Refrigeration cycle apparatus

Abstract

A refrigeration cycle apparatus includes a first circuit that circulates a first refrigerant; a second circuit that circulates a carbon dioxide refrigerant and refrigerating machine oil; a cascade heat exchanger that performs heat exchange between the first refrigerant and the carbon dioxide refrigerant; and a control unit. The second circuit includes: a container that stores the carbon dioxide refrigerant and the refrigerating machine oil; a suction pipe that connects the suction side of a second compressor to the container; an oil return passage for returning the refrigerating machine oil from a lower portion of the container to the suction pipe; and a valve in the oil return passage. During defrost operation, the control unit ensures the valve is closed under a predetermined condition but open if the condition is not met.

1 . A refrigeration cycle apparatus comprising:
 a first circuit that circulates a first refrigerant;   a second circuit that circulates a carbon dioxide refrigerant and refrigerating machine oil;   a cascade heat exchanger that performs heat exchange between the first refrigerant and the carbon dioxide refrigerant; and   a control unit having processor circuitry,   wherein the first circuit includes a first heat exchanger that performs heat exchange between outside air and the first refrigerant,   the second circuit includes
 a second compressor, 
 a container that is provided on a suction side of the second compressor and that stores the carbon dioxide refrigerant and the refrigerating machine oil, 
 a suction pipe that connects the suction side of the second compressor to the container, 
 an oil return passage that returns the refrigerating machine oil from a lower portion of the container to the suction pipe, and 
 a valve that is provided in the oil return passage, and 
   during defrost operation for defrosting the first heat exchanger, under a first condition the valve is in an open state, the processor circuitry of the control unit is configured to close the valve under a second condition in which a temperature or pressure of the carbon dioxide refrigerant and the refrigerating machine oil in the container is below a predetermined temperature or predetermined pressure corresponding to a boundary temperature at which a density of the carbon dioxide refrigerant and a density of the refrigerating machine oil in the container become equal.   
     
     
         2 . The refrigeration cycle apparatus according to  claim 1 , wherein the control unit is configured to close the valve at a start of the defrost operation. 
     
     
         3 . The refrigeration cycle apparatus according to  claim 2 , wherein the control unit is further configured to close the valve when an outside air temperature is equal to or lower than a first temperature. 
     
     
         4 . The refrigeration cycle apparatus according to  claim 3 , wherein the control unit is configured to maintain the valve open in a case where the valve is open when the outside air temperature exceeds the first temperature. 
     
     
         5 . The refrigeration cycle apparatus according to  claim 1 , wherein
 the second circuit further includes
 a second heat exchanger that performs heat exchange between indoor air and the carbon dioxide refrigerant, and 
 a bypass circuit that connects a point between the second heat exchanger and the cascade heat exchanger to the suction pipe, and 
   the control unit is configured to cause the carbon dioxide refrigerant to circulate, in order, through the second compressor, the cascade heat exchanger, and the bypass circuit during the defrost operation.   
     
     
         6 . The refrigeration cycle apparatus according to  claim 2 , wherein
 the second circuit further includes
 a second heat exchanger that performs heat exchange between indoor air and the carbon dioxide refrigerant, and 
 a bypass circuit that connects a point between the second heat exchanger and the cascade heat exchanger to the suction pipe, and 
   the control unit is configured to cause the carbon dioxide refrigerant to circulate, in order, through the second compressor, the cascade heat exchanger, and the bypass circuit during the defrost operation.   
     
     
         7 . The refrigeration cycle apparatus according to  claim 3 , wherein
 the second circuit further includes
 a second heat exchanger that performs heat exchange between indoor air and the carbon dioxide refrigerant, and 
 a bypass circuit that connects a point between the second heat exchanger and the cascade heat exchanger to the suction pipe, and 
   the control unit is configured to cause the carbon dioxide refrigerant to circulate, in order, through the second compressor, the cascade heat exchanger, and the bypass circuit during the defrost operation.   
     
     
         8 . The refrigeration cycle apparatus according to  claim 4 , wherein
 the second circuit further includes
 a second heat exchanger that performs heat exchange between indoor air and the carbon dioxide refrigerant, and 
 a bypass circuit that connects a point between the second heat exchanger and the cascade heat exchanger to the suction pipe, and 
   the control unit is configured to cause the carbon dioxide refrigerant to circulate, in order, through the second compressor, the cascade heat exchanger, and the bypass circuit during the defrost operation.   
     
     
         9 . The refrigeration cycle apparatus according to  claim 1 , wherein the first refrigerant contains R32, R454C, propane, R1234yf, R1234ze, or ammonia. 
     
     
         10 . The refrigeration cycle apparatus according to  claim 2 , wherein the first refrigerant contains R32, R454C, propane, R1234yf, R1234ze, or ammonia. 
     
     
         11 . The refrigeration cycle apparatus according to  claim 3 , wherein the first refrigerant contains R32, R454C, propane, R1234yf, R1234ze, or ammonia. 
     
     
         12 . The refrigeration cycle apparatus according to  claim 4 , wherein the first refrigerant contains R32, R454C, propane, R1234yf, R1234ze, or ammonia. 
     
     
         13 . The refrigeration cycle apparatus according to  claim 5 , wherein the first refrigerant contains R32, R454C, propane, R1234yf, R1234ze, or ammonia. 
     
     
         14 . The refrigeration cycle apparatus according to  claim 6 , wherein the first refrigerant contains R32, R454C, propane, R1234yf, R1234ze, or ammonia. 
     
     
         15 . The refrigeration cycle apparatus according to  claim 7 , wherein the first refrigerant contains R32, R454C, propane, R1234yf, R1234ze, or ammonia. 
     
     
         16 . The refrigeration cycle apparatus according to  claim 8 , wherein the first refrigerant contains R32, R454C, propane, R1234yf, R1234ze, or ammonia. 
     
     
         17 . A refrigeration cycle apparatus comprising:
 a first circuit that circulates a first refrigerant, the first circuit including a first heat exchanger that exchanges heat between outside air and the first refrigerant;   a second circuit that circulates a carbon dioxide refrigerant and refrigerating machine oil, the second circuit includes
 a second compressor, 
 a container that is provided on a suction side of the second compressor and that stores the carbon dioxide refrigerant and the refrigerating machine oil, 
 a suction pipe that connects the suction side of the second compressor to the container, 
 an oil return passage that returns the refrigerating machine oil from a lower portion of the container to the suction pipe, and 
 a valve that is provided in the oil return passage; 
   a cascade heat exchanger that exchanges heat between the first refrigerant and the carbon dioxide refrigerant; and   circuitry configured to
 during a defrost operation that defrosts the first heat exchanger, and under a first condition that the valve is open, control the valve to be closed under a second condition in which a temperature or pressure of the carbon dioxide refrigerant and the refrigerating machine oil in the container is below a predetermined temperature or predetermined pressure corresponding to a boundary temperature at which a density of the carbon dioxide refrigerant and a density of the refrigerating machine oil in the container become equal.