FOR TWO-PHASE COOLING

Cross-Reference to Related Application

This application claims the benefit of U.S. Provisional Application No. 62/658,260 filed April 16, 2018, which is herein incorporated by reference.

Field

This disclosure relates to two-phase fluid heat exchangers in which a fluid is vaporized to remove heat, including micro- and mini-channel two-phase exchangers that are to provide a cooling effect for heat sources with high heat fluxes.

Background

Fluid heat exchangers are used to remove waste heat from high-heat flux heat sources (typically in excess of 5 watts/cm , and often substantially higher) and devices by accepting and dissipating thermal energy therefrom. Examples of such high-heat flux heat sources and devices include microelectronics such as microprocessors and memory devices, solid-state light emitting diodes (LEDs) and lasers, insulated-gate bipolar transistor (IGBT) devices such as power supplies, photovoltaic cells, radioactive thermal generators and fuel rods, internal combustion engines.

The fluid heat exchangers dissipate heat by thermally conducting the heat into internal passages of the exchanger, through which coolant fluid flows, absorbing the heat conducted across the walls of the exchanger, and the fluid is then transported outside the exchanger, where the heat is rejected to an external heat sink. While the coolant fluid flowing through the exchanger may be a gas, it is generally preferable to use a liquid, as liquids have higher heat capacities and heat transfer coefficients than gases. The liquid may remain single phase, or the liquid may partially or completely evaporate within the internal passages of the exchanger.

The flow of coolant liquid fed to the fluid heat exchanger may be driven by a pump, or by natural convection due to density differences and/or elevation between the incoming and exiting fluid (e.g. thermosyphons), or by capillary action in the internal passages of the exchanger or their communicating piping, or by a combination of these mechanisms.

Evaporator-type exchangers rely on the boiling mode, and have the advantages of higher heat transfer coefficients (better heat transfer) per unit of fluid flow rate of the coolant fluid, and also require much less coolant flow, as the majority of the heat is absorbed via the latent heat of vaporization of the boiling fluid, rather than via the sensible heat (heat capacity) of a single-phase liquid or gas.

It is well known that the thermal performance and efficiency of the fluid heat exchangers are greatly enhanced if the internal passages are comprised of microchannels, i.e. having cross-sections with a smallest dimension of less than 1000 microns, and more typically, in the range of 50 - 500 microns.

Evaporative-type coolers typically operate as a closed loop, with liquid entering the evaporator, and a two-phase mixture (vapor and saturated liquid) exiting the evaporator. (It is not generally desirable for all of the liquid to vaporize within the evaporator, as this can lead to dry-out and reduced heat transfer towards the outlet of the cooler, resulting in localized high temperatures / hot spots in the devices in contact with and being cooled by evaporative cooler). The two-phase mixture is sent to a condenser, where the vapor is condensed back to liquid, and the (combined) liquids are returned to the evaporative cooler. The circulation of the boiling fluid may be driven by various means, including but not limited to pumping, natural convection due to density differences (thermosyphon effect), capillary action, gravity flow, and various combinations thereof. However, despite the variety of available configurations, there has still been a need for cooling systems with higher heat transfer rates than those traditionally available.

