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

!8o SEA water distillation. Boiler Pressure. 21. To obtain, however, the advantage of multiple dis- tillation, the pressure of the initial steam—that is, the boiler steam pressure—must be increased or the final pressure be decreased, as each evaporator is added, and the intermediate pressures—that is to say, the pressures in the steam-room or casing of each evaporator—will follow in a gradually diminishing rate from the initial pressure to the final pressure. Thus, in Treble distilla- tion the intermediate pressure, between 75 lbs. of primary pressure in boiler, and zero (or thereabouts) in the 2nd evaporator casing, would be that corresponding to the pressure at the temperature about half-way between the initial and final temperatures of the steam. The tempera- ture of steam at 75 lbs. pressure is 320°, which is 108° above 212°, so that 54 units (or | of 108) would be about midway. Therefore, if 54 be taken off 320°, or 54 be added on to 212°, the result would be 266°, at which temperature we find steam would have a pressure of 25 lbs. per square inch; so that in treble distillation the primary pressure might be 75 lbs. primary pressure, 25 lbs. secondary, and the tertiary pressure zero or | lb. per square inch. Limitation of System. 22. Theoretically, the principle of multiple distillation may be carried on indefinitely ; but, as will be observed, the gain soon reaches proportions negligible in practice. 23. At each successive stage—that is, with every evap- orator added—the pressures must be increased or re- adjusted. The primary pressure must be increased, or the final pressure decreased, to suit the necessity of having the same difference in temperature between that