EP2097686A1

EP2097686A1 - Co2 refrigerant system with tandem compressors, expander and economizer

Info

Publication number: EP2097686A1
Application number: EP06846056A
Authority: EP
Inventors: Alexander Lifson; Michael F. Taras
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2006-12-26
Filing date: 2006-12-26
Publication date: 2009-09-09
Also published as: US20100071391A1; EP2097686A4; CN101568770A; WO2008079128A1

Abstract

A refrigerant system utilizes an expander that provides a more efficient expansion process and recovers at least a portion of energy from this expansion process. At least a portion of refrigerant that has been at least partially expanded in the expander is tapped and passed through an economizer heat exchanger. In the economizer heat exchanger, the tapped refrigerant cools a main circuit refrigerant increasing its thermodynamic potential. Further, a compression section is provided with at least two compressors operating in tandem and allowing for multiple stages of unloading. By selectively utilizing one or more of the tandem compressors, the economizer cycle, and the expander, the refrigerant system can achieve very efficient operation and provide enhanced control flexibility, in particular, for the CO2 refrigerant system operating in the transcritical cycle.

Description

CO₂REFRIGERANT SYSTEM WITH TANDEM COMPRESSORS, EXPANDER AND ECONOMIZER

BACKGROUND QF THE INVENTION This application relates to a refrigerant system wherein an expander is utilized to provide a more efficient expansion process, by recapturing energy from this expansion process and utilizing that energy to power at least one system component. A portion of refrigerant is tapped from a location in the expander at which it has been at least partially expanded, and the tapped refrigerant is then utilized to subcool a main refrigerant flow in an economizer heat exchanger. In this manner, a single expander can provide both main and economizer expansion device functions for the refrigerant system, and enhance these main and economizer expansion device functions. Further, the refrigerant system includes tandem compressors only some of which may be economized. Lastly, the refrigerant system may be charged with CO₂ refrigerant that, at least for a portion of the time, may operate in a transcritical cycle.

Refrigerant compressors circulate a refrigerant through a refrigerant system to condition a secondary fluid. Typically, a compressor compresses a refrigerant and delivers it to a first heat exchanger. Refrigerant from the first heat exchanger passes through an expansion device, in which its pressure and temperature are reduced. Downstream of the expansion device, a refrigerant passes through a second heat exchanger, and then back to the compressor.

One option in a refrigerant system design is the use of an economizer, or so- called vapor injection function. In an economizer function, a portion of refrigerant is tapped from a main refrigerant stream downstream of the first heat exchanger. This tapped refrigerant is passed through an expansion device, to be expanded to an intermediate pressure and temperature, and then this partially expanded tapped refrigerant passes in heat exchange relationship with a main refrigerant flow in an economizer heat exchanger. In this manner, the main refrigerant is subcooled such that it will have a greater thermodynamic potential when it reaches the second heat exchanger. The tapped refrigerant, typically in a superheated thermodynamic state, is returned to an intermediate compression point in the compressor downstream of the economizer heat exchanger.

It is also known to provide at least two compressors operating in parallel and known as "tandem" compressors. Tandem compressor configurations allow the control of the overall capacity provided by the refrigerant system by selectively engaging the tandem compressors. Further, all or only some of the tandem compressors may be economized, and the economizer function may be activated or deactivated to precisely match capacity delivered by a refrigerant system to thermal load demands in the conditioned space. Another feature utilized in refrigerant systems is the use of an expander to provide a more efficient so-called active expansion process (this process can closely follow an isentropic line rather than isenthalpic line for so-called passive expansion devices such as expansion valves and fixed restriction devices). The expanders could be of various designs such as centrifugal, scroll, rotary, screw, shaft-connected piston, free-piston or any other type.

The refrigerants that are being utilized in refrigerant systems are different nowadays, with continuously increasing number of installations charged with natural refrigerants. CO₂ (also known as carbon dioxide or R744) is one such refrigerants that is becoming more widely used in various applications such as supermarket refrigeration, display cases, bottle coolers, transport refrigeration, environmental control units, etc. In particular, CO₂ refrigerant can benefit greatly from any of the above-mentioned system enhancement options, especially in transcritical applications. However, no refrigerant system to date has included an expander, tandem compressors and an economizer cycle combined together to provide a powerful combination of operational options and enhancement features.

