1902 Encyclopedia > Bellows

Bellows




BELLOWS AND BLOWING-MACHINES are machines for producing a current of air, chiefly in order to assist the combustion of a fire.
The common bellows now in use probably represents one of the oldest contrivances for this purpose. It consists of two fiat boards, of oval or triangular shape, connected round their edges by a piece of leather so as to form an air chamber. The leather is kept from collapsing, on separation of the boards, by two or more hoops, which act like the ribs in animals. The lower board has a hole in its centre covered inside by a leather flap or valve opening inwards ; it has also fastened to it a metal pipe or nozzle, of smaller aperture than the valve. On raising the upper board, the air from without lifts the valve and enters the cavity ; then on pressing down the top board, this air it compressed, shuts the valve, and is driven through the pipe with a velocity corresponding to the pressure.


The blast here is, of course, not continuous, but in puffs,— a certain interval being needed for refilling the bellows after each discharge. This drawback was remedied by the invention c-f double bellows. To understand their action, it is only necessary to conceive an additional board with valve, like the lower board of the single bellows, attached by leather under this lower board. Thus two similar cavities are obtained, separated by the lower board of what was the single bellows. The lowest board is held down by a weight, and another weight presses the top board. When the lowest board is raised it forces air into the upper cavity, and the valve of the middle board prevents return of this air. The lowest board being then depressed, air enters the lower cavity from without, and this in its turn is next forced into the upper cavity. The weighted top board is meanwhile continuously pressing the air of the upper cavity through the nozzle. While the blast thus obtained is continuous, it is not wholly free from irregu-larities.
The common smiths' bellows, made on the principle just indicated, are generally of circular form, as shown in figs. 1 and 2. A is the blast pipe, B the movable lowest

Fias. 1 and 2.—Common Smiths' Bellows.
board, C the fixed middle board (into which the pipe is inserted), and D the movable upper board pressed by a weight. The lowest board is moved by means of the lever L and the chain H working on the roller R. The weight required to produce a certain force of blast is easily deter-mined ; if the diameter of the bellows be 1 foot, the area will be 113-19 inches, and the upper board will require a weight of 5 6'5 lb for a blast equivalent to a pressure of £ lb on the square inch, or a velocity of 207 feet per second, which is well suited for a smith's forge. By a simple arrangement for altering the diameter of the pipe the force of the blast may be varied.
It may be noted that in some parts of the Continent a simple form of bellows is made of two wooden boxes, each open on one side, and the one just fitting into the other. The open sides being opposed to each other, the upper enclosing box is made to move up and down over the other, with which it is jointed at one part, and which is provided with a nozzle, and a valve opening inwards. The change of capacity produces a blast. There is con-siderable loss of air, however, from the boxes not exactly fitting.
The blowing-machines now almost exclusively used for blast furnaces are of the cylinder and piston type (which is the principle adopted, it may be remarked, in a small hand bellows used by the Chinese). At first the blowing cylinders were single-acting, that is to say, they had the power of propelling a blast only when the piston was moving in one direction. With two or more of these blowing cylinders attached to one crank-shafs, worked by a water-wheel, a tolerably steady pressure of air was obtained. But in these and other respects considerable progress has been realized.
The cylinder-engines of the present day (which are generally driven by steam) may be classed in two chief systems, according as the cylinder is placed horizontally or vertically. In the former case the steam and blast cylinders are usually in one line, the same rod carrying the pistons of both, and being guided on both sides, while a fly-wheel is employed as regulator. In the vertical systems the steam and blowing cylinders are sometimes similarly connected, but, in the larger engines, they are generally placed one at each end of a beam connecting their pistons. The vertical engines have been most popular in England and in some parts of the Continent (as Silesia), but the other type (almost exclusively used in Westphalia and on the Rhine) is now adopted in several English works.
The general action of many of these machines may be illustrated by the large blowing engine at the Dowlais iron-works, erected in 1851. Fig. 3 is a representation of

FlQ. 3.—Section of Cylinder of Blowing Engine.
its blast cylinder, the piston of which, made air-tight by packing, is moved by the oscillating beam of the engine. The cover of the cylinder, and also its bottom, have several openings, furnished with valves v, which open inwards Other valves «', above and below, open into a lateral chamber B, which is connected by the aperture 0 to the different tuyeres of the furnaces. Suppose, now, the piston is at the top and begins to be forced down. The air in the upper part of the cylinder becomes more and more rarefied, and the difference of density between it and that of the blast in chamber B, causes the upper valve v' to be applied firmly to the metallic surface before which it is hung. The upper valves v, on the other hand, will be raised by the external air which enters to compensate the rarefaction. The same motion of the piston compresses the air below it, causing the lower valves t; (which open inwards) to be firmly closed, while the valve v will be raised and admit the air into chamber B, whence it passes to the furnace. When the piston is raised the reverse takes place; the lower portion of the cylinder receives air from without, and the upper discharges its air through the pipes leading to- the furnace. Thus a nearly continuous flow is obtained. To ensure regularity the pipe 0 is made to communicate with a closed reservoir of wrought iron, where the variations are destroyed by the elasticity of the air itself. The cylinder here figured is 144 inches in diameter, with a stroke of 12 feet, and discharges about



