TANDEM COMPRESSORS WITH PULSE WIDTH MODULATION SUCTION VALVE

Abstract

A refrigerant system is provided with tandem compressors. A tandem compressor arrangement includes at least two compressors operating in parallel and having at least one common manifold. A control may operate the compressors either simultaneously, or in some predetermined sequence to provide control over refrigerant system capacity. At least one of the tandem compressors is provided with a pulse width modulation control on a suction line. In this manner, the amount of refrigerant, compressed by the compressor, can be precisely controlled to exactly meet thermal load demands in the conditioned space.

Description

BACKGROUND OF THE INVENTION

This application relates to a refrigerant system incorporating tandem compressors and pulse width modulation control on a suction line leading to at least one of the tandem compressors.

Refrigerant HVAC&R systems typically include a compressor delivering a compressed refrigerant from a compressor discharge port to a condenser, and then passing the refrigerant from the condenser to an expansion device, an evaporator, and finally back to the compressor suction port throughout a closed-loop circuit. The thermal load demand on the refrigerant system may vary and generally depends on indoor and outdoor operational environments, thermal load generation in a conditioned space and fresh air circulation requirements. At times, there may be a need for a higher system cooling capacity, and hence higher flow of refrigerant circulating throughout the refrigerant system is required. At other times, a lower cooling capacity, and consequently lower refrigerant flow, may be adequate to maintain the conditioned space within the comfort zone. To provide sufficient means of refrigerant flow control, some refrigerant systems utilize tandem compressors to provide unloading capability by switching off one of the tandem compressors to match the system capacity to the thermal load in the conditioned space. In such systems, two or more compressors may simultaneously deliver a compressed refrigerant to a downstream heat exchanger, such as a condenser. Typically, individual discharge lines communicate with the discharge ports of the tandem compressors. These discharge lines are then merged into a single discharge manifold connected to a condenser. Similarly, individual suction lines communicate with the suction ports of the tandem compressors. These suction lines emerge from a single suction manifold connected to a line extending from the evaporator exit. On the other hand, tandem compressor systems are known, wherein separate condensers are associated with each of the compressors, while the compressors are still connected to the same evaporator. Analogously, tandem compressor systems may be connected to separate evaporators, while still communicating to the same condenser. The last two configurations are typically utilized when either condensers or evaporators are associated with separate indoor or outdoor environments that may have different operational characteristics.

A control for a typical tandem compressor system will operate one, or several compressors, depending on system thermal load. Thus, the compressors can be controlled to provide discrete steps in system capacity. Also, as known, tandem compressor arrangements may include pressure and oil equalization lines to prevent oil pumpout from compressors and improve reliability.

One method of providing finer control over the capacity of a refrigerant system is the use of pulse width modulation controls for a refrigerant system compressor. With such a control, a suction pulse width modulation valve is cycled at a predetermined rate between on and off positions to prevent and then allow the flow of refrigerant to the compressor. Since the valve is cycled between open and closed positions, the throttling or any other losses are practically eliminated. In this manner, the amount of refrigerant compressed by the compressor can be finely tailored to a desired capacity, while maintaining efficient system operation. While pulse width modulation controls are known in refrigerant systems, they have not been incorporated into tandem compressor systems.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, two or more tandem compressors are operated in a refrigerant system. A suction line leading to at least one of two compressors is provided with a suction valve having a pulse width modulation control. Thus, the provided capacity of that compressor can be finely tailored to thermal load demands in an environment to be conditioned. In one embodiment, only one of the compressors is provided with such a suction valve controlled by pulse width modulation. In another embodiment, the suction pulse width modulation valve is provided on a manifold leading to each of the compressors.

In various embodiments, the compressors may deliver refrigerant to separate condensers, while still connected to a single evaporator, or may receive refrigerant from separate evaporators, while still communicating compressed refrigerant to a single condenser. In other embodiments, the tandem compressor refrigerant system may be provided with an economizer cycle and/or a bypass feature to achieve even more flexible control over supplied capacity.

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 refrigerant system.

FIG. 2 shows a second schematic.

FIG. 3 shows a third schematic.

FIG. 4 shows a fourth schematic.

FIG. 5 shows a capacity chart for the FIG. 1 schematic.

FIG. 6 shows a fifth schematic.

