Abstract: A variable capacity rotary screw compressor for a refrigeration system which includes a high pressure condenser, a subcooler, a main evaporator and an intermediate evaporator, the compressor including a primary inlet connected to receive low pressure vapor from the main evaporator, a secondary or intermediate inlet connected to receive high pressure vapor from the subcooler and/or intermediate evaporator, and a high pressure discharge port connected to the condenser, the compressor including a pair of oppositely rotating constant mesh helical lobe rotors and a slide valve to vary the capacity of the compressor by changing the points of admission of the low pressure vapor and the high pressure vapor to the rotors of the compressor, the points of admission of low pressure vapor and high pressure vapor being maintained in a fixed relation as the capacity of the compressor is varied.