US20100058781A1

US20100058781A1 - Refrigerant system with economizer, intercooler and multi-stage compressor

Info

Publication number: US20100058781A1
Application number: US12/515,558
Authority: US
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: 2010-03-11
Also published as: CN101568769A; WO2008079129A1

Abstract

A refrigerant system is provided with a multi-stage compression system. An intercooler is positioned between at least two compression stages to cool a refrigerant, by heat transfer interaction with a secondary fluid, after it has been compressed in the lower compression stages to some intermediate pressure. The intercooler enhances refrigerant system performance, improves compressor reliability, and extends operational envelope. Further, at least one economizer circuit is incorporated into the refrigerant system that returns the economized refrigerant flow at the location between at least two compression stages.

Description

BACKGROUND OF THE INVENTION

This application relates to a refrigerant system with a multi-stage compressor that combines the benefits of an intercooler heat exchanger and an economizer cycle. In particular, this application relates to a refrigerant system operating, at least for a portion of the time, in a transcritical cycle.
Refrigerant systems are known, and are utilized to condition a secondary fluid. As an example, an air conditioning system cools and dehumidifies air being delivered into a climate controlled environment.
A basic refrigerant system includes a compressor compressing refrigerant and delivering that refrigerant through a discharge line downstream to a first heat exchanger, a so-called condenser for subcritical applications or a gas cooler for transcritical applications. In the first heat exchanger, the heat is removed from the refrigerant by a secondary media, such as ambient air. From the first heat exchanger, refrigerant passes through an expansion device, where it is expanded to a lower pressure and temperature, and then through a second heat exchanger or so-called evaporator, where the heat is transferred to the refrigerant from other secondary fluid, such as indoor air, to be conditioned and delivered to a climate controlled environment. The refrigerant is then returned to the compressor to repeat the cycle.
To obtain additional capacity, enhance system efficiency and achieve higher compression ratios, it is often the case that a multi-stage compressor is provided in a refrigerant system. With a multi-stage compressor, several separate compression members or several separate compressor units are disposed in series in a refrigerant system. Specifically, for instance, in the case of a two-stage reciprocating compressor, two separate compression members may be represented by different banks of cylinders connected in series. Refrigerant compressed by a lower stage to an intermediate pressure is delivered from a discharge outlet of this lower compression stage to the suction inlet of the higher compression stage. For a multi-stage compression system, this process is repeated. If the compression ratio for the compressor system is high (which is typically the case for multi-stage compression systems) and/or refrigerant suction temperature is high (which is often the case for a refrigerant system equipped with a liquid-suction heat exchanger), then refrigerant discharge temperature can also become extremely high, and may exceed the limit defined by safety or reliability considerations.
Thus, it is known in the art to provide an intercooler heat exchanger (or a so-called intercooler) between the high and low compression stages to extend the operational envelope and/or improve system reliability. In the intercooler, refrigerant flowing between the two compression stages is typically cooled by a secondary fluid. Quite often, additional components and circuitry are required to provide cooling of the refrigerant in the intercooler. As an example, a fan or pump is supplied to move a secondary cooling fluid from a cold temperature source to cool the refrigerant in the intercooler.
Another option that is known in the refrigerant art is the use of an economizer cycle. An economizer cycle taps a portion of refrigerant from a liquid refrigerant line and expands the tapped refrigerant to some intermediate (between suction and discharge) pressure. The partially expanded tapped refrigerant is then passed through a heat exchanger in heat exchange relationship with the liquid refrigerant flow circulating through the main refrigerant circuit and prior to entering main expansion device. In this manner, the main refrigerant flow in the liquid line is cooled, while the tapped portion of refrigerant flow is evaporated and typically superheated. The tapped refrigerant is then returned to an intermediate pressure point in a compression system. As also known, a flash tank separating vapor and liquid phases of refrigerant may be used as the economizer heat exchanger and essentially provide similar benefits to the refrigerant system performance and operation.
