Compressed Air.—Reference has been made to the use of compressed air power, and its facility of adaptation to various requirements, but it is evident from an inspection of some of the devices in use that enthusiasm for new methods, rather than good judgment, has con trolled in many of its applications. For some years compressed air was used only in mines, where it produced marked economics in under ground work. Later, compressed air was intrd duced into manufacturing lines, and to-day its use in railroad and other machine shops, boiler shops, foundries and bridge works is being widely extended. In the Santa Fe Railroad shops at Topeka there are over five miles of pipe in which compressed air is carried to the different machines and labor-saving appliances throughout the works. In such shops air is used to operate riveting machines, punches, stay bolt brealcers, stay-bolt cutters, rotary tapping and drilling machines, flue rollers, rotary grind ers, rotary saw for sawing car roofs, pneumatic hammers, chisels and caulking tools, flue weld ers, boring and valve-facing machines, rail saws, machine for revolving driving-wheels for set ting valves, pneumatic painting and whitewash ing machines, dusters for car seats and the operation of switching engines about the yard. It is also used in the foundry for pressing and ramming molds, and for cleaning castings by the sand blast; but its greatest field of useful ness lies in its application to hoisting and lifting operations in and about the works. More re cently its use has been extended to include the sand blasting of steel passenger coaches. In mines, where exhaust steam or gas engine ex haust would interfere with proper ventilation, compressed-air engines and motors, which dis charge pure air directly into the workings, are used for a variety of purposes, such as loco motive haulage, hoisting, pumping, cutting and drilling. , Probably nothing has called wider attention to the possibilities of the use of com pressed air than the recent installation of a system of compressed air deep-mine hoists in the Butte, Mont., mines. This is especially re markable in view of the fact that the com pressors are operated electrically from trans mitted water power; it was found to be better to use electricity to operate compressors dis charging into storage reservoirs from which the air passed through heaters on the way to hoist ing engines than to operate the hoists electri cally. One factor in this conclusion is the special construction of the engine, which as the cage descends operates as a compressor, storing air by the energy of the falling hoist, and acts as a brake at the same time.
New applications of compressed air are con stantly being made, and each new use suggests another. This has a tendency to increase the number of appliances which are intended to be labor-saving devices, but in many cases the work could be done just as well and much more cheaply by hand. The success and economy which has attended the use of compressed air in so many lines of work has led to its adop tion in fields which are much better covered by electrically operated machines. While com pressed air has been used under certain condi tions very satisfactorily to operate pumps and engines, printing-presses, individual motors for lathes, planers, slotters, dynamos and other work, it does not follow that it is always an economical agent under these various uses or that other methods could not be used even more satisfactorily. It has been proposed to use individual air motors in machine shops and do away with all line shafting, except possibly for some of the heavier machinery. This use of compressed air seems entirely outside the pale of its legitimate field; the general experience thus far indicates that rotary motors are not at all economical and generally are not as satis factory as electric motors. Exceptions are to
be found in the small portable motors for drilling and similar operations to which elec tricity is not so well adapted and where com pressed air has been found to give excellent results. Although these tools are very success ful, they are still rotary motors, not exempt from some of the objectionable features which seem to be inseparable from them. It is not surprising, therefore, to find a tendency to employ reciprocating pistons and cranks in these portable machines, and such tools are now in use weighing only 40 pounds capable of drilling up to two and a half inches diameter. In most cases no attempt has been made to use com pressed air power efficiently; its great con venience and the economy produced by its dis placement of hand labor have, until recently, been accepted as sufficient, and greater econo mies have not been sought.
In the matter of compression, very inefficient pumps are still in use, but manufacturers gen erally have found that it pays to use high-grade economical compressors. The greatest loss is that in the air motor itself. In a large number of cases it is impracticable or, at most, incon venient to employ reheaters, and the air is used very generally at normal temperature for the various purposes to which it is applied. To obtain the most satisfactory results, the air must be used expansively, but usually where the demand for power is intermittent no attempt has been•made to reheat the air, and as a result the combined efficiency of compressor and motor is quite low, varying in general from 20 to 50 per cent. While low-working pressures are more efficient than high, the use of such pres sures would demand larger and heavier motors and other apparatus which is undesirable.
The advantages of higher pressures in re ducing cost of transmission are also well recog nized, and the present tendency is to use air at 100 to 150 pounds instead .of the 60 or 70 pounds formerly used. By reheating the air to a temperature of about 300° F., which may often be accomplished at small expense, the efficiency is greatly increased; in some cases this has been shown to be as high as 80 per cent.
A remarkable development of compressed air power, in which very high compression is has recently been accomplished in the Porter compound compressed-air locomotive for use in mines, lumbering operations, powder works or other places where fire is much fe:.red; also in hauling cars about the yards of large manufacturing plants, where cleanliness as well as avoidance of fire risk is important. In this case the air is stored in the locomotive tanks under a pressure of 800 pounds per square inch; this air thoroughly dried by the high compression then passes through a reduc ing valve, where it produces kinetic energy due to expansion, which it immediately loses by impact, thus avoiding temperature drop when falling from 800 to 250 pounds per square inch. At this pressure and practically atmos pheric temperature the air is admitted to and expanded in the high-pressure cylinder of the locomotive, exhausting into the receiver at as low a temperature below atmosphere as prac ticable. Atmospheric air is blown through a series of tubes in the receiver, so that the working air takes up heat from the atmosphere and, subsequently expanding in the low-pres sure cylinder, converts some of the atmos pheric heat into work. This process is pecu liarly interesting, in that it provides a means of reheating the working air directly from the atmospheric air and obtaining thereby an in crease of 30 per cent in the economy.