Summary

A variety of aspects of the invention are presented in this application and the claims. In one general aspect, the invention features apparatus for minimizing sub-cooling of condensed working fluid returning to one or more parallel two-phase coolers that operate in closed-loop circulation mode. The two-phase mixture exiting the one or more coolers is sent to a chamber in which the vapor is separated from the saturated liquid emerging from the liquid, and the vapor is sent to one or more condensers where the vapor is fully condensed back to saturated liquid, which may then be further sub-cooled. The condensed liquid is returned to the separation chamber. The mixing and contacting of sub-cooled condensate with the incoming vapor and saturated liquid in the chamber re heats the condensate by using the latent heat of condensation of the vapor, preferably substantially to its saturation temperature. A portion of the incoming vapor pre-condenses at its saturation temperature, and the net condensed liquid withdrawn from the chamber and returned to the evaporative cooler, preferably at substantially its saturation temperature. In preferred embodiments, the two-phase evaporative coolers can be of the mini- or micro- channel type, with internal passages comprising the active boiling surfaces have hydraulic diameters of less than 1000 microns. The two-phase mixture from the evaporative coolers of Claim 1 of enters the saturation chamber of Claim 1 above the surface of the liquid, so that the (saturated) liquid disperses into droplets which fall onto to the liquid surface, while the vapor disengages and exits to the condenser inlet from above the liquid surface. The two-phase mixture from the evaporative coolers can enter the saturation chamber below the surface of the liquid, so that the liquid mixes with the liquid in the chamber, while the vapor separates and bubbles up to and emerges from the liquid surface, and then exits to the condenser inlet from above the liquid surface. The condensate can return above the surface of the liquid in the saturation chamber, so that the liquid disperses into droplets, and falls through and contacts the vapor in the chamber. A liquid seal-leg may optionally be used to prevent vapors from being sucked back up into the condensate outlet line. The condensate can return below the surface of the liquid in the saturation chamber, so that so that the liquid mixes with and absorbs heat from saturated liquid in the chamber. The condensate return line can be oriented to promote turbulence in the liquid volume and especially to promote waves or frothing action at the surface, to promote contacting and heat transfer between the liquid and vapor in the chamber. The condenser may be arranged to provide a top-to-bottom flow pattern, with the vapor entering an upper side of the condenser, and the condensate exits by gravity from a lower outlet side of the condenser. The condenser may be oriented substantially horizontally, with working fluid side of the condenser configured in a single-pass or multi-pass down-flow

arrangement. The condenser may be oriented substantially vertically or inclined, with working fluid side of the condenser configured in a single-pass down-flow arrangement. The condenser may be arranged as a“knock-back” condenser, i.e. with the vapor entering and the liquid counter-currently and substantially co-axially exiting by gravity via the underside of the condenser. The saturation chamber may be formed from piping arranged as an tee elevated above the liquid surface, for separating the incoming two-phase mixture from the evaporative coolers and a second tee below the liquid surface, to mix the condensate with the liquid being separated from the incoming two-phase mixture. The saturation chamber may be formed from piping arranged as a cross elevated above the liquid surface, where the incoming two-phase mixture from the evaporative coolers of Claim 1 impinges on, contacts with, and then separates from the condensate returning from the condenser via the sides of the cross, with the vapor exiting to the condenser from the top of the cross, and the net liquid exits via the bottom of the cross. The condenser can be arranged to provide a top-to-bottom flow pattern, wherein the two-phase mixture enters an upper side of the condenser, with the condensate exiting by gravity from a lower outlet side of the condenser. The condenser can be configured for self-venting flow, with the passages where the condensation takes place being sufficiently large such that the condensate does not fully fill the cross sections of the passages, thereby allowing any condensed liquid exiting the condenser to pressure-equalize with the incoming two-phase mixture and thereby remain saturated. The self- venting condenser passages can also serve as the saturation chamber. The liquid transfer line between the saturation chamber and the evaporative coolers can be made from materials with low thermal conductivity and/or are thermally insulated. The pressure differential between the saturation chamber and the evaporative coolers can be minimized by operating the cooling loop as a thermo- syphon, where the circulation is driven by gravity and density differences, and, the elevation difference between the separation chamber and the evaporative cooler can be reduced to the minimum required to ensure adequate circulation without complete vaporization of the fluid in the evaporative cooler. Liquid returning from the saturation chamber to the evaporative coolers that is sub-saturated due to heat looses or pressure increase in the return line, can be re-heated by providing a heat exchange means in the immediate vicinity of the inlet to the evaporative cooler, such that heat is transferred from the two-phase mixture exiting the evaporator into sub-saturated liquid returning to the evaporator. The heat may be rejected from the condenser by any convenient means or cooling media that are cooler than the saturation temperature of the working fluid.

