WO2008150289A1

WO2008150289A1 - Refrigerant system with cascaded circuits and performance enhancement features

Info

Publication number: WO2008150289A1
Application number: PCT/US2007/070288
Authority: WO
Inventors: Michael F. Taras; Alexander Lifson
Original assignee: Carrier Corporation
Priority date: 2007-06-04
Filing date: 2007-06-04
Publication date: 2008-12-11
Also published as: CN101755175A; EP2162686A1; US20100147006A1; EP2162686A4

Abstract

An improved refrigerant system incorporates at least two circuits arranged in a cascaded relationship. Preferably, the upper circuit utilizes a hydrocarbon refrigerant and preferably the lower circuit utilizes CO2 refrigerant. Preferably, the CO2 cascaded circuit mainly operates in a subcritical region. To improve the efficiency and capacity control of the cascaded refrigerant system, at least one of the circuits is equipped with performance enhancement features such as, for example, an economized function provided by a flash tank or economizer heat exchanger. Additional enhancement features can also include a liquid-suction heat exchanger and bypass function.

Description

REFRIGERANT SYSTEM WITH CASCADED CIRCUITS AND PERFORMANCE ENHANCEMENT FEATURES

BACKGROUND OF THE INVENTION This application relates to refrigerant systems with at least two cascaded circuits, and more particularly, to cascade refrigerant systems with performance enhancement features.

For instance, two distinct refrigerants can be utilized in each of the two circuits, with a hydrocarbon refrigerant utilized only in the upper stage circuit and another refrigerant utilized in the lower stage circuit. The hydrocarbon type of refrigerant can be, for example, propane or isobutene refrigerant. Since the upper stage circuit can be located outside of the enclosed conditioned compartment, it would offer an advantage of locating the flammable refrigerant also outside of the enclosed space, which would mitigate fiammability concerns of these refrigerants. By utilizing the two cascaded circuits, the amount of charge in the upper stage circuit would be substantially reduced as compared to a single circuit refrigerant system. Since the amount of charge in the upper circuit is minimized, the concerns for the fiammability of the refrigerant in this circuit are also reduced.

Historically, conventional HFC and HCFC refrigerants such as R22, R123, R407C, Rl 34a, R410A and R404A, have been utilized in air conditioning and refrigeration applications. However, recent, concerns about global warming and, in some cases, ozone depletion promoted usage of natural refrigerants such as R744 (CO₂), R718 (water) and R717 (ammonia). In particular, CO₂ is a promising natural refrigerant that has zero ozone depletion potential and extremely low global warming potential of one. Thus, CO₂ is becoming more widely used as a replacement refrigerant for conventional HFC refrigerants. However, there are challenges for a refrigerant system designer with regard to utilizing CO₂. Due to its low critical point, CO₂ often operates in a transcritical cycle (rejects heat above the two-phase dome or above the critical point) that has certain inefficiencies associated with this heat rejection process. Therefore, refrigerant systems utilizing CO₂ as a refrigerant do not always operate at the efficiency levels of traditional refrigerant systems. One of the approaches to overcome the deficiencies of the CO₂ refrigerant is to utilize a cascaded design of the refrigerant system. For example, each of the two cascaded circuits can be charged with the CO₂ refrigerant. In this case, the system can be designed in such a way that each circuit would have a lower pressure differential across the circuit, than if only a single circuit refrigerant system was utilized. By reducing the pressure differential for each cascaded circuit the reliability and efficiency of the compressors can be increased. In another approach, the lower stage circuit is charged with the CO₂ refrigerant. Since only the lower stage circuit is charged with CO₂, this circuit would operate at much lower pressure as compared to a single circuit refrigerant system (not cascaded) charged with C02 refrigerant. Propane or a like refrigerant would be utilized in the upper circuit. However, even after splitting a single circuit into two independent cascaded circuits, the refrigerant system designer is still faced with many challenges dealing with further improvements of the system efficiency and capacity control.

