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

THE EVAPORATOR. 121 much distilled water. The further use of such exhaust steam in an evaporator coil is, therefore, not a waste of coal fuel, in the manner that the alternative use of live steam would be, as such extra live steam would be converted into primary water without having done any previous work. Note.—To ascertain the economy, it is necessary to see exactly what amount of heat (say per hour) is supplied to the evaporator in the way of primary steam, and then how much is used, and how much is returned by the coil drain, or rather how much, after the primary water has left the heater. Heater Surface. 117. The amount of surface put in a heater differs amongst makers according to the type of heater used, the heat given to the feed, and the temperature at which the discharged primary water is to be. These and other considerations make it impossible to give an inflexible rule for the amount of surface. It is sufficient to say that the heating surface of the heater will always have some relation to the evapor- ative surface of the evaporator. But this great dis- tinction must be kept in view—viz., that in the evaporator coil, the heat that is imparted is the “ latent ” heat of the primary steam yielded when reduced to water, whilst the heat that is imparted in the heater coil is only the “ sensible ” heat left in the primary water which has just become liquefied, and, therefore, deprived of the latent heat that was available in the primary steam. Roughly speaking, the “sensible” heat thus available in the heater coil is about one-eighth of the “ latent ” heat that was imparted in the evaporator coil. It must be borne