Project ForgeHeat Transfer
Convection & Radiation
GATE 2017
Hard
In a counter-flow heat exchanger, water is heated at the rate of 1.5 kg/s from 40°C to 80°C by an oil entering at 120°C and leaving at 60°C. The specific heats of water and oil are 4.2 kJ/kg·K and 2 kJ/kg·K, respectively. The overall heat transfer coefficient is 400 W/m²·K. The required heat transfer surface area (in m²) is
- A0.104
- B0.022
- C10.4
- 21.84
Explanation
Q = ṁwater·Cp,water·ΔT = 1.5×4.2×(80−40) = 252 kW. Counter-flow LMTD: ΔT1=120−80=40°C, ΔT2=60−40=20°C. LMTD=(40−20)/ln(40/20)=20/ln2≈28.85°C. A = Q/(U·LMTD) = 252000/(400×28.85) ≈ 21.84 m².
Source: GATE ME 2017 Session 2 (ME-2) (2017)
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