Project ForgeHeat Transfer
Heat Exchangers
GATE 2011
Hard
Oil at 120°C heats water at 30°C in a 1-1 co-current shell and tube heat exchanger with area S1. Exit temperatures are 90°C (oil) and 60°C (water). The co-current exchanger is replaced by a 1-1 counter-current exchanger with area S2, same exit temperatures and same U. The ratio of S1 to S2 is
- A∞
- 1.1
- C0.91
- D0
Explanation
Co-current: ΔT1=120−30=90, ΔT2=90−60=30. LMTD_co=(90−30)/ln3=60/1.0986≈54.6. Counter-current: ΔT1=120−60=60, ΔT2=90−30=60 (equal), so LMTD_counter=60 exactly. S1/S2=LMTD_counter/LMTD_co=60/54.6≈1.1 (since same Q and U, area is inversely proportional to LMTD).
Source: GATE CH 2011 (2011)
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