THE USE OF COMPRESSED AIR FOR ACTUATING MINING MACHINERY.

Evening Star, Issue 3224, 20 June 1873, Page 3

THE USE OF COMPRESSED AIR FOR ACTUATING MINING MACHINERY.

[From Engineering)

The following short paper is, we believe, the last that the late eminent Professor of Engineering and Mechanics in the University of Glasgow, prepared. It was written, we understand, within a week of his lamented death, and was read at a meeting of the Institution of Engineers in Scotland on the night of his decease, as a supplement to a paper by Mr George Simpson, civil and mining engineer, on “ Recent improvements in mining machinery.” A very melancholy interest attaches to it. Professor Ranking, through the" ecretary, expressed his regret that the state of his health prevented his being present at the meeting, in consequence of which the few remarks he now begged leave to send would be made without the advantage of having previously heard Mr Simpson’s paper. He understood that one of the questions to be considered would be that of the comparative economy of compressed air and wire-ropes in transmitting power to places below ground for the driving of coal hewing or other mining machinery. He thought that it was almost, if not quite to .give a decided answer to this question, which should be universally applicable, and that individual cases or class of cases would have to be decided each on its own merits. From ordinary experience of rope haulage on mineral railways the waste of power may be roughly estimated at about 20 per cent, in a mile. Mr Simpson, no doubt, would be able to give the Institution valuable information on thu point. On the other hand, in transmitting power by means of compressed air, there were great and unavoidable losses of power in the air • compressing engine, arising mainly from the waste of the heat developed by the compression of the air. Those losses had seldom amounted to less than from 60 to 75 per cent, of the whole power of the compressing engine; aud it could be shown that in extreme cases they might even exceed 90 per cent. On the other hand, the part of the loss of power which arose from the friction of the air in the pipe, and which, therefore, increased with the distance to which power had to be transmitted, was comparatively small with well proportioned pipes, and might, he thought, be reduced to about 10 per cent, per mile. From these reasonings, it appeared probable that wire ropes were the more economical means of transmitting power for short distances, and compressed air for long distances. He could give no opinion as to the probable value of the distance at which those two means were equally economical; but, no doubt, the information given by authors so able aud experienced as Mr Simpson would greatly contribute towards the answering of this question iu a satisfactory manner. For comparatively early information as to the transmission of power by compressed air, he might refer to two papers which had appeared is the Transactions of the Institution of the Mechanical Engineers, one by Mr Charles Randolph in 1856, the other by the late Mr Nicholas Wood in 1858. There was

an advantage peculiar to the use of compressed air which deserved serious consideration. It was the ventilating and cooling effects of the air discharged from the underground machinery. He might mention, in conclusion, that the best economy in compressed air apparatus was obtained by the use of moderate pressures ; for with these the heating effect and consequent waste of power were moderate. It may here be explained that when loss of power is stated at a certain percentage per mile—say, for instance, 20 per cent per mile—that does not mean 20 per cent of the whole original power on each mile, but 20 per cent, of the actual power in the first mile—2o per cent, of the power remaining after reduction in the second mile —20 per cent, of the power remaining after the second mile, in the third paile, and so on. It may also be explained that the two rpugh estimates of losses of power already given are based upon ordinary experience with the ordinary apparatus. There is a special form of apparatus in the case of rope traction in which the loss of power per mile, according to public accounts of experiments made in France, is between 1 and 1J per cent, only; but in that form of apparatus the rope which is hung above ground at any required elevation has large supporting pulleys at intervals of about 500 feet, between which it hangs down in curves with a deep deflection, whereas on mineral railways the usual 1 interval is little more than 20 feet. On the other hand, the loss of from 65 to 75 per cent., which occurs in air-com-pressing engines, exceeds many times the loss theoretically due to the waste of heat; and we may, therefore, expect to see it greatly diminished through the gradual im provement of the machinery.