In another general aspect, the invention features a cooling apparatus that includes a heated two-phase evaporator input for receiving a heated two-phase coolant mixture from an output of a two-phase evaporator, a condenser output for providing at least some of the coolant mixture to a at least one condenser, a condenser input for receiving at least some of the received coolant in condensed form from the at least one condenser, and a saturation volume responsive to the heated two-phase evaporator input and to the condenser output, operative to increase the saturation level of the condensed coolant from the condenser, and having an evaporator return output for returning the condensed coolant to an input of the evaporator.

In a further general aspect, the invention features a cooling method that includes receiving a heated two-phase coolant mixture from an evaporator, condensing a first portion of the coolant mixture, using a second portion of the coolant mixture to increase the saturation of the coolant mixture condensed in the step of condensing, and returning the coolant mixture to the evaporator after its saturation has been increased. The various embodiments presented in this application can provide increased cooling efficiency in evaporative microchannel heat exchangers.

It has recently been shown that the localized temperatures of the contact surfaces of two- phase (evaporative) coolers, especially micro- and mini-channel evaporators, are unexpectedly elevated near in the vicinity of the fluid inlets when the entering fluid is sub-cooled, e.g. when returning from a condenser, and that the heat removal efficiency is reduced in this region. This can lead to localized hot spots in the devices being cooled, as well as reduced overall heat removal capability.

The thermal performance of two-phase (evaporative) closed-loop coolers, especially micro- and mini-channel evaporators, can be improved by providing a passive means to re-heat and re saturate sub-cooled working fluid returning from condensers. This is accomplished by providing a chamber to separate the vapor from the saturated liquid emerging from the mixed-phase outlet of the evaporative cooler, sending substantially only the vapor portion to a condenser, and returning the condensed fluid, which may be sub-cooled, to the saturation chamber prior to withdrawing the condensed liquid and returning it to the evaporative cooler.

The mixing and contacting of sub-cooled condensate with the incoming vapor and saturated liquid in the chamber re-heats the condensate substantially to its saturation temperature, by using the latent heat of condensation of the vapor. This causes a portion of the incoming vapor to pre condense at its saturation temperature, and has the beneficial effect of reducing the vapor load on the condenser, increasing the effective cooling capacity of the condenser. The mixed liquid in the chamber is substantially at its boiling (saturation) temperature.

The net condensed liquid withdrawn from the chamber is preferably at substantially its saturation temperature, and is warmer than any sub-cooled liquid emerging from the condenser. To minimize the potential for sub-cooling of the liquid returning to the evaporative cooler from the saturation chamber, it is preferable to minimize heat losses in the transfer line and to minimize the pressure / elevation difference between the separation chamber and the evaporative cooler. Further reheating / re-saturation of the returning liquid can be achieved by providing a heat exchange means in the immediate vicinity of the inlet to the evaporative cooler, whereby heat is transferred from the two-phase mixture exiting the evaporator, to any liquid returning to the evaporator that may be sub saturated due to heat losses or pressure elevation in the return lines.

Because the heat generated by the devices to be cooled may vary over time, condensers 18 are typically sized for a maximum heat load, plus an additional margin (safety factor), so that the condensers are over-sized. As a result, the condensers remove more heat than is simply required to re-condense the vapor (latent heat of vaporization). Once all the vapor is condensed, and only liquid remains, the liquid is cooled further, below its boiling point, i.e. the liquid exiting the condenser and returning to the evaporative cooler is sub-cooled, as shown in Figs. 2A-2C.

It has been traditionally viewed that sub-cooling of the liquid entering the evaporative coolers is beneficial - being colder than the boiling point, it increases the local temperature difference (driving force) for heat transfer in the cooler, and should thereby increase the cooling capacity of the evaporative cooler.

