Abstract:
The present invention provides a loop thermosiphon including an evaporator and a condenser interconnected in flow communication by a vapor conduit and a condensate conduit. A wick is disposed in a portion of the evaporator and a portion of the at least one condensate conduit adjacent to the evaporator to facilitate capillary action to cycle a coolant fluid through the loop thermosiphon. Advantageously, a porous valve is lodged within the condensate conduit so that a first pressure on a condenser side of the porous valve is greater than a second pressure on an evaporator side of the porous valve. In this way, a portion of the liquid coolant fluid disposed within the loop thermosiphon is forced through the porous valve and a remaining portion is forced through the at least one condenser. In one embodiment, the porous valve comprises a plug of sintered material that is lodged within the condensate conduit so as to provide a seepage of coolant fluid during periods of low thermal energy transfer to the evaporator so as to avoid drying out of the system.

Description:
FIELD OF THE INVENTION 
     The present invention relates to thermosiphons, and more particularly to a thermosiphon that resists dry-out conditions and is self starting. 
     BACKGROUND OF THE INVENTION 
     The use of thermosiphons is well known in the art for cooling various types of electronic devices and equipment, such as integrated circuit chips and components. A thermosiphon absorbs heat by vaporizing liquid on an evaporating or boiling surface and transferring the vapor to a condenser where it cools and condenses into a liquid. Gravity then returns the liquid to the evaporator or boiler to repeat the cycle. Thus, a loop thermosiphon is formed by an evaporator and a condenser which are incorporated in a pipe circuit. The circuit is sealed and filled with a suitable working fluid. In order for the circuit to function, it is necessary for the condenser to be located somewhat above the evaporator. When heat is delivered to the evaporator, part of the fluid will boil off so that a mixture of liquid and gas rises to the condenser. The vapor condenses in the condenser and heat is released. The liquid thus formed then runs back to the evaporator under its own weight. 
     Thermosiphon circuits are normally very efficient heat transporters, inasmuch as heat can be transported through long distances at low temperature losses. Thermosiphon circuits can therefore be used advantageously for different cooling purposes. There is also generally a great deal of freedom in the design of the evaporator and condenser. In the context of electronic component cooling, however, the components to be cooled are normally very small, which means that the evaporator must be of comparable size. The external cooling medium used is normally air, which in turn means that the condenser must have a large external surface area. 
     One of the drawbacks with prior art thermosiphons is that the condenser must be sufficiently elevated to allow the condensed working fluid to flow back to the evaporator. It is beneficial to design U-Tubes or liquid tops in the condenser design to allow a higher gravity head during operation or to allow a portion of the condenser to be located below the evaporator. These designs work once they are operating, but can dry out the evaporator when not in use, thus requiring special start up procedures. 
     The wick structure and evaporator portion of the prior art are known to dry out when the thermosiphon is in a non-operating condition. While in this condition the wick structure and evaporator portion dry out to the point that there is not enough liquid in the evaporator portion to evaporate and create enough pressure to force condensate to return to the evaporator. This typically happens when the equipment to be cooled is turned off. When this equipment is turned off, heat is not provided to the evaporator portion. Thus, liquid flow is retarded by the decrease of pressure in the evaporator portion. This allows fluid to accumulate in the condenser region and dry out the evaporator region. Once a prior art loop thermosiphon is in this dry out condition, it can not be restarted until the evaporator portion contains sufficient liquid to evaporate. Simply applying heat to the evaporator portion will not restart thermosiphon flow. If insufficient liquid exists in the evaporator portion, applying heat may damage the thermosiphon, and possibly damage the equipment to be cooled. 
     One possible restarting means is to pump liquid to the evaporator portion. Alternatively, a heater can be added to the condenser section to drive the liquid back to the evaporator prior to startup. Adding pumps or adjunct heaters to a prior art loop thermosiphon alters the system from a passive system to an active system. A loop thermosiphon may be operated as a passive system, requiring no external electrical power. As a passive system, heat is provided to the evaporator portion by the equipment to be cooled, and the condenser portion is cooled by the ambient surroundings. Disadvantages of implementing adjunct heaters and/or pumps to loop thermosiphons include the additional power required, the additional space consumed, the additional system costs, and the increased possibility of malfunctioning components. Thus, a need exists for a thermosiphon which does not suffer the above disadvantages. 
