Show USE OF compressed AIR IN HINES of late years compressed air has gained greatly in prominence as a means for the transmission of power and now occupies a broad field of usefulness in addition to its value and convenience as a power transmitter for engineering purposes says the western mining alining world it possesses also characteristics which make it applicable to a great variety of uses in the manufacturing industries and mechanic arts in which the element of actual transmission of power does not at all enter A discussion of these however would not be appropriate here in connection with mining tunneling shaft t sinking and kindred operations compressed air has found se several veral of its most important applications here for some purposes it has made a place for itself in competition with steam but it is in conjunction with steam steam and other prime movers that compressed air acting purely as an agency for trans power has bas frequently become bridis pen sible for under ground work As compared with steam the employment of cow corp pressed air for such purposes is particularly valuable and convenient for four reasons first its transmission loss is small second the troublesome question of the disposal of exhaust steam und bader er ground is avoided third the exhaust of air is of direct assistance in the ventilation of the confined working places and fourth its capacity for stor ing power makes it well adapted for intermittent work these points will be brielly briefly considered I 1 I 1 the c conveyance onvey ance of steam long underground involves serious and unavoidable loss from radiation and consequent condensation densa tion the loss due to friction is common to both steam and compressed air and although not equal at the same pres pressure sure on account of the greater density of compressed air the one may be taken as approximately offsetting the other in a steam pipe of proper diameter under given conditions the frictional loss should be not more than one fifth the loss from radiation condensation may be reduced by carefully covering the pipe with good conducting non material but even with the best covering the effective pressure at a distant underground engine Is 9 greatly diminished and very uneconomical working is the result in conveying steam a distance of several thousand feet as is by no means uncommon in extensive collieries colli eries the pressure may be reduced reduced to half the boiler pressure or even less take for example a pump situated 2000 feet from the boiler and using cubic feet of steam per minute ata at a boiler pressure of 75 pounds with a ral wool covered pipe 4 inches in diameter the effective pressure at the pump would be only 38 pounds or with a poor covering like some of the asbestos lagging often used it might easily be as low as 35 pounds POUDIs for compressed air transmission the of pressure for the same volume of air size of pipe and initial pressure would be 93 pounds giving a terminal pros pressure sure of pounds but as the speed of flow in pipes for economical tran mission is greater for steam than for aira comparison based on piping of the same diameter cannot justly be made if in tho the above example the diameter of pipe were smaller the gain in reduced radiation would outweigh the increased frictional loss and the net loss would be diminished tho the frictional loss varies invers inversely elv and the loss los from radiation directly with the diameter therefore under conditions the diameter of the pipe can be so proportioned as to produce a minimum loss with coin pressed air transmission however the case is different for if the diameter of the pipe in the above case be increased to 5 inches the loss of pressure or head required to overcome friction is reduced to 28 pounds and increasing the distance to one mile it would be only 74 pounds furthermore the increased cost of the larger air pipe would be offset by the expense of the conducting covering no account has been taken here of the loss of leakage attention may be called to the fact that little or no danger is to be apprehended from the rupture of a compressed air pipe while the bursting of a steam pipe in a shaft shaf t or in mine workings working may be a serious matter |