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GATE 2017 ME (ME2) – Question 56

Heat Transfer · Heat exchangers: LMTD and NTU methods · 2 marks · Multiple choice

In a counterflow exchanger, water at 1.5 kg/s heats from 40 to 80°C. Oil enters at 120°C and leaves at 60°C. Water and oil specific heats are 4.2 and 2 kJ/kg K; overall heat-transfer coefficient is 400 W/m² K. Required area is ______ m².

  1. 0.104
  2. 0.022
  3. 10.4
  4. 21.84

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Correct answer: (D) 21.84

Explanation

Heat duty is1.5·4200·(80−40)=252000W. Counterflow terminal differences are120−80=40K and60−40=20K. LMTD=(40−20)/ln(40/20)=28.854K. Required area isQ/(U·LMTD)=252000/(400·28.854)=**21.84m², D**.