Expansion valve control for heat transfer system

Abstract

A compressor is connected with an evaporator, a condenser, and an electrically controlled valve for circulating a working fluid in a system for recovering waste heat to provide heated water for human use. A suction superheat temperature is determined from a measured compressor suction temperature and a suction saturation temperature. The electrically controlled valve is adjusted to maintain the suction superheat temperature at a suction superheat set point. The electrically controlled valve may be incrementally closed when a compressor suction pressure exceeds a maximum suction pressure. A discharge superheat temperature can be determined from a measured compressor discharge temperature and a discharge saturation temperature. The electrically controlled valve may be incrementally closed when the discharge superheat temperature falls below a minimum discharge superheat temperature.

What is claimed is: 
     
         1 . A heat transfer system comprising:
 a compressor for circulating a working fluid, the compressor having an inlet and an outlet;   a condenser connected to the outlet of the compressor;   an electrically controlled valve positioned to receive working fluid from the outlet of the condenser;   an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor;   a suction pressure sensor located between the outlet of the electrically controlled valve and the inlet of the compressor;   a suction temperature sensor located at the inlet of the compressor; and   a controller connected to the suction pressure sensor, the suction temperature sensor, and the electrically controlled valve, the controller configured to adjust the electrically controlled valve to maintain output of the suction pressure sensor and the suction temperature sensor at above a saturation point of the working fluid.   
     
     
         2 . The system of  claim 1 , wherein the controller is configured to adjust the electrically controlled valve to maintain output of the suction temperature sensor at a suction superheat set point that is based on a suction saturation temperature that the controller determines from output of the suction pressure sensor. 
     
     
         3 . The system of  claim 1 , wherein the controller is configured to adjust the electrically controlled valve to maintain output of the suction pressure sensor to below a maximum suction pressure. 
     
     
         4 . The system of  claim 1 , further comprising:
 a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve; and   a discharge temperature sensor located at the outlet of the compressor.   
     
     
         5 . The system of  claim 4 , wherein the controller is configured to adjust the electrically controlled valve to maintain output of the discharge pressure sensor and the discharge temperature sensor at above saturation of the working fluid. 
     
     
         6 . The system of  claim 5 , wherein the controller is configured to adjust the electrically controlled valve to maintain output of the discharge temperature sensor above a minimum discharge superheat temperature based on a discharge saturation temperature that the controller determines from output of the discharge pressure sensor. 
     
     
         7 . The system of  claim 1 , further comprising a subcooler connected between the condenser and the electrically controlled valve. 
     
     
         8 . The system of  claim 1 , wherein the evaporator is configured to receive flow of waste-heat bearing fluid. 
     
     
         9 . The system of  claim 8 , wherein the condenser is configured to receive flow of water to be heated. 
     
     
         10 . A method of controlling a heat transfer system, the method comprising:
 determining a suction saturation temperature of a compressor, the compressor connected with an evaporator, a condenser, and an electrically controlled valve for circulating a working fluid;   measuring a suction temperature for the compressor;   determining a suction superheat temperature from the measured suction temperature and the suction saturation temperature; and   adjusting the electrically controlled valve to maintain the suction superheat temperature at a suction superheat set point.   
     
     
         11 . The method of  claim 10 , further comprising:
 determining a suction pressure of the compressor; and   incrementally closing the electrically controlled valve when the suction pressure exceeds a maximum suction pressure.   
     
     
         12 . The method of  claim 10 , further comprising:
 determining a discharge saturation temperature of the compressor;   measuring a discharge temperature for the compressor;   determining a discharge superheat temperature from the measured discharge temperature and the discharge saturation temperature; and   incrementally closing the electrically controlled valve when the discharge superheat temperature is below a minimum discharge superheat temperature.   
     
     
         13 . The method of  claim 10 , further comprising feeding waste-heat bearing fluid to the evaporator. 
     
     
         14 . The method of  claim 13 , further comprising feeding water to the condenser and outputting heated water from the condenser. 
     
     
         15 . A heat transfer system for heating water using waste heat, the system comprising:
 a compressor for circulating a working fluid, the compressor having an inlet and an outlet;   a condenser connected to the outlet of the compressor, the condenser configured to receive flow of water to be heated;   an electrically controlled valve positioned to receive working fluid from the outlet of the condenser;   an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor, the evaporator configured to receive flow of waste-heat bearing fluid;   a suction pressure sensor located between the outlet of the electrically controlled valve and the inlet of the compressor;   a suction temperature sensor located at the inlet of the compressor;   a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve;   a discharge temperature sensor located at the outlet of the compressor; and   a controller connected to the suction pressure sensor, the suction temperature sensor, the discharge pressure sensor, the discharge temperature sensor, and the electrically controlled valve, the controller configured to adjust the electrically controlled valve to maintain output of the suction temperature sensor at a suction superheat set point above a suction saturation temperature determined from output of the suction pressure sensor, except when one or more of output of the suction pressure sensor exceeds a maximum suction pressure and output of the discharge temperature sensor falls below a minimum discharge superheat temperature determined from output of the discharge pressure sensor, in which case the controller incrementally closes the electrically controlled valve.   
     
     
         16 . A heat transfer system comprising a plurality of the systems of  claim 15  operating at different pressures, in which water to be heated flows from the condenser of a lower pressure system to the condenser of a higher pressure system. 
     
     
         17 . The heat transfer system of  claim 16 , wherein water to be heated flows in parallel through subcoolers of the lower pressure system and the higher pressure system before flowing into the condenser of the lower pressure system.