Behaviour of a thermostatic expansion valve in non-steady 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 behaviour of various thermostatic expansion valves (ballast charge, adsorbent charge and maximum operating pressure or MOP charge) has been studied for compressor speed variations (between 20% and 200%) of a refrigerating 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 either for the low evaporating temperatures or for great variations of the compressor speed, more than 50%. The stable operating limits of different thermostatic expansion valves have been determined as a function of the evaporation temperature and the rotational speed of the compressor during the non-steady state.