Water flows and heats across a series of four tanks as illustrated. To design a cascade control system for regulating the exit temperature of tank 4 amidst disturbances in a secondary stream entering tank 2, where is the optimal location for the secondary loop measurement?
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Answer: C. Tank 3
Explanation
In cascade control schemes, the secondary measurement loop must be positioned close to the primary disturbance source. Placing the secondary sensor in tank 3 intercepts upstream thermal disturbances before they severely impact tank 4. This configuration allows the secondary controller to take rapid corrective action long before the upset reaches the final exit. Cascade control utilizes a secondary inner control loop to improve control performance against intermediate process disturbances. Multi-tank fluid systems exhibit distributed thermal capacitances that transmit temperature upsets sequentially from tank to tank. Upstream secondary measurements significantly reduce the effective time constant experienced by the primary control loop.
Tank 1 is located too far upstream, making its measurement insensitive to disturbances entering specifically through the secondary stream. Measuring in tank 2 places the secondary loop directly at the disturbance source, but tank 3 provides a better intermediate cascade balance. Tank 4 serves as the primary controlled variable location, which must remain at the outer master controller output.
- Subject
- Engineering
- Category
- Chemical Engineering
- Topic
- —
- Difficulty
- medium
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