Evaporatively cooled thermosiphon

Abstract

A thermosiphon cooling assembly cools an electronic device with a first refrigerant disposed in the lower boiling chamber of a housing for liquid-to-vapor transformation and a second refrigerant disposed in an upper evaporating chamber of a housing for liquid-to-vapor transformation. The partition separating the lower boiling chamber of the housing from the upper evaporating chamber of the housing creates a series of vapor chambers within the lower boiling portion for condensing vapor boiled off the first refrigerant. The upper evaporating chamber contains a series of refrigerant pockets interleaved vertically with the vapor chambers to increase the surface area for heat transfer between the refrigerant vapor and the second refrigerant for absorbing heat by the second refrigerant for liquid-to-vapor transformation.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention relates to a thermosiphon cooling assembly for cooling an electronic device.

2. Description of the Prior Art

The operating speed of computers is constantly being improved to create faster and faster computers. With this, comes increased heat generation and a need to effectively dissipate that heat.

Heat exchangers and heat sink assemblies have been used that apply natural or forced convection cooling methods to dissipate heat from electronic devices that are highly concentrated heat sources such as microprocessors and computer chips. These heat exchangers typically use air to directly remove heat from the electronic devices; however air has a relatively low heat capacity. Thus, liquid-cooled units called LCUs employing a cold plate in conjunction with high heat capacity fluids have been used to remove heat from these types of heat sources. Although LCUs are satisfactory for moderate heat flux, increasing computing speeds have required more effective heat sink assemblies.

Accordingly, thermosiphon cooling units (TCUs) have been used for cooling electronic devices having a high heat flux. A typical TCU absorbs heat generated by the electronic device by vaporizing the working fluid housed on the boiler plate of the unit. The boiling of the working fluid constitutes a phase change from liquid-to-vapor state and as such the working fluid of the TCU is considered to be a two-phase fluid. The vapor generated during boiling of the working fluid is then transferred to a condenser, where it is liquified by the process of film condensation over the condensing surface of the TCU. The heat is rejected into a stream of air flowing through a tube running through the condenser or flowing over fins extending from the condenser. The condensed liquid is returned back to the boiler plate by gravity to continue the boiling-condensing cycle.

The ability of a TCU to dissipate heat can be further improved by adding an additional working fluid cycle. The TCU would absorb heat generated by the electronic device by vaporizing the first working fluid housed on the boiler plate of the unit. The vapor generated during boiling of the first working fluid is then transferred to a first condenser, where it is liquified by the process of a second evaporator chamber absorbing the heat generated by the vapor. The second evaporator chamber would absorb the heat generated by the vapor by vaporizing the second working fluid housed within the second evaporator chamber. The vapor generated during the boiling of the second working fluid is then transferred to a second condenser, where it is liquified by the process of film condensation. The heat is rejected into air flowing over or through the condenser, or alternatively into another refrigerant.

Additionally, with the decreasing size of computers and the increasing portability, new challenges have been presented regarding the ability of cooling units to effectively dissipate heat. The decreasing size has created a demand for smaller cooling units that are able to be fit into tighter spaces. To meet this demand, many TCUs have been created out of a flexible material to allow for an increased ability to be positioned in cramped spaces. The portability of computers has created the need for TCUs that are able to function without the assistance of gravity. Wicking materials have been developed which allow a TCU to transport a liquid regardless of the effect of gravity. A typical TCU utilizing a wicking material contains an evaporator, a condenser, and capillary tubes lined with a wicking material interconnecting the evaporator and condenser. The capillary tubes lined with the wicking material allow refrigerant vapor to flow from the evaporator to the condenser while the wicking material returns the condensed refrigerant in the condenser to the evaporator without the assistance of gravity.

An example of a cooling system for electronic devices is disclosed in U.S. Pat. No. 5,647,429 to Oktay. The Oktay patent discloses an assembly for cooling an electronic device having a flexible housing including an evaporator and a condenser joined by a hollow adiabatic section containing a wicking material. The assembly is additionally adapted so that two or more consecutive assemblies can be joined together by connecting the condenser of one housing to the evaporator of a subsequent housing. The Oktay patent provides for multiple TCUs to be joined together in series to effectively cool a single electronic device.

