An expansion valve is controlled in response to sensing conditions at the outlet of at least one compressor within a refrigeration loop in a manner that achieves low suction superheat operation of the compressor. In particular, a discharge superheat is computed using data obtained from a specific mathematical model of the compression process corresponding to the current capacity stage of the compressor. The position of the expansion valve is controlled so as to result in an actual discharge superheat being within a predetermined dead band amount of the computed discharge superheat.
