Cascade refrigeration system with modular ammonia chiller units

A cascade refrigeration system, comprising:

• an upper portion having at least one modular chiller unit that provides cooling to at least one of a low temperature subsystem having a plurality of low temperature loads, and a medium temperature subsystem having a plurality of medium temperature loads;
• the modular chiller unit comprising:
• a refrigerant circuit having at least a compressor, a condenser, an expansion device, and an evaporator;
• an ammonia refrigerant configured for circulation within the refrigerant circuit;
• an ammonia refrigerant accumulator configured to receive the ammonia refrigerant from the evaporator;
• an oil management system downstream of the compressor and configured to remove oil from the ammonia refrigerant, the oil management system having an oil separator disposed between the compressor and the condenser, and an oil return line coupled to the evaporator and the ammonia refrigerant accumulator.