A method for controlling a vapour compression system at low superheat

Abstract

A method for controlling a vapour compression system ( 1 ) includes a compressor unit ( 2 ), a heat rejecting heat exchanger ( 3 ), an expansion device ( 4 ) and an evaporator ( 5 ) arranged in a refrigerant path. A superheat value of refrigerant leaving the evaporator ( 5 ) is derived, and a quantity being representative for a variance of the derived superheat value is calculated. A reference superheat value is calculated, based on the calculated quantity and on a minimum acceptable superheat value, by adding the calculated quantity to the minimum acceptable superheat value. The expansion device ( 4 ) is operated in accordance with the calculated reference superheat value, and in order to obtain a superheat of refrigerant leaving the evaporator ( 5 ) which is equal to the reference superheat value.

1 . A method for controlling a vapour compression system, the vapour compression system comprising a compressor unit, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant path, the method comprising the steps of:
 deriving a superheat value of refrigerant leaving the evaporator,   calculating a quantity being representative for a variance of the derived superheat value,   calculating a reference superheat value, by adding the calculated quantity to a minimum acceptable superheat value, the minimum acceptable superheat value representing a lower boundary for a range of superheat values which ensure safe operation of the vapour compression system, and   operating the expansion device in accordance with the calculated reference superheat value, and in order to obtain a superheat of refrigerant leaving the evaporator which is equal to the reference superheat value.   
     
     
         2 . The method according to  claim 1 , wherein the step of calculating a quantity being representative for a variance of the derived superheat value comprises calculating the variance of the derived superheat value. 
     
     
         3 . The method according to  claim 1 , wherein the step of calculating a quantity being representative for a reference superheat value further comprises applying a low pass filter. 
     
     
         4 . The method according to  claim 1 , wherein the step of calculating a quantity being representative for a variance of the derived superheat value comprises deriving a standard deviation of the derived superheat value and multiplying the standard deviation by an impact factor. 
     
     
         5 . The method according to  claim 1 , wherein the step of operating the expansion device is performed by means of a proportional integral (PI) controller. 
     
     
         6 . The method according to  claim 1 , wherein the step of deriving a superheat value of refrigerant leaving the evaporator comprises measuring a temperature of refrigerant leaving the evaporator and an evaporating temperature of the evaporator, and calculating the superheat value from the measured temperatures. 
     
     
         7 . The method according to  claim 1 , wherein the step of deriving a superheat value of refrigerant leaving the evaporator comprises measuring a temperature of refrigerant leaving the evaporator and a pressure of refrigerant leaving or entering the evaporator, and calculating the superheat value from the measured temperature and pressure. 
     
     
         8 . The method according to  claim 1 , wherein the step of deriving a superheat value of refrigerant leaving the evaporator comprises measuring a temperature of refrigerant leaving the evaporator and a temperature of refrigerant entering the evaporator, and calculating the superheat value from the measured temperatures. 
     
     
         9 . The method according to  claim 1 , further comprising the steps of:
 opening the expansion device and subsequently operating the expansion device in accordance with a previously stored reference superheat value,   monitoring the superheat value of refrigerant leaving the evaporator, and   in the case that the superheat value of refrigerant leaving the evaporator decreases below a predefined threshold value, performing the steps of calculating a quantity being representative for a variance of the derived superheat value, calculating a reference superheat value, based on the calculated quantity and on a minimum acceptable superheat value, and operating the expansion device in accordance with the calculated reference superheat value, and in order to obtain a superheat of refrigerant leaving the evaporator which is equal to the reference superheat value.   
     
     
         10 . The method according to  claim 9 , wherein the predefined threshold value is a predefined offset above the previously stored reference superheat value. 
     
     
         11 . The method according to  claim 2 , wherein the step of calculating a quantity being representative for a reference superheat value further comprises applying a low pass filter. 
     
     
         12 . The method according to  claim 2 , wherein the step of calculating a quantity being representative for a variance of the derived superheat value comprises deriving a standard deviation of the derived superheat value and multiplying the standard deviation by an impact factor. 
     
     
         13 . The method according to  claim 3 , wherein the step of calculating a quantity being representative for a variance of the derived superheat value comprises deriving a standard deviation of the derived superheat value and multiplying the standard deviation by an impact factor. 
     
     
         14 . The method according to  claim 2 , wherein the step of operating the expansion device is performed by means of a proportional integral (PI) controller. 
     
     
         15 . The method according to  claim 3 , wherein the step of operating the expansion device is performed by means of a proportional integral (PI) controller. 
     
     
         16 . The method according to  claim 4 , wherein the step of operating the expansion device is performed by means of a proportional integral (PI) controller. 
     
     
         17 . The method according to  claim 2 , wherein the step of deriving a superheat value of refrigerant leaving the evaporator comprises measuring a temperature of refrigerant leaving the evaporator and an evaporating temperature of the evaporator, and calculating the superheat value from the measured temperatures. 
     
     
         18 . The method according to  claim 3 , wherein the step of deriving a superheat value of refrigerant leaving the evaporator comprises measuring a temperature of refrigerant leaving the evaporator and an evaporating temperature of the evaporator, and calculating the superheat value from the measured temperatures. 
     
     
         19 . The method according to  claim 4 , wherein the step of deriving a superheat value of refrigerant leaving the evaporator comprises measuring a temperature of refrigerant leaving the evaporator and an evaporating temperature of the evaporator, and calculating the superheat value from the measured temperatures. 
     
     
         20 . The method according to  claim 5 , wherein the step of deriving a superheat value of refrigerant leaving the evaporator comprises measuring a temperature of refrigerant leaving the evaporator and an evaporating temperature of the evaporator, and calculating the superheat value from the measured temperatures.