Cylinder unloading and suction-gas throttling schemes are studied to reduce the capacity of refrigeration and air-conditioning systems when operating at reduced load. In the first scheme, an unloaded valve is used to unload one or more cylinders at part-load conditions. The unloading of cylinders reduces the mass-flow rate of refrigerant in the system and thus reduces the system capacity, whereas in suction-gas throttling, a throttling valve is needed before the compressor to reduce the mass-flow rate through the compressor. These schemes are discussed for HFC-134a and compared on the basis of the first and second laws of thermodynamics. A hot-gas by-pass capacity-control scheme is also examined in terms of system coefficient of performance (COP) and irreversible component losses. It is found that the cylinder-unloading scheme is best and has the highest COP and minimum irreversible losses at any system capacity.