An efficient, technological solution for controlling the heat transfer performance of a heat exchanger is to install on process streams a bypass circuit, whose flow rate is determined in order to keep the outlet temperature of process fluids at target value. Control on outlet temperature is often necessary when working conditions undergo transients and modifications. In this manuscript, a calculation procedure for a heat exchanger and bypass equipment is described, along with relevant numerical model and solving technique. For developing the heat exchanger model in detail and validating the procedure against observed results, the specific case of a process waste heat boiler provided with a bypass is considered. The numerical model is based on mass, momentum and energy balance equations and is applied to a mono-dimensional and stationary type problem. The procedure can be conceptually extended to other specific applications and applied either to rating or design calculations. (C) 2011 Elsevier Ltd. All rights reserved.