Question

Manipulated variable in water heater is:

• A. Flow rate of water
• B. Temperature of outlet water
• C. Heat input
• D. Inlet temperature of water

Hint:

To easily identify the manipulated variable, ask yourself: *"What parameter is directly adjusted by the physical final control element (like a control valve or electrical relay) to maintain stability?"* In a heater, it is always the heat source input.

Answer 1

The Correct Option is C

Concept: In process control architecture, we classify process variables into distinct functional categories:

• Controlled Variable (CV): The variable we want to maintain at a specific value (setpoint).

• Manipulated Variable (MV): The variable that the controller adjusts or varies to counter deviations in the process.

• Disturbance Variable (DV): Uncontrolled inputs that upset the system stability.

Step 1: Identifying the parameters of a water heating system.
Let's analyze a standard continuous water heater loop:

• Our core goal is to keep the exiting water at a comfortable, specified temperature. Thus, the

outlet water temperature is the Controlled Variable.

• Sudden changes in incoming water properties like the

inlet temperature of water or the

flow rate of water act as Disturbance Variables because they unpredictably change the thermal load.

• To fix any drop or spike in outlet temperature, the control valve or heating element alters the steam flow or electrical power supplied to the system. Therefore, the

heat input (or fuel/steam flow rate providing that heat) is the explicitly altered parameter, making it the Manipulated Variable.