The use of these mechanical cleaners is readily understood: (1) they provide a place of accumulation for the sediment; (2) they save the necessity of opening the boilers to remove by hand, the refuse of the boiler; (3) save fuel by avoiding the necessity of frequent blowing off one or two gauges of water, and (4) by the preventing the formation of scale with its attendant evils.
SCUMMING APPARATUS.
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.
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.