SUMMARY QF THE INVENTION

In a disclosed embodiment of this invention, an expander is utilized to provide a more efficient active expansion process to potentially recapture energy from this expansion process, and utilize that energy to power at least one refrigerant system component. The expander increases capacity and enhances efficiency of a refrigerant system through a more efficient expansion process (reduced irreversible losses) and recovering, at least partially, the expansion process energy. Further, a portion of refrigerant that has been at least partially expanded in the expander (or through the first stage of the expander) is tapped and passed through an economizer heat exchanger in heat exchange relationship with a main refrigerant flow. This tapped refrigerant is injected back into the compressor (a single compressor or a multi-stage compressor system consisting of compressors connected in series) at some intermediate pressure. All or only some of the compressors may be economized. Exiting the economizer heat exchanger, the main refrigerant has a higher thermodynamic potential that allows for enhanced refrigerant system performance. In addition, the refrigerant system includes at least two tandem compressors that, along with the economizer function, allow for efficient system unloading and precise matching of delivered system capacity to thermal load demands in the conditioned space. The expander may transfer the recovered energy from the expansion process, directly or indirectly through an energy conversion device, to at least partially power one of the tandem compressors. The refrigerant system, as disclosed, may use a natural refrigerant such as CO₂. The proposed refrigerant system enhancements are particularly beneficial for the CO₂ refrigerant system operating in a transcritical cycle.

The provision of the combination of the economizer cycle, expander, and tandem compressors provides the ability to closely tailor the provided capacity of the refrigerant system to thermal load demands in the conditioned space and to enhance performance of the refrigerant system through a more efficient expansion process, economizer cycle and energy recovery, especially for a CO₂ refrigerant system operating in a transcritical cycle. In one embodiment, a portion of refrigerant is tapped form a single intermediate tap in the expander to flow through the economizer heat exchanger in the economizer cycle. In another embodiment, there are two such taps at two distinct intermediate pressure levels associated with two economizer heat exchangers. Of course, a worker of ordinary skill in the art would recognize that more taps and economizer circuits are within the scope and can equally benefit from the present invention. 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 a first inventive refrigerant system. Figure 2 shows a second schematic. Figure 3A shows a third schematic.

Figure 3B shows an alternate tandem compressor configuration for the refrigerant system depicted in Figure 3A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A refrigerant system 20 is illustrated in Figure 1. As shown, two compressors 22 and 24 operate in tandem to provide compressed refrigerant and deliver it throughout the refrigerant system 20. Obviously, more than two compressors can be

_ connected in parallel arrangement and operated simultaneously or individually, based on thermal load demands in the conditioned space. Although in a conventional configuration shown in Figure 1, the tandem compressors have common suction and common discharge manifolds, as known to a person ordinarily skilled in the art, the tandem compressors may have a common suction manifold and separate discharge lines as well as separate suction lines and a common discharge manifold. Additionally, tandem compressors 22 and 24 may have oil and vapor equalization lines (not shown), as known in the art.

A compressed refrigerant passes through a heat exchanger 26, and downstream to an expander 28. As known to a person skilled in the art, the heat exchanger 26 becomes a condenser, in the subcritical applications, and a single-phase heat exchanger, or a gas cooler, in the transcritical applications. As mentioned above, it is known to use an active expander instead of a passive expansion device, such as an expansion valve or a fixed restriction expansion device, in which energy from a more efficient isentropic expansion process is recaptured and utilized to power or drive (at least partially) at least one refrigerant system component, such as one of the compressors 22 and 24 (as shown schematically at 29). This energy recovery and transfer can be done directly or indirectly. For instance, for a direct energy recovery, the expander 28 and the compressor 22 can be located on the same shaft or connected through a mechanical transmission device. On the other hand, for an indirect energy recovery, the energy from the expansion process may be converted to an electric energy that in turn may assist in driving the compressor 22. Downstream of the expander 28, the refrigerant passes through an evaporator 30, and then back to the compressors 22 and 24. An economizer heat exchanger 32 is incorporated into the circuitry of the refrigerant system 20. As known, an economizer function allows for additional stages of unloading as well as performance (capacity and efficiency) enhancement.

