US20060010907A1

US20060010907A1 - Refrigerant system with tandem compressors and reheat function

Info

Publication number: US20060010907A1
Application number: US10/891,938
Authority: US
Inventors: Michael Taras; Alexander Lifson
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2004-07-15
Filing date: 2004-07-15
Publication date: 2006-01-19
Also published as: WO2006019885A3; WO2006019885A2

Abstract

A refrigerant system incorporates at least two compressors that act in tandem to provide variable control over the refrigerant system performance. The tandem compressors can be of conventional or economized type and are configured for maximum performance utilization. Further, a reheat circuit can be incorporated into the refrigerant system at several different locations. The reheat cycle provides additional control over sensible and latent capacity of the refrigerant system, and is particularly advantageous when utilized in combination with the tandem compressors. As a result, multiple steps of unloading can be implemented in all operation regimes, the external load demands are satisfied with much greater precision, eliminating undesirable variations in temperature and humidity, system efficiency and reliability are augmented and equipment life-cycle cost is reduced.

Description

BACKGROUND OF THE INVENTION

This invention relates to refrigerant cycles incorporating a reheat coil, and wherein a tandem compressor arrangement is utilized. The resulting cycles provide an enhanced control over both humidity and temperature in the conditioned space as well as improve efficiency, reliability and life-cycle cost of the equipment.
Refrigerant cycles are utilized to change the temperature and humidity, or otherwise control the environment, in the conditioned space. In particular, a refrigerant cycle will typically include a compressor delivering a compressed refrigerant to a condenser, and from the condenser to an expansion device. The refrigerant exchanges heat with an outdoor environment at the condenser, and is expanded in the expansion device to a lower pressure and temperature. From the expansion device, the refrigerant continues to an evaporator at which it exchanges heat with an indoor environment, or an environment to be conditioned. From the evaporator, the refrigerant returns to the compressor.
The amount of cooling that can be supplied by a refrigerant cycle is known as its “capacity.” There are numerous ways to provide control over the capacity for a refrigerant cycle. One way is to substitute multiple compressors acting in tandem for a single compressor. In such an arrangement, the compressors can be selectively operated or shut down in response to an external heat load demand. Also, tandem compressors of different sizes can be utilized such that the various steps in the total capacity can be achieved. Moreover, both economized and conventional compressors can be employed in the tandem configuration, allowing for switching between various compressor modes of operation and further increasing a number of the unloading steps. Lastly, the tandem compressor configuration can be selectively chosen for any of the independent circuits of a multi-circuit system.
Another way of controlling an environment in the conditioned space with a refrigerant cycle is the incorporation of a reheat coil. Typically, a reheat coil is provided in the path of air that has been blown over the evaporator. The air passes over the reheat coil to regain heat from a refrigerant that is at a temperature hotter than the temperature of air leaving the evaporator. The reheat coil is thus able to raise the temperature of the air leaving the evaporator. Hence, the air dehumidified and overcooled in the evaporator is reheated back to a comfortable temperature level in the reheat coil. In other words, in the system with the reheat coil, the humidity in the conditioned space is mainly controlled by the evaporator and its temperature—by the reheat coil.
The reheat coil has not been utilized in a combination with tandem compressors, however. Thus, the refrigerant cycles with tandem compressors have not had as complete control over temperature and humidity levels as may be desired. It has to be noted that this invention is not related to any particular reheat concept or tandem compressor configuration, but rather provides advantages, that could not be obtained before for a refrigerant system, by integrating both design features in a single cycle. Consequently, a system with any reheat schematic or tandem compressor configuration can take advantages from the invention.

