The most important advantage of the feedback control scheme is that regardless of the disturbance source, the controller will take corrective action.
In the feedback control scheme, the process variable, T1 Out, is measured and applied to a proportional-integral-derivative (PID)-based feedback temperature controller (fbTC), which compares the process variable with the desired temperature setpoint and in turn calculates and generates the control action required to open or close the steam control valve (see Figure 4). The control objective is to maintain process fluid outlet temperature T1 Out at the desired setpoint-regardless of disturbances-by manipulating the steam flow rate F2. Changes in steam pressure, causing a change in steam flow rate, F2.Changes in process fluid inlet temperature, T1 In.Three major disturbances can affect the process fluid outlet temperature: Control of the steam flow F2 (manipulated variable) is achieved by throttling a modulating valve installed on the steam inlet side. The more steam passing through the tubes, the more heat is transferred to the process fluid, and vice versa. Heating is achieved by passing steam through the tube side. The resulting temperature is measured at the outlet of the heat exchanger T1 Out (controlled variable). The shell side fluid is the process fluid that is required to be heated to a certain temperature setpoint. To develop a comprehensive control strategy for any control loop, it’s important to identify the process variable of interest-called the "controlled variable," the manipulated variable, and the different disturbance variables that directly affect the controlled variable.Ĭonsider the heat exchanger shown in Figure 3. To transfer heat efficiently, many tubes are used, which increases the heat-transfer surface area between the two fluids. Fluids can be liquids or gases on either the shell or the tube side. Heat is transferred from the tube fluid to shell fluid to remove heat, or from the shell fluid to the tube fluid to heat the material inside. Heat is transferred from one fluid to the other through the walls of the tubes.