On reducing evaporator superheat nonlinearity with control architecture

Evaporator superheat control is an important aspect of the operation of refrigeration and air conditioning systems; since the majority of cooling in these systems occurs through evaporation of two-phase refrigerant, the energy efficiency is dramatically improved by reducing the amount of superheat present. However, allowing refrigerant to leave the evaporator without completely vaporizing risks catastrophic damage to the compressor, so superior control is required at low superheat levels. One of the most significant challenges present in this control problem is the presence of significant nonlinearities in the response from the control input, e.g. expansion valve position, to evaporator superheat. This paper reveals how a particular control architecture inherently compensates for both the static and dynamic nonlinearities that dominate the valve-to superheat transient response. Furthermore, the control implementation only requires temperature measurements, which are frequently available in ordinary HVAC systems. Experimental results confirm the reduction of nonlinearities using the proposed approach, and the authors discuss the effect of actuator limitations on the nonlinearity compensation.