Objectives

1. To demonstrate the application of a Low Select (LS) override control strategy for protecting a heat exchanger tube from overheating due to excessive heating.
   
2. To develop a MATLAB Simulink model for the LS control strategy and evaluate its performance under different process variations.

Process Information

A process fluid is heated using steam in a heat exchanger.

If the process fluid flow rate suddenly increases, there will be a large drop in the process fluid temperature $T$. To bring the temperature back to its setpoint, the heat transfer to the process fluid must be increased. Since $U$ (overall heat transfer coefficient) and $A$ (heat exchanger area) are fixed, the only way to increase $Q$ is by increasing $T_s$ (tube skin temperature).

However, if $T_s$ rises above a maximum allowable limit (depending on the tube material), the tube may be damaged due to overheating. To prevent this, an override control strategy is used, where the LS block selects the lower of two control signals, one from the process fluid temperature controller and one from the tube skin temperature controller, to prevent overheating.

Figure 1: Heat transfer in a heat exchanger

Process fluid temperature transfer function

$$\begin{array}{}\text{(1)}& G_p(s)=\frac{4\exp (-2s)}{7s+1}\end{array}$$

Heat exchanger tube temperature transfer function

$$\begin{array}{}\text{(2)}& G_e(s)=\frac{3\exp (-1.5s)}{6s+1}\end{array}$$

At steady-state:

• Process fluid temperature = 300°C
  
• Tube skin temperature = 350°C
  
• Maximum allowable tube skin temperature = 400°C

Methodology

1. Model Development
   • Create a MATLAB Simulink model for the LS control strategy.
     
   • Implement two controllers:
     • Process fluid temperature controller
     • Tube skin temperature controller
       
   • The LS block selects the lower output to prevent overheating.
2. Simulation Cases
   • Nominal operation: Observe the system response to setpoint changes in process fluid temperature.
   • High steam flow rate: Simulate a rapid increase in steam flow rate causing potential overheating.
   • Process gain variation: Change the process gain of $G_e(s)$ by +10% and -10%.
   • Process time constant variation: Change the time constant of $G_p(s)$ by +10% and -10%.

Report Format

Your report (5 pages maximum) should include the following:

0. Submission Details

Include a brief table at the beginning of the report with the following information:

Lab Title:	Lab 03 - Override Control	Student Name	ID
Unit:	CHEN4011	Student 1	12345678
Date:	12 August 2025	Student 2	87654321

1. Objective & Problem Statement

   Briefly describe the LS control strategy and its purpose in preventing heat exchanger damage.

2. Methodology & Implementation

   • Provide a Simulink diagram of the LS control strategy.
   • Explain the roles of both temperature controllers.
   • Describe how the LS block operates in the model.

3. Results

   • Show the system responses for:
     • Setpoint tracking
     • Tube skin temperature protection under excessive heating
   • Include well-labeled plots for:
     • Nominal operation
     • High steam flow rate disturbance
     • Process gain ±10% for $G_e(s)$
     • Process time constant ±10% for $G_p(s)$
   • Summarize relevant performance metrics (IAE, overshoot, settling time).

4. Analysis and Discussion

   Address the following:

   • Under what conditions does the LS controller select the tube skin temperature controller’s output?
   • How does a ±10% change in the gain of $G_e(s)$ affect performance and protection?
   • How does a ±10% change in the time constant of $G_p(s)$ affect system performance?
   • Quantitatively compare performance metrics between nominal and altered conditions.

5. Conclusion

   • Summarize your findings on LS override control effectiveness.
   • Discuss practical applications in industrial safety control.

Assessment Rubric (20 Marks Total)

No	Section	Marks	Evaluation basis
1.	Objectives & Problem	2	Clarity of problem definition; articulation of objectives
2.	Methodology and Implementation	4	Correctness and clarity of Simulink model; explanation of LS control strategy
3.	Results	4	Quality, relevance, and labeling of plots; completeness of performance data
4.	Analysis and Discussion	6	Insightful interpretation; robustness; impact of process variations
5.	Conclusion and Presentation	4	Coherent summary; quality of writing, formatting, and visual presentation