At the back of the grates a bridge wall is run up to the bottom of the inclined water tubes, so that the hot gases that arise over it must circulate among the tubes.
A counter wall is laid on an incline from the top of the tubes to the back of the drum. This is laid on perforated plates or bars and is covered with fire brick. A wall is also built at the lower and back end of the tubes to carry them.
Back of the whole is the outer wall with openings for giving access to the tubes and smoke chambers. Side walls are raised to enclose the same and are arched at the top to come nearly in contact with the drum, which is carried partly by brackets and partly by the connections to the tubes.
Points Relating to Boiler Setting.
Long and heavy boilers are best suspended from two beams or girders by two or three bolts at each end. Boilers over 40 feet long should have three or even four sets of hangers, as the case may require.
Side brackets resting on masonry may be used for short boilers. If used on long boilers, side plates or expansion rollers should be used at one end of boiler. There ought to be not more than two brackets on one side, so divided that the distance between them is about three-fifths of the total length of the boiler, or the distance from ends of boiler to center of bracket is equal to one-fifth the length of boiler.
The side walls in boiler-setting should not be less than twenty inches with a two inch air space; the rear wall may vary from 12 to 16 inches according to the size of the boiler; the front wall 9 inches and the bridge wall may be from 18 to 24 and perfectly straight across the rear of the furnace. If the boilers are supported by side walls, the outside walls should be not less than 13 inches thick and have pilasters where the boiler is resting.
Flues touching the boiler above the water space should be emphatically condemned.
Unless the boiler walls are very heavy, they should be stayed by cast or wrought iron bunch stays, held together by rods at tops and bottoms.