volume change experienced by the phase change material upon its phase change. For the refrigeration devices of the present invention, as heat is transferred from sorbent to the solid phase change-type heat sink material, the heat sink material melts. Some of the heat sink materials useful in the present invention experience a volume increase upon melting. For example, sodium acetate trihydrate undergoes an approximately 17% volume increase upon melting. By decreasing the density (e. g., by introducing void volume within the solid) of the solid sodium acetate trihydrate by approximately 17% before encapsulation, the volume will not change upon melting, and minimal pressure will be exerted upon the capsule. Slight variations in this density change will be tolerated to the extent that the capsule can withstand a volume change of its contents.
The density can be changed by applying large compacting forces to the material. For example, a pressure of up to 24,000 pounds can be applied to a compacted granular heat sink material to increase its density to the desired value, which, in the case of sodium acetate trihydrate, is about 83% of the solid density (density of a crystal of the heat sink material, with no void volume). Coupons or pills of sodium acetate trihydrate, when compressed to this density, appear solid and are strong enough to be easily handled for further processing.
Encapsulation of the heat sink material must be carried out to exclude contaminant materials, such as air or other gases, or water which can degrade the performance of the heat sink. The encapsulation can be carried out by a number of methods including roll coating, metal foil barriers, spray coating, and other methods known to those skilled in the art.
One encapsulation method which is useful for the encapsulation of heat sink materials makes use of a drop tower and W-curable monomers. The result of this process is a polymer-coated heat sink material. Heat sink material is partitioned into particles such as flakes, crystals, pellets or drops and placed at the top of a drop tower. The individual particles are passed, or allowed to fall, through a solution of UV- curable monomer, so as to entirely coat the particles, preferably individually. The monomer-coated particles of heat sink material are then exposed to UV light of sufficient energy and intensity to cure the monomers into a polymeric coating of the particles. This is a UV-induced polymerization which can take place on a time scale of a few seconds. The UV-curable monomers which can be used in the practice of this aspect of the invention are epoxy-based resins, olefins such as vinyl monomers and vinylic monomers, epoxy monomers, and alkyl-, aryl-, and amino-substituted derivatives of these monomers. These processes may require the use of sensitizers, which help to initiate the reactions, usually involving singlet or triplet radical or biradical production. These processes can be carried out generally with light energy in the energy region of from about 220 nm to about 460nm, or from about 280 nm to about 420 nm. Light intensities should be sufficient to affect polymerization of the monomer to a continuous layer around the heat sink material within 30 seconds, preferably within 15 seconds or most preferably within 5 seconds. The monomercoated particles generally continue to fall through the drop tower as the photopolymerization proceeds. They may be maintained in suspension in the light path of the tower by means such as an updraft of air or other gas, at room temperature or at a temperature which will accelerate or decelerate the reaction as desired. Upon formation of the polymer coating, the particles are collected at the bottom of the drop tower. The coating must be of sufficient strength to allow for handling, and to account for any expansion or contraction that the phase change-type heat sink material undergoes during its phase change.
The amount of heat sink material required depends on the amount of refrigerant vapor to be absorbed or adsorbed by the sorbent, the chemical reaction heat of the sorbent and refrigerant vapor binding reaction, the specific heat of the heat sink (or specific heat-latent heat combination in a phase-change material), and the chosen final temperature of the sorber. Since some sorbents decrease in refrigerant vapor sorption capability as the temperature increases, there is a ratio of sorbent to
heat sink which yields minimum system mass, and which depends on the properties of the chosen pair.
The methods of producing heat sink material and the heat sink materials described herein are useful not only for the refrigeration devices described, but also for other refrigeration devices, such as those disclosed in U. S. Patent Nos. 5,197,302 and 5,048,301.
The refrigeration device also includes a means for preventing refrigerant vapor flow from the evaporator chamber to the sorbent chamber before operation of the device. Upon activation of this means, which subsequently allows the foxy of refrigerant vapor from the evaporator chamber to the sorbent chamber, desorption and cooling of product begins. The means for preventing vapor flow can take the form of any of the various types shown in the prior art, The means can be located at any location between the evaporator chamber and the sorber, so long as it prevents refrigerant vapor, or vapor of any kind from being sorbed by the sorbent. However, if the entire refrigeration device is contained within a pressurized container, a pressure responsive valve can be used which can actuate the device upon the release of the pressure within the container.
The device can be constructed of a variety of materials, with the restriction that certain portions must be able to afford good thermal contact with certain other portions. These portions must be made of a relatively good thermal conductor such as a metal or metallic material. Preferred materials for the evaporator chamber, and sorber include metals such as aluminum, copper, tin, steel, and metal alloys such as aluminum alloy. For some applications, corrosion protection will be required on the outer surface of the evaporator. Corrosion protection can include a thin coating of a lacquer specially designed for that purpose. Those of skill in the art will be able to provide suitable materials. The thickness of such coatings generally does not interfere with thermal transfer, but the choice of corrosion protectant will be dictated by the effect such protectant has on the heat transfer. Portions of the refrigerator which are not crucial to thermal transfer include the means for preventing/allowing refrigerant vapor flow. This portion can be made of a polymeric material, such as a thermoplastic material.
