Significant aspects in the design of variable speed reciprocating compressor used in refrigeration plants

An experimental analysis of a variable speed semihermetic reciprocating compressor is presented. The semihermetic compressor tested, working with the fluids R22, R507 and R407C and designed for a revolution speed corresponding to a 50Hz, is a component of a vapour compression refrigeration plant subjected to a cold-store whose refrigeration power is controlled varying the compressor electric motor speed. Different temperatures are selected for the air in the cold-store and at the condenser. To vary the compressor speed a pulse-width modulated source inverter PWM has been used. The volumetric and isoentropic efficiencies together with specific indexes related to the performances of the semihermetic compressor have been considered when the compressor speed decreases. These quantities allow to explain the energy saving obtainable using a variable speed compressor instead of the on/off compressor cycles deriving from a classical thermostatic device. A comparison is performed using the working fluid R22 and its most suitable substitutes as R407C (R32/R125/R134a 23/25/52% in mass), R507 (R125/R143A 50/50% in mass) and R417A (R125/R134a/R600 46.6/50/3.4% in mass). The best compressor performance varying its speed is obtainable with R22, while among the fluids candidate to its substitution, the best suitable seems to be the non azeotropic mixture R407C as it happens also at the compressor nominal speed. The study can be useful to improve specific aspects of the variable speed semihermetic compressor.