Water vaporization type cooling apparatus for heat-generating unit

A water vaporization type cooling apparatus for cooling a heat-generating unit comprises a container which is made of a good thermal conductive material and has an opening, a selective water vapor permeable membrane which is mounted to the container so as to cover the opening and forms a closed space cooperatively with the container, and water charged in the closed space. In the water vaporization type cooling apparatus, the container is thermally connected to the heat-generating unit and dehydrated air flows along the outer surface of the selective water vapor permeable membrane, whereby the heat-generating unit is cooled down.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a cooling apparatus for a computer storage
 unit or an electronic board mounting an LSI and other electronic devices
 to be mounted on an electronic equipment or an electrical power equipment.
 More particularly, the invention relates to a compact water vaporization
 type cooling apparatus excellent in cooling property, which permits
 inhibition of a temperature increase by eliminating heat generation from
 electronic parts or a computer storage unit, and ensures normal operation
 even in an environment of a temperature over the maximum working
 temperature of an electronic equipment.
 2. Description of the Related Art
 For the purpose of cooling electronic parts including LSI mounted on an
 electronic equipment or an electrical power equipment, it has been the
 conventional practice to dissipate the heat generated from heating members
 such as an LSI through combination of a refrigerant bag and a heat pipe,
 as disclosed, for example, in Japanese Unexamined Patent Publication No.
 6-21,279.
 FIG. 8 is a configuration diagram illustrating a conventional heat transfer
 apparatus, for example, disclosed in Japanese Unexamined Patent
 Publication No. 6-21,279.
 In the drawing, a protective metallic container 1 has an opening 2 provided
 in the bottom thereof. A refrigerant bag 3 is housed in the lower part of
 the protective metallic container 1. The refrigerant bag 3 has a
 configuration in which the both ends of a cylinder made of a soft plastic
 material such as polyethylene are sealed by heat sealing, and filled with
 an operating liquid 4, with the upper space filled with a gas. When this
 refrigerant bag 3 is housed in the protective metallic container 1, a part
 of the refrigerant bag 3 projects from the opening 2, and there is formed
 a contact portion 5 coming into contact with an object 8 of cooling such
 as the LSI.
 Further, a heat transfer pipe 6 is housed in the protective metallic
 container 1 as if it were wrapped by the refrigerant bag 3. A radiator fin
 7 is attached to an end of the heat transfer pipe 6 projecting outside
 from the protective metallic container 1.
 Applicable operating liquids 4 include halogen-based solvents such as flon
 and p-fluorocarbon (C.sub.6 F.sub.4).
 Operations of the conventional heat transfer apparatus will now be
 described.
 The heat transfer apparatus is installed so that the contact portion 5
 comes into contact with the object 8 of cooling such as an LSI. Heat
 generated by the object 8 of cooling is transferred from the contact
 portion 5 to the operating liquid 4. The operating liquid 4 is evaporated
 by the heat transferred from the contact portion 5. The thus generated
 vapor rises up through the upper space of the refrigerant bag 3, and upon
 reaching the portion in contact with the heat transfer pipe 6, the heat is
 absorbed by the heat transfer pipe 6 there, the condensed vapor being
 liquefied and dropping. Through this exchange of latent heat, the heat is
 absorbed by the heat transfer pipe 6. Then, the heat is dissipated from
 the radiator fin 7 provided at an end of the heat transfer pipe 6. By
 repeating this process of heat exchange, the object 8 of cooling is
 cooled.
 In the conventional heat transfer apparatus having the configuration as
 described above, the object 8 of cooling cannot be cooled beyond the outer
 periphery temperature of the radiating section, and therefore, the
 apparatus cannot be operated in an environment including a temperature of
 over the maximum working temperature of the electronic equipment. There is
 therefore a problem of limited environments of use.
