This boiler differs from the Heine principally in having the water leg subdivided into as many sections as there are vertical rows of tubes, this being a still further extension of the sectional idea for localising and minimizing the effect of failure of any part. As the arrangement of the tubes in a vertical plane is staggered, these headers are made sinuous, as shown in Fig. 63.

A saddle or cross-box shown by Fig. 64, is riveted to the drum, and each of the headers is connected to this by means of a straight tube expanded into the top of the header at one end and into the bottom of the cross-box at the other.

The boiler as a whole is well shown by Fig. 65. This boiler is suspended from a structural steel frame instead of being supported directly on the brick, as is the case in the Heine boiler. Another difference from the latter boiler is that in this case the passes are at right angles to the line of the tubes instead of parallel with them.

Fig. 65. Babcock & Wilcox boiler in setting.

The baffles are built up of fire brick supported by clamps around the tubes. These fire bricks make a continuous wall with the tubes passing through it, and supporting it. One of these walls run up from the lower bridge wall, the other comes down from the suspended upper bridge wall, the passage of the gases being first up, then down, and then up again, either out above, or through the suspension tubes of the rear headers, and into an underground flue.

The arrangement of having the gas pass off upward is very much to be preferred to that in which it passes downward because the latter involves the use of underground fines, and when these become obstructed by dust as they ordinarily do in the course of a few month's run, they can in general only be cleaned by shutting down the plant and putting men into them to shovel them out. In some cases the arrangement is such that a heavy stream of water can be turned into the flue and the dust washed out with this, but even this arrangement is inferior to the overhead flue, which may be provided with dust pockets through which the dust can be drawn into carts or wheelbarrows before it accumulates to an objectionable extent, and without any interruption to operation.

The boiler here shown is equipped with a super-heater, this being the U-shaped coil lying on its side in the space between the bottom of the drum and the top of the water tubes. Super-heaters have never been used in blast-furnace practice until recently, probably for the reason that they have the reputation of requiring a good deal of maintenance, and a correspondingly large number of interruptions to service, both of which would go far to offset any advantage they might produce in steam economy, interruptions to continuous service being particularly objectionable, but in modern plants super-heaters are being installed.

These boilers can be arranged for gas firing exactly as described for the Heine, by the introduction of a gas burner immediately above the fire door, but my preference is strongly in favor of the use of a combustion chamber for burning furnace gas, and as the tiles surrounding the tubes cannot be used in this boiler because they would cut off the passage of the gas and flame completely, I am strongly in favor of the use of a Dutch oven in front of these boilers when they are to be fired with furnace gas.

Such a Dutch oven will be described and illustrated in connection with the Rust boiler, and therefore need not be described in detail here. In order to apply it successfully the boiler generally requires to be set somewhat higher than when no Dutch oven is used, but this is a matter of slight first cost and no subsequent disadvantage.

The sinuous headers fit into each other very closely and no opportunity is permitted to introduce a steam jet longitudinally between the tubes. Consequently, in order to permit these boilers to be blown off, three sets of blow-off doors are provided whose location is indicated by the three vertical rows of white dots between the tubes in the illustration. The brickwork is cut away from these doors at an angle of 45 deg. on each side, and individual blow-off holes are provided opposite the space between each two rows of tubes. By this arrangement a steam jet can be inserted between each pair of horizontal rows, and also above the top one, and swung through a wide angle. By doing this from both sides the dust may be blown from the tubes quite effectively.

The headers are provided with holes in each side exactly like the water legs of the Heine boiler, the outside One being covered by a cap, held to place against the pressure of the steam with a bolt pulling against an inside dog. This puts the joint on the outside of the header, and these are all ground so that a metal-to-metal joint can be made.

These caps give access to the inside of the tubes for cleaning exactly as in the Heine boiler, and like it at the expense of two hand-hole joints to make for each tube, to which some operators object on account of the pains necessary to keep these joints in good order, although if they are made thoroughly tight before the boiler starts up, they seldom give trouble in operation.

Fig. 66. Babcock & Wilcox boiler in course of erection showing how headers fit into one another.