Although the prior art dissipates heat from electronic devices, as computing speeds increase, there is a continuing need for alternative cooling devices having more efficient heat transfer capabilities.

SUMMARY OF THE INVENTION AND ADVANTAGES

In accordance with the subject invention, heat generated by an electronic device is transferred to a housing having a lower boiling chamber and an upper evaporating chamber. A first refrigerant is disposed within the lower boiling chamber of the housing for liquid-to-vapor transformation, and a second refrigerant is disposed in the upper evaporating chamber of the housing for liquid-to-vapor transformation. The assembly employs a partition to define the lower boiling chamber of the housing, and the partition forms a series of vapor chambers in the lower boiling chamber. The heat generated by the electrical device is transferred to the lower boiling chamber of the housing and is absorbed by the first refrigerant causing the first refrigerant to boil. The vapor boiled off the first refrigerant as a result of the heat absorbed from the electronic device gathers in the vapor chambers and heat is transferred from the vapor contained within the vapor chambers to a plurality of refrigerant pockets contained within the upper boiling portion of the housing that are interleaved with the vapor chambers. The second refrigerant that is disposed within the upper evaporating chamber of the housing gathers in these refrigerant pockets and absorbs the heat transferred from the vapor chambers. The vapor boiled off the second refrigerant as a result of the heat condenses in an open space above the refrigerant pockets and returns to the refrigerant pools occupying the pockets.

The present invention utilizes the series of vapor chambers in the lower boiling portion of the housing interleaved with the series of refrigerant pockets in the upper evaporating portion of the housing to increase the surface area between the lower boiling portion of the housing and the upper evaporating portion of the housing to enhance heat transfer between the lower and upper portions of the housing. The series of vapor chambers and refrigerant pockets increases the rate of heat transfer providing for smaller TCUs that more effectively dissipate heat from an electronic device.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a perspective view of the thermosiphon cooling assembly showing the housing and condensing unit cut away;

FIG. 2 is a cross sectional view of the assembly shown in FIG. 1; and

FIG. 3 is a cross sectional view of the assembly shown in FIG. 1 with the condensing unit placed below the housing.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a thermosiphon cooling assembly 20 is shown generally for cooling an electronic device 22.

A thermosiphon cooling assembly 20 comprises a housing 24 generally indicated having a partition 26 defining a lower boiling chamber 28 for receiving heat from the electronic device 22 and an upper evaporating chamber 30. The assembly 20 is used to cool the electronic device 22 engaging or secured to the exterior of the lower boiling chamber 28 of the housing 24. The housing 24 is generally rectangular with the lower boiling chamber 28 being hermetically sealed to contain a first refrigerant 32 for liquid-to-vapor transformation.

The first refrigerant 32 is disposed below the partition 26 in the lower boiling chamber 28 of the housing 24 for liquid-to-vapor transformation. The first refrigerant 32 is confined to the lower boiling chamber 28 of the housing 24 and absorbs heat generated by the electronic device 22 and transferred into the lower boiling chamber 28 of the housing 24. The first refrigerant 32 is boiled and evaporated by the heat transferred from the electronic device 22 and the resultant vapor is later condensed and returned to the refrigerant pool. The first refrigerant 32 is preferably essentially dielectric in character to prevent any electrocution hazard in the event of leakage of the fluid. A refrigerant like R-134a is suitable for this purpose.

A plurality of boiler fins 34 extend from the bottom of the lower boiling chamber 28 of the housing 24 for increasing heat transfer from the electronic device 22 to the interior of the lower boiling chamber 28 of the housing 24. The boiler fins 34 extend upwardly toward the upper evaporating chamber 30 of the housing 24. The boiler fins 34 are disposed in the lower boiling chamber 28 of the housing 24 for transferring heat from the electronic device 22 disposed on the exterior of the lower boiling chamber 28 of the housing 24 by boiling the first refrigerant 32 in the lower boiling chamber 28 of the housing 24.

