A solution of organic colloids in water is to be concentrated from 8 to 45 percent solids in a single-effect evaporator. Steam is available at a gauge pressure of 1.03 atm (120SC). A pressure of 102 mm Hg ab, is to be maintained in the vapor space: this corresponds to a boiling point for water of 51.7°C. The feed rate to the evaporator is 20,000 kgfh. The overall heat -transfer coefficient can be taken as 2800 W/m2_ cc. The solution has a negligible elevation in boiling point and a negligible heat of dilution. Calculate the steam consumption, the economy. and the heating surface required if the temperature of the feed is

(a) 51.7°C,

(b) 21.1'C,

(c) 93.3°C.

The specific heat of the feed solution is 3.77 J/g_OC, and the latent heat of vaporization of the solution may be taken equal to that of water. Radiation losses may be neglected.