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

MULTIPLE DISTILLATION. 18 r of the steam inside the coils of each evaporator and the water outside such coils. Thus, in the foregoing explan- ation the difference (at each stage) is 54° of temperature —i.e., the difference in the “ sensible ” heat of the generat- ing steam mside of the coil and of the generated steam owfeide of the coil is 54°, the primary pressure of 75 lbs. per square inch having a net temperature of 54° (320 - 266 = 54) above the secondary pressure, and the secondary pressure of 25 lbs. a net temperature of 54° (266 - 212 = 54) above that of the tertiary pressure. The evaporators should, therefore, be constructed—i.e., provided with a heating surface—on the above basis. If, however, this basis is altered to suit requirements, the construction must be altered accordingly. The heating surface may be de- creased if the temperature difference is more than 54°, but increased if less, according to the rule already ex- plained—viz., that the amount of evaporator heating surface is governed by the amount that the sensible heat of the steam inside the coils is above that of the steam or water (which is the same), outside of the coils. 24. From the above observations it will be seen that a practical limit is soon found whether the pressures be increased or decreased, as the maximum primary pressure and the minimum final pressure available are soon reached. The practical result of these considerations is to avoid carrying the principle beyond the limits dictated by practi- cal economy, as the cost of the plant increases with every evaporator added, while the advantage gained with each lessens. The quadruple distilling apparatus, which is capable of producing about 30 tons of water to the ton of coal, working steadily and easily, will be found to be a good limit. Beyond this point, the actual return for the outlay is a doubtful advantage.