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

70 SEA WATER DISTILLATION. 10. Economy is, therefore, a totally different matter from the evaporative power of an evaporator as against time, which we have just been considering with regard to the amount of heating surface required. Although large evaporators are naturally rather more economical than small ones, as relatively less heat is wasted, the primary steam required for a large production of secondary steam, or a small one, is proportionately the same. Thus, if it takes, say, 12 lbs. of primary steam to obtain 1 lb. of secondary steam (irrespective of the time taken in the operation), it will take 1*2 tons of primary steam to pro- duce 1 ton of secondary steam, no matter what may be the amount of heating surface in operation, provided the type of apparatus and all the other conditions of working are kept to. Note.—In specifying the economy of multiple distilling machinery, where the proportion of distilled water to coal consumption is specified, it is usual to state the economy as being so many lbs. (or tons) of water per 1 lb. (or ton) of coal. 11. Thus, take the three cases before referred to, and first take the case where the primary pressure is at 10 lbs. per square inch. Every lb. of the Ij tons of feed-water will have to be heated from, say, 100° to 212°, and then f of the 1| tons (i.e., 1 ton) will have to be converted into steam. This will, therefore, require heat as follows :— 168 units of sensible heat to heat up the feed-water (212 - 100 X = 168). 966 units of latent heat for evaporation. 1,134 B.T.U. (total) to be found (per lb. evaporated).