Refrigeration system with warm defrost

Abstract

A refrigeration system includes a low temperature (LT) compressor, a medium temperature (MT) compressor having a suction side fluidly coupled with a discharge side of the LT compressor, a low temperature evaporator, and a medium temperature evaporator. Refrigerant from the low temperature evaporator is returned to a suction side of the LT compressor and refrigerant from the medium temperature evaporator is returned to a connection point downstream of the discharge side of the LT compressor and upstream of the suction side of the MT compressor. The refrigeration system performs a defrost cycle by directing warm refrigerant from the suction side of the MT compressor to pass through the low temperature evaporator in a reverse direction. The warm refrigerant is drawn from a position downstream of the connection point and upstream of the suction side of the MT compressor and returned to the suction side of the LT compressor.

What is claimed is: 
     
         1 . A refrigeration system comprising:
 a low temperature (LT) compressor;   a medium temperature (MT) compressor, a suction side of the MT compressor fluidly coupled with a discharge side of the LT compressor;   a low temperature evaporator configured to provide cooling to a low temperature space; and   a medium temperature evaporator configured to provide cooling to a medium temperature space;   wherein the refrigeration system is configured to perform a refrigeration cycle to maintain the medium temperature space at a first temperature and the low temperature space at a second temperature lower than the first temperature, refrigerant from the low temperature evaporator returned to a suction side of the LT compressor, and refrigerant from the medium temperature evaporator returned to a connection point downstream of the discharge side of the LT compressor and upstream of the suction side of the MT compressor; and   wherein the refrigeration system is configured to perform a defrost cycle by using warm refrigerant from a suction side of the MT compressor and directing the warm refrigerant to pass through the low temperature evaporator in a reverse direction, the warm refrigerant used for the defrost cycle drawn from a position downstream of the connection point and upstream of the suction side of the MT compressor and returned to the suction side of the LT compressor.   
     
     
         2 . The refrigeration system of  claim 1 , further comprising:
 a flash tank fluidly coupled with an outlet of a gas cooler fluidly coupled with a discharge side of the MT compressor, the flash tank fluidly coupled with an inlet of the medium temperature evaporator and, through a header, an inlet of the low temperature evaporator; and   a hot gas tank fluidly coupled at a lower end with an inlet of the low temperature evaporator and configured to receive the warm refrigerant returned from the inlet of the low temperature evaporator during the defrost cycle, an outlet of the hot gas tank fluidly coupled with the suction side of the LT compressor to return the warm refrigerant to the LT compressor.   
     
     
         3 . The refrigeration system of  claim 2 , wherein in response to a presence of fluid being detected in the hot gas tank, the refrigeration system is configured to allow refrigerant from the hot gas tank to return to the header by closing a valve that fluidly couples an outlet of the flash tank with the header, closing the valve causing a pressure drop to occur in the header which draws the refrigerant from the hot gas tank into the header. 
     
     
         4 . The refrigeration system of  claim 2 , wherein in response to fluid in the hot gas tank being at an upper threshold of fill level, the refrigeration system is configured to implement a flush cycle by (1) closing a return valve to restrict the return of refrigerant from the hot gas tank to the suction side of the LT compressor, and (2) modulating a valve that fluidly couples the hot gas tank with the discharge side of the MT compressor to maintain a pressure in the hot gas tank at least a specific amount above a pressure of the flash tank such that refrigerant exits the hot gas tank and is flushed into the flash tank. 
     
     
         5 . The refrigeration system of  claim 1 , wherein the refrigerant is carbon dioxide (CO2). 
     
     
         6 . The refrigeration system of  claim 1 , further comprising a heat exchanger through which the warm refrigerant passes before entering the low temperature evaporator for defrosting, the warm refrigerant configured to absorb latent heat while passing through the heat exchanger. 
     
     
         7 . The refrigeration system of  claim 6 , wherein the refrigeration system further comprises a three-way valve positioned downstream of the discharge side of the MT compressor, the three-way valve configured to transition into a position to direct hot refrigerant from the MT compressor into the heat exchanger to provide latent heat to the warm refrigerant for defrosting. 
     
     
         8 . A method for operating and defrosting a refrigeration system, the method comprising:
 discharging refrigerant from a discharge side of a low temperature (LT) compressor of the refrigeration system to a suction side of a medium temperature (MT) compressor of the refrigeration system;   providing cooling to a medium temperature space using a medium temperature evaporator of the refrigeration system to maintain the medium temperature space at a first temperature;   providing cooling to a low temperature space using a low temperature evaporator of the refrigeration system to maintain the low temperature space at a second temperature lower than the first temperature;   returning refrigerant from the low temperature evaporator to a suction side of the LT compressor;   returning refrigerant from the medium temperature evaporator to a connection point downstream of the discharge side of the LT compressor and upstream of the suction side of the MT compressor; and   performing a defrost cycle by using warm refrigerant from the suction side of the MT compressor and directing the warm refrigerant to pass through the low temperature evaporator in a reverse direction, the warm refrigerant used for the defrost cycle drawn from a position downstream of the connection point and upstream of the suction side of the MT compressor and returned to the suction side of the LT compressor.   
     
     
         9 . The method of  claim 8 , the refrigeration system further comprising:
 a flash tank fluidly coupled with an outlet of a gas cooler fluidly coupled with a discharge side of the MT compressor, the flash tank fluidly coupled with an inlet of the medium temperature evaporator and, through a header, an inlet of the low temperature evaporator; and   a hot gas tank fluidly coupled at a lower end with an inlet of the low temperature evaporator and configured to receive the warm refrigerant returned from the inlet of the low temperature evaporator during the defrost cycle, an outlet of the hot gas tank fluidly coupled with the suction side of the LT compressor to return the warm refrigerant to the LT compressor.   
     
     
         10 . The method of  claim 9 , further comprising, in response to a presence of fluid being detected in the hot gas tank, allowing refrigerant from the hot gas tank to return to the header by closing a valve that fluidly couples an outlet of the flash tank with the header, wherein closing the valve causes a pressure drop to occur in the header which draws the refrigerant from the hot gas tank into the header. 
     
     
         11 . The method of  claim 9 , further comprising, in response to fluid in the hot gas tank being at an upper threshold of fill level, implementing a flush cycle by (1) closing a return valve to restrict the return of refrigerant from the hot gas tank to the suction side of the LT compressor, and (2) modulating a valve that fluidly couples the hot gas tank with the discharge side of the MT compressor to maintain a pressure in the hot gas tank at least a specific amount above a pressure of the flash tank such that refrigerant exits the hot gas tank and is flushed into the flash tank. 
     
     
         12 . The method of  claim 8 , wherein the refrigerant is carbon dioxide (CO2). 
     
     
         13 . The method of  claim 8 , further comprising passing the warm refrigerant through a heat exchanger before the warm refrigerant enters the low temperature evaporator for defrosting, wherein the warm refrigerant absorbs latent heat while passing through the heat exchanger. 
     
     
         14 . The method of  claim 13 , further comprising transitioning a three-way valve positioned downstream of the discharge side of the MT compressor into a position to direct hot refrigerant from the MT compressor into the heat exchanger to provide latent heat to the warm refrigerant for defrosting. 
     
     
         15 . A controller for a refrigeration system, the controller comprising processing circuitry configured to operate equipment of the refrigeration system to:
 discharge refrigerant from a discharge side of a low temperature (LT) compressor of the refrigeration system to a suction side of a medium temperature (MT) compressor of the refrigeration system;   provide cooling to a medium temperature space using a medium temperature evaporator of the refrigeration system to maintain the medium temperature space at a first temperature;   provide cooling to a low temperature space using a low temperature evaporator of the refrigeration system to maintain the low temperature space at a second temperature lower than the first temperature;   return refrigerant from the low temperature evaporator to a suction side of the LT compressor;   return refrigerant from the medium temperature evaporator to a connection point downstream of the discharge side of the LT compressor and upstream of the suction side of the MT compressor; and   perform a defrost cycle by using warm refrigerant from the suction side of the MT compressor and directing the warm refrigerant to pass through the low temperature evaporator in a reverse direction, the warm refrigerant used for the defrost cycle drawn from a position downstream of the connection point and upstream of the suction side of the MT compressor and returned to the suction side of the LT compressor.   
     
     
         16 . The controller of  claim 15 , the refrigeration system further comprising:
 a flash tank fluidly coupled with an outlet of a gas cooler fluidly coupled with a discharge side of the MT compressor, the flash tank fluidly coupled with an inlet of the medium temperature evaporator and, through a header, an inlet of the low temperature evaporator; and   a hot gas tank fluidly coupled at a lower end with an inlet of the low temperature evaporator and configured to receive the warm refrigerant returned from the inlet of the low temperature evaporator during the defrost cycle, an outlet of the hot gas tank fluidly coupled with the suction side of the LT compressor to return the warm refrigerant to the LT compressor.   
     
     
         17 . The controller of  claim 16 , configured to operate the equipment of the refrigeration system to, in response to a presence of fluid being detected in the hot gas tank, allow refrigerant from the hot gas tank to return to the header by closing a valve that fluidly couples an outlet of the flash tank with the header, wherein closing the valve causes a pressure drop to occur in the header which draws the refrigerant from the hot gas tank into the header. 
     
     
         18 . The controller of  claim 16 , configured to operate the equipment of the refrigeration system to, in response to fluid in the hot gas tank being at an upper threshold of fill level, implement a flush cycle by (1) closing a return valve to restrict the return of refrigerant from the hot gas tank to the suction side of the LT compressor, and (2) modulating a valve that fluidly couples the hot gas tank with the discharge side of the MT compressor to maintain a pressure in the hot gas tank at least a specific amount above a pressure of the flash tank such that refrigerant exits the hot gas tank and is flushed into the flash tank. 
     
     
         19 . The controller of  claim 15 , wherein the refrigerant is carbon dioxide (CO2). 
     
     
         20 . The controller of  claim 15 , configured to operate the equipment of the refrigeration system to pass the warm refrigerant through a heat exchanger before the warm refrigerant enters the low temperature evaporator for defrosting, wherein the warm refrigerant absorbs latent heat while passing through the heat exchanger.