Compressed Air Work And Diving 1909

io8 COMPRESSED AIR WORK. When two-stage compression is usecl, the low-pressure cylinder compresses up to 20 Ibs., represented by b, g. b rom g the piston travels to l, whilst the air is discharged into the intercooler at a pressure of 20 Ibs. On the return stroke the clearance air expands to M, and atmospheric air is taken in from j to B. It will be thus seen that there is an increase in free air clelivery equal to m, j. In the meantime the conipressecl air in the receiver is cooled, and contracts to d, a point on the curve of isothermal compression, and follows the cycle d, n, o, l. The air will always contract to approximately K on the isothermal line before it can be usecl, and the useful discharge with two-stage compression is, therefore, k, o, instead of k, i, as given by one-stage compression, or a gain represented by o, 1, which is the same when recluced to atmospheric pressure as M, j, which represents the gain in free air clelivery. e - a represents 3 per cent, clearance in the large cylinder, and f - c 3 per cent, clearance in the small cylinder. It will be seen, therefore, that two-stage compression gives greater economy in power and an increase in free air capacity. This increase in free air capacity is theo- retically of no advantage when working rock drills, because the increased volume due to heating and expan- sion is as useful as an increased volume of free air. Practically it is so because the heated air would become cooled to approximately its original temperature when passing through a long line of pipes, and thus there would be a loss in volume and power. In itself there is no clisadvantage for this class of work in having the air at ci high terminal pressure, and sometimes the practice is adopted of after-heating in order to get increased power.