STABILIZING CONTROL OF HEAT EXCHANGERS

Abstract

This paper investigates stabilizing control of heat exchangers which are apt to be unstable for a simple feedback control law. The instability is caused by the time delay effect due to the pure delay time element and the right-hand side zeros in their transfer functions. It is shown that parallel type heat exchangers such as a parallel heat exchanger or one-shell two-pass heat exchangers may have these difficulties. A Smith type predictor which can be realized on a small micro-computer is derived to eliminate the time delay effect. A control system using it shows considerable improvement of stability and control performance. Because a heat exchanger has an irrational transfer function which does not express a pure time delay element explicitly, the dynamics of such a heat exchanger is approximated with a transfer function which consists of a pure time delay element and a rational transfer function to apply the predictor. By using an asymptotic approximation method, such approximate transfer functions are derived to extract a pure delay element.