Lead poisoning and lead absorption

by Sir Thomas Morison Legge; Kenneth Weldon Goadby

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.

Fig. 2.—Davidson’s Dust Centrifugal Fan.

Centrifugal Fans.

—Generally, in the removal of dust, a strong suction has to be set up in a system of narrow ducts by means of a centrifugal fan—i.e., a fan-wheel formed by a number of vanes attached to an axle mounted in a spiral-shaped casing—so that when the wheel rotates air is carried along by the vanes, and flies off tangentially into the space between the blades and the casing, and thence to the outlet ([Fig. 2]). The air inlet or junction of the fan with the exhaust duct is at the centre of the fan, an arrangement by which the kinetic energy created by the rapid motion of the air leads to increase of draught instead of being wasted in production of eddies in the surrounding spaces. They are made in many different patterns, according to the nature of the work to be done. Their advantage over the propeller type in the removal of dust lies in the fact that they overcome greater internal resistance, and a uniform high velocity in a complicated system of pipes can thus more easily be maintained.

Fig. 3 shows adjustable hoods and ducts fitting closely over rollers for mixing coloured inks, and serving not only to prevent inhalation of lead dust by the workers, but also the colour from one machine affecting that on another. In the particular room where the installation is fitted there are thirteen separate sets of rollers; the diameter of the branch duct of each machine is about 5 inches, and that of the main duct close to the fan about 20 inches. The special points we have considered as to entrance of all branch ducts into the main duct tangentially, gradual tapering of the main trunk, and collection of the dust in filter-bags, are noticeable. Further, when one set of rollers is not in use the raising of the hood automatically cuts off the draught through it. (Drawing supplied by the Sturtevant Engineering Company, Limited, London.)