Feedback-feed-forward control scheme used in heat exchanger for temperature control

Feedback control

Feedback control is a control action of altering the manipulated variable in response to the deviation between the controlled variable and its set point in such a way to eliminate the deviation.

Temperature Control Using Feedback Control

Consider a process outlet temperature control problem in a furnace. The furnace is used to heat the process fluid stream and its outlet temperature is to be controlled. In feedback control, the outlet temperature is measured and it is compared against the reference set point. The deviation between the desired and measured temperatures is found. The flow rate of the fuel gas is adjusted in accordance with the magnitude of deviation by the controller through the control valve. The outlet stream temperature is kept at the desired value by controlling fuel gas flow rate

Limitations of feedback control

• Feedback control takes corrective action only after the disturbances has affected the process and generated an error.
• The feedback control corrects the output deviations by trial and error method which results in the oscillatory response of the feedback loop.
• Feedback control continues to change its output until the error is zero. There exists a finite delay between the detection of disturbance and the effect of control action on the controlled variable.
• Feedback control cannot achieve perfect control because its effectiveness is limited by the process response to continuous manipulation of the process variable.
• The inclusion of feedback control to a process affects stability, sensitivity and overall gain.

Feed forward control

Feed forward control provides a direct solution to the control problem by measuring the disturbances and compensating the controlled variable before it deviates from the set point. Feedforward control or Open loop control is one of the basic automatic control strategies.

Process Fluid Temperature Control with Feedforward Control

Consider the process outlet temperature control problem in a furnace. Here, the inlet flow rate of the process fluid is the major disturbance. To implement the feedforward control, the major disturbance is first. Depending on the measurement, he feedforward controller decides how to manipulate the control valve for fuel gas in order to maintain the controlled variable at set point. In this example, the inlet temperature of the process fluid can also be included as a next major disturbance.

Advantages of Feedforward Control

• Feedforward control takes corrective action before the disturbances entering into the process.
• The feedforward control does not affect the stability of the system. A stable system without feedforward control would remain stable even after including it.
• It works well for multi-capacity systems or with significant dead time.

Limitations of Feedforward Control

• The concept of feedforward control demands the identification and direct measurement of all possible disturbances. This is not possible for many processes. Also, it cannot deal with unmeasured or unexpected disturbances.
• The effectiveness of feedforward control depends on a good design of process model. Perfect process model is not practically possible for many systems.
• The feedforward controller requires sophisticated instruments and control equipments making it expensive.
• The inability to prepare perfect model and take accurate measurements would make the feedforward control accumulate errors. The accumulation of errors would self-destruct the control mechanism.
• The process parameter changes (like catalyst deactivation, reduction of a heat transfer coefficient) cannot be compensated by the feedforward controller.