Drying time for a batch dryer. A slab with a wet weight of $18kg$ originally contains $65\%$ moisture $($wet basis$).$ The slab is $1100$ by $700$ by $80mm$ thick. The equilibrium moisture content is $6\mathrm{.}5\%$of the total weight when in contact with air of $20C$ and $20\%$ humidity. Experiments have determined that the drying rate is constant at $4\mathrm{.}75k\frac{g}{m^2}$ hr until the total weight reaches $7\mathrm{.}2kg$ when in contact with air of the above conditions at a constant velocity.

Drying is from both faces. It may be assumed that the drying rate is proportional to the free water content of the slab during the falling rate period. Determine the total drying time for to reduce the moisture in the slab to

$10\%($wet basis$).$ How long would it take if the air rate were to be increased by $50\%?$