A method and apparatus for separating air in which production of the liquid products can be selectively varied between high and low production rates by varying the pressure ratio across a turboexpander (64) used in imparting refrigeration with the use of a branched flow path (34). The branched flow path (34) has a system of valves to selectively and gradually introduce a compressed refrigerant air stream into either a booster compressor branch (40) having a booster compressor (42) to increase the pressure ratio during high modes of liquid production or a bypass branch (38) that bypasses the booster compressor (42) to decrease the pressure ratio during low modes of liquid production. A recycle branch (44) is connected to the booster compressor branch(40) to allow compressed air to be independently recycled from the outlet to the inlet of the booster compressor (42) during turndown from the high to the low liquid mode of liquid production to prevent surge.