WO2008130412A1

WO2008130412A1 - Co2 refrigerant system with booster circuit

Info

Publication number: WO2008130412A1
Application number: PCT/US2007/067168
Authority: WO
Inventors: Michael F. Taras; Alexander Lifson
Original assignee: Carrier Corporation
Priority date: 2007-04-23
Filing date: 2007-04-23
Publication date: 2008-10-30
Also published as: US20100043475A1; CN101688695A; CN101688695B; EP2150755A1; HK1142662A1; EP2150755A4

Abstract

A refrigerant system, which utilizes CO2 as a refrigerant, includes a main closed-loop refrigerant circuit and a booster closed-loop refrigerant circuit. A heat accepting heat exchanger, which provides extra cooling for the refrigerant circulating through the main circuit, and thus improves refrigerant system performance, also serves as a shared component coupling the two circuits through heat transfer interaction. Various schematics and configurations for the booster circuit, which may be combined with other performance enhancement features, are disclosed. Additional benefits for economizer function, 'liquid-to-suction' heat exchanger, intercooling and liquid injection are also presented. The booster circuit may also contain CO2 refrigerant.

Description

CO₂ REFRIGERANT SYSTEM WITH BOOSTER CIRCUIT

BACKGROUND OF THE INVENTION

This application relates to refrigerant systems which utilize CO₂ refrigerant and are provided with a booster circuit to enhance operational performance.

Refrigerant systems are known in the HVAC&R (heating, ventilation, air conditioning and refrigeration) art, and operate to compress and circulate a refrigerant throughout a closed-loop refrigerant circuit, connecting a plurality of components, to condition a secondary fluid to be delivered to a climate-controlled space. In a basic refrigerant system, refrigerant is compressed in a compressor from a lower to a higher pressure and delivered to a downstream heat rejection heat exchanger, which is a so-called gas cooler, in transcritical applications, or a so-called condenser, in subcritical applications. From the heat rejection heat exchanger, where heat is typically transferred from the refrigerant to ambient environment, a high-pressure refrigerant flows to an expansion device where it is expanded to a lower pressure and temperature and then is routed to an evaporator, where refrigerant cools a secondary fluid to be delivered to the conditioned environment. From the evaporator, refrigerant is returned to the compressor. One common example of refrigerant systems is an air conditioning system, which operates to condition (cool and often dehumidify) air to be delivered into a climate-controlled zone or space.

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, recently, 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 one of these promising natural refrigerants that have 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 inherit inefficiencies associated with the heat rejection process. Therefore, refrigerant systems utilizing CO₂ as a refrigerant do not always operate at the efficiency levels of traditional refrigerant systems. Thus, it is desirable to provide design features enhancing CO₂ system performance to become comparable to the traditional refrigerant systems for a wide spectrum of operating and environmental conditions.

SUMMARY OF THE INVENTION

A separate closed-loop booster circuit is provided in combination with a main refrigerant circuit utilizing CO₂ as a refrigerant. The booster circuit provides extra cooling for the high pressure refrigerant, in addition to the cooling provided in the heat rejection heat exchanger of the main CO₂ system. The booster circuit may also utilize CO₂ as a refrigerant.

In various features, the booster system may cool the refrigerant in the main liquid line, in the main heat rejection heat exchanger, or in a separate heat exchanger positioned downstream of the main heat rejection heat exchanger, with respect to the refrigerant flow. Moreover, the heat rejection heat exchanger of the booster circuit can be combined with the heat rejection heat exchanger of the main circuit in a single construction, such that a single air management (fan) system may be utilized to move air over both heat exchangers. Both heat rejection heat exchangers are preferably positioned to provide a more efficient counterflow configuration, with respect to the airflow.

The compressor for the booster circuit may be combined with the main circuit compression system, such as, for instance, some of the cylinder banks of a multi-piston compressor system, or may comprise a separate compressor unit.

Additionally, the booster circuit may be provided to enhance or assist with other features of the refrigerant system, such as an economizer function, "liquid-to-suction" heat exchanger, intercooling and liquid injection. 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 first schematic of the present invention.

Figure 2 shows a second schematic of the present invention. Figure 3 shows a third schematic of the present invention. Figure 4 shows a fourth schematic of the present invention. Figure 5 shows a fifth schematic of the present invention. Figure 6 shows a sixth schematic of the present invention.

