THERMODYNAMIC ANALYSIS OF CAPACITY-CONTROL SCHEMES FOR REFRIGERATION AND AIR-CONDITIONING SYSTEMS

Cylinder unloading and suction-gas throttling schemes are studied to r educe the capacity of refrigeration and air-conditioning systems when operating at reduced load. In the first scheme, an unloaded valve is u sed to unload one or more cylinders at part-load conditions. The unloa ding of cylinders reduces the mass-flow rate of refrigerant in the sys tem and thus reduces the system capacity, whereas in suction-gas throt tling, a throttling valve is needed before the compressor to reduce th e mass-flow rate through the compressor. These schemes are discussed f or 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 exam ined in terms of system coefficient of performance (COP) and irreversi ble component losses. It is found that the cylinder-unloading scheme i s best and has the highest COP and minimum irreversible losses at any system capacity. Copyright (C) 1996 Elsevier Science Limited.