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 (120.5°C). A pressure of 102 mm Hg abs is to be maintained in the vapor space. The feed rate to the evaporator is 20,000 kg/h. The overall heat-transfer coefficient can be taken as 2,800 W/m²·°C. 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. °C, and the latent heat of vaporization of the solution may be taken equal to that of water. Radiation losses may be neglected.