 Since a halogen-based solvent such as flon or perfluorocarbon is used as
 the operating liquid 4, the refrigerant must be collected upon abolishing
 the apparatus for environmental protection purposes. However, many of
 electronic equipments are supplied to a market composed of unspecified
 users, and this has posed the problem of establishing a method of
 collection.
 In general, an electronic equipment should meet the requirement for
 downsizing. The aforementioned structure of the heat transfer apparatus
 however comprises many components near the board, and this has prevented
 the problem of downsizing from being solved.
 The object 8 of cooling is in mechanical contact with the refrigerant bag
 3. This results in a large contact heat resistance, leading to a further
 larger heat density. As a result, there is posed another problem of
 impossibility to take sufficient actions to satisfy the requirement for a
 cooling method excellent in cooling performance.
 SUMMARY OF THE INVENTION
 The present invention was developed to solve the aforementioned problems
 and has an object to provide a compact water vaporization type cooling
 apparatus of a heating element, which permits cooling of the heating
 element to a temperature lower than the outer periphery temperature of the
 heat-generating unit, without limitation of the working environment, and
 is suitable for environmental protection purposes.
 In order to achieve the above object, according to one aspect of the
 present invention, there is provided a water vaporization type cooling
 apparatus for cooling a heat-generating unit, comprising a container which
 is made of a good thermal conductive material and has an opening; a
 selective water vapor permeable membrane which is mounted to the container
 so as to cover the opening and forms a closed space cooperatively with the
 container; and water charged in the closed space. In the water evaporative
 cooling device, the container is thermally connected to the
 heat-generating unit and dehydrated air flows along the outer surface of
 the selective water vapor permeable membrane, whereby the heat-generating
 unit is cooled down.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments according to the present invention will be explained with
 reference to the accompanying drawings.
 First Embodiment
 FIG. 1 shows a system arrangement of a water vaporization type cooling
 apparatus according to the first embodiment of the present invention for a
 heat-generating unit. FIG. 2 is a sectional view of a major portion of the
 water vaporization type cooling apparatus according to the first
 embodiment of the present invention for the heat-generating unit.
 In FIGS. 1 and 2, the heat-generating unit 50 is composed of electronic
 parts, such as a CPU 50b used for a computer, mounted on a board 50a. A
 heat sink 51 is composed of a container 52 made of a good thermal
 conductive material and a selective water vapor permeable membrane 53
 mounted to the container 52 so as to cover an opening 52a provided at a
 part of the container 52, wherein a space formed by the container 52 and
 the selective water vapor permeable membrane 53 is filled with water 54.
 A wall 52b of the container 52 opposing the selective water vapor permeable
 membrane 53 is thermally connected to the heat-generating unit 50 tightly
 by a good thermal conductive adhesive agent. On the outer surface of the
 selective water vapor permeable membrane 53, dehydrated air 55 flows in a
 direction shown by an arrow A. The heat sink 51 is connected to a water
 supply tank 56 through conduits 57a and 57b, and the water 54 is to be fed
 continuously into the heat sink 51.
 The term "good thermal conductive material" means a material having high
 thermal conductivity; metal such as copper, silver, gold, and aluminum are
 used for this purpose.
 Next, an operation of the water vaporization type cooling apparatus
 according to the first embodiment will be explained.
 Heat generated by the heat-generating unit 50 flows into the heat sink 51
 thermally connected to the heat-generating unit 50, so that the water 54
 in the heat sink 51 is heated. Consequently, water vapor evolves at the
 interface between the water 54 and the selective water vapor permeable
 membrane 53, and afterwards the water vapor passing through the selective
 water vapor permeable membrane 53 is absorbed in the dehydrated air 55
 flowing along the outer surface of the selective water vapor permeable
 membrane 53.
 Hence, the heat generated by the heat-generating unit 50 is absorbed in the
 dehydrated air 55, that is, the heat-generating unit 50 is cooled down.
 Now, the selective water vapor permeable membrane 53 used for the
 embodiment of the present invention will be explained.
