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

68 SEA WATER DISTILLATION. opinions do not, in fact cannot, vary much, as all are governed by the laws of nature—the nature and properties of sea water as well as the properties of steam, which we have had under consideration, necessarily apply to all types of evaporators, although different makers may have their own special way of carrying out the details of their own make of evaporator. Steam Pressure—Inside and Outside the Coil. 4. We have seen that the production, in a specified time, is increased or diminished according as the primary steam pressure (inside the coil) is raised or lowered, and that the back pressure (outside the coil) has the reverse effect—viz., that as the secondary, or back pressure, rises, so the production of secondary steam falls ; and as the back pressure lowers, so the production of secondary steam rises, the “ primary ” pressure, of course, being the same in both cases. Use of Exhaust Steam in Coils. 5. A pressure of 20 lbs. per square inch inside the coil is suitable, when working with exhaust steam limited to 25 lbs. pressure in the exhaust steam pipes on board a large ship, such as a cruiser or battleship. When, how- ever, the evaporator is placed on smaller ships, such as a destroyer or torpedo boat, it is found better to take the primary steam direct from the boiler at a considerably higher pressure. The secondary steam can then be raised to a pressure varying from, say, | lb. to 15 lbs. per sq. inch. 6. On ocean liners, yachts, and ordinary steam ships, boiler steam is invariably supplied to the evaporator, which generates its secondary steam mostly at or about atmospheric pressure. This, as will be shown later on,