Based on the balance equations for enthalpy, mass, and momentum a theoretical model of a refrigerant evaporator has been developed. The distributed parameter process is approximated by several lumped parameter models. The model is completed by equations for the expansion valve, the compressor and the superheater. Various effects, e.g., the random fluctuations of the liquid-dry-out-point can be explained by the model. The dynamic behavior of the evaporator is investigated as a function of the manipulating signal UEV (position of the expansion valve) and various disturbances (air temperature ϑA, condenser pressure pCd and compressor rotation speed nc), considering the superheating temperature ϑs as control variable and the evaporator performance Q˙E, which has to be optimized. Two controllers are considered. First, the control behavior with a conventional thermostatic expansion valve is shown, which often operates unstable. The control performance can be considerably improved by a controller whose structure and parameters are better adapted to the evaporation process. For the experiments a process computer is connected on-line to the process. It will be demonstrated that the performance of the evaporator and therefore its efficiency can be increased by at least 5 percent.
On-Line PI Controller Tuning Using Closed-Loop Setpoint Response