44,000 cubic feet per minute, at a pressure of ft> to the square inch.
Where it is desirable to make small blast engines do the work of large ones, compensating smallness of size with velocity, it becomes necessary that the air valves be moved otherwise than by the simple action of the air itself. The best form of such an arrangement is that devised by Mr Slate, in which there is an annular slide valve placed outside the blast cylinder; it receives its motion from a crank connected with the fly-wheel shaft. Thus, with lap and lead of the valve properly proportioned, a high velocity can be attained, and the tremor and jar that are observable in some of the larger engines are entirely absent. Two such engines working together, with their cranks at right angles, give such a uniform blast that no regulator of any kind is needed. In Fossey's engine, which appeared in the Exhibition of 1862, the slide valves are replaced by discs with radial perforations, which are put in slow rotatory motion by gearing connected with the main shaft.
The blast engines with slide valves, however, have not proved so advantageous in practice as was anticipated, owing to the large amount of friction on the valve surfaces, greater liability to derangement, and the wear and tear resulting from such rapid motion.
As a recent example of engines of the vertical type, with steam and air cylinders in one line (which have now come a good deal into use in the north of England) we may briefly notice the compound cylinder blowing engines at the Lackenby Iron-Works, Middlesborough. These engines were described by Mr Alfred Hill before the Institution of Mechanical Engineers in 1871. Fig 4 (copied from the drawings by permission of the Institute and of Mr Hill) presents them in vertical section.
They consist of a high pressure non-condensing engine and a low pressure condensing engine, the latter supplied by steam from the former,—this arrangement being adopted for economical reasons. A is the high pressure cylinder (32 inches in diameter) and C the low pressure (80 inches). Both engines have a stroke of 54 inches; and a peculiarity is that they are coupled by cranks placed directly opposite each other instead of, as usual, at right angles,—a light fly-wheel being relied on to carry them over the dead centres. This secures a better balance of the engines, and expansion of the steam in both cylinders in the most advantageous manner; it also obviates the danger of breakages common in the case of right-angle cranks, which probably arises from the tendency to sudden accelera-tion of one engine over the other at the commencement of each stroke,—full steam pressure being then upon both pistons simultaneously, whilst the resistance of the blast pressure is acting against only one of the blowing pistons. In the blowing cylinders B, the inlet valves in the bottom are circular disc valves of leather, eighteen in number. The inlet valves T on the top of the cyUnder are arranged in ten rectangular boxes, having openings in their vertical sides, inside which are hung leather flap valves. The box covers are made hollow, and are carried down between the backs of the leathern flaps (so as to diminish the air-space as much as possible). The outlet valves o for air are ten in number, at each end of the cylinders, and are hung against flat gratings, which are fixed round the circumference of the cylinder. Enclosing each cylinder is an air-tight wrought-iron case M, into which the blast is delivered, and a branch at one side (not shown in figure) conveys the blast to the main. The area of the inlet valves is 860 square inches, or about £th the area of the piston; that of the outlet valves is about |th. For details of the balanced slide valves of the steam cylinders, the surface condenser D, the circulating pump E, the air-pump F, the feed pumps G, <fcc., we must refer to Mr Hill's paper.
cylinders at the regular speed of 24 revolutions per minute is 15,072 feet per minute, measured at atmospheric pressure; thus the supply of blast, including loss by leakage, amounts to 190,000 cubic feet per ton of iron made. The pressure of blast in the blast-main is very free from fluctuations,—owing, doubtless, to its large size, 12^ times the joint

The capacity of each blowing cylinder is 157 cubic feet: consequently, the total quantity of blast supplied from both

capacity of the two blowing cylinders. The indicated power of the engine is found to give a total of 290 horse; that of the two blowing cylinders 258.
Among the more powerful blowing engines of piston and cylinder type at present in use, may be mentioned, besides that at Dowlais referred to above, those of Woolwich dockyard, employed for supplying air to forty forge fires, the Kirkless Hail engines, constructed from Robert Wilson's designs for the Wigan Iron and Coal Company, and the seven engines of Schneider and Co. at Creusot, three of which are horizontal engines of an old type, and the other four direct-acting vertical engines. Descriptions of these will be found in various standard works on metallurgy and engineering. For a description of the large blowing and exhausting engines lately constructed for the new Post-Office in London, see Engineering, 20th February 1874.
An ingenious mode of obtaining a blast is adopted in Savoy, Carniola, and in some parts of America; it is the trompe or water blowing engine. A flow of a few yards of water is required. From the bottom of a reservoir water is admitted, by removal of a plug from a conical-shaped aperture, into a large vertical wooden pipe, which terminates below in a wind chest. The water, falling