Various enhancement features are known to increase the functionality and performance of refrigerant systems. As one example, an economizer cycle may be incorporated into a refrigerant system for its performance boost. An economizer cycle operates to subcool a main refrigerant flow, and does so, in one variation, by tapping a portion of refrigerant from the main refrigerant flow and expanding this tapped refrigerant to some intermediate pressure. This expanded refrigerant is at a cooler temperature, and passes in a heat exchange relationship with the main refrigerant flow in an economizer heat exchanger. In a variation of the economizer cycle, a flash tank replaces the heat exchanger, where vapor and liquid refrigerant phases are separated, with the liquid flow continuing through the main circuit and the vapor flow injected into the compression process at some intermediate pressure. In either variation, a vapor refrigerant is returned to the compressor.

Another enhancement feature is a refrigerant bypass function. In a bypass function, at least a portion of partially compressed refrigerant is returned to a refrigerant suction line, allowing for unloading of the refrigerant system.

Still another enhancement feature is a liquid-suction heat exchanger. In a liquid- suction heat exchanger, refrigerant downstream of an evaporator passes in heat exchange relationship with a refrigerant downstream of the condenser, allowing for additional subcooling and capacity increase of the refrigerant system. In the past, these enhancement features were associated with a standard circuit, where the circuit had an evaporator and gas cooler (or condenser).

However, none of the above-referenced enhancement features has been incorporated into a cascaded refrigerant system. In cascaded refrigerant systems, each of the cascaded circuits does not operate with an evaporator and gas cooler. Instead, the lower stage circuit has an evaporator and shares the common refrigerant-to-refrigerant heat exchanger with the upper stage circuit. The upper circuit has the gas cooler and shares the same common refrigerant-to-refrigerant heat exchanger with the lower circuit. In other words, there is no evaporator associated with the upper circuit and there is no gas cooler associated with the lower circuit.

This invention provides additional design features enhancing the cascaded system performance and functionality to become comparable to the traditional refrigerant systems for a wide spectrum of operating and environmental conditions as described in the main body of this application.

SUMMARY OF THE INVENTION

In this invention, cascaded refrigerant circuits are incorporated into a refrigerant system design. As one particular application, an upper stage circuit includes a hydrocarbon refrigerant, such as for example propane or isobutene, which can be located outdoors. The upper stage circuit is positioned in a cascaded relationship with a lower stage circuit, which would normally utilize the CO₂ refrigerant. The upper stage circuit is mainly located in the outdoor environment, while the lower stage circuit is normally located in the indoor environment. However, other locations would also fall within the scope of this invention. As one of the features of this invention, the lower stage inside CO₂ circuit operates in a subcritical region while the upper stage outside cascaded circuit would operate in a transcritical region if it was charged with the same CO₂ refrigerant. The combination of the two circuits provides performance enhancements for the supercritical region operation of the CO₂ circuit. To enhance the operation of the cascaded refrigerant system at least one of the circuits can be equipped with the economized cycle, utilizing either economizer heat exchanger or flash tank arrangements. Additionally, or as a stand alone feature, at least one of the circuits can be equipped with a liquid suction heat exchanger. Further, an unloading feature can be provided for one or both cascaded refrigerant circuits.

These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic of prior art system Figure 2 generally illustrates a feature of this prior art. Figure 3 shows a first embodiment of the present invention. Figure 4 shows a second embodiment of the present invention.

Figure 5 shows a third embodiment of the present invention. Figure 6 shows a fourth embodiment of the present invention. Figure 7 shows a fifth embodiment of the present invention. Figure 8 shows a sixth embodiment of the present invention. Figure 9 shows a seventh embodiment of the present invention.

Figure 10 shows an eighth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a prior art refrigerant system 20 incorporating two cascaded circuits 21 and 23. A lower stage circuit 23 includes a compressor 22 delivering a compressed refrigerant into a refrigerant-to-refrigerant heat exchanger 24. Heat exchanger

24 is preferably positioned outside of an environment 32 to be conditioned. Refrigerant passes from the heat exchanger 24 through an expansion device 26, and to an indoor heat exchanger 28. As known, a fan 30 blows air over external surfaces of the indoor heat exchanger 28 and delivers that conditioned air into the environment 32. The lower stage circuit 23 would normally be charged with a refrigerant that would operate in a subcritical region. One such refrigerant that can be used for this circuit would be CO₂ refrigerant that, while in the lower cascaded circuit, would still be in the subcritical region. If this same

CO₂ refrigerant would have been used in the upper cascaded circuit, it is likely to operate at transcritical regime.

