US20100024470A1

US20100024470A1 - Refrigerant injection above critical point in a transcritical refrigerant system

Info

Publication number: US20100024470A1
Application number: US12/528,699
Authority: US
Inventors: Alexander Lifson; Michael F. Taras
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2007-05-23
Filing date: 2007-05-23
Publication date: 2010-02-04
Also published as: EP2153139A4; CN101688711A; WO2008150284A1; EP2153139A1

Abstract

A refrigerant system operates in a transcritical regime. An economizer circuit is incorporated into the refrigerant system, and includes an economizer injection line for refrigerant injection above the critical point. In one of the disclosed embodiments, the refrigerant utilized is CO₂.

Description

BACKGROUND OF THE INVENTION

This application relates to a refrigerant system having an economizer circuit, wherein the refrigerant injection to the compressor occurs above a critical point in a transcritical operation.
Refrigerant compressors circulate a refrigerant through a refrigerant system to condition a secondary fluid. In a basic refrigerant system, a compressor compresses a refrigerant and delivers it downstream 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 cycle, or so-called refrigerant injection function. In the economizer cycle, 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 cooled further such that it will have a greater thermodynamic cooling potential when it reaches the second heat exchanger, enhancing refrigerant system performance. 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.
The use of economizer cycles is especially advantageous in refrigerant systems utilizing carbon dioxide (CO₂) as a refrigerant, because of increased capacity and efficiency gains in transcritical operation. The use of a two-stage (or multi-stage) economized cycle is even more beneficial, although the performance gain at each stage is typically smaller.
Carbon dioxide is an environmentally friendly natural refrigerant that nowadays is more commonly used in vapor compression systems operating for at least a portion of the time in a transcritical region. Since carbon dioxide has a low critical point, most vapor compression systems utilizing carbon dioxide as the refrigerant operate in a transcritical region (or above the critical point on a high pressure side) in at least some environmental conditions. The pressure of a subcritical fluid is essentially a function of temperature only under saturated conditions (when both liquid and vapor are present), or in other words, one is directly interrelated to the other. However, when the temperature of the fluid is higher than the critical temperature (supercritical), the saturated state no longer exists, and pressure and temperature become independent thermodynamic variables defining a thermodynamic state of the fluid. Therefore, for any set of heat sink temperature conditions, it is possible to operate a refrigerant system at many high side pressure conditions.
In the prior art, the economizer refrigerant injection occurred at an intermediate compression point early in the compression cycle, and below a critical point. For an economizer circuit in a refrigerant system operating transcritically, this may not be achievable. A similar situation would occur when partially expanded refrigerant tapped at the exit of the first heat exchanger is injected into a compression process to reduce the discharge temperature. The cooling refrigerant may be injected at supercritical conditions.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, a refrigerant system operates in a transcritical regime, at least for a portion of the time. An economizer cycle is incorporated into the refrigerant system. Refrigerant from the economizer cycle is reinjected back into the compressor at a point designed and selected such that the reinjection may occur above the critical point. In this manner, the benefits of an economizer function can be achieved in a transcritical refrigerant system operating at high discharge pressure that is well above the critical pressure. In one embodiment, there is a single stage compressor and the intermediate pressure injection point is selected and designed into the compressor to allow the injection to occur above the critical point. In another embodiment, the refrigerant system incorporates multiple economizer circuits, and the compressor has a multi-stage refrigerant injection. In the upper injection stage, the refrigerant injection occurs above the critical point and at the lower injection stage the injection may occur below the critical point. The compressor can be represented by two separate compression stages (such as two reciprocating compressor cylinders placed in serial arrangement) or two compressors connected in series. In this case, refrigerant injection is taking place between the compression stages. On the other hand, refrigerant injection can be internal to the compression pockets located within a single compressor.
In still another embodiment, the refrigerant system may have a refrigerant injection cooling branch, that is typically utilized to reduce the discharge temperature of the refrigerant exiting the compressor, through which a partially expanded portion of refrigerant tapped from the exit of the heat rejection heat exchanger is directed and injected at a point in the compression process designed and selected such that the injection may take place above the critical point.
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

FIG. 1A shows a first schematic.