in streamlets, carries down with it air drawn in through sloping holes near the top of the pipe. The wind chest below has an opening for escape of the water, and the air passes out from another part, in a regular stream, by a nozzle pipe. To facilitate separation of the water and the air, it is found advantageous to fix a small platform under the bottom of the pipe, on which the water may impinge in its fall. The tension of the blast is determined by the height from which the water fails ; but this height seldom exceeds 27 feet, which gives a pressure of from 1| to 2 ft> to the square inch While the blast obtained is very equable, there is the serious drawback that the air supplied is always more or less laden with moisture. The action of the trompc has been investigated by Mr Piodwell (Philoso-vhical Mag., 1864, 1867).
Another kind of blowing engine, in which water is employed, is that invented by Mr Street; in its simpler form it consists of a barrel-shaped vessel, supported hori-zontally by the two ends of its axis. The cylinder is divided longitudinally by a plane extending from the middle of the internal surface above (the barrel being in its position of rest) to near the opposite side. Suppose the cylinder partly filled with water and made to turn a little way round on its axis, the air on one side will be compressed by the water, while that on the other will be rarefied. A valve opening outwards from the condensed side admits the air to a cavity from which a nozzle pipe proceeds, while a valve opening inwards on the rarefied side admits external air. With additional and correspond-ing valves, the process is repeated on the reverse oscillation of the cylinder. Thus by swinging the cylinder from side to side, by a crank and rod connected with the engine, alternate puffs of air are propelled into a regulative air chest of special construction, which then supplies a steady blast.
Fan-blast machines are frequently employed, especially to urge the fire of steam boilers, and in puddling and reheating, and in the cupola furnaces where anthracite is burnt, or coke used for remelting pig-iron in foundries. In one common form the fan consists of four spokes of a rimless wheel, tipped with vanes and made to rotate in a cylindrical chest, in which it has often a slightly eccentric position. There are openings on both sides round the spindle for admission of air, which, sucked in by the centrifugal action of the fan as it quickly rotates, flows towards the vanes, and is driven through an exit pipe attached to another part of the cylinder.
There are numerous varieties of these engines. An American machine, introduced into England a few years :igo by Mr Ellis, has found considerable favour. It is represented in section in fig. 5. It consists of an iron cylindrical casing A, open about a fourth part of its circumference (a to 6) for admission of air, and an exit pipe' B. Inside the casing is another cylinder, placed eccen-trically to it, and which always fits close up against the wooden packing C. This cylinder acts as driver for the three fan blades or pistons D, which are capable of passing out and in through longitudinal slits in its circumference. There is a shaft passing through the small cylinder, and concentric with it at the ends, but cranked in the middle part so as to become concentric with the casing. The inner cylinder revolves round the axis of the ends of the shaft, and on the cranked part revolve the fan blades or pistons, driven by the cylinder. The outer extremities of the fan blades follow closely the inside face of the casing. The crank is placed opposite to the point where the inner cylinder touches the inside of the casing, always retaining, it must be remembered, the same position; when passing this point, the blades are wholly withdrawn inside the cylinder, but when passing the opposite point they are thrust out to the fullest extent, and are always working into or out of the inner cylinder as it revolves. The air
The rotary blower, invented by Messrs Root of Con-nersville, Ind., is one which has of late years found extensive use both in America and

is thus continually being drawn in at the upper opening, compressed, and delivered by the lower one.

and Europe. The arrange-ment differs in some essential features from that of the ordinary fan ; it acts by regular displacement of the air at each revolution, as shown in fig 6. A pair of horizontal shafts geared together at both ends traverse a case of the form of two semi-cylinders separated by a rectangle equal in depth to the diameter of the semi-cylinders, and in width to the distance be-tween the centres of the shafts. These shafts carry a pair of solid arms, each hav-ing a section somewhat re-sembling a figure of eight;
the action of which, as they revolve, takes the air in by an aperture at the bottom of the machine, and expels it with considerable pressure, if required, at the top. The gearing outside serves merely to keep the revolving pieces in their proper position, and the power is applied directly to each shaft. One of these machines, employed to give the blast in a pneumatic railway under Broadway, New York, delivers, when worked to maximum speed, a volume of 100,000 cubic feet of air per minute. The engine is also much used in the Bessemer steel-works of this country.
Among the exhibits at a recent exhibition of the Franklin Institute in America, was shown a new form of blower, acting much on the same principle as the Root blower, but, according to the report of the committee, offering certain advantages over the latter. From a cross section of the chamber it appears that three drums of equal size are enclosed in it, two in a line below and one above; the upper one is provided with wings, and the two lower have wide slots along their entire length, allowing the wings to enter in the course of rotation. The function of the two lower drums is to supply alternately abutments to prevent the escape of the air. They are caused to revolve in proper relation with the motion of the upper drum by spur-wheels on the journals, which mesh into another spur-wheel on the shaft of the upper drum. In the moving parts of this machine there are no parts that come into actual contact except the teeth of the spur-wheels. The report allows the

superiority of this rotary blower of Baker, inter alia, as regards durability, little pulsation, absence of internal fric-tion and of the need of lubrication, suitability for blowing either hot or cold air, and less power required for the amount of air discharged. A fuller account of it will be found in the American Artisan for March 1875.
For the arrangement of bellows in organs see the article
ORGAN. (A. B. M.)








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