226 THE COLLIERY GUARDIAN. Januaby 31, 1913. per cent, losses 429-horse power. For practical pur- poses the engine losses must be added to these, and will depend on the type of engine. The author then takes a few typical machines by leading English builders and describes in some detail the valve arrangements, and the useful effect obtained. For slow-speed horizontal engines those manufactured by Messrs. Walker Brothers, of Wigan, and Messrs. Robey and Co., of Lincoln, were taken, these two firms building both simple and stage compressors ; for high-speed horizontal engines, Messrs. Fraser and Chalmers, of Firth, who have taken up the Riedler type of high- speed compressor ; and for high-speed vertical engines, Messrs. Alley and MacLellan, of Glasgow; Messrs. Beiliss and Morcom, of Birmingham, and Messrs. Peter Brother- hood and Co., of Peterborough. There remain the rotary compressors, of which that of Messrs. Reavell and Co,, of Ipswich, is well known; and lastly, the turbo- compressor. These above-named firms are fully repre- sentative of English engineering. Other countries, especially the United States of America and Germany, put their air-compressors on the English market, and very excellent machines many of them are. AL'TO;UNLOADING DEVICE. Delivery. WATER INLET." AUCTION. Inlet. MH Parts of Reavell Air-compressor. Fig. 5.—Arrangement of Working 1 i I Fig. 6.—Section of A.E.G. Turbo-compressor. Fig. 7.—Section of Impeller (A.E.G. Turbo-compressor.) Speaking of turbo-compressors, the author said its great utility is where a large and constant quantity of compressed air is required. For small quantities, and for intermittent deliveries, it is altogether unsuited ; but as mechanical contrivances are being rapidly introduced into collieries, in substitution for human and animal labour, the requirements for some form of power are continually increasing, and in large collieries very large amounts of power are required day and night. The recent means which have been devised for utilising the exhaust steam from colliery engines provides a cheap form of power; but as exhaust steam turbines must run at 3,000 revolutions or more per minute, and this speed cannot be satisfactorily geared down to drive com- pressors of the ordinary reciprocating type, a compressor which will run at the high speed of an exhaust steam turbine, and give a reasonable degree of efficiency, enables the exhaust steam to be utilised for the purpose of generating power required in the mine, if it is obligatory to use compressed air, just in the same way as the exhaust steam turbine is now largely used for generating electricity for power purposes underground and elsewhere. Thus, if under the legal restrictions compressed air is the only available power, it is doubtful whether any other method of obtaining it is so economical on a large scale as with a turbo-compressor. It has, however, to be borne in mind that, in addition to the cooling water required for the exhaust steam turbine, large quantities of water are also required for cooling purposes in connection with turbo-compressors. There is nothing special in the system which renders this cooling water more necessary than in reciprocating engines, but the large volumes of air which are com- pressed give off large quantities of heat, and it is an essential condition to compressing by turbines that this heat should be removed. What is, therefore, under all circumstances a highly economical thing to do, namely, to thoroughly cool the air during compression, is merely rendered obligatory in the case of the turbo- compressor. Utilisation of Compressed Air. The author next dealt briefly with the way in which compressed air is utilised. It is when the actual practical results obtained in return for the cost of producing compressed air are estimated that disappointment is felt at obtaining so poor a return for the expenditure. With ordinary dry compressors fitted with jacket water- coolers, there is usually a loss of 25 per cent, by the time the air enters into the receiver. This includes clearance losses, and the actual amount of the loss varies according to the superiority or inferiority of the cylinder Fig. 8.—A.E.G. Steam-condensing Plant. Fig. 9.—“ Meco ” Double-valve Hammer Drill. and valve design. The losses in the motors actuated by the compressed air is quite as much as, or greater than, it is in the generator. In the one case heat is given off by compression; in the other, cold is produced on expansion. It happens frequently that the air actually converted into work underground is only 30 per cent, of the energy created by the steam engine.