Injectors are usually only employed for special technical or economical reasons. A jet of steam or compressed air which acts on the injector creates a partial vacuum and so produces a powerful exhaust behind. [Fig. 45] shows the mechanism of an injector. They are used for exhausting acid fumes which would corrode metal fans and pipes, and for explosive dust mixtures where fans are inadmissible.
Fig. 46.—Propeller Fan coupled to Electromotor (Davidson & Co., Ltd.)
In the industries described in this book fans are most commonly used. These are, in the main, wheels with two or more wing-shaped flattened blades. Some are encased, others are open and fitted by means of annular frames in the ducts according to the intended effect and kind of fan. Fans are of two kinds, propeller and centrifugal, and, according to the pressure they exert, of low, medium, or high pressure. They are now often driven electrically, in which case there is advantage in coupling them directly with the motor.
Propeller fans have curved screw-shaped blades and are set at right angles in the duct upon the column of air in which they act by suction. The air is moved in the direction of the axis of the fan, and generally it is possible, by reversing the action, to force air in instead of extracting it. The draught produced is a low-pressure one (generally less than 15 mm. of water). The current of air set in motion travels at a relatively slow speed, yet such fans are capable, when suitably proportioned, of moving large volumes of air. Propeller fans are specially suitable for the general ventilation of rooms when the necessary change of air is not being effected by natural means.
Fig. 47.—The Blackman (Belt-driven) Fan.
Centrifugal or high-pressure fans (see figs. [48a] and [48b] ) are always encased in such a way that the exhaust ducts enter on one or both sides of the axis. The air thus drawn in is thrown by the quickly rotating numerous straight blades to the periphery and escapes at the outlet. The centrifugal fan travels at a great speed, and the air current has therefore great velocity and high pressure. When the pressure is less than 120 mm. it is described as a medium, and when greater, a high-pressure fan. For the former a galvanised iron casing suffices; for the latter the casing requires to be of cast iron. Medium pressure centrifugal fans are used to exhaust dust or fumes locally from the point at which they are produced. They play a great part in industrial hygiene.