A portion of refrigerant is tapped from a tap line 51 connected to an intermediate expansion port in the expander 28 and passed through the economizer heat exchanger 48. The tap line 51 is communicated to a point in the expansion process at which the refrigerant has been at least partially expanded to a pressure intermediate of suction and discharge pressures. The refrigerant flowing in the tap line 51 is at lower pressure and temperature than the refrigerant in the main refrigerant circuit leaving the heat exchanger 26 and cools this refrigerant passing through the economizer heat exchanger 48, preferably in a counterflow heat transfer relationship. The tapped refrigerant is returned through a vapor injection line 53 to the compressors 22 and 24, typically in a superheated state. As is known, the vapor injection line 53 manifolds refrigerant to the compressors 22 and 24 and delivers it into the compression chambers within the compressor at an intermediate point in the compression process. As known, each of individual compressors 22 and 24 can be replaced with two compressor stages connected in series.

This embodiment achieves the inclusion of the economizer circuit without the need for a separate economizer expansion device, since the expander 28 utilizes an intermediate expansion port for delivering of the economizer branch refrigerant. In this manner, a single expander can provide both main and economizer expansion device functions at higher efficiency levels than in conventional refrigerant systems equipped with passive expansion devices. Obviously, two separate expansion stages for the main and economized refrigerant flow can be used instead, and in this case, the economizer branch refrigerant and the tap line 51 can be positioned downstream as well as upstream of the economizer heat exchanger 32.

The use of the tandem compressors 22 and 24 allows for a more flexible system control by selectively actuating one or both of the compressors to provide a necessary capacity to adequately satisfy thermal load requirements in the conditioned space. Compressors 22 and 24 may have different sizes to allow for more stages of capacity adjustment. In addition, the economizer function can be enabled or disabled, for each compressor individually or simultaneously for both compressors, to provide additional capacity stages when necessary. As shown in Figure 1, valves 62 and 64 allow for selective engagement and disengagement of the economizer function for the compressors 22 and 24 respectively.

The refrigerant system 20 may utilize CO₂ as the refrigerant. In this case, the refrigerant system 20 may operate in the transcritical cycle, at least for a portion of the time. As known, the transcritical cycle is normally less efficient than the subcritical cycle, so the abovementioned enhancement features and control options of the present invention (the tandem compressors in combination with the expander and the economizer cycle) will be the most beneficial for the transcritical cycle.

Figure 2 shows another embodiment 70, which is similar to the embodiment 20, however, three tandem compressors 72, 74 and 76, with only one compressor 72 being economized, are illustrated. Again, the refrigerant system 70 would provide additional benefits in operation control and performance enhancement. The combination of the expander 28, the economizer circuit and economizer heat exchanger 32, along with the tandem compressor allows close tailoring of the refrigerant system capacity to achieve desired conditions in the climate controlled environment in the most efficient manner. The control (not shown) for the refrigerant system 70 may be configured to operate the individual compressors 72, 74 and 76 (which, once again, may be of different sizes) or any combinations of them to provide a desired number of unloading staged. As mentioned above and shown in Figure 2, the economized refrigerant is returned to only one compressor 72. A worker skilled in this art would recognize the various features and benefits, such as performance, control flexibility, control complexity and cost, that would be provided by returning the refrigerant to one or more than one of the compressors, and can select an appropriate design accordingly.

