More particularly, this invention relates to an apparatus (and methods therefor) using volatile fluids for two-phase heat transfer without first de-gassing the fluid or having to maintain the apparatus in a de-gassed state.

Background Two-phase heat transfer can generally be described as a process wherein the heat- transfer fluid changes phase from a liquid to a vapor (or vice-versa) thus utilizing the latent heat of vaporization for the heat-transfer fluid to either cool or heat a surface.

Applications utilizing two-phase heat transfer include, but are not limited to, direct contact electronic cooling (for example, cooling of supercomputers, avionics, transformers, train- traction electronics, and power electronics), heat pipes/thermosyphons (that is, saturated devices designed to operate as thermal superconductor or as thermo diodes), cooling of fuel cells (that is, electrochemical cells which produce useable electrical energy from a chemical reaction utilizing an external chemical fuel source), electrochemical batteries (that is, similar to fuel cells except they contain their own fuel), and other applications.

Fluids used in such applications are typically required to possess low or zero ozone depletion potential, low toxicity, be preferably non-flammable, be non-aqueous, be inert, be dielectric, etc.

Volatile halogenated organic compounds, such as perfluorocarbons (PFCs), perfluoropolyethers (PFPEs), hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluoroethers (HFEs), hydrohalofluoroethers (HHFEs), hydrochlorocarbons (HCCs), hydrobromocarbons (HBCs), perfluoroalkyl iodides (PFIs), perfluoroolefins (PFOs), fluorinated compounds containing at least one aromatic moiety, and combinations thereof, can

be used as two-phase heat-transfer fluids. These halogenated organic compounds are often used as heat-transfer fluids because many of them possess the requisite properties. These properties ensure, for example, that the heat-transfer fluids will not degrade sensitive components or allow electric discharge or parasitic current drains.

Fluorinated organic compounds (for example, PFCs, PFPEs, HFCs, HCFCs, HFEs, HHFEs, PFIs, and PFOs) are particularly suitable as heat-transfer fluids because of the combination of their physical properties and their safety of use. Fluorinated organic compounds are chemically inert (defined herein as being non-reacting with sensitive components (for example, capacitors, diodes, transistors, process fluids, etc.)) and have excellent dielectric properties (defined herein as being non-conductive). Thus, fluorinated organic compounds will not degrade sensitive components or allow electric discharge or parasitic current drains.

A common practice when using such fluids in a two-phase mode is to first de-gas the fluid to remove any dissolved air or other non-condensable gases and further to use the fluid in a vacuum-tight system to ensure that such gases do not re-enter the system if it reaches a vacuum condition (that is, the internal pressure is less than the ambient pressure). The presence of air in a system is commonly believed to adversely affect condenser performance by reducing the partial pressure of the fluid vapor and thus reducing the temperature required to condense the vapor.

For example, C4F9OCH3 boils and condenses at about 61 °C at one atmosphere (absolute pressure). Assuming ideal gas behavior, the presence of 20 percent by volume air in the condenser lowers the partial pressure of the C4F9OCH3 vapor to approximately 0.8 atmosphere. Though the boiling temperature remains at about 61 °C, the temperature required to condense the vapor present in the resultant vapor/air mixture is now about 55 °C. The affect of air on required condenser temperature is shown in Fig. 1. Thus, with air present in the vapor, it is a common belief that vapor can be condensed only by lowering the condenser temperature or by increasing the condenser surface area. These alterations are undesirable because they

increase the system cost, for example, by necessitating larger condenser surfaces, larger fans, and possibly even requiring mechanical refrigeration.

Unfortunately, keeping air out of most of these systems is difficult if not impossible. The boiling points of the fluids which are desirable for these applications are almost always such that the system will be under vacuum during some period of time when the system is not operational and possibly even when it is operational. Under vacuum conditions, non-condensable gases such as air will find their way into all but the most hermetic systems. Because systems of the type listed above inherently require wires, connections, and access ports into the cooled region, they are by nature non-hermetic to the degree required to properly operate in a de-gassed state. Air generally leaks in eventually and thus degrades system performance.

Operating a two-phase system in a de-gassed state introduces other complications. Because the system is gas free, the temperature of boiling depends upon the temperature of the condenser and the condenser capacity. Operation at a specified temperature is possible only when the condenser capacity can be precisely modulated to keep the boiling temperature constant. In practice, this can be accomplished only with complicated active control schemes that modulate the heat-transfer fluid flow rate or temperature (in single phase systems) or the condenser capacity in two-phase systems.

This dependency may not be a problem for applications which are not sensitive to the boiling temperature. This is, however, a problem for applications such as electrochemical fuel cell cooling where the operating temperature which is dictated by the evaporator temperature is a critical operating parameter and where the temperature of the air cooling the condenser can range from about-50 °C to about 55 °C, for example, in Arizona (55 °C) to Alaska (-50 °C). In such applications, the requisite modulation of the condenser capacity can be accomplished only with expensive measures such as proportional, integral, derivative (PID) controllers, valves, louvers, variable speed fans, etc. Without these measures, evaporator temperatures can broadly vary.

Thus, the need exists for an apparatus and method for two-phase cooling which can operate without de-gassing the heat-transfer fluid and without needing to maintain the

system in the de-gassed state while having the ability to maintain a substantially constant evaporation temperature.

Summarv of the Invention The present invention provides an apparatus for two-phase heat transfer where volatile fluids such as volatile halogenated organic compounds may be used without first de-gassing the volatile fluid or having to maintain the apparatus/system in a de-gassed state. The apparatus of the present invention allows for self-modulating two-phase heat transfer and for evaporation temperatures which, once the system is operating, are stable to within a couple of °C regardless of the temperature of the heat sink (for example, the fluid cooling the condenser). Expensive condenser capacity modulation schemes are unnecessary when the apparatus of the present invention is employed.

The present invention provides an apparatus for two-phase heat transfer comprising a heat source comprising energy; an evaporator through which said energy is dissipated containing a heat-transfer fluid as a condensate and a vapor; a condenser comprising a first end and a second end, said condenser first end in fluid connection with said evaporator, said condenser being of a size such that said condensate is not entrained by said vapor; a heat sink for dissipating said energy from said condenser; and an expansion device in fluid connection with said condenser second end, said expansion device containing heat-transfer fluid vapor and non-condensable gas and being of a size to allow for expansion of said vapor and said gas into said expansion device while maintaining a substantially constant operating pressure by causing said vapor and said gas to flow into and out of said condenser second end to optimize dissipation of said energy; wherein said heat source is maintained at substantially constant temperature and said apparatus is maintained at substantially constant operating pressure.

The apparatus of the present invention may further comprise an adjuster device.

Another embodiment of the present invention is a method of two-phase heat transfer which comprises a method suitable for two-phase heat transfer comprising the steps of : (a) providing non-condensable gas; (b) causing energy to flow from a heat source to an evaporator which contains heat-transfer fluid as a condensate; (c) after step (b), said condensate absorbing energy from said evaporator and forming a vapor; (d) after step (c), causing a heat sink to come into contact with a condenser containing said vapor to form

in-part condensate of said vapor and to remove energy; (e) after step (d), causing the condensate to return to said evaporator; (f) after step (c), causing some vapor to flow through to an expansion device where said expansion device adjusts to provide sufficient volume for said vapor; (g) causing some non-condensable gas to flow through to an expansion device where said expansion device adjusts to provide sufficient volume for said gas; (h) after step (f), causing some vapor to flow through to said condenser by the expansion device contracting; and (i) after step (g), causing some gas to flow through to said condenser by the expansion device contracting, wherein said heat source is maintained at substantially constant temperature and said apparatus is maintained as substantially constant operating temperature.

