_Exhaust by Fans._--The draught for removal of dust, and frequently also of fumes, is produced by a fan, of which there are two types: (1) low-pressure volume fans and (2) high-pressure centrifugal fans. In the first the draught is created by the rotation of a wheel with inclined vanes, causing the air to be driven transversely through the wheel parallel to the axis of rotation (Fig. 1). During a revolution a portion of the air is cut off from one side of the wheel, and transferred through the wheel to the other. Such fans are light, run easily, and are cheap. They are of many forms, both with regard to the number of blades--from two to eight--and general manner in which they are arranged. Some closely resemble the screw-propeller of a ship, while others have blades turned over and fastened on an outer rim. Their main defect is inability to overcome any but slight resistance in the course of suction behind, as from constriction in, or friction along the sides of, the ducts and right-angled bends, or of outflow in front, as from wind-pressure. Under favourable conditions, however, and when carefully fitted, a volume fan will exhaust dust and fumes through a system of ducts several feet in length, as, for example, from mono and linotype machines and electro melting-pots in letterpress printing works. But, in order to avoid resistance from friction, the ducts have to be somewhat larger in diameter than when a centrifugal fan is used. With nine[A] linotype machines connected up to a 14-inch propeller fan, the branch ducts should be about 4 inches in diameter, and the main duct 12 inches, increasing from 12 to 15 inches within 2 feet of the fan-box. The shorter and straighter the course of the duct to the propeller fan, the more efficiently it works. Wind-guards are necessary to overcome resistance from this source in front, but their position requires to be carefully considered, so as to prevent the screen itself crippling the outflow. [A] If gratings are also inserted in the same duct for general ventilation the number of machines must be decreased _pro ratâ_. All fans require frequent cleaning, and in this respect propeller fans have the advantage over centrifugal, in that they are usually more accessible. [Illustration: FIG. 2.--DAVIDSON’S DUST CENTRIFUGAL FAN.]