DRYING PROBLEM NO4 - LEWIS RELATION SHIP IN AIR DRYING -
A value of mass transfer coefficient kg = 0.0150 kg /m2-s . it was also found that the corresponding heat transfer co efficient = hc = 15 j /m2-s
Hc = hc = 15 j/m2-s oc = 1000 * 0.015
Hc = kg ( mass transfer coefficient ) *1000
Mass transfer co efficient from a free water surface into air ( g -/m2-s ) = heat transfer coefficient from the air to the surface expressed in j/m2-soc.
Recalling that Lewis n0 = hc / Cs * Kg =1
( for air water system )
The Lewis no , which arises in gaseous diffusion theory = hc /kg. Cs , but for air of the humidity encountered in ordinary practice , Cs = Cp = 1.02 kj / kg oc , there fore ,numerically . if hc( heat transfer coefficient ) is in j/m2 -s -oc and kg ( mass transfer coefficient ) in kg /m2 -s ,
Kg ( mass transfer coefficient ) = hc /1000
Where hc = heat transfer coefficient
Kg = mass transfer co efficient
The value of hc predicted , using , the standard relationship for heat transfer co efficient is written above as Mass transfer coefficient ( kg ) = heat transfer coefficient ( hc )