As can be recognized by a person ordinarily skilled in the art, more than two sequential compression-expansion stages and more than one economizer circuit may be integrated into the refrigerant systems 20 and 70. The number of stages is primarily defined by a cost-performance benefit tradeoff. These sequential stages may be represented by stand-alone devices or integrated into a single device. One of such refrigerant systems having two economizer circuits and three compression-expansion stages is shown in Figure 3A. In the refrigerant system 170 depicted in Figure 3A, an expander 128 has two intermediate pressure ports associated with two economizer circuits. A higher pressure port 136 communicates with an economizer heat exchanger 134 operating at a higher pressure and temperature, and a lower pressure port 138 communicates with an economizer heat exchanger 132 operating at a lower pressure and temperature. It has to be noted that both higher and lower intermediate pressures mentioned above are positioned between the suction and discharge operating pressures for the refrigerant system 170. Further, there are three compression stages incorporated into the refrigerant system 170, and a vapor injection line 157 for the higher pressure economizer circuit is positioned between the second and third compression stages, while a vapor injection line 153 for the lower pressure economizer circuit is positioned between the first and second compression stages. Refrigerant flow control devices such as a valve 166 and valves 162 and 164 selectively activate the higher and lower pressure economizer circuits respectively. The valve 164 deactivates the economizer function for both lower pressure compressors 172 and 174, while the valve 162 deactivates the economizer function just for the compressor 172. Once again, only two (out of three) lower pressure tandem compressors 172, 174 and 176 are equipped with the economizer ports, the compressors 172 and 174 in particular. The first and second compression stage of the economized tandem compressors 172 and 174 represent the first and second compression stages of the refrigerant system 170, while the non-economized tandem compressor 176 operates between the same manifold pressures as the two sequential stages of the economized compressors 172 and 174. Two higher pressure non- economized tandem compressors 182 and 184 represent the third compression stage of the refrigerant system 170. Furthermore, the recovered portion of the expansion process energy in the expander 128 is transmitted to at least partially power the non- economized lower pressure compressor 176. Obviously, any other compressor of the refrigerant system 170 can be connected to the power source provided by the expender 128, in a similar manner. In all other aspects, the embodiment 170 of Figure 3A is similar to the Figure 1 and Figure 2 embodiments 20 and 70 respectively.

Figure 3B embodiment shows an alternate tandem compressor configuration for the refrigerant system depicted in Figure 3A. In this embodiment, the compressor 182 of the third compression stage directly communicates with the downstream compressors 172 and 174, while the compressor 184 of the same third compression stage directly communicates with the downstream compressor 176. Two refrigerant flow control valves 192 and 194 in Figure 3B replace a single refrigerant flow control valve 166 of Figure 3A, to selectively engage and disengage the economizer function for each of the compressors 182 and 184 respectively. Lastly, the compressor 184 now takes advantage of the energy recovery from the refrigerant system expander, as shown at 29 in Figure 3B. It is understood that a person skilled in the art would recognize that many other variations of tandem compressor and economizer circuit configurations, as well as energy recovery arrangements, are feasible and can equally benefit from the invention.

The tandem compressors described in this application can be separate compressor units, or the compression elements of each of those compressors can be combined within a single compressor shell (as, for example, the compression elements positioned on the opposite ends of the rotating shaft or separate compression elements and associated motors simply enclosed within a single shell).

It should be noted that this invention is not limited to the system shown in the Figure 1, as the actual refrigerant system may include additional components, such as, for example, a liquid-suction heat exchanger, a reheat coil, an intercooler, additional economizer heat exchangers or flash tanks. The individual tandem compressors can be of variable capacity type, including variable speed and multi-speed configurations. Further, the compressors may have various unloading options, including intermediate pressure to suction pressure bypass arrangements. The compressors may be unloaded internally, as for example, by separating fixed and orbiting scrolls from each on an intermittent basis. These system configurations are also not limited to a particular compressor type and may include scroll compressors, screw compressors (single or multi-rotor configurations), reciprocating compressors (where, for example, some of the cylinders are used as a low compression stage and the other cylinders are used as a high compression stage) and rotary compressors. The refrigerant system may also consist of multiple separate circuits. The present invention would also apply to a broad range of systems, for example, including mobile container, truck-trailer and automotive systems, packaged commercial rooftop units, supermarket installations, residential units, environmental control units, etc.

Although a preferred embodiment of this invention has 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

We claim: 1. A refrigerant system comprising: at least two compressors operating in tandem, said compressors compressing refrigerant and delivering it downstream to a first heat exchanger, refrigerant passing from said first heat exchanger through a first pass of an economizer heat exchanger, and then to an expander, refrigerant expanding in said expander, and said expander recovering at least a portion of energy from the expansion process of said refrigerant, and said recovered energy utilized to assist in driving of at least one of the refrigerant system components; a second heat exchanger positioned downstream of said expander, and refrigerant passing from said expander through said second heat exchanger, and then back to said at least two compressors; and a portion of refrigerant being tapped at a location where it has been at least partially expanded in said expander, and passed through a second pass of said economizer heat exchanger to cool refrigerant in a main refrigerant circuit, said tapped portion of refrigerant then being returned to at least one of said at least two compressors.

