A Practical Manual On Sea Water Distillation
With A Description Of The Necessary Machinery For The Process

74 SEA WATER DISTILLATION. there is still heat in the primary water draining from the coils, which is generally cooled down to below 150°, so that the heat in such primary water (from the point at which it was converted into water) down to, say, 100°, is available for heating the feed from, say, 100°, and this should, therefore, be taken into account in a comparison of the economy. Note.—In practice the feed-water is not heated quite up to the boiling point, nor is the primary water dis- charged quite at the same temperature as the initial temperature of the feed (100°). 18. We will, therefore, compare three typical cases, as follows :— («) ( Primary pressure, 75 lbs. per square inch. I Secondary pressure, 25 lbs „ (&) J Primary pressure, 25 lbs. I Secondary pressure, 0 lb. (c) J Primary pressure, 10 lbs. I Secondary pressure, - 6 lbs. (12" of mercury). 1st. Take the secondary heat required. I n case (a) the secondary heat required is as follows :— 249 B.T.U. 927 „ For heating the feed-water from 100° to 266° = 166°, and as the quantity is 1| the evaporation, this would be 266 - 100 x 1| = 249 B.T.U. Latent heat to evaporate 1 lb. of water at 25 lbs. pressure. 1,176 Total heat required.