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GATE 2022 CH – Question 22

Instrumentation and Process Control · Controller modes, control valves and tuning · 1 mark · Multiple choice

Liquid flowing through a heat exchanger (HX) is heated. A bypass stream is provided to control the temperature of the heated exit stream. From the given plumbing options, the one that provides the most effective temperature control for large disturbances while avoiding vaporization in the heat exchanger is

four plumbing layouts A to D. Each has a pump, a heat exchanger HX and a bypass line with a valve. In A and B there is also a valve upstream of the exchanger (at the entry, or in the branch to it), in C there is a valve in the exit of the exchanger branch before the bypass joins, and in D a valve in the line after the bypass has rejoined.
  1. Option A in the figure
  2. Option B in the figure
  3. Option C in the figure
  4. Option D in the figure

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Show answer and explanation

Correct answer: (C) Option C in the figure

Explanation

Effective control needs valves in both branches, so that the flow can be split between the exchanger and the bypass. To avoid vaporisation in the exchanger, its pressure must stay high, so the valve in the exchanger branch has to be on the outlet side of the exchanger. Layout C has this, and it also has the bypass valve. In D the valve is after the two streams join, so it does not split the flow, and in A and B the valve is upstream of the exchanger and lowers its pressure.