FIG. 7 shows a sixth schematic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a refrigerant system 20 having two compressors 22 and 24 operating in tandem to compress refrigerant and deliver the refrigerant to a common discharge manifold 28. A single condenser 30 receives refrigerant from the common discharge manifold 28 and delivers that refrigerant to an expansion device 32. An evaporator 34 is positioned downstream of the expansion device 32. Refrigerant from the evaporator 34 passes into a common suction manifold 26 and back to the compressors 22 and 24. As is well known in the art, a control 38 for the system operates to drive one or both of the compressors 22 and 24 to provide a desired capacity for the refrigerant system 20. The compressors 22 and 24 can be of the same size, or can be of different sizes. The control and operation of a tandem compressor refrigerant system are, as known in the art, and thus no further description is deemed necessary. What is inventive is the provision of a suction valve 36 having pulse width modulation control by the control 38, or by a dedicated controller. Thus, as known, the control 38 cycles the valve 36 at a predetermined rate between an open (normally fully open position) and closed position (normally either fully closed or nearly closed position) to control the flow of refrigerant to the compressors 22 and 24. In this manner, the amount of refrigerant delivered to the compressors 22 and 24 can be finely tailored to achieve precise capacity values, no matter how many compressors are operated at a particular moment of time. Since the pulse width modulation valve 36 is cycled from an open to closed position, there are minimal throttling or other losses that are associated with the valve 36. The cycling rate and time interval for the valve 36 to stay in an open position are determined by the capacity to be delivered to a conditioned environment, reliability requirements, allowable comfort zone parameter variations and refrigerant system thermal inertia. While two tandem compressors are shown, refrigerant systems are known with three, four or even higher number of tandem compressors. The present invention thus provides a very powerful and efficient means of achieving a precisely tailored refrigerant system capacity from a tandem compressor configuration, while minimizing temperature and humidity variation in the conditioned space, improving occupant's comfort and reducing power consumption.

FIG. 2 shows another embodiment 220, wherein the tandem compressors 122 and 124 again deliver refrigerant to a common discharge manifold 28 and a downstream condenser 30. This system configuration is distinct in that the refrigerant branches to two separate expansion devices 132, and two separate evaporators 134A and 134B. Such an arrangement is typically provided when the evaporators are serving different conditioned/refrigerated zones and having different operational characteristics. Thus, there is no common suction manifold delivering refrigerant to each of the compressors 122 and 124. In this embodiment, only one of the compressors, the compressor 122, has a suction pulse width modulated valve 136, mounted on its suction line leading to that compressor, which is controlled by a pulse width modulation control 138. As above, the control 138 can be a separate control or integrated into a system control for the refrigerant system 220. Again, when fine tailoring of the capacity delivered by the refrigerant system 220, and the evaporator 134A in particular, is desired, the compressor 122 would be operated, as it is able to provide precise refrigerant flow control by its associated pulse width modulation suction valve 136. Once again, although only two tandem compressors are show in FIG. 2, with only one of them having an associated pulse width modulation valve, a higher number of tandem compressors, with several of them having associated pulse width modulation valves, would be within the scope of the invention.

FIG. 3 shows yet another embodiment 221, wherein each of the compressors 22 and 24 deliver refrigerant to separate condensers 230 and 232, while still receiving refrigerant from the same evaporator 34. Such an arrangement would be typically used when the condensers reject heat into different environments (such as, for instance, indoors and outdoors) and have different operational characteristics. The condensers 230 and 232 deliver refrigerant to individual expansion devices 233, and the refrigerant flows are then combined before returning to the evaporator 34, and the common suction manifold 26. Again, a pulse width modulation valve 36 is mounted on the suction line leading to the tandem compressors 22 and 24 and controls refrigerant flow through the refrigerant system 221, no matter how many compressors are in operation. Similar to the FIG. 2 embodiment, the pulse width modulation valve 36 can be associated with only one of the compressors 22 or 24 and control the refrigerant flow through that particular compressor and an associated condenser, if desired (see further explanation below). Once again, this embodiment can be extended to any number of tandem compressors.

FIG. 4 shows a refrigerant system 222. This system is somewhat similar to the FIG. 1 system; however, the pulse width modulation valve 136 is positioned downstream of the common suction manifold 26, and on a suction line delivering refrigerant only to the compressor 122. With this arrangement, it may be desirable to only operate one of the compressors 122 and 124 at any instant of time, since compressor oil sumps of the two compressors 122 and 124 would be at a considerable pressure differential, when the pulse width modulation control is activated for the valve 136. Otherwise, cross-leakage between the compressors, oil pumpout and reliability problems may take place. Again, the compressor 122 is capable of providing precise control to the capacity provided by the refrigerant system 222. Any number of tandem compressors may have the associated pulse width modulated valves in this arrangement as well.

