The outer system, embraces the short manifold sections which completely encircle the fire-box walls. The top end of each section is screwed and suspended from the flanged part of the shell, and the lower end is stayed by direct connection with the leg of the fire-box. The tubes are “staggered” in their manifolds, thereby exposing the greatest possible surface to the fire, and filling out the space due to the difference in the width of the water-leg and steam space of the shell.
The direct application of heat to the tubes causes a natural and active upward current therein, which in turn induces a corresponding downward movement of the water in the leg of the fire-box, and promotes the flow into the feed pipes.
Top View.—Fig. 401.
Bottom View.—Fig. 402.
The inner-tube system comprises those tube sections which extend to the upper limits of the boiler, their number and arrangement being such as to completely fill the interior of the shell above the space required for the combustion of the fuel. The construction of the vertical inner-tube system is simple, and consists of the required number of manifold sections, suitably arranged to conform to the circular space occupied, the flat inner end of each upper manifold being rigidly bolted to a heavy transverse beam, which in turn is supported in suitable pockets secured to the upper part of the shell.
At the top of the boiler, each section has its own connection with the steam space, and it is easy to remove either one of the sections separately without disturbing the others; or the entire inner-tube system can be raised out of the boiler as a whole, after breaking the proper connections, all of which are accessible. The current of steam and water carried over through the top connections of the inner system is generally sufficient to keep the tubes clear of scale; and the point of discharge and disengagement is brought down low, to prevent its mixture with the drier steam contained in the highest part of the shell.
When connected to a stationary boiler, as is now the general practice in fire departments, the circulative currents of water reach all parts of the boiler, hence its contents may be kept uniformly at any desirable temperature.