Development of a superheat controller for mitigating hunting in a direct expansion air conditioning system

This paper reports a development of a new degree of superheat (DS) controller which can slow down the rate of DS signal transfer in a direct expansion (DX) air conditioning (A/C) system for help mitigate operational instability. Through analyzing the heat transfer occurred at the outlet of an evaporator where the DS was evaluated, the dynamics of the DS signal transfer could be simply characterized by a first-order transfer function model (FOM). The new DS controller was thus developed by incorporating a FOM into a conventional proportional-integral (PI) controlled EEV-evaporator control loop, so that the rate of DS signal transfer could be regulated through varying the time constant of the FOM. Using an experimental DX A/C system, controllability tests for the new DS controller were carried out. Experimental results showed that the new DS controller would help mitigate possible system hunting when the experimental DX A/C system was unstably operated. The developed control strategy provides an alternative way to regulate the DS with an emphasis on operational stability.