HK1121798B - A refrigerant system comprising a variable speed compressor motor control for low speed operation, a compressor and a method of operating the refrigerant system - Google Patents

Description

Refrigerant system including variable speed motor controller for low speed operation, compressor and method of operating refrigerant system

Technical Field

The present invention relates to a controller for a variable speed compressor motor in which the compressor can be operated at very low speeds for extended periods of time while maintaining adequate lubrication at all times.

Background

Refrigerant systems are used in many applications to condition an environment. In particular, air conditioners and heat pumps are used to cool and/or heat air entering an environment. The cooling or heating load of the environment may vary as environmental conditions, occupancy levels, sensitive and latent load demands change, and the temperature and/or humidity set points are adjusted by the occupant of the environment.

A feature that has found widespread use in improving refrigerant system efficiency is the use of a variable speed drive for the compressor motor. Typically, the compressor need not operate at full speed, such as when the cooling load of the refrigerant system is relatively low. In such a situation, it may be desirable to reduce the compressor speed, and thus the overall energy consumption of the refrigerant system. The application of variable speed drives is one of the most effective techniques to increase system performance and reduce life cycle cost of the device over a wide range of operating environments and potential applications, particularly under localized loading conditions.

However, the required reliability issues set a lower limit for the required compressor speed reduction. In particular, insufficient lubrication of the compressor elements poses problems at low operating speeds. This situation often occurs because the delivery of compressor oil is dependent on the operation of a pump installed within the compressor, where the oil pump delivery head is strongly affected by the operating speed. If the compressor operating speed is reduced below a certain level, the oil pump cannot generate the required head to deliver oil to the components requiring lubrication within the compressor. This results in inadequate lubrication of these components and subsequent compressor failure. The most affected internal components are those located away from the pump inlet. Therefore, the compressor is typically set to the lowest speed (typically 45Hz) required to ensure adequate lubrication of the compressor. Even compressors specifically designed for variable speed operation and incorporating special structures for oil enhanced lubrication (e.g., special oil pumps) typically cannot operate below 30 Hz. However, the minimum speed limit created by the proper lubrication requirements may also need to be reduced below 30Hz in order to achieve efficient operation under local loads. That is, much of the energy efficiency that can be provided by a variable speed drive is largely eliminated due to the minimum speed limitation. Therefore, there is a need to provide a compressor that operates at a lower average speed than is achievable with current configurations.

Disclosure of Invention

In the disclosed embodiment of the invention, the compressor is provided with a variable speed drive. Upon detection of a low load, the compressor is moved to a low speed to maintain adequate conditions in the environment without transitioning to a start-stop mode of operation. In fact, due to reliability issues, the compressor may move to a speed far below what is normally recommended as the lowest speed. The compressor may be operated at this low speed for a period of time. The compressor speed is periodically increased to a level that ensures adequate lubrication of the compressor elements. By periodically driving the compressor at higher speeds, the present invention ensures that an overall adequate supply of lubricant is provided to the compressor elements. The present invention relies on residual oil remaining in the components that need to be lubricated when the compressor is operating at low speeds. The short burst of high velocity delivers oil to the parts requiring lubrication. By having the compressor operate at low speed and at high speed for short periods of time, the average compressor operating speed of the present invention can be much lower than the minimum speed established by the prior art. In a preferred embodiment of the present invention, a pulse width modulation technique is used to vary the motor speed of the compressor.

Although, for purposes of illustration, the present invention is described with respect to a refrigerant system incorporating a scroll compressor, the present invention is applicable to variable speed oil lubricated compressors whose oil delivery mechanism is dependent upon the compressor operating speed. 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 is a schematic view of a refrigerant system incorporating the present invention;

FIG. 2 is a simplified flow diagram of the present invention;

FIG. 3 is a graph showing average velocity according to the present invention;

FIG. 4 illustrates another schematic diagram of a refrigerant system.

