$5.$ Air having DBT $=12$C$,$ WBT $=8\mathrm{.}5$C$,$ is heated to $43$C before being passed over trays of wet solid in a $2-$pass tray drier. The air leaves the first pass at $45\%$ saturation and is reheated to $45$C before passing over trays in the second pass, from where it finally exhausts at $60\%$ saturation. The volume of exit air is $8\mathrm{.}5$ m$3/$s$.$

Assuming the dryer operation is at constant enthalpy,

a$)$ Estimate the surface temperature of the wet solid in each pass,

b$)$ What is the dew point of the air after each pass?

c$)$ Calculate the mass of water evaporated per hour

d$)$ If the operation is to be completed in a single pass, what will be the required inlet air temperature?