Figure 7 shows system performance improvement obtained by the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refrigerant system 20 is illustrated in Figure 1 and includes a main closed-loop refrigerant circuit 21 and a booster closed-loop refrigerant circuit 32. A main circuit compressor 22 compresses a refrigerant and delivers it downstream to a main circuit heat rejection heat exchanger 24, which is a so-called gas cooler in transcritical applications or a so-called condenser in subcritical applications. A separate heat exchanger 26 is positioned downstream of the heat rejection heat exchanger 24, with respect to the refrigerant flow, to provide extra cooling to the main circuit refrigerant. A main circuit expansion device 28 is positioned downstream of the heat exchanger 26, and a main circuit evaporator 30 is located downstream of the expansion device 26. As known, the evaporator 30 operates with an associated air-moving device, such as fan, to condition (cool and often dehumidify) air being delivered into a climate-controlled zone or space of the indoor environment.

A separate closed-loop booster circuit 32 is associated with the heat exchanger 26. A booster circuit compressor 34 compresses refrigerant and delivers it to a booster circuit heat rejection heat exchanger 36, a booster circuit expansion device 38 and then through the heat accepting heat exchanger 26. The main circuit 21 operates with CO₂ as a refrigerant. CO₂ refrigerant has some challenges in providing adequate cooling performance levels, and in particular as compared to the cooling performance levels provided by the prior art traditional refrigerants. As noted above, since the CO₂ refrigerant has a low critical point, it quite often operates in a transcritical cycle, which has certain inherent inefficiencies, in comparison to a traditional subcritical vapor compression cycle. The implementation of the heat exchanger 26 provides extra cooling for the main circuit refrigerant, prior to entering the expansion device 28, and a subsequent capacity boost in the evaporator 30 as well as potential thermodynamic efficiency augmentation for the entire refrigerant system 20. Thus, the employment of the heat accepting heat exchanger 26 allows CO₂ refrigerant systems to enhance performance requirements (capacity and thermodynamic efficiency) of modern refrigerant systems, and, in particular, air conditioning systems. Additionally, the compressor 34 of the booster circuit 32 operates at much lower pressure ratios (as well as pressure differentials), in comparison to the compressor 22 of the main circuit 21, and should have better performance characteristics (isentropic and volumetric efficiencies). Further, the booster circuit compressor 34 will take advantage of a steeper slope of constant entropy lines in its operational domain, translating into lower compressor power consumption. Both phenomena described above improve overall performance characteristics (capacity and thermodynamic efficiency) of the refrigerant system 20. The booster circuit 32 may also operate with CO₂ as its refrigerant.

Refrigerant flows in the heat exchanger 26 are preferably arranged in a counterflow configuration, in order to improve the heat exchanger effectiveness. Also, the heat exchanger 26 could be incorporated into the design of the heat rejection heat exchanger 24. For instance, a tube-and-shell heat exchanger 26 may be configured as an outlet manifold of the heat exchanger 24. Alternatively, the heat exchanger 26 may be a separate heat exchange unit, such as a brazed plate heat exchanger. Further, although the booster circuit heat rejection heat exchanger 36 is shown in Figure 1 as a separate unit, it may be combined with the main circuit heat rejection heat exchanger 24. In this case, a single air management (fan) system can be provided to move air over both heat exchangers 24 and 36 that are preferably arranged in a counterflow configuration, with respect to the airflow.

As know other secondary heat transfer fluids may be used instead of air. For instance, water or brine could be employed, with liquid pumps replacing air-moving fans. All these system configurations are within the scope and can equally benefit from the present invention.

As shown in Figure 2, another embodiment 44 provides tandem compressor stages 46 and 48 which circulate refrigerant through a main circuit 41, including a heat rejection heat exchanger 50, a heat exchanger 52, the expansion device 28, and the evaporator 30. A separate compressor stage 54 compresses the refrigerant in a booster circuit 42 and circulates it through a heat rejection heat exchanger 56, the heat exchanger 52, an expansion device 43 and back to the compressor 54. As shown, a fan system 57 may be designed to move air over both heat rejection heat exchangers 50 and 56. In this manner, there is not the requirement of a separate air-moving device for each heat exchanger. Although heat rejection heat exchangers 50 and 56 are shown in a sequential arrangement, with respect to the airflow, they can also be positioned in a parallel configuration.

The tandem compressors 46 and 48 of the main circuit 41 and the compressor 54 of the booster circuit 42 may all be receiving power from the same source of energy or be driven by the same mechanism. For instance, a common eccentric drive may be provided for a multi-piston reciprocating compressor arrangement. In other words, the compressors 46, 48 and 54, although in general operating at different pressures, may be represented by separate compressor banks of the same reciprocating compressor. In all other aspect the Figure 2 embodiment provides similar benefits to the embodiment shown in Figure 1.

