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

THE EVAPORATOR. 8l paring the effect of the three primary pressures, let us first take the primary pressure of 25 lbs. per square inch, which has a “ sensible ” heat of 266° (i.e., 54° above 212°, the temperature of the sea water outside the coils). 30. Say also that, from experience of a particular make of evaporator, it has been found that x square feet of heating surface (i.e., outer coil surface) suffices for evaporating the required quantity of 1 ton per hour, with the specified primary steam—that is, with primary steam 54° hotter (sensibly) than the boiling sea water outside the coils. 31. Now, take for comparison a primary steam of 10 lbs. pressure, whose sensible heat is 239°—i.e., only 27° hotter than 212°, the temperature of the boiling sea water outside the coil. It is clear that to generate the same amount of secondary steam—viz., 1 ton in the hour—double the surface will be required, so that for this primary pressure of 10 lbs. we shall require^ surface of 2x (whatever x may be), as the intensity of the heat presented to the coil surface (27°) is only half^what it was with 25 lbs. pressure (viz., 54°). 32. For the same reason, if the primary pressure is raised to, say, 75 lbs., the temperature of such steam being 320°, or 108° hotter than 212°, the temperature of the secondary steam—all conditions remaining the same— as 108° is double the intensity of the heat of the steam at 25 lbs. pressure, the heating surface can be halved, and is, therefore, only | x. 33. This is carried out in practice, so that whatever number of square feet of surface is allowed for a pres- sure inside the coil of 25 lbs. per square inch, double that amount must be allowed, if the evaporator is to have a 6