Feedback control is a control method in which a measured value of a process variable is compared with its set point, and action us taken to correct any error.
The illustration of a exchange process in Fig 4.2.1 can be used to explain manual feedback control.
In this process, unheated water is supplied to a tank to keep the tank full; steam flows through coil in the tank and heats the water and the hot water is drawn out to supply another process.
The amount of steam that flows through the coil is regulated to keep the water temperature at its set point.
The process variable being controlled is the temperature of the water.
In order to keep the water temperature at its set point, the flow of steam can be increased or decreased.