For smelting and refining furnaces, where a blast with a pressure of 3 or 4 lbs. per sq. in. is required, engines of large size are usually employed. In our article InoN, this kind of engine is referred to, and a small figure of one given; but we shall here describe the blowing apparatus itself more in detail. A blowing-engine consists of a steam-engine, with the ordinary steam-cylinder at one end, and a blast-cylinder at the other end of the beam. Such, at least, is the construction preferred for the larger-sized engines; but sometimes a horizontal arrangement of the cylinders is adopted for those of smaller size. The blowing-cylinder is of cast iron, with an air-tight piston, which, as it ascends and descends with the motion of the engine, alternately inhales and expels the air at each end. To affect this, a series of valves are provided, and these are arranged as follows: Inlet valves are placed on the top of the cylinder, and also on throe sides of the box. but on the fourth side of this box there are two outlet valves. These valves consist of numerous openings, against which leather flaps lie when they are shut. Valves of a similar nature are placed at the bottom of the cylinder. When the piston descends, it would create a vacuum in the upper portion of the cylinder, provided there were no openings in it; but the exiernal air pressing on the inlet valves, opens them, and fills the space above the piston; at the same time, the outlet valves, which only open outwards, are tightly closed by the air pressing inwards from the pipe. Again, when the piston ascends, it compresses the air above it, and exactly reverses the action of the valves. The valves at the bottom of the cylinder work exactly in the same way, the inlet valves opening when the piston ascends, and shutting when it descends, thus com pelling the inhaled air to pass into the pipe by the lower outlet valves. The air is con ducted by the pipe into a receiver of large capacity, which serves to equalize the blast before it passes to the to -ergs. ' A blast-engine at Shelton iron-works, of which the blowing cylinder is 8 ft. 4 in. In diameter, and has a 9-ft. stroke, working with 186 horse-power, and making n single strokes of the piston per minute, inhales 15.700 cubic feet of atmospheric air per minute; but this is compressed by the blowing-cylinder to a pressure of 3 lbs. per sq. in. above the atmosphere, which reduces the volume supplied by the cylinder to 13,0S3 cubic feet. Its volume, however, is largely increased again, when raised to the hot-blast temperature, before entering the furnace. Much valuable information respecting blow ing-engines and blast apparatus will be found in Dr. Percy's large work on 3felallurgy, vol. ii.
In the Catalan forges of Spain and the s. of France, there is a very ingenious water blowing machine in use called a 7'rompe; but it can only be advantageously employed where a fall of a few yards of water is available. A strong wooden cistern, to act as a reservoir for the water; wooden pipes (generally two in number), through which it descends; and a wind-chest, to allow the air and water to separate, constitute the essen tial parts of the apparatus. It is put in operation by lifting the wedge with a lever; this allows the water to rush down the pipe, and in doing so, it draws in air through sloping boles, called aspirators, at the throat of the pipe. A continuous current of water and air is thus supplied to the wind-chest, which is provided with an opening for the escape of the water, while the air passes out in a regular stream by the nozzle-pipe. The height from which the water falls determines the tension of the blast; but the height seldom exceeds 27 ft., which gives a pressure of from lf to 2 lbs. to the sq. inch. It is
asserted that no other blowing-machine so equable a blast as the trompe, and it is the least costly of any; but it has the serious defect of supplying air more or less saturated -with 'moisture. The theory of this singular machine has never been satisfactorily explained, although one or two able philosophers, who have specially studied the mat ter, incline to the belief that much of the air is carried down the pipe by becoming entangled in water. It is found that the separation of the air from the water is greatly promoted by allowing the current to impinge on a narrow platform.
The fan, or fanners (q.v.), as it is 'sometimes called, is another machine of great value for producing currents of air. It has leng been in use as a winnowing-machine for agri cultural purposes, and also for creating. a blast to melt pig-iron in foundries. More recently, it has been employed instead of bellows in smithies, on account of its greater convenience and the steadier blast which it yields. A domestic bellows has even been introduced on the fan principle. The fan is also much used in.the ventilation of build ings, ships, and mines. .For the last, it is now considered preferable to the plan of fur nace ventilation; especially where there are fiery seams of coal.
In its construction, the fan a wheel, having the arms tipped with vanes, instead of being joined by a rim. It is placed inside a chest—usually in an eccentric position— with openings on each side round•the spindle for the admission of air. The motion is given by steam or other power; and as it revolves, the centrifugal action sucks in air at the center, draws it towards the tips of the vanes, and these impel it forward through the exit-pipe. Engineers differ as to the proportions which should be adopted for the fan,- and as to the extent of spiral which the fan-case should have. For foundries and smithies where the pressure of the blast required is from 4 to 5 ozs. per square inch, the following have been found to suit very well in practice: the width of the vanes, as well as their length, made one fourth of the diameter of, the fan; the inlet openings in the sides of the fan-chest, one half, and the degree of eccentricity, one tenth of this diameter. There is a segmental slide by which the opening into the delivery-pipe may be increased or diminished. For such purposes, fans vary from 3 to 6 feet in diameter, and they are entirely constructed of iron. Double fans have been introduced by Mr. Chaplin in Eng land, and by M. Perrigault in France. In these, two simple fans are so disposed on one spindle that the blast produced by one passes in its compressed state through a tube to the other, which largely augments the working pressure. In Platt and Schiele's silent fan, 'the air enters by a central entrance at one side only,' and is expelled from the case at the opposite side. The vanes are a peculiar shape, and describe what the inventor (Schiele) calls an anti-friction curve. It is said to be very efficient, and so also is another form of noiseless fan by Mr. George Lloyd, London., For the use of the fan in ventilation, see that head. In some cases, fans are of large size; some also are of peculiar construction. Agricultural fan's (see F.ANNERS) are not usually placed in an eccentric position in their cases, and only some kinds of ventilating fans are. One of the happiest applications of the 'fan has been to draw off and render harmless the fine steel dust in the operation of needle-grinding.