US20050274139A1

US20050274139A1 - Sub-ambient refrigerating cycle

Info

Publication number: US20050274139A1
Application number: US10/867,331
Authority: US
Inventors: William Wyatt
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 2004-06-14
Filing date: 2004-06-14
Publication date: 2005-12-15
Also published as: EP1756488A1; WO2005124248A1; NZ552033A; MXPA06014594A; JP2008502878A; CA2570038A1; AU2005255442A1; BRPI0512095A

Abstract

According to one embodiment of the invention, a method for cooling heat-generating structure disposed in an environment having an ambient pressure includes providing a fluid refrigerant and reducing a pressure of the refrigerant to a first sub-ambient pressure at which the refrigerant has a boiling temperature less than a temperature of the heat-generating structure. The method also includes bringing the refrigerant at the first sub-ambient pressure into thermal communication with the heat-generating structure, so that the refrigerant boils and vaporizes to thereby absorb heat from the heat-generating structure. The method further includes increasing a pressure of the vaporized refrigerant above the first sub-ambient pressure to a second sub-ambient pressure.

Description

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to cooling techniques and, more particularly to a method and apparatus for cooling a system that generates a substantial amount of heat.

BACKGROUND OF THE INVENTION

Some types of electronic circuits use relatively little power, and produce little heat. Circuits of this type can usually be cooled satisfactorily through a passive approach, such as convection cooling. In contrast, there are other circuits that consume large amounts of power, and produce large amounts of heat. One example is the circuitry used in a phased array antenna system.
Electronic circuits and other structures that generate relatively large amounts of heat may be cooled through well know refrigeration systems. However, suitable refrigeration units are large, hefty and consume many kilowatts of power in order to provide adequate cooling. One reason for this is that typical refrigerants in these types of systems tend to have a low change of phase energy, requiring a large flow rate for removal of heat. The combination of high flow rates and high pressure causes high compressor work. Thus, although refrigeration units of the above type have been generally adequate for their intended purposes, they have not been satisfactory in all respects.
In this regard the size, weight and power consumption characteristics of the known refrigeration systems are all significantly larger than desirable for certain apparatuses generating large amounts of heat. Given that there is an industry trend towards even greater power consumption in heat dissipation in certain types of electronics, such as phased array antenna systems, continued use of conventional refrigeration-based cooling systems would continue to result in even greater size, weight and power consumption, which is undesirable.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, a method for cooling heat-generating structure disposed in an environment having an ambient pressure includes providing a fluid refrigerant and reducing a pressure of the refrigerant to a first sub-ambient pressure at which the refrigerant has a boiling temperature less than a temperature of the heat-generating structure. The method also includes bringing the refrigerant at the first sub-ambient pressure into thermal communication with the heat-generating structure, so that the refrigerant boils and vaporizes to thereby absorb heat from the heat-generating structure. The method further includes increasing a pressure of the vaporized refrigerant above the first sub-ambient pressure to a second sub-ambient pressure.
Embodiments of the invention may provide numerous technical advantages. Some embodiments may benefit from some, none, or all of the following advantages. According to one embodiment, an efficient, lightweight refrigeration system is provided that has a large cooling capacity, but requires less power than conventional refrigeration systems. In particular, cooling may occur in an ambient environment having a temperature greater than the heat-generating structure that is being cooled. In some embodiments water is used as a refrigerant and provides a high degree of heat transfer, enabling an efficient heat transfer system. In addition, water does not result in harmful effects to the environment associated with many common refrigerants. Such a system may also allow for the use of a smaller heat exchanger than would otherwise be required.
Other advantages may be readily apparent to one skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and its advantages will be apparent from the detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram of an apparatus according to the teachings of the invention; and
FIG. 2 is a graph illustrating a thermal cooling cycle according to the teachings of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Example embodiments of the present invention and its advantages are best understood by referring to FIGS. 1 and 2 of the drawings, like numerals being used for like and corresponding parts of the various drawings.
FIG. 1 is a block diagram of a system 10 for cooling according to the teachings of the invention. As illustrated, system 10 includes heat-generating structure 12, which in this example is electronic circuitry and, in particular, is a phased array antenna. Although electronic circuitry is used as an example for heat-generating structure 12, system 10 may be used to cool any suitable heat-generating structure, including use as a home cooling system. In that example, an electronic cold plate may be in thermal communication with both the phased array antenna and refrigerant within a refrigeration loop 17, as described below. System 10 also has a compressor 14 and a heat exchanger 16 included within the refrigeration loop 17.
A refrigerant within cooling loop 17 is maintained at a sub-ambient pressure that is less than the pressure of the ambient environment, represented by reference numeral 19. By maintaining the pressure of the refrigerant in loop 17 at a sub-ambient pressure, refrigerants such as water, which typically boil at temperatures too high to be used as a refrigerant, may be utilized. The use of water as a refrigerant provides several advantages. In particular, the boiling of water provides a high degree of heat transfer, enabling an efficient heat transfer system. In addition, water does not result in harmful effects to the environment associated with many common refrigerants. Ethylene glycol may also be added to water and the mixture used as the refrigerant. Other refrigerants may also be used, including conventional ones, depending on the saturation pressure of the refrigerant and the desired cooling temperature. In general, the refrigerant may be selected by any standard selection criteria used in the industry.
The pressure of the refrigerant between heat-generating structure 12 and compressor 14 is maintained approximately at a first sub-ambient pressure. Without the use of compressor 14, a refrigerant at the first such sub-ambient pressure could provide a good cooling system in which heat-generating structure 12 is cooled by coming into thermal communication with the liquid refrigerant, causing the liquid refrigerant to boil at its saturation temperature and change into its vapor form. The heat stored in the vapor refrigerant is then transferred by exchanger 16 to the outside environment. However, a problem with such a system is that the ambient temperature of ambient environment 19 with which heat exchanger 16 exchanges heat could not practically be greater than the temperature of the heat-generating structure 12. This is satisfactory in some circumstances; however, there are many instances in which heat-generating structure 12 is at a temperature that is near or less than the temperature of ambient environment 19.