A tailored refrigerant system capacity achieved by a pulse width modulation valve of FIG. 1 is illustrated in FIG. 5. As shown in this Figure, if full capacity is required then two tandem compressors operate together with pulse width modulation valve fully open. When the capacity requirements are reduced, the pulse width modulation valve will cycle between open and closed positions. As more capacity reduction is required, the valve would stay in a closed position for longer periods of time than in an open position. When the capacity requirements are sufficiently low, then one of the tandem compressors is shut off, and the pulse width modulation valve cycle is altered once again to be most of the time in an open position. As the capacity requirements are reduced even further, the system continues to operate with one compressor turned off, and the cycle of the pulse width modulation valve adjusted, such that the pulse width modulation valve stays in a closed position for longer periods of time. The most appropriate time when the controller turns off one of the tandem compressors is determined and predominately based on the system capacity requirements, while it can be adjusted further to take system efficiency and reliability into consideration. While FIG. 5 illustrates adjustments in the pulse width modulation valve position for schematic shown in FIG. 1, it can be applied in a similar fashion to other embodiments of this invention as shown in other Figures.

FIG. 6 shows enhancement features that can be incorporated into any of the above schematics, in a system 300. The tandem compressors 302 and 304 are similar to the above embodiments, and the compressor 302 has an associated pulse width modulation valve 303 with a pulse width modulation control 305. However, even more flexibility in capacity control is provided in this arrangement. A condenser 306 receives refrigerant from the tandem compressors 302 and 304, and delivers it to a liquid line. A portion of refrigerant is tapped from the liquid line downstream of the condenser 306, and the tapped refrigerant passes through an economizer expansion device 310, where it is expanded to a lower pressure and temperature, and into an economizer heat exchanger 312 for the heat transfer interaction with the refrigerant circulating through the main circuit. As is known, the refrigerant passing through a tap line 308 subcools the refrigerant in a liquid line 314 flowing into the main expansion device (not shown). This provides a greater thermal potential for the refrigerant entering an evaporator (also, not shown) and consequently enhances refrigerant system cooling capacity and/or efficiency. The use of an economizer cycle is known in the art. Although the flow from the tap line 308 and the liquid line 314 are shown passing through the economizer heat exchanger 312 in the same direction, this is for illustration simplicity only. In practice, they are preferably flowing in a counterflow arrangement. The refrigerant from the tap line 308 is then flown through a return line 316, having a shutoff valve 318, and a vapor injection line 320. As is known in the art, the vapor injection line 320 would inject the returned refrigerant from the line 316 to an intermediate compression point in the economized compressor 302. It should be noted that the shutoff valve 318 may not be need, if the economizer expansion device 310 is equipped with a shutoff capability.

In addition, and as shown schematically in FIG. 5, a return line 330, shown in phantom, can also return refrigerant to the second compressor 304, if the compressor 304 is of an economized type as well. It should be noted that the compressors 302 and 304 may not necessarily share the same economizer branch components such as economizer heat exchanger 312 and economizer expansion device 310.

A bypass valve 322 may be opened to selectively bypass at least a portion of partially compressed refrigerant back from the compressor 302, through the vapor injection line 320, and a bypass line 324 to a suction line. Typically (but not always), a bypass function is utilized in a non-economized mode of operation. Further, the compressor 304 may be also equipped with a bypass function. Again, the use of the bypass for compressor unloading is as known in the art. However, the addition of the bypass and/or economizer function into a tandem compressor system having at least one compressor provided with a pulse width modulation control allows for even more flexible control over the provided capacity. Since an economizer and bypass functions offer two additional discrete steps of capacity control, the pulse width modulation technique offers precision control for refrigerant system operation within the economizer stage (when an economizer circuit is engaged) and bypass stage (when the bypass function is activated). As a result, superior accuracy control is provided in the conditioned space and efficiency boost in the refrigerant system operation is achieved. Once again, more than a single tandem compressor can be equipped with economizer and bypass features (whether or not sharing the economizer branch components such an economizer heat exchanger and economizer expansion device) and associated with the pulse width modulation control. Also, the economizer and bypass function do not need to be combined with each other. For example, only an economizer feature or only a bypass feature can be associated with a particular compressor. Further, as known in the art, it might be desirable to provide a small leak through a pulse width modulation valve (or a small bypass around the valve), when the valve is in a closed position to prevent the compressor operating in a vacuum.

