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BELLOWS
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BELLOWS and BLOWING See also:MACHINES, appliances used for producing currents of See also:air, or for moving volumes of air from one M. 2.3 See also:place to another. Formerly all such artificially-produced currents of air were used to assist the See also:combustion of fires and furnaces, but now this purpose only forms a See also:part of the uses to which they are put. Blowing appliances, among which are included bellows, rotary fans, blowing engines, rotary blowers and See also:steam-See also:jet blowers, are now also employed for forcing pure air into buildings and mines for purposes of See also:ventilation, for withdrawing vitiated air for the same See also:reason, and for supplying the air or other See also:gas which is required in some chemical processes. Appliances of this See also:kind differ from air compressors in that they are primarily intended for the See also:transfer of quantities of air at See also:low pressures, very little above that of the See also:atmosphere, whereas the latter are used for supplying air which has previously been raised to a pressure which may be many times that of the atmosphere (see See also:POWER TRANSMISSION: Pneumatic). Among the earliest contrivances employed for producing the See also:movement of air under a small pressure were those used in See also:Egypt during the See also:Greek occupation. These depended upon the See also:heating of the air, which, being raised in pressure and bulk, was made to force See also:water out of closed vessels, the water being afterwards employed for moving some kind of mechanism. In the See also:process of See also:iron smelting there is still used in some parts of See also:India an artificial blast, produced by a See also:simple See also:form of bellows made from the skins of goats; bellows of this kind probably represent one of the earliest contrivances used for producing currents of air. The bellows' now in use consists, in its simplest form, of two See also:flat boards, of rectangular, circular or See also:pear shape, connected See also:round their edges by a wide See also:band of See also:leather so as to include an air chamber, which can be increased or diminished in See also:volume by separating the boards or bringing them nearer together. The leather is kept from collapsing, on the separation of the boards, by several rings of See also:wire which See also:act like the ribs of animals. The See also:lower See also:board has a hole in the centre, covered inside by a leather flap or See also:valve which can only open inwards; there is also an open outlet, generally in the form of a See also:pipe or nozzle, whose See also:aperture is much smaller than that of the valve. When the upper board is raised air rushes into the cavity through the valve to fill up the partial vacuum produced; on again depressing the upper board the valve is closed by the air attempting to See also:rush out again, and this air is discharged through the open nozzle with a velocity depending on the pressure exerted. The current of air produced is evidently not continuous but intermittent or in puffs, because an See also:interval is needed to refill the cavity after each See also:discharge. In See also:order to remedy this draw-back the See also:double bellows are used. To understand their See also:action it is only necessary to conceive an additional board with valve, like the lower board of the single bellows, attached in the same way by leather below this lower board. Thus there are three boards, forming two cavities, the two lower boards being fitted with air-valves. The lowest board is held down by a See also:weight and another weight rests on the See also:top board. In working these double bellows the lowest board is raised, and drives the air from the lower cavity into the upper. On lowering the bottom board again a fresh See also:supply of air is See also:drawn in through the bottom valve, to be again discharged when the board is raised. As the air passes from the lower to the upper cavity it is prevented from returning by the valve in the See also:middle board, and in this way a quantity of air is sent into the upper cavity each See also:time the lowest board is raised. The weight on the top board provides the necessary pressure for the blast, and at the same time causes the current of air delivered to be fairly continuous. When the air is being forced into the upper cavity the weight is being ' The Old See also:English word for this appliance was bldstbaelig, i.e. " See also:blow-bag," cf. See also:German Blasebalg. By the 11th See also:century the first part of the word apparently dropped out of use, and baelig, bylig, bag, is found in See also:early glossaries as the See also:equivalent of the Latin follis. Baelig became in Middle English bely, i.e. " belly," a See also:sack or bag, and so the See also:general word for the lower part of the See also:trunk in See also:man and animals, the See also:stomach, and another form, probably See also:northern in origin, belu, belw, became the See also:regular word for the appliance, the plural " bellies " being still used till the 16th century, when "bellows " appears, and the word in the singular ceases to be used. The verb " to bellow " of the roar of a See also:bull, or the low of a cow, is from Old English bellan, to See also:bell, roar. 1I raised, and, during the interval when the lowest board is descending, the weight is slowly forcing the top board down and thus keeping up the flow of air. See also:Hand-bellows for domestic use are generally shaped like a pear, with the See also:hinge at the narrow end. The same shape was adopted for the older forms of smiths' bellows, with the difference that two bellows were used superposed, in a manner similar to that just described, so as to provide for a continuous blast. In the later form of smiths' bellows the same principle is employed, but the boards are made circular in shape and are always maintained roughly parallel to one another. These are shown on See also:figs. z and 2. Here A is the blast pipe, B the movable lowest board, C the fixed middle board, See also:close to which the pipe A is inserted, and D is