Cascade refrigeration system with modular ammonia chiller units

A cascade refrigeration system including 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 includes a refrigerant circuit having at least a compressor, a condenser, an expansion device, and an evaporator. An ammonia refrigerant which may have entrained oil from the compressor circulates within the refrigerant circuit. An oil recycling circuit removes some oil from the ammonia refrigerant for return to the compressor. An oil pot collects oil accumulated in the evaporator and an oil return line drains oil from the oil pot to an ammonia accumulator or directly to the compressor.