Refrigeration apparatus

Abstract

An extra low temperature refrigeration system consisting of a two-stage compressor that has an electronic controller. The electronic controller controls an electric expansion valve in the evaporator, which modulates the true superheat. There is also an electric pressure control valve in the condenser, which modulates the head pressure in cool mode, and the suction pressure in defrost mode. The defrost cycle uses a reversing valve. The liquid refrigerant for the inter-stage sub-cooling circuit is constantly provided by a sub-condenser separated from a conventional one condenser system. Therefore, the system provides constant cooling for the compressor motor windings as the compressor runs. There is a one-way pathway from the main condenser circuit to the sub-condenser circuit. This pathway will call for more refrigerant from the main circuit to the sub-cooling circuit as demanded. The inter-stage sub-cooling circuit uses an electric expansion valve true superheat control for its sub-cooler.

What is claimed is: 
     
         1 . A refrigeration system for cooling a space, said system having a refrigeration mode and having a defrost mode, said system comprising:
 a two-stage compressor to compress a refrigerant;   a two-circuit condenser connected to said two-stage compressor, said condenser comprising a sub-condenser circuit and a main condenser circuit;   an evaporator connected to said main condenser circuit of said two-circuit condenser, said evaporator absorbs heat from the space that is to be cooled during refrigeration mode;   a sub-cooling heat exchanger having a first connection between said sub-condenser circuit of said two-circuit condenser and said two-stage compressor and a second connection that is between said main condenser and said evaporator;   a first electric expansion valve connected between said sub-condenser circuit of said two-circuit condenser and said sub-cooling heat exchanger;   a second electric expansion valve connected between said sub-cooling heat exchanger and said evaporator;   a third electric expansion valve connected between main condenser circuit of said two-circuit condenser; said third electric expansion valve also controls head pressure of said main condenser circuit of said two-circuit condenser during refrigeration mode;   means for electronic controlling said system wherein said controller means controls said first electric expansion valve to modulate the true superheat of said sub-cooling heat exchanger and wherein said controller means controls said second electric expansion valve to modulate the true superheat of said evaporator when said system is in the refrigeration mode and wherein said controller means controls said third electric expansion valve to modulate the suction pressure of said main condenser circuit of said two-circuit condenser when said system is in the defrost mode and modulates the head pressure when said system is in the refrigeration mode.   
     
     
         2 . The system of  claim 1  further comprising:
 a differential oil pressure switch associated with said controller means such that said defrost mode is initiated when oil level is low. 
 
     
     
         3 . The system of  claim 1  further comprising:
 a drain pan beneath said evaporator; 
 a drain pan loop associated with said evaporator, wherein a portion of said refrigerant enters said loop to melt the ice or frost and the major portion of said refrigerant enters said evaporator to melt the ice or frost when said system is in the defrost mode. 
 
     
     
         4 . The system of  claim 1  further comprising a refrigerant reservoir interposed between said sub-cooling heat exchanger and said main condenser in the connection connecting said sub-cooling heat exchanger and said main condenser. 
     
     
         5 . The system of  claim 1  further comprising an oil separator connected between said two-stage compressor and said two-circuit condenser such that refrigerant oil is separated by said oil separator and discharged back to a crankcase in said two-stage compressor.