Question

PCT3601 OCT/NOV 2018 QUESTION 1 Your company is designing a plant to produce biodiesel Several of the unit processos require heated water that is under tight temperature control The desired temperature of the water is 65°C: 1'C To accomplish this, steam will be used in a shell and tube heat exchanger to heat the incoming water A member on the project team suggests the following feed forward control system to regulate the temperature of the water stream Temperature and flow rate data for the incoming steam and water streams will be used along with energy balance calculations to determine the flow rate of steam required to keep the exiting water stream at the appropriate temperature Controller Shell and Tube heat exchanger Torestos Steam out Figure 1 You on the other hand suggest a feedback control system a) Place the appropriate sensors, actuators, and controllers to implement a feedback control system that will allow for the control of the exting water temperature Draw and discuss your suggested feedback control system of the above process (12) b) What are some of the advantages and disadvantages of using a feedback control system over a feed forward control system? [20 marks] [TURN OVERI

Answer 1

(a) Please follow the image of Shell & Tube HE feed backward control scheme:

flow controller -- temperature controller (primary te control value Temperatul tremwitter water in > Waterat Condenlate out

The outlet water temperature will be measured by Temperature transmitter & will be controlled by temperature controller (primary controller) if it is less than the set point (65C) then control valve will open slowly to increase the steam flow & if it is more then control valve will close slowly to reduce the steam flow which will measured by Flow transmitter & controlled by Flow controller (secondary controller).

(b) Advantages of Feedback control:

1.Even if input adjustments failed to produce correct output, feedback control can identify the error and do adjustments gradually to reach the set point. In Feed forward control, If the input is failed to produce correct output, then the process will continue to produce the wrong output.

Disadvantages of Feedback control:

1.Feedback control takes corrective action only after the disturbances has affected the process and generated an error whereas Feed forward control takes corrective action before the disturbances entering into the process.

2.The feedback control affects stability of the system because of its sensitivity and overall gain. The feed forward control does not affect the stability of the system. A stable system without feed forward control would remain stable even after including it.