     SUMMARY OF THE INVENTION 
     The present invention provides a loop thermosiphon comprising of an evaporator and a condenser interconnected in flow communication by at least one vapor conduit and at least one condensate conduit. A wick is disposed in a portion of the evaporator and a portion of the at least one condensate conduit adjacent to the evaporator to facilitate capillary action to cycle a coolant fluid through the loop thermosiphon. Advantageously, a porous valve is lodged within the condensate conduit. This porous valve will act as a pressure barrier for vapor, forcing the vapor through an alternate condenser flow path. This the vapor pressure within this alternate flow path increases the gravity head of the condensed working fluid. During periods of inactivity, the porous valve will allow liquid to flow freely in both directions preventing a buildup of liquid in the condenser and a potential dry out condition in the evaporator system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be more fully disclosed in, or rendered obvious by, the following detailed description of the preferred embodiment of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein: 
     FIG. 1 is a schematic diagram of a loop thermosiphon having a porous valve formed in accordance with the present invention and representing a normal operating condition; 
     FIG. 2 is a schematic diagram of the loop thermosiphon shown in FIG. 1, but showing a non-operating condition. 
     FIG. 3 is an enlarged broken-away and partially sectional view of a portion of the loop thermosiphon shown in FIGS. 1 and 2, showing a porous valve formed in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This description of preferred embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. In the description, relative terms such as “horizontal,” “vertical,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms including “inwardly” versus “outwardly,” “longitudinal” versus “lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. The term “operatively connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship. 
     Referring to FIG. 1, a loop thermosiphon  5  formed in accordance with the present invention comprises one or more evaporators  14 , one or more condensers  16 , at least one vapor conduit  18 , at least one condensate conduit  20 , a wick  22 , and a porous valve  24 . Loop thermosiphon  5  is charged with a suitable coolant fluid  7 , e.g., water, freon, alcohol, acetone, or some other fluid known in the art for use in heat transfer devices, and which is capable of rapid vaporization and condensation within a closed loop environment. Parameters to be considered when selecting coolant fluid  7  include the amount of pressure that can be safely applied to each evaporator, the operating temperature of the equipment to be cooled, the rate of heat transfer, the temperatures reached within each evaporator, the viscosity of coolant fluid  7 , and the boiling point of coolant fluid  7 . Loop thermosiphon  5  is sealed to the ambient atmosphere so as to form a closed loop system. 
     Evaporators  14  comprise at least one chambered enclosure  30  having an inlet opening  32  and an outlet opening  34 . Inlet opening  32  is arranged in flow communication with condenser  16 , via condensate conduit  20 , and outlet opening  34  is arranged in flow communication with condenser  16 , via vapor conduit  18 . 
     Chambered enclosures  30  are arranged in intimate thermal engagement with a source of thermal energy, such as an integrated circuit chip or chips, or an electronic device comprising such chips or other heat generating structures known in the art (not shown). Evaporators  14  may include external and/or internal features and structures to aid in the rapid vaporization of coolant fluid  7 . For example, an externally applied thermally conductive coating may used to enhance heat transfer and spreading from the heat source throughout evaporator  14 , or a sintered internal surface coating or heat pipe structures may be included in evaporator  14  for the purpose of spreading and transferring heat generated by the electronic components evenly throughout the evaporator. 
