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

STEAM. 51 water will boil below 212°, and the specific volume of the steam will be correspondingly greater. There is clearly no vacuum in either case. Yet, if the vessel be closed, instead of open, and the water made to boil at the same low temperature as when boiling in an open vessel at an altitude, by drawing away the steam by the action of a condenser and air pump, the evaporator is said to work at a vacuum, whereas it is only working at a specific pressure below ordinary atmospheric pressure. In both cases the upper part of the vessel is entirely filled with steam, but in the one case the steam is denser than in the other—that is, its specific volume varies according as the load is added to or taken from off the water that is being evaporated, and the temperature corresponds with the pressure, whatever it may be. Note.—The boiling point at altitudes may be taken as 1° F. less than 212° for every 590 feet of ascent. 32. The object of this slight digression was only to make the subject of pressures above and below that of the atmosphere at ocean level easily understood, and it would be more convenient if all pressures were under- stood, as being above the weight of the atmosphere at the level of the ocean, unless denoted by a minus indica- tion, to be below atmospheric weight. There would then be no confusion of ideas as to the condition of things when pressures are above or below atmospheric pressure. The same rules equally apply in both cases, and calcu- lations with regard to sensible heat, latent heat, and total heat at different pressures are ascertainable by exactly the same process, and by reference to the Table E (p. 33) of the properties of saturated steam. 33. It will, therefore, be seen that as the pressure be- DANMARKS TEKNISKE BIBLIOTEK