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GATE 2023 CH – Question 27

Heat Transfer · Heat exchangers and evaporators: types and design · 1 mark · Multiple choice

An isothermal jacketed continous stirred tank reactor (CSTR) operating at 150 °C is shown in the figure below. The cold feed entering the system at 30 °C is preheated to a temperature $T$ ($T <$ 150 °C) using a heat exchanger $HX_1$. This preheated feed is further heated to 150 °C using the utility heater $HX_2$. The mass flow rate and heat capacity are same for all the process streams, and the overall heat transfer coefficient is independent of temperature. Which one of the following statements is the CORRECT action to take if it is desired to increase the value of $T$ ?

the cold feed at 30 °C goes through the heat exchanger $HX_1$ (the hot fluid in it is the reactor effluent at 150 °C), leaves at $T$ °C, goes through the heater $HX_2$ and enters the CSTR at 150 °C.
  1. Increase both heat transfer area of $HX_1$ and heat duty of $HX_2$.
  2. Decrease both heat transfer area of $HX_1$ and heat duty of $HX_2$.
  3. Increase the heat transfer area of $HX_1$ and decrease the heat duty of $HX_2$.
  4. Decrease the heat transfer area of $HX_1$ and increase the heat duty of $HX_2$.

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Show answer and explanation

Correct answer: (C) Increase the heat transfer area of $HX_1$ and decrease the heat duty of $HX_2$.

Explanation

The preheater $HX_1$ uses the hot reactor effluent. A larger area for $HX_1$ transfers more heat, so the feed leaves at a higher temperature $T$. The heater $HX_2$ then has to supply only the rest of the heat up to 150 °C, so its duty falls. The correct action is to increase the area of $HX_1$ and decrease the duty of $HX_2$.