Termperature Control Logic For Refrigeration System

Abstract

A refrigeration system includes a compressor having a first stage ( 20 a ) and a second stage ( 20 b ); a motor ( 22 ) driving the compressor; a heat rejecting heat exchanger having a fan ( 44 ) drawing ambient fluid over the heat rejecting heat exchanger, the heat rejecting heat exchanger including an intercooler ( 43 ) and a gas cooler, the inter-cooler coupled to an outlet of the first stage and the gas cooler ( 41 ) coupled to an outlet of the second stage; a flash tank ( 70 ) coupled to an outlet of the gas cooler; a primary expansion device ( 55 ) coupled to an outlet of the flash tank; a heat absorbing heat exchanger ( 50 ) coupled to an outlet of the primary expansion device, an outlet of the heat absorbing heat exchanger coupled to the suction port of the first stage; and a controller ( 100 ) for implementing a pulldown mode, a control mode and a staging logic mode.

What is claimed is: 
     
         1 . A refrigeration system comprising:
 a compressor having a first stage and a second stage;   a motor driving the compressor;   a heat rejecting heat exchanger having a fan drawing ambient fluid over the heat rejecting heat exchanger, the heat rejecting heat exchanger including an intercooler and a gas cooler, the intercooler coupled to an outlet of the first stage and the gas cooler coupled to an outlet of the second stage;   a flash tank coupled to an outlet of the gas cooler;   a primary expansion device coupled to an outlet of the flash tank;   a heat absorbing heat exchanger coupled to an outlet of the primary expansion device, an outlet of the heat absorbing heat exchanger coupled to the suction port of the first stage;   a controller for implementing a pulldown mode for cooling a space to a control temperature within a band of a control temperature setpoint, a control mode for maintaining the control temperature within the band of the control temperature setpoint and a staging logic mode for reducing system capacity while maintaining the control temperature within the band range of the control temperature setpoint.   
     
     
         2 . The refrigeration system of  claim 1  further comprising:
 a secondary expansion device positioned between the gas cooler and the flash tank; 
 the pulldown mode includes controlling the secondary expansion device to control discharge pressure at the compressor. 
 
     
     
         3 . The refrigeration system of  claim 1  wherein:
 the pulldown mode includes controlling the primary expansion device to control the heat absorbing heat exchanger superheat temperature. 
 
     
     
         4 . The refrigeration system of  claim 1  wherein:
 the control mode includes controlling drive signals to the motor to control the control temperature. 
 
     
     
         5 . The refrigeration system of  claim 4  wherein:
 the motor is a variable frequency drive (VFD) motor. 
 
     
     
         6 . The refrigeration system of  claim 1  further comprising:
 a secondary expansion device positioned between the gas cooler and the flash tank; 
 the control mode includes controlling the secondary expansion device to control discharge pressure at the compressor. 
 
     
     
         7 . The refrigeration system of  claim 1  wherein:
 the control mode includes controlling the primary expansion device to control the heat absorbing heat exchanger superheat temperature. 
 
     
     
         8 . The refrigeration system of  claim 1  further comprising:
 a heater; 
 the control mode includes controlling the heater to maintain the control temperature within the band of the control temperature setpoint. 
 
     
     
         9 . The refrigeration system of  claim 1  wherein:
 the staging logic mode includes modulating the fan to reduce system capacity. 
 
     
     
         10 . The refrigeration system of  claim 1  further comprising:
 an economizer valve coupling the flash tank to an inlet of the second stage; 
 wherein the staging logic mode includes closing the economizer valve. 
 
     
     
         11 . The refrigeration system of  claim 1  further comprising:
 an unload valve coupling an outlet of the intercooler to the suction port of the first stage; 
 wherein the staging logic mode includes opening the unload valve. 
 
     
     
         12 . The refrigeration system of  claim 1  wherein:
 the staging logic mode includes modulating the fan to control the control temperature. 
 
     
     
         13 . The refrigeration system of  claim 1  wherein:
 the controller implements an off integrator, the off integrator tracking an amount of error accumulated by the system when the control temperature is below the control temperature setpoint. 
 
     
     
         14 . The refrigeration system of  claim 13  wherein:
 the off integrator sums the difference between the control temperature and the control temperature setpoint over time. 
 
     
     
         15 . The refrigeration system of  claim 14  wherein:
 wherein when the sum difference exceeds an off integrator limit, the controller implements a shut down mode.