A very low temperature mixed refrigeration system containing refrigeration process comprising a supply flow path (116/120) flowing high-pressure refrigerant, a return flow path (148/164) flowing low-pressure refrigerant, and at least one heat exchanger (202, 206, 208) exchanging heat between the high-pressure refrigerant and the low-pressure refrigerant, characterized by a freeze out prevention circuit comprising: a bypass loop connected either a) from a point in the refrigeration process where the warm high pressure refrigerant enters the refrigeration process and prior to the exit of the high pressure refrigerant from a cold end of the refrigeration process to the coldest point of the low pressure refrigerant in the system, and wherein the freezing point of the warm high pressure refrigerant is no higher than the freezing point of the low pressure refrigerant at the cold end of the refrigeration process whereby freeze out is being prevented; b) from a point in the refrigeration process where the high pressure refrigerant is at its coldest temperature directly to a point where the low pressure refrigerant is exiting the coldest heat exchanger in the refrigeration process; or c) from a room temperature high-pressure refrigerant line to a compressor suction line; and a control valve (218, 318, 418) arranged in said bypass loop; and a system control that: (i) opens the control valve (218, 318, 418) based on the mode of operation being a standby mode, a defrost mode, a bakeout mode or a cool mode, or (ii) comprises a closed loop feedback control system comprising a temperature sensor at the coldest part of the system where freezeout is to be prevented and a control device programmed with appropriate set points whose outputs are used to control the control valve (218, 318, 418). a bypass loop connected either a) from a point in the refrigeration process where the warm high pressure refrigerant enters the refrigeration process and prior to the exit of the high pressure refrigerant from a cold end of the refrigeration process to the coldest point of the low pressure refrigerant in the system, and wherein the freezing point of the warm high pressure refrigerant is no higher than the freezing point of the low pressure refrigerant at the cold end of the refrigeration process whereby freeze out is being prevented; b) from a point in the refrigeration process where the high pressure refrigerant is at its coldest temperature directly to a point where the low pressure refrigerant is exiting the coldest heat exchanger in the refrigeration process; or c) from a room temperature high-pressure refrigerant line to a compressor suction line; a) from a point in the refrigeration process where the warm high pressure refrigerant enters the refrigeration process and prior to the exit of the high pressure refrigerant from a cold end of the refrigeration process to the coldest point of the low pressure refrigerant in the system, and wherein the freezing point of the warm high pressure refrigerant is no higher than the freezing point of the low pressure refrigerant at the cold end of the refrigeration process whereby freeze out is being prevented; b) from a point in the refrigeration process where the high pressure refrigerant is at its coldest temperature directly to a point where the low pressure refrigerant is exiting the coldest heat exchanger in the refrigeration process; or c) from a room temperature high-pressure refrigerant line to a compressor suction line; and a control valve (218, 318, 418) arranged in said bypass loop; and a system control that: (i) opens the control valve (218, 318, 418) based on the mode of operation being a standby mode, a defrost mode, a bakeout mode or a cool mode, or (ii) comprises a closed loop feedback control system comprising a temperature sensor at the coldest part of the system where freezeout is to be prevented and a control device programmed with appropriate set points whose outputs are used to control the control valve (218, 318, 418). (i) opens the control valve (218, 318, 418) based on the mode of operation being a standby mode, a defrost mode, a bakeout mode or a cool mode, or (ii) comprises a closed loop feedback control system comprising a temperature sensor at the coldest part of the system where freezeout is to be prevented and a control device programmed with appropriate set points whose outputs are used to control the control valve (218, 318, 418).