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

44 SEA WATER DISTILLATION. primary water, thus resulting, is measured against the secondary water resulting from the condensation of the secondary steam, we have at once the proportion of the one to the other, which represents the economy of the process ; because the primary steam was necessarily obtained by the agency of fuel (say coal). One has really, therefore, to consider the weight of coal or other fuel required first to obtain a sufficiency of primary steam to produce the secondary steam. Such secondary steam is consequently termed (as its name would imply) gained steam—i.e., extra to the primary steam directly obtained by the consumption of coal or other fuel. Compound Distillation. 20. From the foregoing explanation, it will be gathered that the relationship between primary and secondary steam is such that by raising the pressure of the primary steam, and then getting a pressure on the secondary steam, this new steam may in its turn be used as prim- ary steam to another evaporator, working on the same lines, and thus be the means of obtaining a further supply of gained steam, which will also be obtained for the same consumption of fuel, because we have seen that it takes practically the same amount of heat—that is, fuel con- sumed—to generate steam at any pressure. Note.—The pursuance of this matter leads to an ex- planation of multiple distillation, which is best dealt with läter on under the title of “ Multiple Distillation.” The Heat of Primary Steam at Varying Pressures. 21. We have now to compare the effect of different pressures of primary steam, supplied to the interior of a coil, all other conditions being the same. The pressures