FIG. 1B is a P-h chart of the FIG. 1A embodiment.

FIG. 2A shows a second embodiment.

FIG. 2B is a P-h chart for the second embodiment.

FIG. 3A shows a third embodiment.

FIG. 3B is a P-h chart for the third embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A refrigerant system 70 is illustrated in FIG. 1A having a compressor 20 compressing a refrigerant and delivering it downstream to a heat rejection heat exchanger 24, and then through an economizer heat exchanger 26. As shown, in the economized mode of operation, at least a portion refrigerant is tapped into a tap refrigerant line 28 from a main refrigerant line 30, downstream of the economizer heat exchanger 26, and directed through an economizer expansion device 32, and once again, through the economizer heat exchanger 26. This refrigerant further cools the refrigerant in the main refrigerant line 30. Refrigerant in the main refrigerant line 30 then passes through a main expansion device 34, and then to a heat accepting heat exchanger 36, before returning to the compressor 20.
The refrigerant from the tap refrigerant line 28 passes back into the compressor 20 at an injection point 27 through an injection refrigerant line 29. The injection point 27 is selected during the design of the compressor to accommodate the conditions at which the injection may take place above the critical point in the transcritical operation of the refrigerant system 70. In other words, the injected refrigerant would not be transiting through a two-phase evaporation process while traveling through the economizer heat exchanger 26. In the prior art, the injected economized refrigerant would always transit through a two-phase evaporation process while traveling through the economizer heat exchanger.
As shown, for example, in FIG. 1B, the economizer injection occurs above the critical point CP. Thus, the entirety of the economizer flow heating during heat transfer interaction in the economizer heat exchanger 26 will occur above the critical point CP and along the line X. The distance Z represents additional cooling provided by the economizer function to the main refrigerant flow in the economizer heat exchanger 26, and, as shown, also occurs entirely above the critical point CP. The refrigerant systems may operate in a transcritical regime at certain environmental conditions, at least for a portion of the time. The modification to the location of the injection of the economized refrigerant into the compression process provides the ability to efficiently achieve the economizer function in a refrigerant system 70 operating in transcritical regime. The economizer heat exchanger 26 is also designed to accommodate a single-phase refrigerant flow on its economized leg. The economizer expansion device 32 is also operated and designed to handle a single-phase refrigerant, both upstream and downstream of this expansion device. In the past, the expansion devices were not intended to operate with single-phase refrigerant at both locations upstream and downstream of the expansion device.
As shown in FIG. 2A, another refrigerant system 50 includes two compressors (or two compression stages) 52 and 54 operating in series. The refrigerant from compressor 54 is directed downstream into a heat rejection heat exchanger 56. A first economizer function occurs within an economizer heat exchanger 58, and a second economizer function occurs within an economizer heat exchanger 60. The refrigerant from a main refrigerant line 62 serially passes through the two heat exchangers 58 and 60, through a main expansion device 64, and into a heat accepting heat exchanger 66, before returning to the lower stage compressor 52. As shown, tap refrigerant lines associated with each heat exchanger 58 and 60 pass through economizer expansion devices 94 and 92 respectively. The tap refrigerant lines are shown to flow refrigerant in the same direction as the main refrigerant flow, although, this is generally for simplicity of illustration. In practice, it is often the case that the refrigerant is best to be arranged to flow in a counterflow direction, relative to the main flow, as illustrated in FIG. 1A.
Refrigerant from the second downstream economizer heat exchanger 60 is directed through an injection refrigerant line 78 back to a point 72 intermediate the compression stages 52 and 54. As shown in FIG. 2B, the heat absorption by this economized refrigerant flow extends along the line Y, and is below the critical point CP. However, the upstream economizer circuit directs refrigerant through an injection refrigerant line 74 to a point 76 and along the line C, which is positioned above the critical point CP. Again, the design of the compressor 54, and the location of the injection point 76 are selectively and carefully designed to optimize operation of the compressor 54, and to accommodate that the refrigerant injection above the critical point CP. As stated earlier, when the refrigerant injection occurs above the critical point CP, there is no transition to a predominantly vapor state through a two-phase dome in the economizer heat exchanger 58. The additional amount of cooling gained in the economizer heat exchangers 58 and 60 is shown by the corresponding distances A and B.
