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

82 SEA WATER DISTILLATION. pressure of only 10 lbs. per square inch inside the coil, other conditions being the same. 34. Having a fixed basis to work with—that is, say a surface of x square feet per ton of evaporation per hour, when the primary and secondary pressures are, say, 25 lbs. for the former, and practically zero for the latter—there is no difficulty in calculating what surface would be required for a pressure on the secondary steam greater or less than the atmosphere. 35. Thus, say the primary pressure is 25 lbs. per square inch, and the secondary pressure is, say, 10 lbs. per square inch, it will be seen that the excess heat of the primary steam over that of the secondary steam is the same as when the primary steam was at 10 lbs. pressure, and the secondary steam at a pressure of, say, | lb. per square inch (viz., 27°), so that x surface would do in either case, other conditions remaining the same. 36. If evaporators are worked with a primary steam pressure suitable for secondary steam generated below ’ atmospheric pressure, the heating surface has to be cal- culated in the same way according to circumstances. Probably, taking all circumstances into consideration, the most convenient pressures to take, for ordinary working, are about 20 lbs. pressure for the primary steam, and | to | lb. pressure for the secondary- steam—when one evaporator only is brought into requisition. This is when exhaust steam is being used inside the coils. If live steam is used, then the 20 lbs. pressure may be considerably increased. 37. The relationship of pressures inside and outside the coils is a subject which needs much consideration in multiple distillation, where it will be seen there are two, three, or more evaporators linked together, so that the