In addition to the bottom blow-out apparatus every boiler should be
provided with means for blowing out water from the surface in order
to remove the fine particles of foreign matter floating there, which
afterward settle and consolidate as scale on the heating surfaces.
[Illustration: Fig. 70.
_Scum Cock_]
It consists, in its simplest form, of a pan, or a conical scoop, near
the surface of the water, but below it, connected with a pipe passing
through the boiler-shell, on which is a cock, or valve, for regulating
the escape of the water laden with the impurities deposited in the pan.
There are patented apparatus for this purpose which are well designed
and easily fitted to a boiler.
The office of the surface blow-off, illustrated in Fig. 70, is to
remove the foreign matter which is precipitated from its solution in
the water.
A surface blow-off used occasionally will remove the greater portion
of this scum and keep the boilers reasonably free from scale and mud.
Where dirty or muddy water is fed into the boilers the surface blow-off
is one of the cheapest and most efficient means for keeping the boiler
clean. The efficiency of the surface blow-off is not so great as that
of some of the mechanical boiler-cleaners, as by their use it is not
required that any hot water shall be wasted, and this is the greatest
objection to the surface blow-off, as in the hands of some people a
large amount of boiling water is wasted each time it is used. But
both of these arrangements are virtually skimmers, as they remove the
precipitated mineral and vegetable matter from the surface of the water
in the boiler. One does it by blowing out the scum and some water at
the same time, while the mechanical boiler-cleaner removes the scum,
but returns the water to the boiler.
There are several efficient ways of arranging a surface blow-off. The
principal part of the blow-off is a pan or perforated pipe placed
horizontally at the water level having a pipe leading outside the
boiler to any convenient place where the scum may be blown. When a
perforated pipe is used the action is to force the scum from the top of
the water during the time the valve is open, and blow it through the
pipe. In using an apparatus of this kind it should be blown often, but
only for a moment at a time, as all the scum near the pipe is removed
immediately, and to keep the valve open longer than necessary to remove
the scum near the pipe would allow the escape of clean water or steam
which would be wasteful. If a pan is used and is fastened so that the
top is secured at the ordinary water level, as shown in Fig. 70, the
blow-off pipe leading from near the bottom of the pan, it will be more
efficient than the perforated pipe arrangement as it will not require
to be used so often, and the waste of water and steam will not be so
great. The pan, by producing an eddy in the water, causes all the
scum to gather over the top, and as the water is quiet there it will
gradually settle into the pan, where it will remain as mud. When the
blow-off valve is opened the greater part of the mud which is gathered
is blown out, and but very little water is carried with it.