WO2009048464A1

WO2009048464A1 - Tandem compressors of different types

Info

Publication number: WO2009048464A1
Application number: PCT/US2007/080871
Authority: WO
Inventors: Alexander Lifson; Michael F. Taras
Original assignee: Carrier Corporation
Priority date: 2007-10-10
Filing date: 2007-10-10
Publication date: 2009-04-16
Also published as: US20110214439A1

Abstract

A refrigerant system having tandem compressors includes at least two compressors of different types. By utilizing the two distinct compressor types, a greater difference in the provided capacity of the two compressors can be achieved at part-load conditions, as well as a particular compressor type can be engaged at specific environmental conditions to provide the most efficient operation of the refrigerant system.

Description

TANDEM COMPRESSORS OF DIFFERENT TYPES

BACKGROUND OF THE INVENTION

This application relates to refrigerant systems having tandem compressors, wherein the compressors are of distinct types.

Heating, ventilation, air conditioning and refrigeration (HVAC&R) systems are utilized to condition various environments. The HVAC&R systems typically use a refrigerant circulating throughout a closed-loop circuit and are applied as air conditioners, heat pumps, refrigeration units, etc. Various enhancement techniques and system configurations are known and implemented to provide a required performance over a wide spectrum of environmental conditions to satisfy diverse thermal load demands.

In a very basic refrigerant system, a compressor compresses a refrigerant and delivers it downstream to a heat rejection heat exchanger (a condenser in subcritical applications and a gas cooler in transcritical applications). Refrigerant passes from the condenser to an expansion device, and from the expansion device to an evaporator. From the evaporator, refrigerant returns to the compressor. This basic system is typically supplemented and enhanced by a number of different options and features to satisfy application requirements.

One such enhancement is the use of tandem compressors. Tandem compressor configurations include a plurality of compressors each receiving refrigerant from the refrigerant system, each separately compressing the refrigerant and delivering the refrigerant back to the refrigerant system. Tandem compressors have at least one common manifold such, as for instance, a suction manifold or a discharge manifold. Tandem compressors equipped with the vapor injection function may also have a common intermediate pressure manifold. Each of these compressors may be independently turned on or off to vary refrigerant system capacity. In this manner, the capacity provided by the compressor subsystem to the overall refrigerant system can be tailored to the thermal load demands in the conditioned space and environmental conditions. Quite often, tandem compressor configurations include oil and vapor equalization lines for functionality and reliability enhancement.

To date, tandem compressors have always relied on the compressors of the same type. As an example, there may be two or more scroll compressors operating in tandem, two or more reciprocating compressors operating in tandem, two or more screw compressors operating in tandem, etc.

Since the compressor types were of the same design, their sizes have tended to span a limited range of capacities. Normally, the capacity would not differ by more than a ratio of 1 :5, for example.

For optimum capacity and pull down control, it would often be desirable to have one of the compressors to be of a fairly large size, and the other to be significantly smaller. This has been difficult to achieve with the prior art tandem compressors.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, a tandem compressor system incorporates at least two compressors that are of distinct types. The compressors operate in tandem to receive a refrigerant from a refrigerant system and to deliver the compressed refrigerant back to the refrigerant system. Various refrigerant system and tandem compressor subsystem enhancements may be incorporated into this basic design.

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.

Figure 2 shows a second schematic. Figure 3 shows a third schematic. Figure 4 shows a fourth schematic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A refrigerant system 20 is illustrated in Figure 1, and has a relatively large compressor 22, and a smaller compressor 24 operating in tandem and receiving a refrigerant from a common suction manifold 36 while delivering a compressed refrigerant to a common discharge manifold 28. As shown in Figure 1, an oil equalization line 26 may be installed to connect oil sumps of tandem compressors 22 and 24 to prevent oil pumpout from one of the tandem compressors while having it accumulated within the other tandem compressor. As known, refrigerant vapor equalization line (not shown) may be also installed to equalize refrigerant pressure between the tandem compressors.

The compressors 22 and 24 are of a distinct type. Any one of the two compressors could be, for instance, a scroll compressor, a rotary compressor, a screw compressor or a reciprocating compressor. In particular, in disclosed embodiments, one of the tandem compressors is a scroll compressor and the other is a rotary compressor, or one is a scroll compressor and the other is a screw compressor. By utilizing tandem compressors of different types, greater differences in compressor sizes and capacity can be achieved. Also, the overall unit cost can be substantially reduced, as some compressor types are more cost effective to be manufactured in certain capacity ranges (sizes) in comparison to the other types. For example, rotary compressors are very cost effective for manufacturing in smaller sizes, with the electrical power consumption of the compressor motor being in the range of one to three kilowatts. On the other hand, scroll compressors might be the most cost effective for manufacturing in the compressor motor size range from three to fifteen kilowatts. Therefore, it might be cost effective to build refrigerant system where a rotary compressor is operated at light operational loads and the scroll compressor is operated at high operational load.