Brief Description of the Drawings Fig. 1 is a graph of condensation temperature versus volume percent of non- condensable gas in an ideal vapor/non-condensable gas mixture.

Fig. 2a is a graph of condenser operating temperature over time for varying cooling water temperatures.

Fig. 2b is a graph of condenser operating temperature over time in a saturated system (that is, air removed) for varying condenser water temperatures.

Fig. 2c is a graph of evaporator operating temperature over time using a similar configuration as the apparatus depicted in Fig. 5.

Fig. 3 is a schematic of an apparatus of the present invention comprising a heat source 30, evaporator 32, vertical pipe 34, condenser 36, heat sink 37, and an expansion device 38.

Fig. 4 is a schematic of an apparatus of the present invention comprising a heat source 40, evaporator 42, vapor line 44, liquid return line 45, condenser 46, heat sink 47, and an expansion device 48.

Fig. 5 is a schematic of an apparatus of the present invention comprising a heat source 50, an evaporator 52, a condenser 56, an expansion device 58, a vapor line 54, a liquid return line 55, and a heat sink 57.

Fig. 6 is a schematic of an apparatus of the present invention comprising a heat source 60, thermocouple 61, evaporator 62, condenser 66, thermocouple 65, cooling water in 67a, cooling water out 67b, and an expansion device 68.

These figures are not to scale and are intended to be merely illustrative and non- limiting.

Detailed Description Of Illustrative Embodiments The present invention provides an apparatus for two-phase heat transfer. The apparatus of the present invention comprises a heat source, evaporator, condenser, heat sink, expansion device, and a heat-transfer fluid. The evaporator and condenser may be one unit. Moreover, the condenser and expansion device may be one unit. Optionally, the apparatus may further comprise an adjuster device. The apparatus of the present invention may be used with applications/systems requiring either heating or cooling, for example an application requiring a constant surface temperature which generates or absorbs heat during use.

The apparatus of the present invention provides for the evaporation or condensation temperature to be held substantially constant at the saturation temperature of the heat-transfer fluid at the pressure selected by the user and maintained by the expansion device. There are no active controls required, thus complexity and cost associated therewith can be avoided. In addition, no costly and complex heat-transfer fluid de-gassing is required. Further, no costly and complex measures are required to ensure that no air enters into the system. If the heat-transfer fluid is properly selected, the system, including the apparatus of the present invention, may run at zero to slight gage pressure and therefore, high pressure components may not be required. Therefore, less expensive, low pressure components such as thin-walled or plastic heat exchangers may be used.

Optionally, an adjuster device which will increase the system pressure may be used if the use of a low boiling temperature fluid is desirable or if operating temperature modulation is desirable.

Heat-Transfer Fluid As discussed above, the present invention utilizes a heat-transfer fluid. The particular heat-transfer fluid is largely chosen based on the application/system with which the apparatus is used. Generally, the heat-transfer fluids are inert, dielectric, non- flammable, volatile, non-aqueous, and environmentally acceptable.

The heat-transfer fluids are preferably non-flammable, which is defined herein as having a flash point significantly greater (for example, 10 to 20 °C) than the operating temperature for the application/system. Preferably, the flash point is above about 60 °C and more preferably the flash point is above about 100 °C.

The heat-transfer fluids are volatile under the application conditions. Preferably, the boiling point will be less than about 120 °C at atmospheric pressure. More preferably, the boiling point of the heat-transfer fluids will range from about 50 °C to about 80 °C at the system pressure.

Suitable heat-transfer fluids for use in this invention include, but are not limited to, halogenated (that is, fluorine-, chlorine-, bromine and/or iodine-substituted) and non- halogenated organic compounds. Classes of non-halogenated organic compounds which can be used include linear, branched and cyclic aliphatic hydrocarbons; aromatic hydrocarbons; alcohols; ethers; ketones; and esters. Examples of such non-halogenated organic compounds include n-heptane, n-octane, cyclohexane, toluene, ethanol, n- propanol, isopropanol, methyl ethyl ketone, ethyl acetate and diisopropyl ether. However, these relatively low boiling non-halogenated organic compounds are flammable and toxic so are not desirable as heat-transfer fluids. Halogenated organic compounds are preferred as heat-transfer fluids. Useful halogenated organic compounds include perfluorocarbons (PFCs), perfluoropolyethers (PFPEs), hydrofluorocarbons (HFCs), hydrofluoroethers (HFEs), hydrochlorofluorocarbons (HCFCs), hydrohalofluoroethers (HHFEs), chlorofluorocarbons (CFCs), hydrochlorocarbons (HCCs), hydrobromocarbons (HBCs), perfluoroiodides (PFIs), perfluoroolefins (PFOs), fluorinated compounds containing at least one aromatic moiety, or a combination thereof. Preferably, the halogenated organic compound (s) comprise a fluorinated organic compound. Smaller amounts of flammable halogenated or flammable non-halogenated organic compounds can be incorporated in the heat-transfer fluid (for example, azeotropic or near-azeotropic compositions), provided that the resulting mixture is non-flammable and has a very narrow boiling point range.

Until recently, liquid CFCs such as CFC-113 (CCIF2CC12F) and CFC-11 (CC13F) were considered ideal candidates for heat-transfer applications, exhibiting excellent performance, low cost, and no safety drawbacks. However, as of the 1987 Montreal Protocol, CFCs have been legislated out of production due to their proven degradation of the stratospheric ozone layer.

HCFCs useful as heat-transfer fluids include CF3CHC12, CH3CC12F, CF3CF2CHC12 and CCIF2CF2CHCIF. However, in the long term, HCFCs may also be legislated out of production due to ozone layer degradation.

Useful PFCs include perfluorinated fluids that can be single compounds, but usually will be a mixture of such compounds. The PFCs have molecular structures which can be straight-chained, branched-chained or cyclic, or a combination thereof, such as perfluoroalkylcycloaliphatic, are fluorinated to greater than at least 95 molar percent substitution of the carbon chain, and are preferably free of ethylenic unsaturation. The skeletal chain of the molecular structure can contain catenary (that is,"in-chain") oxygen, trivalent nitrogen or hexavalent sulfur heteroatoms bonded only to carbon atoms, such heteroatoms providing stable linkages between fluorocarbon groups and not interfering with the inert character of the fluid. The perfluorinated fluid will preferably have about 5 to about 9 carbon atoms, the maximum number being dictated by the desired boiling point.

Preferred PFCs typically contain about 60 to about 76 weight percent carbon-bonded fluorine. U. S. Patent Nos. 2,500,388 (Simons); 2,519,983 (Simons); 2,594,272 (Kauck et al.); 2,616,927 (Kauck et al.); and 4,788,339 (Moore et al.), describe the preparation of inert perfluorinated compounds, such as perfluorinated hydrocarbons, ethers, tertiary amines and aminoethers, said preparation involving electrochemical fluorination in anhydrous HF medium. PFCs useful in this invention also include those described in Encyclopedia of Chemical Technology, Kirk-Othmer, Third Ed., Vol. 10, pages 874-81, John Wiley & Sons (1980).

Useful PFCs include perfluoro-4-methylmorpholine, perfluorotriethylamine, perfluoro-2-ethyltetrahydrofuran, perfluoro-2-butyltetrahydrofuran, perfluoropentane, perfluorohexane, perfluoro-4-methylmorpholine, perfluorodibutyl ether, perfluoroheptane, perfluorooctane, perfluorotripropylamine, perfluorononane, and mixtures thereof.

