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Apart from its use in motor ships and on railways, compressed air is an excellent source of power for rock drills, riveting hammers and other engineering implements. It is easily produced and distributed, and is perfectly safe in mines or other dangerous situations

ROTARY AIR COMPRESSOR ON TEST in the maker’s works

ROTARY COMPRESSOR ON TEST in the maker’s works. This installation supplies 2,471 cubic feet of air a minute at a pressure of 50 lb. per sq. in. The rotor of the compressor revolves at a speed of 485 revolutions a minute, but speeds of nearly 3,000 revolutions a minute are attained in some compressors.

NOT long ago, compressed air was but little used and was even looked on with, suspicion as a suitable form either for the transmission or for the application of power. Now, however, it is scarcely possible to find a single field of industry in which compressed air does not play an important part.

Steam as a motive power was for long universal wherever it could be applied. Its properties were well understood, but its functions were almost entirely confined to the driving of engines of various kinds, for certain heating processes and for the operation of such machines as steam hammers. Steam cannot, however, without careful provision against condensation, be easily conveyed long distances through pipes.

Electricity, although it can conveniently be transmitted by wires and cables over long distances with little loss, has its limitations also. The ubiquitous nature of compressed air is astonishing, for its applications are met with not only in the engineering world, but also in every industry. In the same way as steam and electricity, compressed air has first to be produced by independent mechanical means. The power is not ready made; it does not exist, as water power does, in natural conditions, ready to be tapped from waterfalls with their potential energy.

Air, however, is plentiful and even cheaper than water. It can be compressed to the moderate pressures at which it is most commonly used in a simple form of engine. It can be transmitted in that state through long pipe lines with little loss in the process, and loses little, unless by leakage (which can be prevented) when left standing a long time. For use in coal mines, in explosives factories and in other dangerous surroundings, compressed air is safe. The earliest practical use of compressed air on a large scale was in 1845, when it was conveyed a distance of 750 feet to the workings in a French colliery. Since that time it has been an increasingly recognized necessity for supplying the power to machines in collieries all over the world.

When work was started on the Mont Cenis Tunnel through the Alps in 1857, it soon became apparent that it would take a lifetime to complete it by hand-drilling methods. In 1861-62 an air-operated rock drill was used with such success that by the installation of large numbers of these the tunnel was completed in 1871, or only nine years after these tools had been introduced. The improvements in such methods, assisted by other modern appliances, enabled the 15-miles Ben Nevis Tunnel to be completed in only three years, athough it is nearly twice the length of the Mont Cenis Tunnel.

It would be impossible to carry out diving on the underwater works involved in the laying of the foundations of piers and bridges without the extensive use of compressed air. The modern ship, especially the motorship, would be paralysed without compressed-air auxiliary machinery. Every navy depends on highly compressed air for propelling torpedoes and for operating other machines.

Compressed air is most extensively used, in industrial applications, at a pressure of from 80 lb. to 120 lb. per sq. in. This makes for safe conditions in everyday work, and involves no special mechanical difficulties such as accompany the use of high pressures.

The act of compressing air, or any gas, increases its temperature. Conversely, expansion cools it down. To generate such heat where it is not wanted, or without being able to use it effectively, means the consumption of energy to no purpose, and therefore is wasteful. Thus, one of the prime factors in the design of efficient air compressors is to provide for the dissipation of the heat generated during the process of compression.

Common terms used in the design and construction of air compressors are “isothermal” and “adiabatic” compression.

Roughly, in an isothermal design heat caused by compression is removed as quickly as it is generated. Adiabatic is the converse of isothermal, none of the heat being carried away during the process. The conditions relating to the use of steam are almost diametrically opposite, for everything possible is done to avoid the slightest loss of heat.

