Cooling system with oil return to accumulator

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;
• using, by a first low side heat exchanger, primary refrigerant from the flash tank to cool a secondary refrigerant, wherein both the first valve and the second valve are each associated with the first low side heat exchanger;
• receiving, by an accumulator, primary refrigerant from the first low side heat exchanger, wherein the accumulator comprises a U-shaped pipe having an end configured to receive the primary refrigerant, and wherein the U-shaped pipe comprises a hole located towards a bottom of the accumulator;
• receiving, by a first cooling system fluidly coupled to the first low side heat exchanger, the cooled secondary refrigerant;
• compressing, by a first compressor, primary refrigerant from the accumulator;
• and
• compressing, by a second compressor disposed downstream of the first compressor, primary refrigerant from the first compressor, during a second mode of operation:
• opening the first valve;
• closing the second valve;
• and
• 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 first vessel;
• and
• during a third mode of operation:
• closing the first valve;
• opening the second valve;
• directing, by the second valve, primary refrigerant from the first compressor to the first vessel;
• and
• pushing, by the primary refrigerant from the first compressor, the oil in the first vessel to the accumulator and wherein the oil collected at the bottom of the accumulator is sucked through the hole in the U-shaped pipe and directed to the first compressor.

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 that comprises a U-shaped pipe having an end configured to receive the primary refrigerant, and wherein the U-shaped pipe comprises a hole located towards a bottom of the accumulator;
• a first compressor;
• a second compressor;
• a first valve;
• a first cooling system fluidly coupled to the first low side heat exchanger;
• and
• a second valve associated with the first low side heat exchanger;
• wherein during a first mode of operation:
• the first and second valves are closed;
• the first low side heat exchanger is configured to use primary refrigerant from the flash tank to cool a secondary refrigerant;
• the accumulator is configured to receive the primary refrigerant from the first low side heat exchanger;
• the first cooling system is configured to receive the cooled secondary refrigerant;
• the first compressor is configured to compress the primary refrigerant from the accumulator;
• and
• the second compressor is configured to compress the primary refrigerant from the first compressor,
• wherein during a second mode of operation:
• the first valve is open and configured to direct the primary refrigerant from the first low side heat exchanger and an oil from the first low side heat exchanger to the vessel;
• and
• the second valve is closed,
• wherein during a third mode of operation:
• the first valve is closed;
• and
• the second valve is open and directs the primary refrigerant from the first compressor to the vessel, wherein the primary refrigerant from the first compressor pushes the oil in the vessel to the accumulator, and wherein the oil collected at the bottom of the accumulator is sucked through the hole in the U-shaped pipe and directed to the first compressor.