Predictive functional control of an expansion valve for minimizing the superheat of an evaporator

In a previous paper, a Predictive Functional Control (PFC) method was proposed to control the evaporator superheat with an electronic expansion valve. It has been shown that superheat may be more accurately controlled by PFC than the conventional Proportional-Integral-Derivative (PID) control. In this paper, the proposed methodology is extended to regulate the condensing pressure. In order to study the influence of this control method on the Coefficient of Performance (COP), experiments are conducted on a refrigerating machine by changing the cooling capacity from 120 to 30 kW. As PFC improves disturbance rejection compared to a PID control, it is possible to reduce the superheat setting value and to prevent any unevaporated refrigerant liquid from reaching the compressor. As a consequence the use of PFC leads to an increase of COP which depends on operating conditions.