This chapter discusses the control and optimization of each of these systems. While an increase in load tends to increase the heat-transfer efficiency by increasing the Reynolds number, the overall effect is a drop because the fixed area of the exchanger makes the transfer of heat less efficient as the load rises. The variation in dead time can sometimes be minimized by using recirculation to keep the flow through the exchanger constant. In the majority of installations, a three-mode controller would be used for heat exchanger service. The derivative or rate action becomes essential in long time-lag systems or when sudden changes in heat exchanger throughput are expected. For best performance, both the dead time and the time constant of the temperature sensor should be minimized. The dead time of the sensor is minimized by locating it close to the outlet of the heat exchanger.