The combination of the intercooler heat exchanger and the economizer cycle has not been fully realized for multi-stage compression systems and would be especially beneficial in modern refrigerant systems that are operating, at least for portion of the time, in transcritical cycle and utilizing natural refrigerants such as carbon dioxide (also known as CO₂or R744).
In particular, with the CO₂refrigerant systems, the intercooler heat exchanger and the economizer cycle become even more important, as these systems tend to operate at high discharge temperatures due to high operating pressure ratios, and, in general, by the transcritical nature of the CO₂cycle, as well as a high value of the polytropic compression exponent for the CO₂refrigerant. However, the additional cost and complexity of the circuitry and components associated with the intercooler and economizer, makes the provision of the intercooler less feasible. However, it become desirable to provide proper intercooler and economizer configurations for multi-stage compressor refrigerant systems, and particularly for CO₂refrigerant systems, for the reasons described above.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, a refrigerant system is provided with at least two compression stages connected in series. The refrigerant is progressively compressed to a higher pressure while flowing from a lower compression stage to a higher compression stage. At least one intercooler is placed between at least the two compression stages to cool the refrigerant after it exits the lower stage and before it enters the higher stage. In addition, the refrigerant system incorporates at least one economizer cycle, with the tapped portion of refrigerant returned from the economizer branch to an intermediate compression point between the higher and lower compression stages. In one embodiment, there are at least three compression stages connected in series, and the tapped portion of refrigerant is returned to a point in the compression cycle between the two compression stages that are different from the compression stages between which the intercooler is located. In this embodiment, the intercooler is preferably positioned downstream of the return point of the tapped economized refrigerant. In another embodiment, the intercooler and the economizer branch tapped refrigerant return point are positioned between the same two compression stages, with the economized refrigerant return point being preferably located downstream of the intercooler heat exchanger.
Preferably, the intercooler heat exchanger is positioned between the higher compression stages of the multi-stage compression system (with more than two compression stages), where refrigerant temperatures have reached higher values, allowing for larger temperature differentials between the refrigerant and a secondary fluid, enhancing heat rejection capability, and improving performance of the refrigerant system. This is especially beneficial when ambient air is utilized directly or indirectly (e.g., through auxiliary loops with an intermediate fluid, such as city water) as a secondary fluid, in particular, at high ambient temperatures. The locations for the intercooler and the economizer circuit can be interchanged, with the intercooler positioned between the lower compression stages and the economizer circuit positioned between the higher compression stages, depending on the temperature of the secondary fluid utilized in the intercooler and capacity vs. efficiency tradeoff for the economizer circuit.
On the other hand, strategically positioning the economizer circuit between the lower compression stages allows for larger temperature differentials in the economizer heat exchanger and thus for higher refrigerant cooling potential in the evaporator and the refrigerant system performance (capacity and/or efficiency) enhancement. Also, the colder refrigerant injected between the compression stages further reduces discharge temperature, improves reliability of the entire compression system, extends an operational envelope for the refrigerant system and enhances evaporator dehumidification capability. Lastly, positioning the economizer circuit between lower compression stages allows for a larger step in refrigerant system unloading strategy, which is desired in most of the applications.
This is especially important in case of transcritical operation, where the high side temperature and pressure are independent from each other. In the transcritical operation, the discharge pressure is not limited by the discharge temperature anymore and can be adjusted to the value providing an optimum performance level. Thus, in such circumstances, the refrigerant system efficiency and capacity can be enhanced even further by optimizing the discharge pressure.
In another embodiment, both intercooler and economizer circuit are positioned between the same compression stages. Once again, the relative position of the intercooler and the economizer circuit, with respect to refrigerant flow primarily depends on the temperature of the secondary fluid utilized in the intercooler and capacity-efficiency tradeoff for the economizer circuit.
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. 1 shows a first schematic view of a refrigerant system incorporating the present invention.

