DISTILLERS
We now come to a part of a ship's plant very necessary for both machines and human beings. Many a time have people been in the position of the Ancient Mariner, who exclaimed:—
"Water, water, everywhere,
But not a drop to drink!"
Water is so weighty that a ship cannot carry more than a very limited quantity, and that for the immediate needs of her passengers. The boilers, in spite of their condensers, waste a good deal of steam at safety valves through leaking joints and packings, and in other ways. This loss must be made good, for, as already remarked, salt water spells the speedy ruin of any boiler it enters.
The distiller in its simplest form combines a boiler for changing water into vapour, with a condenser for reconverting it to liquid. Solids in impure water do not pass off with the steam, so that the latter, if condensed in clean vessels, is fit for drinking or for use in the engine boilers.
A pound of steam will, under this system, give a pound of water. But as such procedure would be extravagant of fuel, compound condensers are used, which act in the following manner.
High-pressure steam is passed from the engine boilers into the tubes of an evaporator, and converts the salt water surrounding it into steam. The boiler steam then travels into its own condenser or into the feed water heater, while the steam it generated passes into the coils of a second evaporator, converts water there into steam, and itself goes to a condenser. The steam generated in the second evaporator does similar duty in a third evaporator. So that one pound of high-pressure steam is directly reconverted to water, and also indirectly produces between two and three pounds of fresh water.
The condensers used are similar to those already described in connection with the engines, and need no further comment. About the evaporators, it may be said that they are so constructed that they can be cleaned out easily as soon as the accumulation of salt and other matter renders the operation necessary. Usually one side is hinged, and provided with a number of bolts all round the edges which are quickly removed and replaced.
The United States Navy includes a ship, the Iris, whose sole duty is to supply the fleet she attends with plenty of fresh water. She was built in 1885 by Messrs. R. and W. Hawthorn, of Newcastle-on-Tyne, and measures 310 feet in length, 38 1 / 2 feet beam. For her size she has remarkable bunker capacity, and can accommodate nearly 2,500 tons of coal. Fore and aft are huge storage tanks to hold between them about 170,000 gallons of fresh water. Her stills can produce a maximum of 60,000 gallons a day. It has been reckoned that each ton of water distilled costs only 18 cents; or, stated otherwise, that 40 gallons cost one penny. At many ports fresh water costs three or four times this figure; and even when procured is of doubtful purity. During the Spanish-American War the Iris and a sister ship, the Rainbow, proved most useful.