Air

Air

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Fig. 26.-Sectional Arrangement of Direct-Coupled, Double-Ended, Electrically Driven Portable Mine Compressor Reavell & Co., Ltd., Ipswich.

are mounted on a truck base with wheels, and a high-speed engine on the bed plate drives the compressor through gear wheels.

The Reavell rotary compressor lends itself admirably to electric driving. In the smaller sizes the motor drives the compressor through gear wheels, but in the larger it is coupled direct. A small reservoir only is necessary, because of the high speed of rotation, and the fact that there are four deliveries of air per revolution. The sectional view of a Reavell portable mine compressor is given in Fig. 26, and a general view of the same by the photograph, Fig. 27, on Plate II.

The subject of Compressed Air will be treated under that heading. For the present, therefore, we are only concerned with the bearing of that subject on the design of air compressors.

The first thing to be guarded against in the design is the prevention of useless work in the transformation of energy into heat, or in other words, the aim is the production of the largest volume of air under pressure, with the smallest expenditure of fuel.

The difference between the adiabatic and isothermal quantities of work done is the amount of work that is turned into heat in the work of compression. Practically adiabatic compression alone is considered in design, partly because air is so nearly a perfect gas that nearly all the work done upon it appears as heat, and partly because when high compression is necessary, it is produced in two or more stages And this is in spite of the fact that the cylinders are jacketed with circulating water to lessen as far as possible the loss of heat through the cylinder walls.

Beyond about 60 or 801b. per square inch, when two-stage or multi-stage compression is adopted, with intercoolers (which take the form of receivers, or of tubes), the work of compression is divided equally between the compressing cylinders. These are designed either for the volume, or weight of air to be taken in, or of that of discharge. If for admission, the speed and piston displacement and the volume of free air at atmospheric pressure at 13 cubic feet to the pound are the factors required. If for discharge, the relation becomes that of a volume

of 1 lb. of free air at atmospheric pressure to that of a volume of 1 lb. of air when compressed adiabatically at the given pressure.

The reason for the water jacket is clear from the fact that as the temperature of the air rises under compression, the work of compression is increased proportionally, because the heated air is attempting to expand against the piston pressure and to do external work. The necessity for intercooling is also explained in the same way. The aim in intercooling is as nearly as possible to reduce the temperature of the outlet air from one cylinder to the atmospheric temperature, at the inlet of the succeeding stage.

The water jacket is only a crude device, but it is the best attainable at present. Air is a bad conductor, and loses its heat slowly. Only a small portion of the volume of the air comes in contact with the walls of the jacket; hence, in large cylinders the jacket economy is less than in small ones.

End clearance in cylinders is reduced to the minimum, because any volume of air left there expands on the return stroke of the piston, and checks the admission of the volume of free air. If there were no clearance, which is impracticable, there would be no limit to the pressure that could be employed in one cylinder, except that of the heat generated, or the strength of the cylinder.

The governing of compressors is a matter of much economical importance. If less air is being used, as frequently happens, than the compressor is delivering, the surplus goes off through the safety valve of the air receiver, and represents so much wasted. The object of governing is to prevent this. waste by causing the compressor to respond to the demands made upon the supplies stored in the receiver.

Governing is done in two ways, either by reducing the speed of the compressor, though still allowing it to supply reduced quantities of air to the receiver, or by reducing speed, and stopping the supply of air entirely. The objection to the first is that the air is constantly escaping from the safety valve of the receiver. In an ideal regulating device the air supply to the air cylinder is cut off. In the Ingersoll-Sergeant machines this is effected by the opening of a passage between both ends of the air cylinder,

by which air is admitted on both sides of the piston, tightly closing the inlet valves. At the same time the supply of steam to the engines is automatically throttled to a stage which just permits the engine to keep turning at a slow speed sufficient to overcome friction or dead points.

The precise functions and forms of governors therefore vary. In those fitted to steam-driven compressors the governor operates on the steam supply pipe, regulating the volume delivered to the cylinder or cylinders. The governor can be set for a particular air pressure desired. In belt-driven compressors the governor opens a relief valve. A useful form is that of a combined speed and pressure governor which combines with the air-pressure governor to prevent racing when exceptionally heavy demands are made upon the air supply, which often occurs in intermittent service.

Governors should not be set to stop the compressor entirely, but adjusted to merely keep the cranks moving over dead points. If a compressor stops on the centre it may fail to respond to the governor when a demand is made on the supply. In duplex compressors where there are no dead points the adjustment must be just sufficient to overcome the friction of the moving parts.

The fact that the demand for air is nearly always of an intermittent character, though to a greater or less extent, renders it desirable that compressors should be driven independently of all other machinery.

If air compressors have to be used in mountainous districts, allowance must be made for loss of efficiency due to the lower pressure of the atmosphere. See Barometer.

Air Cooling. See Air Compressor, Petrol Engines, Ventilation.

Air Cushion.-The volume of air under compression in an air vessel, and by which the pulsations of the water are checked. See Air Vessel.

Also a small volume of air imprisoned and compressed behind a retreating piston in pneumatic mechanisms. It is obtained by blocking the further escape of air just before the termination of the stroke. The result is similar to that of cushioning obtained in engine cylinders, and

for the same reason, to prevent shock, and act as a spring buffer in bringing the piston to

rest.

Air Cylinder. The cylinder of a blowing engine through which the air is compressed. The cylinder of an air compressor, in which the air is compressed by the piston.

Air Drills. See Pneumatic Drills.

Air Engine, or Air Motor. The term is given to a type which is actuated by compressed air, and used both for hoists and travelling cranes. Two double oscillating cylinders have their axes set at right angles in an air-tight case, the air ports being controlled by a slide valve operated by the oscillations of the cylinders. Reversals are effected by an eccentric motion attached to a hand wheel, by which the direction of movement of the valve is controlled. A small quantity of oil is enclosed in the case, and the revolving crank is thus lubricated, and it also dashes oil on the valve seats. See also Compressed Air Locomotives, Hot Air Engine.

Air, Free.-Air at atmospheric pressure. See Air Compressor, Compressed Air.

Air Friction.-This has its applications in the loss of pressure due to the transmission of air through long pipes, and to the ventilation of mines.

The loss of pressure in passing through pipes is slight. The only effect is to lessen the pressure, and not the volume. Sharp bends must be avoided, as in water pipes, and for the same

reason.

Globe valves reduce pressure considerably, and elbows and tees rather less. A globe valve inserted in a 4-inch pipe is equivalent to increasing the pipe length by 20 feet. The Ingersoll-Sergeant formula is:

Additional length of pipe =
114 x diam. of pipe
1+(3.6÷diam.)

Using elbows, and tees, the reduction of pressure equals two-thirds that of globe valves, so that these in a 4-inch pipe would be equivalent to increasing the length of the pipe by 13 feet.

The experiments of M. D. Murgue in mines proved that the smoothness or roughness of