Figure 3 shows an embodiment 60, wherein the compressor 62 of the main circuit 61 delivers refrigerant sequentially to a heat rejection heat exchanger 64, a heat exchanger 66, an expansion device 68 and an evaporator 70. As shown in Figure 3, the refrigerant in the main circuit 61 flows from the evaporator 70 back through the heat exchanger 66, before returning to the compressor 62. In this manner, the heat exchanger 66 performs a function similar to a "liquid-to-suction" heat exchanger function (since, in transcritical operation, there may not be any liquid at the exit of the heat rejection heat exchanger 64 of the main circuit 61). This function is assisted and enhanced by a booster circuit 75, since extra cooling is obtained by the main circuit refrigerant entering an expansion device 68. This increase in the cooling capacity would normally be substantially higher then subsequent decrease in the cooling capacity caused by density reduction of the refrigerant vapor entering the compressor 62 of the main circuit 61. As described before, the booster circuit 75 includes a compressor 74 circulating the refrigerant through a heat rejection heat exchanger 76 and the heat exchanger 66. In this embodiment, the booster circuit refrigerant in the heat rejection heat exchanger 76 is cooled by a separate secondary fluid flowing through a conduit 80. For example, this secondary fluid flowing through the conduit 80, which could be, for instance, water, may be utilized as a source of heat for other needs. The refrigerant in the booster circuit 75 continues through the expansion device 77, the heat exchanger 66 and returns to the compressor 74. Therefore, the booster circuit 75 enhances the "liquid-to-suction" heat exchanger function, providing augmentation in performance characteristics of the refrigerant system 60. In all other aspects, this embodiment is similar to the embodiment of Figure 1.

An embodiment 90 is illustrated in Figure 4. In the embodiment 90, two sequential stages of compression 92 and 94 are associated with the main circuit 91. Although these two compression stages 92 and 94 are depicted as separate compressor units, they may be represented as the two compression stages within the same compressor housing. The heat rejection heat exchanger 96 is positioned downstream of the second stage 94. A tap line 100 taps a portion of refrigerant from a liquid line 106 in the main refrigerant circuit 91 and passes this tapped portion of the refrigerant through an auxiliary expansion device 102, where it is expanded to a lower pressure and temperature. Then, the tapped refrigerant passes in heat exchanger relationship with the main refrigerant flow in an economizer heat exchanger 98, to provide additional cooling to the main refrigerant, as known. As also known, while the refrigerant flows in the lines 100 and 106 through the economizer heat exchanger 98 are shown in the same direction, in practice, they are preferably arranged in a counterflow configuration, to enhance the effectiveness of the heat exchanger 98. As known, instead of using a conventional economizer heat exchanger, a flash tank arrangement can be utilized to provide similar functionality. Refrigerant in the tap line 100 is returned through a vapor injection line 104 to an intermediate pressure point between the compressors 92 and 94. The booster circuit 108 serves to enhance the economizer function and to provide additional cooling in the economizer heat exchanger 98 to the refrigerant in the main circuit 91. Therefore, the main circuit refrigerant would have a greater cooling thermal potential in the evaporator 107, while refrigerant in the vapor injection line would have a lower temperature, enhancing the compression process. The refrigerant in the main circuit 91 continues through the expansion device 28 and the evaporator 107 and returns to the first compression stage 92. As before, in the booster circuit 115, the compressor 110 compresses the refrigerant and delivers it through a heat rejection heat exchanger 112. Then, the refrigerant passes through an expansion device 116 and through the economizer heat exchanger 98 and returns to the compressor 110. Again, the purpose of this arrangement would be to provide addition cooling of the CO₂ refrigerant in the main circuit 91 and to reduce the temperature of the refrigerant vapor in the vapor injection line 104. The booster circuit 115 allows for the enhancement of the economizer function, due the two phenomena described above, and subsequent performance augmentation of the refrigerant system 90. It should be noted that, although only one economizer circuit and two compression stages are shown in Figure 4, there may be any number of economizer circuits, compression stages and associated booster circuits incorporated into a single refrigerant system design. Also, as known, there are many variations of the economizer circuit arrangement, all of which can benefit from the present invention.

