An Investigation Into the Dynamics and Control of a Liquid Heat Exchanger

Abstract

Parametric transfer-function matrices are derived completely analytically for the symmetrically operated liquid heat exchanger to provide a basis for the assessment of approximate models obtained by joining low-and high frequency asymptotic behaviour. The complex form of transfer function, make them inconvenient, to use for simulation and conventional control system design. This paper therefore addresses itself mainly to the problem of finding simple and suitable analytical transfer function models to represent the heat exchanger dynamics.It is shown that, whereas multivariable first order models readily produce excellent approximation to the low and fairly high frequency behaviours of the heat exchanger, the travelling wave phenomena can nevertheless give rise to instability when high performance proportional plus integral controllers are employed. To overcome the problem of failure to predict the travelling-wave effect, the fundamental idea of matching high - and low frequency asymptotic behaviour is developed beyond the level of first order lag approximation to produce a single and multivariable lag-delay model structure for the process. Such structures are shown to be derivable with almost the same facility as the multivariable lag models and to reproduce the above mentioned resonance effects very closely indeed.The lag-delay model is verified by using it to design very satisfactory feed-forward controllers.