A second refrigerant 36 is disposed above the partition 26 in the upper evaporating chamber 30 of the housing 24 for liquid-to-vapor transformation. The second refrigerant 36 absorbs heat transferred to the upper evaporating chamber 30 by the first refrigerant 32 disposed within the lower boiling chamber 28. The second refrigerant 36 has a thermal conductivity larger or greater than that of air for increasing the heat transfer between refrigerant vapor and the second refrigerant 36. The second refrigerant 36 is also preferably essentially dielectric in character to prevent any electrocution hazard in the event of leakage of the fluid. A refrigerant like R-134a is suitable for this purpose.

The assembly 20 has a condensing unit 38 generally indicated having a lower portion 40 and an upper portion 42 with the upper portion 42 having a plurality of spaced radiation chambers 44 extending through the condensing unit 38. The lower portion 40 of the housing 24 is generally rectangular and the upper portion 42 generally consists of a plurality of rectangular radiation chambers 44 extending perpendicular to the lower portion 40 of the condensing unit 38 at spaced intervals. The radiation chambers 44 house refrigerant vapor awaiting condensation while the lower portion 40 provides a reservoir for the cooled liquid refrigerant. The condensing unit 38 is preferably oriented as illustrated in FIG. 2 so that the radiation chambers 44 are positioned above the lower portion 40.

A plurality of condensing fins 46 extend from the exterior of the radiation chambers 44 of the condensing unit 38 for increasing the heat transfer from refrigerant vapor contained within the radiation chambers 44 to the exterior environment. The condensing fins 46 as illustrated in FIG. 1 preferably extend from one radiation chamber 44 to an adjacent radiation chamber 44 and back to the original radiation chamber 44 forming a channel for air to pass through. The condensing fins 46 are disposed within the radiation chambers 44 for transferring heat contained within the condensing unit 38 to the exterior environment of the assembly 20.

As illustrated in FIG. 1, an air moving device 48 for moving air through the radiation chambers 44 and over the condensing fins 46, here shown as a single axial fan, can be positioned within the proximity of the condensing unit 38. The air moving device 48 increases the flow of air over the condensing fins 46 to increase the rate at which heat contained within the condensing unit 38 is transferred to the surrounding environment. The air moving device 48 can either be of the push or pull type.

A capillary tube 50 interconnects the upper evaporating chamber 30 of the housing 24 and the lower portion 40 of the condensing unit 38 to transfer vapor from the upper evaporating chamber 30 of the housing 24 to the lower portion 40 of the condensing unit 38 and to transfer condensed vapor from the lower portion 40 of the condensing unit 38 to the upper evaporating chamber 30 of the housing 24. The upper evaporating chamber 30 of the housing 24 and the condensing unit 38 are hermetically sealed about the capillary tube 50 to contain the second refrigerant 36 for liquid-to-vapor transformation. The capillary tube 50 is preferably operatively attached to the top surface of the upper evaporating chamber 30 opposite the second refrigerant 36 and preferably operatively attached to the bottom surface of the lower portion 40 of the condensing unit 38 opposite the radiation chambers 44 as illustrated in FIG. 2.

The capillary tube 50 is flexible to move the condensing unit 38 relative to the housing 24 for increasing the ability to position the assembly 20 in tight spaces. The condensing unit 38 may be either positioned above the housing 24 as illustrated in FIG. 2 or the condensing unit 38 may be positioned below the housing 24 as illustrated in FIG. 3.

A plurality of capillary fins 52 extend from the exterior of the capillary tube 50 for increasing heat transfer. The capillary fins 52 preferably extend perpendicularly from the capillary tube 50. The capillary fins 52 are disposed on the capillary tube 50 for transferring heat from refrigerant vapor contained within the capillary tube 50 to the exterior environment of the assembly 20.