However, it has recently been discovered that sub-cooling of liquid entering evaporative coolers, especially in those (“cold plates”) with mini- or micro-channel where the boiling takes place can instead be deleterious to the localized heat transfer and overall cooling capacity of the coolers. It has been shown than in the vicinity of the zone where the liquid in cold plates transitions from the sub-cooled liquid to saturated / boiling flow regime, the localized temperatures of the cooler surfaces in contact with hot devices being cooled are substantially higher than the surface temperatures of the cooler in the region where boiling occurs (see Figures 1A and 1B). This effect is described in EXPERIMENTS AND MODELING OF A TWO-PHASE THERMOSYPHON BASED

THERMAL MANAGEMENT SYSTEM FOR RACK MOUNTED SYSTEMS”, Benjamin Zuk, Alfonso Ortega, Steven Schon, and David Santoleri, presented at SEMTHERM 2014, which is herein incorporated by reference.

The unexpected increase in the wall temperatures in the vicinity of the sub-cooled zone of the cooler reduces the temperature driving force for removing heat (via conduction through the wall to the fluid), despite the lower fluid temperate of the sub-cooled liquid compared to its temperature in the boiling zone. The higher wall temperatures are attributed to the inferior local heat transfer coefficients where the fluid is single (liquid) phase, compared to the heat transfer coefficients where the fluid is in the boiling regime.

Thus, sub-cooling can be deleterious to the performance of evaporative coolers, particularly high-performance coolers using mini- or micro-channels.

These results indicate that the thermal performance of evaporative coolers, especially mini- or micro-channel cold plates, can be improved if the sub-cooling of the liquid returning from condensers can be minimized, and preferably if the liquid temperature can be increased to substantially its saturation temperature. While it is certainly possible to re-heat the sub-cooled liquid using external heat sources, this can be undesirable both from the point of view of additional system complexity and cost (for additional heating devices as well as a means for controlling to heat input, to prevent overheating and premature boiling), and from an energy efficiency perspective, as the additional heat input consumes energy.

Another option would be control the cooling media to the condensers, to limit or prevent the sub-cooling effect. But this would seem to require some kind of active cooling control means, again adding to the complexity and cost of the system, e.g. for temperature sensors, control solver, and mechanical means (flow or speed throttling) of adjusting the flow of coolant.

It is therefore desirable to provide a means to passively minimize the sub-cooling of liquid condensed in of evaporative coolers, especially mini- or micro-channel cold plates, using only fluid- dynamic phenomena inherently available to the evaporative cooling loop. Such a means would be self-regulating while maximizing the thermal performance of the evaporative coolers.

In accordance with a broad aspect of the invention, there are provided one or more parallel two-phase (evaporative) coolers operating in closed-loop circulation mode, in which the two-phase mixture exiting the one or more coolers is sent to a chamber, and in which the vapor is separated from the saturated liquid emerging from

the liquid, the vapor is sent to one or more condensers where the vapor is fully condensed back to saturated liquid, which may then be further sub-cooled, and the condensed liquid is returned to the separation chamber. The mixing and contacting of sub-cooled condensate with the incoming vapor and saturated liquid in the chamber re-heats the condensate substantially to its saturation temperature, by using the latent heat of condensation of the vapor. This also causes a portion of the incoming vapor to pre-condense at its saturation temperature, reducing the vapor load on the condenser.

The mixed liquid in the chamber is substantially at its boiling (saturation) temperature. The net condensed liquid withdrawn from the chamber and returning to the evaporative cooler is warmer than any sub-cooled liquid emerging from the condenser.

To minimize the potential for sub-cooling of the liquid returning to the evaporative cooler from the saturation chamber, it is preferable to minimize heat losses in the transfer line, e.g. using thermally non-conductive or insulated lines, and to minimize the elevation difference (i.e., liquid pressure head, which would increase the saturating temperature) between the separation chamber and the evaporative cooler. In the event that some sub-cooling occurs as the liquid transfers from the saturation chamber to the evaporative coolers, further reheating / re- saturation of the returning liquid can be achieved by providing a heat exchange means in the immediate vicinity of the inlet to the evaporative cooler, whereby heat is transferred from the two-phase mixture exiting the evaporator, back to the sub saturated liquid returning to the evaporator.

Various configurations may be employed for supplying and returning the various streams to and from the condensers and the saturation chambers. These include, but are not limited to:

• The two-phase mixture from the evaporative cooler enters the saturation chamber above the surface of the liquid, so that the (saturated) liquid disperses into droplets which fall onto to the liquid surface, while the vapor disengages and exits to the condenser inlet from above the liquid surface.