 The selective water vapor permeable membrane 53 has a function that a
 permeation rate of water vapor is markedly larger than that of the
 components of air. A functional membrane composed of a fluorinated resin,
 having a hydrophilic functional group, laminated with or impregnated into
 a porous carrier, as described in Japanese Unexamined Patent Application
 Publication No. 1-194927, can be used as the selective water vapor
 permeable membrane 53.
 Since the functional membrane made of the porous carrier laminated or
 impregnated with the fluorinated resin becomes non-porous material at
 least in a thickness direction thereof, air, nitrogen, and hydrocarbons
 such as methane are substantially not allowed to permeate through the
 membrane 53. On the other hand, the water vapor absorbs on the surface of
 the membrane made of the fluorinated resin by means of the hydrophilic
 functional group thereof, diffuses in the layer of the fluorinated resin,
 and rapidly permeates through the membrane 53. Driving force by which the
 water vapor permeates is a difference of partial pressure of the water
 vapor across the membrane. The larger the difference of the partial
 pressure is, the greater the permeation rate of the water vapor is.
 A cellulose, a polyolefin, a polyester, a polysulfone, and a fluorinated
 type porous sheets, nonwoven fabric, and woven fabric can be used as the
 porous carrier. Of those listed above, the fluorinated type resin
 associated with heat stability and chemical resistance is preferable.
 The hydrophilic functional group connected to the fluorinated resin
 includes a sulfonic, a sulfonic salt, a sulfate, a sulfate salt, a
 carboxyl, and/or a carboxyl salt groups.
 The selective water vapor permeable membrane 53 utilized in the first
 embodiment is prepared by processes as described below. Ten micrometer
 thickness of the fluorinated polymer having the sulfate salt group is
 formed on a porous membrane (thickness: 40 micrometer, porosity: 75%, the
 maximum pore diameter: 0.5 micrometer) made of a drawn membrane of a
 polytetrafluoroethylene, and after air-drying the membrane is dried at
 100.degree. C. for 180 minutes. In FIG. 3, gas permeation rate of water
 vapor, oxygen, nitrogen, hydrogen, and methane by use of the selective
 water vapor permeable membrane 53 prepared as described above, and the
 ratios of the permeation rate are shown. It is appreciated from FIG. 3
 that oxygen and nitrogen, i.e., components of the air, scarcely permeate,
 and water vapor selectively permeates through the membrane 53.
 For confirming the function of the selective water vapor permeable membrane
 53, a measurement instrument is prepared, wherein the instrument is
 composed of a closed container of 15 liter volume having an opening of 50
 cm.sup.2 area on the wall of the container and the selective water vapor
 permeable membrane 53 mounted to the container so as to cover the opening.
 The measurement result of humidity in the closed container with humidified
 condition therein, when dehydrated air of 20.degree. C. flows along the
 outer surface of the selective water vapor permeable membrane 53, is shown
 in FIG. 4. It is understood from FIG. 4 that the water vapor is drawn from
 the closed container by the dehydrated air flowing along the outer surface
 of the selective water vapor permeable membrane 53, so that the humidity
 therein becomes lower. It is also appreciated that, by the dehydrated air
 flowing along the outer surface of the selective water vapor permeable
 membrane 53 for a predetermined period of time, the humidity in the closed
 container can be reduced to the level of the dehydrated air.
 The principle, in which the water can be cooled down below the temperature
 of the air by contacting the dehydrated air with the water, is now
 explained.
 When the temperature and the humidity of the dehydrated air are 32.degree.
 C. and 20 percent respectively (corresponding to the point P in FIG. 5) as
 shown in a graph of FIG. 5, the temperature of the water contacting with
 this air isoenthalpically varies along the straight line between P and Q
 and decreases to the wet-bulb temperature (TW) of 17.degree. C.
