Steam
Setpoint
Controller
Output
Process
Variable
TC
Process
Process Water In
Process Water Out
TT
TT = Temperature Transmitter

TC = Temperature Controller
Figure 8.1 Feedback control loop for heating process water with live
steam injection.
74 ◾  Distillation Control, Optimization, and Tuning
8.1 The Feedback Control Loop
Feedback controllers have been used for many years, starting
with speed control on windmills and then steam engines 1 and
now modern-day cruise control on automobiles. The concept
is to increase the flow of energy when the speed falls below
the desired value or reduce the flow of energy when the
speed is too high.
An example of a feedback control loop is shown in
Figure 8.1 for heating the process water with live steam injection. The temperature of the water leaving the process is
measured by a temperature sensor, and a transmitter sends
the signal to the controller. The desired temperature setpoint
is adjusted in the controller, and the difference between the
water temperature out of the process and the setpoint is called
the error. In this example, the process water temperature out
of the process is the controlled process variable, that is, controlled to a setpoint. The manipulated variable is the steam
flow rate, which is manipulated by the automatic valve that is
connected to the controller output.
The controller output is determined by the three controller
actions P, I, and D in response to the error and how it changes
with time.