Two-Stage Compression

The object of the designer of an air compressor is to attain as closely as possible to the isothermal condition. In practice, the performance of air compressors falls between the two extremes. Although the most generally adopted pressure range is only between 80 lb. and 120 lb. per sq. in., there is a diversity of opinions among air-compressor manufacturers as to whether the final pressure should be obtained in a single stage of compression or in two stages. Some obtain it all in one stage, but many others consider that the additional complication of using two stages is made worth while by the improved results obtained. The main difference in the respective forms of construction is that, as far as the air-compressing cylinder portion is concerned, single-stage compression can be effected by one cylinder and piston, whereas two stages generally require two cylinders and pistons, with their additional accessories.

Two-stage compression is also carried out, in some designs, by combining the two stages in a single differential trunk form of piston, with a correspondingly stepped cylinder, each portion having a different diameter. This enables the usual piston rod to be discarded and makes possible a shorter engine, though some makers prefer to retain the differential cylinder as a separate unit and retain the piston rod, considering that the extra cost is justified by improved performance.

When two stages are adopted it is usual to introduce an “intercooler” between the two stages of compression to reduce the temperature by extracting the heat generated in the first stage before the second stage begins. This intercooler contains a nest of small diameter tubes through which a constant supply of cold water is circulated, and round which the air circulates on its way from the low-pressure to the high-pressure cylinder.

AIR COMPRESSORS FOR A GOLD MINEApart from this feature, the general construction of low- and of moderate pressure machines is in principle somewhat similar, though the design and construction of details may differ. In some instances the reciprocating compressor is built as a horizontal, in others as a vertical machine. Some are totally enclosed, others only partly so. Speeds of rotation vary according to the particular maker’s own practice or according to the conditions in which the compressor has to work.

COMPRESSORS FOR A GOLD MINE. This set of four motor-driven air compressors was installed in the New Modderfontein Gold Mine, in Transvaal, South Africa. Each compressor has an output of 6,500 cubic feet a minute. Compressors of this type can be driven directly, without gearing by electric motors using either direct or alternating current.

In the early days of compressed air an ordinary steam engine was sometimes, with slight alterations, converted into an air compressor. When the questions of efficiency and working conditions received more consideration the air cylinder portion had to be specially designed and made to meet the specific requirements. Below the air cylinder (in vertical designs) or beyond it (in the horizontal types) the design still follows steam engine practice.

In an ordinary single-stage machine, the air cylinder is water-jacketed, in the same way as the water-cooled cylinder of a motor car engine. The one end cover, or cylinder head, of a single-acting compressor is also water-jacketed; in a double-acting compressor both end covers are water-jacketed. The valves controlling the admission and the delivery of the air in most compressors take the form of light disks made of the best quality steel about in. thick, and they are held lightly on their seats by small springs. The disks generally have two or three concentric circular slots through which the air passes as they are pressed off or on to their seats by the air, and they have a lift of little more than their own thickness. Separate valves are used for admission and delivery.

Running in a Vacuum

For a two-stage machine, with separate cylinders, the larger, or low-pressure cylinder forms the first stage of compression and a smaller cylinder the second stage, the air already being partly compressed when it enters the second stage. In the differential trunk piston type of two-stage compressor the top or larger diameter portion of the piston does the first-stage compression. It passes the discharge to the annular space between the larger bore of the cylinder and bottom portion of the piston, which has a smaller diameter and compresses the air further. Each portion of the piston is, however, only single-acting, whereas in the double-acting machine each piston functions on either side alternately. For doubling or trebling the capacity at which one cylinder may be rated the number of cylinders and cranks is accordingly increased.

As the possible output of the compressor must always be in excess of the average demand, some form of governing is necessary. Generally this is carried out in a simple way by an automatically controlled valve admitting the air to the compressor. This valve, which is balanced for pressure and therefore requires little effort to move it, remains fully open in its normal position, being held in that position by a light spring. When the pressure in the air receiver, into which the compressed air is discharged, reaches the working limit, say 100 lb., a small piston fixed to the spindle of the valve is made to lift the main throttle valve and close- it, stopping the supply of air to the compressor. As the throttle valve closes, air is admitted to another small valve which unloads the second-stage cylinder and then until the pressure falls to, say, 95 lb., the compressor runs in a vacuum.

