_A view of a Pelton water wheel_ operating a compressor it shown in Fig. 363. The cut represents a compound air compressor in which the valves are operated mechanically. The water which drives the wheel enters through the pipe and nozzle secured in the wheel pit, as represented.
Fig. 364 exhibits a belted duplex air compressor built by Allis-Chalmers & Co.
Fig. 365 shows a vertical duplex compressor driven by a belt.
All the latter, as may be seen by the engravings, have the positive valve motion operated by an eccentric. In selecting an air compressor the following points need consideration: 1, Number of cubic feet of free air required per minute; 2, Altitude, _i.e._, the number of feet above the sea level; 3, Steam pressure and air pressure.
_The use of compressed air for operating mining pumps_, while having advantages in some cases, _is not to be recommended in all_, particularly on account of the low efficiency of the plant as a whole. The loss due to leaks is serious, and the long line of piping with its numerous joints causes much trouble, delay and expense.
_In Fig. 369 is shown a direct acting steam single air compressor_; simplicity in its construction is a leading feature and there are few parts in this pump that are liable to wear.
This apparatus is designed for working pressures up to twenty pounds; it is intended for use in oil refineries, smelting works, blast furnaces and in all situations where compressed air of medium pressure is required. They are variously used for sand blasts, ventilating purposes, and for pneumatic deliveries.
The steam end and valve motion are the regular Deane pattern, assuring positive operation. The air cylinder is provided with a water jacket.
_A power wall or post air compressor_ is shown in Figs. 370-372. The machine is single acting and is recommended where little space is available, as it can be bolted to the wall or to a post, or on the under side of the ceiling. The crank shaft and connecting rod are of cast steel. The bearings are babbitted and adjustable. The piston is of the trunk form, carrying a pin for the connecting rod, and is of extra length to act as a guide for the lower end of the connecting rod. The valves are of the poppet type. These compressors are extensively used in electric power stations for supplying air for removing dust from electric machinery, in bicycle shops for inflating pneumatic tires, maintaining a supply of air in pressure tanks, and for various purposes where a limited supply of air is needed.