Preferred inert fluorochemical liquids include perfluorohexane, perfluoro-2- butyltetrahydrofuran, perfluoroheptane, perfluorooctane, and mixtures thereof, especially perfluoroheptane and perfluorooctane. Commercially available PFCs useful in this invention include FLUORINERTTM liquids, for example, FC-72, FC-75, FC-77 and FC- 84, described in the 1990 product bulletin #98-0211-5347-7 (101.5) NPI, and mixtures thereof. Also useful as fluids are FLUORINERTTM liquids FC-3284 and FC-6003.

FLUORINERTTM liquids are available from Minnesota Mining and Manufacturing Company, St. Paul, Minnesota.

Useful PFPEs are described in U. S. Patent Nos. 3,250,807 (Fritz et al.); 3,250,808 (Moore et al.); and 3,274,239 (Selman). PFPEs derived by polymerization of perfluoropropylene oxide followed by stabilization, for example, with fluorinating agents are available as KRYTOXT K fluorinated oils from E. 1. du Pont de Nemours & Co., Wilmington, Delaware. Fluids derived from tetrafluoroethylene and hexafluoropropylene oxide are available as GALDENTM HT fluids from Ausimont Corp., Thorofare, New Jersey.

Useful HFCs include organic compounds having a 3-to 8-carbon saturated backbone substituted with both hydrogen and fluorine atoms, but essentially no other atoms, such as chlorine. HFCs having a 4-to 8-carbon backbone are preferred. The carbon backbone can be straight, branched, cyclic, or mixtures of these. Useful HFCs include compounds having more than approximately 5 molar percent fluorine substitution, or less than 95 molar percent fluorine substitution, based on the total number of hydrogen and fluorine atoms bonded to carbon, and specifically excludes PFCs, PFOs, PFPEs, CFCs, HCFCs, and HHFEs.

Useful HFCs can be selected from: (1) linear or branched compounds of Formula I: C4HnF 10-n, wherein n < 5 representative compounds of Formula I include CHF2 (CF2) 2CF2H, CF3CF2CH2CH2F, CF3CH2CF2CH2F, CH3CHFCF2CF3, CF3CH2CH2CF3, CH2FCF2CF2CH2F, CF3CH2CF2CH3, CHF2CH (CF3) CF3, and CHF (CF3) CF2CF3; (2) linear or branched compounds of Formula II: (II) CSHnF12-n, wherein n < 6

representative compounds of Formula II include CF3CH2CHFCF2CF3, CF3CHFCH2CF2CF3 CF3CH2CF2CH2CF3, CF3CHFCHFCF2CF3, CF3CH2CH2CF2CF3, CH3CHFCF2CF2CF3, CF3CF2CF2CH2CH3, CH3CF2CF2CF2CF3, CF3CH2CHFCH2CF3, CH2FCF2CF2CF2CF3, CHF2CF2CF2CF2CF3, CH3CF (CHFCHF2) CF3, CH3CH (CF2CF3) CF3, CHF2CH (CHF2) CF2CF3, CHF2CF (CHF2) CF2CF3, CHF2CF2CF (CF3) 2, and C5F I I H ; (3) linear or branched compounds of Formula III: (III) C6HnFl4-n, wherein n < 7 representative compounds of Formula III include CHF2 (CF2) 4CF2H, (CF3CH2) 2CHCF3, CH3CHFCF2CHFCHFCF3, HCF2CHFCF2CF2CHFCF2H, H2CFCF2CF2CF2CF2CF2H, CHF2CF2CF2CF2CF2CHF2, CH3CF (CF2H) CHFCHFCF3, CH3CF (CF3) CHFCHFCF3, CH3CF (CF3) CF2CF2CF3, CHF2CF2CH (CF3) CF2CF3, CHF2CF2CF (CF3) CF2CF3 and C6F13H; (4) linear or branched compounds of Formula IV: (IV) C7HnF 16-n wherein n S 8 representative compounds of Formula IV include CH3CHFCH2CF2CHFCF2CF3, CH3 (CF2) 5CH3, CH3CH2 (CF2) 4CF3, CF3CH2CH2 (CF2) 3CF3, CH2FCF2CHF (CF2) 3CF3, CF3CF2CF2CHFCHFCF2CF3, CH3CF2C (CF3) 2CF2CH3, CF3CF2CF2CHFCF2CF2CF3, CH3CH (CF3) CF2CF2CF2CH3, CH3CF (CF3) CH2CFHCF2CF3, CH3CF (CF2CF3) CHFCF2CF3, CH3CH2CH (CF3) CF2CF2CF3, CHF2CF (CF3) (CF2) 3CH2F, CHF2CF (CF3) (CF2) 3CF3 <BR> <BR> CF3CHFCHFC4F9, CF3CF2CHFCHFC3F7, CF3CHFCH2C4F9, CF3CH2CHFC4F9,<BR> <BR> <BR> <BR> <BR> CF3CF2CH2CHFC3F7 and CF3CF2CHFCH2C3F7;

(5) highly fluorinated cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl compounds having at least as many fluorine atoms as hydrogen atoms, such as: CF3-c- (- CF-CF2-CH2-CH2-), (6) Linear or branched HFCs as represented below in Formula V: (V) C 8HnF 18-n, wherein n < 9 Useful HFCs of Formula V include CH3CH2CH2CH2CF2CF2CF2CF3, CH3 (CF2) 6CH3, CHF2CF (CF3) (CF2) 4CHF2, CHF2CF (CF3) (CF2) 4CHF2, CH3CH2CH (CF3) CF2CF2CF2CF3, CH3CF (CF2CF3) CHFCF2CF2CF3, CH3CH2CH2CHFC (CF3) 2CF3, CH3C (CF3) 2CF2CF2CF2CH3, CH3CH2CH2CF (CF3) CF (CF3) 2 and CH2FCF2CF2CHF (CF2) 3CF3.

Preferred HFCs include CF3CFHCFHCF2CF3, CsF 11 H, C6F 13H, CF3CF2CH2CH2F, CHF2CF2CF2CHF2,1,2-dihydroperfluorocyclopentane and 1,1,2- trihydroperfluorocyclopentane. Useful HFCs include HFCs available under the VERTRELTM, available from E. I. duPont de Nemours & Co., and under the ZEORORA- HTM, available from Nippon Zeon Co. Ltd., Tokyo, Japan.

Generally the most suitable fluorinated compounds will be hydrofluoroethers, as they exhibit the best combination of good fluid heat transfer performance along with optimum safety (non-flammability and low toxicity) and environmental (non-ozone depleting and low global warming) properties. HFEs are chemical compounds containing carbon, fluorine, hydrogen, one or more ether oxygen atoms, and optionally one or more additional catenary heteroatoms within the carbon backbone, such as sulfur or trivalent

nitrogen. The HFE can be straight-chained, branched-chained, or cyclic, or a combination thereof, such as alkylcycloaliphatic. Preferably, the HFE is free of unsaturation.

These highly fluorinated ethers may be depicted by Formula VI: (VI) (Rl-0)R2 where, in reference to Formula VI, n is a number from 1 to 3 inclusive and RI and R2 are the same or are different from one another and are selected from the group consisting of alkyl, aryl, and alkylaryl groups. At least one of RI and R2 contains at least one fluorine atom, and at least one of R1 and R2 contains at least one hydrogen atom, either or both groups RI and R2 can optionally contain one or more catenary heteroatoms, and preferably the total number of fluorine atoms in the HFE at least equals the total number of hydrogen atoms. RI and R2 may also be linear, branched, or cyclic, and may contain one or more unsaturated carbon-carbon bonds, though preferably R1 and R2 are both saturated.

