_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. [Illustration: FIG. 48A.—‘Sirocco’ Centrifugal Fan] [Illustration: FIG. 48B.—Showing exhaust aperture and fan blades] High-pressure fans are used mainly for technical purposes, as, for example, the driving of air or gas at high pressure. Localised ventilation is needed to limit diffusion of dust and fumes, which is attained in a measure also by separation of those workrooms in which persons come into contact with poisonous materials from others. Separation of workrooms, however, is not enough, as it is the individual who manipulates the poison for whom protection is desired. To enclose or hood over a dusty machine or fume-producing apparatus completely, or to close hermetically a whole series of operations by complicated technical arrangements, is only possible when no opening or hand feeding is required. Dangerous substances can only be wholly shut in by substitution of machinery for handwork. [Illustration: FIG. 49.—Localised Exhaust Ventilation in a Colour Factory (_Sturtevant Engineering Co., Ltd._)] [Illustration: FIG. 50A. FIG. 50B. Ball Mills] Where, however, absolute contact is unavoidable the dust or fume must be carried away at its source. This is done by exhaust ventilation, locally applied, in the following manner: A suitable hood or air guide of metal or wood is arranged over the point where the dust is produced, leaving as small an opening as possible for necessary manipulations. The hood is connected with a duct through which the current of air travels. An exhaust current dependent upon heat will only suffice in the case of slight development of dust or fumes. As a rule exhaust by a fan is necessary. Where exhaust ventilation has to be arranged at several points all these are connected up by branch ducts with the main duct and centrifugal fan. Where the ducts lie near the floor it is advisable to fix adjustable openings in them close to the floor to remove the sweepings. [Illustration: FIG. 51.—Ventilated Packing Machine (_after Albrecht_) _A_ Worm; _B_ Collector; _D_ Fan; _E_ Filter bag; _J_, _F_ Movable shutters; _H_ Jolting arrangement]