In the upper stage circuit 21, a compressor 34 compresses a refrigerant and delivers it to a second outdoor heat exchanger 36. A fan 38 blows air over the heat exchanger 36. Refrigerant passes from the heat exchanger 36 downstream to an expansion device 40, and then back through the refrigerant-to-refrigerant heat exchanger 24 to the compressor 22. Figure 2 shows a P-h chart for the refrigerant system 20.

The upper stage circuit 21 can be charged with a hydrocarbon refrigerant, and in particular, this refrigerant is disclosed as one of propane or isobutene. It is known that propane and isobutene have great thermo-physical properties as refrigerants, however, they are both potentially explosive, and there are safety concerns to use them, especially in confined environments. By limiting hydrocarbon refrigerant applications to the outdoor heat exchangers, the problem of explosiveness is significantly reduced. Further, by charging only the upper stage cascaded circuit 21 with the hydrocarbon refrigerant, the refrigerant system designer reduces the total amount of the hydrocarbon refrigerant used within the refrigerant system 20, consequently decreasing the flammability risk from using hydrocarbon refrigerants. Moreover, by positioning the fans 38 in an optimum orientation with respect to heat exchanger 36, any leakage or accidental discharge of the hydrocarbon refrigerant into the conditioned space can be directed toward the outdoor environment, thus further minimizing risks of explosion.

The lower stage cascaded circuit 23 preferably operates in a subcritical region. Further, while it is disclosed that the upper stage cascaded circuit 21 operates with a hydrocarbon refrigerant, the circuit 21 can operate with other suitable refrigerants. In disclosed embodiments, additional enhancement features are provided to allow the cascaded circuits to perform more efficiently.

As shown in Figure 3, the upper stage cascaded circuit 100 is equipped with an economizer function 102 that would increase the capacity and amount of subcooling to the main refrigerant flow for this upper stage cascaded economized circuit 100. Consequently, the performance of the lower stage cascaded circuit 101 is also enhanced, since the performance of the refrigerant-to-refrigerant heat exchanger 104, that provides heat transfer interaction means between the upper stage cascaded circuit 100 and the lower stage cascaded circuit 101 and serves as a condenser for the lower stage cascaded circuit 101, is increased. An economizer heat exchanger 109 and an economizer expansion device 99 are shown. Therefore, the capacity and efficiency of the overall cascaded refrigerant system shown in Figure 3 is augmented. As an additional enhancement feature, a bypass valve 106 can be installed to connect an intermediate pressure side 107 of the upper stage cascaded circuit 100 to the suction pressure side 108 of this circuit. Selective opening of the bypass valve 106 provides the compressor unloading and capacity control means for the upper stage cascaded circuit 100, and therefore for the entire refrigerant system.

The economizer function 102 provided for the upper stage cascaded circuit 100 by the economizer heat exchanger 109 in the Figure 3 embodiment can be also provided by a flash tank 112, as shown in Figure 4, and an expansion device 199.

In general, the use of economized circuits improves the system efficiency, but also provides capacity control by selectively engaging these circuits. As known, there are many variations of the economizer cycle schematics, all of which can benefit and are within the scope of the invention. Also, selectively opening and closing the bypass valve would provide additional flexibility for the capacity control for the cascaded refrigerant system.

The upper stage cascaded circuit 100 can also be equipped with a liquid-suction heat exchanger (LSHE) 114, as shown in Figure 5, once again for the purpose of improving the capacity and amount of subcooling achieved in this upper stage cascaded circuit 100, by transferring heat from the hot refrigerant in a refrigerant line 116 to the suction refrigerant vapor in a refrigerant line 108.

