US20080190126A1

US20080190126A1 - Capacity Control For Refrigerant System With Multiple Compressors

Info

Publication number: US20080190126A1
Application number: US11/908,977
Authority: US
Inventors: Steven James Holden; Erric Layne Heitmann
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2005-06-03
Filing date: 2005-06-03
Publication date: 2008-08-14
Also published as: HK1121523A1; CN101194130B; CN101194130A; CA2606308A1; EP1886074A4; EP1886074A1; WO2006132626A1

Abstract

A refrigerant system operates with tandem compressors (22, 24). A control (41) for the refrigerant system is operable to change the capacities of each of the compressors dependent upon a load on the system. The control determines the percentage of maximum load capacity currently being utilized by each of the compressors. A compressor with the highest percentage is decreased in capacity should the control determine that excess capacity is being provided. Conversely, the control will increase the capacity of a compressor having a lower percentage, if insufficient capacity is being provided to meet a load on the system.

Description

BACKGROUND OF THE INVENTION

This application relates to a method and a control for controlling a refrigerant system having multiple compressors to operate in a most efficient manner.
Refrigerant systems are known, and are typically utilized to condition the air in an environment, such as an air conditioner or heat pump. A refrigerant system typically is operated at a capacity, which is selected to most efficiently control the temperature in the environment to be conditioned. Thus, as a cooling load may increase on an air conditioning system, a higher capacity would be desirably provided by the refrigerant system. Alternatively, as the cooling load would lessen, a lower capacity may be selected. By selecting the proper capacity, the efficiency, and energy cost can be optimized.
One type of refrigerant system operates with tandem compressors. While the term tandem is utilized, and two compressors will be discussed, systems having three or even more compressors are also known and would benefit from this invention. The tandem compressors are controlled to together provide a desired amount of compressed refrigerant within the refrigerant system. Compressors can have their capacities varied, and the tandem compressors are no exception. Thus, and as for example with screw compressors, a slide valve is positioned adjacent compression chambers, and may be opened to control how much refrigerant is compressed by the screw compressor. To achieve a desired capacity, a control for the refrigerant system can move the slide valve to achieve a desired amount of capacity as is required to condition the environment to be cooled (or heated).
To date, a control for the tandem compressors based upon an algorithm that will provide the most efficient operation has not been developed.

SUMMARY OF THE INVENTION

In the disclosed embodiment of this invention, a control makes an estimate of which of the tandem compressors is operating at a highest percentage of its capacity. The compressor operating at the highest percentage of its capacity will be lowered in capacity should there be excess capacity. Alternatively, should additional capacity be necessary, the compressor utilizing the lowest percentage of its maximum capacity will have its capacity increased. In this manner, the tandem compressors will operate close to the same percentage of their maximum capacity. It has been found that this state of operation will maximize the overall efficiency of the system.
In a disclosed embodiment of this invention, the current leading to a motor is monitored, and compared to a maximum current for the compressor motor. As an example, motors are provided with a rated load amps (RLA). By comparing the actual current to the RLA, a percentage of capacity can be approximated. The ratios between the tandem compressors can be compared to identify which compressor is operating at the highest percentage of its capacity. As mentioned above, the control then changes capacity in view of which motor is operating at the highest percentage.
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 is a schematic of a refrigerant system incorporating the present invention.

