11 Process Control CTR

CTR.1 FEEDBACK CONTROL

A heavy oil is heated by condensing steam in a shell-and-tube heat exchanger. Which sketch in Figure 34 contains a feedback control loop that will deliver the oil at the proper temperature by adjusting the amount of steam going to the exchanger? Note that TRC = temperature recorder and controller.

Solution

A control system or scheme is characterized by an output variable (e.g., temperature, pressure, liquid level, etc.) that is automatically controlled through the manipulation of inputs (input variables). Suppressing the influence of external disturbances on a process is the most common objective of a controller in a chemical plant. Such disturbances, which denote the effect that the external world has on a process, are usually out of reach of the human operator. Consequently, a control mechanism must be introduced that will make the proper changes on the process to cancel the negative impact that such disturbances may have on the desired operation of the process. Control engineers usually refer to the combination of a sensing element and a control device with a set point as a "control loop."

The central element in any control loop is the process to be controlled. Therefore, the control objectives must be defined (e.g., maintain a desired outlet temperature and/or composition, maintain the level in a tank at a certain height, etc.). Once the control objective is specified, variables are measured in order to monitor the operational performance of the process (sensing element). Next, the input variables that are to be manipulated are determined. Finally, after the control objectives, the possible measurements, and the available manipulated variables have been identified, the control configuration is defined. The control configuration is the information structure used to connect the available measurements to the available manipulated variables. The two general types of control configurations are feedback control and feedforward control. Details on feedback control are discussed below in this problem. Feedforward control is discussed in the next problem, CTR.2.

Feedback control, the general structure of which is illustrated in Figure 35, uses direct measurements of the controlled variable to adjust the values of the manipulated variable. The objective is to keep the controlled variable at a desired level (set point). In a feedback control loop, a measurement is made downstream of the ra

STEAM

CONDENSATE

STEAM

STEAM

CONDENSATE

STEAM

STEAM

CONDENSATE

D

IMPOSSIBLE

TO

SKETCH

Figure 34. Answers to Problem CTR. 1.

Disturbance

Manipulated Variable

PROCESS

Controlled Variable (Measured Output)

-1 CONTROLLER j"

Figure 35. Feedback control.

process being controlled in order to manipulate a variable and achieve the desired set point.

Since the controlled variable (the outlet oil temperature) in Figure 34A is being measured, this is a feedback control system. Since the disturbance (the inlet oil temperature) in Figure 34B is being measured, this is a feedforward control system and not a feedback control system. Since both the outlet and inlet oil temperatures in Figure 34C are being measured, this is not a feedback control system. Therefore, the correct answer is Figure 34A.

CTR.2 FEEDFORWARD CONTROL

Consider a tank's contents that are heated by steam in a coil. Which sketch in Figure 36 contains a feedforward control loop that will deliver the fluid at the proper temperature by adjusting the flowrate of the steam in the heating coil? Note once again that TRC = temperature recorder and controller.

Solution

Feedforward control, the general structure of which is illustrated in Figure 37, uses direct measurement of the disturbances to adjust the values of the manipulated variables. The objective is to keep the values of the controlled output variables at desired levels.

Feedforward control does not wait until the effect of the disturbances has been felt by the system but acts appropriately before the external disturbance affects the system by anticipating what its effect will be.

Since the controlled variable (the outlet fluid temperature) in Figure 36A is being measured, this is a feedback control system and not a feedforward control system. Since the disturbance (the inlet fluid temperature) in Figure 36B is being measured, this is a feedforward control system. Since both the outlet and inlet fluid temperatures in Figure 36C are being measured, this is not a feedforward control system. Therefore, the correct answer is Figure 36B.

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Renewable Energy Eco Friendly

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