A refrigeration system for long term continuous operation in cooling and defrost modes, comprising: a compression unit having an inlet and an outlet, and taking in refrigerant at said inlet at a low pressure and discharging high pressure refrigerant at said outlet; a refrigeration process unit having a high pressure circuit and low pressure circuit, said high pressure circuit receiving said high pressure refrigerant from said compression unit, said low pressure circuit delivering said low pressure refrigerant to said low pressure circuit of said compression unit, heat exchange occurring between the refrigerant in said high and low pressure circuits; a primary throttle unit having an inlet and an outlet, said primary throttle unit inlet receiving high pressure refrigerant from said high pressure circuit of said refrigeration process unit and discharging low pressure refrigerant at said primary throttle unit outlet for connection to an evaporation unit having an inlet and an outlet for selectively cooling or heating a load, and returning to said low pressure circuit of said refrigeration process unit; a condenser unit upstream of said primary throttle unit and said refrigeration process unit, said condenser unit removing heat from said refrigerant at said high pressure from said compressor unit and rejecting said heat externally of said refrigeration system; a first bypass circuit (176, 178, 180, 182, 184) including at least one high pressure branch circuit for circumventing refrigerant flow around a downstream portion of said refrigeration process unit high pressure circuit; and a second bypass circuit (186, 188, 190) including at least one low pressure branch circuit for circumventing refrigerant flow around said refrigeration process unit low pressure circuit; characterised by a control system for directing, in selected sequences, said refrigerant in selected closed cycles between said compression unit and said evaporation unit; by the refrigeration system providing refrigeration in the temperature range of - 50° C to -250° C; in that said control system has a first controllable device in said second bypass circuit (186, 188, 190) regulating refrigerant flow through said second bypass circuit (186, 188, 190), said first controllable device having at least one of on/off operation and variable flow operation; and by said control system further having first blocking means in series with said low pressure circuit of said refrigeration process unit, said first blocking means obstructing return refrigerant flow through said low pressure circuit of said refrigeration process unit when said first controllable device permits flow. a compression unit having an inlet and an outlet, and taking in refrigerant at said inlet at a low pressure and discharging high pressure refrigerant at said outlet; a refrigeration process unit having a high pressure circuit and low pressure circuit, said high pressure circuit receiving said high pressure refrigerant from said compression unit, said low pressure circuit delivering said low pressure refrigerant to said low pressure circuit of said compression unit, heat exchange occurring between the refrigerant in said high and low pressure circuits; a primary throttle unit having an inlet and an outlet, said primary throttle unit inlet receiving high pressure refrigerant from said high pressure circuit of said refrigeration process unit and discharging low pressure refrigerant at said primary throttle unit outlet for connection to an evaporation unit having an inlet and an outlet for selectively cooling or heating a load, and returning to said low pressure circuit of said refrigeration process unit; a condenser unit upstream of said primary throttle unit and said refrigeration process unit, said condenser unit removing heat from said refrigerant at said high pressure from said compressor unit and rejecting said heat externally of said refrigeration system; a first bypass circuit (176, 178, 180, 182, 184) including at least one high pressure branch circuit for circumventing refrigerant flow around a downstream portion of said refrigeration process unit high pressure circuit; and a second bypass circuit (186, 188, 190) including at least one low pressure branch circuit for circumventing refrigerant flow around said refrigeration process unit low pressure circuit; characterised by a control system for directing, in selected sequences, said refrigerant in selected closed cycles between said compression unit and said evaporation unit; by the refrigeration system providing refrigeration in the temperature range of - 50° C to -250° C; in that said control system has a first controllable device in said second bypass circuit (186, 188, 190) regulating refrigerant flow through said second bypass circuit (186, 188, 190), said first controllable device having at least one of on/off operation and variable flow operation; and by said control system further having first blocking means in series with said low pressure circuit of said refrigeration process unit, said first blocking means obstructing return refrigerant flow through said low pressure circuit of said refrigeration process unit when said first controllable device permits flow.