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Mass Transfer
Diffusion
GATE 2010
Medium

At 25°C and 90% relative humidity, water evaporates from the surface of a lake at the rate of 1.0 kg/m².h. The relative humidity that will lead to an evaporation rate of 3.0 kg/m².h, with other conditions remaining the same, is

  • A30%
  • B50%
  • C60%
  • 70%

Explanation

Evaporation rate ∝ Psat×(1−RH) (driving force = saturation vapor pressure minus actual partial pressure). 1.0=k·Psat×(1−0.9)=k·Psat×0.1. For rate=3.0: 3.0=k·Psat×(1−x). Ratio: 3=（1−x)/0.1 ⟹ 1−x=0.3 ⟹ x=0.7=70%.

Source: GATE CH 2010 (2010)

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