Page:The New International Encyclopædia 1st ed. v. 01.djvu/289

AIR BRAKE. quick action brake is tlic picvailins equipment of railway trains in .Vmerica. it lia ini; replaced practically entirely the i)lain automatic brake. The very hi^h passenger train speeds of recent years led Mr. Wcstinghouse, in lSt)7. to place on the market a high speed brake. This brake is designed to use very high air pressure when the brake is applied with the train at full speed, which pressure is gradually reduced by an auto- niiitic reducing valve on the brake cylinder as the speed diminishes. This l)rakc has not been extensively used. Tests made with the regular high speed brake attached to a fifty-car train ghowed the following among other results: Emergency stop of train running at 40 miles per hour made in about 075 feet, in 20 seconds; breaking the train in two at a speed of from 20 to 2.5 miles per hour, the two sections stopped at distances of from 32 feet to ISO feet apart; applying brakes with train standing to show rapidity of action, all brakes applied within two seconds; comparison of emergency air brake stop and hand brake stop at 20 miles per hour; air brake stop in 158 to 104 feet, hand brake stop in 1000 feet to 1720 feet; service stop test to de- termine time of release of brakes, all brakes released in four seconds. Several forms of air brake besides the Wcstinghouse have been em- ployed to some extent in .merica, but they are exactly similar in their principles of operation. The air brake has been applied to electric street cars and to cable cars. In this application the air pump is operated from one of the axles, and usually the straight air system is used, in ex- actly similar form as it was formerly used for ste.a'ra railway trains. Those desiring further and more technical information on this subject are referred to Blackall. Air Brake ('atrchism (New York. 1000) ; Synnestvedt, Air Brake Dis- eases (New York. 1900) ; Proceedings Master Car Buildi-rx' Association.

AIR CELLS, or AIR SACS. See.

AIR COMPRES'SOR, or AIR PUMP. A machine for coinpressing air. Compressed air is used for a nuiltitude of purposes in the arts and in manufacturing, and to catalogue all of its uses would require a great amount of space. The simplest fmin of air pump is the little apparatus for inflating bicycle tires, with which nearly every one is familiar. These bicycle pumps are made both single and double acting, the single acting pump being the simplest form of air com- pres.sor. Compared with the enormous air com- pressing machines used in shops and mines, this little device seems almost too trivial to merit notice, but by carefully observing its actions and their ell'ects we have brought to our attention several phenomena which ace inijiortant facts in air compressing on a large scale. One of these phenomena is the power required to pump against the resistance of the compressed air in the nearly inflated tire: the second and more important is the fact that a very [)erceptible development of heat results as the pumping pro- ceeds. The bearing of both these observations will appear as we proceed. For the present it need only be observed that hand air pumps .of the simple form indicated are used for a variety of purposes where only a small amount of com- pressed air is required. Where a somewhat larger volume of air is required, hand pumps provided with fly-wheels and operated by one or more men by means of a crank, are employed. With these large hand pumps we arrive at power air compressors.

The air pump was invented by Otto von Cluericke of Magdeburg, Germany, about 1654. In 1753 Hoi! used an air engine for raising water, and in 1788 .Smeaton invented a pump for use with diving apparatus. In 1851 compressed air was used by William Cubitt for bridge work, and a little later it was used by Brunei for the same jnirpose. In 1852 Colladon patented the application of compressed air for driving machine drills in tunnel construction. Sommeiller developed CoUadon's idea, and constructed an air compressing plant for the Mont Cenis Tunnel work. (See Tunnel.s.) The Sonimeiller compressor was operated as a ram. utilizing a natural head of water to force air at 80 pounds pressure into a receiver. The column of water contained in a long pipe on the side of the hill was started and stopped automatically by valves controlled by engines. The weight and momentum of the water forced a volume of air with such a shock against a discharge valve that it was opened, and the air was discharged into the tank. The valve was then closed and the water checked, and a portion of it was al- lowed to discharge and the space to fill with air, which was in turn forced into the tank. The injection of water in the form of a spray into the compressor cylinder was first introduced on the St. (iothard funnel work begun in 1872. The first compressor used in .merica was developed by Mr. Thoiuas Doane, the chief engineer of the Hoosac Tunnel, and was employed on that work. This compressor had four single acting cylinders, and was cooled by the injection of water through the inlet valves into the cylinders. These early compressors are of historical interest only at the present time. As the necessity for com- pressed air i)Ower grew, inventors turned their attention to the design and construction of com- pressors which would combine efficiency with light weight and economy of space and cost. As the result of this work, the modern air compres- sor has been developed.

The simplest form of power air compressor is the air brake ])um]i, with which practically every American locomotive is equipped. In this pump, it will be readily understood, the main considera- tions are economy of space, light weight, and absolute reliability of action: economy of steam consumption being quite a secondary matter. -V 9'/; inch air brake i)ump, for example, will give 1.8*5 cubic feet of air at 90 pounds pressure, with a consumption of 1 pound of steam at 140 pounds pressure, while a two-stage Corliss air compressor will give 13.7 cubic leet of air at 90 pounds pressure with the same steam consumption. The standard air brake has a steam cylinder and an air cylinder of the same size, viz., 9^4 inches diameter and 10 inch stroke, set vertically one above the other, with a common piston rod. See .

It has been stated above, in referring to the bicycle pump, that air is heated by compression. .s heat causes air to expand, a cubic foot of hot air, at, say, 75 pounds pressure, will decrease in volume When cooled, and thus bring about a reduction in pressure to something less than 75 pounds. Evidently, therefore, a loss of work done in compression results from the heating of the air. The amount of the loss is estimated at 21.3 per cent, of the total work done in compressing air to 75 pounds pressure. To save this loss.