A wick material 54 is disposed in the upper evaporating chamber 30 of the housing 24 and extending into the capillary tube 50 for conveying liquid from the condensing unit 38 to the upper evaporating chamber 30 of the housing 24. Refrigerant vapor travels through the capillary tube 50 to the condensing unit 38 to be cooled whereupon the wick material 54 is able to return the liquid refrigerant back to the upper evaporating chamber 30 of the housing 24 substantially unaided by gravity. A wick material 54 formed by a cladless metal coating process, which is capable of yielding about fifty percent porosity required to wick the refrigerant liquid, could be used.

As illustrated by FIG. 2, the partition 26 separating the lower boiling chamber 28 of the housing 24 from the upper boiling chamber of the housing 24 defines a plurality of spaced vapor chambers 56 extending upwardly above the lower boiling chamber 28 for condensing vapor boiled off the first refrigerant 32. The vapor chambers 56 are rectangular and extend from the lower boiling chamber 28 towards the upper evaporating chamber 30. The vapor chambers 56 provide a receptacle for refrigerant vapor boiled off the first refrigerant 32.

The refrigerant vapor is able to occupy the vapor chambers 56 until it is condensed to liquid form whereupon gravity will return the condensed refrigerant to the reservoir for the first refrigerant 32. The vapor chambers 56 preferably extend perpendicularly above the lower boiling chamber 28.

The upper evaporating chamber 30 of the housing 24 includes a plurality of refrigerant pockets 58 interleaved with the vapor chambers 56 for holding the second refrigerant 36 and an open space 60 above the refrigerant pockets 58 for condensing vapor boiled off the second refrigerant 36. FIG. 2 illustrates the refrigerant pockets 58 containing the second refrigerant 36. The refrigerant pockets 58 are preferably rectangular in shape and extend from the upper evaporating chamber 30 towards the lower boiling chamber 28 and are adjacent to two separate vapor chambers 56. The refrigerant pockets 58 also preferably extend downwardly from the upper evaporating chamber 30 creating a depth that is equal to the height the vapor chambers 56 extend upwardly from the lower boiling chamber 28. As shown in FIG. 2, the vapor chambers 56 and refrigerant pockets 58 are located adjacent to one and other where the partition 26 is a common side to two adjacent chambers 28, 30, 44, 56. The refrigerant pockets 58 increase the surface area between the upper evaporating chamber 30 and the lower boiling chamber 28 for increasing the heat transfer from the evaporated refrigerant collected within the vapor chambers 56 and the second refrigerant 36 contained within the refrigerant pockets 58.

The wick material 54 is disposed on the refrigerant pockets 58 for enhancing heat transfer from the vapor chambers 56 to the refrigerant pockets 58. The wick material 54 lining the refrigerant pockets 58 enhances heat transfer by promoting nucleate boiling within the upper evaporating chamber 30. A wick material 54 formed by a cladless metal coating process, which is capable of yielding about fifty percent porosity required to wick the refrigerant liquid, could be used.

The present invention utilizes the first refrigerant 32 disposed in the lower boiling chamber 28 of the housing 24 and the second refrigerant 36 disposed in the upper evaporating chamber 30 of the housing 24 to remove heat by ebullition. The heat transfer rate of the second refrigerant 36 for liquid-to-vapor transformation is inherently higher than that of air or a single-phase fluid enhancing the cooling capacity of the TCU. Thus to ensure the second refrigerant 36 will undergo liquid-to-vapor transformation, the second refrigerant 36 preferably has a boiling temperature less than the boiling temperature of the first refrigerant 32. Assuming the assembly 20 initially starts with a negligible amount of energy, a certain amount of energy will be transferred to the assembly 20 as heat from the electronic device 22 causing the first refrigerant 32 to boil, and a specific amount of energy determined by the heat of vaporization of the first refrigerant 32 will be used up to completely vaporize a unit of the first refrigerant 32. As a result, an amount of energy less than that required to boil the first refrigerant 32 will be transferred from the refrigerant vapor to the second refrigerant 36. Therefore, having a second refrigerant 36 with a lower boiling temperature than the first refrigerant 32 will ensure ebullition of the second refrigerant 36 and increase the effectiveness of the assembly 20.

