Example of a Proportional Process Control System:

Above figure describes a process system using a proportional temperature controller for giving hot water.

Figure: Proportional Temperature Control System

Steam is admitted to the heat exchanger to increase the temperature of the cold water supply.  A temperature detector monitors the hot water outlet and generates a 3 to 15 psi output signal which represents a controlled variable range of 100^o to 300^oF.  A controller compares the measured variable signal along with the setpoint and sends a 3 to 15 psi output to the last control element that is a 3-in control valve.

The controller has been set for a proportional band of 50 percent. Thus, a 50 percent change in the 200^oF span, or a change of 100^oF, causes a 100 percent controller outcome change.