A ‘‘cooling bag,’’ commonly used for storing water in hot, arid environments is made of a thin porous fabric. A small amount of water diffusesthroughthe fabric and evaporates from the surface of the bag. The rate of evaporation is controlled by convective mass transfer from the outer surface of the fabric to the surrounding dry air. The energy for evaporation is supplied by the surrounding hot air. The evaporation of the water cools the remaining liquid water within the bag and a temperature driving force is established. Determine the temperature of the ambient air using mass-transfer considerations, if the following values hold
A ‘‘cooling bag,’’ commonly used for storing water in hot,
arid environments is made of a thin porous fabric. A small amount
of water diffusesthroughthe fabric and evaporates from the surface
of the bag. The rate of evaporation is controlled by convective mass
transfer from the outer surface of the fabric to the surrounding dry
air. The energy for evaporation is supplied by the surrounding hot
air. The evaporation of the water cools the remaining liquid water
within the bag and a temperature driving force is established.
Determine the temperature of the ambient air using mass-transfer
considerations, if the following values hold:
surface temperature of the bag ¼ 298 K
heat capacity of air ¼ 1 J/g K
viscosity of air ¼ 1:84 10-
4 g/cm s
density of air ¼ 1:17 10-
3 g/cm3
thermal conductivity of air ¼ 2:62 10-
4 J/cm s K
mass diffusivity of water in air ¼ 3 10-
5 m2/s
latent heat of vaporization of ¼ 2:45 kJ/g
water at 293 K
vapor pressure of water at 293 K ¼ 1:3 103 Pa
bulk velocity of air stream ¼ 2:2 10-
1 m/s
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