Cascade refrigeration system with modular ammonia chiller units

A modular ammonia chiller unit for a refrigeration system, comprising:

• a refrigerant circuit having at least a compressor, a condenser, an expansion device, an evaporator, and a header leading to an outdoor location;
• an ammonia refrigerant;
• a soluble oil mixed with the ammonia refrigerant;
• and
• a control device configured to operate the expansion device according to a control scheme comprising:
• modulating a position of the expansion device such that a superheat temperature of the ammonia refrigerant proximate an outlet of the evaporator is intentionally oscillated within a superheat temperature range, and
• causing an accumulation of the soluble oil in the evaporator to be at least partially reabsorbed by the ammonia refrigerant as a result of the superheat temperature oscillation and flushed from the evaporator via the ammonia refrigerant;
• wherein the header is configured to facilitate selective venting of the modular ammonia chiller unit.

A method of providing a cascade refrigeration system that is substantially HFC-free, comprising:

• providing a lower portion having at least one of a low temperature subsystem that uses carbon dioxide as a refrigerant to cool a plurality of low temperature loads, and a medium temperature subsystem that uses one of CO2 and a water-glycol mixture as a liquid coolant to cool a plurality of medium temperature loads;
• providing an upper portion having at least one modular chiller unit that provides cooling to the low temperature subsystem and the medium temperature subsystem, the modular chiller unit comprising a refrigerant circuit having at least a compressor, a condenser, an expansion device, and an evaporator;
• charging the refrigerant circuit of the modular chiller unit with a critical charge amount of an ammonia refrigerant;
• and
• programming a control device to operate according to a control scheme comprising modulating a position of the expansion device such that a superheat temperature of the ammonia refrigerant proximate an outlet of the evaporator is intentionally oscillated within a superheat temperature range;
• wherein the at least one modular chiller unit is packaged within at least one transportable enclosure configured for shipping, direct installation at a facility, and operation of the at least one modular chiller unit within the at least one transportable enclosure after installation;
• and
• wherein at least one of the plurality of low temperature loads and the plurality of medium temperature loads is external to the at least one transportable enclosure.