RIVETING COPPER STAY BOLTS by which a firebox is fixed to a boilerIn another design a slightly different result is aimed at. The unloading device operates on the inlet valves of the cylinder by lifting them off their seats and holding them so. Thus the air drawn into the cylinder is freely discharged again without being compressed. When the pressure has dropped to the desired minimum the unloader closes, allowing the inlet valves to seat and compression to be resumed. In certain instances, where the compressor is driven by an electric motor, means are adopted either for automatically slowing down the motor when the demand is reduced or for stopping it altogether, starting it up again automatically when necessary. The motive power required for driving the compressor may be of whatever form is most suitable for the conditions obtaining. Thus it may be derived from a steam or oil engine or from an electric motor, either direct-coupled or driving through suitable gearing.

RIVETING COPPER STAY BOLTS by which a firebox is fixed to a boiler. The work is being done by a pneumatic riveting hammer, which can close a stay bolt head in twenty seconds or less. Pneumatic riveters generally work by percussion and are noisy.

To avoid excessive wear in the cylinders from dust-laden air, an air filter is generally fitted on the air inlet of the cylinder. In a moist atmosphere there may be a considerable water content in the air, and in the compressing process this water content may accumulate sufficiently to become troublesome in the pipe lines and machines driven by the compressed air. For many purposes the rotary type of compressor has advantages over the reciprocating kind, especially as the rotary type can be driven at high speeds, enabling a small compact machine to be made. The rotary type is also commonly used when dealing with low pressures and large volumes, though it can be made for fairly high pressure conditions. Most compressors of this type comprise a rotor composed of a cylindrical body fixed to the driving shaft, and fitted with a number of radiating blades which fit loosely in corresponding slots. The axis of the rotor shaft is placed eccentrically in the cylindrical fixed stator casing inside which it runs.

As the rotor revolves the blades are flung out by centrifugal force against the casing. These blades divide the space between rotor and stator casing into a number of spaces which diminish wedge-like in depth as they approach the delivery port on the side opposite to the admission port. In this way the air is sucked in at the thick end of the space and becomes more and more compressed until it reaches the thin end, when it is discharged through the delivery port of the compressor.

These machines are suitable for pressures up to 30 lb. in a single stage, and can be made in two stages for pressures between 30 lb. and 120 lb. The speeds vary from rather less than 3,000 revolutions a minute for the smallest sizes, to about 500 revolutions in the largest of the type. Thus they are suitable for direct coupling to high-speed motors, an arrangement also conducive to small space and weight.

Applications in Mines

Probably the largest users of compressed air on land are the coal mines. Coal-cutting machines driven by air, performing their work either by percussive or by rotary cutting action, give an enormously increased output compared with hand cutting. Though sometimes of great weight, they are compact and adaptable for working in thin seams. The cutting tool may be driven either by a two-cylinder reciprocating air engine or by an air turbine. Compressed-air locomotives are not used so much in collieries in Great Britain as in America and on the Continent, probably because of the narrowness and shallowness of the underground roadways. Compressed-air locomotives are also used in some paper mills and other works where fire dangers have to be guarded against.

An air reservoir takes the place of the ordinary steam locomotive boiler. The main reservoir is charged with air at a pressure of 800 lb. to 1,000 lb. from a supply pipe. From there it passes to an auxiliary reservoir, the pressure in this being automatically reduced to about 150 lb., at which pressure the locomotive cylinders are supplied. Only short runs are possible, their duration depending on the storage capacity of the main reservoir.

Other pneumatic tools used by the miner are the implement for drilling shot holes after the coal has been undercut, the coal pick, in place of the hand pick, and other appliances such as fans driven by air turbines, and lamps having individual air-driven dynamos.