• The two-phase mixture from the evaporative cooler enters the saturation chamber below the surface of the liquid, so that the (saturated) liquid mixes with the liquid in the chamber, while the vapor separates and bubbles up to and emerges from the liquid surface, and then exits to the condenser inlet from above the liquid surface.

• Returning the condensate above the surface of the liquid in the saturation chamber, so that the (sub-cooled) liquid disperses into droplets, and falls through and contacts the vapor in the chamber. A liquid seal-leg may optionally be used to prevent vapors from being sucked back up into the condensate outlet line (due to difference in vapor pressures of the sub-cooled liquid in the condenser and saturated liquid in the chamber).

• Returning the condensate below the surface of the liquid in the saturation chamber, so that so that the (sub-cooled) liquid mixes with and absorbs heat from saturated liquid in the chamber. The condensate return line may be oriented to promote turbulence in the liquid volume and especially to promote waves or frothing action at the surface, to promote contacting and heat transfer between the liquid and vapor in the chamber.

• Arranging the condenser as top-to-bottom flow pattern, in which the vapor enters an upper side of the condenser, and the condensate exits by gravity from a lower outlet side of the condenser. The working fluid side of the condenser may be configured in a single -pass or multi pass arrangement.

• Arranging the condenser as a“knock-back” condenser, i.e. with the vapor entering and the liquid counter-currently and substantially co-axially exiting by gravity via the underside of the condenser. • The saturation chamber may be formed from piping arranged as a tee elevated above the liquid surface, for separating the incoming two-phase mixture from the evaporative cooler, and a second tee below the liquid surface, to mix the condensate with the liquid being separated from the incoming two-phase mixture.

• The saturation chamber may be formed from piping arranged as a cross elevated above the liquid surface, where the incoming two-phase mixture from the evaporative cooler impinges on, contacts with, and then separates from the condensate returning from the condenser via the sides of the cross, with the vapor exiting to the condenser from the top of the cross, and the net liquid exits via the bottom of the cross.

• Arranging the condenser as top-to-bottom flow pattern, in which the two-phase mixture enters an upper side of the condenser, and the condensate exits by gravity from a lower outlet side of the condenser. The condenser is configured to be“self-venting”, i.e. the passages where the condensation takes place are sufficiently large such that the condensate does not fully fill the cross sections of the passages, thereby allowing any condensed liquid exiting the condenser to pressure-equalize with the incoming two-phase mixture and thereby remain saturated. With the self-venting arrangement, the condenser passages also serve as saturation chambers. The working fluid side of the condenser may be configured in a single -pass or multi pass arrangement.

One advantage of embodiments of the present invention is that they can passively facilitate the re-heating of sub-cooled condensate to return it to the saturation temperature, without requiring external heating means or controls. They can also reduce the volume of vapor entering the condensers, thereby increasing the available cooling capacity of the condensers, so that they can handle higher heat loads from the evaporative coolers. In addition, the saturation chamber can serve as a reservoir for the working fluid, and can accommodate changes in fluid volume due to thermal expansion effects at different operating temperatures.

It is to be understood that other aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, in which various embodiments of the invention are shown and described by way of illustration. As will be realized, the invention is capable for other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention.

Accordingly the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. Brief Description Of The Drawings

Fig. 1A is a first compound figure showing local boiler temperature measurements plotted against temperature, reproduced from Fig. 7 of the Zuk et al. paper;

Fig. 1B is a second figure showing local boiler temperature measurements plotted against temperature, reproduced from Fig. 11 of the Zuk et al. paper;

Fig. 2A is a schematic view of a first conventional condenser arrangement in which there is a direct return from a condenser;

Fig. 2B is a schematic view of a second conventional condenser arrangement in which there is an external reservoir;

Fig. 2C is a schematic view of a third conventional condenser arrangement in which a condenser serves as a reservoir;

