Modeling and Control of a Refrigerant Evaporator

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 behaviour of the evaporator is investigated as a function of the manipulating signal UEV (position of the expansion valve) and various disturbances (air temperature ¿A, condenser pressure PCd and compressor rotation speed nC), considering the superheating temperature ¿s as control variable and the evaporator performance QE, which has to be optimized. Two controllers are considered. First, the control behaviour 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 efficiency of the evaporator can be increased by at least 5%.