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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/ kgK and 2 kJ/kgK, respectively. The overall heat transfer coefficient is 400 W/m2K. The required heat transfer surface area (in m2) is
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- 0.104
- 0.022
- 10.4
- 21.84
Correct Option: D
Heat exchanger is a counterflow type.
∆Ti = 120 – 80 = 40°C (θ1)
∆Te = 60 – 40 = 20°C (θ2)
| LMTD = | |||||
| In |  |  | |||
| θ2 | |||||
| = | = 28.86°C | ||||
| In | | | |||
| 20 | |||||
Q = mc Cpc(Tce - Tci)
= 1.5 × 4.2 × 103 (80 – 40) watt
Q = 252000 W
Q = UA∆Tm .
| A = | = | |||
| U∆Tm | 400 × 28.86 |
= 21.83 m2.
