AIR COMPRESSORS.

Compressed airis air compressed by mechanical force into a state of more or less increased density.The power obtained from the expansionof greatly compressed air in a cylinder, on being set free is used in many applications as a substitute for steam or other force as in operating drills, shop tools andengineswhich are driven by the elastic force of compressed air.

A compressoris a machine usually driven by steam by which air is compressed in a receiver so that its expansion may be utilized as a source of power at distances where an ordinary engine could not be conveniently used.

The compressor proper comprises two sets of valves, usually designed to be opened automatically by excess of pressure under them and to be closed by gravity or by the action of springs when the pressures become equal. The inlet valves open just after the piston commences its stroke, when the expansion of the compressed air remaining in the cylinder behind the piston has lowered the pressure above the valves. They close at the end of the intake stroke, just as the piston comes to rest. The outlet valve lifts during the compression stroke, at about the time the rising pressure in the cylinder becomes equal to that in the outlet passage above the valves; and they close when the flow of air ceases as the piston completes its stroke.

Any of the accurately fitted steam engine valve gears may be used for compressors, observing only that the compressor is in every way a reversed steam engine.

Compressed air is already used in the operation of1. Cranes, hoists and motors of all types and of all capacities.2. Portable drilling, reaming and tapping machines.3. Riveters and stay-bolt cutters, calking and chipping tools.4. Shop tools of all kinds.5. Air brakes.6. Sand blasts.7. Rock drills and coal mining machines.8. Pneumatic locomotives and street cars.

Fig. 362.—see page70.

and also for the following diversified uses,1. Pumping water, sewage, oil and acids.2. Raising sunken vessels.3. Refrigerating and ice making.4. Transmitting messages through pneumatic tubes.5. Cleaning carpets and railroad cars and seats.6. Sinking caissons and driving tunnels through silt and soft earth.7. Tapping iron furnaces.8. Transmitting power for all purposes.

The office of the air compressor is to store up air under high pressures, which can be utilized at a greater or less distance, without sustaining any loss by condensation in the pipes, as is the case of carrying steam in pipes long distances.

Air stored under pressure in a reservoir can be used expansively, in an ordinary steam engine returning an equivalent amount of work that was required to compress it—less the friction.

The admission of the air being through a single tube, it creates a constant flow of air in one direction only, thus filling the cylinder at each stroke with air at atmospheric pressure. This movement gives a momentum to the air which causes it to fill the cylinder to its fullest extent at each stroke.

Air compressors may be driven in various ways, but the most commonly used are those which are directly connected to a steam engine, thus doing away with intermediate machinery. When the air piston draws in a charge of air, the air fills the cylinder at atmospheric pressure, or a little below, and on the return stroke of the piston it has to be compressed to the same pressure as in the receiver before it can lift the delivery valve, and as the valve is held to its seat by a spring, and also by its own weight, the pressure has to be considerably above that of the receiver before the valve will lift. To overcome this the valves are operated by mechanical means, which lifts them at a point of the stroke, when the pressure in the cylinder corresponds with that of the receiver.

Fig. 363.—See page71.

This arrangement avoids pounding of the valves as well as the noise caused by the air when rushing at much higher pressure from the cylinder into the receiver.

For the sake of economy, air compressors are compounded, as for example, by drawing the air into a large cylinder and compressing it to a certain stage, whence it passes into a smaller cylinder, which compresses it to a much higher pressure.

In a simple compressor, for very high pressures, there is at the end of the stroke a large volume of air left in the clearance space, which expands on the return stroke, to atmospheric pressure, before another charge of air can be drawn in.

But in the compound compressor, the air is delivered from the low pressure receiver to the high pressure cylinder far above atmospheric pressure, thus the remaining air need not expand so much and allows the cylinder to take a larger volume of air. The load is also distributed more evenly.

The following are valuable “points” relating to the care and management of air compressors.

As in a steam pipe line, elbows should be avoided in an air pipe line but unlike a steam pipe it should be larger.

A mistake is sometimes made in purchasing a compressor built for a low altitude and trying to run it in a higher elevation; the machine then experiences the same trouble that some people do, in not being able to get breath enough under the changed conditions.

The use of cheap oils, especially in an air cylinder is a most serious mistake, as the least tendency to gum will prevent the valves from properly seating, and even with the best of oils, it is well to use a small amount of mineral oil at times.

In localities where the water is bad, the water jacket will require extra attention, as it gets as badly scaled like steam boilers, principally due to a very slow or retarded circulation, which allows the sediment to settle, and should the water supply be shut off, even for a few minutes, the cylinder heat will bake it so hard as to give considerable trouble. It is a good plan to put a good boiler compound in the water jacket, and run the machine for some time without any circulation.

Fig. 364.(See page71.)

In this case good judgment must be used not to run too long or too fast, as the cylinder will heat very quickly and is liable to be damaged.

