Investigation on the fractional nature of a refrigeration evaporator

The frequency response of an evaporator for commercial refrigeration is investigated to develop a control-oriented model which balances simplicity with accuracy. The dynamics of such devices is highly nonlinear; the response curve was experimentally identified around an operating point by recording the output of the system for several sinusoidal input signals. Among the possible linear approximating models, the collected data turn out to be best fitted with a fractional order transfer function. The proposed identification and process model allow for online tuning at different operating points for an efficient control design.