The present invention additionally utilizes a criterion that the heat capacity of the first refrigerant 32 must be equal to the heat capacity of the second refrigerant 36 in order to ensure a proper charge balance. The efficiency of any cooling system suffers if there is too much or too little refrigerant charge. In a single coolant system, if there is too little charge the evaporator capacity is reduced because less of its surface is wetted and the average evaporator temperature differential increases across the evaporator. If there is too much charge, refrigerant may back up in the condenser reducing the effective surface area of the condenser and increasing the average temperature differential across the condenser. Therefore, to ensure efficient heat transfer in the present assembly 20, the charge between the first refrigerant 32 and the second refrigerant 36 must be balanced so as to not affect the charge balance of the assembly 20. If the charge between the first refrigerant 32 and the second refrigerant 36 is too high, the evaporator capacity of the lower boiling portion of the housing 24 will be reduced. If the charge between the first refrigerant 32 and the second refrigerant 36 is too low, the evaporator capacity of the upper evaporating chamber 30 of the housing 24 will be reduced. Therefore, to ensure efficient heat transfer, the charges of the two refrigerants 32, 36 must be balanced by equating the heat capacities of the two refrigerants 32, 36. The first refrigerant 32 has a heat capacity equal to (mh_fg)₁ and the second refrigerant 36 has a second heat capacity equal to (mh_fg)₂ and the charge balance of the two refrigerants 32, 36 is based on the criterion that the heat capacity of the two refrigerants 32, 36 must be equal. Thus
(mh_fg)₁=(mh_fg)₂  (1)

where the subscripts 1 and 2 refer to the two refrigerants 32, 36 and

m is the refrigerant mass, lb_m

h_fg is the latent heat of evaporation of the refrigerant, Btu/lb_m

The latent heat of evaporation of the refrigerant is given by the relation
h_fgα(1−T/T_c)^3/8  (2)

where

α is a constant defined below, Btu/lb_m

T is the absolute temperature of interest, °R

T_c is the critical temperature of the refrigerant, °R
α=βRT_c/J  (3)

where

β is a dimensionless constant defined below

R is the gas constant of the refrigerant, ft*lb_f/lb_m*°R

T_c is the critical temperature of the refrigerant, °R

J is mechanical-to-thermal energy conversion factor=778.163 ft*lb_f/Btu
β=ln(P_n/P_c)/(1−T_c/T_n)  (4)

where

P_n is the atmospheric pressure, lbf/ft²

P_c is the critical pressure of the refrigerant, lbf/ft²

T_c is the critical temperature of the refrigerant, °R

T_n is the normal boiling temperature of the refrigerant, °R

Based on the foregoing simple relations the charge balance for the two refrigerants 32, 36 can be determined in a straightforward fashion as follows:

1. Determine the dimensionless constant β for the first refrigerant 32 from Eq. (4) knowing the refrigerant specific constant values.

2. Determine the dimensionless constant β for the second refrigerant 36 from Eq. (4) knowing the refrigerant specific constant values.

3. Determine the dimensional constant α for the first refrigerant 32 from Eq. (3) knowing the refrigerant specific constant values.

4. Determine the dimensional constant α for the second refrigerant 36 from Eq. (3) knowing the refrigerant specific constant values.

5. Determine the latent heat of evaporation h_fg of the first refrigerant 32 in the lower boiling chamber 28 corresponding to T=T_b where T_b is the boiler plate temperature.

6. Determine the latent heat of evaporation h_fg of the second refrigerant 36 in the upper evaporating chamber 30 corresponding to T=T₆₀ where T_α is the cooling medium temperature.

7. Knowing h_fg values of the two refrigerants 32, 36 from steps 5 and 6 determine the mass ratio of the two refrigerants 32, 36 from Eq. (1).

