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

232 SEA WATER DISTILLATION. the burning mass, so that all this radiant heat is utilised. (Some experts say 70 per cent, of the heat is thus imported.) The remaining 50 per'cent. of the heat given off is trans- ferred to the air admitted to the furnace to support com- bustion. This latter heat, therefore, passes through the flues, and what is not absorbed passes away, and is lost up the chimney. Hence it is necessary for these flues to be long enough to abstract as much heat as possible before the chimney is reached, but not too long, as it entails extra length of boiler, and a corresponding loss of heat by radiation from its exterior surface. A correct balance between the two extremes is gained by experience only. A properly proportioned boiler is essential to an economical distilling apparatus. If, therefore, coal is taken as capable of imparting, say, 13,000 B.T.U. per lb., 6,500 B.T.U. (i.e., 50 per cent.) will be directly imparted as radiant heat, and none of this will be wasted; but the balance, 6,500 (i.e., the remaining 50 per cent.), which has to travel along the flues, will have a large proportion of unused heat when he chimney is reached. Thus, if in practice, only 10,000 B.T.U. are used out of the 13,000 B.T.U., which is approximately the heat required to evaporate 10 lbs. of water (see p. 66), it would show that the remaining 3,000 B.T.U. (13,000-10,000 = 3,000) are lost by radia- tion or gone up the chimney; i.e., instead of the balance 6,500 B.T.U. being also utilised, only 3,500 B.T.U. are used, the remaining 3,000 B.T.U. being wasted. Hence the necessity of a properly proportioned boiler to minimise the loss inevitably caused, either by heat uselessly going up the chimney or by radiation, by the boiler being too long to effectively retrieve any part of such loss.