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

52 SEA WATER DISTILLATION. comes lower, over the water from which the secondary steam is being evaporated, the lower may be the pressure of the primary steam to generate it. Thus with, say, the utmost minus pressure that can be obtained (even down to a vacuum, when water will boil at below 80° F.), if the water outside the coil is made to boil by a suitable pres- sure of steam inside the coil, there is no reason for not carrying the system of evaporation by steam heat thus far—that is, if it can be done advantageously in practice. The subject is, however, better dealt with in Chapter vi., which explains the evaporator in detail. Some evapora- tors are arranged for a primary pressure of, say, 6 to 8 lbs. per square inch with a minus pressure indicated by 8 to 10 inches for the secondary pressure, whilst in others the primary pressure is less, say, 4 to 6 lbs. per square inch, and a correspondingly lower pressure for the generating of the secondary steam outside the coil. This will be referred to again later on. 34. Having treated of “ Steam ” and its properties, generally, before dealing with the evaporator, which both uses and generates more of it, it is well first to treat of “ Fuel ” by which the initial heat is obtained to produce the primary steam.