Many kinds of filtering media have been tried—fabrics of silk, calico, cocoanut fibre, towelling, sawdust, cork dust, charcoal, coke; but the ideal substance, at once cheap, easily obtainable, durable, and completely effective, yet remains to be found.
A filter should be so constructed that the filtering substance is very accessible for cleansing or renewal.
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.