Front for Manhole.—Fig. 75.
[Fig. 75] shows a boiler the setting of which is similar in general design to the other three, except that in the place of a cast-iron front it has bolted to it a sheet iron breeching that comes down over the tubes and receives the gases of combustion from them. In [Fig. 75] a manhole is shown under the tubes. This, of course, is not an essential feature of the breeching, but it will be seen that manholes can readily be put below the tubes on fronts of this kind, in such a manner as to be very convenient of access.
In addition to these more general styles of boiler fronts, there are fronts designed particularly for patent boilers, water-front boilers, etc., which are made, very often, in ornamental and attractive designs. In [Fig. 71] is shown a beautiful and appropriate design in use in connection with water tubular boilers.
FURNACE DOORS.
The chief points to be considered in the design of furnace doors are to prevent the radiation of heat through them, and to provide for the admission of air above the burning fuel in order to aid in the consumption of smoke and unburnt gases.
In all cases where the doors are exposed to very rough usage—such, for instance, as in locomotive and marine boilers—the means for admitting air must be of the simplest, and consist generally of small perforations as shown in [Fig. 76] which represents a front view, and section of the furnace door of a locomotive boiler. The heat from the burning fuel is prevented from radiating through the perforation in the outer door, by attaching to it a second or baffle plate, a, at a distance of about 11⁄2 inches, the holes in which do not coincide in direction with the door proper. By the constant entry of cold air from the outside the greater part of any heat which may be communicated to the door by radiation or conduction is returned to the furnace.
Fig. 76.
Doors similar to the above provide for the constant addition of limited quantities of fresh air above the fuel, but in actual practice, however, air is only needed above the fire for a few minutes after fresh fuel has been thrown on the grates and then is required in considerable quantities. In the case of land boilers, the furnace doors of which undergo comparatively mild treatment, it is possible to introduce the necessary complications to effect this object.