FIG. 2 shows a second schematic view of a refrigerant system incorporating the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A refrigerant system 20 is illustrated in FIG. 1. Three stages of compression 24, 26, and 28 are positioned in series within the refrigerant system 20 to progressively compress refrigerant from suction to discharge pressure. Although a multi-stage compressor system is represented by separate compressor units that are disposed in series, as shown in FIG. 1, separate compression members can be utilized instead of some or all of the compressor units. Specifically, for instance, in the case of a three-stage reciprocating compressor, the three separate compression members may represent different banks of cylinders connected in series. Refrigerant, compressed by the first stage from a suction pressure to a first intermediate pressure, is delivered from a discharge outlet of this first stage to the suction inlet of the second stage. Refrigerant vapor is compressed by the second stage to a second intermediate pressure and delivered from a discharge outlet of this second stage to the suction inlet of the third stage. Lastly, refrigerant, compressed by the third stage to a discharge pressure, is delivered from a discharge outlet of this third stage to a discharge line of a refrigerant system. An intercooler heat exchanger 30 is positioned between the second and third compression stages 26 and 28. Secondary fluid, such as air blown by a fan 32, passes over the intercooler 30 to cool the refrigerant.
Cooling refrigerant in the intercooler 30 increases system capacity and efficiency, since the compressor discharge temperature is reduced and the first or outdoor heat exchanger 34 (a condenser in the subcritical cycle and a gas cooler in the transcritical cycle) will be capable of cooling refrigerant to a lower temperature, eventually providing a higher cooling potential for the refrigerant entering the evaporator 50. Compressor power is also reduced, as heat removed from the compression process decreases the operating pressure of the outdoor heat exchanger 34. Additionally, if the refrigerant system 20 operates in a transcritical cycle, where the high side temperature and pressure are independent from each other, the discharge pressure is not limited by a discharge temperature anymore and can be adjusted to a value corresponding to an optimum performance level. Moreover, in both subcritical and transcritical cycles, the temperature of the refrigerant discharged from the highest, third compression stage 28 is reduced, improving overall reliability of the compression system. Thus, performance (efficiency and capacity) of the refrigerant system 20 is increased and compressor reliability is improved.
The present invention is particularly useful in refrigerant systems that utilize CO₂as a refrigerant, since CO₂refrigerant has a high value of a polytropic compression exponent, and the discharge operating pressures and pressure ratios of such systems can be very high, promoting higher than normal discharge temperatures. Still, the invention would extend to refrigerant systems utilizing other refrigerants.
Preferably, the intercooler heat exchanger 30 is positioned between the higher compression stages, such as the compression stages 26 and 28 in FIG. 1, where refrigerant temperature have reached the higher values, allowing for the larger temperature differentials between the refrigerant and secondary fluid, enhanced heat rejection capability, and superior performance of the refrigerant system 20. This is especially beneficial when ambient air is utilized directly or indirectly (e.g., through auxiliary loops with an intermediate fluid, such as city water) as a secondary fluid, in particular, at high ambient temperatures.
From the third compression stage 28, the refrigerant passes through the outdoor heat exchanger 34, and then to an economizer heat exchanger 36. As known, a tapped portion of refrigerant in a tap line 38 is tapped from a liquid line 40. The tapped refrigerant in the tap line 38 passes through an economizer expansion device 42, where it is expanded to some intermediate (between suction and discharge) pressure. During the expansion process in the economizer expansion device 42, the temperature of the tapped portion of refrigerant is reduced as well. Therefore, the tapped expanded refrigerant flowing through the economizer heat exchanger 36 is able to cool refrigerant in the liquid line 40. Although, for illustration simplicity, the two refrigerant streams are shown flowing in the same direction, in this embodiment, in practice, it is desirable to arrange the two flows in the counterflow configuration. The tapped portion of refrigerant is evaporated and typically superheated, during heat transfer interaction with the liquid refrigerant in the liquid line 40 in the economizer heat exchanger 36, and is returned through a vapor injection refrigerant line 44 to an intermediate point 46 between the first and second compression stages 24 and 26.
Downstream of the economizer heat exchanger 36, refrigerant in the liquid line 40, having been cooled to a lower temperature in the economizer heat exchanger 36 and therefore having higher cooling potential, passes through a main expansion device 48, where it is expanded to a pressure approximated the suction pressure, and then through an evaporator 50, where it conditions a secondary fluid supplied to a climate controlled environment, while the refrigerant is evaporated and typically superheated prior to entering the compression system. From the evaporator 50, the refrigerant is returned to the first compressor stage 24 to repeat the cycle.
