HK1119229A - Refrigerant system with variable speed compressor in tandem compressor application - Google Patents

Description

Refrigeration system with variable speed compressor in tandem compressor application

Technical Field

The present invention relates to a variable speed motor for driving compressors installed in a refrigeration system having tandem compressors.

Background

Refrigerant systems are employed in many air conditioning and heat pump applications to cool and/or heat ambient air. The cooling or heating load of the environment varies with ambient conditions and with the temperature and/or humidity levels required by the occupants of the building.

In some refrigeration systems, a single compressor is employed to compress the refrigerant and move the refrigerant in a closed loop through cyclically connected indoor and outdoor heat exchangers. However, in many cases, it is desirable to be able to vary the capacity or cooling or heating provided by the refrigeration system. Therefore, known refrigeration systems are provided with tandem compressors. Tandem compressors are essentially at least two compressors operating in parallel, wherein the compressors are connected to each other via a common suction pump and/or discharge manifold. For example, a controller for a dual compressor system activates both compressors or one of the compressors. The two compressors are of different sizes, providing distinct capacity stages during part load operation. Rather than having a single capacity level, a refrigerant system having tandem compressors has several discrete capacity levels.

In the prior art, the control is programmed to selectively activate the tandem compressors. However, the capacity control provided by tandem compressors is increased or decreased in large discrete steps. It would be desirable to be able to improve system control capacity so that capacity can be varied between these discrete steps to accurately match the external load requirements over a wide range of environmental conditions.

It is common knowledge to use a variable speed drive to drive a variable speed compressor in a refrigeration system. By driving the compressor at a higher or lower speed, the amount of refrigerant compressed per unit time varies, and therefore the system capacity can be adjusted.

Variable speed drives have not been employed in refrigerant systems containing tandem compressors, wherein a selected number of the tandem compressors are driven by the variable speed drives in order to vary the system capacity controlling the temperature and humidity levels within the conditioned space.

Disclosure of Invention

In a disclosed embodiment of the invention, a variable speed drive compressor is provided in at least one compressor in a refrigeration system having tandem compressors. By selectively utilizing this compressor, capacity modulation between discrete steps provided by tandem compressor operation can be achieved.

The control device identifies a desired cooling capacity, and then achieves the desired capacity by: first, in the most efficient and reliable manner, tandem compressors are activated to accurately approximate the desired capacity. Then, the speed of the at least one compressor having a variable speed function is incrementally changed. The capacity is then monitored. When the desired level is finally achieved, the at least one compressor is operated at the new speed. If the capacity still needs to be adjusted, the speed is again incrementally adjusted and the resulting conditions are again monitored.

In the disclosed embodiment, one of the tandem compressors is provided with a variable speed drive, while the other tandem compressor is not. In other embodiments, both compressors are provided with variable speed drives.

The disclosed embodiments incorporate an economizer cycle and an unloader cycle along with a variable speed drive into the illustration.

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.

Drawings

Fig. 1 shows a refrigeration system of a first embodiment.

Fig. 1A shows other possible circuit diagrams.

Fig. 1B shows other possible circuit diagrams.

Fig. 1C shows other possible circuit diagrams.

Fig. 2 shows a refrigeration system of a second embodiment.

Fig. 3 illustrates the capacity control provided by the prior art.

Fig. 4 illustrates the capacity control provided by the present invention.

Fig. 5 is a flow chart of a control algorithm according to the present invention.

Detailed Description

A refrigeration system 20 is shown in fig. 1. The compressor 22 is provided with a variable speed drive 24. The second compressor 26 is not provided with a variable speed drive and operates in conjunction with the compressor 22. As shown, the shutoff valve 28 isolates the compressor 26 from the discharge manifold, and the control for the system should determine that only the compressor 22 is needed to achieve a given capacity. As is well known, the compressors 22 and 26 deliver refrigerant to a common discharge line 30 which leads to a condenser 32. While the system 20 is used as an air conditioning system, it should be understood that the present invention may also be applied to heat pumps and chillers.

As is well known, the two compressors 22 and 26 are preferably provided with distinct capacities so that a change in the total level of capacity can be achieved by operating one or both of the compressors 22 and 26. In this case, the choice of whether a larger or smaller compressor is provided with a variable speed drive is made at the discretion of the system designer. The choice will depend on many factors including (but not limited to) application requirements, cost, system operating efficiency, etc. An expansion device 34 is disposed downstream of the condenser 32, and an evaporator 36 is disposed downstream of the expansion device 34. The common suction line 38 leads to a separate suction line 39 for returning refrigerant to the compressors 22 and 26.