     Evaporator  14  acts as a heat exchanger transferring the heat given off by the equipment being cooled to coolant fluid  7 . As coolant fluid  7  is heated, the pressure within each chambered enclosure  30  increases, vaporizing the saturated fluid contained in the evaporator. The vapor flows through vapor conduit  18 , toward condenser  16 , i.e., in the direction of arrows  50  in FIG.  1 . Evaporator  14  may comprise any type of evaporator having the capability to facilitate the transfer of thermal energy to coolant fluid  7 . Some types of evaporators that have been found to be useful when used in connection with this invention include, tube evaporators, rising film evaporators, falling film evaporators, plate evaporators, and layered wick evaporators. For example, in one embodiment of the invention, evaporator  14  comprises a layered wick evaporator, having a wick formed on the interior surfaces of chambered enclosure  30 , and in flow communication with wick  22 . 
     Vapor conduit  18  and condensate conduit  20  may have a conventional structure that is capable of transferring coolant fluid  7  between evaporators  14  and condenser  16 . For example, vapor conduit  18  and condensate conduit  20  may be separate structures (e.g., tubes or pipes), or may be formed from a single structure, e.g., multiple channels molded or cut into single or multiple blocks. 
     Wick  22  is positioned on the inner surfaces of each inlet opening  32  and the inner surfaces of the portion of condensate conduit  20  that engages inlet opening  32 . Wick  22  may comprise any of the typical heat pipe wick structures such as grooves screen, cables, adjacent layers of screening, felt, or sintered powders, and may extend onto the inner surfaces of chambered enclosure  30 . Wick  22  draws liquid into evaporator  14  from condensate conduit  20  by capillary action. 
     Condensers  16  typically comprise a plurality of ducts  40  having an inlet opening  42  and an outlet opening  44 . Inlet opening  42  is arranged in flow communication with evaporator  14 , via vapor conduit  18 , and outlet opening  44  is arranged in flow communication with evaporator  14 , via return duct  45  and condensate conduit  20 . Condenser  16  acts as a heat exchanger transferring heat contained in a mixture of vaporous coolant fluid  7  and liquid coolant fluid  7  to the ambient surroundings. Condenser  16  may comprise a conventional condenser having the capability to facilitate transfer of thermal energy. Plurality of ducts  40  are often arranged within a heat transfer device, such as a fin stack, cold plate or heat exchanger of the type well known in the art. In one embodiment of the invention, plurality of ducts  40  are thermally engaged with a conventional fin stack that is adapted to utilize air flow for the transfer of heat. In another embodiment, condenser  16  comprises cooling fins, each having a large surface area for efficient transfer of thermal energy, and with a portion of each cooling fin thermally engaged with at least one of plurality of ducts  40 . 
     In operation, as the mixture of vaporous coolant fluid  7  and liquid coolant fluid  7  enters plurality of ducts  40 , the mixture condenses into a liquid as a result of the heat transferred from the mixture to the ambient surroundings via the cooling fins. Condenser  16  may be cooled by various other methods known in the art, such as forced liquid or air, or large surface areas of condenser  16  exposed to ambient surroundings. 
     Referring to FIGS. 1,  2 , and  3 , porous valve  24  comprises a plug of poriferous material, lodged within condensate conduit  20 , that is permeable to coolant fluid  7 , but at a significantly reduced rate as compared to an unobstructed portion of condensate conduit  20 . As such, porous valve  24  forms a seeping barrier to liquid coolant fluid  7  within condensate conduit  20 . In one embodiment of the present invention, porous valve  24  may be formed from a sintered material, e.g., copper, with pores sized in a range from about 25 um to about 150 um, with pores sized in the range of 50 um to about 80 um being preferred for most applications using water for coolant fluid  7 . The length of porous valve  24  may be set according to the flow rate through the valve that is needed to prevent drying out of wick  22 , as will hereinafter be disclosed in further detail. Porous valve  24  is positioned within condensate conduit  20 , adjacent to outlet opening  46  of return duct  45 . 