 (corresponding to the point Q in FIG. 5). This is caused by the fact that
 the water evaporated is absorbed in the dehydrated air, thus the
 temperature decreases as a result of the deprivation of the evaporation
 latent heat when the water evaporates. The water is cooled down according
 to the principle explained above and absorbs the heat from the CPU 50b of
 the heat-generating unit 50, so that the CPU 50b is cooled down. Hence,
 even when the heat-generating unit 50 is disposed in higher temperature
 condition, the temperature of the heat sink 51 can be maintained less than
 that in the condition. That means, if necessary, the CPU 50b can be cooled
 down below the ambient temperature.
 The cooling according to the first embodiment utilizes the evaporation
 latent heat of the water instead of the sensible heat of air. Therefore, a
 small amount of circulating water gives a high cooling effect since the
 water has the large evaporation latent heat of 590 Kcal/kg. Moreover,
 since the circulation of the air acts only as a medium for transporting
 the water vapor evolved through the water evaporation, volume of air to be
 circulated is extremely small comparing with a wind cooling which utilizes
 the sensible heat of the air with the specific heat of 0.24
 Kcal/kg..degree. C. Accordingly, no large air passage for the aeration is
 required, so that the arrangement of the cooling apparatus can be
 miniaturized.
 In the first embodiment as explained above, the selective water vapor
 permeable membrane 53 is mounted to the container 52 so as to cover the
 opening 52a provided at the part of the wall of the container 52 composing
 the heat sink 51, and the wall 52b of the container 52, opposing the
 selective water vapor permeable membrane 53, is thermally connected to the
 heat-generating unit 50. The water evaporates at the interface between the
 selective water vapor permeable membrane 53 and the water by the
 dehydrated air 55 flowing along the outer surface of the selective water
 vapor permeable membrane 53, thus the temperature decreases as a result of
 the deprivation of the evaporation latent heat when the water evaporates,
 whereby the water 54 can be cooled down less than that in the ambience.
 Hence, the water vaporization type cooling apparatus can be operated in
 the condition of the temperature being above the maximum working
 temperature of the electronic equipment, that means, no ambient limitation
 of the usage exists.
 Since the selective water vapor permeable membrane 53 composing a part of
 the container 52 does not allow the permeation of the water, there is no
 risk of leaking water outside when the water 54 is supplied in the heat
 sink 51. Therefore, a plurality of the heat sinks 51 can be mounted on the
 board 50a without any risk of the short circuit caused by the water
 leakage, so that the safe and compact cooling can be effected.
 In addition, since the air and the water are utilized as the cooling
 coolant, the recovery of the coolant is not required for the environmental
 protection as in the case of utilizing the halogenated solvent such as
 flon or perfluorocarbons. As a result, the cooling apparatus free of the
 problem on the environmental protection can be obtained.
 As the cooling mechanism is based on the evaporation latent heat of the
 water, high cooling efficiency can be achieved by the small amount of the
 coolant to be circulated. In addition, since the air circulated acts only
 as the transporting medium for transporting the evaporated water, the air
 volume to be circulated is extremely small. Hence, no larger air
 circulator or larger passage for the aeration is required, so that the
 arrangement of the cooling apparatus required from the cooling point of
 view can be miniaturized.
 Second Embodiment
 In the first embodiment described above, the water evaporated in the heat
 sink 51 is charged with the water stored in the water supply tank 56
 through the conduits 57a and 57b which interconnect the heat sink 51 and
 the water supply tank 56. In the second embodiment, on the recognition
 that the dehydrated air is humidified by absorbing the water vapor through
 the process that the dehydrated air flows on the outer surface of the heat
 sink 51, the water is designed to be recovered for reuse from the
 humidified air by absorbing the water vapor through the process that the
 dehydrated air flows on the outer surface of the heat sink 51 so that the
 dehydrated air can be obtained simultaneously.
 FIG. 6 shows a system arrangement of a water vaporization type cooling
 apparatus according to the second embodiment of the present invention for
 the heat-generating unit.
 In FIG. 6, a dehumidifying device 65 is composed of a coolant compressor
 71, a coolant condenser 72, an expansion valve 73, a coolant evaporator
 74, an air heater 75, and a coolant conduit 76 connecting these units
 mentioned above to form a closed loop. The circulating air circulated by
 an air circulator 67 is designed to flow in a secondary side of the
 coolant evaporator 74 and the air heater 75.
 The coolant is circulated in the closed loop. The coolant in the coolant
 evaporator 74 is compressed in the coolant compressor 71 and afterwards is
 fed to the coolant condenser 72 through the air heater 75. The compressed
 coolant is condensed in the coolant condenser 72 to radiate the heat
 outside the system, and then is adiabatically expanded freely in the
 expansion valve 73. The coolant expanded adiabatically is fed in the
 coolant evaporator 74, so that the air circulating in the secondary side
 is cooled down below the dew point thereof by the endothermic cooling
 effect of the coolant developed through the process of the adiabatic
 expansion thereof.
 There is provided a water pit 78 as a water reservoir at the bottom of the
 coolant evaporator 74. When the air flowing in the secondary side is
 cooled down, the condensed water evolved at the condensation of the
 moisture contained in the air is stored in the water pit 78.
 The heat-generating unit 50 in the state of being thermally connected to
 the heat sink 51 is received in the closed space 58. The closed space 58
 and the dehumidifying device 65 are interconnected through air conduits
 77a and 77b so as to form the closed circuit of the air circulation as a
 gas circulation passage. An air circulator 67 as a gas circulating means
 such as a fan is disposed in the course of the air conduit 77a. By the
 operation of the air circulator 67, the air in the closed space 58 is fed
 into the dehumidifying device 65 through the air conduit 77a, so that the
 air becomes the dehydrated air through separation and condensation of the
 water vapor contained in the air. The resulting dehydrated air is returned
 to the closed space 58 through the air conduit 77b and is forced to flow
 in the closed circuit along the selective water vapor permeable membrane
 53 as shown by the arrow A in FIG. 6.
 The water pit 78 in the dehumidifying device 65 and the heat sink 51 are
 interconnected through a conduit 57a as a mean for returning water so that
 the water separated and recovered in the dehumidifying device 65 is
 returned to the heat sink 51.
 Next, the cooling operation of the water vaporization type cooling
 apparatus according to the second embodiment will be explained.
 Upon actuation of the air circulator 67 the air in the closed space 58 is
 fed in the dehumidifying device 65 through the air conduit 77a to flow
 through the secondary side of the circulating coolant evaporator 74 and
 the air heater 75, and then is fed in the closed space 58 through the air
 conduit 77b. When flowing through the secondary side of the circulating
 coolant evaporator 74, the air is cooled down below the dew point thereof.
 During the cooling the moisture contained in the air is condensed and
 stored as the condensed water in the water pit 78. When the air cooled
 down below the dew point thereof flows through the secondary side of the
 air heater 75, the air is heated up to the room temperature and fed as the
 dehydrated air in the closed space 58 through the air conduit 77b.
 The dehydrated air 55 fed in the closed space 58 flows along the outer
 surface of the selective water vapor permeable membrane 53 as shown by the
 arrow A in FIG. 6. The water vapor in the heat sink 51 are drawn into the
 closed space 58 through the selective water vapor permeable membrane 53,
 so that the air flowing in the closed space 58 is moistened by the water
 vapor and then is fed in the dehumidifying device 65 through the air
 conduit 77a.
 In the meantime, heat generated by the CPU 50b is transferred to the heat
 sink 51 through the wall 52b of the container 52, so that the water in the
 container 52 is heated to evaporate. The water vapor generated as
 described above passes through the selective water vapor permeable
 membrane 53 and is absorbed in the dehydrated air 55 flowing in the closed
 space 58, so that the humidity in the heat sink 51 becomes to an
 equivalent level of the dehydrated air flowing in the closed space 58. The
 water vapor drawn in the closed space 58 is condensed and recovered in the
 dehumidifying device 65, and is stored as the condensed water in the water
 pit 78. Afterwards, the condensed water is returned sequentially to the
 heat sink 51 through the conduit 57a.
