Patent ID: 8011191

Claim:
A two-stage cascade refrigeration system comprising: a first refrigeration stage defining a first fluid circuit for circulating a first refrigerant, the first refrigeration stage having a first compressor, a condenser, and a first expansion device in fluid communication with the first fluid circuit; a second refrigeration stage defining a second fluid circuit for circulating a second refrigerant, the second refrigeration stage having a variable speed second compressor, a second expansion device, and an evaporator in fluid communication with the second fluid circuit; a heat exchanger in fluid communication with the first and second fluid circuits to exchange heat between the first and second refrigerants; at least one controller operatively coupled to the first and second compressors for independently controlling operation thereof; and a sensor operatively coupled to the at least one controller, the sensor being configured to sense a temperature of the first refrigerant at an outlet of the heat exchanger and to generate a signal indicative of the sensed temperature to the at least one controller, the at least one controller being operable to vary the speed of the second compressor in response to the signal.