$(20)$ A sodium hydroxide solution with $20\%$ content will be concentrated to $50\%$ by a single$-$effect

evaporator. The feeding flow rate is $9070kg$ per hour with a temperature of $37\mathrm{.}8C.$ The gauge pressure

of the steam is to be $138$kPa $(1\mathrm{.}38$ bar$)$; the absolute pressure in the vapor space is to be

bar$).$ The overall coefficient is $1400\frac{W}{m^2}*C.$ By using the D$$hring lines for sodium hydroxide solution

given in Figure $1$ and steam table to find boiling point of water under $13$kPa $(1\mathrm{.}38$ bar$),$ heat of

vaporization of steam under $138$kPa $(\frac{0\mathrm{.}133}{b}ar).$

a$.(10)$ The consumed amount of steam

b$.(5)$ The economy

c$.(5)$ The required heating surface.

Figure $1$ D$$hring lines for sodium hydroxide $($Temperature is given in $F)$