Detailed Description

The refrigerant system 19 is shown in fig. 1 as having a scroll compressor 21 incorporating a non-orbiting scroll member 22 and an orbiting scroll member 24. As is well known, shaft 26 is driven by an electric motor 28 to cause orbiting scroll member 24 to orbit. As indicated above, the variable speed drive 30 is schematically connected to drive the electric motor 28. An oil pump 32 and an oil passage 34 in the shaft 26 supply oil to various moving elements in the compressor 21 as is well known.

As is known, the condenser 36 is positioned downstream of the compressor 21, the expansion device 38 is positioned downstream of the condenser 36, and the evaporator 40 is positioned downstream of the expansion device 38. The compressor 21 is driven by an electric motor 28 to compress refrigerant vapor and drive it through the refrigerant system 19, also as is known. Oil from the oil pump 32 is delivered to the compressor elements to provide proper lubrication of the compressor components, such as the crankcase bearing 100, the orbiting scroll bearing 102, the non-orbiting scroll member 22, and the orbiting scroll member 24, while a quantity of oil exits the compressor 21 with the refrigerant and circulates through the refrigerant system 19. One of the most typical oil delivery systems with scroll compressors is also shown in fig. 1, where oil from the oil pump 32 is picked up by the oil pick-up tube 110 and delivered to the various compressor components along the oil passage 34 as described above. Some oil may be delivered through the refrigerant entering the compressor via the suction port 120. However, most oil delivery is achieved by delivering oil from an oil pump as described above. In the prior art, where a variable speed drive is used in a refrigerant system, the designer is limited by the minimum operating speed of the shaft 26 of the compressor 21 (the operating speed of the shaft is very close to the operating frequency). If the speed is reduced to a certain extent over a long period of time, an insufficient amount of oil is delivered via the oil channel to the compressor parts that need to be lubricated. Thus, for low cooling load situations where a small amount of compressed refrigerant needs to be circulated through the system, a minimum speed of, for example, 45Hz is often the limiting factor in reducing the amount of refrigerant circulated to the required amount while ensuring adequate lubrication.

FIG. 1 illustrates additional features that may be incorporated into the refrigerant system 19. As an example, an economizer circuit is included and has an economizer heat exchanger 18. The main liquid line 13 has a tap line 11 that branches off from the main liquid line and passes through an economizer expansion device 115. Both the tap line 11 and the main liquid line 13 pass through an economizer heat exchanger 18. In practice, and in use, the refrigerant flow in the tap line typically passes through the economizer heat exchanger in a counter-current direction relative to the flow in the main liquid line 13. However, to simplify the description in the drawings, they are shown in the same direction. As is known, the economizer circuit subcools the refrigerant in the main liquid line and thus increases the performance (capacity and/or efficiency) of the refrigerant system 19. An economizer injection line 20 is shown extending back to the compressor 21 and injecting an intermediate pressure refrigerant into the compressor chamber via a passage, such as passage 23. The function and structure of the economizer circuit is well known, but it includes an innovative motor control 30 to provide a refrigerant system with greater flexibility to enhance operation of the refrigerant system 19. The unloader line 17 includes an unloader valve 200. The unloader valve 200 is selectively opened to return partially compressed refrigerant from the compression chamber to the suction port 120 of the compressor 21 via passage 23. The unloader function provides the refrigerant system designer with additional freedom to adjust and optimize performance.

Basically, when greater capacity is required, the economizer function is utilized with the unloader valve closed. Alternatively, if lower capacity is desired, the economizer expansion device 115 (or a separate shut-off device) is closed and the unloader valve 200 is opened. In this way, the amount of compressed refrigerant delivered to the condenser 36 is reduced. Likewise, if it is desired to provide the capacity of another intermediate stage of the refrigerant system 19, the economizer function is combined with the unloader function by opening both the economizer expansion device 115 and the unloader valve 200.

