It is desired to concentrate a NaOH solution from 12.5 % to 40% in a double effect evaporator. 50,000 lb/hr of feed solution enters the evaporator at 120 ^oF, and steam at 15 psig is available for effecting the concentration. Vacuum in the 2nd effect is held at 24in. Hg, and water from a cooling tower is available at 85 ^oF. Estimate

(a) quantity of feed and water needs for the forward feed operation,

(b) quantity of feed and condenser water need for the backward feed operation,

(c) Surface need for the operation in (a), suppose U is 400 and 250 Btu/hr(ft2) (^oF) for the first and second effect respectively. UD equal to 450 and 350 in two effects correspondingly.