Cooling system with oil return to oil reservoir

A method comprising:

• storing, by a flash tank, a primary refrigerant;
• during a first mode of operation:
• closing a first valve and a second valve;
• opening a third valve;
• using, by a first low side heat exchanger, primary refrigerant from the flash tank to cool a secondary refrigerant;
• receiving, by an accumulator, primary refrigerant from the first low side heat exchanger;
• compressing, by a first compressor, primary refrigerant from the accumulator;
• and
• compressing, by a second compressor, primary refrigerant from the first compressor,
• and
• during a second mode of operation:
• opening the first valve;
• directing, by the first valve, primary refrigerant from the first low side heat exchanger and an oil from the first low side heat exchanger to a vessel;
• closing the second valve;
• opening the third valve;
• and
• directing, by the third valve, primary refrigerant from the vessel to the accumulator.

A system comprising:

• a high side heat exchanger configured to remove heat from a primary refrigerant;
• a flash tank configured to store the primary refrigerant;
• a first low side heat exchanger;
• an accumulator;
• a first compressor;
• a second compressor;
• an oil reservoir;
• a first valve;
• a second valve;
• and
• a third valve, during a first mode of operation:
• the first and second valves are closed;
• the third valve is open;
• the first low side heat exchanger uses primary refrigerant from the flash tank to cool a secondary refrigerant;
• the accumulator receives primary refrigerant from the first low side heat exchanger;
• the first compressor compresses primary refrigerant from the accumulator;
• and
• the second compressor compresses primary refrigerant from the first compressor,
• and
• during a second mode of operation:
• the first valve is open and directs primary refrigerant from the first low side heat exchanger and an oil from the first low side heat exchanger to a vessel;
• the second valve is closed;
• and
• the third valve is open and directs primary refrigerant from the vessel to the accumulator.