Preferred HFEs include: (1) segregated HFEs, wherein ether-bonded alkyl or alkylene, etc., segments of the HFE are either perfluorinated (for example, perfluorocarbon) or non-fluorinated (for example, hydrocarbon), but not partially fluorinated; and (2) non-segregated HFEs, wherein at least one of the ether-bonded segments is neither perfluorinated nor fluorine-free but is partially fluorinated (that is, contains a mixture of fluorine and hydrogen atoms).

Segregated HFEs include HFEs which comprise at least one mono-, di-, or trialkoxy-substituted perfluoroalkane, perfluorocycloalkane, perfluorocycloalkyl- containing perfluoroalkane, or perfluorocycloalkylene-containing perfluoroalkane compound. These HFEs are described, for example, in PCT Publication No. WO 96/22356, and can be represented below in Formula VII: (VII) Rf- (O-Rh) x wherein:

x is from 1 to about 3, and Rf is a perfluorinated hydrocarbon group having a valency x, which can be straight, branched, or cyclic, etc., and preferably contains from 3 to about 7 carbon atoms, and more preferably contains from 3 to about 6 carbon atoms; each Rh is independently a linear or branched alkyl group having from 1 to about 3 carbon atoms; wherein either or both of the groups Rf and Rh can optionally contain one or more catenary heteroatoms; and wherein the sum of the number of carbon atoms in the Rf group and the number of carbon atoms in the Rh group (s) is preferably between 4 and about 9.

Preferably, x is 1. Most preferable Rf groups include C3F7-isomers (that is, n-, iso-), C4F9-isomers (that is, n-, iso-, sec-, tert-), CsF1 1-isomers, C6F13-isomers, and perfluorocyclohexyl; and most preferable Rh groups include methyl and ethyl.

Representative compounds described by Formula VII useful in the present invention include, but are not limited to, the following compounds:

n-C4F9OC2H5<BR> <BR> n-C3F70CH3<BR> <BR> n-C3F70C2HS

C5F11OC2H5 CF3OC2F4OC2H5 (CF3) 2CFOCH3 (CF3)3C-OCH3 (CF3) 3C-OC2H5 (C2F5)2NCF2CF2OCH3 (CF3) 2N (CF2) 30CH3 (CF3)2N(CF2)2OC2H5 (C2F5)2NCF2CF2OCH3

C2F5CF (OCH3) CF (CF3) 2 C2F5CF (OC2H5) CF (CF3) 2 CF3CF (OCH3) CF (CF3) 2 CF3CF (OC2H5) CF (CF3) 2 wherein cyclic structures designated with an interior"F"are perfluorinated.

Particularly preferred segregated HFEs of Formula VII include n-C3F7OCH3, (CF3) 2CFOCH3, n-C4F90CH3, (CF3) 2CFCF20CH3, n-C3F7OC2Hs n-C4F90C2H5, (CF3) 2CFCF20C2H5, (CF3) 3COCH3, (CF3) 3COC2H5, CF3CF (OCH3) CF (CF3) 2, CF3CF (OC2H5) CF (CF3) 2, and mixtures thereof. Segregated HFEs are available as 3MTM NOVECTM HFE-7100 and HFE-7200 specialty fluids from Minnesota Mining and Manufacturing Company.

Also useful as heat-transfer fluids are azeotropes and azeotrope-like compositions which are blends of segregated HFEs with non-halogenated organic compounds.

Especially useful are the azeotropes and azeotrope-like compositions consisting of blends of C4FgOCH3, C4FgOC2Hs and C3F70CH3 with organic solvents.

Such blends of C4F9OCH3 with organic solvents are described in PCT Publication No. WO 96/36689. Useful binary C4F9OCH3/solvent azeotropes and azeotrope-like compositions include blends of C4FgOCH3 with the following solvents: straight chain, branched chain and cyclic alkanes having from 6 to 8 carbon atoms; cyclic and acyclic ethers having from 4 to 6 carbon atoms; acetone; chlorinated alkanes having 1,3 or 4 carbon atoms; chlorinated alkenes having 2 carbon atoms; alcohols having from 1 to 4 carbon atoms; partially fluorinated alcohols having 2 to 3 carbon atoms; 1-bromopropane; acetonitrile; HCFC-225ca (1,1-dichloro-2,2,3,3,3- pentafluoropropane); and HCFC-225cb 2,2,3- pertafluoropropane). Useful ternary C4FgOCH3/solvent azeotropes and azeotrope-like compositions include blends of C4F9OCH3 with the following solvents

pairs: trans-1,2-dichloroethylene and alcohols having from 1 to 4 carbon atoms; trans-1,2- dichloroethylene and partially fluorinated alcohols having 2 to 3 carbon atoms; trans-1,2- dichloroethylene and acetonitrile; and HCFC-225 and alcohols having from 1 to 2 carbon atoms.

Such blends of C4F9OC2H5 with organic solvents are described in PCT Publication No. WO 96/36688. Useful binary C4F9OC2H5/solvent azeotropes and azeotrope-like compositions include blends of C4FgOC2Hs with the following solvents: straight chain, branched chain and cyclic alkanes having from 6 to 8 carbon atoms; esters having 4 carbon atoms; ketones having 4 carbon atoms; disiloxanes having 6 carbon atoms; cyclic and acyclic ethers having from 4 to 6 carbon atoms; alcohols having from 1 to 4 carbon atoms; partially fluorinated alcohols having 3 carbon atoms; chlorinated alkanes having 3 or 4 carbon atoms; chlorinated alkenes having 2 or 3 carbon atoms; 1- bromopropane; and acetonitrile.

Such blends of C3F70CH3 with organic solvents are described in PCT Publication No. WO 98/37163. Useful binary C3F70CH3/solvent azeotropes and azeotrope-like compositions include blends of C3F70CH3 with the following solvents: straight chain, branched chain and cyclic alkanes having from 5 to 7 carbon atoms; methyl formate; acetone; methanol; 1,1,1,3,3,3-hexafluoro-2-propanol; methylene chloride and trans-1,2-dichloroethylene. Useful ternary C3F70CH3/solvent azeotropes and azeotrope- like compositions include blends of C3F70CH3 with the following solvents pairs: trans- 1,2-dichloroethylene and methanol; trans-1,2-dichloroethylene and 1,1,1,3,3,3-hexafluoro- 2-propanol; methylene chloride and methanol; and methylene chloride and 1,1,1,3,3,3- hexafluoro-2-propanol.

Useful non-segregated HFEs include alpha-, beta-and omega-substituted hydrofluoroalkyl ethers such as those described in U. S. Patent No. 5,658,962 (Moore et al.), and those described by Marchionni et al. in"Hydrofluoropolyethers,"Journal of Fluorine Chemistry 95 (1999), pp. 41-50, which can be described by the general structure shown in Formula VIII: <BR> <BR> <BR> <BR> (VIII)<BR> <BR> <BR> <BR> <BR> X- [R-O] yR"H wherein:

X is either F, H, or a perfluoroalkyl group containing from 1 to 3 carbon atoms which is optionally hydro-substituted in the omega position; each Rf is independently selected from the group consisting of-CF2-,-C2F4-, and -C3F6-, wherein, when X is perfluorinated, X and at least a portion of the adjacent Rf' group taken together can form a perfluorocycloalkyl group; R"is a divalent organic radical having from 1 to about 3 carbon atoms, and may be perfluorinated, unfluorinated or partially fluorinated; and y is an integer from 1 to 7; wherein when X is F, R"contains at least one F atom; and wherein preferably the total number of carbon atoms is between about 3 and about 8.

