Refrigerant System With Variable Capacity Expander

Abstract

A refrigerant system incorporates a variable capacity expander. A bypass line selectively bypasses at least a portion of the refrigerant approaching the expander to the intermediate expansion point within the expander. In this manner, the refrigerant expansion process is controlled more efficiently than in the prior art.

Description

BACKGROUND OF THE INVENTION

[0001] This application relates to refrigerant systems, wherein the expansion process is provided by an expander. The capacity of the expander can be varied by controlling the amount of refrigerant that is diverted to an intermediate expansion point in the expander. By controlling the amount of diverted refrigerant, the overall refrigerant system can be more efficiently controlled and operated, as will be explained below.

[0002] Refrigerant systems are known in the air conditioning and refrigeration art, and are utilized to condition a secondary fluid, such as air, water or glycol solution, that is delivered to a climate-controlled zone or space. In a basic refrigerant system, a compressor compresses a refrigerant and delivers it downstream to a first heat exchanger, where heat is rejected, directly or indirectly, from the refrigerant to an ambient environment. From this first heat exchanger, the refrigerant passes through an expansion process, where it is expanded to a lower pressure and temperature, and then through a second heat exchanger, where heat is accepted by the refrigerant from a secondary fluid to cool this secondary fluid to be delivered to an indoor environment. The first heat exchanger is normally called a condenser, for system operation below the refrigerant critical point, or so-called subcritical operation, and is called a gas cooler, for system operation above the refrigerant critical point, or so-called supercritical operation. The second heat exchanger typically operates in a subcritical two-phase region and is called an evaporator.

[0003] One performance enhancement option that is utilized in known refrigerant systems is the use of an expander. As compared to restriction type expansion devices, whether of fixed or adjustable type, the expander offers advantages of a more efficient isentropic expansion process, resulting in a higher refrigerant cooling potential in the evaporator, as compared to an isenthalpic process for restriction type expansion devices. Also, some expansion work can be recovered to assist in driving at least one of refrigerant system components. Both expansion work recovery and additional refrigerant cooling potential realized in the evaporator are beneficial to the refrigerant system operation, since they augment refrigerant system capacity and efficiency.

[0004] The use of the expanders for CO.sub.2 refrigerant applications is especially important, as on a relative basis, the expanders provide much larger thermodynamic cycle improvements for CO.sub.2 refrigerant than for the traditional refrigerants. It is also important to use expanders within the CO.sub.2 systems, as these systems are not as thermodynamically efficient, on an absolute scale, as the systems with conventional refrigerants, such as R22, R410A, R404A, R407C, R134a, etc.

[0005] One problem in using the expanders is related to a difficulty of controlling the amount of refrigerant passing through the expander. Further, because of their transcritical nature of the CO.sub.2 cycle, these systems are more sensitive to the refrigerant charge management than systems with conventional refrigerants.

[0006] The prior art systems relied on the refrigerant bypass around the expander (from its inlet to its outlet) to adjust the refrigerant flow throughout the system. In other words, a portion of the refrigerant flow was short-circuited to pass directly from the heat rejection heat exchanger outlet into the evaporator inlet. The use of this bypass proved to be inefficient, as the bypassed refrigerant represents a direct "leakage" from the expander inlet to the expander outlet, does not participate in the work recovery and is known to be one of the major contributors to the expander inefficiency.

SUMMARY OF THE INVENTION

[0007] In a disclosed embodiment of this invention, an expander capacity is adjusted by providing an intermediate pressure port in the expander. If it is desirable to pass more refrigerant through the expander, then a portion of the refrigerant flow from an expander inlet is diverted to the intermediate expansion port. The amount of refrigerant passing through the expander is controlled by appropriate sizing and/or adequate restriction placed in the bypass line. Preferably, the flow of refrigerant in the bypass line is controlled by a flow control device such as a valve. For instance, this valve can be of an ON/OFF type, such as a solenoid valve. The valve can also be of an adjustable restriction (modulation) type or of a pulsation type, for even more precise control of the refrigerant flow through the bypass line. A similar technique can be used if an expander consists of multiple expansion stages or expanders that are installed in series with each other. In this case, some of the refrigerant is diverted from the inlet of the upstream expansion stage into the inlet of the expansion stage located downstream. In other words, in this case, the refrigerant is injected between the expansion stages.

