Methods of freezeout prevention for very low temperature mixed refrigerant systems

Abstract

Refrigerant freezeout is prevented by the use of a controlled bypass flow that causes a warming of the lowest temperature refrigerant in a refrigeration system that achieves very low temperatures by using a mixture of refrigerants comprising at least two refrigerants with boiling points that differ by at least 50° C. This control capability enables reliable operation of the very low temperature system.

1 . A very low temperature refrigeration system utilizing mixed refrigerants, the system comprising: 
 a compressor in fluid communication with a refrigeration process, the refrigeration process including a high pressure line on a high pressure side of the refrigeration system between the compressor and an evaporator, a low pressure line on a low pressure side of the refrigeration system in a refrigerant return path between the evaporator and the compressor, and at least one heat exchanger cooling refrigerant in the high pressure line from refrigerant in the low pressure line; and    a freeze out prevention circuit including a valve, the freeze out prevention circuit bypassing at least a portion of the refrigeration process to prevent freeze out in the mixed refrigerants;    the freeze out prevention circuit comprising a bypass loop connected either:    a) from a point in the refrigeration process where high pressure refrigerant flows, prior to where the high pressure line exits a cold end of the refrigeration process, and to a point in the refrigeration process where the coldest low pressure refrigerant in the system flows, and wherein the freezing point of the warm high pressure refrigerant is no higher than the freezing point of refrigerant at the point in the refrigeration process where the coldest low pressure refrigerant in the system flows; or    b) from a compressor high-pressure refrigerant line between the compressor and an entrance to the high pressure line of the refrigeration process, and to a suction line of the compressor; or    c) from a point in the refrigeration process where high pressure refrigerant is at its coldest temperature, and to a point in the refrigeration process where low pressure refrigerant exits the coldest of the at least one heat exchangers in the refrigeration process, the bypassed refrigerant not passing through a heat exchanger between the point where high pressure refrigerant is at its coldest temperature and the point where low pressure refrigerant exits the coldest of the at least one heat exchanger.    
   
   
       2 . The refrigeration system of  claim 1  wherein the freeze out prevention circuit comprises a bypass loop connected from the compressor high-pressure refrigerant line between the compressor and the entrance to the high pressure line of the refrigeration process, and to the suction line of the compressor.  
   
   
       3 . The refrigeration system of  claim 1  wherein the bypass circuit includes a means of controlling fluid flow through the circuit and wherein the fluid flow is controlled utilizing an on-off valve and a flow-metering device.