Low suction pressure protection for refrigerant vapor compression system

1. A refrigerant vapor compression system comprising:

a compressor for compressing a refrigerant, the compressor having a suction port and a discharge port; a refrigerant heat rejection heat exchanger operatively coupled downstream to the discharge port of the compressor; a refrigerant heat absorption heat exchanger operatively coupled downstream to the refrigerant heat rejection heat exchanger; a compressor suction inlet line connecting the refrigerant heat absorption heat exchanger to the suction port of the compressor; an adiabatic expansion device operatively coupled to the suction inlet line;

a sensor operatively coupled to the suction inlet line for measuring a superheat value of the refrigerant;

a bypass line connecting the discharge port of the compressor to the suction port of the compressor, the bypass line having an unload valve to regulate the refrigerant flow therethrough;

wherein the adiabatic expansion device is positioned upstream with respect to refrigerant flow of a point where the bypass line connects to the suction inlet line;

a controller in communication with the sensor, the controller configured to operate the refrigerant vapor compression system in a first cooling mode when a pressure of the refrigerant downstream of the adiabatic expansion device is greater than a threshold value, and operate the refrigerant vapor compression system in a second cooling mode when the pressure of the refrigerant downstream of the adiabatic expansion device is less than the threshold value, the threshold value indicative of a pressure at which expansion across the adiabatic expansion device would result in formation of solid-phase refrigerant;

wherein the second cooling mode of operation includes opening the unload valve to admit higher-temperature refrigerant to the suction port of the compressor; and

wherein the compressor continues to operate in both cooling modes, wherein the refrigerant vapor compression system operates in a transcritical mode in both the first cooling mode and the second cooling mode.

2. The refrigerant vapor compression system of claim 1 , wherein the adiabatic expansion device is a suction modulation valve.

3. The refrigerant vapor compression system of claim 1 , wherein the second cooling mode comprises increasing the superheat value of the refrigerant in the suction inlet line.

4. The refrigerant vapor compression system of claim 1 , wherein the second cooling mode comprises increasing a pressure upstream of the adiabatic expansion device until a pressure downstream of the adiabatic expansion device is above the triple point pressure of the refrigerant.

5. The refrigerant vapor compression system of claim 1 , wherein the threshold value is a fixed value greater than a triple point pressure of the refrigerant.

6. The refrigerant vapor compression system of claim 5 , wherein the threshold value is equal to the triple point pressure of the refrigerant.

7. The refrigerant vapor compression system of claim 1 , further including an expansion valve upstream of the refrigerant heat absorption heat exchanger, wherein increasing the superheat value comprises adjusting the expansion valve.

8. The refrigerant vapor compression system of claim 7 , wherein adjusting the expansion valve comprises closing down the expansion valve.

9. The refrigerant vapor compression system of claim 1 , further comprising a heater disposed in thermal contact with the suction inlet line, wherein increasing the superheat value of the refrigerant comprises powering the heater.

10. The refrigerant vapor compression system of claim 1 , wherein operating the refrigerant vapor compression system in the second cooling mode further comprises operating with a pressure at the suction port of the compressor being less than the triple point pressure of the refrigerant.

11. The refrigerant vapor compression system of claim 1 , wherein the refrigerant is carbon dioxide.