As mentioned above, the refrigerant in the refrigerant systems 70 and 50 may be CO₂. The refrigerant systems may operate in a transcritical regime, at least at some environmental conditions and for a portion of the time, and the modification to the location of the injection point of the economizer refrigerant provides the ability to efficiently execute the economizer function in a refrigerant system operating in a transcritical regime. The economizer heat exchanger 58 is designed to accommodate a single-phase refrigerant flow on its economized leg. The expansion device 94 is also designed to handle a single-phase refrigerant at both locations upstream and downstream of this expansion device.
It should be emphasized that a two-stage economized design can also be accomplished using a single compressor where upper and lower injection stages are placed within the same compression element. It is also possible to have three independent compression stages where both upper and lower refrigerant injections will take place between the compression stages. Furthermore, the number of compression stages can be extended to more than three, and is only limited by practical cost, size, weight, etc. and diminishing performance return considerations.
It should be pointed out that many different compressor types could be used in this invention. For example, scroll, screw, rotary, centrifugal or reciprocating compressors can be employed.
Another embodiment is shown in FIG. 3A where a portion of refrigerant can be selectively tapped through a refrigerant tap line 91 from the main refrigerant line 62 at the exit of the heat rejection heat exchanger 56. As before, this refrigerant is expanded to a lower intermediate pressure and temperature in an expansion device 90 and injected into the compression process. This refrigerant transverses the injection refrigerant line 74 but does not transverse a heat exchanger economizer 88 and therefore has a higher thermodynamic cooling potential while injected into the compression process. This tapped refrigerant is injected into the compression process at the intermediate point 76 and is typically used to reduce the refrigerant discharge temperature, in order to avoid the situations where it exceeds the allowable limit, which is likely to occur in a transcritical regime of operation. Such refrigerant injection is carefully and selectively designed to provide sufficiently low discharge temperature while conducting the injection process above the critical pressure, as shown in FIG. 3B, at least at some environmental conditions and for at least a portion of the operational time. This would also allow reduction of the compressor power, since the injected refrigerant is only compressed from a higher supercritical pressure to a discharge pressure (also supercritical).
Although in FIG. 3A the refrigerant injection is associated with a higher compression stage 54, it also can be done between the higher and lower compression stages 54 and 52 respectively. As mentioned before, multiple sequential compression stages can be represented by separate compressors, a single compression device or a combination of both. Further, as shown in FIG. 3A, the economizer and injection cooling features can be incorporated in combination with each other into the same refrigerant system, and the two tapped refrigerant flows may be injected through the same or different injection points into the compression process. Further, these two performance enhancement features can be operated on demand, simultaneously or separately.
As illustrated in FIG. 3A, the heat exchanger economizer 88 having a separate refrigerant tap line 82 and a separate expansion device 84 can be associated with the same (as the refrigerant injection cooling feature) refrigerant injection line 74. In such a combination of performance enhancement options, the economizer feature can be utilized to provide the performance boost for the refrigerant system 80, and the refrigerant injection cooling feature can be used for the refrigerant discharge temperature reduction. In the configuration shown in FIG. 3A, the same refrigerant tap line can be utilized for both performance enhancement features. If the expansion devices 84 and 90 are not equipped with a refrigerant flow shutoff capability, separate flow control devices may be provided.
It should also be noted that the system shown above can operate as a heating unit or as a cooling unit. It can also operate as a combination of a heating and cooling unit with an appropriate location of a four-way reversing valve (not shown), as known in the art. A single compressor or compression stage as shown in FIGS. 1A and 2A can be substituted by several compressor operating in tandem or as compressor banks operating in parallel (not shown).
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.
Also, the expansion devices may be a fixed orifice, capillary tube, TXV, EXV or expanders. While the economizer is illustrated as a heat exchanger economizer, it should be understood that a flash tank economizer may provide similar economizer function, as known in the art. As also known in the art, the refrigerant injection lines can be equipped with shutoff valves (not shown) to selectively turn the refrigerant injection ON and OFF, in case the economizer expansion devices are not provided with a shutoff function. The optional bypass lines and corresponding valves (not shown) can also be included to selectively bypass at least a portion of refrigerant from the refrigerant injection line to a lower compression point. Further, both economized refrigerant systems 70 and 50 may tap a portion of a refrigerant flow for the economizer function either upstream or downstream of the first refrigerant pass of the economizer heat exchanger.
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

