Blowing Machine

Blowing Machine is the general term for any force-pump arrangement to produce a current of gas. The chief types of blowing machines are on the principle of the common bellows, the ordinary pump, the fan, or the injector. In the ordinary bellows a flexible-sided chamber is made of wood and leather, and is provided with a nozzle, a flap-valve, and a handle or lever to enlarge and diminish the cavity alternately. When the cavity is enlarged, the flap-valve opens and air rushes in; when the air is compressed, it closes the flap-valve and is forced out at the nozzle. Thus a succession of intermittent puffs is given. The employment of two air-chambers in the double bellows enables us to obtain a continuous blast instead of the series of puffs. Blowing machines on the pump principle are much used in blast-furnaces and in the Bessemer process. They consist essentially of an air cylinder and a large air-chamber. In the former a piston is worked backwards and forwards by a separate steam-engine, and alternately draws air into the cylinder and forces it into the air-chamber, whose function is to act as an accumulator and ensure a steady blast. From this the air passes out by pipes to the furnace or to the converter, at a pressure of from 3 to 30 lbs. per square inch. In the fan, which is much adopted for the ventilation of mines, ships and public buildings, for forge fires and for the melting of pig-iron, we have a wheel supplied with vanes, rotating inside a cylindrical chest at a speed of from 600 to 2,000 revolutions per minute. Air is drawn in at the centre of each face of the chest, and is forced out tangentially through a suitable exit-pipe. The fan is analogous in principle to the centrifugal pump. The trompe is a blowing machine on the injector principle (q.v.) employed in France, Spain and America, where a head of water is available. Water flows out from a cistern through a nozzle at the bottom, and then into a vertical pipe of somewhat larger dimensions. Air is drawn into the pipe at the nozzle by the flowing water; it is carried down to a cistern below, and is forced out at a suitable orifice.

Roots's rotary blower has a chamber in which two solid pieces rotate together in such a way as to make always a close fit with each other and with the sides of the chamber. A volume of air is drawn in on one side of the rotating pieces during part of a revolution, and is forced out at the other side during the rest of the revolution.