As shown, there is an economizer circuit in the schematic of fig. 1. The economizer heat exchanger 40 receives tapped refrigerant from a line 42 through an economizer expansion device 44. As is well known, by passing the tapped refrigerant through the expansion device 44, its pressure and temperature are reduced. Thus, in the economizer heat exchanger 40, the tapped refrigerant subcools the refrigerant in the main liquid line 45, which also passes through the economizer heat exchanger 40. The economizer function is well known in the art and can increase the capacity and/or efficiency of the refrigerant system 20.

As shown, in at least one compressor, here shown as compressor 22, the tapped refrigerant is returned to an intermediate compression point 48 via line 46. Although the refrigerant in the branch line 42 is shown flowing through the economizer heat exchanger 40 in the same direction as the refrigerant in the main liquid line 45, it should be understood that in a preferred embodiment, the two flows may actually be in a counter-flow configuration.

A bypass line 50 is also included that returns a portion of the refrigerant from the intermediate compression point 48 in the compressor 22 to the suction line 39. When it is desired to have unloaded operation, valve 52 is opened while expansion device 44 is preferably (but not necessarily) closed. In this manner, refrigerant that has been partially compressed by the compressor 22 will return to the suction line 39, thus providing an unloading function.

It should be understood that the economizer compressor 22 has more than one injection port 48 and more than one associated economizer heat exchanger 40. And, as is well known, the economizer heat exchanger configuration can be replaced by a flash tank. Also, a multi-stage compression system may be employed instead of a single economizer compressor. In such a multi-stage compressor system, one or more stages may be provided using a variable speed drive.

As shown, the motor 200 is connected to a fan for blowing air over the condenser 32 and evaporator 36. One of the motors 200 or the other motor 200 may be provided with a variable speed drive 202. Those skilled in the art will appreciate that when a variable speed control fan or other component, such as a secondary loop pump, is desired, it is desirable to provide a motor connected to the refrigeration system.

Fig. 1A shows another circuit diagram 100 in which one of the compressors, for example compressor 22, is replaced by two compressor stages 104 and 106. Although both compressor stages 104 and 106 are shown connected to the variable speed drive 102, only one stage may be connected thereto. As shown, the return line 108 from the economizer heat exchanger simply extends between the two stages, rather than into the compression chamber in one of the stages.

FIG. 1B shows another embodiment 110 in which there are three compressor stages 112, 114, and 116. The variable speed drive 118 controls two of the stages 114 and 116. Each stage is shown connected to an unloader valve 120. Two separate economizer heat exchangers 122 selectively route refrigerant back to a point between the compressor stages via line 124. It is well known to those skilled in the art that the selection of multiple compressor stages (and multiple compressor stages operating at variable speeds and their specific locations), multiple unloader valves, and multiple economizer heat exchangers can be made according to the desires of the designer and the requirements of the particular application.

FIG. 1C shows another embodiment 130 in which the first stage compressor is provided by a pair of tandem compressors 134 and 136 fed to a second compressor stage 138. As shown, an intermediate-pressure refrigerant return line 140 extends between the stages. The variable speed drive 132 is connected only to the compressor 134. Of course, many other illustrations are within the scope of the invention, including (but not limited to) a variable number of tandem cylinders and variable speed compressors.

Fig. 2 shows another different embodiment 60 in which two tandem compressors are replaced by a bank of four compressors. As shown, the compressors 64 are each provided with a variable speed drive 62. Shut-off valves 66 are provided in the discharge lines of the three compressors 64, 68 and 70 to isolate these compressors when they are stopped by the system control. A common discharge manifold 72 leads to a condenser 74, an expansion device 76, and an evaporator 78. The control for the refrigerant system 60 is used to move the two compressors 64 at variable speeds, and the two compressors 68 and 70 are brought to a desired capacity at a fixed speed.

The control for each refrigerant system 20 and 60 is capable of identifying the required cooling capacity, and operating the tandem compressors and/or the economizer, and the unloader function as needed. Thus, as shown in FIG. 3, a prior art system that incorporates the FIG. 1 illustration and does not have a variable speed drive may provide at least three capacity control stages A, B and A + B. In practice, the illustration of FIG. 1 may have even more stages, as operation of the unloader valve and economizer function provides additional capacity stages. However, the simplified illustration of FIG. 3 will suffice for an understanding of the remainder of the present invention. It can be seen that there are several values between the value A, B and a + B that the prior art system cannot provide. This is of course an oversimplified version of the system, but this does not provide a good basis for understanding the present invention. Many other levels of capacity control may also be obtained with the embodiment of fig. 2.