2. The refrigerant system as set forth in claim 1, wherein said tapped portion of refrigerant is returned to both of at least two compressors.

3. The refrigerant system as set forth in claim 1, wherein said at least two compressors have at least one common manifold.

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

5. The refrigerant system as set forth in claim 1, wherein at least one of said at least two compressors consists of at least two separate stages.

6. The refrigerant system as set forth in claim 1, wherein said expander has separate expansion stages for the main and tapped refrigerant flows.

7. The refrigerant system as set forth in claim 1, wherein there are at least three of said compressors operating in tandem.

8. The refrigerant system as set forth in claim 1, wherein said tapped portion of refrigerant being returned to at least one of said at least two compressors.

9. The refrigerant system as set forth in claim 1, wherein said refrigerant is CO₂-

10. The refrigerant system as set forth in claim 1, wherein said refrigerant system operates in a transcritical cycle for at least a portion of the time.

11. The refrigerant system as set forth in claim 1, wherein said refrigerant system operates in a subcritical cycle for at least a portion of the time.

12. The refrigerant system as set forth in claim 1, wherein said at least two compressors can be controlled individually or in combination with each other.

13. The refrigerant system as set forth in claim 1, wherein the economizer circuit can be selectively actuated.

_*

14. The refrigerant system as set forth in claim 1, wherein more than two sequential compression stages and more than two economizer circuits are utilized.

15. The refrigerant system as set forth in claim 1, wherein said recovered energy is utilized to at least partially drive at least one of said at least two compressors.

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

(1) providing at least two compressors operating in tandem, said compressors compressing refrigerant and delivering it downstream to a first heat exchanger, refrigerant passing from said first heat exchanger through a first pass of an economizer heat exchanger, and then to an expander, refrigerant expanding in said expander, and said expander recovering at least a portion of energy from the expansion process of said refrigerant, and said recovered energy utilized to assist in driving of at least one of the refrigerant system components;

(2) providing a second heat exchanger positioned downstream of said expander, and refrigerant passing from said expander through said second heat exchanger, and then back to said at least two compressors; and

(3) tapping a portion of refrigerant from a location where it has been at least partially expanded in said expander, and passing it through a second pass of said economizer heat exchanger to cool refrigerant in a main refrigerant circuit, said tapped portion of refrigerant then being returned to at least one of said at least two compressors.

17. The method as set forth in claim 16, wherein said tapped portion of refrigerant is returned to both of at least two compressors.

18. The method as set forth in claim 16, wherein said at least two compressors have at least one common manifold.

19. The method as set forth in claim 16, wherein said at least two compressors are of different sizes.

20. The method as set forth in claim 16, wherein at least one of said at least two compressors consists of at least two separate stages.

21. The method as set forth in claim. 16, wherein said expander has separate expansion stages for the main and tapped refrigerant flows.

22. The method as set forth in claim 16, wherein there are at least three of said compressors operating in tandem.

23. The method as set forth in claim 16, wherein said tapped portion of refrigerant being returned to at least one of said at least two compressors.

24. The method as set forth in claim 16, wherein said refrigerant is CO₂.

25. The method as set forth in claim 16, wherein the refrigerant system operates in a transcritical cycle for at least a portion of the time.

26. The method as set forth in claim 16, wherein the refrigerant system operates in a subcritical cycle for at least a portion of the time.

27. The method as set forth in claim 16, wherein said at least two compressors can be controlled individually or in combination with each other.

28. The method as set forth in claim 16, wherein the economizer circuit can be selectively actuated.

29. The method as set forth in claim 16, wherein more than two sequential compression stages and more than two economizer circuits are utilized.

30. The method as set forth in claim 16, wherein said recovered energy is utilized to at least partially drive at least one of said at least two compressors.