Another embodiment 120 is illustrated in Figure 5. Again, in the main refrigerant circuit 121, there are two sequential stages of compression 122 and 124 that may or may not be represented by separate compressor units, a heat rejection heat exchanger 126, a heat accepting heat exchanger 128, and an evaporator 136. At a point 130 upstream of the expansion device 28, a portion the refrigerant is selectively diverted through an auxiliary expansion device 132, and into a liquid injection point 134 intermediate the compression stages 122 and 124. By metering the flow of the cold and partially expanded refrigerant from the point 130 to the point 134 in the main refrigerant circuit 121, the overall temperature of the refrigerant reaching the second compression stage 124 can be controlled. As described above, a booster circuit 138 provides additional cooling in the heat exchanger 128 for the refrigerant circulating through the main circuit 121. The booster circuit 138 includes a compressor 140, a heat rejection heat exchanger 142 and an expansion device 144. Therefore, the main circuit refrigerant reaching the diversion point 130 has a lower temperature, allowing not only for the performance enhancement in the evaporator 136, but also providing a greater cooling potential for the refrigerant injected between the compression stages 122 and 124. As a result, compression process is improved, discharge temperature control is provided and operational envelope for the refrigerant system 120 is extended. It has to be noted that there could be more then two compression stages and more than a single liquid injection point incorporated into the design of the refrigerant system 120.

Yet, another embodiment 220 is illustrated in Figure 6. Once again, in the main refrigerant circuit 221 there are two sequential compression stages 222 and 224 that, as before, may or may not be represented by separate compressor units. A heat rejection heat exchanger 226 is located downstream of the second compression stage and a heat accepting heat exchanger 228 is positioned downstream of the heat rejection heat exchanger 226, both with respect to the refrigerant flow. An expansion device 28 and then an evaporator 236 are located in series downstream of the heat accepting heat exchanger 228, also with respect to the refrigerant flow. An intercooler is positioned between the compression stages 222 and 224 and is an integral part of the heat accepting heat exchanger 228. The intercooler provides cooling to the refrigerant vapor compressed in the first compression stage 222 and routed to the second compression stage 224. As a result, the compression process is improved and the discharge temperature at the exit of the second compression stage 224 does not exceed the specified limit. Further, in transcritical applications, where temperature and pressure are independent from each other, the overall system performance can be maximized by the discharge temperature reduction. Therefore, the booster circuit 238, containing a compressor 240, a heat rejection heat exchanger 242, an expansion device 244 and the heat accepting heat exchanger 228, connected in sequence by refrigerant lines, not only improves performance of the refrigerant system 220 by providing extra cooling to the refrigerant exiting the heat rejection heat exchanger 226, but also enhances operation by providing an intercooler function, as explained above. As described earlier, it should be noted that there could be more then two compression stages and more than a single intercooler incorporated into the design of the refrigerant system 220.

In the pressure-enthalpy (P-h) graph shown in Figure 7, additional capacity obtained due to extra cooling provided by the booster circuit is depicted as Δh. Although, as known, the economizer, liquid injection and intercooler cycles are somewhat different from the basic cycle exhibited in Figure 7, in general, the performance benefits obtained from the booster circuit are similar.

It should be understood that although in the embodiments of Figures 3-6 the design of the heat accepting heat exchanger 228 has all three refrigerant streams arranged in parallel, in some embodiments, the two refrigerant streams of the main circuit may be configured in sequence with each other to provide sequential heat transfer interaction with the booster circuit, preferably in a counterflow manner. Further, in the latter arrangement, the heat exchanger 228 may be represented by two separate heat exchanger units.

In summary, the present invention discloses various schematics and techniques which can be utilized to provide a booster circuit for obtaining an extra cooling of a CO₂ refrigerant in a main refrigerant circuit. The additional benefits of enhancement of other features of the refrigerant systems, such as an economizer function, "liquid-to-suction" heat exchanger, intercooling and liquid injection are also disclosed.

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. Lastly, the booster circuit itself may have various performance enhancement features, if desired. While several embodiments are disclosed, a worker of ordinary skill in this art would recognize that certain modifications 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: a main closed-loop refrigerant circuit including a compressor for compressing refrigerant and delivering it downstream to a heat rejection heat exchanger, refrigerant from said heat rejection heat exchanger passing through an expansion device, then through an evaporator and returning to said compressor; and a booster closed-loop refrigerant circuit, said booster circuit including a compressor, a first heat exchanger to reject heat from said booster circuit, an expansion device and a heat accepting heat exchanger, and refrigerant in said booster circuit cooling refrigerant in said main circuit in said heat accepting heat exchanger, said main circuit being charged with CO₂ refrigerant.