The operation of the assembly 20 is incorporated into a liquid cooling system as illustrated in FIG. 1. The electronic device 22 generates an amount of heat to be dissipated and the heat is transferred from the electronic device 22 to the lower boiling chamber 28 of the housing 24. The heat is then conducted from the lower boiling chamber 28 to the boiler fins 34 and thence to the first refrigerant 32. Vapor boiled off the first refrigerant 32 generates an amount of heat to be dissipated and the heat is transferred from the vapor to the upper evaporating chamber 30 of the housing 24. The heat is then conducted from the upper evaporating chamber 30 to the second refrigerant 36. Vapor boiled off the second refrigerant 36 generates an amount of heat to be dissipated and the heat is transferred from the vapor through a capillary tube 50 to a condensing unit 38. The heat is then conducted from the condensing unit 38 to condensing fins 46 to air forced through the condensing fins 46 by an air moving device 48.

The invention therefore provides a method of cooling an electronic device 22 by transferring heat generated by an electronic device 22 to a lower boiling chamber 28 of a housing 24, boiling a first refrigerant 32 in the lower boiling portion of the housing 24 from liquid-to-vapor at a first temperature, and transferring heat from the vapor of the first refrigerant 32 to a second refrigerant 36 in an upper evaporating chamber 30 of the housing 24. The method is distinguished by boiling the second refrigerant 36 in the upper evaporating chamber 30 of the housing 24 from liquid-to-vapor at a second temperature lower than the first temperature. The method is further distinguished by condensing the vapor of the second refrigerant 36 in a condensing unit 38 and wicking the condensed liquid refrigerant through a capillary tube 50 to the upper evaporating chamber 30 of the housing 24.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. The invention may be practiced otherwise than as specifically described within the scope of the appended claims.

Claims

1. A thermosiphon cooling assembly for cooling an electronic device, comprising;

a housing having a partition defining a lower boiling chamber for receiving heat from the electronic device and an upper evaporating chamber,

a first refrigerant disposed below said partition in said lower boiling chamber of said housing for liquid-to-vapor transformation,

a second refrigerant disposed above said partition in said upper evaporating chamber of said housing for liquid-to-vapor transformation,

said partition defining a plurality of vapor chambers extending upwardly above said lower boiling chamber for condensing vapor boiled off said first refrigerant, and

said upper evaporating chamber including a plurality of refrigerant pockets interleaved with said vapor chamber for holding said second refrigerant and an open space above said refrigerant pockets for condensing vapor boiled off said second refrigerant.

2. An assembly as set forth in claim 1 wherein said first refrigerant has a higher boiling temperature than the boiling temperature of said second refrigerant.

3. An assembly as set forth in claim 2 wherein said first refrigerant has a first heat capacity (mhfg)1 and said second refrigerant has a second heat capacity (mhfg)2 and (mhfg)1=(mhfg)2

wherein

m is the refrigerant mass and

hfg is the latent heat of evaporation of the refrigerant and is defined as

hfg=α(1−T/Tc)3/8

wherein

T is the absolute temperature of interest and

Tc is the critical temperature of the refrigerant and

α is a constant and is defined as

α=βRTc/J

wherein

R is the gas constant of the refrigerant and

Tc is the critical temperature of the refrigerant and

J is the mechanical-to-thermal energy conversion factor and is

J=778.163 ft*lbf/Btu

β is a dimensionless constant and is defined as

β=ln(Pn/Pc)/(1−Tc/Tn)

wherein

Pn is the atmospheric pressure and

Pc is the critical pressure for the refrigerant and

Tc is the critical temperature of the refrigerant and

Tn is the normal boiling temperature of the refrigerant.

4. An assembly as set forth in claim 2 including a condensing unit operatively attached to said upper evaporating unit.

5. An assembly as set forth in claim 4 including a capillary tube interconnecting said upper evaporating chamber of said housing and said condensing unit.

6. An assembly as set forth in claim 5 including a wick material disposed on said refrigerant pockets and said wick material disposed in said upper evaporating chamber of said housing and extending into said capillary tube for conveying liquid from said condensing unit to said refrigerant pockets.

7. An assembly as set forth in claim 6 wherein said capillary tube is flexible for moving said condensing unit relative to said housing.

