BEHAVIOR OF A THERMOSTATIC EXPANSION VALV E IN NONSTEADY STATE FOR A REFRIGERATING MACHINE

A good adaptation of refrigerating machine components operating with a variable-speed compressor requires knowledge of the dynamic behaviour of each component, more particularly of the expansion valve. The beha viour of various thermostatic expansion valves (ballast charge, adsorb ent charge and maximum operating pressure or MOP charge) has been stud ied for compressor speed variations (between 20% and 200%) of a refrig erating machine, with R22 as refrigerant, and a cooling capacity of 6 kW. This study has shown the disadvantages of these expansion valves, during dynamic operation, without modification of the static superheat . The decrease of the evaporating pressure, consecutive to a step or a ramp of the compressor speed, can lead to the suction of a two-phase fluid in the compressor. This phenomenon takes place particularly eith er for the low evaporating temperatures or for great variations of the compressor speed, more than 50%. The stable operating limits of diffe rent thermostatic expansion valves have been determined as a function of the evaporation temperature and the rotational speed of the compres sor during the non-steady state.