2. The refrigerant system as set forth in Claim 1, wherein said heat accepting heat exchanger and said heat rejection heat exchanger for said main circuit comprise a single heat exchanger unit.

3. The refrigerant system as set forth in Claim 2, wherein said heat accepting heat exchanger is a shell-and-tube heat exchanger and is incorporated into an outlet manifold of said heat rejection heat exchanger.

4. The refrigerant system as set forth in Claim 1, wherein said booster circuit and said main circuit are being charged with different refrigerants.

5. The refrigerant system as set forth in Claim 1, wherein said booster circuit is also being charged with CO₂ refrigerant.

6. The refrigerant system as set forth in Claim 1, wherein said first heat exchanger for said booster circuit and said heat rejection heat exchanger are aligned such that a single air-moving device can be utilized to move air over both heat exchangers.

7. The refrigerant system as set forth in Claim 6, wherein said first heat exchanger for said booster circuit and said heat rejection heat exchanger comprise a single heat exchanger unit.

8. The refrigerant system as set forth in Claim 1, wherein said main circuit compressor and said booster circuit compressor are separate compressor units.

9. The refrigerant system as set forth in Claim 1, wherein a single drive is utilized to drive both said main circuit compressor and said booster circuit compressor.

10. The refrigerant system as set forth in Claim 1, wherein said main circuit compressor and said booster circuit compressor comprise a single compressor unit.

11. The refrigerant system as set forth in Claim 10, wherein said main circuit compressor and said booster circuit compressor are represented by different banks of cylinders of the same reciprocating compressor.

12. The refrigerant system as set forth in Claim 1, wherein said first heat exchanger of said booster circuit is utilized for heating purposes.

13. The refrigerant system as set forth in Claim 1, wherein refrigerant downstream of said evaporator in said main circuit also passes through said heat accepting heat exchanger before being returned to said compressor.

14. The refrigerant system as set forth in Claim 13, wherein said heat accepting heat exchanger is a three-stream refrigerant-to-refrigerant heat exchanger.

15. The refrigerant system as set forth in Claim 14, wherein all three refrigerant streams are arranged in parallel relationship.

16. The refrigerant system as set forth in Claim 14, wherein two refrigerant streams of said main circuit are arranged in sequence with each other and in parallel to the refrigerant stream of said booster circuit.

17. The refrigerant system as set forth in Claim 14, wherein said heat accepting heat exchanger consists of two heat exchanger units arranged in sequence with respect to refrigerant flow in said main circuit.

18. The refrigerant system as set forth in Claim 1, wherein said main circuit is provided with an economizer function.

19. The refrigerant system as set forth in Claim 18, wherein said heat accepting heat exchanger of said booster circuit is also utilized as an economizer heat exchanger of said economizer function.

20. The refrigerant system as set forth in Claim 19, wherein said heat accepting heat exchanger is a three-stream refrigerant-to-refrigerant heat exchanger.

21. The refrigerant system as set forth in Claim 20, wherein all three refrigerant streams are arranged in parallel relationship.

22. The refrigerant system as set forth in Claim 1, wherein said compressor in said main circuit has at least two sequential compression stages, and at least a portion of refrigerant downstream of said heat accepting heat exchanger being selectively tapped through an auxiliary expansion device and returned to a point intermediate of said two compression stages to provide an additional cooling for the refrigerant exiting the first compression stage and entering the second compression stage.

23. The refrigerant system as set forth in Claim 1, wherein said main circuit is provided with an intercooler heat exchanger; and said heat accepting heat exchanger of said booster circuit also being utilized as an intercooler heat exchanger.

24. The refrigerant system as set forth in Claim 23, wherein said heat accepting heat exchanger is a three-stream refrigerant-to-refrigerant heat exchanger.

25. The refrigerant system as set forth in Claim 24, wherein all three refrigerant streams are arranged in parallel relationship.

26. The refrigerant system as set forth in Claim 24, wherein two refrigerant streams of said main circuit are arranged in sequence with each other and in parallel to the refrigerant stream of said booster circuit.

27. The refrigerant system as set forth in Claim 23, wherein said heat accepting heat exchanger consists of two heat exchanger units arranged in sequence with respect to refrigerant flow in said main circuit.

