CHAPTER XXVIII

COMPRESSED AIR

An aid to the miner, quarryman and sculptor . . . An actuator for pumps . . . Engraves glass and cleans castings . . . Dust and dirt removed by air exhaustion . . . Westinghouse air-brakes and signals.

Some recent noteworthy advances of invention have been due to co-operation by many workers, not however on such lines of definite group attack as have just been remarked. Among these advances may be chosen for rapid survey the applications of compressed air, of plain and reinforced concrete, the economy of power-production and of fuel for whatever purpose employed. Let us begin with compressed air.

Compressed Air. In Effect Cold Steam for Driving Hammers, Drills, and Picks.

Hammers, drills, and picks, all working by percussion, are among the most effective tools. They may be attached to a steam piston, as are Nasmyth hammers and common quarry drills, yielding a much cheaper product than does hand labor. In many places where it is not feasible to use steam in this direct and most economical way, it is best to employ compressed air which works much as steam does, so that a motor or a drill with no change of build may be operated by one or other motive power at will. Compressed air, unlike steam, may be taken long distances without condensation; in tight receivers it may be kept without any loss as long as we like, and used in mines and tunnels where steam heat would be a nuisance, or where electricity would be unsafe. Electrical drills and cutters, moreover, are liable to have their insulation harmed by working shocks, and by surrounding grit, sand or chips. In mines after a blast of gunpowder, a direct current from the main pipe quickly freshens the air; at all times the cool, pure breeze from the exhaust pipe is a welcome aid to ventilation. Steam, one of the chief servants of industry, must be kept and used hot. When its energy is used to compress air we have at command a substance with all the working quality of steam, without having to keep it warm. As it toils at common temperatures, we can imagine compressed air to be, in effect, cold steam.

Of late years cutters driven by compressed air have been largely adopted throughout the coal mines of the United States. A cutter weighing ten pounds, with air at seventy-five pounds behind it, strikes a blow 160 to 250 times a minute, beginning at the floor and making as little slack as a hand pick intelligently wielded. Other tools, in great diversity, actuated in the same way, ask only skill in guidance instead of muscular drudgery. Air drills are used in mines, wells, tunnels, and rock foundations; at will the mechanism impels a hammer instead of a drill. Air riveters build ships and bridges, as well as fasten together the comparatively small plates of boilers and fire-boxes. With a little variation in its form we have a tool which caulks boilers, tanks, and ships. Air-hammers light and strong have revolutionized the art of cutting and carving stone, the force of a stroke being regulated by a touch. Pneumatic hammers are of two kinds: Valveless hammers in which the piston is the hammer, opening and shutting the inlet and exhaust parts; and valve hammers, in which there is a distinct moving valve. Hammers without valves are always short of stroke, and are chiefly used in caulking and chipping. Some of them yield as many as 250 strokes per minute. Valve hammers do not move at this high pace, rarely exceeding thirty-five strokes per minute, but each stroke is comparatively long and forcible for riveting and the like severe work. In the Keller hammer the valve moves longitudinally with the hammer barrel and in the same direction with the hammer piston, instead of in the opposite direction as is usually the case. A blow, therefore, tends to seat the valve all the more firmly, instead of jarring it off its seat. Another result is that the tool works efficiently even when the valve is loosened by much use. This hammer is manufactured by the Philadelphia Pneumatic Tool Co., Philadelphia.

It is interesting to learn from Mr. W. L. Saunders, of New York, how the air-tools just considered were introduced. He says:--

“Mr. McCoy is entitled to the credit of first applying pneumatic tools to heavy work, such as chipping metals, caulking boilers, cutting stone and so on. He was not, however, the originator of the broad idea, as long before he perfected the tool for heavy work it had been used as a dental plugger, a device working compressed air in a cylinder so that a piston struck the end of a tamping tool, used to insert gold into the cavities of teeth.”