FIG. 2 is a flowchart of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A refrigerant system 20 is illustrated in FIG. 1 having tandem compressors 22 and 24, which each compress and deliver a refrigerant into a discharge line 25 leading to a common discharge 26. Discharge line 26 passes through an outdoor heat exchanger 28 (if the refrigerant system 20 is an air conditioner) and then through an expansion device 30. Downstream of the expansion device 30 is an indoor heat exchanger 32 (again in air conditioning mode). If the refrigerant system 20 were operated in a heating mode, the refrigerant would first pass through the indoor heat exchanger 32 directly downstream of the compressors 22, 24. The present invention would extend to either type mode.
As shown, an environment to be cooled 34 receives air driven over the evaporator 32. The environment 34 is provided with a control 41, such as a thermostat, and including a temperature sensor. The control 41 communicates with, or incorporates, a system control. The control 41 will provide an indication of the cooling load (or heating load) on the system. In general, an occupant of the environment 34 sets a desired temperature at the control 41, and the control 41 operates the refrigerant system 20 to achieve that desired temperature. As known, humidity control, etc. can also be provided.
Downstream of the evaporator 32, refrigerant passes into a common suction line 35, and then into separate suction lines 36 leading to the tandem compressors 22, 24.
Slide valves 38 are positioned on the compressors 22, and each communicate with the control 41. The control 41 is operable to drive the slide valves 38 more open or less open dependent upon a perceived need to increase or decrease the capacity provided by the refrigerant system 20.
A power supply 39 supplies power to an electric motor for each of the compressors 22 and 24. As shown, a sensor is placed on the power supply 39, and communicates with the control 41.
As mentioned above, the control 41 identifies a cooling (or heating) load to achieve a desired temperature within the environment 34. The control 41 then operates the system, including stopping/starting compressors 22, 24 and opening/closing the slide valves 38. This aspect of the invention is as known.
The present invention is directed to an algorithm explained in the flowchart of FIG. 2. In general, the present invention is directed to moving each of the tandem compressors (of course the invention can extend to more than two compressors) to be close to each other in a percentage of total capacity at which they operate. Thus, the present invention monitors the current being supplied by the current supplies 39 to the motors for the compressors 22 and 24. As shown in the flowchart, a ratio of this actual current to the RLA for each motor is determined. The control 41 then asks if the capacity is sufficient to meet the load presented by the environment 34. If the capacity is proper, then the algorithm returns to the first step. However, if additional capacity is needed, the compressor having the motor with the lower ratio has its load increased. In the disclosed embodiment, this would mean closing its slide valve somewhat.
On the other hand, if there is excess capacity, then compressor with the highest ratio has its capacity reduced. Again, this would mean opening its slide valve.
While a compressor with a slide valve is disclosed, other ways of changing the capacity can be utilized with the present invention. As an example, refrigerant systems are known having electric motors that can be variable in speed. This type compressor motor can be utilized in the inventive method. Also, while one way is disclosed to determine the percentage of capacity, other ways would come within the scope of this invention.
While a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in the 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, said at least two compressors each having a variable capacity;

an indoor heat exchanger, an expansion device and an outdoor heat exchanger, and a control for controlling a capacity provided by each of said at least two compressors, said control being able to determine whether a capacity provided by said at least two compressors is sufficient to meet a load on the refrigerant system, and increase or decrease the capacity dependent upon this determination, and said control further determining which of said at least two compressors is operating at a highest percentage of its maximum capacity, said control utilizing said determination of which of said compressors is operating at said highest percentage of its maximum capacity when determining which of said at least two compressors to control when said control determines that a change in capacity is desirable.

2. The refrigerant system as set forth in claim 1, wherein said control decreasing a capacity of one of said at least two compressors having a higher percentage if excess capacity is identified.

3. The refrigerant system as set forth in claim 1, wherein said control increasing a capacity of one of said at least two compressors having a lower percentage if insufficient capacity is being provided.

4. The refrigerant system as set forth in claim 1, wherein said percentage is determined by monitoring an actual current leading to a motor associated with each of said at least two compressors, and comparing each said actual current to a rated load for each of said motors.

5. A method of operating a refrigerant system including the steps of:

providing at least two compressors, said at least two compressors each having a variable capacity;

providing an indoor heat exchanger, an expansion device and an outdoor heat exchanger, and a control for controlling a capacity provided by each of said at least two compressors;

determining whether a capacity provided by said at least two compressors is sufficient to meet a load on the refrigerant system;

increasing or decreasing capacity dependent upon this determination;

determining which of said at least two compressors is operating at a highest percentage of its maximum capacity; and

said control utilizing said determination of which of said compressors is operating at said highest percentage of its maximum capacity when determining which of said at least two compressors to control when said control determines that a change in capacity is desirable.

6. The method as set forth in claim 5, wherein said control decreasing a capacity of one of said at least two compressors having said highest percentage if excess capacity is identified.

7. The method as set forth in claim 5, wherein said control increasing a capacity of one of said at least two compressors having a lower percentage if insufficient capacity is being provided.

8. The method as set forth in claim 5, wherein said percentage is determined by monitoring an actual current leading to a motor associated with each of said at least two compressors, and comparing said actual current to a rated load for each of said motors.