Rofary which have rotary pistons are generally called "blowers." They are in most essentials very similar to many rotary engines and rotary pumps for liquids. The Root blower (jigs. 3, 4) is of this class, as is also the Disston blower.
centrifugal type of blower is the most common. As in many centrifugal pumps, the fans of the blower consist of wheels pro vided with several straight, curved, or peculiarly-shaped vanes enclosed in casings: by the quick action of the vanes the air in the casings is com pressed and whirled into the outlet-pipe, fresh air being sucked in through openings in the centre of the apparatus. Of this type one of the best known is the Sturtevant (fig. 6).
Grtibal's shown in three-quarter section in Figure 8, as arranged for mine or tunnel ventilation has a diameter of 36 feet, and is operated by a steam-engine whose crank-shaft is a prolongation of the axis of the wheel. The casing of the air-duct leading from its periphery is of brick. The wheel is of wood, made by fastening joints parallel with the diameter of the casing to the periphery of three octag onal iron hubs, and fixing boards across these joists.
"Pro/Seller" Fans.--Fans such as the Wing and the Blackman 118, jig. 5) do not act upon the centrifugal principle, but work like a screw propeller, drawing the air in at one side of a circular wheel-case charging it at the other side. Figure 7 shows a turbine ventilator having several wheels, each being entirely composed of helical paddles. A single
wheel has the disadvantage of allowing considerable of the moved fluid to run back through the spaces between the wheel and the inner surface of the casing, so that the compression cannot be increased to a very high degree. To do away with this evil, several such wheels are arranged one after the other, each taking in its suction the air compressed by the one which precedes it. Such a type is called a "multiplying" ventilator.
nearly the same principle as the jet injector there has been perfected an arrangement by which the end of the blast-pipe of a ventilator or blowing-engine provided with a nozzle termi nates in the same axial line as a relatively wide channel. The friction of the air-current in the smaller and central jet draws in air through orifices in the side of the larger channel. In the tromp or water-blowing machine a jet of water is substituted for the jet of air in the last-named jet-ex hauster.
In the blast-pipe of a locomotive a jet of steam draws the gases of combustion from the fire-box. In Figure i (pi. a' is the smoke-pipe, c the smoke-box, and h the exhaust-nozzle which receives the steam from the exhaust of the cylinder and ejects it in the direction of the axis of the smoke-pipe a'. When the engine is not running the same effect may be produced in less degree by a blast-pipe of live steam in the smoke-box, pointing up time stack.