SUMMARY OF THE INVENTION

In disclosed embodiments of this invention, a refrigerant system incorporates tandem compressors. At least two compressors each separately compress a refrigerant and deliver it into the refrigerant cycle. A reheat coil is also incorporated into the refrigerant system, and receives air having passed over the evaporator to reheat the air to a desired temperature when required. In one embodiment, the reheat coil is positioned in the refrigerant cycle to receive a hot gas. Other embodiments may position the reheat coil such that it utilizes a two-phase mixture of refrigerant, or a liquid refrigerant. Also, some embodiments allow the condenser to be bypassed by at least a portion of refrigerant flow, when desired. Further, the reheat cycle may be incorporated in a system utilizing either economized or conventional tandem compressors, or a combination of both, in other embodiments. Also, in one embodiment, at least one compressor may be unloaded to provide even greater capacity control.
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 embodiment of the present invention.
FIG. 2 shows a second embodiment of the present invention.
FIG. 3 shows a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a refrigerant system 20 incorporating a pair of conventional tandem compressors 22 and 24 for compressing a refrigerant and passing the refrigerant downstream to a discharge manifold 30, and then to condenser 36. The compressors 22 and 24 may or may not have oil equalization line 12 and pressure equalization line 13 connecting the two compressors for the purpose of potentially improving the oil management in the connected compressors. Furthermore, compressors 22 and 24 can be of different sizes and may have shutoff or check valves 32 located on the compressor discharge line to enhance system performance upon shutdown of one of the compressors.
A three-way valve 34 selectively communicates at least a portion of the refrigerant to a reheat coil 44. The valve 34 can be of a fixed orifice design, a regulating device, or can be substituted by a pair of solenoid valves. The refrigerant passes to a line 39 downstream of the reheat coil 44 through the check valve 46, and rejoins the main refrigerant circuit at point 41. Point 41 is upstream of condenser 36. A main expansion device 38, and an evaporator 40 are downstream of condenser 36. Thus, refrigerant flows from the main expansion device 38 to the evaporator 40, and then is returned to a suction manifold 26, communicating through lines 28 to the suction port of each compressor 22 and 24.
As is known, an indoor airflow 45 is driven over the evaporator 40. As the air is cooled, the moisture content in the air stream is typically reduced, and, thus, the air supplied to the conditioned space has been dehumidified. If the temperature of air leaving the evaporator is lower than desired for the conditioned space, reheat coil 44 is utilized to reheat the air stream 45 to a required temperature level. As shown, air moving device 42 drives air stream 45.
A control 19 for the refrigerant system 20 can operate the system to achieve various goals. In particular, one or both of the compressors 22 and 24 can be operated by controlling motors and closing shut-off valves 32. Thus, the total amount of refrigerant being compressed and circulated throughout the refrigerant system can be controlled to achieve different capacity levels. Moreover, the three-way valve 34 may be controlled to direct at least a portion of refrigerant into the reheat coil 44. Under many conditions it is not necessary to circulate refrigerant through the reheat coil. However, when it is desirable to achieve dehumidification to the extent that the air stream 45 would otherwise be cooled below the desired temperature, then circulating at least some refrigerant through the reheat coil 44 will allow the temperature of the air stream 45 to be raised to a comfortable level, while the moisture content had been reduced in the evaporator 40. Thus, system sensible and latent capacity can be controlled in both cooling and dehumidification modes of operation to a much greater extent by selectively operating tandem compressors 22 and 24.
As known, each of the tandem compressors 22 and 24 may be provided with a means of unloading, where part of the compressed refrigerant is by-passed back to the compressor suction (internal or external of compressor) and allow for additional steps in capacity control. This is particularly true when the capacities of the two compressors are selected to be different. Control 19 can close either shut-off valve 32 or leave both valves open (in conjunction to controlling the respective compressor motor shutdowns) to achieve several capacity steps. Further, at each capacity step, there is greater humidification control by selectively opening valve 34, utilizing the reheat coil 44.
Also, more than two compressors can be configured into a tandem arrangement offering multiple steps of unloading and control over various system operation parameters.