As known, in a majority of the cases, the economizer cycle allows for enhanced performance (capacity and/or efficiency), reduced discharge temperature, improved reliability, more flexible unloading strategy and better dehumidification capability. Strategically positioning the economizer circuit return line 44 between the lower compression stages, such as the compressor stages 24 and 26 in FIG. 1, allows expansion of the tapped portion of refrigerant in the economizer expansion device 42 to a lower intermediate pressure, and thus obtaining larger temperature differentials in the economizer heat exchanger 36 between the refrigerant in the liquid line 40 to be cooled and the tapped portion of refrigerant. These higher temperature differentials in turn allow for lower temperatures of the refrigerant in the liquid line 40 and higher cooling potential in the evaporator 50. Therefore, the system performance (capacity and/or efficiency), as well as its dehumidification capability, can be increased significantly, by locating the vapor injection line 44 of the economizer cycle between lower compression stages. Also, the colder refrigerant injected between the compression stages 24 and 26 further reduces discharge temperature, improves reliability of the entire compression system and extends the operational envelope for the refrigerant system 20. Once again, this is especially important in case of transcritical operation, where the high side temperature and pressure are not directly related to each other, i.e. the discharge pressure is not limited by the discharge temperature anymore and can be adjusted to the value providing an optimum performance level. Thus, in such circumstances, refrigerant system efficiency and capacity can be enhanced even further by optimizing the discharge pressure. Additionally, it is beneficial in situations where the intercooler heat exchanger 30 alone is not capable of performing the desired function and assuring efficient and reliable operation of the refrigerant system 20. Lastly, positioning the vapor injection line 44 between the lower compression stages 24 and 26 allows for a large step in refrigerant system unloading, which is desired in most of the applications.
By incorporating the intercooler heat exchanger 30 and the economizer cycle, and utilizing strategic locations for both enhancement option, the present invention provides maximum benefits in performance (capacity and/or efficiency), reliability, operational envelope extension, unloading capability, dehumidification flexibility and ability to achieve precise control over the temperature and humidity in the conditioned environment
Although only three compression stages are shown in FIG. 1, refrigerant systems having more than three compression stages, with the economizer circuit preferably positioned between the lower compression stages and the intercooler heat exchanger positioned between the higher compression stages, can equally benefit and are within the scope of the present invention. Further, depending on the temperature of the fluid utilized to cool refrigerant in the intercooler 30 (to obtain an overall counterflow configuration) and a tradeoff between refrigerant system capacity and efficiency related to the economizer circuit, the locations of the intercooler and the return point of the vapor injection line 44 can be interchanged, with the intercooler 30 being positioned between the lower compression stages and the economizer circuit positioned between higher compression stages.
FIG. 2 shows another embodiment 60, wherein the refrigerant system incorporates a higher stage and a lower stage of compression 62 and 64 respectively, with the intercooler heat exchanger 66 and the return point 68 for the vapor injection line 44 of the economizer branch both being positioned intermediate the two compression stages. As shown in this embodiment, the return point 68 of the vapor injection line 44 is located downstream of the intercooler 66, with respect to refrigerant flow. Further, in this embodiment, the tap line 70 for tapping the portion of refrigerant to pass through the economizer heat exchanger 36 is positioned downstream of the economizer heat exchanger 36. The economizer circuit and economizer expansion device 72 operate as in the FIG. 1 embodiment. Also, rather than utilizing the fan 32 of FIG. 1, a fluid conduit 80 is used to cool the refrigerant in the intercooler heat exchanger 66. The fluid in the conduit 80 can be supplied, for instance, by a pump (not shown). Although the refrigerant system 60 shown in FIG. 2 has less flexibility and potential for operation enhancement, in comparison to the FIG. 1 embodiment, the benefits obtained from the combination of the intercooler 66 and economizer circuit are still significant. Obviously, the location of the return point 68 of the vapor injection line 44 can also be upstream of the intercooler heat exchanger 66, with respect to refrigerant flow, and depends on the temperature of cooling fluid in the conduit 80, in order to provide most efficient overall conterflow configuration. Refrigerant systems with more than two compression stages can equally benefit from this embodiment, where the intercooler heat exchanger 66 and the economizer circuit positioned between the same compression stages.
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 use of a lower and upper compression stage can be combined within a single compressor, where the vapor injection would take place at the intermediate location in the compression cycle for this compressor. Alternatively, the upper and lower compression stages can be represented by a separate compression elements, with the vapor injection or intercooling taking place between the stages. The compression elements can be separate compressor units or the compression elements can be a part of a single compressor, as it is the case for a reciprocating compressor where each compression element can be represented by a single bank of cylinders for this reciprocating compressor. The refrigerant systems that utilize this invention can be applied in many different applications, including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigerated truck-trailer systems, and supermarket refrigeration applications.
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:

