TRANSPORT MACHINES FOR GASES.
It is required at times to carry air or other gaseous bodies from one place to another in closed vessels, as for lighting railway-cars, making, the hydrogen light, etc. It is also required to empty a closed vessel of air or other gas, or to compress such air or gas at high tension in a closed vessel, or to replace one body of air or gas with another. The means generally employed in all the cases cited except the first consist of a pipe-system connecting the initial and terminal points, and being in communication with a machine for producing rarefaction or compression of the fluid, or both. Rarefaction causes a current of the gas to pass toward the appa ratus; comprc•ssion causes a current of air to pass from the apparatus. Rarefaction may be produced by heat, as in ventilating-shafts and in chimneys, or by friction of moving liquid, as in the tromp; but heat and friction are much less used than those special apparatuses called fans, air pumps, blowing-engines, and air-compressors, all of which show consid erable resemblance to those employed to transport liquids.
are closely allied to the ordinary piston-pumps for liquids. Figure r (A/. 'IS) shows a blowing-engine with a horizontal cyl inder connected with the steam-engine. The blowing-cylinder is at the left, the steam-cylinder in the centre, and the crank-shaft and fly-wheel at the right, the steam-piston having a rod projecting through each end of the steam-cylinder. The great weight of the piston renders it desirable that there be a back-rod and all extra cross-head. This engine, which is in the Middlesbrough Iron Works, England, is one of the largest hori zontal blowing--engines in the world, the blowing-cylinder being 9 feet in diameter and the steam-cylinder 4 feet, with a stroke of about 9 feet.
The valves of blowing-engines are generally sectional, as may be seen in Figure 2 (pl. 118), this being arranged for a blowing-engine with two cylinders (to the right and left of the Figure respectively). In the centre is shown the blast-pipe for one section. These valves are long cylindrical
metal tubes covered with vulcanized rubber. They rest upon rectangular valve-seats, and are covered by straps to limit their motion.
furnishing compressed air to drive rock-drills or motors, or to keep out the water from air-caissons in bridge-bu ildi ng, there are needed compressors delivering air under a tension of several atmospheres; and these differ from blowing-engines more in the proportion of parts than in anything else, requiring of course where steam is used larger steam cylinders in proportion to the air-cylinders. Air-compressors may be driven by steam, by water-power, by wind-wheels, or by animal power. Those which are steam driven may have a steam-piston upon the same rod as that of the blowing-cylinder, or a steam-cylinder parallel with the air cylinder, the pistons and their connecting-rods acting upon a common crank-shaft. Where the blowing-cylinder is not directly steam driven, the power may be conveyed by belts or by gears, or, as in the case of water-wheels, directly through a shaft one end of which bears a water wheel and the other a crank which actuates the connecting-rod of the blast-cylinder piston-rod.
In the air-compressor shown in Figure 9 the air- and steam-cylin ders are horizontal and in line. There are an in-take air-cylinder and a compressing air-cylinder, into which the former discharges and which increases the pressure obtained thereby. Both air-cylinders are water jacketed, to cool the air heated by compression, and there is between the two cylinders an " inter-cooler," a large pipe or receiver filled with thin brass water-pipes, between and among- which the air passes. Both the air-discharge valves and the air-inlet valves are of the type used in the Corliss engine, and are operated by the main shaft, instead of the suction valves being lifted by the external air-pressure and the discharge by the internal tension. The discharge-valves stand still during the period that they are closed with more pressure above than below them.