Effects of superheat nonlinearity on the operational stability of a direct expansion (DX) air conditioning (A/C) system

The paper reports a study on examining the influences of superheat nonlinearity on the operational stability of a direct expansion (DX) air conditioning (A/C) system. Using an experimental DX A/C system, the static nonlinearity of the electronic expansion valve (EEV) and superheat nonlinearity of the evaporator exhibited under different operating conditions were obtained. A first-order plus dead time (FOPDT) model was used to characterize the dynamic responses of the evaporator when the experimental system was operated at different compressor speeds and evaporating temperatures. Using Nyquist stability criterion, the effects of superheat nonlinearity on the system operational stability were theoretically analysed. The study results showed that a higher chance to instability would be resulted in when the system was operated at a smaller EEV's opening, a lower compressor speed or evaporating temperature.