For metal mining, especially gold mining, large installations of compressed-air plant are necessary to cope with the extensive use of pneumatic rock drills for drilling holes for the insertion of explosives for blasting purposes. Drills have developed much since the days of the Mont Cenis Tunnel, and vary in size, using from 70 to 200 cubic feet a minute of free air. The usual rating of the capacity of any air compressor is given as the quantity, or volume, of free air at atmospheric pressure, which it will compress up to the specified pressure in one minute.

PNEUMATIC DRILLS at workSuch rock drills are powerful and will drill holes up to 4 in. diameter and up to 10 feet deep or more through solid rock. Somewhat similar tools are also used in stone quarrying work and rock excavations of various kinds.

PNEUMATIC DRILLS at work, breaking up ground for the foundations of a new pier on the Hudson River, New York. Long flexible pipes deliver compressed air to the drills from an air compressor situated at a convenient central point.

Perhaps the most general use to which compressed air is put is the operation of small tools of various kinds, such as are common in all branches of the shipbuilding and engineering industries. Although hydraulic riveting is still extensively adopted, especially for large fixed structures and for heavier work, pneumatic riveting is largely used also. Hydraulic machines require a high water pressure, provided by special pumps of a more or less stationary form, not easily transported about. Quite small portable air-compressing plants, however, driven by internal combustion engines, can easily be moved from place to place and located near the work to be done. Where electric current is available such portable machines are driven by motors supplied through trailing cables.

Further, hydraulic tools require rigid pipes having to stand high pressures up to 800 lb. or 1,000 lb., but the air pipe from a compressor has to stand a pressure of only about 100 lb., and can be made flexible and convenient for handling in fairly long lengths. Pneumatic riveters, caulkers, chipping tools, concrete breakers and so on generally work by percussion, and these, especially the concrete breakers, compel the attention of the public by their noise. Their general design is much the same for all percussion tools.

The main body of the tool forms the cylinder, made of case-hardened nickel alloy steel, and contains the working piston which strikes the blow on the end of the loose hammer head or the rivet “set”. Behind the piston is an ingenious yet simple little valve which controls the distribution of the air into and out of the cylinder. Enclosed in the hollow handle is a throttle valve which is operated by the workman. In certain designs this valve is provided with a throttle adjuster to regulate the amount of air admitted, on which depends the force of the blow.

Braking on Railways

In closing up hot rivets the rivet set gets hot, conveying this heat more or less to the body. In some designs a system of air-cooling ports is provided to counteract this. This enables more work to be done in a given time and gives the tool a longer life, with less discomfort to the operator.

Allied to these are all forms of cutting tools, the main applications of which are for such purposes as cutting steel plates, removing rivet heads, cutting and dressing stone work of all kinds, scaling or scraping off old paint and rust, in another class are the compressed-air tools which require a rotary motion, such as drilling machines, grinders, small hoists, polishers and so on. In these a small air motor is used, of the two- or three-cylinder reciprocating type for the larger tools, or of the rotary type with vanes for smaller ones or those in which high speeds are wanted. Various forms of pumping are carried out by the use of compressed air, either by merely forcing it through the pipes of an artesian well — the air-lift method — or by direct displacement under pressure.

The castings in an iron or brass foundry may involve work of a delicate nature. However firmly the sand may be rammed round a pattern in its moulding box, if the pattern is not gently withdrawn the mould will often be spoilt and will have to be done over again. For small castings and repetition work pneumatic moulding machines are often used. The sand is pressed to the right consistency by a quick-acting compressed-air ram which raises the table carrying the pattern, moulding box and sand, against a fixed head. Most moulding machines also have a device known as a vibrator, whose purpose is to give the pattern a definite vibratory movement, and allow the pattern to be withdrawn without damage or fracture of the mould, giving a more accurate result. Castings as they come out of the mould are often rough in places. One of the most effective means of removing that roughness and giving a clean smooth finish is the use of a compressed-air sandblast. The process consists of blowing, at a pressure of 15 lb. to 30 lb., a stream of sand or other abrasive material on to the surface of the object to be cleaned. Other pneumatically operated machines and tools found in a foundry are sand riddlers, sifters, chipping hammers, grinders and hoists.