The refrigerators are subjected to external pressure, since they are evacuated internally. In order to avoid the necessity of fabricating a heavy structure, selfsupporting arch designs or ribbed designs can be used. Materials with similar gauge to those employed in the construction of carbonated beverage cans are able to find application in the construction of the inventive refrigerators. A particular embodiment of aself-supporting arch design is depicted in Fig. 6. Sorber 30 is shown having sorber 32 and heat sink material 40 included in its interior. On outer surface 46 of sorber 30 are a series of spacers 48. They generally continue around the circumference of surface 46, but some are omitted from Fig. 6 for clarity. There is intermediate material 50, which can be a polymeric material such as a thermoplastic, attached to spacers 48 across the entire circumference of surface 46. The assembly is meant to be placed in a cylindrical product container, with the terminal portions of spacers 48 abutting the inner walls of the cylindrical product container. This assembly assists the sorber to maintain its structure, preventing collapse from pressure inequalities between the interior and exterior of the sorber.
The product which can be cooled can be a liquid, gas or solid, as long as good thermal contact is made with the outer surface of the evaporator. Preferred products to be cooled are liquids or gases, most preferably liquids. Among the liquids which can be cooled using the refrigeration device of the invention are those comprising water, such as those comprising at least 20% water, those comprising at least 40% water, and those comprising at least 60% water, Included among such water-containing liquids are water itself, milk, fruit and vegetable juices, soft-drinks, beer, wine, and mixed drinks. These products can be contained in vessels of various sizes and shapes, and those made of various materials. As mentioned above, certain applications will involve the cooling of liquids which can, over lengthy storage times, corrode the containers in which they are stored. Corrosion protection, known to those skilled in the art, is available in such instances.
The invention also includes a method of using the refrigeration device described herein. The method includes the step of providing a refrigeration device of the type set fort herein, opening the means for preventing vapor flow, whereby the pressure in the evaporator is reduced, causing the refrigerant to be vaporized, which vapor is collected by the sorbent, removing the vapor from the evaporator by collecting
the vapor until an equilibrium condition is reached wherein the sorbent is substantially
saturated or substantially all the refrigerant originally in the evaporator chamber has been
collected in the sorbent, and simultaneously removing heat from the sorbent by means of
the heat sink material described above. The process is preferably a one-shot process;
thus, opening the'means for preventing/allowing flow is preferably irreversible. At the
same time, the system is a closed system; in other words, the refrigerant does not escape
from the system, and ther is no means by which the refrigerant or the sorbent may escape
either the evaporator chamber or the sorber.
It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.
A method for preparing phase change-type heat sink material for a portable, single-use, refrigerator that cools by evaporation of a refrigerant and does not release gas or vapor to the atmosphere, the method comprising:
(A) providing a quantity of solid phase change heat sink material that undergoes a volume increase upon changing from a solid phase to a liquid phase;
(B) encapsulating said phase change-type heat sink material in, gas-impermeable capsules essentially free of air or other gases, said capsules containing void volume when the material is a solid, said void volume being sufficient to accommodate the volume increase upon melting without rupturing said capsules; and
(C) placing the encapsulated phase change-type heat sink material in thermal contact with a sorbent material,
wherein the sorbent material receives refrigerant vapor during operation of said refrigerator, subsequently evolving heat.
The method as claimed in claim 1, wherein the phase change heat sink material undergoes a phase change at a temperature between 350C and 600C.
The method as claimed in claim 1, wherein said void volume is introduced into the solid phase of a phase change type heat sink material before encapsulation.
The method as claimed in claim 3, wherein said void volume reduces the density of the phase change-type heat sink material in its solid phase by a percentage that is within 2% of its percentage volume increase on melting.
The method as claimed in any of claims 1 to 4, wherein the gas-impermeable capsule comprises a metallic layer.
The method as claimed in claim 1, wherein the gas-impermeable
capsule comprises a polymeric layer.
7. The method as claimed in claim 6, wherein the polymeric layer is formed by light-induced polymerization with UV-light having energy in the range of from 220 nm to 460 nm.
8. The method as claimed in claim 7, wherein the polymeric layer is made from at least one UV-curable monomer selected from the group of epoxy-based resins, vinyl monomers, vinylic monomers, epoxy monomers, and alkyl-, aryl- and amino substituted derivatives of these monomers.
9. The method as claimed in any of claims 1 to 4, wherein the heat sink material is selected from the group consisting of paraffin hydrocarbons, sodium acetate trihydrate, sodium thiosulfate pentahydrate, and disodium phosphate dodecahydrate.
10. The method as claimed in claim 1, wherein encapsulation is performed by:
A) providing falling drops of phase change-type heat sink material from a drop tower apparatus;
B) passing said falling drops through a solution of UV-curable monomer selected from the group of epoxy-based resins, vinyl monomers, vinylic monomers, epoxy monomers, and alkyl-, aryl- and amino-substituted derivatives of these monomers;
C) providing UV-light having energy in the range 220 nm to 460 nm and intensity sufficient to cure said UV-curable monomer, so as to produce a polymer-encapsulated falling drop of heat sink material.
Dated this the 16th day of August, 2001
Of R&otry & Sagar
ATTORNEY FOR THE APPLICANTS
|Indian Patent Application Number||IN/PCT/2001/00984/MUM|
|PG Journal Number||38/2007|
|Date of Filing||16-Aug-2001|
|Name of Patentee||TEMPRA TECHNOLOGY INC.|
|Applicant Address||6140 15TH STREET EAST, BRADENTON , FLORIDA 34203, UNITED STATES OF AMERICA.|
|PCT International Classification Number||F25D 3/08|
|PCT International Application Number||PCT/US00/04637|
|PCT International Filing date||2000-02-24|