From the common discharge manifold, the refrigerant flows to a heat rejection heat exchanger 30 and passes through an expansion device 32 and an evaporator 34 in sequence. From the evaporator 34, the refrigerant returns through a common suction manifold 36 to the tandem compressors 22 and 24. As is known, dependent upon environmental conditions and thermal load demand in a climate-controlled space, either of the tandem compressors 22 and 24 can be shut down, or both of the compressors 22 and 24 can be operated at the same time. In this manner, the performance of the refrigerant system 20 can be controlled to tailor the provided capacity to thermal load demands in the climate-controlled space. By utilizing the distinct compressor types, a greater difference in sizes and provided capacity between the two compressors is achieved at part-load operating conditions. This allows the refrigerant system 20 to be better respond to a wide range of potential thermal loads in the climate-controlled space without cycling compressors on and off, and consequently provide better temperature and humidity control, enhance operational thermodynamic efficiency of the refrigerant system 20 and improve reliability of the tandem compressors 22 and 24. Of course, more than two compressors and more than two compressor types can be utilized.

Also, different compressor types have a "sweet" spot at different operating conditions. For instance, fixed volume ratio compressors such as screw and scroll compressors provide the most efficient full-load operation in the region of the pressure ratios corresponding to the built-in volume ratio in accordance to the polytropic compression process. On the other hand, reciprocating compressors provide poor volumetric efficiency at large pressure ratios, due to the clearance volume refrigerant re- expansion. Therefore, even having the same size (capacity) tandem compressors 22 and 24 within the refrigerant system 20 benefits the system operation, since different compressors could be operated at different environmental conditions to optimize the refrigerant system efficiency at these environmental conditions.

An economized refrigerant system 40 is illustrated in Figure 2. Again, the tandem compressors 22 and 24 are of distinct types, as mentioned above. The tandem compressors 22 and 24 have the common discharge manifold 28. Downstream of the heat rejection heat exchanger 30, a portion of refrigerant is tapped into a tap refrigerant line 42, passed through an economizer expansion device 44, and through an economizer heat exchanger 46. In the economizer heat exchanger 46, the refrigerant that has been tapped and expanded to an intermediate pressure and temperature, cools refrigerant in a liquid refrigerant line 38 passing through the main expansion device 32, and then through the evaporator 34. While the tapped refrigerant is shown passing through the economizer heat exchanger 46 in the same direction as the main refrigerant flow, in practice, it is typically desirable to arrange the two refrigerant passes in counterflow relationship. However, for illustration simplicity, they are shown flowing in the same direction. Downstream of the economizer heat exchanger 46, the tapped refrigerant passes through an economized flow return refrigerant line 48, and then into an economized flow injection refrigerant line 50 leading to an intermediate pressure port in the compressor 22. An optional bypass refrigerant line 52 has a shutoff valve 54 that selectively allows for a return of at least a portion of the refrigerant from the economized flow return refrigerant line 48 back to a common suction manifold 136. Alternatively, in a non-economized mode of operation, the shutoff valve 54 may allow for a return of at least a portion of a partially compressed refrigerant from an intermediate pressure port to the suction port of the compressor 22, unloading the compressor 22. Further, the two refrigerant flows mentioned above may be combined and delivered to the suction port of the compressor 22.

As known, there are different configurations of economized refrigerant systems, including (but not limited to) refrigerant systems where the economized refrigerant flow is tapped downstream of the economizer heat exchanger or refrigerant systems with a flash tank (in place of the economizer heat exchanger). Further, if there are more than two economized compressors, they may have a common intermediate pressure manifold. As explained above, vapor equalization lines may be provided as well. These systems are within the scope and can equally benefit from the invention. All the advantages of the refrigerant system 20 of the Figure 1 embodiment outlined above are also applicable to the refrigerant system 40 of the Figure 2 embodiment.

Figure 3 shows another embodiment 60, wherein a liquid line 62 passes through an expansion device 64, an evaporator 66, and back to the compressor 24. Another liquid line 67 passes through an expansion device 68, an evaporator 70, and back to the compressor 22. In this embodiment, the two compressors 22 and 24 of different types are associated with their own evaporators 70 and 66 and expansion devices 68 and 64 respectively. Therefore, the tandem compressors of different types 22 and 24 have distinct suction manifolds 236A and 236B but still have the common discharge manifold 28. Once again, all the benefits of the Figure 1 embodiment are equally applicable to the refrigerant system 60.