A rock drill, on occasion, may serve as a blacksmith’s hammer. The drill, detached from its tripod, is fastened to a vertical support. The ram, duly supplied with compressed air, is fixed in position over the anvil, upon which it descends more frequently if less forcibly than a steam hammer. A rock drill may also serve to drive drift bolts into the timbers of caissons. This task when effected by ordinary sledge hammers is slow and costly, while with compressed air as a servant capital work is done at much lower expense. The drill is provided with handles so as to be readily managed by two men, who place the anvil, with its cupped end, on the head of the bolt to be driven. Pneumatic energy does the rest.

With dimensions much enlarged an air-driven piston becomes a rammer for foundry sand, for roads and pavements, for tamping the beds of railroads. In foundries a moulder is furnished with a small sand-sifter, vibrated by compressed air; he is now free to use his shovel all the time, so that he does five times as much work as before. Hoists small and large are actuated by the same agency; in every case the mechanism is so simple that rough usage is withstood and repairs, when needed, are easily effected. If a ratchet, a pawl, a bearing, wears out, a new one can be bought at small cost and at once fitted into place. Designers have produced rotary as well as reciprocating air tools; of these a wood-borer is a capital example.

Sometimes it is well worth while to employ compressed air simply as a blast to keep a milling-cutter free from its chips; when the blast is cold, as it usually is, the cutter may turn all the quicker.

Compressed air can do much else than impel pistons of familiar type. In one remarkable device it has put pistons out of business altogether.

Air-Lifts.

Fill a tumbler to the brim with water, take a straw and dip it to the bottom of the glass, blowing as heartily as you can. At once the water overflows because displaced by rising bubbles of air. Instead of a tumbler take a long upright pipe filled with water, send to its base compressed air of adequate pressure, and you have the Pohle air-lift, which carries water into the reservoirs of Fort Madison, Iowa, of Dixon, Illinois, of Asbury Park, New Jersey, and many other towns and villages. On a smaller scale the air-lift brings up water from thousands of wells, rivers, and lakes. Aboard ship it moves water ballast from one compartment to another, so as to give the vessel just the trim or inclination desired. In chemical works it raises liquids so corrosive that no other lifter is feasible. It has no valves or other moving parts to be deranged or hurt in case its stream bears sand or dirt, so that it is a capital drainage pump; after serving thus it may bring sewage to farms and distribute it thoroughly. To be fairly efficient the air-lift requires that two thirds of the length of its upright pipe be immersed below the surface of the liquid to be raised.

Liquids Lifted by Expanding Air.

For oil wells, which may be 2000 or more feet in depth, a lifter not so simple is employed. A pipe, comparatively large, is lowered to the oil. Its base forms a receiver which, at will, may be closed on its earthward side, then through a small inner tube compressed air reaches the oil to force it bodily to the surface of the ground. The Harris pump lifts oil, water, or other liquids with high efficiency: it allows the compressed air after use to act expansively; this helps to drive the compressor; then this expanded air is once more highly compressed, and so recurrently.

A Jack-of-All-Trades.

Compressed air readily moves liquids as masses; it as easily impels them as particles. A lady’s toilet table usually displays an atomizer. Its rubber bulb, sharply squeezed, emits a tiny stream of perfume as a quick air blast breaks a drop of liquid into spray. Magnify this apparatus and you have a painting machine for freight and passenger cars, fences, and out-buildings. Driven as it is with projectile force the pigment penetrates further than if laid on by hand, reaching crannies and crevices which evade a brush. On the same principle Hook’s spraying machine sends Bordeaux mixture into the foliage of an orchard, or delivers a solution of carbolic acid upon the floors, walls, and ceilings of a hospital or a sick-room. Strengthen such a blast and you can elevate, dry, and aerate grain, or lift the culm from a coal heap to a furnace, and then discharge the ashes as they tumble from a grate. Where stretches of water are sandy and muddy, compressed air dredges a channel by stirring up deposits at the bottom.

Removing Dust and Dirt.