FIG. 2 shows another embodiment 50. There are tandem economized compressors 22 and 24 delivering refrigerant to a discharge manifold 30. Downstream of this manifold is a condenser 36, a main expansion device 38, and an evaporator 40. An air moving device 42 blows air over evaporator 40. Refrigerant is returned to a suction manifold 26, and through lines 28 back to the individual suction ports of compressors 22 and 24. Compressors 22 and 24 also have intermediate pressure, or economizer, ports communicating through lines 61 and economizer manifold 57 to the refrigerant system. Economizer lines 61 may incorporate shutoff valves 59 in order to switch between economizer and conventional modes of operation for each individual compressor. Bypass valves 51 allow the compressors to be unloaded, such that either or both of the compressors can be operated at a reduced capacity.
An economizer loop is incorporated in the refrigerant system 50 downstream of the condenser 36. In the economizer cycle, an economizer heat exchanger 52 receives a tapped refrigerant flow 54, and a main refrigerant flow 55. As can be seen, the tapped refrigerant flow in this embodiment is tapped from the main refrigerant flow 55 downstream of condenser 36. The tapped refrigerant passes through an economizer expansion device 56. After having passed through the economizer expansion device 56, the tapped refrigerant is at a lower pressure and temperature, and is able to cool the main refrigerant flow 55 in the economizer heat exchanger 52. In a preferred embodiment, the flow of the tapped refrigerant through the economizer heat exchanger 52 is preferably in the reverse direction to that illustrated (that is in the opposed direction to the flow 55). However, the flows are illustrated in the same direction to simplify the drawing.
The tapped refrigerant is typically returned as a vapor to be injected into the compressors 22 and 24 through the economizer manifold 57 communicating with separate economizer return lines 61, each having a shut off valve 59.
In this embodiment, the control 19 has the ability to open the valves 59 and adjust expansion device 56 to control the economizer loop. This control strategy is employed in combination with the abovementioned control methodology for operating the valves 32 to utilize either or both of the compressors. In addition, the bypass or unloader valves 51 provide further ability to reduce the refrigerant flow into the cycle. A worker of ordinary skill in the art would recognize the times when a controller would like to utilize each of these options. Valves 32, 51 and 59 may be either shutoff valves or regulating flow control devices, allowing for more flexibility in the refrigerant flow control.
The economizer loop may or may not be engaged. To turn off the economizer loop, the economizer expansion device 56 may be closed down such that no refrigerant is tapped. Furthermore, to disengage an economizer cycle for each individual compressor, a respective valve 59 needs to be closed. Similarly, to turn off the reheat coil 60, which is now positioned downstream of the condenser 36 and utilizes a two-phase mixture or liquid refrigerant for the reheat purpose, the three-way valve 58 may be moved to such a position that no refrigerant is tapped through the reheat coil 38. It has to be noted, that the illustrated position of the reheat coil 60 and the economizer heat exchanger 52 is not critical to take advantage of the invention benefits, and various possible arrangements become obvious to a person ordinarily skilled in the art.
Further, as is shown, a bypass line 64 with a valve 65 selectively bypasses refrigerant around the condenser 36. The valve 65 preferably allows a metering of flow around the condenser 36. Now, when the system is utilized under conditions such that humidity control is desirable, but no significant temperature change is necessary, a significant portion of the refrigerant may be bypassed through line 64, through valve 65, and around the condenser 36. This refrigerant is not cooled in the condenser 36, and when mixed with the refrigerant passing through the condenser has more heating capacity to be realized in the reheat coil 60, reheating refrigerant overcooled in the evaporator 40. On the other hand, if greater temperature reduction is desired, then more or most of the refrigerant passes through the condenser 36, and the system operates as in a standard cooling and dehumidification cycle, utilizing liquid refrigerant in the reheat coil 60. Thus, any of these reheat loop controls may be employed independent of each other, in combination with the economizer loop controls and tandem compressor control strategy, or none of them need be used. The present invention is mainly directed to providing the ability to use all techniques in combination with each other, while providing better control over the humidity and temperature, while enhancing system efficiency by matching latent and sensible load demands more closely and improving component reliability by reducing a number of start-stop cycles. Also, it has to be understood that the three-way valve 58 can be substituted by a pair of conventional valves. If the expansion device 56 is of such a type that it cannot be closed down completely, an additional shutoff valve may be placed on the tap line 54.