the movable uppermost board pressed upon by the weight shown. The board B is raised by means of a hand See also:lever L, through either a See also:chain or a See also:con- necting See also:rod, and lowered by a weight. The See also:size of the weight on D depends on the air pressure required. For instance, if a blast pressure of See also:half a See also:pound per square See also:inch is wanted and the boards are 18 in. in See also:diameter, and therefore have an See also:area of 254 sq. in., on each of the 254 sq. in. there is to be a pressure of half a pound, so that the weight to See also:balance this must be half multiplied by 254, or 127 lb. The diameter of the air-pipe can be varied to suit the required conditions. Instead of bellows with flexible sides, a sliding arrangement is sometimes used; this consists of what are really two boxes fitting into one another with the open sides both facing inwards, as if one were acting as a lid to the other. By having a valve and outlet pipe fitted as in the bellows and sliding them alternately apart and together, an intermittent blast is produced. The See also:chief defect of this arrangement is the leakage of air caused by the difficulty in making the See also:joint a sufficiently See also:good See also:fit to be air-tight. Blowing Engines.—Where larger quantities of air at higher pressures than can conveniently be supplied by bellows are required, as for blast furnaces and the See also:Bessemer process of See also:steel-making, what are termed " blowing engines " are used. The mode of action of a blowing See also:engine is simple. When a See also:piston, accurately fitting a See also:cylinder which has one end closed, is forcibly moved towards the other end, a partial vacuum is formed between the piston and the See also:blank end, and if this space be allowed to communicate with the See also:outer atmosphere air will flow in to fill the vacuum. When the piston has completed its movement or " stroke," the cylinder will have been filled with air. On the return of the piston, if the valve through which the air entered is now closed and a second one communicating with a chamber or pipe is opened, the air in the cylinder is expelled through this second valve. The action is similar to. that of the bellows, but is carried out in a See also:machine which is much better able to resist higher pressures and which is more convenient for dealing with large quantities of air. The valves through which the atmosphere or " See also:free " air is admitted are called " See also:admission " or " suction " valves, and those through which the air is driven from the cylinder are the " discharge " or " delivery " valves. Formerly one See also:side only of the blowing piston was used, the engine working " single-acting "; but now both sides of the piston are utilized, so that when it is moving in either direction suction will be taking place on one side and delivery on the other. All processes in connexion with whichblowing engines are used require the air to be above the pressure of the outer atmosphere. This means that the discharge valves do not open quite at the beginning of the delivery stroke, but remain closed until the air in the cylinder has been reduced in volume and so increased in pressure to that of the air in the discharge chamber. The power used to actuate these blowing-engines is in most cases steam, the steam cylinder being placed in See also:line or " tandem " with the air cylinder, so that the steam piston rod is continuous with or directly joined to the piston rod of the air cylinder. This See also:plan is always adopted where the cylinders are placed horizontally, and often in the See also:case of See also:vertical engines. The engines are generally built in pairs, with two blowing cylinders and one high-pressure and one low-pressure steam cylinder, the piston rods terminating in connecting rods which are attached to the pins of the two cranks on the See also:shaft. In the centre of this shaft, midway between the two engines, there is usually placed a heavy flywheel which See also:helps to maintain a See also:uniform See also:speed of turtling. Some of the largest blowing engines built in. See also:Great See also:Britain are arranged as See also:beam engines; that is to say, there is a heavy rocking beam of See also:cast iron which in its middle position is See also:horizontal. One end of this beam is linked by a See also:short connecting rod to the end of the piston rod of the blowing cylinder, while the other end is similarly linked to the top of the steam piston rod, so that as the steam piston comes up the air piston goes down and See also:vice versa. At the steam end of the beam a third connecting rod See also:works the See also:crank of a flywheel shaft. About the end of the 19th century an important development took place which consisted in using the See also:waste gas from blast furnaces to form with air an explosive mixture, and employing this mixture to drive the piston of the actuating cylinder in precisely the same manner as the explosive mixture of See also:coal gas and air is used in a gas engine. Since the See also:majority of blowing engines are used for providing the air required in iron blast furnaces, considerable saving should be effected in this way, because the gas which escapes from the top of the See also:furnace is a waste product and See also:costs nothing to produce. The general action of a blowing engine may be illustrated by the sectional view shown' on fig. 3, which represents the Flo. 3.-See also:Section of Cylinder of Early Blowing Engine (1851). See also:internal view of one of the blowing cylinders of the engines erected at the Dowlais Ironworks as far back as 1851. Many of the details are now obsolete, but the general See also:scheme is the same as in all blowing engines. Here A is the air cylinder; in this is a piston whose rod is marked R; this piston is usually made air-tight by some form of packing fitted into the groove which runs round its edge. In this particular case the cylinder is placed vertically and its piston rod is actuated from the end of a rocking beam. Additional information and CommentsThere are no comments yet for this article.
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