Representative compounds described by Formula VIII useful in the present invention include, but are not limited to, the following compounds: <BR> <BR> <BR> <BR> <BR> C4F90C2F4H<BR> <BR> <BR> <BR> <BR> HC3F60C3F6H HC3F60CH3 <BR> <BR> <BR> <BR> CsF 11 OC2F4H<BR> <BR> <BR> <BR> <BR> C6F 1 3°CF2H<BR> <BR> <BR> <BR> <BR> C3F70CH2F HCF20CF20CF2H <BR> <BR> <BR> <BR> HCF20CF20C2F40CF2H<BR> <BR> <BR> <BR> <BR> HCF20C2F40C2F40CF2H C3F70 [CF (CF3) CF20] pCF (CF3) H, wherein p = 0 to 1 HCF20C2F40CF2H HCF20CF20CF20CF2H <BR> <BR> <BR> HCF20C2F40C2F40CF2H<BR> <BR> <BR> <BR> <BR> c-C6F 11 OCF2H<BR> <BR> <BR> <BR> <BR> c-C6F 11 OCH2F

Preferred non-flammable, non-segregated HFEs include C4F9OC2F4H, C6F130CF2H, HC3F60C3F6H, C3F70CH2F, HCF20CF20CF2H, HCF20C2F40CF2H, HCF20CF20CF20CF2H, HCF20CF2CF20CF2H, HC3F60CH3, HCF20CF20C2F40CF2H, and mixtures thereof. Non-segregated HFEs are available from Ausimont Corp., Milano, Italy, under the H-GALDENTM.

For the present invention, HHFEs are defined as ether compounds containing fluorine, non-fluorine halogen (that is, chlorine, bromine, and/or iodine) and hydrogen atoms. An important subclass of HHFEs is perfluoroalkylhaloethers (PFAHEs). PFAHEs are defined as ether compounds wherein on one side of the ether oxygen atom is a perfluoroalkyl group and on the other side of the ether oxygen atom is a carbon backbone substituted with carbon-bonded hydrogen atoms and halogen atoms, wherein at least one of the halogen atoms is chlorine, bromine, or iodine. Useful PFAHEs include those described by the general structure shown in Formula IX: (IX) wherein Rf"is a perfluoroalkyl group preferably having at least about 3 carbon atoms, most preferably from 3 to 6 carbon atoms, and optionally containing a catenary heteroatom such as nitrogen or oxygen; X is a halogen atom selected from the group consisting of bromine, iodine, and chlorine;"a"preferably is from about 1 to 4;"b"is at least 1;"c"can range from 0 to about 2;"d"is at least 1; and b+c+d is equal to 2a+1.

Such PFAHEs are described in PCT Publication No. WO 99/14175. Useful PFAHEs include c-C6F11-OCH2Cl, (CF3) 2CFOCHCl2, (CF3) 2CFOCH2C1, CF3CF2CF20CH2C1, CF3CF2CF20CHC12, (CF3) 2CFCF20CHC12, (CF3) 2CFCF20CH2C1, CF3CF2CF2CF20CHC12, CF3CF2CF2CF20CH2CI, (CF3) 2CFCF20CHC1CH3, CF3CF2CF2CF20CHC1CH3, (CF3) 2CFCF (C2F5) OCH2Cl, (CF3) 2CFCF20CH2Br, and CF3CF2CF20CH2I.

Suitable hydrochlorocarbons and hydrobromocarbons include HCCs and HBCs such as trans-1,2-dichloroethylene, trichloroethylene, perchloroethylene, 1,1,1- trichloroethane and n-propyl bromide.

Suitable fluorinated compounds containing at least one aromatic moiety include fluorinated monoalkyl-, dialkyl-and trialkyl-substituted aromatic compounds, including toluene and xylene derivatives. Preferred among these compounds are fluoroalkyl substituted compounds, such as hexafluoroxylene, benzotrifluoride and p- chlorobenzotrifluoride. Such compounds are commercially available, for example, under the OXSOLTM, available from Occidental Chemical Corp., Niagara Falls, New York.

Suitable perfluoroiodides include PFIs such as perfluoropropyl iodide (C3F7I) and perfluorobutyl iodide (C4FgI).

Perfluoroolefins (PFOs) suitable for use as heat-transfer fluids are normally liquid perfluoroolefin compounds, perfluoroaromatic compounds, and perfluorocycloolefin compounds. The PFOs can contain some residual carbon-bonded hydrogen (generally less than about 0.4 mg/g and preferably less than about 0.1 mg/g, for example, 0.01 to 0.05 mg/g) but are preferably substantially completely fluorinated. The PFOs can contain from about 5 to about 10 carbon atoms and can contain one or more catenary heteroatoms, for example, trivalent nitrogen or divalent oxygen atoms. Representative examples of suitable blowing agent compounds include hexafluoropropene dimers, for example, perfluoro (4- methylpent-2-ene) and perfluoro (2-methylpent-2-ene); hexafluoropropene trimers, for example, perfluoro (4-methyl-3-isopropylpent-2-ene) and perfluoro (2,4-dimethyl-3- ethylpent-2-ene); tetrafluoroethylene oligomers, for example, perfluoro (3-methylpent-2- ene), perfluoro (3,4-dimethylhex-3-ene), and perfluoro (2,4-dimethyl-4-ethylhex-2-ene); perfluoro (1-pentene); perfluoro (2-pentene); perfluoro (1-hexene); perfluoro (2-hexene); perfluoro (3-hexene); perfluoro (1-heptene); perfluoro (2-heptene); perfluoro (3-heptene); perfluorocyclopentene; isomers of C6F103 for example, perfluorocyclohexene, perfluoro (1-methylcyclopentene), perfluoro (3-methylcyclopentene), and perfluoro (4methylcyclopentene); perfluoro (1-methylcyclohexene); perfluoro (3- methylcyclohexene); perfluoro (4-methylcyclohexene); perfluoro (oxaalkenes), for example, perfluoro (3-oxahex-1-ene), perfluoro (3-oxahept-1-ene), and perfluoro (3-oxa-4- methylpent-1-ene); perfluoro (3-ethyl-3-azapent-1-ene); and mixtures thereof. Suitable PFOs are described in U. S. Pat. No. 5,631,306.

Heat Source The heat source depends on the system or application for which the apparatus of the present invention is being used. For example, for a fuel cell, the heat source is the electrochemical reaction which transfers heat to the heat-transfer fluid. Other examples include, but are not limited to the windings in an electrical transformer, integrated circuits in an electronics module, and power electronics in a rectifier.

Generally the heat-transfer fluid receives energy from the heat source in the evaporator.

Evaporator and Condenser Generally, the evaporator comprises the surface from which heat is being removed or dissipated and a container for heat-transfer fluid. The surface and container can be the same. The condenser comprises the surface on which heat is being deposited. Generally, the condenser comprises a first and a second end.

The heat-transfer fluid receives energy from the heat source in the evaporator (if the apparatus is to be used for cooling). For example, in the case of a transformer, the windings which generate heat comprise the evaporator. In the case of an electronics module, the integrated circuits and various other components comprise the evaporator. The size and shape of the evaporator depend upon the system/application. The heat-transfer fluid boils or evaporates to form a vapor.

The vapor then travels to the condenser. The heat-transfer fluid has an evaporation temperature. The evaporating surface temperature is preferably at a temperature at least about 10 °C more than the evaporation temperature of the heat-transfer fluid.

The condenser is typically connected directly to the evaporator such that the evaporator is in fluid connection with the condenser. For example, there may be one large open tube such that falling condensate is not entrained by the rising vapor. This type of connection may not be practical for commercial applications because these applications often require remote location of the condenser if it is not desirable to dissipate the condenser heat in the same location as the evaporator.

