A process involving heat exchange that employs heat exchangers where the process material flows over a heat transfer surface in which a process material passes through the process conduit of a heat exchanger wherein the desired velocity profile of the process material passing through the process conduit is achieved by varying the cross sectional area of the process conduit in 3 or more locations along the path length of the process conduit which allows better regulation of heating or cooling within the system which, for some processes, will result in improved temperature control.