AIR COMPRESSOR IN A COLLIERY near Chesterfield, Derbyshire. Driven by steam, the compressor has an output of 4,000 cubic feet of air a minute. Compressed air is used extensively in coal mines, because the danger of fire is minimized. Coal-cutting machines, drills and other equipment are driven by this means.

As the demand for higher speeds on railways developed, one of the most important problems connected with its achievement was that of ample braking power. Here the compressed-air brake system — mostly favoured in America — and also what might be termed the negative pressure, or vacuum brake so extensively used in Great Britain, have played an important part. They both function by the application of air pressure to the brake cylinder containing the piston which actuates the brake levers; but opposite methods are used. In the compressed-air system air is compressed by a small compressor carried on the locomotive, and applied through pipes connecting up all the brake cylinders along the train, which normally are at atmospheric pressure. The opening of a small valve admits the higher pressure to the cylinders and applies the brake.

In the vacuum system the cylinder and train pipes are connected up to an “injector” on the locomotive. This creates a vacuum by means of a steam jet which, by blowing through a specially designed nozzle, sucks in the surrounding air with the steam. The air contained in the whole pipe system and brake cylinders of the train is exhausted by this method, a vacuum being thus created. As long as that vacuum is maintained the pistons in the brake cylinders remain at the bottom of the cylinders, and the brakes are in

the off position. To apply the brakes, all that is required is to admit air from outside the pipe system. Then the in-rushing air at atmospheric pressure forces the piston of the brake cylinder up to the top of its stroke, the movement applying the brake. One advantage of the vacuum system is that, should one or more of the coaches become detached from the rest by the breaking of a coupling link or from any other cause, the train pipe will at once part and admit air from outside, thereby automatically putting on the brakes on every part of the train, as well as on the detached coaches.

1½ Tons per Square Inch

Another factor in enabling high train speeds to be obtained is an efficient signalling system. Many railways use the electro-pneumatic system, in which electricity is the controlling power, and compressed air does the work of operating the signals. The automatic signalling system of the London underground railways is carried out by this means.

The manufacture of glass bottles depends almost entirely on compressed air. In modern machines for this work the “gob” of glass as it issues from the furnace is discharged through an orifice in a measured quantity by an air-operated plunger. The glass is cut off by pneumatic shears, the bottle is blown by compressed air, and the whole complicated mechanism is automatically operated and controlled by the same medium. A bottle-making machine is shown in the chapter “The Manufacture of Glass”.

It is almost impossible to enumerate all the applications of compressed air in the thousand spheres of manufacture, including the brewing, dyeing, chemical and other industries.

Motorships driven by oil engines and steam-driven ships using oil fuel now rival in tonnage the steam-driven ships using coal as fuel. All these oil engines require air-compressor auxiliaries of some kind. For those engines using air injection, compressed air at from 800 lb. to 1,000 lb. pressure is needed, and for the more modern motors, working with airless injection, compressors are needed for starting purposes.

Much higher pressures are used for naval work; for example, in submarines and for the propulsion of torpedoes. The demand for highly concentrated power in a restricted space makes this necessary, and for this purpose air pressures reach 3,500 lb., or 1| tons per sq. In.


TWO-CRANK STEAM-DRIVEN AIR COMPRESSORS in the Horden Colliery, Co. Durham. Each of these compressors has an output of 3,000 cubic feet a minute at pressures varying from 80 lb. to 120 lb. per square inch.

You can read more on “How Gold is Mined”, “The Manufacture of Glass” and “The Oil-Engined Ship” on this website.

The Uses of Compressed Air