These system configurations, in combination with the variable speed motor controller described below, provide greater freedom and flexibility to the refrigerant system designer.

It should be understood that the motor controller 30 includes a routine that receives inputs from various locations of the refrigerant system and determines when a lower speed of the compressor motor is required.

Those skilled in the art will appreciate when a lower speed is required and preferred as compared to other variable parameters.

As shown in fig. 2, the controller of the refrigerant system 19 determines the load demand of the refrigerant system 19 and, if the load demand is low, the speed is reduced to an appropriate level. In the reduced compressor speed mode, if a very low speed (e.g., below 45Hz) is employed, the speed is periodically increased (e.g., to the extent above 45Hz) to ensure that sufficient lubrication is provided to the compressor elements.

For example, as shown in FIG. 3, a pulse width modulation technique may be used to periodically increase the compressor speed from a level below 20Hz to a level of 50 Hz. As shown in the embodiment of fig. 3, this may result in an average speed as low as 25Hz, while always ensuring adequate lubrication of the compressor elements. Of course, the particular frequency and modulation time interval are examples only and are dependent on compressor design specifications. The main object of the present invention is to have the compressor operate at a significantly reduced speed when low load is required for the refrigerant system 19 and the compressor 21. The compressor speed may be reduced to a speed well below that required to ensure proper lubricant circulation, and then the compressor speed is increased periodically.

It will be appreciated that the average velocity may be calculated as follows:

average speed ═ low speed × (% of low speed time) + high speed × (% of low speed time)

In the example, the low speed is used for a longer time than the high speed. In the disclosed example, a speed of 20Hz lasts 20 seconds, while a speed of 50Hz lasts only 5 seconds. In other words, the lower speed may last twice as long as the higher speed. In addition, sufficient lubrication is achieved. Again, the desired modulation interval and the maximum and minimum compressor speeds may vary with compressor design, desired average operating speed, and system operating conditions.

It will also be appreciated from fig. 3 that there will be multiple high speed levels (high 1, high 2) and multiple low speed levels (low 1, low 2) associated with, for example, different operating conditions. Again, the program incorporated into the controller 30 sets these variables. Further, multiple speeds may be selected based on system considerations.

Fig. 4 shows another schematic diagram of a refrigerant system having multiple independent refrigerant circuits each including a compressor 21, a condenser 36, an expansion device 38, and an evaporator 40. The motor of the compressor 21 is provided with a variable speed drive as the controller 30. While these two circuits are shown in a simplified manner, it should be understood that various additional elements such as economizer and unloader functions may be incorporated into the system. It will be apparent that the refrigerant system may incorporate more than two independent circuits and that it is not necessary to provide a variable speed drive for each compressor.

It should be understood that while the present invention is described with respect to a refrigerant system incorporating a scroll compressor, the present invention is also applicable to any variable speed oil lubricated compressor employing an oil delivery mechanism that operates according to speed.

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 (33)

1. A refrigerant system, comprising:

a compressor and an electric motor for driving the compressor, a variable speed drive for varying the speed of the electric motor;

a condenser downstream of the compressor, an expansion device downstream of the condenser, and an evaporator downstream of the expansion device; and

the variable speed drive moves the compressor to low speed operation and the variable speed drive operates the compressor at a low level of speed and then periodically increases the speed of the compressor toward a higher level of speed, wherein the higher level of speed is selected to ensure adequate lubrication of compressor elements.

2. The refrigerant system as set forth in claim 1, wherein said variable speed drive is programmed to determine when said compressor needs to operate at said low speed.

3. The refrigerant system as set forth in claim 1, wherein said variable speed drive is provided with a pulse width modulation control to periodically drive the compressor at said higher level of speed.

4. The refrigerant system as set forth in claim 1, wherein said compressor is selected from the group consisting of a rotary compressor and a reciprocating compressor.

5. The refrigerant system as set forth in claim 1, wherein a shaft for driving the compressor has a lubrication passage through which lubricant moves upwardly from a sump in the compressor housing to components of said compressor.