1. A refrigerant system comprising:

a compressor for compressing refrigerant and delivering it downstream to a heat rejection heat exchanger, the refrigerant from said heat rejection heat exchanger passing through a main expansion device, and then to a heat accepting heat exchanger, before being returned to said compressor;

an injection line injecting a portion of a refrigerant tapped downstream of the heat rejection heat exchanger back into the compressor at an injection point; and

said compressor compressing a refrigerant through a transcritical cycle, where the injection may occur, for at least a portion of the time, above a critical point.

2. The refrigerant system as set forth in claim 1, wherein an economizer circuit is incorporated between the heat rejection heat exchanger and the main expansion device, said economizer circuit including at least an economizer and an economizer expansion device, receiving a main refrigerant flow from the heat rejection heat exchanger and delivering it downstream through the main expansion device, tapping a portion of the refrigerant flow upstream of said main expansion device into the injection line and injecting the tapped refrigerant into said compressor at the injection point.

3. The refrigerant system as set forth in claim 2, wherein the economizer circuit is a pair of economizer circuits having a pair of economizers, with an upstream economizer and a downstream economizer, and a pair of associated refrigerant injection points into the compression process, with an upstream refrigerant injection point and a downstream refrigerant injection point.

4. The refrigerant system as set forth in claim 3, wherein said compressor includes a two-stage compressor.

5. The refrigerant system as set forth in claim 3, wherein one of said two economizer circuits delivers the tapped refrigerant back to the compressor a point above the critical point, and the other of the two economizer circuits delivers a refrigerant back to the compressor at a point below the critical point.

6. The refrigerant system as set forth in claim 5, wherein the economizer circuit that delivers the refrigerant to a point below the critical point, delivers the refrigerant to a point intermediate the two compression stages.

7. The refrigerant system as set forth in claim 5, wherein the economizer circuit that delivers the refrigerant back to the compressor at a point below the critical point is a downstream economizer circuit, and the economizer circuit that delivers the refrigerant back to the compressor at a point above the critical point is an upstream economizer circuit.

8. The refrigerant system as set forth in claim 2, wherein said economizer is a heat exchanger type economizer.

9. The refrigerant system as set forth in claim 8, wherein said tapped portion of refrigerant flow is tapped at a location upstream of the economizer and downstream of the heat rejection heat exchanger.

10. The refrigerant system as set forth in claim 8, wherein said tapped portion of refrigerant flow is tapped at a location downstream of the economizer and upstream of the main expansion device.

11. The refrigerant system as set forth in claim 2, wherein said economizer is a flash tank type economizer.

12. The refrigerant system as set forth in claim 2, wherein a second portion of refrigerant is tapped downstream of heat rejection heat exchanger, expanded and injected back into the compressor, without passing through the economizer.

13. The refrigerant system as set forth in claim 12, wherein the two tapped refrigerant flows are injected into the compression process at the same injection point.

14. The refrigerant system as set forth in claim 12, wherein the two tapped refrigerant flows are injected into the compression process at different injection points.

15. The refrigerant system as set forth in claim 14, wherein the injection point for the tapped refrigerant flow that does not pass through the economizer is upstream of the injection point of the refrigerant that has passed through the economizer.

16. The refrigerant system as set forth in claim 14, wherein the injection point for the tapped refrigerant flow that does not pass through the economizer is downstream of the injection point of the refrigerant that has passed through the economizer.

17. The refrigerant system as set forth in claim 1, wherein said compressor is a two-stage compressor.

18. The refrigerant system as set forth in claim 1, wherein said compressor is a pair of separate compressors.

19. The refrigerant system as set forth in claim 1, wherein the refrigerant utilized in the refrigerant system is CO2.

20. The refrigerant system as set forth in claim 1, wherein said tapped portion of the refrigerant is expanded and directly injected back into the compressor at said injection point, without passing through an economizer.

21-40. (canceled)