There are many emergency ways of stopping small pipe leaks; any good sticky substance, such as tar, wax, tallow candles, or even chewing gum, melted and applied on narrow strips of cloth and wound as a bandage, will be found handy.

It should be remembered that leaks in an air pipe line are as bad as in a steam pipe line, and should receive as much care.

The theoretical operation of air compressors may be thus explained:

If a tight cylindrical vessel, containing one cubic foot of air at atmospheric pressure, be fitted with a piston which is free to move up and down but yet perfectly tight, the air in the vessel will have no means of escape, and the pressure within and without the vessel, both being atmospheric, are balanced.

Now, if the piston should be loaded with a weight, the pressure on the outside would be that due to the atmosphere, plus the weight, while the pressure from the inside is simply equal to atmospheric pressure; thus the piston is forced to descend, but as the air inside of the cylinder has no means of escape, the volume it fills being diminished, its pressure rises until the pressure under the piston balances that above it.

If, for example, the area of the piston should be 100 square inches, and the weight with which it is loaded be 100 pounds, assuming the piston to be without weight, the pressure below will have to react with an equal force to hold the piston stationary, which in this case would be 1 pound to the square inch above atmospheric pressure, and the piston would have to descend sufficiently to cause this increase of pressure, which descent would be equal to1⁄16of the total fall of the piston. By adding another 100 pounds above, the pressure would rise to 2 pounds to the square inch. The cylinder is thus charged with compressed air.

Liquid Airis a marvelous result of compression. It liquefies at a pressure of 573 pounds per square inch, at the reduced temperature of -220° F.; at atmospheric pressure it boils at -312° F., at which temperature it can be handled like water.Airis the vapor of a liquid, and acts in its properties like the vapor of other liquids. Liquid air in color is like that of a blue sky on a cloudless day.

Fig. 365.(See page71.)

“Denys Papinwas the first to propose and make, in 1653, an actual trial of the transmission of power to a distance by compressed air. It was the fertile and mechanical brain of Papin that first conceived the idea of the pneumatic tube for transmitting parcels by air pressure.” Historical Note by Hiscox.

If now the bottom of the cylinder should be connected by means of a pipe to another vessel of larger capacity called a receiver, the pipe having been closed by a valve in it during compression, and the valve should be opened, the piston would at once commence a further descent, the compressed air escaping into the receiver, until the pressure in the receiver and cylinder is equalized, or the piston reaches the bottom of the cylinder, which it will do, if the receiver is large enough. Then the valve closes, stopping communication between cylinder and receiver, and the piston is drawn upward; at the same time air is again admitted to the cylinder by another valve, which closes when the piston reaches the top, and the same operation is again repeated.

The receiver can thus be charged with compressed air and by loading the piston very heavy the pressure can be raised quite high.

Now, if the piston, instead of being loaded by weights, be connected to the piston rod of a steam engine, or by means of a connecting rod to a crank (which is rotated by a belt or some other driving mechanism, and the valves be operated automatically, as the valves on a water pump), the simple apparatus is converted into a perfect air compressor,which really is nothing else than an air pump, and the air can be pumped into the receiver against a high pressure the same as water is forced into an elevated reservoir by a pump.

As air is a compressible gas, it acts a little different in the air cylinder from the almost incompressible water in a pump.

To lift the valves of an air compressor by the compressed air pressure in the cylinder (added to the pressure of their springs besides the receiver pressure), the air would have to be compressed considerably above the receiver pressure before it would lift the valve which allows it to flow from the cylinder into the receiver, and then the valve would not open freely as a pump valve, but would chatter, causing a disagreeable noise, and damaging the valve.

To avoid this, the valves of an air compressor are operated by mechanical means.Some devices operate the valve directly as soon as the pressure in the cylinder reaches that of the receiver, while others simply release it of the spring pressure, the valve itself being lifted by the air itself. Such devices generally give the valves a full free opening, without noise.

Fig. 366.

The blowing engineis almost identical with the air compressor. The chief difference between them being the ratio of steam cylinder to air cylinder. While the air compressor furnishes a comparatively small amount of air at very high pressures,the blowing engine delivers a very large volume at lower pressures.

Blowing engines are mainly used in large blast furnaces, smelting works and foundries, to furnish the air pressure for cupolas, air furnaces and smelting ovens.

In Fig.366is shown a blowing engine of very large size; the steam cylinder is 42 inches in diameter, the air cylinder 84 inches, and the stroke 60 inches.

The valve gear is of the Reynolds-Corliss type.The piston rod is attached to a cross-head extending through the guides, which are formed by the frame, with wrist pins upon each end, from which the two connecting rods are suspended with their lower ends connected to the cranks, as shown in Fig.366. There are two air piston rods attached to the main piston and held to the cross-head by nuts at points near the guides.

