Refrigeration system with oil control system

1 . A controller for a refrigeration system that provides cooling for a refrigeration load using carbon dioxide (CO 2 ) as a refrigerant, the controller comprising:

one or more processors; and

one or more non-transitory computer-readable media storing instructions that, when executed by the one or more processors, cause the one or more processors to perform operations comprising:

controlling a plurality of compressors to circulate the CO 2 refrigerant within the CO 2 refrigeration system;

obtaining a measurement of an amount of oil within a suction line coupled to an inlet of the plurality of compressors, the suction line configured to contain a mixture of oil and the CO 2 refrigerant and to deliver at least a portion of the mixture of the oil and the CO 2 refrigerant to at least one of the plurality of compressors positioned downstream of the suction line;

operating a plurality of oil equalization valves in a plurality of oil equalization lines connecting the plurality of compressors to an oil return line to control the CO 2 refrigerant flow through the plurality of oil equalization lines to the plurality of compressors;

comparing the measured amount of the oil within the suction line to an oil threshold;

based on a result of the comparison, periodically opening the plurality of oil equalization valves to cause a flow of excess oil from the plurality of compressors into the plurality of oil equalization lines to equalize an amount of oil within each compressor of the plurality of compressors;

opening the oil control valve in response to a determination that the amount of the oil within the suction line is less than the oil threshold; and

keeping the oil control valve closed in response to a determination that the amount of the oil within the suction line is greater than or equal to the oil threshold.

2 . The controller of claim 1 , wherein the operations comprise:

obtaining a second measurement of the amount of the oil within the suction line after opening the oil control valve to release the oil from the oil separator into the suction line; and

deactivating one or more components of the CO 2 refrigeration system in response to the second measurement of the amount of the oil within the suction line being less than the oil threshold.

3 . The controller of claim 2 , wherein the operations comprise stopping operation of the CO 2 refrigeration system when the amount of the oil is less than a shutdown oil threshold.

4 . The controller of claim 1 , wherein operating the plurality of oil equalization valves comprises:

opening the plurality of oil equalization valves to equalize the oil within each compressor of the plurality of compressors; and

closing the plurality of oil equalization valves after a predetermined amount of time has elapsed.

5 . The controller of claim 4 , wherein the operations comprise stopping operation of the CO 2 refrigeration system when the amount of the oil is less than a shutdown oil threshold.

6 . The controller of claim 1 , wherein operating the plurality of oil equalization valves comprises:

after a predetermined amount of time, adjusting the plurality of oil equalization valves to an open state; and

subsequent to adjusting the plurality of oil equalization valves to an open state, adjusting the plurality of oil equalization valves to a closed state.

7 . The controller of claim 6 , wherein the operations comprise stopping operation of the CO 2 refrigeration system when the amount of the oil is less than a shutdown oil threshold.

8 . The controller of claim 1 , wherein the operations comprise operating the plurality of oil equalization valves to control the CO 2 refrigerant flow through the plurality of oil equalization lines to, when the amount of the oil is below the oil threshold, only return the CO 2 refrigerant from the plurality of compressors to the suction line.

9 . The controller of claim 1 , wherein the operations comprise:

operating the oil control valve to flow the oil from the oil separator into a portion of the oil return line; and

operating the plurality of oil equalization valves to flow the oil from the plurality of oil equalization lines into the portion of the oil return line such that oil is delivered from the oil separator and from the plurality of oil equalization lines in the portion of the oil return line to the suction line.

10 . The controller of claim 1 , wherein the operation of opening the plurality of oil equalization valves causes any excess oil from the plurality of compressors to return to the oil return line via a plurality of oil equalization lines, each oil equalization line of the plurality of oil equalization lines connecting one of the plurality of compressors to the oil return line.

11 . The controller of claim 10 , wherein operation of the plurality of oil equalization valves and oil control valve causes the oil return line to receive both the flow of the oil from the oil separator and a flow of the oil from the plurality of oil equalization lines to deliver the oil to the suction line.

12 . The controller of claim 1 , wherein the operation of obtaining the measurement of the amount of the oil within the suction line comprises:

receiving a signal from an oil sensor fluidly coupled to the suction line and configured to measure the amount of the oil within the suction line upstream of the plurality of compressors, the signal representing the amount of the oil within the suction line upstream of the plurality of compressors; and

based on the signal representing the amount of the oil within the suction line upstream of the plurality of compressors, determining the amount of the oil within the suction line.

13 . The controller of claim 12 , wherein the oil sensor comprises one of: an optical oil sensor, a capacitive oil sensor, a dielectrical oil sensor, a resonance frequency oil sensor, a floating bulb oil sensor, or a Baumer frequency sweep sensor.

14 . The controller of claim 1 , wherein the amount of the oil comprises one or more of an oil mist or an oil liquid.

15 . The controller of claim 1 , wherein the operation of operating the oil control valve to control the flow of the oil from the oil separator to the suction line comprises at least one of:

adjusting the oil control valve to move to an open state to permit the flow of the oil from the oil separator to the suction line; or

adjusting the oil control valve to move to a closed state to prevent the flow of the oil from the oil separator to the suction line.

16 . The controller of claim 15 , wherein the operation of operating the oil control valve to control the flow of the oil from the oil separator to the suction line further comprises:

adjusting the oil control valve to the open state in response to a determination that the amount of the oil within the suction line is less than the oil threshold; and

maintaining the oil control valve in the closed state in response to a determination that the amount of the oil within the suction line is greater than or equal to the oil threshold.

17 . The controller of claim 1 , wherein the operations further comprise:

operating a plurality of expansion valves, each expansion valve located downstream of a respective evaporator of a plurality of evaporators; and

responsive to operating each expansion valve, controlling a pressure in each of the evaporators.

18 . The controller of claim 17 , wherein the operation of controlling the pressure in each of the evaporators comprises expanding the CO 2 refrigerant to a lower pressure, lower temperature state.

19 . The controller of claim 1 , wherein the operations comprise operating at least one of the oil control valve or the plurality of oil equalization valves to balance a flow of the oil back to each of the plurality of compressors such that a level of the oil is maintained above a minimum level in each of the plurality of compressors.

20 . The controller of claim 1 , wherein the oil threshold comprises a maximum allowable oil level or a maximum desirable oil level within each of the plurality of compressors.

21 . The controller of claim 1 , wherein the operations comprise stopping operation of the CO 2 refrigeration system when the amount of the oil is less than a shutdown oil threshold.