Fig. 3 and 4 are oversimplified versions of the embodiment of fig. 1 and are capable of providing capacity ratings. As described above, additional capacity steps can be achieved by operating the unloader valve and economizer function. However, the control of the system may operate a compressor (e.g., compressor 26) that is smaller than compressor 22 to provide level a. Another compressor 22 may be operated to provide class B, wherein the compressor 26 is stopped. By operating both compressors 22 and 26, a level a + B can be achieved. In each of the stages, by increasing the speed of the driving motor of the compressor 22, the slope R above the stage A, B or a + B can be achieved. On the other hand, by reducing the speed, the opposite occurs, namely moving the ramp downwards starting from these values. The switching decisions between compressor speed adjustment and moving to different operating modes are typically made based on the amount of cooling required, efficiency and reliability considerations. For example, it may not be safe to operate the compressor at a particular speed because of insufficient lubrication between the compressor components. On the other hand, operating the compressor at a relatively high speed is less efficient than switching to the economizer mode of operation.

Fig. 5 shows how these ramps are typically implemented using standard variable speed motor controls known in the art. The ramp R shown in fig. 4 is an oversimplified version. In practice, the control process is typically performed in incremental steps, and then the operation of the refrigerant cycle is monitored after the incremental change. Thus, there are multiple step changes along each ramp R, rather than an infinite number of changes as shown in FIG. 4. However, fig. 4 does illustrate very well how the invention provides a process of varying capacity.

It is noted that the variable speed tandem compressors may be employed in conjunction with other system components, such as fans or pumps, which also operate at variable speeds.

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 (20)

1. A refrigeration system comprising:

at least two tandem compressors operating in parallel, wherein at least one compressor has a variable speed drive for varying the speed of the at least one compressor;

a condenser disposed downstream of the compressor and an evaporator disposed downstream of the condenser; and

a control for selectively varying the speed of the at least one compressor.

2. The refrigeration system of claim 1, wherein an economizer heat exchanger is disposed intermediate the condenser and the evaporator, the economizer heat exchanger selectively receiving a tapped refrigerant to subcool a main refrigerant flow passing through the economizer heat exchanger, and the tapped refrigerant being returned to at least one compressor, and the control being operable to vary the speed of the at least one compressor to provide a capacity control variation between a level at which the economizer heat exchanger is operated and a level at which the economizer heat exchanger is not operated.

3. The refrigeration system of claim 2, wherein a plurality of intermediate ports are provided, wherein the tapped refrigerant is returned to the at least one of the compressors via the intermediate ports.

4. The refrigerant system as set forth in claim 1, wherein at least one of said tandem compressors is provided by a multi-stage compressor.

5. The refrigerant system as set forth in claim 1, wherein said control varies said speed of said at least one compressor in incremental steps.

6. The refrigeration system of claim 1, wherein at least one of the at least two compressors is provided with an unloader function.

7. The refrigerant system as set forth in claim 1, wherein at least one of said at least two compressors is not provided with a variable speed drive.

8. The refrigerant system as set forth in claim 1, wherein there are more than two of said at least two compressors, and at least two of said compressors are provided with variable speed drives.

9. The refrigerant system as set forth in claim 1, wherein a fan or pump connected to a component other than the compressor is also provided with the variable speed drive.

10. The refrigeration system of claim 1, wherein the at least two compressors have different capacities.

11. A method of controlling a refrigeration system comprising the steps of:

(1) providing at least two tandem compressors operating in parallel, wherein at least one compressor has a variable speed drive for varying the speed of said at least one compressor, providing a condenser downstream of said compressor and an evaporator downstream of said condenser, and providing a control means for selectively varying said speed of said at least one compressor to effect varying levels of capacity control; and

(2) determining a required capacity, and operating one or the other or both of the at least two compressors, and varying the speed of the at least one compressor to achieve the determined desired capacity.

12. The method of claim 11, wherein an economizer function is provided with the refrigeration system and selectively activated to provide additional capacity or increase operating efficiency if needed to achieve the desired capacity of step 2.

13. The method of claim 12, wherein refrigerant from a corresponding economizer heat exchanger is returned to a plurality of ports associated with the at least two tandem compressors.

14. The method of claim 11, wherein at least one of the at least two compressors is provided by a multi-stage compressor.

15. The method of claim 11, wherein the control varies the speed of the at least one compressor in incremental steps.

16. The method of claim 11, wherein an unloader function is provided to unload at least one of the at least two compressors to achieve the desired capacity of step 2.

17. The method of claim 11, wherein at least one of the at least two compressors is not provided with a variable speed drive.

18. The method of claim 11 wherein there are more than two of said at least two compressors and at least two of said compressors are provided with variable speed drives and said control varies the speed of said at least two variable speed driven compressors.

19. The method of claim 11, wherein the at least two compressors are provided with different capacities.

20. The method of claim 11, wherein at least one fan or pump connected to another component in the refrigeration system is provided with a variable speed drive.