6. The refrigerant system as set forth in claim 1, wherein said variable speed drive drives said compressor at said low level speed for a longer period of time than said compressor is driven at said higher level speed.

7. The refrigerant system as set forth in claim 6, wherein said low level of speed continues for more than twice said high level of speed.

8. The refrigerant system as set forth in claim 1, wherein the refrigerant system includes a plurality of circuits, at least one of said circuits having a variable speed drive for controlling said compressor as described above.

9. The refrigerant system as set forth in claim 1, wherein an economizer circuit is incorporated into the refrigerant system.

10. The refrigerant system as set forth in claim 1, wherein said compressor is provided with an unloader circuit.

11. The refrigerant system as set forth in claim 1, wherein the refrigerant system is provided with both an economizer circuit and an unloader circuit.

12. The refrigerant system as set forth in claim 1, wherein said higher level of speed varies as a function of refrigerant system operating conditions.

13. The refrigerant system as set forth in claim 1, wherein said low level speed varies as a function of refrigerant system operating conditions.

14. The refrigerant system as set forth in claim 1, wherein said higher level of speed comprises a plurality of levels of speed that can be selected.

15. The refrigerant system as set forth in claim 1, wherein said low level of speed comprises a plurality of levels of speed that can be selected.

16. The refrigerant system as set forth in claim 1, wherein said compressor is selected from the group consisting of scroll compressors and screw compressors.

17. A compressor, comprising:

a compressor pump unit and an electric motor for driving the compressor pump unit, the electric motor being provided with a variable speed drive; and

the variable speed drive moves the compressor to low speed operation and the variable speed drive operates the compressor at a low level of speed and then periodically increases the speed of the compressor toward a higher level of speed, wherein the second higher level of speed is selected to ensure adequate lubrication of compressor elements.

18. The compressor as set forth in claim 17, wherein said variable speed drive is provided with a pulse width modulation control to periodically drive the compressor at said higher level of speed.

19. The compressor of claim 17, wherein the compressor is selected from the group consisting of a rotary compressor and a reciprocating compressor.

20. The compressor of claim 17, wherein a shaft for driving the compressor has a lubrication passage through which lubricant moves upwardly from a sump in a compressor housing to components of the compressor.

21. The compressor as set forth in claim 17, wherein said variable speed drive drives said compressor at said low level of speed for a longer period of time than said compressor is driven at said higher level of speed.

22. The compressor of claim 21 wherein said low level of speed is for more than twice said higher level of speed.

23. The compressor of claim 17, wherein the economizer injection port extends into the compressor.

24. The compressor of claim 17, wherein the compressor is provided with an unloader circuit.

25. The compressor of claim 17, wherein the compressor is provided with both an economizer circuit and an unloader circuit.

26. The compressor of claim 17 wherein the higher level of speed varies based on refrigerant system operating conditions.

27. The compressor of claim 17 wherein the low stage speed varies based on refrigerant system operating conditions.

28. The compressor of claim 17 wherein said higher level of speed comprises a plurality of levels of speed that can be selected.

29. The compressor of claim 17 wherein said low stage speeds comprise a plurality of stages of speeds that can be selected.

30. The compressor of claim 17, wherein the compressor is selected from the group consisting of scroll compressors and screw compressors.

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

(1) providing a variable speed drive for a compressor and detecting a load on a refrigerant system associated with the compressor;

(2) identifying a low load condition and moving the compressor to low speed operation when a low load condition has been identified;

(3) the compressor is operated at a low level of speed and the compressor speed is periodically moved up to a higher level of speed, wherein the higher level of speed is selected to ensure adequate lubrication of the compressor elements.

32. The method of claim 31, wherein the low level speed lasts for a longer period of time than the higher level speed.

33. The method of claim 32 wherein said low level of speed is for more than twice said high level of speed.