Fig. 3 is a schematic view of a first embodiment according to the invention in which there is an above-surface two-phase inlet and condensate return;

Fig. 4 is a schematic view of a second embodiment according to the invention in which there is a sub-surface two-phase inlet and condensate return;

Fig. 5 is a schematic view of a third embodiment according to the invention in which there is an above-surface two-phase inlet and sub-surface condensate return;

Fig. 6 is a schematic view of a fourth embodiment according to the invention in which there is a sub-surface two-phase inlet and sub-surface condensate return;

Fig. 7 is a schematic view of a fifth embodiment according to the invention in which there is an above- surface two-phase inlet and condensate return with seal-leg;

Fig. 8 is a schematic view of a sixth embodiment according to the invention in which there is a sub-surface two-phase inlet and condensate return with seal-leg;

Fig. 9 is a schematic view of a seventh embodiment according to the invention in which there is an above- surface two-phase inlet and knock-back condenser;

Fig. 10 is a schematic view of an eighth embodiment according to the invention in which there is a sub-surface two-phase inlet and knock-back condenser;

Fig. 11 is a schematic view of a ninth embodiment according to the invention in which there is a bottom two-phase inlet and outlet of knock-back condenser;

Fig. 12 is a schematic view of a tenth embodiment according to the invention in which there are two piping tees as saturation chamber; Fig. 13 is a schematic view of an eleventh embodiment according to the invention in which there is a piping cross as saturation chamber;

Fig. 14 is a schematic view of a twelfth embodiment according to the invention in which there is a top-entering self-venting condenser and tubes that keep condensate at saturation, with a condenser outlet serving as reservoir;

Fig. 15 is a schematic view of a thirteenth embodiment according to the invention in which there is a self-venting condenser with external phase separator riser and tubes that keep condensate at saturation, with a condenser outlet serving as reservoir; and

Fig. 16 is a schematic view of a fourteenth embodiment according to the invention in which there is a top-entering self-venting multi-pass condenser and tube that keep condensate at saturation, with a condenser outlet pipe serving as reservoir.

Description of Various Embodiments

Referring to Fig. 3, in one general embodiment of a cooling system 10 according to the invention, one or more parallel two-phase (evaporative) coolers 14 operate in closed-loop circulation mode, in which a two-phase mixture 16 exiting the one or more coolers is sent to a saturation chamber 38, where the saturated vapor 36 is separated from the saturated liquid 21 emerging from the liquid. The vapor is sent to one or more condensers 18 where the vapor is fully condensed back to saturated liquid by a coolant 22, 24, and the saturated liquid may then be further sub-cooled. This condensed liquid is returned to the separation chamber 38. The mixing and contacting of sub-cooled condensate with the incoming vapor and saturated liquid in the chamber re-heats the condensate substantially to its saturation temperature, by using the latent heat of condensation of the vapor, and a portion of the incoming vapor pre-condenses at its saturation temperature. The net condensed liquid 21 is withdrawn from the chamber and returned to the evaporative cooler 14 at substantially its saturation temperature.

In a specific embodiment, the two-phase mixture from the evaporative cooler 14 enters the saturation chamber 38 above the surface of the liquid, so that the (saturated) liquid disperses into droplets 32 which fall onto to the liquid surface, while the vapor disengages and exits to the condenser inlet from above the liquid surface.

Referring to Fig. 4, in another embodiment, the two-phase mixture from the evaporative cooler enters the saturation chamber 38 below the surface of the liquid, so that the (saturated) liquid mixes with the liquid in the chamber, while the vapor separates and forms bubbles 33 that rise up to and emerge from the liquid surface, and then exit to the condenser inlet from above the liquid surface.

Referring to Figs. 5-8, in other embodiments, the condensate can also return above the surface of the liquid in the saturation chamber, so that the (sub-cooled) liquid disperses into droplets 32, and falls through and contacts the vapor 36 in the chamber. A liquid seal-leg 40 may optionally be used to prevent vapors from being sucked back up into the condensate outlet line (due to difference in vapor pressures of the sub-cooled liquid in the condenser and saturated liquid in the chamber).