 Hence, the effect obtained in the second embodiment is equivalent to that
 in the first embodiment described above.
 According to the second embodiment, the water vapor evaporated in the heat
 sink 51 is absorbed in the dehydrated air 55 after passing through the
 selective water vapor permeable membrane 53 and is recovered as the
 condensed water in the dehumidifying device 65 to be fed in the heat sink
 51. Consequently, no water supply from the outside of the system is
 required, that is, the cooling of the heat-generating unit 50 in the
 closed loop can be performed.
 Third Embodiment
 In the third embodiment, a shape of an outer surface 52b of the container
 52 is formed in conformity with the surface to be cooled of the
 heat-generating unit 50 as shown in FIG. 7.
 Accordingly, the outer surface 52b of the container 52 can be disposed in a
 close contact with the surface to be cooled of the heat-generating unit
 50, so that the thermal resistance at the interface between the container
 52 and the heat-generating unit 50 can be markedly reduced, i.e., the
 heat-generating unit 50 can be effectively cooled down.
 Fourth Embodiment
 In the third embodiment described above, the shape of the outer surface 52b
 of the container 52 is formed in conformity with the surface to be cooled
 of the heat-generating unit 50. While in the fourth embodiment, an
 equivalent effect to the third embodiment can be obtained when the
 container 52 is made of a flexible material. The flexible material
 composing the container 52 is such as a film laminated a good thermal
 conductive plastic and a metal film, or an embossed metal film.
 In each embodiment described heretofore, the heat-generating unit 50 to be
 cooled is explained by use of the CPU 50b mounted on the board 50a.
 However, it is appreciated that the present invention is not limited to
 the above application and can be applied to, for example, a power electric
 semiconductor, a thyristor, and a laser related apparatus. More
 particularly, the present invention can be effectively applied to the
 cooling for a device associated with large thermal flux of radiation.
 In order to achieve the above object, according to one aspect of the
 present invention, there is provided a water vaporization type cooling
 apparatus for cooling a heat-generating unit, comprising a container which
 is made of a good thermal conductive material and has an opening; a
 selective water vapor permeable membrane which is mounted to the container
 so as to cover the opening and forms a closed space cooperatively with the
 container; and water charged in the closed space. In the water evaporative
 cooling device, the container is thermally connected to the
 heat-generating unit and dehydrated air flows along the outer surface of
 the selective water vapor permeable membrane, whereby the heat-generating
 unit is cooled down. Hence, the cooling apparatus described above can be
 cooled down below an ambient temperature, i.e., the usage can not be
 restricted by the ambient condition, and is suitable for the environment
 protection.
 The cooling apparatus may have a dehumidifying device which
 separates/recovers water vapor contained in air by condensation thereof in
 a water reservoir and obtains the air with low humidity, an air
 circulation circuit which circulates the air with low humidity obtained in
 the dehumidifying device along the outer surface of the selective water
 vapor permeable membrane and then returns the air to the dehumidifying
 device, and water return means for returning the water recovered in the
 water reservoir of the dehumidifying device to the closed space.
 Accordingly, no water supply from the outside of the system is required
 and the heat-generating unit can be cooled down in the closed loop.
 An outer surface of a part of the container to be thermally connected to
 the heat-generating unit may be formed in conformity with a surface to be
 cooled of the heat-generating unit, so that the thermal resistance at the
 interface between the container and the heat-generating unit can be
 reduced to be able to perform the highly efficient cooling.
 The container may be made of a flexible material, so that, similar to the
 effect mentioned above, the thermal resistance at the interface between
 the container and the heat-generating unit can be reduced to be able to
 perform the highly efficient cooling.