Fresh eggplant slices $(907$ kg$)$ at $87\%$ MCwb are dried in a tunnel dryer with air heated to

$80$oC$.$ The surface area available for heat and mass transfer is $1\mathrm{.}858$ m$2.$ The latent heat of

vaporisation is $2,257$ kJ$/$kg$.$ The fresh eggplant slices were placed on a weighing platform

and a thermocouple was placed on the surface of the eggplant slices to monitor their surface

temperature. The table below shows changes in the weight and surface temperature of

eggplant slices during hot air drying.

Table. Changes in the surface temperature and weight for eggplant slices during drying.

Time $($hr$)$ Weight $($kg$)$ Surface temperature $($oC$)$

$090768$

$189168$

$287568$

$386068$

$484468$

$582868$

$677870$

$777071$

The following formula is provided for your use:

where, Rc $=$ drying rate, kg$/$s

hc $=$ surface heat transfer coefficient for heating, W$/($m$2.$oC$)$

A $=$ surface area available for drying, m$2$

Lv $=$ latent heat of vaporisation, J$/$kg

Ta $=$ average dry bulb temperature of drying air, oC

Tw $=$ average wet bulb temperature of drying air, oC

$($a$)$ Determine the critical moisture content $(\%$CMC wet basis$).$

$($b$)$ Calculate the drying rate of constant rate period and the surface heat transfer

coefficient for heating $($in W$/($m$2.$oC$)).$