Thus, in a commercial setting, the condenser is likely plumbed to the evaporator by some type of line. Ideally a large, vertical pipe which runs from the evaporator to the condenser would carry the vapor and returning condensate in a counterflow

arrangement as is shown in Fig. 3. In Fig. 3, the heat-transfer fluid in the evaporator 32 is heated from the heat source 30 and forms a vapor. This vaporized fluid then flows through a vertical pipe 34 from 34b (that is, evaporator first connector) to 34a (that is, condenser first connector) into the condenser 36. In the condenser, the vapor condenses into a liquid condensate and flows down vertical pipe 34 from 34a to 34b into the evaporator. Heat flows out the condenser 36 into the heat sink 37.

If one line or passage is used for vapor and condensate, then this line should be large enough to accommodate these counterflows without entrainment of liquid. The criteria for sizing these lines is one commonly used in designing industrial distillation columns: <BR> <BR> (A)<BR> Vl1/2Dl1/4#0.22(gd(Dl-Dv))1/4Vv1/2Dv1/4+ where Dl = liquid density (kg/m3) Dv liquid density (kg/m3) Vl = liquid velocity (m/s) Vu liquid velocity (m/s) d = diameter of the fluid passage (m) g = acceleration of gravity (m/s2) However, a small vapor line such as those used in commercial refrigeration systems can be used in conjunction with a separate liquid return line. This configuration is shown in Fig. 4. In Fig. 4, the heat-transfer fluid in the evaporator 42 is heated from the heat source 40 and forms a vapor. This vaporized fluid then flows through a vertical pipe 44 (vapor line) from 44a (that is, evaporator first connector outlet) to 44b (that is, condenser first connector inlet) into the condenser 46. In the condenser, the vapor condenses into a liquid condensate and flows down

the vertical pipe 45 (liquid return line) from 45a (that is, condenser first connector outlet) to 45b (that is, evaporator first connector inlet) into the evaporator 42. Heat flows out the condenser 46 into the heat sink 47.

In a third configuration shown in Fig. 5, the heat-transfer fluid in the evaporator 52 is heated from the heat source 50 and forms a vapor. This vaporized fluid then flows through the vertical pipe 54 (vapor line) from 54a (that is, evaporation first connector outlet) to 54b (that is, condenser first connector inlet) into the condenser 56 (that is, the top region of the condenser). In the condenser 56, the vapor condenses into liquid condensate and flows downward through the condenser 56 and then flows down the vertical pipe 55 (liquid return line) from 55a to 55b into the evaporator 52. Heat flows out the condenser into the heat sink 57.

A natural result of separate vapor and liquid lines (Figs. 4 and 5) is the propensity of liquid condensate in the evaporator to"back up"in the direction of the condenser through the liquid return line. Any time the vapor line is sized too small, the resulting pressure drop in the vapor line results in liquid back-flowing up the liquid return line.

Commercially available condensers are typically flow-though type condensers often with very small fluid passages. If these condensers are used with the configuration shown in Fig. 5, the non-condensable gas may be entrained right through the condenser by the fast moving vapors. Because the non-condensable gas separates and collects into the expansion device, a condenser with rather large, open passages is preferred. When oriented vertically, these passages permit condensed liquid to form a film on the interior walls of the condenser tubes and flow back to the evaporator as required by the configurations shown in Figs. 3 and 4. The same criteria outlined in Equation A for sizing the plumbing preferably are followed when sizing the condenser tubes and headers so that rising vapors will not entrain the falling liquid film. Furthermore, the condenser is sized to dissipate the maximum expected evaporator heat input.

During operation with the configurations shown in Figs. 3 and 4, a stable interface forms between saturated vapor and a mixture of non-condensable gas and vapor which is above the saturated vapor. No condensation occurs above this interface. This interface is automatically located such that enough of the condenser

has been wetted with saturated vapor to condense the vapor and to keep the system operating at the selected operating pressure.

If the condenser temperature drops (for example, if the water temperature is lowered in a water-cooled condenser) the interface drops (as the expansion device contracts) and thus reduces the effective condenser surface area. The system remains at the selected operating pressure.

If the condenser temperature rises (for example, if the water temperature is raised in a water-cooled condenser), the interface rises (as the expansion device rises) and increases the effective condenser surface area. The system remains at atmospheric pressure.

Expansion Device The expansion device is a passively controlled device which allows the above-mentioned interface to rise and fall as previously described without causing the system pressure to change to an extent that would cause the evaporator temperature to go out of the specified/desired range and without allowing fluid vapor to leak out.

In Figs. 3,4, and 5, non-condensable gas and heat-transfer fluid vapor enters the condenser and is in fluid connection with the expansion device 38,48, and 58. The expansion device will self-regulate. Thus, the expansion device will expand as the system pressure increases and contracts when this pressure decreases 39,49, and 59.

The condenser comprises a second connector and the expansion device comprises a first connector. A tube (for example, Tygon tubing, brazed copper, steel, polyvinylchloride, etc.) is interconnected with the condenser second connector and the expansion device first connector.

The expansion device may be a large ground glass syringe which maintains the system at atmospheric pressure. However, this type of expansion device is not practical for commercial applications because it is relatively fragile and cannot be used with systems having pressurization. Commercially available bellows or bladder-type expansion vessels may be used for this purpose. Commercially available products include: metal bellows such as those manufactured by Senior

Flexonics, Sharon, Massachusetts; reinforced polyethylene expansion devices with polymeric bladders such as those manufactured by WellMate Division of Structural Group, Chardon, Ohio; or a polymeric bellows, available from Marsh Bellofram Corp., Newell, West Virginia.

The bladder is chosen preferably such that non-condensable gases, such as air, do not diffuse into the system and accumulate in such a way as to raise the system pressure above its intended level. Such gas accumulation can destroy the functionality of the design unless some type of purge is incorporated which removes only an acceptable amount of gas. Purging gas when the system is non- operational is one way to accomplish this. Another way to purge gas that has accumulated due to diffusion through a membrane or seal is to allow the system to vent to atmosphere (purge) in a controlled manner, expelling the accumulated gas.

The purging preferably occurs in a manner that will conserve the heat-transfer fluid whilst allowing the excess non-condensable gases to escape. There are several methods that allow this purging to occur in a controlled manner.

In addition, there are at least two indicators that can be used to determine when purging is needed, including pressure indication and indication based on volume of non-condensable gas. A volume-based purge indicator works as follows: a piston/cylinder expansion chamber is embodied as a syringe with a barrel and plunger located in a gravitational field, and having the plunger oriented such that gravity acts to push the plunger into the barrel of the syringe. A small hole is drilled into the syringe barrel. The position of the hole along the barrel determines the expansion reservoir volume at which purging begins. As non- condensable gas infiltrates the system, it collects (under normal two-phase operation) in the piston/cylinder expansion chamber. As the non-condensable gas volume increases with time, the plunger of the syringe extends axially from the cylinder of the syringe barrel. When the volume of non-condensable gas exceeds a design point, the piston exposes the hole in the barrel allowing the excess non- condensable gas to be purged from the expansion chamber. After purging the excess non-condensable gas, the plunger, under the action of an external force (for example, a spring force or the influence of gravity), is drawn back into the barrel

and re-seals the hole in the barrel, again yielding a hermetic system. Note that this type of purge results in the loss of minimal vapor of the heat-transfer fluid.

Another indicator that can be used to determine whether to purge excess non-condensable gas is based on the internal pressure of the expansion chamber.

