GATE 2020 ME (ME1) – Question 35
In a concentric-tube counterflow heat exchanger, hot oil enters at 102°C and leaves at 65°C. Cold water enters at 25°C and leaves at 42°C. Find the log mean temperature difference (°C), to one decimal.
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Correct answer: 48.8 to 49.8
Explanation
The counterflow terminal differences are ΔT₁=102−42=60°C and ΔT₂=65−25=40°C. Therefore $\Delta T_{lm}=(60-40)/\ln(60/40)=49.3261$°C, or **49.3°C**. Pair opposite ends, not both inlet temperatures.