The crank shaft carrying the flywheels, which also form the cranks attached to the ends of this shaft, is located below the steam cylinder. This construction is of the return connecting rod engine design, to economize space.

Both the air and steam valve gears are worked from eccentrics on an auxiliary shaft, driven from the main shaft by bevel gears underneath the steam cylinder.

The “Imperial” air compressoris presented herewith in Figs.367and368.

The “Imperial” compressor is especially designed for use in machine shops, foundries and other industrial establishments where it is not convenient to use a steam driven compressor.

The machine has two vertical, single-acting cylinders, each employing long trunk pistons that act as guides for the lower ends of the connecting rods. By this design, the height of the machine is reduced, stuffing-boxes and crossheads are eliminated, and a minimum number of bearings required. Thecranks are set opposite to each other, so that when the piston on one side is ascending, the other side is descending.

Fig. 367.

The machine is made with duplex cylinders for the low pressures used in sand blast work and the like, and with either duplex or compound cylinders for higher pressures. In the compound type, an intercooler is supplied, through which the air passes from the low pressure to the high pressure cylinder.

The air cylindersare water-jacketed and provided with hooded heads, so that air may be supplied to them from outside the compressor-room; the cylinders are cast in one piece with the frame.

Fig. 368.

The air-valves, both inlet and outlet, are of the poppet type, fitted with light springs, and work vertically. On account of their position at the bottom of the cylinder, they are welllubricated, and, acting vertically, they have little tendency to wear out of line with their seats.

The air intake passageis tapped to receive a supply pipe leading from out-of-doors, or from some place where cool and clean air is obtainable. The compressed air is discharged into a passage which is tapped for a pipe to convey it to the air-receiver.

All parts of the compressor are easily accessiblefor inspection, adjustment, or repair. The air-heads may be removed without disturbing any of the pipe connections. The valves may be taken out by unscrewing the bonnets.

Tableof parts of the Imperial Compressor.

Numberof PartName of Part1Frame2Main-bearing cap3Main-bearing cap-bushing4Fly-wheel5Fly-wheel key6Fly-wheel key set-screw7Crank-shaft8Crank-disc9Crank-disc key10Crank-pin11Crank-pin cap12Crank-pin cap set-screw13Connecting-rod cap14Connecting-rod cap-bushing15Connecting-rod16Piston17Piston-pin18Piston-pin bushing19Piston-pin set-screw20Piston, inside ring21Piston, outside ring22Adjusting-bolt for piston-pin end of connecting-rod23Connecting-rod bolt24Guard-plate25Air-head26Air-head gasket27Air-head studs28Inlet valve and stem29Inlet valve-seat30Inlet valve-spring31Inlet valve-stem head32Inlet valve-stem cotter33Inlet valve-bonnet34Outlet valve35Outlet valve-spring36Outlet valve-bonnet37Water inlet pipe38Water outlet pipe39Air inlet pipe40Air outlet pipe41Unloader42Main-bearing grease-cup43Crank-pin grease-cup44Piston-pin grease-cup45Air-cylinder lubricator46Main-bearing studs47Main-bearing liners48Unloader regulating-cylinder

Frame

Main-bearing cap

Main-bearing cap-bushing

Fly-wheel

Fly-wheel key

Fly-wheel key set-screw

Crank-shaft

Crank-disc

Crank-disc key

Crank-pin

Crank-pin cap

Crank-pin cap set-screw

Connecting-rod cap

Connecting-rod cap-bushing

Connecting-rod

Piston

Piston-pin

Piston-pin bushing

Piston-pin set-screw

Piston, inside ring

Piston, outside ring

Adjusting-bolt for piston-pin end of connecting-rod

Connecting-rod bolt

Guard-plate

Air-head

Air-head gasket

Air-head studs

Inlet valve and stem

Inlet valve-seat

Inlet valve-spring

Inlet valve-stem head

Inlet valve-stem cotter

Inlet valve-bonnet

Outlet valve

Outlet valve-spring

Outlet valve-bonnet

Water inlet pipe

Water outlet pipe

Air inlet pipe

Air outlet pipe

Unloader

Main-bearing grease-cup

Crank-pin grease-cup

Piston-pin grease-cup

Air-cylinder lubricator

Main-bearing studs

Main-bearing liners

Unloader regulating-cylinder

The Norwalk standard compressoris shown in Figs.361and362, the latter being a longitudinal section; Fig.361is a perspective view; the two compressors are driven by a single steamcylinder having an adjustable cut-off. The air valves are operated by a positive crank motion.

A view of a Pelton water wheeloperating a compressor it shown in Fig.363. The cut represents a compound air compressor in which the valves are operated mechanically. The water which drives the wheel enters through the pipe and nozzle secured in the wheel pit, as represented.

Fig.364exhibits a belted duplex air compressor built by Allis-Chalmers & Co.

