This is the latest developed of any boiler in extensive modern use. It was invented to solve what had seemed for many years an insoluble problem, that of making a boiler containing nothing but straight tubes, and yet using only circular drums. This seemed impossible, because it is necessary for the tubes to enter the drum radially in all the older constructions, and this means that all the tubes except one row must be bent to reach this direction, as is clearly shown by the illustration of the Stirling boiler.

Mr. E. G. Rust, then chief engineer and later general superintendent of the Colorado Fuel & Iron Company, overcame this difficulty by making a forged steel tube sheet for the drum, which is circular in its principal section, but which has forged into it cup-like depressions whose axis is not radial to the drum, but in the line which the tubes are desired to have. This is shown by Fig. 68. These tube sheets are made of sufficiently heavy material so that even after the drawing necessary to form these pockets the material is amply strong.

The boiler in its setting is well shown by Fig. 69. It consists of four drums, two below and two vertically above, connected by five rows of straight vertical tubes. A bridge wall extends up from the foundation between the two elements of the boiler. The flame comes in on one side, passes vertically up, surrounding one nest of tubes, over the bridge wall and down, surrounding the other, thence to the stack. The bottom drums are connected by a single row of straight tubes, the top drums by an upwardly curved set of tubes for steam, and a downwardly curved set for water, the latter carrying baffle tiles to keep flame out of contact with the drums above the water line.

One of the advantages claimed for this boiler is that the tubes being set vertically the dust has a minimum tendency to adhere to them, nevertheless three horizontal blow-off doors are provided which are of the same type as those described for the Babcock & Wilcox boiler, but set horizontally in this case so as to be at right angles to the line of the tubes. The setting of this boiler precludes the possibility of having the flame burn directly upward against the tubes, and owing to this construction a Dutch oven must be provided even with coal firing, which makes its adaptation to blast-furnace gas very easy.

Fig. 70 shows the front of a battery of these boilers equipped with gas burners above the fire doors. The gas main may be seen in the upper right-hand corner of the illustration, with its connections to the individual pairs of burners. The valves controlling the latter, operated by wheel and hand-chain, are also shown.

It seems no more than fair to say that these boilers are generally preferred above any other type by blast-furnace managers who have used them.