Furthermore, Figure 6 shows another embodiment where the economizer heat exchanger 109 and the liquid- suction heat exchanger 114 features are combined to achieve even further capacity and efficiency improvements for the upper stage cascaded circuit 100, and thus for the entire cascaded refrigerant system.

Figure 7 represents another cascaded schematic, where an economizer heat exchanger 120 is incorporated into the lower stage cascaded circuit 101. As an illustration of the additional optional feature, this lower stage cascaded circuit 101 can also be equipped with an unloader valve 122, which would allow for bypass of a portion of refrigerant from an intermediate pressure side to suction pressure side.

Figure 8 shows yet another cascaded schematic where a flash tank 130 is incorporated into the lower stage cascaded circuit 101.

Figure 9 shows still another cascaded schematic where a liquid-suction heat exchanger 132 is incorporated into the lower stage cascaded circuit 101. Figure 10 yet shows another yet another cascaded schematic where both functions of the liquid-suction heat exchanger 132 and economizer heat exchanger 120 are incorporated into the lower stage cascaded circuit 101. These enhancement features can be used independently or in combination with each other. This embodiment shows a lower stage compressor 202 and an upper stage compressor 201.

Figure 10 also schematically shows a "black box" 300, which illustrates a performance enhancement feature such as disclosed in any of the above embodiments. That is, both circuits can be provided with such a feature.

It should be noted that the performance enhancement features described above could be incorporated and operated independently or in combination with each other for each of the cascaded circuits within the refrigerant system. Also, it has to be understood that there could be more than two cascaded circuits operating within a refrigerant system. Obviously, in many cases, it would make more sense to apply performance enhancement features listed above to the cascaded circuits charged with the refrigerants that don't operate well in the basic refrigerant cycle.

It should be pointed out that many different compressor types could be used in this invention. For example, scroll, screw, rotary, or reciprocating compressors can be employed.

The refrigerant systems that utilize this invention can be used in many different applications, including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigeration truck-trailer units, and supermarket refrigeration systems. Although embodiments of this invention have been disclosed, a worker of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.

Claims

CLAIMSWhat is claimed is:

1. A refrigerant system comprising: at least one pair of circuits operated in a cascade fashion, with a first circuit including a first compressor compressing a refrigerant and delivering the refrigerant to a heat rejection heat exchanger, refrigerant passing from said heat rejection heat exchanger through an expansion device, and then to a refrigerant-to-refrigerant heat exchanger, refrigerant from said refrigerant-to-refrigerant heat exchanger being returned to said first compressor; a second circuit, said second circuit including a second compressor, said second compressor delivering refrigerant to said refrigerant-to-refrigerant heat exchanger, refrigerant from said refrigerant-to-refrigerant heat exchanger passing through an expansion device, and then through said heat accepting heat exchanger; said refrigerant-to-refrigerant heat exchanger providing heat transfer communication between said first circuit and said second circuit; said first circuit utilizing a first refrigerant, and said second circuit utilizing a second refrigerant; and at least one additional performance enhancement feature incorporated in at least one of said first circuit or said second circuit.

2. The refrigerant system as set forth in claim 1, wherein said additional performance enhancement feature is an economizer.

3. The refrigerant system as set forth in claim 2, wherein the economizer function is provided by an economizer heat exchanger.

4. The refrigerant system as set forth in claim 2, wherein said economizer function is provided by a flash tank.

5. The refrigerant system as set forth in claim 2, wherein a bypass function is also incorporated into the refrigerant system.

6. The refrigerant system as set forth in claim 5, wherein the economizer and the bypass function are both included in the first circuit.

7. The refrigerant system as set forth in claim 5, wherein the economizer and the bypass function are both included in the second circuit.

8. The refrigerant system as set forth in claim 2, wherein the economizer is included in the first circuit.

9. The refrigerant system as set forth in claim 2, wherein the economizer is included in the second circuit.

10. The refrigerant system as set forth in claim 1, wherein the additional performance enhancement feature is a liquid-suction heat exchanger.

11. The refrigerant system as set forth in claim 10, wherein an economizer is also included in the refrigerant system.

12. The refrigerant system as set forth in claim 11 , wherein the liquid-suction heat exchanger and the economizer are both included in the first circuit.