at least two compression stages connected in series with respect to a refrigerant flow, a condenser positioned downstream of said at least two compression

stages, an expansion device positioned downstream of said condenser and an evaporator positioned downstream of said expansion device, refrigerant passing through said at least two compression stages, to said condenser, to said main expansion device, to said evaporator, and then returning to a lower compression stage of said at least two compression stages; and

an economizer circuit incorporated into the refrigerant system, said economizer circuit including an economizer heat exchanger for receiving a portion of refrigerant tapped from a liquid line in the main circuit, and expanded to an intermediate pressure, said tapped expanded refrigerant cooling a refrigerant flowing in said liquid line through said economizer heat exchanger, and said tapped refrigerant

then being returned to an injection point intermediate said at least two compression stages, and an intercooler heat exchanger positioned between said at least two compression stages.

2. The refrigerant system as set forth in claim 1, wherein at least two compression stages are represented by separate compressor units.

3. The refrigerant system as set forth in claim 1, wherein at least two compression stages are represented by compression members within the same compressor.

4. The refrigerant system as set forth in claim 1, wherein there are at least three of said compression stages, and said injection point for said tapped refrigerant and said intercooler are positioned between different compression stages.

5. The refrigerant system as set forth in claim 4, wherein said intercooler is positioned between higher compression stages and said injection point is positioned between lower compression stages with respect to refrigerant flow.

6. The refrigerant system as set forth in claim 4, wherein said intercooler is

positioned between lower compression stages and said injection point is positioned between higher compression stages with respect to refrigerant flow.

7. The refrigerant system as set forth in claim 1, wherein said injection point for said tapped refrigerant and said intercooler are positioned between the same compression stages.

8. The refrigerant system as set forth in claim 7, wherein said intercooler is positioned downstream of said injection point with respect to refrigerant flow.

9. The refrigerant system as set forth in claim 7, wherein said intercooler is positioned upstream of said injection point with respect to refrigerant flow.

10. The refrigerant system as set forth in claim 1, wherein said tapped refrigerant is tapped from a location upstream of said economizer heat exchanger.

11. The refrigerant system as set forth in claim 1, wherein said tapped refrigerant is tapped from a point downstream of said economizer heat exchanger.

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

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

14. The refrigerant system as set forth in claim 1, wherein said refrigerant system utilizes CO₂as a refrigerant.

15. The refrigerant system as set forth in claim 1, wherein a relative position of said intercooler and said injection point with respect to refrigerant flow is at least partially defined based on at least one of a temperature of a source for a secondary fluid supplied to the intercooler and relative importance of capacity and efficiency provided by the economizer circuit.

16. The refrigerant system as set forth in claim 1, wherein an absolute position of said intercooler and said injection point with respect to compression stages is at least partially defined based on at least one of discharge temperature, environmental conditions, required refrigerant system efficiency, required refrigerant system capacity, required unloading options and a temperature of a source for a secondary fluid supplied to the intercooler.

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

(1) providing at least two compression stages connected in series with respect to a refrigerant flow, a condenser positioned downstream of said at least two compression stages, an expansion device positioned downstream of said condenser and an evaporator positioned downstream of said expansion device, refrigerant passing through said at least two compression stages, to said condenser, to said main expansion device, to said evaporator, and then returning to a lower compression stage of said at least two compression stages; and

(2) providing an economizer circuit incorporated into the refrigerant system, said economizer circuit including an economizer heat exchanger for receiving a portion of refrigerant tapped from a liquid line in the main circuit, and expanded to an intermediate pressure, using said tapped expanded refrigerant to cool a refrigerant flowing in said liquid line through said economizer heat exchanger, and said tapped refrigerant then being returned to an injection point intermediate said at least two compression stages, and positioning an intercooler heat exchanger between said at least two compression stages.

18. The method as set forth in claim 17, wherein there are at least three of said compression stages, and said injection point for said tapped refrigerant and said intercooler are positioned between different compression stages.

19. The method as set forth in claim 18, wherein said intercooler is positioned between higher compression stages and said injection point is positioned between lower compression stages with respect to refrigerant flow.

20. The method as set forth in claim 18, wherein said intercooler is positioned between lower compression stages and said injection point is positioned between higher compression stages with respect to refrigerant flow.

21. The method as set forth in claim 17, wherein said injection point for said tapped refrigerant and said intercooler are positioned between the same compression stages.

22. The method as set forth in claim 21, wherein said intercooler is positioned downstream of said injection point with respect to refrigerant flow.

23. The method as set forth in claim 21, wherein said intercooler is positioned upstream of said injection point with respect to refrigerant flow.

24. The method as set forth in claim 17, wherein said tapped refrigerant is tapped from a location upstream of said economizer heat exchanger.

25. The method as set forth in claim 17, wherein said tapped refrigerant is tapped from a point downstream of said economizer heat exchanger.

26. The method as set forth in claim 17, wherein said refrigerant system operates at least for a portion of the time in the transcritical cycle.

27. The method as set forth in claim 17, wherein said refrigerant system operates at least for a portion of the time in the subcritical cycle.

28. The method as set forth in claim 17, wherein said refrigerant system utilizes CO₂as a refrigerant.

29. The method as set forth in claim 17, wherein a relative position of said intercooler and said injection point with respect to refrigerant flow is at least partially defined based on at least one of a temperature of a source for a secondary fluid supplied to the intercooler and relative importance of capacity and efficiency provided by the economizer circuit.

30. The method as set forth in claim 17, wherein an absolute position of said intercooler and said injection point with respect to compression stages is at least partially defined based on at least one of discharge temperature, environmental conditions, required refrigerant system efficiency, required refrigerant system capacity, required unloading options and a temperature of a source for a secondary fluid supplied to the intercooler.