The power first applied to blowing furnaces was that of water wheels, but something over a generation ago these passed out of use except at very small plants working under peculiar conditions, on account of the lack of water powers of sufficient capacity to drive furnaces with the increasing volumes of wind desired and increasing pressures which those volumes necessitated. This brought about the introduction of the steam engine for blowing purposes. The number of types of such engines, and the method by which they were applied to blowing purposes, was almost unlimited.

About thirty years ago one type became predominant and held the lead for a long term of years. This is known as the long-cross-head type. It consists of a vertical steam-cylinder with a vertical air-cylinder immediately over it, both pistons being connected to the same piston rod. The shaft passes immediately underneath the center of the steam cylinder with a fly-wheel at each end just outside the bedplate which supports the whole. These fly-wheels carry crank pins exactly in line with one another so as to act as one.

The cross-head is fastened to the piston rod midway between the two pistons and extends parallel with the main shaft far enough for the wrist pins on its ends to come in line with the crank pins in the fly-wheels. The housings which support the air cylinder from the bed-plate are cut out in the center so that the cross-head can pass through, and carry on each side of the opening the slides which guide the latter and take up the horizontal component of the thrust of the connecting rods.

Modern designs of this type of engine are shown in Figs. 71 and 72. This engine has the advantage of securing a direct transmission of the stresses arising in the work of compression directly from one cylinder to the other, and also of having its pistons move in a vertical line and therefore avoids their dragging on the bottoms of the cylinders with resultant friction and wear, a condition which at one time was considered unavoidable with horizontal engines.

The engine is also compact; it requires only a small foundation and small space in the engine house. It is under the disadvantage of requiring two connecting rods, and of requiring very careful workmanship in securing the absolute correctness of alignment of the two crank pins. It also labors under the disadvantages in regard to supervision to which all vertical engines are subject. The advantages were long sufficient to outweigh the disadvantages, and this type of engine held an absolutely predominant place for a long term of years, and while changes in the conditions have brought about the development of many other types, these engines, when of modern design and economical type, are not only kept in use, but new engines of this type continue to be built in considerable numbers. It is, therefore, with the development of this type that the modern blowing engine has its real beginning.

Fig. 71. Southwark vertical long cross-head blowing engine.

At the time when these engines obtained their predominance outputs per furnace were small with consequent small requirements for volume of blast, and pressures were quite low, probably, on the average, less than one-half of what they are to-day.

As a result the power required per furnace was small and economy, as I pointed out in the last chapter, was a factor almost absolutely ignored. The small blast volume required made it possible for few engines of moderate size to supply the blast required for a furnace at a very moderate speed. Hence, there was little or no demand for high speed to cut down the number and size of engines required.

Fig. 72. Installation of "Mesta" long cross-head steam blowing engines.

Under these conditions engines were always fitted with some type of self-actuating air valves, in which a slight difference of pressure within and without the cylinder operated both the inlet and the outlet valves in the required direction, and when this difference in pressure was ended and then reversed, as the piston reached the end of its stroke and started on the return, these valves were closed by the return currents of air so set up.

With low pressures and moderate speeds these conditions of operation produced no bad results, especially as the pressure and temperature to which the air was raised by compression were low and therefore valve faces and piston rings could be made of leather and wood respectively, or of other natural fibrous materials. These materials, however, while perfectly satisfactory at low temperature, undergo chemical change with increasing rapidity as the temperature rises, and so the time came, as pressures increased, when such materials could no longer satisfactorily be used for these purposes.

This forced the substitution of metal faces for these valves and thus emphasized the two difficulties with automatic valves, which had gradually come to be understood. First, if these valves are operated entirely by the currents of air which pass through them, they do not seat themselves until these currents are reversed in direction, and then are brought to their seats with a violent shock at any but very low speeds. As long as leather could be used this shock was much cushioned by the leather, but when metal faces became necessary operation under these conditions became increasingly difficult.

Second, the area inside these valves on which the excess pressure operates to lift them from their seats is less than the area outside them, tending to hold them closed, by an amount equal to the area of the seat itself, which must always be a considerable percentage of the total area. Therefore, the pressure inside the valve must always be greater than the pressure outside it, in the inversed ratio of the internal and external areas, before the valve will open. This leads to a delayed operation of the valve and necessitates a much greater difference between the internal and external pressures than would otherwise be nenessary with corresponding loss of power.