DRYING PROBLEM NO6 -
Xc = 100 %
Xe = equilibrium moisture content = 15%
And drying curve is that .
Estimate total time today to dry up to 17%.
T =t = w ( x ) / f ( dw /dt ) constant
% Moist ure 0.5 0.4 0.3 0.2 0.18 0.17
Content ( x )
W ( x1 - x2 ) 2 2 2 0.4 0.3
F (f ) 0.87 0.57 0.28 0.11 0.08
1/ f ( dw /dt) 1.63 *10^2 2.46* 10^2 4.8*10^2 1.28 *10^2 2.81* 10^2
T ( t) 326 492 968 512 5620
∑ t = 326 + 492 + 968 +512 + 5620 = 7918 s = 7918 /60 *60 = 7918/3600 = 2.2 hr = falling rate period
Total drying time = 2.3 + 2.2 = 4.5 hrs
This shows -
1 . as the moisture level descends towards the equilibrium value , so the drying rate becomes more slow.
2. in mass transfer terms , humidity , humidity and partial pressure driving force tends to zero as equilibrium constant approaches .
3. in heat transfer terms , the surface temperature rises above the wet bulb surface ceases to behave as a wet surface - the surface temperature rises to dry bulb temperature and air as the moisture level falls. This leading to continuous diminishing temperature driving force for the surface heat transfer.