At the Illinois Steel Works in South Chicago, where coal was expensive as compared with Pittsburgh, and where a furnace plant and steel mill were early operated in conjunction, they used at one time a great number of horizontal tubular boilers; but this is the only large blast-furnace plant, to my knowledge, where extensive use was ever made of this type of boiler. These have now all been superseded by water-tube boilers.

In New England, where fuel is expensive and where the textile mills use in the aggregate almost as much power as do the steel mills of the country, so that their operators have been forced to use every possible economy in steam generation and utilization, they continue to use the horizontal tubular boiler to a great extent, even down to the present time.

It is, therefore, not on the ground of economy that this boiler has been driven out, but rather by two demands of modern practice which jointly have set conditions impossible for it to meet. These two conditions are, first, increasing steam pressure; second, a demand for large individual units so as to keep down as low as possible the total number of units for large powers. Both these conditions necessitate heavier shells for horizontal tubular boilers, and since these are exposed to the direct action of the fire their safe thickness is limited to that which will permit the cooling effect on the inside to prevent the rapid burning or cutting away of the outside, particularly at the seam with its double thickness and projecting rivets.

Water cooling is effective against the direct action of fire only within certain definite limits of thickness, and these are approximately reached in the horizontal tubular boiler at a capacity of 300 horsepower and a pressure of 125 lb. to the square inch, and though by special construction these conditions have been exceeded in horizontal tubular boilers, this pressure is lower than that demanded by modern practice, and the units are smaller than is desirable in large plants. Hence the horizontal tubular boiler is eliminated from the competition.

Moreover, the question of safety is always to be considered, and while it must by no means be thought that water-tube boilers are free from accidents, it must be admitted that the consequences of such accidents are probably on the average less destructive to life and property than those of horizontal tubular boilers.

The number of types of water-tube boilers which have been introduced in the last twenty or thirty years is literally legion. Many, if not most of them, have been applied from time to time and from place to place, in blast-furnace service. Many of them are still in such service. But the development of modern plants has settled down to the use of half a dozen types, of which we may consider four.

Before proceeding to a description of these, however, a general review of the conditions of operation of boilers at blast-furnace plants will not be amiss. These differ in some respects from those prevailing in ordinary installations. First, they are normally gas-fired, though provision is always made for the use of coal should it become necessary. Second, the furnace gas with which they are fired generally contains a considerable quantity of dust which settles on the tubes and heating surfaces and, if not removed, rapidly impairs their conducting power. Third, the boilers are operated twenty-four hours a day and seven days a week. There is no stoppage of the plant over Sunday during which the boilers can be cleaned, as is customary in other industries, and for this reason a sufficient reserve of boiler power must be installed to allow at least one unit to be constantly out of service for cleaning. Other than these, the conditions do not differ materially from those of economical boiler operation in general.

General Considerations Concerning Water-Tube Boilers

The first and most important condition is, of course, that the tubes must be kept clean, not only externally but internally, and until very recent years it was practically impossible to do this in any except perfectly straight tubes. This is no longer true, but at the same time straight tubes are greatly to be preferred to curved ones, as they are less expensive and generally far less troublesome to replace than curved tubes, and in everything except length are interchangeable, which cannot be the case when curved tubes are used.

Owing to the nature of hydrostatic pressure, any vessel subject to such pressure tends to become circular, and if any other form than this be used, in order to resist deformation it must be given greater strength than would be required by the circular form in a proportion greater than its deviation from the latter. For this reason flat, parallel surfaces on upper and lower drums, until recently the only ones between which straight tubes could pass, have been frowned upon by boiler insurance companies and boiler inspectors, particularly during recent years when pressures have risen far beyond what they were ten years ago, because of the difficulty of staying such flat surfaces so that they will stay flat, and the liability to excessive stresses and leakage, and even failure if they distort under pressure.

Moreover, it was for a long time very desirable to have the tubes not only straight, but to have them so set that their ends could be exposed and a cleaner, carried by a rigid rod a little longer than the tube, inserted and driven through. This could not be done if the tubes were set in drums, because the drum would prevent the insertion of a cleaner so mounted into the tube. Therefore, two of the most successful of the earlier types of boilers had parallel water legs depending from opposite ends of a combined steam and water drum above, and from one of these water legs to the other ran the straight tubes. The water legs could obviously not be circular in shape and yet permit the use of straight tubes, so they were made in various ways to give the necessary strength and access to the tube ends, and at the same time permit the use of straight tubes.