Figure 4 shows yet another embodiment 72, wherein the refrigerant flowing to the tandem compressors 22 and 24 passes in series through a heat rejection heat exchanger 30, a main expansion device 74 and an evaporator 76, returning to a common suction manifold 136. A refrigerant tap line 79 passes through an auxiliary expansion device 78 connecting to a refrigerant injection line 110 that leads to an intermediate pressure point in the compressor 24. This refrigerant injection through the intermediate pressure port in the compressor 24 can be utilized for various functions, such as, for instance, lowering the refrigerant discharge temperature. Further, a bypass valve 82 positioned in a refrigerant bypass line 80 can be utilized to return at least a portion of that refrigerant to the suction ports of the compressors 22 and 24. Again, a worker ordinarily skilled in the refrigerant art would be aware of why and when such an operational regime would be valuable. Alternatively, a portion of partially compressed refrigerant in the compressor 24 can be passed from the intermediate pressure port of the compressor 24 into the suction ports of the compressors 22 and 24, through the bypass valve 82 and the bypass line 80, to unload the compressor 24 and to reduce capacity of the refrigerant system 72. Further, in another mode of operation, the two refrigerant flows from the refrigerant tap line 79 and from the intermediate pressure port of the compressor 24 can be combined to be diverted to the suction side of the compressors 22 and 24. As described above, all the advantages of the Figure 1 embodiment are equally applicable to the refrigerant system 72.

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 this 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: a tandem compressor unit including at least two compressors receiving refrigerant from the refrigerant system, compressing the refrigerant and delivering the refrigerant into the refrigerant system; a heat rejection heat exchanger positioned downstream of said tandem compressor unit, an expansion device positioned downstream of said heat rejection heat exchanger, and an evaporator positioned downstream of said expansion device, refrigerant passing from said tandem compressor unit to said heat rejection heat exchanger, through said expansion device to said evaporator, and back to said tandem compressor unit; and said at least two compressors of said tandem compressor unit being of distinct types.

2. The refrigerant system as set forth in claim 1, wherein said at least two compressors of said tandem compressor unit have a common suction manifold and a common discharge manifold.

3. The refrigerant system as set forth in claim 1, wherein said at least two compressors of said tandem compressor unit have at least one of a common suction manifold and a common discharge manifold.

4. The refrigerant system as set forth in claim 1, wherein said at least two compressors of said tandem compressor unit are of different sizes.

5. The refrigerant system as set forth in claim 1, wherein an economizer cycle is incorporated into the refrigerant system.

6. The refrigerant system as set forth in claim 5, wherein said economizer cycle returns a tapped refrigerant to an intermediate compression point in at least one of said at least two compressors of said tandem compressor unit.

7. The refrigerant system as set forth in claim 6, wherein said economizer circuit returns refrigerant to an intermediate compression point only in some of said at least two compressors of said tandem compressor unit.

8. The refrigerant system as set forth in claim 7, wherein said some of said at least two compressors have a common intermediate pressure manifold.

9. The refrigerant system as set forth in claim 5, wherein said at least two compressors of said tandem compressor unit have a common suction manifold and a common discharge manifold.

10. The refrigerant system as set forth in claim 5, wherein said at least two compressors of said tandem compressor unit have at least one of a common suction manifold and a common discharge manifold.

11. The refrigerant system as set forth in claim 5, wherein said at least two tandem compressors of said tandem compressor unit are of different sizes.

12. The refrigerant system as set forth in claim 1, wherein a bypass function is included into the refrigerant system.

13. The refrigerant system as set forth in claim 1, wherein at least one of said two compressors of said tandem compressor unit is a scroll compressor.

14. The refrigerant system as set forth in claim 1, wherein at least one of said at least two compressors of said tandem compressor unit is a screw compressor.

15. The refrigerant system as set forth in claim 1, wherein at least one of said at least two compressors of said tandem compressor unit is a rotary compressor.

16. The refrigerant system as set forth in claim 1, wherein at least one of said at least two compressors of said tandem compressor unit is a reciprocating compressor.

17. The refrigerant system as set forth in claim 1, wherein said at least two compressors of distinct types being utilized for selective unloading of the refrigerant system.

18. The refrigerant system as set forth in claim 1, wherein said at least two compressors of distinct types being utilized for the refrigerant system efficiency boost by selectively engaging a certain compressor at certain environmental conditions.

19. The refrigerant system as set forth in claim 1, wherein a tap selectively taps refrigerant upstream of said expansion device, expands that tapped refrigerant to an intermediate pressure, and returns it to an intermediate compression point in at least one of said at least two compressors of said tandem compressor unit.

20. A method of operating a refrigerant system comprising the steps of: providing a tandem compressor unit that includes at least two compressors receiving refrigerant from the refrigerant system, compressing the refrigerant and delivering the refrigerant into the refrigerant system; positioning a heat rejection heat exchanger downstream of said tandem compressor unit, positioning an expansion device downstream of said heat rejection heat exchanger, and positioning an evaporator downstream of said expansion device, refrigerant passing from said tandem compressor unit to said heat rejection heat exchanger, through said expansion device to said evaporator, and back to said tandem compressor unit; and said at least two compressors of said tandem compressor unit being of distinct types.

21. The method as set forth in claim 20, wherein said at least two compressors of distinct types being utilized for selective unloading of the refrigerant system.

22. The method as set forth in claim 20, wherein said at least two compressors of distinct types being utilized for the refrigerant system efficiency boost by selectively engaging a certain compressor at certain environmental conditions.