It should be pointed out that while the present invention provides illustration for only two tandem compressors, as known in the art, more than two tandem compressors can be connected together in a tandem configuration. Also each compressor shown in the above Figures can represent a bank of compressors connected together in tandem and providing a nested arrangement. Within this nested arrangement, any compressor can be equipped with a pulse width modulated valve. For instance, a two-level nested tandem compressor system, incorporating two compressor banks 422 and 424 and suction modulation valves 436, is shown in FIG. 7. It also should be pointed out that, in general, the pulse width modulation valve can be associated with any compressor in the system or with any compressor type, such as a scroll compressor, a rotary compressor, a reciprocating compressor, a screw compressor, etc. Furthermore, the described refrigerant systems can be of a single circuit configuration or can be a part of a multi-circuit arrangement. In multi-circuit configurations, only a single circuit may be equipped with tandem compressors having a suction pulse width modulation valve or multiple circuits may have tandem compressors and an associated suction pulse width modulation valve.

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 compressors operating in parallel to compress refrigerant and deliver refrigerant downstream to at least one condenser, at least one expansion device positioned downstream of said at least one condenser, and at least one evaporator positioned downstream of said at least one expansion device; and

refrigerant returning from said at least one evaporator through at least one suction line to said at least two compressors, and a suction valve provided between said evaporator and at least one of said at least two compressors, said suction valve being provided with a pulse width modulation control to control the amount of refrigerant being delivered to said at least one of said at least two compressors.

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

3. The refrigerant system as set forth in claim 2, wherein said at least one common manifold is a suction manifold.

4. The refrigerant system as set forth in claim 2, wherein said at least one common manifold is a discharge manifold.

5. The refrigerant system as set forth in claim 1, wherein at least two of said at least two compressors deliver refrigerant to a single condenser.

6. The refrigerant system as set forth in claim 1, wherein a single expansion device receives refrigerant from at least two of said at least two compressors.

7. The refrigerant system as set forth in claim 1, wherein a single evaporator delivers refrigerant to a suction manifold returning refrigerant to at least two of said at least two compressors.

8. The refrigerant system as set forth in claim 7, wherein said suction valve is provided on said suction manifold, and thus regulates the flow of refrigerant to at least two of said at least two compressors.

9. The refrigerant system as set forth in claim 7, wherein said suction valve is provided downstream of said suction manifold and on a line delivering refrigerant to at least one of said at least two compressors.

10. The refrigerant system as set forth in claim 1, wherein there are at least two of said condensers separately receiving refrigerant from said at least two compressors.

11. The refrigerant system as set forth in claim 10, wherein each of said at least two condensers is provided with a separate expansion device.

12. The refrigerant system as set forth in claim 1, wherein there are at least two evaporators, and said at least two evaporators separately returning refrigerant to said at least two compressors.

13. The refrigerant system as set forth in claim 12, wherein at least two expansion devices are provided and separately pass refrigerant to said at least two evaporators.

14. The refrigerant system as set forth in claim 1, wherein at least one of said at least two compressors is provided with a bypass function.

15. The refrigerant system as set forth in claim 14, wherein at least one of said at least two compressors is provided with a bypass function and said at least one compressor is associated with said suction valve.

16. The refrigerant system as set forth in claim 1, wherein at least one of said at least two compressors is provided with an economizer cycle.

17. The refrigerant system as set forth in claim 16, wherein at least one of said at least two compressors is provided with an economizer cycle and said at least one compressor is associated with said suction valve.

18. The refrigerant system as set forth in claim 16, wherein more than one of said at least two compressors is provided with an economizer cycle and these economized compressors are sharing at least one of the economizer branch components.

19. The refrigerant system as set forth in claim 1, wherein at least one of said at least two compressors is a compressor bank and said suction valve is associated with at least one compressor in the bank.

20. The refrigerant system as set forth in claim 1, wherein at least one of said at least two compressors is selected from a set of a scroll compressor, a rotary compressor, a screw compressor, and a reciprocating compressor.

21.-39. (canceled)