In further embodiments, the condensate returns below the surface of the liquid in the saturation chamber, so that so that the (sub-cooled) liquid mixes with and absorbs heat from saturated liquid in the chamber. The condensate return line may be oriented to promote turbulence in the liquid volume and especially to promote waves or frothing action at the surface, to promote contacting and heat transfer between the liquid and vapor in the chamber.

Referring to Figs. 9-11, in other embodiments, the condenser is arranged as a“knock-back” condenser 48, i.e. with the vapor entering and the liquid counter-currently and substantially co axially exiting by gravity via the underside of the condenser.

Referring to Fig. 12, in a further embodiment, the saturation chamber may be formed from piping arranged as a first tee 50 elevated above the liquid surface, for separating the incoming two- phase mixture from the evaporative cooler, and a second tee 52 below the liquid surface, to mix the condensate with the liquid being separated from the incoming two-phase mixture.

Referring to Fig. 13, in another embodiment, the saturation chamber may be formed from piping arranged as a cross 54 elevated above the liquid surface, where the incoming two-phase mixture from the evaporative cooler impinges on, contacts with, and then separates from the condensate returning from the condenser via the sides of the cross, with the vapor exiting to the condenser from the top of the cross, and the net liquid exits via the bottom of the cross.

Referring to Fig. 14-15, in a further embodiment, the condenser 58 is arranged to provide a top-to-bottom flow pattern, where the two-phase mixture enters an upper side of the condenser, and the condensate exits by gravity from a lower outlet side of the condenser. The condenser is configured to be“self-venting”, i.e. the passages 62 where the condensation takes place are sufficiently large such that the condensate does not fully fill the cross sections of the passages, thereby allowing any condensed liquid exiting the condenser to pressure-equalize with the incoming two-phase mixture and thereby remain saturated. With the self-venting arrangement, the condenser passages also serve as saturation chambers. The working fluid side of the condenser may be configured in a single-pass or multi-pass arrangement.

In another embodiment of the invention, the potential for sub-cooling of the liquid returning to the evaporative cooler from the saturation chamber is minimized by minimizing heat losses in the transfer line, e.g. using thermally non-conductive or insulated lines

In a further embodiment of the invention, the potential for sub-cooling of the liquid returning to the evaporative cooler from the saturation chamber is minimized by minimizing the pressure elevation between the separation chamber and the evaporative cooler. This is preferably

accomplished by operating the cooling loop as a thermo-syphon, where the circulation is driven by gravity and density differences, rather than pumping the liquid to the evaporative cooler. In the thermo-syphon mode, the elevation difference [liquid pressure head] between the separation chamber and the evaporative cooler is reduced to the minimum required to ensure adequate circulation.

In another embodiment of the invention, in the event that some sub-cooling occurs as the liquid transfers from the saturation chamber to the evaporative coolers, further reheating / re saturation of the returning liquid can be achieved by providing a heat exchange means in the immediate vicinity of the inlet to the evaporative cooler, whereby heat is transferred from the two- phase mixture exiting the evaporator, back to the sub-saturated liquid returning to the evaporator.

As shown in Fig. 15, a riser 64 can be placed upstream of the condenser 58 to separate vapor from the two-phase flow and direct it to the top of the condenser while combining the separated liquid with the liquid exiting the condenser.

Referring to Fig. 16, in a further embodiment, the condenser 68 is arranged to provide a top- to-bottom flow pattern, where the vapor enters an upper side of the condenser, and the condensate exits by gravity from a lower outlet side of the condenser, such as through a serpentine path 70. The working fluid side of the condenser may be configured in a single-pass or multi-pass arrangement.

The present invention has now been described in connection with a number of specific embodiments thereof. However, numerous modifications which are contemplated as falling within the scope of the present invention should now be apparent to those skilled in the art. Therefore, it is intended that the scope of the present invention be limited only by the scope of the claims appended hereto. In addition, the order of presentation of the claims should not be construed to limit the scope of any particular term in the claims.

What is claimed is: