Hot gas defrost using medium temperature discharge gas

Abstract

A refrigeration system includes evaporators, one or more medium temperature compressors, and a three-way valve positioned downstream from the one or more medium temperature compressors. The three-way valve receives the compressed refrigerant from the medium temperature compressors and directs flow of the received compressed refrigerant to (i) a gas cooler and/or (ii) one or more of the evaporators based on an operation mode of the evaporators. A controller determines that operation of a first evaporator in a defrost mode is indicated and causes the first evaporator to operate in the defrost mode by adjusting the three-way valve to direct a portion of the received compressed refrigerant to the first evaporator.

What is claimed is: 
     
         1 . A refrigeration system, comprising:
 a plurality of evaporators configured to transfer heat from a space to a refrigerant;   one or more medium temperature compressors configured to compress the refrigerant;   a three-way valve positioned downstream from the one or more medium temperature compressors, the three-way valve configured to receive the compressed refrigerant from the one or more medium temperature compressors, wherein the three-way valve is operable to direct flow of the received compressed refrigerant to one or both of a gas cooler and one or more of the evaporators of the plurality of evaporators based on an operation mode of the plurality of evaporators;   and a controller communicatively coupled to the three-way valve, wherein the controller is configured to:
 determine that operation of a first evaporator of the plurality of evaporators in a defrost mode is indicated; 
 after determining that operation of the first evaporator in the defrost mode is indicated, cause the first evaporator to operate in the defrost mode by adjusting the three-way valve to direct a portion of the received compressed refrigerant to the first evaporator; 
 determine that defrost mode operation of the first evaporator is complete; 
 after determining that defrost mode operation of the first evaporator is complete, cause the first evaporator to operate in a transition mode, wherein during operation in the transition mode at least a portion of the compressed refrigerant provided to the first evaporator is provided back toward the one or more medium temperature compressors; and 
 during operation of the first evaporator in the transition mode:
 determine a pressure associated with the first evaporator; 
 determine that the pressure is less than a threshold value; and 
 after determining that the pressure is less than the threshold value, cause the first evaporator to operate in a refrigeration mode. 
 
   
     
     
         2 . The refrigeration system of  claim 1 , further comprising:
 the gas cooler configured to receive refrigerant and facilitate heat transfer from the received refrigerant, thereby cooling the refrigerant; and   a flash tank located downstream from the gas cooler and configured to receive the refrigerant cooled by the gas cooler and store at least a portion of the received refrigerant.   
     
     
         3 . The refrigeration system of  claim 2 , further comprising:
 a first valve located upstream from the first evaporator in refrigerant conduit coupling a liquid outlet of the flash tank to the first evaporator, wherein, when the first evaporator is operating in a refrigeration mode, the first valve is open; and   a second valve located downstream from the first evaporator in refrigerant conduit allowing flow of refrigerant towards the one or more medium temperature compressors, wherein, when the first evaporator is operating in the refrigeration mode, the second valve is open; and   the controller is further configured to cause the first evaporator to operate in the defrost mode by causing the first valve to close and causing the second valve to close.   
     
     
         4 . The refrigeration system of  claim 3 , further comprising:
 a third valve located upstream from the first evaporator in refrigerant conduit coupling an inlet of the flash tank to the first evaporator, wherein, when the first evaporator is operating in a refrigeration mode, the third valve is closed; and   a fourth valve located downstream from the first evaporator in refrigerant conduit allowing flow of refrigerant from the one or more medium temperature compressors to the first evaporator, wherein, when the first evaporator is operating in the refrigeration mode, the fourth valve is closed;   wherein the controller is further configured to cause the first evaporator to operate in the defrost mode by causing the third valve to open and causing the fourth valve to open.   
     
     
         5 . The refrigeration system of  claim 4 , wherein the controller is further configured to, during operation of the first evaporator in the transition mode:
 determine a pressure associated with the first evaporator;   determine that the pressure is less than a threshold value; and   after determining that the pressure is less than the threshold value, cause the first evaporator to operate in a refrigeration mode.   
     
     
         6 . The refrigeration system of  claim 1 , wherein, while the first evaporator is caused to operate in the defrost mode, a second evaporator of the plurality of evaporators is caused to operate in a refrigeration mode. 
     
     
         7 . The refrigeration system of  claim 1 , wherein the controller is further configured to:
 at least partially open a third valve located upstream from the first evaporator in refrigerant conduit coupling an inlet of the flash tank to the first evaporator; and   at least partially open a fourth valve located downstream from the first evaporator in refrigerant conduit allowing flow of refrigerant from the one or more medium temperature compressors to the first evaporator.