* In such a case 25 per cent, may be due to compressor losses, 25 per cent, to motor losses, and the balance to friction in the air-mains and leakages. This is where electric power shows itself more economical than compressed air, because the losses in the generator, in the conductor, and in the motor are all much less. Where, however, electric power is not permissible, compressed air, if somewhat more costly, becomes indispensable. Where the units are small and dispersed throughout a mine, it has been attempted to distribute these losses by installing small electrically-driven air-compressors as near as possible to the places where air-power is required. This is particularly the case with air for driving rock drills. The system is less cumbrous and more convenient, but the writer questions whether the actual power economy is much greater than with direct air transmission. The small portable air-compressors can scarcely, by the nature of the case, be as efficient * If a steam engine converts 8 per cent, of the energy in the fuel into power, 30 per cent, of 8 per cent, is 2’6 per cent. only. and economical as larger compressors. There are the motor losses, the gearing or belt-drive, and the drop in current to allow for; and these little compressors must have some supervision, as well as maintenance and stores. When all comes to be reckoned out, the balance in favour of small underground compressors cannot be large. At Wharncliffe Silkstone Colliery, a modern two-stage compressor placed underground, driven by a 70-horse power motor, was insufficient to drive two coal- cutting machines which required about 25-horse power each. It would just drive one coal-cutting machine and one face-conveyor, the latter requiring 8-horse power. The principal purposes for which compressed air is required in mines is in connection with coal-cutting machines, haulage motors, conveyor motors, pumps and rock drills. Most compressed-air motors are running with cut-offs at three-quarters and seven-eighths. It is very deplorable from the standpoint of the theorist and the mechanical engineer, but, to the best of the writer’s knowledge, it is so. The haulage motors give the best scope if they are provided with gear for notching up; but, as a rule, the youths who attend to these engines understand only the reversing lever and the throttle valve. With a pump one has a steady load, but to get expansion the cylinder must be of ample size. With expansion, too, the tendency to freeze in the exhaust ports is increased, and one often hears the pump wheezing as if it had bronchitis. With coal-cutting machines, hard substances are constantly being met with, and often the full cylinder of air is not sufficient to prevent the wheel from sticking. The writer does not think that any system of variable cut-off is appli- cable to coal-cutters. Hence, with all these machines it is inevitable that a very low efficiency should be obtained. The following is a typical case of comparative economic efficiencies of air and electric motors in con- nection with coal-cutting at Wharncliffe Silkstone Colliery, when working at the same distance from the Fig. 10.—Section of 1912 Model Hardy Puncher Coal-cutter, with new Valve action. I® plant supplying the power : An electrical series-wound coal-cutting machine was ascertained to take 30 amperes at 550 volts at the switchboard on the surface, equal to 22’1-horse power. This machine cuts, on an average, 30 ft. an hour. An air machine, supplied by air from an electrically-driven compressor, with double cylinders measuring 20 by 30 inches, driven by a motor of 150- brake horse-power, absorbed 169 electrical horse-power at the switchboard. The mechanical efficiency of the compressor and belt drive is 87 per cent, by indication, leaving 130‘5-horse power as work done on the air, and the adiabatic loss on compression accounts for another 29'3-horse power, leaving 101*2-horse power in the air. This air, at the coal face, is barely capable of driving two coal-cutting machines doing roughly an average of 18 ft. an hour. The comparison is, therefore, 22-horse power to drive an electrical machine, and 65 electrical horse power converted into compressed air to drive one air machine, the work done by the former being as 10*6 of the latter. Rock Drills. The use of compressed air for actuating rock drilling and boring machines is larger than for all other purposes combined, more particularly in the case of metal mines such as the goldmines on the Rand of South Africa. Here the whole system of mining depends on the use and efficiency of rock drills; and economy in air consumption is a very important factor in the reduction of working costs. Of late years, portable boring, punching and shearing machines have been introduced into coalmining both in this country and in America, and it seems probable that their use will be greatly increased year by year as thinner seams are more largely worked and labour difficulties increase. Pneumatic rock drills should be divided into two classes, namely : (1) The percussive type of machine, in which the drill steel is firmly attached to the piston and pounds against the rock, as in the hand jumper bar used in quarries, which is lifted up and allowed to fall by its own weight; and (2) the hammer drill type, in which the piston strikes the drill steel on the down stroke and