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

THE EVAPORATOR. 73 i .e., T07 lbs. of primary steam to obtain 1 lb. of gained steam (weight for weight). 14. From the above calculations it will be seen that the economy of working at various primary pressures is not very different, if none of the heat is wasted, or not accounted for. The variation of primary pressure, how- ever, makes an enormous difference in the time production. Economy of Low Pressures. 15. Up to the present, we have been treating of various primary pressures working with the same secondary pres- sure at or just above that of the atmosphere. This is undoubtedly the most convenient pressure inside the evaporator. But sometimes it is desired to reduce the pressure outside the coil to a minus pressure—i.e., some lbs. below the weight of atmosphere, so that when work- ing at what is called a vacuum, the sea water will boil at a lower temperature, and will do so rather more economically with a lower primary steam pressure than by evaporating the sea water at atmospheric pressure. 16. There is a saving of heat by working at low pres- sures—that is to say, it is more economical to work with a low primary pressure than with a high primary pressure, and, consequently, with a low secondary pressure than’a high secondary pressure. A larger amount of surface may be required, but that is not the point now ; economy is being regarded, and the question is how much. 17. If the two primary pressures, 10 lbs. and 25 lbs., be compared, the latent heat of the former being 946 B.T.U., and of the latter 927 B.T.U., the economy would appear to be simply—as 946 : 927 :: 100 : 98—viz., 2 per cent, gained in economy. But it must not be forgotten that