The internal pressure can be monitored and controlled by a simple, self re-sealing pressure relief device commonly available as commercial products.

The ability to accommodate non-condensable gas in a two-phase heat transfer system provides utility in the following ways: (1) accommodating some non-condensable gas in the heat-transfer fluid allows normal handling of the heat- transfer fluid during fill, shipment, power down etc. Normal handling means the heat-transfer fluid can be exposed to atmospheric air during the filling process or while the system is opened for maintenance; (2) some non-condensable gas in the two-phase heat transfer system, when properly accommodated, allows for a robust heat transfer system with consistent evaporator and condenser behavior.

The utility of controlling the volume of non-condensable gas by a volume- indicated or pressure-indicated purge allows the system to operate properly despite the infiltration of non-condensable gas that may occur over time; and allows the sizing of a smaller expansion reservoir than is required had not a purge mechanism been provided.

The volume of the expansion device is preferably sized according to one of the following equations: (B) VexP _ op"o (7om&) TcoM TopW'satTamb) Tcold Vexp (/' TCond, Yh + x YJ VPat/n-Psat (Teold)pop-Psat (Tcond) Tcold where: Pmax is the maximum system design pressure;

Psat (T) is the heat-transfer fluid saturation pressure at temperature T; Patm is atmospheric pressure; Pexp is the effective pressure deliberately applied to the expansion device in an effort to raise Pop; Pop is the operating pressure of the system= Psat (Top) = Patm+Pexp ; qop is the heat flux during operation; qcrit is the critical film boiling heat flux of the heat-transfer fluid for the application geometry; qinc is the incipient heat flux required for the application geometry and the incipient heat flux is that required to initiate boiling; Qev is the maximum total heat being produced at the evaporating surface; Qcond is the condenser capacity at the hottest heat sink temperature when all available condensing area is being utilized; Tamb is the ambient temperature in the environment of the expansion device; Tboil is the heat-transfer fluid boiling temperature at 1 atm; Tcold is the coldest conceivable temperature the system can reach when it is non-operational; Tsat (P) is the heat-transfer fluid saturation temperature at pressure P; Tev is the desired operating temperature of the evaporating surface; Tcond is the condenser temperature; #Tsat is the wall superheat during operation/nucleate boiling = Tev-Top;

Top is the heat-transfer fluid boiling temperature during operation= TeV-ATsat ; Vu vis the volume of heat-transfer fluid when system is non- operational Tcold ; Vh is the free volume or headspace volume when the system is non-operational at Tcold ; Vexp is the expansion device volume; and x is the solubility (volume percent) of air in the heat-transfer fluid at TCold- (D) Pop<Pmax (E) qinc<qop<qcrit (F) QcondQev In Equation (B), which is relevant for configurations such as those depicted in Figs. 3 and 4, the expansion device is large enough to accommodate whatever non- condensable gas may have been in the system before startup assuming the system leaked to atmospheric pressure. After startup, this non-condensable gas migrates into the expansion device and this gas is saturated with vapor at the expansion device ambient temperature (as a worst case).

In Equation (C), which is relevant for the configuration shown in Fig. 5, the expansion device is again large enough to accommodate whatever non-condensable gas may have been in the system before startup assuming the system leaked to atmospheric pressure. However, during operation, saturated vapor may condense in the expansion device heating it to the condensation temperature, thereby raising the partial pressure of vapor in the expansion device and necessitating a larger expansion device.

The expansion device may be sized smaller than described above if the temperature fluctuations expected from this are manageable for the application. These specifications are not, therefore, intended to be limiting.

Equation (D) indicates that the system operating pressure is chosen such that no structural or mechanical limitations are violated. An example is the safe operating pressure for a seal or component.

Equation (E) indicates that the heat flux at the evaporation surface is large enough to permit two-phase heat transfer. This incipient heat flux is well known in the art and is strongly dependent not only on the heat-transfer fluid being used, but on the geometry of the particular application. The heat flux is lower than the critical or film boiling heat flux which is also commonly known in the art. If this constraint is not met, evaporator surface temperatures can rise sharply and lead to system failure.

Equation (F) indicates that the condenser has sufficient capacity to dissipate all heat generated by the evaporator at a time when the heat sink temperature is the warmest conceivable, generally 55 °C.

Heat Sink The heat sink depends on the system or application for which the apparatus of the present invention is being used. For example, in a commercial setting, the heat sink may be an air stream. Other examples include, but are not limited to, cooling fluids in the case of liquid or gas (for example, air) cooled condensers and a process stream in the case when the process stream is heated at a constant temperature.

The heat-transfer fluid has an evaporation temperature. The heat sink is preferably at a temperature at least about 10 °C less than the evaporation temperature.

Apparatus Fig. 6 depicts an example of the apparatus of the present invention as built in a laboratory. In this case, the evaporator 62 is a 50 milliliter, 3-neck spherical Pyrex flask which is heated with a conventional mantle 60. The condenser 66 is a standard 12 centimeter long, 1.2 centimeter I. D. Pyrex single tube-in-shell (tube 66a, shell 66b), water-cooled condenser. The water flows in 67a and out 67b of

the shell of the condenser. The expansion device 68 is a 75 ml Perfectum MicromateTM ground glass syringe, available from Popper and Sons, New Hyde Park, New York. When the system is not powered, the syringe is held closed by gravity. The expansion device 68 does, however, float freely so that any rise in system pressure will permit the syringe plunger to rise 69. The heat-transfer fluid may be used as a lubricant and sealant for the syringe plunger.

Prior to startup, the system contained a significant amount of air. There was air in the condenser 66 as well as in the headspace of the evaporator.

Additionally, air dissolved into the heat-transfer fluid, in this case C6F14, was used and may be 50 volume percent air under ambient pressure and at room temperature. Upon startup, the dissolved air was liberated. Because the air was less dense than the vapor being generated, the air migrated toward the top of the system. An interface between saturated vapor and the air/vapor mixture formed as the vapor pushed up into the condenser. This is a phenomenon commonly observed during distillations and reflux experiments. As this happened, the expansion device 68 expanded and accommodated the air/vapor mixture.

Fig. 2a shows actual operating temperatures during changes in the cooling water temperature using the laboratory apparatus described above. If the same system is operated in a saturation state with all of the air removed, the system behaves as shown in Fig. 2b (comparative). Fig. 2c shows actual evaporator operating temperature over time using a configuration similar to Fig. 5.

Thus, a totally passive but completely isothermal system is realizable. The evaporator will equilibrate at the atmospheric boiling point of the heat-transfer fluid and the heat-transfer fluid will not leak.

Alternatively, an adjuster device can be added which applies pressure to the expansion device. The adjuster device may be a mechanical pressure regulator or gas source which induces a system operating pressure, throughout the operating range of the expansion device, sufficiently adjustable that the system pressure and thus the operating temperature do not fall outside of specifications. Though the system now operates at a positive pressure, the applied pressure can be modulated to adjust the operating temperature. This further device permits the use of heat- transfer fluids with much lower boiling points.

Method The present invention also provides a method for two-phase heat transfer without de-gassing the heat-transfer fluid. This method comprises the steps of a method suitable for two-phase heat transfer comprising the steps of : (a) providing non-condensable gas; (b) causing energy to flow from a heat source to an evaporator which contains heat-transfer fluid as a condensate; (c) after step (b), said condensate absorbing energy from said evaporator and forming a vapor; (d) after step (c), causing a heat sink to come into contact with a condenser containing said vapor to form in-part condensate of said vapor and to remove energy; (e) after step (d), causing the condensate to return to said evaporator; (f) after step (c), causing some vapor to flow through to an expansion device where said expansion device adjusts to provide sufficient volume for said vapor; (g) causing some non-condensable gas to flow through to an expansion device where said expansion device adjusts to provide sufficient volume for said gas; (h) after step (f), causing some vapor to flow through to said condenser by the expansion device contracting; and (i) after step (g), causing some gas to flow through to said condenser by the expansion device contracting, wherein said heat source is maintained at substantially constant temperature and said apparatus is maintained as substantially constant operating temperature.

