space A, and thence, through the spring-controlled steel disk valves v′, into the discharge chamber C, which ultimately leads to the blast pipe. It will be seen that the valves v on the other side of the annular chamber are closed. At the same time a partial vacuum is being formed in the space B, to be filled by the inflow of air through the valves v which are now open, the corresponding discharge valves v′ being closed. These valves on the inside and outside of the annular spaces referred to are arranged so as to form a circle round the ends of the barrel of the cylinder. The free air, instead of being drawn into the valves v direct from the air of the engine house, is taken from an enclosed annular chamber E, which may be in communication with the clean, cool air outside. It will be seen that the piston is made deep so as to allow for a long bearing surface in the cylinder. Two metal packing rings are provided to render the piston air-tight. The horse-power of this engine, which is designed on the Cockerell system, is 750.
Air valves of other types than those which have been mentioned have been tried, such as sliding grid valves, rotatory slide valves and piston valves, but it has been found that either flap or disk lift valves are more satisfactory for air on account of the grit which is liable to get between slide valves and their seatings. In some of the blowing engines made by Messrs Fraser & Chalmers (see Engineer, June 15, 1906), sheets of flexible bronze act as flap valves both for admission and delivery, the part which actually closes the opening being thickened for strength.
The pressure of the air supplied by blowing engines depends upon the purposes for which it is to be used. In charcoal furnaces the pressure is very low, being less than 1 ℔ per sq. in.; for blast furnaces using coal an average value of 4 ℔ is common; for American blast furnaces using coke or anthracite coal the pressure is as high as 10 ℔; while for the air required in the Bessemer process of steel-making pressures up to 25 or 30 ℔ per sq. in. are not uncommon. According to British practice one large blowing engine is used to supply several blast furnaces, while in America a number of smaller ones is used, one for each furnace.
|Fig. 6.—Thwaites’ Improved Roots’ Blower.|
Rotary blowers occupy a position midway between blowing engines and fan blowers, being used for purposes requiring the delivery of large volumes of air at pressures lower than those of blowing engines, but higher than those of fan blowers. The blowing engine draws in, compresses and delivers its air by the direct action of air-tight pistons; the same effect is aimed at in a rotary blower with the difference that the piston revolves instead of moving up and down a cylinder.
Two of the best-known machines of this kind are Roots’ and Baker’s, both American devices. The mode of action of Roots’ blower, as made by Messrs Thwaites Bros. of Bradford, will be clear from the section shown on fig. 6. The moving parts work in a closed casing B, which consists of half-cylindrical curved plates placed a little more than their own radius apart, the ends being enclosed by two plates. Within the casing, and barely touching the curved part of the casing and each other, revolve two parts C, D, called “revolvers,” the speed of rotation of which is the same, but the direction opposite. They are compelled to keep their proper relative positions by a pair of equal spur wheels fixed on the ends of the shafts on which they run. The free air enters the casing through a wire screen at A and passes into the space E.
As the space E increases in volume owing to the movement of the revolvers, air is drawn in; it is then imprisoned between D and the casing, as shown at G, and is carried round until it is free to enter F, from which it is in turn expelled by the lessening of this space as the lower ends of the revolvers come together. In this way a series of volumes of air is drawn in through A, to be afterwards expelled from H in an almost perfectly continuous stream, this result being brought about by the relative variation in volume of the spaces E, F and G. In their most improved form the revolvers are made hollow, of cast iron, and accurately machined to a form such that they always keep close to one another and to the end casing without actually touching, there being never more space for the escape of air than 1⁄32nd of an inch. Machines after this design are made from the smallest size, delivering 25 cub. ft., to the largest, with a capacity of 25,000 cub. ft. per minute working up to a pressure of 3 ℔ per sq. in. It is not found economical to attempt to work at higher pressures, as the leakage between the revolvers and the casing becomes too great; where a higher pressure is desired two or more blowers can be worked in series, the air being raised in pressure by steps. A blower using 1 H.P. will deliver 350 cub. ft. of air per minute and one using 2¾ H.P. will deliver 800 cub. ft., at a pressure suitable for smiths’ fires. At the higher pressure required for cupola work—somewhere about ¾ ℔ per sq. in.—6½ H.P. will deliver 1300, and 123 H.P. 25,000 cub. ft. per minute. In the Baker blower three revolvers are used—a large one which acts as the rotating piston and two smaller ones forming air locks or valves.
Rotary Fans.—Now that power for driving them is so generally available, rotary blowing fans have for many purposes taken the place of bellows. They are used for blowing smiths’ fires, for supplying the blast for iron melting cupolas and furnaces and the forced draught for boiler fires, and for any other purpose requiring a strong blast of air. Their construction will be clear from the two views (figs. 7 and 8) of the form made by Messrs Günther of Oldham, Lancashire. The fan consists of a circular casing A having the general appearance of a snail shell. Within this casing revolves a series of vanes B—in this case five—curved as shown, and attached together so as to form a wheel whose centre is a boss or hub. This boss is fixed to a shaft or spindle which revolves in bearings supported on brackets outside the casing. As the shaft is rotated, the vanes B are compelled to revolve in the direction indicated by the arrow on fig. 7, and their rotation causes the air within the casing to rotate also. Thus a centrifugal action is set up by which there is a diminution of pressure at the centre of the fan and an increase against the outer casing. In consequence air is sucked in, as shown by the arrows on fig. 8, through the openings C, C, at the centre of the casing around the spindle. At the same time the air which has been forced towards the outside of the casing and given a rotary motion is expelled from the opening at D (fig. 8). All blowing fans work on the same principle, though differences in detail are adopted by different makers to meet the variety of conditions under which they are to be used. Where the fan is to be employed for producing a delivery or blast of air the opening D is connected to an air pipe which serves to transmit the current of air, and C is left open to the atmosphere; when, however, the main object is suction, as in the case where the fan is used for ventilation, the aperture C is connected through a suction pipe with the space to be exhausted, D being usually left open. Günther fans range in size from those which have a diameter of fan disk of 8 in. and make 5500 revolutions per minute, to those which have a diameter of 50 in. and run at from 950 to 1200 revolutions per minute. For exhausting the fans are run less quickly than for blowing, the speed for a fan of 10 in. diameter being 4800 revolutions for blowing and 3300-4000 for exhausting, while the 50-in. fan only runs at 550-700 when exhausting. These two exhausting fans remove 400-500 and 12,000-15,000 cub. ft. of air per minute respectively.