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A balanced counter flow heat exchanger has a surface area of 20 m2 and overall heat transfer coefficient of 20 W/m2K. Air (cp = 1000 J/kgK) entering at 0.4 kg/s and 280 K is to be preheated by the air leaving the system at 0.4 kg/s and 300 K. The temperature (in K) of the preheated air is
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- 290
- 300
- 320
- 350
Correct Option: A
Counter flow heat exchanged
Surface Area A = 20 m2, mass flow rate = 0.4 kg/s
| u = | ; | m2 |
Temperature Tci = 280K
| Cp of air = 1000 | kg K |
Since m is same for both flow = 0.4 kg/s
Assume Cp is same = 100 J/kg.K
Hence ∆T1 = Tci – Tco = ∆T2 = Th0 – Tci
∆T1 = 300 – Tco = Th0 – 280
∆Tm = ∆T1 = ∆T2
uA∆Tm = mCp (Tco – Tci)
20 × 20 × (300 – Tco) = 0.4 × 1000(Tco – 280)
2Tco = 300 + 280
| Tco = | = 290 K | 2 |