Examples The present invention will be further described with reference to the following nonlimiting examples and test methods. All parts, percentages, and ratios are by weight unless otherwise specified.

(1) In the first example, the present invention is used to cool a proton exchange membrane fuel cell (PEMFC) using a hydrofluoroether fluid methoxynonafluorobutane which is sold by Minnesota Mining and Manufacturing as 3MTM NOVECTM HFE-7100.

This heat-transfer fluid has an atmospheric boiling point of 61 °C and possesses the requisite dielectric properties to keep adjacent cooling plates electrically isolated. The fuel cell is used to power a 2000 square foot home in Arizona where ambient temperatures are expected to range between 32 °F (0 °C) and 120 °F (48.9 °C).

The fuel cell stack is composed of a series of 100+ electrochemical cells between which are placed roughly as many cooling plates which contain the HFE fluid. These plates are in fluid connection with one another such that they share a bottom header and a top header. The top header is connected to the condenser as shown in Fig. 3. The condenser is constructed such that the vapors can rise into the vertically oriented condenser tubes without entraining the falling condensate as described by Equation (A).

The outdoor-located expansion reservoir is sized such that it can accommodate any air which might leak into the system at 32 °F (0 °C) when the system is non operational and accommodate that air at 120 °F (48.9 °C). The system volume is Vf=l liter and the headspace volume is Vh=0.75 liter and the solubility of air is 50 percent. By Equation (B), the volume must be roughly 4.3 liters.

Because of the numerous seals required to seal the cooling plates, it is desirable to operate this system at a pressure below PmaX=lOO Pa gage. There is, therefore, no pressure applied to the expansion device and PeXp=0 Pa and the system will operate at Pop=1 atm which satisfies Equation (D). For the evaporator geometry described, the heat flux required to initiate boiling is qinc=0.2 W/cm2and the critical heat flux is qcrit=10 W/cm2. Since the heat flux during operation is expected to be qOp=0. 5 W/cm2, this satisfies Equation (E).

The total heat evolved at the evaporator is expected to be 10 kW so the condenser, which is located outside the home, is designed to accommodate 15 kW at the maximum expected ambient temperature of 120 °F thus satisfying Equation (F).

It is desirable to maintain an evaporator temperature of TeV=70 °C for this application because a conduction heat transfer analysis indicates that this will keep the cell reaction temperature at the desired level. Since the heat-transfer fluid has a saturation temperature at the operating pressure of Top=61 °C and requires a wall superheat of ATSatw10 °C to maintain nucleate boiling for this geometry with prescribed heat flux, the fluid is well chosen (for example, Tgy z Top+ATsat)

(2) The proton exchange membrane fuel cell (PEMFC) described above may also be cooled using the lower boiling hydrofluorocarbon fluid 1,1,1,3,3- pentafluoropropane. In such a configuration, the system operating pressure is above atmospheric and the seals are designed to accommodate this increased pressure. The variables described above now take on the following values: Pmax 4 atm gage Pexp 3.25 atm Pop 4.25 atm qop 0.5 W/cm2 qcrit 10 W/cm2 qinc 0.2 W/cm2 Qev 10 kW Qcond 15 kW Tboil 15 °C Tev 70 °C ATsat 10 °C Top 60 °C Vf 1 liter Vh 0.75 liter Vexp 0.6 liter x <0.5 This time, the system is designed using the configuration shown in Fig. 4.

Because the vapor and returning condensate flow through separate lines, there is danger of condensate backflow if the vapor lines are not large enough.

Calculations indicate that for the 10 kW capacity and a vapor line 2.54 cm in diameter, the pressure drop per meter length of vapor line is less than 0.5 cm of saturated liquid head. This liquid head is negligible and so the vapor will not backflow through the condensate return line.

Input HT Capacity [W] 10000 Estimated Vapor Density [kg/m3] 20.8 Liquid Density [kg/m3] 1320 Estimated Vapor Viscosity [m2/s] 5.29E-07 Latent Heat [J/kg] 208000 Flow Rate [kg/s] 0.04807692

Results Vapor Line Diameter [cm] 7.62 5. 08 2. 54 1.27 Vapor Line Diameter [m] 7.62E-02 5. 08E-02 2. 54E-02 1.27E-02 Vapor Line Area [m2] 4.56E-03 2. 03E-03 5. 07E-04 1.27E-04 Vapor Velocity [m/s] 5. 07E-01 1.14E+00 4.56+00 1.82E+01 Liquid Velocity [m/s] 7.99E-031. 80E-027. 19E-022. 87E-01 Vapor Red 730E+04 1. 09E+OS 2. 19E+OS 4. 38E+OS Vapor Friction Factor 1. 96E-02 1. 81E-02 1. 57E-02 1.37E-02 Vapor dP/dx [Pa/m] 6.87E-01 4. 81E+00 1.34E+02 3.73E+03 Vapor dP/dx as cm Liquid 5. 30E-03 3. 71E-02 1. 03E+00 2. 88E+OI Head [cm/m]

(3) The proton exchange membrane fuel cell (PEMFC) described above may also be cooled using the lower hydrofluorocarbon fluid 2,3- dihydrodecafluoropentane, available from E. I. du Pont de Nemours & Co. In such a configuration, the system operating pressure is still slightly above atmospheric and the various variables described above take on the following values: Pmax 1 atm gage Pexp 0.21 atm Pop 1.21 atm

qop 0.5 W/cm2 qcrit 10 W/cm2 qinc 0.2 W/cm2 Qev 10 kW Qcond 15 kW Tboil 55 °C Tev 70 °C #Tsat 10 °C Top 60 °C Vf 1 liter Vh 0.75 liter Vexp 3.54 liter x <0.5 (4) In this example the cooling of a 200 kVA electrical transformer is accomplished using the dielectric perfluorocarbon C6F14 available from Minnesota Mining and Manufacturing Company. This system is located in Minnesota where the ambient temperature can be expected to vary between-40 °F (-40 °C) and 100°F (37. 8 °C). This machine operates at a slight positive pressure relative to ambient. The variables now take on the following values: Pmax 1 atm gage Pexp 0.08 atm Pop 1.08 atm qop 1.2 W/cm2 qcrit 15 W/cm2 qinc 1.0 W/cm2 Qev 40 kW

Qcond 50 kW Tboil 56 °C Tev 70 °C ATsat 10 °C Top 60 °C Vf 390 liter Vh 300 liter Vexp 1180 liter x 0.55 (5) In this example, the cooling of an electronics module on board a hybrid electric vehicle is accomplished using the hydrofluoroether methoxyheptafluoropropane, available from Minnesota Mining and Manufacturing Company. This system is expected to operate in a wide range of ambient temperatures between-40°F (-40 °C) and 130 °F (54.4 °C).

Pmax 3.5 atm gage Pexp 1.98 atm Pop 2.98 atm qop 10.0 W/cm2 qcrit 15 W/cm2 qinc 1.0 W/cm2 Qev 1.5 kW Qcond 2.0 kW Tboil 34 °C Tev 80 °C ATsat 10 °C Top 70 °C

Vf 1 liter Vh 2 liter Vexp 3.5 liter x 0.50 Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.