     In order to operate loop thermosiphon  5  according to the present invention, the equipment to be cooled (not shown) is thermally coupled to a portion of evaporator  14 . A portion of the packaging containing the equipment to be cooled is often attached directly to evaporator  14  by a thermally conductive material or fastener of the type well known in the art. As thermal energy is transferred from the equipment to be cooled to evaporator  14 , coolant fluid  7  within chambered enclosure  30  begins to evaporate (i.e., boil). As coolant fluid  7  boils, the pressure within evaporator  14  increases, which in turn forces a mixture of vaporous coolant fluid  7  and liquid coolant fluid  7  to flow along vapor conduit  18  toward condenser  16 . Slugs of liquid  51  are formed by the condensation of the mixture of vapor/liquid coolant  7  within plurality of ducts  40 . As the vapor pressure within evaporator  14  increases, it also forces slugs of liquid  51  to flow up each of plurality of ducts  40  in condenser  16 , as indicated by arrows  52  in FIGS. 1 and 3. As slugs of liquid  51  reach the top of condenser  16 , they are forced to flow out of outlet opening  44 , into return duct  45 , and downwardly through outlet opening  46  to condensate conduit  20  by gravity. 
     Referring to FIGS. 1 and 3, during normal operating conditions mixture of vaporous coolant fluid  7  and/or liquid coolant  7  flows in the direction of arrows  50  (FIG.  1 ). Liquid level  58  marks an approximate level of liquid coolant fluid  7  within condenser  16  and condensate conduit  20  while loop thermosiphon  5  is operating normally. When liquid coolant fluid  7  is at level  58 , wick  22  is sufficiently moistened to maintain thermosiphon operation. In this operating condition porous valve  24  prevents vapor  50  from flowing directly from vapor conduit  18  to condensate conduit  20 , and forces it through plurality of ducts  40 . Referring to FIG. 3, also during normal operation of loop thermosiphon  5 , pressure P 1 , on the condenser side of porous valve  24  is greater than the pressure P 2 , on the evaporator side of porous valve  24 . When P 1  is greater than P 2 , the capillary forces generated by the saturated porous valve are equal to 2 T/r c ⊖, where T equals the surface tension of the fluid, r c  equals pore radius, and ⊖ wetting angle. This capillary force prevents vapor from flowing through the porous valve  24 . Thus, the mixture of liquid and vapor is forced up through plurality of ducts  40  as slugs  51 . 
     Referring to FIGS. 1 and 2, porous valve  24  advantageously eliminates drying out of wick  22  and evaporator  14  when loop thermosiphon  5  is not operating by allowing a portion of liquid coolant fluid  7  to seep into condensate conduit  20  from condenser  16 , and thereby to maintain wick  22  in a moistened condition. More particularly, and referring to FIG. 2, loop thermosiphon  5  is not operating when evaporators  14  are not being heated and there is no liquid coolant fluid  7  flowing between condensers  16  and evaporators  14 . For example, this situation typically occurs when the equipment to be cooled is not operating or generating thermal energy. While loop thermosiphon  5  is in this non-operating condition, it would be possible for the working fluid to accumulate in plurality of ducts  40 . This would allow the liquid level to rise to level  61  with no flow back to evaporators  14 , completely drying them out. However, with the porous valve  24  in place, the force of gravity exerted on these columns of liquid increases P 1 . In turn, liquid seeps through porous valve  24  from the condenser side to the evaporator side, until P 1  is approximately equal to P 2 . At this point the level of liquid in plurality of ducts  40  is approximately at level  60  (FIG.  2 ). This is also the approximate level of liquid at wick  22 . Because liquid is always present at wick  22 , liquid is always available to be drawn into evaporator  14 . Thus, the dry out condition that is associated with prior art loop thermosiphons during the non-operating condition is eliminated. 
     Loop thermosiphon  5  may be restarted by simply starting the equipment to be cooled. Because sufficient liquid is present in evaporator  14 , as heat is transferred to evaporator  14 , thermosiphon action begins and liquid coolant fluid  7  starts to flow. Thus, a loop thermosiphon  5  in accordance with the present invention may be restarted without any active components (e.g., pumps, adjunct heaters). 
     It is to be understood that the present invention is by no means limited only to the particular constructions herein disclosed and shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims.