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

50 SEA WATER DISTILLATION. Compound Distillation. 30. The above observations have been confined to a constant pressure (that of the atmosphere) outside of the coils. It will, however, be found, when we consider com- pound or multiple distillation, that the rules above given are equally applicable when the outside pressure is varied. Pressures below Atmospheric Pressure. 31. Hitherto secondary pressures, more or less low, have been dealt with. It is true, they have been very low, but they have been at or just above atmospheric pressure. It is, however, possible to evaporate at what is called a vacuum or minus pressure, often described as so many inches of mercury—that is, taking the height of 30 inches of mercury as counterbalancing the atmosphere. As atmospheric pressure is equal to 14’7 lbs. per square inch, every | lb. per square inch below the atmosphere would, therefore, equal about 1 inch in a column of mercury. Table E., on p. 33 (in the first column) gives the pro- perties of steam at such negative pressures. Note.—The term “ vacuum ” is frequently used as being any pressure below that of the atmosphere at ocean level. It would be less confusing if a pressure which is beloio that of ordinary atmospheric pressure were referred to by another name than that of a vacuum. Jf water is converted into steam in an open vessel at ocean level, the evaporation will take place at what is called atmo- spheric pressure, so that the temperature of the water when boiling will be at 212°, and the specific volume of the steam will be 1,643. If, however, that open vessel be taken to a considerable altitude above ocean level, the