13. The refrigerant system as set forth in claim 11, wherein the liquid-suction heat exchanger and the economizer are both included in a second circuit.

14. The refrigerant system as set forth in claim 10, wherein the liquid-suction heat exchanger is included in the first circuit.

15. The refrigerant system as set forth in claim 10, wherein the liquid-suction heat exchanger is included in the second circuit.

16. The refrigerant system as set forth in claim 1, wherein said first refrigerant is hydrocarbon.

17. The refrigerant system as set forth in claim 1, wherein said second refrigerant is CO₂.

18. The refrigerant system as set forth in claim 1, wherein said first refrigerant and said second refrigerant are the same refrigerants.

19. The refrigerant system as set forth in claim 1, wherein said first refrigerant and said second refrigerant are different refrigerants.

20. The refrigerant system as set forth in claim 1, wherein at least one circuit from said pair of circuits itself is represented by a pair of circuits.

21. The refrigerant system as set forth in claim 1, wherein there is at least one additional performance enhancement feature in said first circuit and at least one said performance enhancement feature in said second circuit.

22. A method of operating a refrigerant system comprising the steps of: providing at least one pair of circuits operated in a cascade fashion, with a first circuit including a first compressor compressing a refrigerant and delivering the refrigerant to a heat rejection heat exchanger, refrigerant passing from said heat rejection heat exchanger through an expansion device, and then to a refrigerant-to-refrigerant heat exchanger, refrigerant from said refrigerant-to-refrigerant heat exchanger being returned to said first compressor; providing a second circuit, said second circuit including a second compressor, said second compressor delivering refrigerant to said refrigerant-to-refrigerant heat exchanger, refrigerant from said refrigerant-to-refrigerant heat exchanger passing through an expansion device, and then through said heat accepting heat exchanger; said refrigerant-to-refrigerant heat exchanger providing heat transfer communication between said first circuit and said second circuit; said first circuit utilizing a first refrigerant, and said second circuit utilizing a second refrigerant; and operating at least one additional performance enhancement feature incorporated in at least one of said first circuit or said second circuit.

23. The method as set forth in claim 22, wherein there is at least one additional performance enhancement feature in said first circuit and at least one said performance enhancement feature in said second circuit.

24. The method as set forth in claim 22, wherein said additional performance enhancement feature is an economizer.

25. The method as set forth in claim 24, wherein the economizer function is provided by an economizer heat exchanger.

26. The method as set forth in claim 24, wherein said economizer function is provided by a flash tank.

27. The method as set forth in claim 24, wherein a bypass function is also incorporated into the refrigerant system.

28. The method as set forth in claim 27, wherein the economizer and the bypass function are both included in the first circuit.

29. The method as set forth in claim 27, wherein the economizer and the bypass function are both included in the second circuit.

30. The method as set forth in claim 23, wherein the economizer is included in the first circuit.

31. The method as set forth in claim 23, wherein the economizer is included in the second circuit.

32. The method as set forth in claim 22, wherein the additional performance enhancement feature is a liquid-suction heat exchanger.

33. The method as set forth in claim 32, wherein an economizer is also included in the refrigerant system.

34. The method as set forth in claim 33, wherein the liquid-suction heat exchanger and the economizer are both included in the first circuit.

35. The method as set forth in claim 33, wherein the liquid- suction heat exchanger and the economizer are both included in a second circuit.

36. The method as set forth in claim 32, wherein the liquid-suction heat exchanger is included in the first circuit.

37. The method as set forth in claim 32, wherein the liquid-suction heat exchanger is included in the second circuit.

38. The method as set forth in claim 22, wherein said first refrigerant is hydrocarbon.

39. The method as set forth in claim 22, wherein said second refrigerant is CO₂.

40. The method as set forth in claim 22, wherein said first refrigerant and said second refrigerant are the same refrigerants.

41. The method as set forth in claim 22, wherein said first refrigerant and said second refrigerant are different refrigerants.

42. The method as set forth in claim 22, wherein at least one circuit from said pair of circuits itself is represented by a pair of circuits.