Machines employed for producing a rapid combustion of fuel, by furnishing a more copious supply of air than can be obtained by the mere draft of the ordinary chimneys. Although the common bellows is undoubtedly a blowing machine, yet the term is generally restricted in its application to those machines which are employed at large furnaces, as in foundries forges, etc. Blowing machines are constructed of various forms, the great object in all being, that the blast should be as continuous and uniform as possible. The method of producing such blast by a centrifugal force has long been known, but the first blowing machine on this principle, of which we have a distinct account, is that invented by Mr. Teral, in 1729. It consists of a number of vanes or fanners, radiating from a horizontal shaft, and enclosed within a cylindrical box, having two apertures at opposite sides of the cylinder, to one of which is fitted a conical pipe leading to the furnace, whilst the air enters by the other aperture, and the shaft being turned with great rapidity, a copious and uniform current of air may be impelled through the conical pipe to the furnace.
From the great simplicity and cheapness of these machines they have recently been coming into more general notice.
Another kind of blowing machine, and which is very extensively used for smiths' forges, is the double bellows. This machine in form resembles an ordinary single bellows, but is divided into two parts by a middle board, similar to the bottom board, and like it furnished with a valve opening upwards. The upper and under boards are each loaded with weights, which compress the upper and distend the lower compartments, and the middle board is supported in a horizontal position upon a frame. The pipe or nozzle of the bellows communicates with the upper compartment of the bellows only whilst the air is admitted by the valve in the lower compartment. The action of the machine is as follows: The lower board being raised by the brake or handle, the air contained in the lower compartment is driven through the valve in the middle board into the upper compartment, and not escaping from it through the nozzle, as fast as it is forced into it, it elevates the upper board, and thus distends the upper compartment.
Upon the descent of the lower board, the valve in the middle board closes, and the upper board descending by the pressure of the weights upon it, the air beneath it is urged through the nozzle in a continuous current.
During the descent of the lower board, the air enters by the valve in the board, and fills the lower chamber of the bellows; and upon the rise of the board it is forced into the upper chamber as before, and thus a continuous blast is maintained. But although continuous, it is not quite equal, or of a uniform force; for during the up-stroke the air is compressed by a force exceeding that of the weights on the upper board, since it causes the upper board to ascend; but upon the descent of the lower board, the air is expelled by the pressure of the weights alone, which, being at all times the same, the current is then nearly uniform.
Another species of blowing machine is the water bellows, invented by Hornblower, several of which machines have been erected in various parts of the county. The nature of these machines will be readily understood by the help of the following diagram The side figure is a vertical section of the machine, a is the fulcrum of the lever or beam, with two inverted vessels b and c suspended from its extremities; these vessels are open underneath, but air-tight above, d and e are two larger vessels, filled with water to the same level in which the vessels b and c rise and fall alternately. g k i is a tube or pipe, which passes through the vessels d and e, and reaches above the surface of the water; at the extremities are two valves, (omitted by mistake) which respectively open outwards into the inverted vessels, with a pipe at h open to the atmosphere, k and l are pipes passing through the bottom of d and e, and extending a little above the surface of the water; they are open at top, and have valves at bottom opening into the trunk o, to which the pipe is fitted which conducts the blast to the furnace.
An alternating motion being imparted to the beam by a steam engine or other first mover, the air passes up the tubes g h i and fills each inverted vessel as they are successively drawn up out of the water; the descent of the inverted vessel closes the valves at g and i, and opens those at the bottom of the tubes k and l, through which the air is driven forward by the trunk o, and thus, by the reciprocation of the beam, a continual blast is maintained through the trunk o and the tuyere of the furnace.
But the most perfect blowing machines are those in which the blast is produced by the motion of pistons in a cylinder. The annexed engraving represents a blowing machine of this description, erected by Mr. Paterson, of Lanark. It consists of two double acting force pumps, placed at right angles to each other, to equalize the draft; they are driven by a water-wheel of 5-horse power, a is the vertical cylinder, b the horizontal cylinder; c c two connecting rods united to the crank d; e the working beam; f the parallel motion; g the pipe for conveying the blast to the cupola or furnace h; i a small wheel, running in a groove in a cast iron plate; j frame supporting the vertical cylinder, between which the lowermost connecting rod c passes. At k k are placed valves, to admit the air into the vertical cylinder; similar valves are placed at the ends of the horizontal cylinder into it The operation is simply this: by the revolution of the crank the air is drawn in at each end alternately of both cylinders, and at the same time it is forced out at the opposite extremity along the pipe g into the furnace; and the cylinders being placed at right angles, one piston will be moving with its greatest velocity whilst the other is moving with its least velocity, by which means the blast is rendered nearly uniform, and an air chamber or reservoir rendered unnecessary.
The first cylinders of magnitude used as blowing machines, were erected by Mr. Smeaton, in 1760, at the Carron Iron Works, the cylinders being four in number, 4 feet 6 inches in diameter, and the piston making a stroke of 4 feet 6 inches in length; but the blowing machine lately erected at the Smithery, in the Royal Dockyard at Woolwich, is perhaps the most powerful and the most complete in the kingdom.
In this machine there are three blowing cylinders, of 4 feet 8 inches diameter, with a stroke of 4 feet 8 inches, and each cylinder making 20 strokes per minute, expelling near 5000 feet of air per minute. Over the wind chest is fixed a regulating cylinder, which has no bottom, being open to the wind chest; and its piston, which weighs 700 lbs. serves only to regulate the pressure, which amounts to about 1/4 lb. per square inch. When the pressure exceeds this, the piston rises, and opens an escape valve at the back of the cylinder.