A triple$-$effect evaporator with backward feed is evaporating aqueous solution with negligible boiling$-$point rise and containing $10wt\%$ solids to $50\%$ solids. Fresh saturated steam at $1\mathrm{.}02$ atm abs $($corresponding to $120C$ of temperature$)$ is used for heating in the first effect. The pressure in the vapor space of the third effect is $10cmHg.$ At given pressure, the boiling point of the solution in the third effect is $51C.$ The feed rate $25000k\frac{g}{h}$ and the temperature $21C.$ The liquid heat capacity is $4\mathrm{.}18k\frac{J}{k}gC$ for all liquid concentrations. The heat$-$transfer coefficients are $U_l=2250,U_2=1750,$ and $U_3=1375$kca$\frac{l}{m^2}hC.$ Calculate

A$)$ The heating surface area,

B$)$ The mass flow rate of the steam,

C$)$ The temperatures drop in each effect

D$)$ The steam economy of system.