8. An assembly as set forth in claim 4 wherein said condensing unit defines a lower portion and an upper portion with said upper portion having a plurality of spaced radiation chambers extending through said condensing unit and a plurality of condensing fins disposed within said radiation chambers.

9. An assembly as set forth in claim 8 including an air moving device for moving air through said radiation chambers and over said condensing fins.

10. An assembly as set forth in claim 2 including a plurality of boiler fins disposed in said lower boiling chamber of said housing for enhancing heat transfer to said first refrigerant.

11. An assembly as set forth in claim 5 including a plurality of capillary fins disposed on said capillary tube for enhancing heat transfer from said second refrigerant.

12. A thermosiphon cooling assembly for cooling an electronic device, comprising;

a housing having a partition defining a lower boiling chamber for receiving heat from the electronic device and an upper evaporating chamber,

a first refrigerant disposed below said partition in said lower boiling chamber of said housing for liquid-to-vapor transformation,

a plurality of boiler fins disposed in said lower boiling chamber of said housing for enhancing heat transfer to said first refrigerant,

a second refrigerant disposed above said partition in said upper evaporating chamber of said housing for liquid-to-vapor transformation,

a condensing unit defining a lower portion and an upper portion with said upper portion having a plurality of spaced radiation chambers extending through said condensing unit,

a plurality of condensing fins disposed within said radiation chambers,

an air moving device for moving air through said radiation chambers and over said condensing fins,

a capillary tube interconnecting said upper evaporating chamber of said housing and said lower portion of said condensing unit,

said capillary tube being flexible for moving said condensing unit relative to said housing,

a plurality of capillary fins disposed on said capillary tube for enhancing heat transfer from said second refrigerant,

a wick material disposed in said upper evaporating chamber of said housing and extending into said capillary tube for conveying liquid from said condensing unit to said upper evaporating chamber of said housing,

said partition defining a plurality of spaced vapor chambers extending upwardly above said lower boiling chamber for condensing vapor boiled off said first refrigerant,

said upper evaporating chamber including a plurality of refrigerant pockets interleaved with said vapor chambers for holding said second refrigerant and an open space above said refrigerant pockets for condensing vapor boiled off said second refrigerant,

said wick material disposed on said refrigerant pockets, and

said first refrigerant having a higher boiling temperature than the boiling temperature of said second refrigerant,

13. An assembly as set forth in claim 12 wherein said first refrigerant has a first heat capacity (mhfg)1 and said second refrigerant has a second heat capacity (mhfg)2 and (mhfg)1=(mhfg)2

wherein

m is the refrigerant mass and

hfg is the latent heat of evaporation of the refrigerant and is defined as

hfg=α(1−T/Tc)3/8

wherein

T is the absolute temperature of interest and

Tc is the critical temperature of the refrigerant and

α is a constant and is defined as

α=βRTc/J

wherein

R is the gas constant of the refrigerant and

Tc is the critical temperature of the refrigerant and

J is the mechanical-to-thermal energy conversion factor and is

J=778.163 ft*lbf/Btu

β=is a dimensionless constant and is defined as

β=ln(Pn/Pc)(1−Tc/Tn)

wherein

Pn is the atmospheric pressure and

Pc is the critical pressure for the refrigerant and

Tc is the critical temperature of the refrigerant and

Tn is the normal boiling temperature of the refrigerant.

14. A method of cooling an electronic device comprising the steps of;

generating heat by an electronic device,

transferring the heat generated by the electronic device to the lower boiling chamber of a housing,

a boiling a first refrigerant in the lower boiling chamber of the housing from liquid-to-vapor at a first temperature,

transferring heat from the vapor of the first refrigerant to a second refrigerant in the upper evaporating chamber of the housing, and

boiling the second refrigerant in the upper evaporating chamber of the housing from liquid-to-vapor at a second temperature lower than the first temperature.

15. A method as set forth in claim 14 including condensing the vapor of the second refrigerant in a condensing unit and wicking the condensed liquid refrigerant through a capillary tube to the upper evaporating chamber of the housing.