To address this problem, according to the teachings of the invention, compressor 14 is provided in loop 17 between heat-generating structure 12 and heat exchanger 16. Providing such a compressor 14 results in lowering the saturation temperature on the low pressure side of compressor 14 and thus the temperature at which heat is exchanged at heat-generating structure 12 can be lowered, such that when heat is exchanged by heat exchanger 16 to the outside environment, it may be exchanged at a temperature that is greater than the temperature of heat-generating structure 12. However, the pressure on the high pressure side of compressor 14 remains at sub-ambient levels. In contrast to conventional refrigeration systems, compressor 14 does not result in a large pressure differential, and in many applications provides only a few psi pressure increase. This is often large enough to allow exchange of heat by heat exchanger 16 to the outside environment at a higher temperature than possible without compressor 14.
As merely one example, the temperature at which heat-generating structure 12 is generating heat is 50° C. and the temperature within loop 17 at heat-generating structure 12 is 1.8 psia. However, the pressure on the high pressure side of compressor 14 is 4.54 psia at a temperature of 70° C. According to one embodiment, loop 17 is filled with an appropriate amount of refrigerant and loop 17 is evacuated until the desired saturation pressure (which is below ambient pressure) is achieved. This could be performed with any suitable structure, including a vacuum pump (not explicitly shown).
Thus, through the use of a refrigerant that is maintained at a sub-ambient pressure in conjunction with the use of a compressor with a relatively small power input, an efficient, refrigeration system may be provided that has a large cooling capacity. In particular, cooling may occur in an ambient environment having a temperature greater than the heat-generating structure that is being cooled.
Such a system may also allow for use of a smaller heat exchanger, such as heat exchanger 16, than would otherwise be required. Such a system may be applied in any suitable context, including military applications as well as an alternative to commercial air conditioning systems using high pressure compressors. Further, use of a sub-ambient refrigeration system allows use of water as a refrigerant, which has an associated high value of phase change energy and is more environmentally friendly than conventional refrigerants.
In certain embodiments, a pump 18 may be desired to circulate the refrigerant within loop 17. In particular, use of pump 18 may be combined with orifices 22. Orifices 22 may be provided to allow selective control of cooling of various portions of electronics 12. As described above, electronics 12 may take the form of a phased array antenna in which selective cooling of various portions of antenna system may be desired (separate portions not explicitly shown). Such a phased array antenna system is described in greater detail in co-pending application entitled “Method and Apparatus for Cooling With Coolant at a Subambient Pressure”, filed Jul. 11, 2002, having a Ser. No. of 10/192,891, and an attorney docket number of 004578.1262, which is incorporated herein by reference for all purposes. Such cooling may be effected by pumping selective amounts of the refrigerant in loop 17 through orifices 22 to selected portions of electronics 12. Pump 18 boosts the liquid refrigerant pressure for orifice control.
In conjunction with orifices 22, a temperature sensor and feedback system 24 may be provided at the output of electronics 12 to measure the temperature of the refrigerant flowing through various portions of electronics 12. This temperature may be fed back to an orifice flow controller 25 to allow modification of the amount of refrigerant flow through orifices 22 and therefore to various portions of heat-generating structure 12.
After thermal communication with heat-generating structure 12, it is likely that portions of the refrigerant within loop 17 will remain in liquid form. An accumulator 20 is provided, in one embodiment, to accumulate portions of refrigerant that have not vaporized and to minimize liquid flow to compressor 14, however, it may be desirable to have some liquid flow past such an accumulator to compressor 14. In an example in which the refrigerant is a combination of water and ethylene glycol, accumulator 20 accumulates the ethylene glycol, which is likely to remain in liquid form. In this embodiment, a second pump 26, which may be a relatively small pump with low pressure differential, may be provided to raise the pressure of the accumulated liquid such that it may be reintroduced into loop 17 and provided back to heat-generating structure 12 for further cooling.
According to another aspect of the invention, the teachings of the invention recognize that use of a sub-ambient refrigeration system could result in leaks of non-condensable ambient air into loop 17. Sub-ambient systems are counter-intuitive because of the potential for leaks into the system. However, the teachings of the invention recognize the benefits of such a system and ways to address such leaks. The teachings of the invention recognize that such air would tend to be trapped on the hot side of heat exchanger 16 and could result in a lower inner heat transfer coefficient.
To address this problem, in one embodiment, a gas removal system 28 is provided. Alternatively, gas removal system 28 could be replaced with a port that allows periodic discharge of air with a vacuum pump or other suitable device on a periodic basis. In the embodiment in which gas removal system 28 is provided, gas removal system 28 may include a small volume but large pressure differential compressor 30 that raises the pressure of the entrapped air above ambient for venting to the environment. In another embodiment, this may be combined with providing the resultant air at an increased pressure along with vapor to heat exchanger 32 to transfer some of the heat in the mixture to the ambient environment. This results in condensing some of the vapor to liquid form. The resulting mixture is provided to separator 34. Restriction valve 36 allows communication of the condensed liquid back into loop 17 from separator 34, while a vent 38 allows venting of the air as well as vapor to the atmosphere. As described above, compressor 30 increases the pressure of the trapped air to above ambient, so that it may be vented outside system 10 to the outside environment. Gas removal system 28 may be operated periodically as needed, or may operate continuously.
FIG. 2 is a graph illustrating a thermal cooling cycle according to the teachings of the invention. Reference to this graph illustrates advantages of the use of the above-described sub-ambient refrigeration system using water as a refrigerant, as opposed to a conventional refrigeration system using R22 as a refrigerant. The following calculations for the cooling system according to the teachings of the invention utilizing water has the refrigerant and for a conventional refrigeration system using R22 illustrate a fifty percent improvement in the coefficient of performance of the cooling system of the present invention as compared to a conventional system. FIG. 2 also illustrates an alternative thermal cooling cycle utilizing points 3′ and 4′, in which liquid is carried over to avoid high temperatures in the compression process and potentially make the process more efficient.
Water:

- h₁=h₂=125.89 Btu/lb (Ref. P₁=4.54 PSIA, P₂=1.8 PSIA)
- h₃=1114.5 Btu/lb, v₃=194.1 Ft³/lb
- s₃=s₄=1.9294 Btu/lb-R, h₄=1180. Btu/lb
- COP=Useful Heat Removal/Work Required=(h₃−h₂)/(h₄−h₃)=(1114.5−125.89)/(1180.−1114.5)
- COP=15.1

R22:

- h₁=h₂=59.2 Btu/lb (Ref. P₁=434.75 PSIA, P₂=281.84 PSIA)
- h₃=112.45 Btu/lb, s₃=0.20517 Btu/lb-R
- h₄=117.5 Btu/lb, v₃=0.18573 lb/Ft³
- COP=(112.45−59.2)/(117.5−112.45)=10.54

Ideal Comparison of Water and R22:
Heat in at 50C, Heat Rejection at 70C, Three Ton Refrigeration (10.55 kW)

Water R22

COP 15.1 10.5

Work (Hp) 0.937 1.34

PD (Ft3/min) 117.8 2.09

Mass (lb/min) 0.607 11.3
Thus, in addition to providing a lighter weight and more environmentally friendly system, according to one embodiment, a more efficient cooling system is provided.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A method for cooling heat-generating structure disposed in an environment having an ambient pressure, comprising the acts of:

providing a fluid refrigerant;

reducing a pressure of the refrigerant to a first sub-ambient pressure at which the refrigerant has a boiling temperature less than a temperature of the heat-generating structure;

bringing the refrigerant at the first sub-ambient pressure into thermal communication with the heat-generating structure, so that the refrigerant boils and vaporizes to thereby absorb heat from the heat-generating structure; and

increasing a pressure of the vaporized refrigerant above the first sub-ambient pressure to a second sub-ambient pressure.

2. The method of claim 1, and further comprising the act of selecting for use as the refrigerant one of water and a mixture of water and ethylene glycol.

3. The method of claim 1, and further comprising the act of circulating the refrigerant through a flow loop while maintaining the pressure of the refrigerant within a range having an upper bound less than the ambient pressure.

4. The method of claim 3, and further comprising the act of configuring the loop to include a heat exchanger for removing heat from the refrigerant so as to condense the refrigerant to a liquid.