When relatively low humidity and temperature levels are desired in the air stream 45, or the capability to provide a significant amount of sensible and latent capacity is required, both economizer expansion device 56 and the three-way valve 58 are moved to an open position to operate both the economizer heat exchanger 56 and the reheat coil 60 and both tandem compressors 22 and 24 are controlled to provide maximum refrigerant flow with valves 32 and 59 in open positions and valves 51 closed. Refrigerant passing through the main line 55 will be subcooled by the refrigerant from the tap 54. Thus, that refrigerant will have a higher cooling capacity (both sensible and latent) when reaching the evaporator 40. Consequently an air stream 45 leaving evaporator 40 can be cooled to a lower temperature. At this lower temperature, more moisture can be removed from the air. Then, refrigerant in the refrigerant cycle 50 passes through the reheat coil 60, where its temperature is reduced further during the heat transfer interaction with the indoor air stream 46 leaving the evaporator 40. As a result, the refrigerant cooling capacity is boosted even further, allowing for even more dehumidification in the evaporator 40. This drier air then passes over the reheat coil 60, which will have a higher temperature refrigerant, as it is positioned upstream of the main expansion device 38. An air moving device 42, shown schematically, drives air over the evaporator 40 and reheat coil 60. This hotter refrigerant in the reheat coil will reheat the air stream 45 such that the desired temperature is reached. Moisture has already been removed from this air stream in the evaporator 40. Thus, by utilizing the combination of the economizer cycle, tandem compressor configuration and the reheat coil, a refrigerant system designer is able to achieve both desired temperature and humidity levels, especially in hot and humid environments. Moreover, the higher efficiency levels are achieved due to implementation of the economizer cycle concept. Obviously, other external load demand scenarios can be considered along with corresponding control strategies for the refrigerant cycle 50. These scenarios are well known to a person ordinarily skilled in the art.
Furthermore, this invention offers additional steps of unloading. Turning a tapped refrigerant flow in the economizer heat exchanger 26 on and off, the system capacity can be correspondingly increased or decreased, depending on the external load requirements. Also, one or both compressors 22 and 24 can be operated in the conventional, economized or unloaded modes. Again, this provides several distinct capacity control steps. This will allow matching the desired temperature and humidity levels with a greater precision as well as improve system reliability through the reduction of the start-stop cycles. Also it has to be understood that various configurations of this embodiment are possible including (but not limited to) more than two compressors, and more than one economizer heat exchanger.
FIG. 3 shows yet another embodiment 70. In embodiment 70, the reheat coil 74 communicates with the main cycle refrigerant by means of a three-way valve 72 downstream of the economizer heat exchanger 52. Thus, there would be a warm liquid refrigerant circulated through the reheat coil. Further, the compressors 22 and 24 are provided with a bypass line through valve 51 on only one of the two compressors (here compressor 24). Further, the economizer fluid is returned through line 28 to only one of the two compressors (here compressor 24 as well). While this embodiment does not provide the extreme number of steps of control provided by FIG. 2 embodiment, it does provide additional control when compared to a system that does not have tandem compressors, an economizer cycle or a reheat coil. The two tandem compressors 24 and 22 can still be operated independently, or in combination. The economizer cycle would only be utilized in combination with operation of compressor 24. Any number of economized and conventional compressors can be employed in this embodiment and will function and communicated to the refrigerant system 70 in the manner described above.
It should be understood that a refrigerant cycle designer would be able to identify many different options that would flow from the several embodiments in this invention. Additionally, an equivalent approach can be applied to each independent circuit of a multi-circuit system to obtain similar benefits.
Thus, the invention presents a significantly enhanced control over temperature and humidity precisely satisfying latent and sensible capacity demands, while reaching superior efficiency and reliability levels and reducing life-cycle cost of equipment.
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:

an inlet manifold providing a supply of refrigerant to at least two compressors, and a discharge manifold receiving compressed refrigerant from said at least two compressors, said discharge manifold supplying refrigerant to a condenser;

refrigerant flowing to said condenser from said at least two compressors, and refrigerant flowing from said condenser to a main expansion device, refrigerant from said main expansion device flowing to an evaporator, and refrigerant from said evaporator returning to said inlet manifold;

an air moving device for moving air to an environment to be conditioned over said evaporator; and

a reheat coil communicating to a refrigerant line in said system, said reheat coil being positioned in a path of at least a portion of air having passed over said evaporator.

2. The refrigerant system as set forth in claim 1, wherein said reheat coil communicating to the refrigerant line at a position between said discharge manifold and said condenser.

3. The refrigerant system as set forth in claim 2, wherein an inlet to said reheat coil includes a selectively opened valve for controlling flow of refrigerant to said reheat coil.

4. The refrigerant system as set forth in claim 2, wherein said reheat coil returns the refrigerant to a location upstream of said condenser.

5. The refrigerant system as set forth in claim 3, wherein an outlet of said reheat coil is positioned downstream of said condenser and downstream of said valve, but upstream of said main expansion device.

6. The refrigerant system as set forth in claim 1, wherein said refrigerant system further includes an economizer circuit, said economizer circuit receiving a tapped flow of refrigerant from a location downstream of said condenser, and passing said tapped flow of refrigerant through an economizer expansion device, said tapped flow of refrigerant and a main flow of refrigerant passing through an economizer heat exchanger.

7. The refrigerant system as set forth in claim 1, wherein a bypass line selectively bypasses said condenser, and a control bypassing fluid around said condenser if humidification control is desired with less temperature control.

8. The refrigerant system as set forth in claim 6, wherein said economizer circuit is upstream of said position at which said reheat coil communicates with said refrigerant line but downstream of said condenser.

9. The refrigerant system as set forth in claim 6, wherein said economizer circuit is downstream of said position at which said reheat coil communicates with said refrigerant line.

10. The refrigerant system as set forth in claim 6, wherein said economizer circuit is in parallel arrangement with said reheat coil.

12. The refrigerant system as set forth in claim 6, wherein at least one said compressor has an intermediate injection port.

13. The refrigerant system as set forth in claim 1, wherein an unloader allows selective unloading of at least one of said at least two compressors.

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

(1) providing an inlet manifold to supply refrigerant to at least two compressors and a discharge manifold receiving compressed refrigerant from said at least two compressors, said discharge manifold supplying refrigerant to a condenser, providing an expansion device downstream of the condenser, an evaporator downstream of the expansion device, and a reheat coil for selectively receiving refrigerant, and an air moving device for passing air into an environment to be conditioned, and over said evaporator and said reheat coil; and

(2) selectively operating one or both of said at least two compressors to achieve a desired cooling capacity, and selectively communicating refrigerant into said reheat coil to provide a desired temperature to said air passing into said environment to be conditioned.

15. The method as set forth in claim 14, wherein said reheat coil is communicated into a refrigerant line at a position between said discharge manifold and said condenser.

16. The method as set forth in claim 15, wherein said reheat coil returns refrigerant to a location upstream of said condenser.

17. The method as set forth in claim 14, wherein an outlet of said reheat coil is positioned downstream of said condenser, but upstream of said main expansion device.

18. The method as set forth in claim 14, further providing an economizer circuit, said economizer circuit receiving a tapped flow of refrigerant from a location downstream of said condenser, and including the steps of selectively passing said tapped flow of refrigerant through an economizer expansion device, and passing a main flow of refrigerant through an economizer heat exchanger along with selectively passing said tapped flow of refrigerant through said economizer heat exchanger.

19. The method as set forth in claim 18, wherein said tapped flow of refrigerant is returned to at least one of said at least two compressors at an intermediate compression point.

20. The method as set forth in claim 18, wherein said economizer circuit is positioned to be upstream of a position at which said reheat coil communicates with said refrigerant line, and downstream of said condenser.

21. The method as set forth in claim 18, wherein said economizer circuit is positioned to be downstream of said position at which said reheat coil communicates with said refrigerant line.

22. The method as set forth in claim 18, wherein said economizer circuit is positioned to be in a parallel arrangement with said reheat coil.

23. The method as set forth in claim 14, wherein at least one of said at least two compressors is provided with an unloader, and said unloader is controlled to provide variable system performance.