Fig.365shows a vertical duplex compressor driven by a belt.

Fig. 369.

All the latter, as may be seen by the engravings, have the positive valve motion operated by an eccentric. In selecting an air compressor the following points need consideration: 1, Number of cubic feet of free air required per minute; 2, Altitude,i.e., the number of feet above the sea level; 3, Steam pressure and air pressure.

The use of compressed air for operating mining pumps, while having advantages in some cases,is not to be recommended in all, particularly on account of the low efficiency of the plant as a whole. The loss due to leaks is serious, and the long line of piping with its numerous joints causes much trouble, delay and expense.

In Fig.369is shown a direct acting steam single air compressor; simplicity in its construction is a leading feature and there are few parts in this pump that are liable to wear.

Figs. 370 and 371.

This apparatus is designed for working pressures up to twenty pounds; it is intended for use in oil refineries, smelting works, blast furnaces and in all situations where compressed air of medium pressure is required. They are variously used for sand blasts, ventilating purposes, and for pneumatic deliveries.

The steam end and valve motion are the regular Deane pattern, assuring positive operation. The air cylinder is provided with a water jacket.

Fig. 372.

A power wall or post air compressoris shown in Figs.370-372. The machine is single acting and is recommended where little space is available, as it can be bolted to the wall or to a post, or on the under side of the ceiling. The crank shaft and connecting rod are of cast steel. The bearings are babbitted and adjustable. The piston is of the trunk form, carrying a pin for the connecting rod, and is of extra length toact as a guide for the lower end of the connecting rod. The valves are of the poppet type. These compressors are extensively used in electric power stations for supplying air for removing dust from electric machinery, in bicycle shops for inflating pneumatic tires, maintaining a supply of air in pressure tanks, and for various purposes where a limited supply of air is needed.

These compressors are ofthe “Blake” designand the following particulars will be of interest.

Table.

Diameter of Cylinder.21⁄434567Stroke.666666Revolutions Per Minute.150150140140130130Piston Speed in Feet Per Minute.150150140140130130Cubic Feet of Free Air Per Minute.23691217Working Pressure.150150100100100100Horse Power Required.5⁄83⁄411⁄2221⁄24Pipes.Suction.3⁄4111⁄22221⁄2Discharge.3⁄4111⁄211⁄211⁄211⁄2Dimensions.Length.22″22″24″30″36″36″Width.13″14″15″16″18″191⁄2″Height.32″331⁄2″38″401⁄2″43″44″Diameter of Cylinder.21⁄434567Stroke.666666Revolutions Per Minute.150150140140130130Piston Speed in Feet Per Minute.150150140140130130Cubic Feet of Free Air Per Minute.23691217Working Pressure.10010090856060Horse Power Required.1⁄25⁄811⁄413⁄4221⁄2Pipes.Suction.3⁄4111⁄22221⁄2Discharge.3⁄4111⁄211⁄211⁄211⁄2Dimensions.Length.16″16″16″24″24″30″Width.13″14″14″14″14″18″Height.28″291⁄2″331⁄2″371⁄2″371⁄2″42″

Diameter of Cylinder.

Stroke.

Revolutions Per Minute.

Piston Speed in Feet Per Minute.

Cubic Feet of Free Air Per Minute.

Working Pressure.

Horse Power Required.

Suction.

Discharge.

Length.

Width.

Height.

Diameter of Cylinder.

Stroke.

Revolutions Per Minute.

Piston Speed in Feet Per Minute.

Cubic Feet of Free Air Per Minute.

Working Pressure.

Horse Power Required.

Suction.

Discharge.

Length.

Width.

Height.

With increase in altitudethe barometric or atmospheric pressure falls from 14.7 pounds per square inch at sea level to about 10 pounds at 10,000 feet above sea level. Since the density of the air decreases with its pressure it is obvious that at such an altitude the total weight of air handled by a given displacement is considerably less than at sea level; and that to fill any volume—a rock drill cylinder, for instance—with air compressed to 90 pounds, a greater free-air displacement will be necessary than would be required at sea level. The relativecapacities of a given displacement to do work—as through rock drills or pumps—at varying altitudes are figured in the following table:

Capacities at Varying Heights above Sea Level.

Feet aboveSea LevelBarometerInchesRelativeCapacities030.001.00050029.42.983100028.87.967150028.33.954200027.79.938250027.27.924300026.76.909350026.25.894400025.75.879450025.26.867500024.78.856600023.85.827700022.95.800800022.10.772900021.22.7501000020.43.7251200018.92.677

The fact that the heating effect of compressing air from an initial pressure of 10 pounds absolute to 90 pounds gauge pressure is theoretically equivalent to that of compression to 132 pounds at sea level,makes a two-stage arrangement more imperative in high level work than under ordinary conditions.

Fig. 373.

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