[0008] In this invention, the efficiency of the expansion process is improved by eliminating the direct "leak" path from a high pressure heat rejection heat exchanger to a low pressure evaporator, while maintaining the ability to provide precise control over the amount of refrigerant passing through the expander. Furthermore, due to additional work recovery obtained from the bypassed refrigerant and more efficient isentropic process, the refrigerant system's operational performance is improved.

[0009] 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

[0010] FIG. 1 schematically shows a prior art refrigerant system.

[0011] FIG. 2 shows an inventive refrigerant system.

[0012] FIG. 3 shows another schematic of an inventive refrigerant system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] A prior art refrigerant system 20 is illustrated in FIG. 1. As known, a compressor 22 compresses a refrigerant and delivers it to a heat rejection heat exchanger 24, which is a condenser, for subcritical applications, and a gas cooler, for transcritical applications. From the heat rejection heat exchanger 24, the refrigerant, expanding to a lower pressure and temperature, drives an expander 26. The expander 26 is shown schematically and includes a moving member that is driven by the expanding fluid. The expansion work recovered by the expander may be utilized (directly or indirectly) to assist in driving at least one of refrigerant system components. In other words, the expander can be connected to other system components, such as a compressor, a fan, or a pump, either directly through a coupling, a clutch, a gearbox, etc., or can be used to drive a generator to produce electric energy.

[0014] In order to control the expansion process through an expander, the prior art refrigerant systems have utilized a bypass line 28 that routed at least a portion of the refrigerant from the outlet of the heat rejection heat exchanger 24 to the inlet of the evaporator 36, at operating conditions when the expander could not handle all of the expanding refrigerant flow. In cases when the expander could not handle all of the expanding refrigerant flow, the refrigerant system performance would have become sub-optimized, with the refrigerant pressure in the heat rejection heat exchanger rising above a desired level and evaporator superheat also potentially increasing above the desired value. An inlet 32 to the bypass line 28 extends to an outlet point 33. When a bypass valve 34 is opened, the refrigerant could flow through the bypass line 28, and thus the amount of refrigerant moving through the circuit can be increased. However, the use of this bypass is ineffective as it essentially creates a high-to-low refrigerant "leak" bypassing the expander 26. In other words, as more refrigerant is bypassed around the expander 26, less useful work can be recovered by the expander. Furthermore, a portion of the refrigerant that flows through the bypass valve 34 undergoes isenthalpic expansion, which is less thermodynamically efficient than an isentropic expansion process in the expander 26.

[0015] The present invention is shown in FIG. 2 as a refrigerant system 50. Here, the bypass inlet 32 leads to a bypass line 52. A restriction 54 can be positioned on the bypass line 52, and the point 56 terminates the bypass line 52 at an intermediate expansion point in the expander 26. The restriction 54 may be an ON/OFF, modulation or pulsation valve. In this invention, when at least a portion of the refrigerant bypasses through the valve 54, the entire refrigerant still moves through and exits the expander 26. A portion of the refrigerant that bypasses through the valve 54 continues to undergo an expansion process from the intermediate expansion point 56 to the expander exit point 58. In this manner, part of the expansion work from the refrigerant passing through the bypass line 52 is still recovered in the expander 26, as well as, at least partially, this bypassed portion of the refrigerant will be expanded isentropically. At the same time, pressure upstream of the expander 26 can be controlled by the same valve 54 to optimize the operation of the refrigerant system 50.

[0016] The present invention increases the efficiency and capacity of a refrigerant system by including a variable capacity expander, while at the same time, controlling the system operation to be in the optimum domain. The present invention can be extended to an expander consisting of several expansion stages, as for example, can be a case for a multi-stage turbine. It can also be extended to a system configuration of expanders installed in series with each other. In this case, as shown in FIG. 3 for an embodiment 70, an intermediate expansion point 156 is located between the expansion stages (or independent expanders) 26A and 26B. Of course, more than two expanders can be installed in series with the bypass line routed into the point between any stages. Further, more than one bypass line can be installed when more than two expansion stages are connected serially.