5. The method of claim 4, and further comprising the act of causing the heat exchanger to transfer heat from the refrigerant to a further medium having an ambient temperature that is less than the boiling temperature of the refrigerant at the second sub-ambient pressure.

6. The method of claim 5, and further comprising the act of selecting for use as the medium one of ambient air and ambient water.

7. The method of claim 3, and further comprising the act of configuring the loop to include a first pump for circulating the refrigerant through the loop.

8. The method of claim 4, and further comprising the act of removing any air accumulated in the heat exchanger from any leak in the loop.

9. The method of claim 8, wherein the act of removing any air comprises condensing at least some vapor to liquid.

10. The method of claim 8, where the act of removing any air comprises raising a pressure of the air above the ambient pressure and venting the air at the raised pressure to the environment.

11. The method of claim 3, and further comprising accumulating any liquid not evaporated by the heat-generating structure and within the loop.

12. The method of claim 11, and further comprising pumping the accumulated liquid back into the loop for further thermal communication with the heat-generating structure.

13. The method of claim 1, including the act of configuring the heat-generating structure to include a plurality of sections that each generate heat; and

wherein the act the of bringing the cooling into thermal communication with the heat-generating structure includes the act of bringing respective portions of the coolant into thermal communication with the respective sections of the heat-generating structure.

14. The method of claim 13, and further comprising the acts of:

providing a plurality of orifices; and

causing each of the portions of the refrigerant to pass through the respective orifice before being brought into thermal communication with the respective section of the heat-generating structure.

15. The method of claim 14, and further comprising the act of configuring the orifices to have respective different sizes or to cause the portions of the refrigerant to have respective different volumetric flow rates.

16. An apparatus, comprising a heat-generating structure disposed in an environment having an ambient pressure, and a refrigeration system for removing heat from the heat-generating structure, the refrigeration system including:

a fluid refrigerant maintained at a pressure less than the ambient pressure at which the refrigerant has a boiling temperature less than a temperature of the heat-generating structure;

a structure that directs a flow of the refrigerant in the form of a liquid at the sub-ambient pressure in a manner causing the liquid refrigerant to be brought into thermal communication with the heat-generating structure, the heat from the heat-generating structure causing the liquid refrigerant to boil and vaporize so that the refrigerant absorbs heat from the heat-generating structure as the refrigerant changes state; and

a compressor that increases the pressure of the vaporized refrigerant to a second sub-ambient pressure.

17. The apparatus of claim 16, wherein the refrigerant is one of water and a mixture and ethylene glycol.

18. The apparatus of claim 16, wherein the structure that directs a flow of the refrigerant is configured to circulate the refrigerant through a flow loop while maintaining the pressure of the refrigerant within a range having an upper bound less than the ambient pressure.

19. The apparatus of claim 18, further comprising a heat exchanger for removing heat from the refrigerant flowing through the loop at the second sub-ambient pressure so as to condense the refrigerant to a liquid.

20. The apparatus of claim 19, wherein the heat exchanger transfers heat from the refrigerant at the second sub-ambient pressure to a further medium having an ambient temperature greater than a temperature of the heat-generating structure.

21. The apparatus of claim 20, wherein the medium is one of ambient air and ambient water.

22. The apparatus of claim 19, wherein the structure that circulates the coolant through the loop includes a pump that effects the circulation of the coolant.

23. The apparatus of claim 16, and further comprising an accumulator for accumulating liquid that is not condensed by the heat-generating structure.

24. The apparatus of claim 23, and further comprising a second pump for returning the accumulated liquid to the heat-generating structure.

25. The apparatus of claim 19, and further comprising an air removal structure attached to the heat exchanger for removing any air trapped in the heat exchanger from leaks in the refrigeration structure.

26. The apparatus of claim 25, wherein the air removal structure comprises:

a second compressor to raise the pressure of the air;

a second heat exchanger to remove heat from the air;

a separator for separating the air into liquid and gas components; and

a vent operable to vent the gas component to the environment.

27. The apparatus of claim 19, wherein the heat exchanger exchanges heat with an environment having a temperature greater than a temperature of the heat-generating structure.

28. The apparatus of claim 16,

wherein the heat-generating structure includes a plurality of sections that each generate heat; and

wherein the structure for directing the flow of the refrigerant brings respective portions of the refrigerant into thermal communication with the respective sections of the heat-generating structure.

29. The apparatus of claim 16, wherein the structure for directing the flow of the fluid includes a plurality of orifices and causes each of the portions of the refrigerant to pass through the respective orifice before being brought into thermal communication with the respective section of the heat-generating structure.

30. The apparatus of claim 29, wherein the orifices have respective different sizes in order to cause the portions of the refrigerant to have respective different volumetric flow rates.