Process control in heat exchangers (shell-and-tube): In a typical feedback/feedback–feedforward temperature-control scheme, which variable is normally chosen as the manipulated variable to meet a specified outlet temperature—the flow rate of the heating or cooling utility, the process load, or something else?

Introduction / Context:
In chemical and process industries, shell-and-tube heat exchangers are frequently operated under temperature control. The outlet temperature of the process stream is the controlled variable, while the utility flow (steam, hot oil, chilled water, brine) is most often used as the lever to achieve the setpoint. Understanding the role of a manipulated variable is fundamental to process control design and tuning.

Given Data / Assumptions:

• The exchanger duty can be adjusted by changing utility flow.
• Process disturbances (feed flow/composition/temperature) appear as load changes.
• Standard PID loop: controller output drives a control valve on utility supply or return.

Concept / Approach:

In control terminology, the controlled variable is the measured process output (here, the outlet temperature). The manipulated variable is what the controller changes to influence that output (commonly utility flow rate). The load encompasses unmeasured disturbances such as feed temperature swings. Choosing a manipulated variable that has a fast, strong, and safe influence on the controlled variable improves loop performance and robustness.

Step-by-Step Solution:

Verification / Alternative check:

Energy balance: Q = m * Cp * (Tout − Tin). Varying the utility flow changes the overall heat transfer coefficient times area and the mean temperature driving force, thus changing Q and stabilizing Tout. Plants universally implement this via a control valve on the utility—i.e., manipulation of utility flow.

Why Other Options Are Wrong:

Load: A disturbance, not something the controller purposely changes. Controlled: This is the target variable (temperature), not the handle. None of these: Incorrect because utility flow is the standard manipulated variable.

Common Pitfalls:

Confusing the process load with the manipulated variable; attempting to manipulate outlet temperature directly rather than the utility supply; overlooking actuator limits causing sluggish response.

Final Answer:

Manipulated