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

104 SEA WATER distillation. venient to have the entry to the steam pipe governed by an orifice of suitable size, so that the pressure of the secondary steam inside the evaporator casing may be recorded on a water pressure gauge, and the height the water as forced up may be made to record the quantity of steam that is passing through the orifice. A water pressure gauge is best for this purpose, as an ordinary steam gauge is not accurate for such very low pressures. 3. Pressure Gauges. 81. The two pressure gauges placed on the evaporator are to indicate the pressures of (1) the primary steam i?iside the coils, and (2) the secondary steam oufeide the coils. The primary steam gauge is the usual Bourdon type, and on its face should be indicated the maximum working pressure. For the secondary steam, if the evaporator works only at atmospheric pressure or just above it, the best and simplest type of gauge is an ordinary water pressure gauge, which records very slight variations in low pressures more accurately than an ordinary steam gauge. If there is to be a vacuum (more or less great) inside the casing, the vapour gauge is often of the compound type—that is, for secondary pressures above and below that of the atmosphere. 4. Safety Valve. 82. The safety valve or valves fitted to the evaporator shell give relief when pressure is increased in the evapor- ator casing for blowing away the brine, or when the casing is tested with all the coils removed and the primary steam allowed to fill the casing. The removal of the coils, as a test, prevents any risk if the coils get leaky, or damaged.