Flush Front.—Fig. 72.
As will be seen from cut 72, the front of the smoke arch, in this form of setting, is flush with the front of the brickwork, and the dry sheet just outside of the front head is built into the brickwork. The heat from the fire, striking through the brickwork, impinges on this sheet, which is unprotected by water on the inside. So long as the furnace walls are in proper condition the heat thus transmitted should not be sufficient to give trouble; but after running some time bricks are very apt to fall away from over the fire door, and thus expose portions of the dry sheet to the direct action of the fire, causing it to be burned or otherwise injured by the heat, and perhaps starting a leakage around the front row of rivets when the head is attached to the shell.
In the overhanging front this tendency is entirely prevented by setting the boiler in such a manner that the dry sheet projects out into the boiler room. If the brickwork over the fire door falls away when a boiler is set in this manner, the only effect is to slightly increase the heating surface. No damage can be done, since the sheet against which the heat would strike is protected by water on the inside.
Overhanging Front.—Fig. 73.
The objection is sometimes raised against the projecting front, that it is in the way of the fireman. To meet this point and yet preserve all the advantages of this kind of front, the cutaway style has come into use. In this form the lower portion or the front sheet is cut obliquely away, so that at the lowest point the boiler projects but little beyond the brickwork.
Cutaway Front—Fig. 74.
It will be noticed that in the flush and overhanging fronts, the doors open sidewise, swing about on vertical hinges; in the cutaway front the best way to arrange the tube door is to run a hinge along the top of it, horizontally, and to have the door open upward. But with such a disposition of things the door is not easy to handle. For the purpose of support a hook and chain, hanging from the roof should be provided.
