Variable Compressor Oil Return

Abstract

A method includes operating a variable-speed compressor in a refrigeration system carrying a refrigerant-oil mixture at a varying compressor speed to provide cooling capacity in proportion to cooling demand on the refrigeration system, determining that increased oil return to the compressor compared to a current oil-return rate is desired, and increasing the speed of the compressor to be disproportionately high relative to the cooling demand to provide increased oil return to the compressor.

1 . A refrigeration system comprising:
 a condenser configured to condense refrigerant in a refrigerant-oil mixture from first gas-phase refrigerant to first liquid-phase refrigerant;   a pressure mechanism in liquid communication with the condenser and configured to receive the first liquid-phase refrigerant from the condenser and to reduce a pressure of the first liquid-phase refrigerant of the mixture to produce second liquid-phase refrigerant;   an evaporator in liquid communication with the pressure mechanism and configured to receive the second liquid-phase refrigerant from the pressure mechanism and to transfer heat to the second liquid-phase refrigerant of the mixture to convert the second liquid-phase refrigerant to second gas-phase refrigerant;   a compressor subsystem in liquid communication with the evaporator and the condenser and configured to receive the second gas-phase refrigerant from the evaporator and to compress the second gas-phase refrigerant vapor refrigerant of the mixture into the first gas-phase refrigerant and send the first gas-phase refrigerant toward the condenser; and   a controller coupled to the compressor subsystem and configured to determine that an amount of oil available to the compressor subsystem is less than desired and to control the compressor subsystem to increase a speed of the compressor subsystem to cause more oil to be provided to the compressor subsystem such that the compressor subsystem has a desirable amount of oil available to the compressor subsystem.   
   
   
       2 . The system of  claim 1  wherein the compressor subsystem comprises a variable-speed compressor. 
   
   
       3 . The system of  claim 2  wherein the controller is configured to determine that the amount of oil available to the compressor subsystem is less than desired if the variable-speed compressor's speed is below a first speed threshold for longer than a first time threshold. 
   
   
       4 . The system of  claim 3  wherein the controller is configured to cause the variable-speed compressor to increase its speed if the variable-speed compressor's speed is below a second speed threshold for longer than a second time threshold, the second speed threshold being lower than the first speed threshold, the second time threshold being shorter than the first time threshold. 
   
   
       5 . The system of  claim 2  wherein the controller is configured to cause the variable-speed compressor to make its speed approximately 60% or higher of a maximum speed associated with the variable-speed compressor if the controller determines that the amount of oil available to the compressor subsystem is less than desired. 
   
   
       6 . The system of  claim 2  wherein the controller is configured to determine that the amount of oil available to the compressor subsystem is less than desired if an oil level at the variable-speed compressor is below a lower oil-level threshold. 
   
   
       7 . The system of  claim 6  wherein the controller is configured to cause the compressor to increase its speed until the oil level at the compressor reaches an upper oil-level threshold that is higher than the lower oil-level threshold. 
   
   
       8 . The system of  claim 1  wherein the compressor subsystem comprises a first compressor and a second compressor, the first compressor being a fixed-speed compressor, and wherein the controller is configured to cause the first compressor to operate if the controller determines that the amount of oil available to the compressor subsystem is less than desired and the first compressor is currently off. 
   
   
       9 . A system comprising:
 a variable-speed compressor including an input configured to receive a refrigerant-oil mixture with the refrigerant being in gas phase and at a first pressure, the compressor including an output configured to transfer the refrigerant-oil mixture with the refrigerant being in gas phase and at a second pressure that is higher than the first pressure, the compressor being further configured to increase the pressure of the refrigerant received via the input from the first pressure to the second pressure and to supply the refrigerant-oil mixture with the refrigerant at the second pressure to the output; and   a controller coupled to the compressor and configured to control the compressor to increase a speed of the compressor from a first speed to a second speed if the compressor speed has been below a first low-speed threshold for longer than a first time, the second speed being substantially faster than the first speed.   
   
   
       10 . The system of  claim 9  wherein the second speed is at least about 60% of a maximum speed of the compressor. 
   
   
       11 . The system of  claim 10  wherein the second speed is at least about 90% of the maximum speed of the compressor. 
   
   
       12 . The system of  claim 11  wherein the second speed is about 100% of the maximum speed of the compressor and the controller is configured to control the compressor speed to be the second speed for a second time that is about 10% of the first time or less. 
   
   
       13 . The system of  claim 12  wherein the controller is configured to cause the compressor to increase its speed if the compressor speed is below a second low-speed threshold for longer than a third time threshold, the second speed threshold being lower than the first speed threshold, the third time threshold being shorter than the first time threshold. 
   
   
       14 . The system of  claim 13  wherein the second speed is about 100% of the maximum speed of the compressor and the second speed is about 10% of the third time or less. 
   
   
       15 . A method comprising:
 operating a variable-speed compressor in a refrigeration system carrying a refrigerant-oil mixture at a varying compressor speed to provide cooling capacity in proportion to cooling demand on the refrigeration system;   determining that increased oil return to the compressor compared to a current oil-return rate is desired; and   increasing the speed of the compressor to be disproportionately high relative to the cooling demand to provide increased oil return to the compressor.   
   
   
       16 . The method of  claim 15  further comprising returning the speed of the compressor to a speed proportionate to the cooling demand after increasing the speed of the compressor to be disproportionately high relative to the cooling demand has resulted in the compressor being desirably lubricated.