[0017] Also, if needed, there can be multiple bypass lines 52. As shown in FIG. 3 embodiment, one bypass line 52 extends through the flow control valve 54 from a point 200 upstream of the first expansion stage 26A to an intermediate expansion point 202 within the same expansion stage 26A, while another bypass line 52, also incorporating the flow control valve 54, extends from a point 32 upstream of the first expansion stage 26A to a point 156 intermediate of two expansion stages 26A and 26B. Obviously, upstream points 32 and 200 can be combined into a single point.

[0018] As mentioned above, the bypass valve 54 can be of a variable area type to provide condition dependant control of how much refrigerant is routed into the bypass line 52. The bypass valve 54 can also operate in a pulse width modulated manner, such that it is rapidly cycling between ON and OFF positions.

[0019] The present invention is particularly well suited for use in refrigerant systems incorporating CO.sub.2 as a refrigerant, where the benefits of using an expander are the most pronounced.

[0020] It should be pointed out that many different expander and compressor types could be used in this invention. For example, scroll, screw, rotary or reciprocating expanders and compressors can be employed.

[0021] 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.

[0022] 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

1. A refrigerant system comprising: a compressor, said compressor compressing a refrigerant and delivering this refrigerant to a downstream heat rejection heat exchanger, refrigerant from the heat rejection heat exchanger passing through an expander, said expander being operable to recover at least a portion of work from the refrigerant expansion process, and a bypass line for selectively bypassing at least a portion of refrigerant from a point upstream of said expander to an intermediate pressure point in said expander.

2. The refrigerant system as set forth in claim 1, wherein said bypass line has a refrigerant flow restriction.

3. The refrigerant system as set forth in claim 2, wherein said refrigerant flow restriction is a valve.

4. The refrigerant system as set forth in claim 3, wherein the valve is an on/off valve.

5. The refrigerant system as set forth in claim 4, wherein said on/off valve has capability to rapidly cycle between on and off positions.

6. The refrigerant system as set forth in claim 5, wherein said cycle rate of said valve is between 1 second and 60 seconds.

7. The refrigerant system as set forth in claim 2, wherein said refrigerant flow restriction has a variable restriction area.

8. The refrigerant system as set forth in claim 1, wherein said recovered portion of work from the refrigerant expansion process is utilized to assist in driving at least one of refrigerant system components.

9. The refrigerant system as set forth in claim 8, wherein said system component is said compressor.

10. The refrigerant system as set forth in claim 1, wherein said expander consists of multiple expansion stages.

11. The refrigerant system as set forth in claim 10, wherein at least one of said expansion stages is an independent expander.

12. The refrigerant system as set forth in claim 10, wherein said bypass line bypasses the refrigerant from an upstream location of one of said expansion stages to an upstream location of a downstream one of said expansion stages.

13. The refrigerant system as set forth in claim 10, wherein there are multiple bypass lines between said expansion stages.

14. The refrigerant system as set forth in claim 1, wherein said refrigerant is CO.sub.2.

15. The refrigerant system as set forth in claim 1, wherein the recovered work is utilized to power another component by providing at least one of rotational energy and electrical power.

16. A method of operating a refrigerant system including the steps of: (1) compressing a refrigerant and delivering this refrigerant to a downstream heat rejection heat exchanger, refrigerant from the heat rejection heat exchanger passing through an expander, said expander being operable to recover at least a portion of work from the refrigerant expansion process; and (2) selectively bypassing at least a portion of refrigerant from a point upstream of said expander to an intermediate pressure point in said expander.

17. The method as set forth in claim 16, wherein a valve controlling the selective bypassing is rapidly cycled between on and off positions.

18. The method as set forth in claim 17, wherein a cycle rate of said valve is between 1 second and 60 seconds.

19. The method as set forth in claim 16, wherein said recovered portion of work from the refrigerant expansion process is utilized to assist in driving at least one of refrigerant system components.

20. The method as set forth in claim 18, wherein said bypass line bypasses the refrigerant from an upstream location of one of said expansion stages to an upstream location of a downstream one of said expansion stages.