THE TRANSIENT-RESPONSE OF AN EVAPORATOR FED THROUGH AN ELECTRONIC EXPANSION VALVE

The primary task of an expansion valve in a refrigerating machine is t o control the mass flowrate into the evaporator to obtain optimal oper ation without 'hunting' under given conditions. A stepper-motor-contro lled expansion valve and an evaporator have been studied. The tests we re carried out on a fixed refrigerating machine of cooling capacity le ss than 6 kW, with constant condensation conditions and variable evapo rating temperature (-20 to + 10 degrees C) and compressor speed (1000 to 3000 r.p.m). Two control algorithms (proportional/derivative and qu alitative optimal regulation) have been developed for opening and clos ing the valve with the stepper motor. The control parameters depend on both the expansion valve and the evaporator transfer functions. In st eady-state conditions, the system is stable with a superheat equal to the set value. Under transient conditions, with step excitations of 30 0 and 1000 r.p.m as well as for cold-start of the machine, the control algorithms are adequate for regulating the refrigerant flowrate into the evaporator. (C) 1997 by John Wiley & Sons, Ltd.