An air compressor reversed in direction is an air exhauster, such as we find carrying money in department stores. The powerful in-draft of this apparatus, often drawing large pieces of paper or card into the pipes, has led to the invention of a means of removing dust and dirt, admirable in thoroughness. A receiver, shaped to suit its special task, is passed over pictures and their frames, upholstery, carpets or bare floors, and through the flexible pipe attached to its handle, dust and dirt are borne into a reservoir where they are caught by water for due removal. Ordinary sweeping with a broom, the usual wielding of a feather duster, or a blast of compressed air, but stir up dust and dirt for harmful redistribution. This “vacuum” cleaning method takes dust and dirt wholly away, and with wonderful celerity. See picture opposite page 164. It is astonishing to see a pound of fine flour removed from a thick carpet in twelve seconds, leaving behind not one visible particle. This plan cleanses carpets without their being lifted from floors, or a billiard cloth just as it stands on a table. This service greatly promotes health; the further the physician goes with his microscope the more convinced is he that dust is one of the chief carriers of disease.

Not only dust but sand may be borne when a breeze rises to a gale.

Sand-blast.

In Lyell’s Bay, near Wellington, New Zealand, and in many other places throughout the world, flints have been found so beautifully and symmetrically polished that they were at first believed to be products of art, yet nothing but wind-blown sand had given them form. Fifty years ago globes for gas jets were frosted by a handful of sand quickly thrown from side to side for a few minutes. Strange to say, gunnery was to supply the link to carry sand to labors of much greater moment.

General B. C. Tilghman, of Philadelphia, one day noticed the much worn touch-hole of an old bronze cannon. He felt sure that the wear had been due not so much to outflowing gases as to bits of unburnt powder driven out at each discharge, identifying this abrasion with the roughening of glass in windows facing sandy shores of the sea. In 1870 he began experiments by blowing sand jets with a fan, soon discovering that he had hit upon a cheap and easy means of frosting glass, carving stone, and scouring castings. He was astonished to find that sand readily pierced materials harder than itself, as corundum and toughened steel. To-day the sand-blast executes many new tasks: it resurfaces stone buildings which have become discolored and grimy; it cleanses metallic surfaces for the welder, the electroplater, the enameler; it renews files and rasps; it removes scale from boilers, paint and rust from steel bridges and other structures. The apparatus manufactured by Mr. C. Drucklieb, of New York, designed much in the form of a steam injector, employs air at a pressure of about twenty pounds to the square inch.

Air Compressors.

Compressed air is at work on so large a scale that its economical production and use are matters of consequence. Mechanism for both purposes, of the best design, involves a few simple principles. Suppose we have a cylinder, fourteen inches long, and that with a piston we force the contained air within one inch of its base, so as to occupy 1/14 of its original volume. This act of compression, which we will imagine to be all but instantaneous, will heat the air through 613° Fahr., so that if at 60° when the operation begins, the air will be 673° at the end. Suppose, further, that this air parts with no heat to surrounding metal, and that the piston moves without friction; the compressed air on being allowed to expand will return all the work expended in compression, and resume its first temperature, 60°. If air would serve us in this ideal way, we would have an agent with all the good points of steam and none of its drawbacks. In actual practice several items left out of our imaginary picture must be reckoned with. Air heated in compression quickly warms surrounding masses and has to be cooled when sent off on distant errands, losing much working power in the process. The very act of compression retards itself: the air, because heated, has additional elasticity for the compressor to overcome.

Plainly, the engineer should begin by sending into his compressor air as cool as possible, and during compression he should keep the temperature of the air as low as he can. Moderate pressures, to fifty pounds per square inch or so, may well be effected at a single stroke, the air as it issues from the compressing cylinder passing through pipes immersed in cold water, a similar chilling stream being sent around the cylinder walls themselves. This air at fifty pounds, duly cooled, may now, if we wish, be brought to say 100 pounds pressure in a second cylinder; its output is in turn cooled as before by conveyance through pipes bathed in cold water. The more thorough the cooling, the less moisture will the air contain to give trouble afterward by condensing in pipes or machinery. If a pressure higher than 100 pounds to the square inch is in request, a third compressor may be linked to the second. In some installations, where extreme pressures are attained, four-fold apparatus is employed; its chief economy rests in cooling the air at four distinct stages, greatly diminishing the work which otherwise would have to be wastefully done.

