The figure below shows a heat exchanger with no phase change. The "Process in" stream comes from upstream units and you can only measure its flow (Fi) and temperature (Ti) but you cannot change these values. CTS is the cooling water from the cooling tower and CTR is the return of the cooling water to the cooling tower. You can readjust the flow (Fw) of the cooling water if needed. The control objective is to keep the "Process out" stream at a target temperature Ts. (a) Considering the variables Fi, Ti, Fo, To and Fw; What is/are the disturbance/s and the manipulated variable/s? (b) Show how you can control this heat exchanger in order to meet the control objective