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Heat Transfer
Heat Exchangers
GATE 2009
Hard

A double-pipe heat exchanger heats 4 kg/s of a cold feed from 20 to 40°C using a hot stream available at 160°C and a flow rate of 1kg/s. The two streams have equal specific heat capacities. Then the ratio of the heat transfer areas required for co-current to counter-current modes of operation is

  • A0.73
  • B0.92
  • 1.085
  • D1.25

Explanation

Q=4×Cp×20=80Cp=1×Cp×(160−Th,out) ⟹ Th,out=80°C. Co-current: ΔT1=140, ΔT2=40, LMTD=(140−40)/ln3.5≈79.8. Counter-current: ΔT1=120, ΔT2=60, LMTD=(120−60)/ln2≈86.6. Area ratio (co/counter, since area∝1/LMTD)=86.6/79.8≈1.085.

Source: GATE CH 2009 (2009)

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