With the energy of steam economically converted into the energy of compressed air, the engineer sends his new servant as far as he pleases. Let us imagine that a mile off he wishes to drive a gang of saws. He will soon notice that the exhaust pipe is very cold, and if the compressed air was not well dried as produced, its moisture will now be deposited not as water merely, but as frost to check the machinery. This is because air, like steam, falls in temperature as it expands at work; that fall measuring the heat-equivalent of the work performed. For the chill which the engineer observes, he has a simple remedy; he surrounds the air pipe, as it enters its machinery, with a small heater, fed with coke, coal, or oil. At once all frost vanishes, and the air with added elasticity is vastly more effective than before. By no other means can so much work be won from fuel as through this device. In some cases a heater has yielded 1.25 horse power for an hour in return for each pound of coal it has burned.

In producing compressed air, inventors step by step have kept in view the best steam practice. It was long ago observed that working steam when wholly expanded in one cylinder chills itself, imparting its chill to the cylinder walls so that they seriously cool the next charge of steam, lowering its value for motive power. In a multiple expansion engine of four successive cylinders, each in turn receives the steam, which with thorough jacketing is maintained at the highest temperature possible. Keeping to converse lines the compressor divides its task into stages, at each of which a desired change of temperature can be easily effected. With steam this change consists in adding heat; with compressed air it consists in abstracting heat.

A Centralized Air Plant.

Thirty miles from Cleveland, at North Amherst, Ohio, is the largest sandstone quarry in the world. Its owners, the Cleveland Stone Company, in their original plant employed steam from no fewer than forty-nine boilers, all machinery, including drills and channelers, being driven by steam. In January, 1904, this was replaced by a centralized air plant which has resulted in marked economy. In the power-house four water-tube boilers, each of 257 horse-power rated capacity, drive compound compressors which deliver air at about 100 pounds pressure. This air, duly piped, is distributed to drills, channelers, hoists, pumps, saws, grindstones, forge fires, and so on. Economies, familiar in electrical centralization, are here paralleled in an interesting way. In the working day not a moment is wasted. When the whistle blows the full working pressure is ready to begin work and maintain duty until night. There is no fluctuation of pressure due to careless boiler attendance; no wheeling coal or water barrels to keep pace with advancing channelers. Some of the old boilers, discarded from steam service, are used as air receivers, these and other reservoirs, together with the pipe line itself, unite their immense storage capacity so that throughout the day there is no peak load. Incidentally the new plant renders the quarry free from smoke-laden steam such as of old darkened its air and soiled its output. Fuel and labor under this system were reduced one half when a month of the old service was compared with a month of the new. In one case steam is used for power outside of the main plant. Close to the power-house is a mill where eleven gang saws are driven by a steam engine of 175 horse-power. The nearness of this engine to the boilers ensures a somewhat higher economy than if compressed air were employed. Here, as everywhere else, the engineer engages whatever servant will do good work at the lowest wages.

Westinghouse Air Brakes and Signals.

By all odds the most important use of compressed air is that developed by Mr. George Westinghouse, of Pittsburg, in his automatic brakes for railroads. For each locomotive he provides an air compressor which fills in the engine itself, and beneath each car, a reservoir of compressed air. Every reservoir aboard a long train in rapid motion may at the same instant, by a touch from the engine-runner, actuate the brakes so as to stop the train in the shortest possible time. This invention has accomplished more for the safety of quick railroad travel than any other device; no wonder, then, that Westinghouse brakes are in all but universal use. They are now being adopted for trolley-cars which often require to be stopped in the briefest possible period. The Westinghouse Company builds and installs elaborate signal systems worked by compressed air and electricity. All these are described and pictured in the “Air Brake Catechism,” by Robert H. Blackall, published by N. W. Henley & Co., New York. This book is constantly appearing in new editions, of which the reader should procure the latest.