$($a$)$ What do you mean by humidification and dehumidification operations? Discuss their industrial applications.

$($b$)$ Partial pressure of water vapor in a mixture of airwater vapor at a total pressure of $106\mathrm{.}6$ kPa and a temperature of $50C$ is $12\mathrm{.}3$ kPa $.$ Express the concentration of water vapor in $($i$)$ absolute humidity, $($ii$)$ mole fraction $($iii$)$ relative humidity and $($iv$)$ g water $\frac{?}{m^3}$ mixture. Assume vapor pressure is $20\mathrm{.}6$ kPa at $60C.$

$5,10$

$($a$)$ Define the following terms : wet bulb temperature, humid heat and humid volume.

$($b$)$ The air supply for a drier has dry bulb temperature of $260$ C and a wet bulb temperature of $180$ C $.$ It is heated to $800$ C by heating coils and introduced into

the drier. In the drier it cools along the adiabatic cooling line and leaves the drier fully saturated:

$($i$)$ What is its initial and final humidity $?$

$($ii$)$ Calculate the dew point of initial air. How much water will be evaporated per $100m^3$ of entering air $?$

$3,12$

Unit III

$($a$)$ With neat diagrams, explain the working of a Spray dryer.

$($b$)$ Find an expression for the determination of total time of drying of a wet solid material under constant drying conditions to a final moisture content well below the critical moisture content.

$8,7$

$6.($a$)$ Explain the effect of operating parameters in drying during constant rate period.

$($b$)$ A woollen cloth is dried in a hot air dryer from initial moisture content of $100\%$ to final content of $10\%.$ If the critical moisture content is $55\%$ and the equilibrium moisture content is $6\%($at dryer condition$),$ calculate the saving in drying time if the material is dried to $16\%$ instead of $10\%.$ All other drying conditions remain the same. All the moisture contents are on the dry basis.

$5,10$