WATER SUPPLY

345

This trouble is most likely to occur if the pipes, etc., have been left empty
for any length of time and have become dry.

When hard water has to be used for steam raising it must first be softened
in order to prevent scale in the boilers and economiser pipes, and one of the
plants in general use is that made by Paterson. The machine consists of a
tower (Fig. 151), at the top of which is a mixing tank for chemicals and also the

[Paterson Engineering Co. Ltd.
FIG. 150.—SECTION OF PRESSURE FILTER, SHOWING UNDER-DRAINING
SYSTEM
intake for untreated water. The water flows down a central tube fixed in the
middle of the tower, and is mixed with the softening chemicals as it passes
down. When it reaches the bottom of the tube it passes up the tower again,
depositing any heavy, insoluble matter at the bottom. On reaching the top
it passes through a filter, where any suspended matter is removed, and is now
softened and cleaned ready to pass to the boiler-feed tank, or any other place
where it may be required. This system is effective and simple in operation
and costs very little to run.