A system and method for providing supplemental refrigeration to an air separation plant is provided. A closed loop supplemental refrigeration circuit that can be easily retrofitted or added onto an air separation plant that increases the liquid product production capability of the air separation plant. The supplemental refrigeration capacity of the supplemental refrigeration circuit is controlled by removing or adding a portion of the refrigerant in the supplemental refrigeration circuit to adjust the inlet pressure while maintaining a substantially constant volumetric flow rate and substantially constant pressure ratio across the compressor. Removing the refrigerant from the supplemental refrigeration circuit decreases the refrigeration imparted by the supplemental refrigeration circuit and thus provides the capacity to turn-down liquid product make without shutting down the compressors and turbo-expanders in the supplemental refrigeration circuit.