28. A method of operating a refrigerant system comprising the steps of:

1) providing a main closed- loop refrigerant circuit including a compressor for compressing refrigerant and delivering it downstream to a heat rejection heat exchanger, refrigerant from said heat rejection heat exchanger passing through an expansion device, then through an evaporator and returning to said compressor; and

2) providing a booster closed-loop refrigerant circuit, said booster circuit including a compressor, a first heat exchanger to reject heat from said booster circuit, an expansion device and a heat accepting heat exchanger, and refrigerant in said booster circuit cooling refrigerant in said main circuit in said heat accepting heat exchanger, said main circuit being charged with CO₂ refrigerant.

29. The method as set forth in Claim 28, wherein said heat accepting heat exchanger and said heat rejection heat exchanger for said main circuit comprise a single heat exchanger unit.

30. The method as set forth in Claim 29, wherein said heat accepting heat exchanger is a shell-and-tube heat exchanger and is incorporated into an outlet manifold of said heat rejection heat exchanger.

31. The method as set forth in Claim 28, wherein said booster circuit and said main circuit are being charged with different refrigerants.

32. The method as set forth in Claim 28, wherein said booster circuit is also being charged with CO₂ refrigerant.

33. The method as set forth in Claim 28, wherein said first heat exchanger for said booster circuit and said heat rejection heat exchanger are aligned such that a single air- moving device is utilized to move air over both heat exchangers.

34. The method as set forth in Claim 33, wherein said first heat exchanger for said booster circuit and said heat rejection heat exchanger comprise a single heat exchanger unit.

35. The method as set forth in Claim 28, wherein said main circuit compressor and said booster circuit compressor are separate compressor units.

36. The method as set forth in Claim 28, wherein a single drive is utilized to drive both said main circuit compressor and said booster circuit compressor.

37. The method as set forth in Claim 28, wherein said main circuit compressor and said booster circuit compressor comprise a single compressor unit.

38. The method as set forth in Claim 37, wherein said main circuit compressor and said booster circuit compressor are represented by different banks of cylinders of the same reciprocating compressor.

39. The method as set forth in Claim 28, wherein said first heat exchanger of said booster circuit is utilized for heating purposes.

40. The method as set forth in Claim 28, wherein refrigerant downstream of said evaporator in said main circuit also passes through said heat accepting heat exchanger before being returned to said compressor.

41. The method as set forth in Claim 40, wherein said heat accepting heat exchanger is a three-stream refrigerant-to-refrigerant heat exchanger.

42. The method as set forth in Claim 41, wherein all three refrigerant streams are arranged in parallel relationship.

43. The method as set forth in Claim 41, wherein two refrigerant streams of said main circuit are arranged in sequence with each other and in parallel to the refrigerant stream of said booster circuit.

44. The method as set forth in Claim 41, wherein said heat accepting heat exchanger consists of two heat exchanger units arranged in sequence with respect to refrigerant flow in said main circuit.

45. The method as set forth in Claim 28, wherein said main circuit is provided with an economizer function.

46. The method as set forth in Claim 45, wherein said heat accepting heat exchanger of said booster circuit is also utilized as an economizer heat exchanger of said economizer function.

47. The method as set forth in Claim 46, wherein said heat accepting heat exchanger is a three-stream refrigerant-to-refrigerant heat exchanger.

48. The method as set forth in Claim 47, wherein all three refrigerant streams are arranged in parallel relationship.

49. The method as set forth in Claim 28, wherein said compressor in said main circuit has at least two sequential compression stages, and at least a portion of refrigerant downstream of said heat accepting heat exchanger being selectively tapped through an auxiliary expansion device and returned to a point intermediate of said two compression stages to provide an additional cooling for the refrigerant exiting the first compression stage and entering the second compression stage.

50. The method as set forth in Claim 28, wherein said main circuit is provided with an intercooler heat exchanger; and said heat accepting heat exchanger of said booster circuit also being utilized as an intercooler heat exchanger.

51. The method as set forth in Claim 50, wherein said heat accepting heat exchanger is a three-stream refrigerant-to-refrigerant heat exchanger.

52. The method as set forth in Claim 51, wherein all three refrigerant streams are arranged in parallel relationship.

53. The method as set forth in Claim 51, wherein two refrigerant streams of said main circuit are arranged in sequence with each other and in parallel to the refrigerant stream of said booster circuit.

54. The method as set forth in Claim 51, wherein said heat accepting heat exchanger consists of two heat exchanger units arranged in sequence with respect to refrigerant flow in said main circuit.