Why is evaporation not mass transfer operation?
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. On a very simplistic point, evaporation is generally due to mass transfer. But why?…answer is, every liquid having a free surface also posseses vapour dome just above its surface. The pressure here is the saturated pressure corresponding to its temperature (which can be thought of as the surface itself). If this pressure is greater than the partial pressure of the same liquid/atmosphere, evaporation proceeds. Hence no business of heat transfer…But
How this vapour zone got created? It is created by taking energy from surface itself. Hence without energy transfer vapour zone can not persist. Further, whenever the surface temperature enhances (may be due to heat transfer) two thing will happen
More amount of vapour will be attached to the corresponding liquid surface.
The saturated pressure for a higher temperature will be more as compared to pressure for lower temperature. (You can see that from T-S or p-v diagram)
Hence a hotter fluid having higher surface temperature will enhance the rate of evaporation, since higher pressure gradient is imposed because of elevation of surface temperature which enlarge the corresponding saturation pressure of vapour. Hence indirectly heat transfer may affect the physics of evaporation.
Another possibility is blowing air over the surface of liquid (preferably dry) will inflenece the evaporation and convective heat transfer may have a significant role to play.
There are many issues also but I have tried to illustrate as much as lucid physics as possible.
Thank You
How this vapour zone got created? It is created by taking energy from surface itself. Hence without energy transfer vapour zone can not persist. Further, whenever the surface temperature enhances (may be due to heat transfer) two thing will happen
More amount of vapour will be attached to the corresponding liquid surface.
The saturated pressure for a higher temperature will be more as compared to pressure for lower temperature. (You can see that from T-S or p-v diagram)
Hence a hotter fluid having higher surface temperature will enhance the rate of evaporation, since higher pressure gradient is imposed because of elevation of surface temperature which enlarge the corresponding saturation pressure of vapour. Hence indirectly heat transfer may affect the physics of evaporation.
Another possibility is blowing air over the surface of liquid (preferably dry) will inflenece the evaporation and convective heat transfer may have a significant role to play.
There are many issues also but I have tried to illustrate as much as lucid physics as possible.
Thank You
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