Axial flow fans deliver the air in a direction parallel to the axle around which the fan revolves and may have either curved or flat blades. They cannot overcome a heavy duct resistance since under such a condition the air delivered at the relatively high speed tips of the blades tends to return against the relatively low speed blade surfaces close to the hubs. Axial flow fans, however, are useful for installation in outside walls of rooms, especially where the emerging air will not be opposed by direct wind action. Axial flow fans are objectionably noisy when run at high speed. Aeroplane propellers are axial flow fans.
Radial flow fans deliver the air at right angles to the shaft and are capable of overcoming very great resistances. They are like centrifugal pumps with paddle wheels and housings. They may have blades curved forward with reference to the direction of rotation or curved backward, and usually have anywhere from 8 to 6o blades. The physical proportions of centrifugal fans and their housings have been the subject of profound research, and strange characteristics have been discovered. For instance, the greater the resistance at a given speed the less air will be delivered, and the less power will be required.
Forward curved blades will deliver more air than backward curved blades at a given speed, with increased noise, and if the resistance is reduced will deliver still more air and may overload the driving mechanism. Backward curved blades require higher
speed for the same air delivery and resistance as forward curved blades, but are less noisy and may be so designed as never to over load when the resistance is reduced. Given several forward curved blade fans working in parallel, one or the other will try to carry more than its share of the burden while two or more backward curved blade fans once placed in proper speed-relation will work together satisfactorily.
The air delivery capacity of any fan varies directly as the speed varies.
The intensity of the resistance overcome by any fan varies as Authority: Guide, American Society of Heating and Ventilating Engineers.
It is desirable at all grinding, polishing, and buffing hoods to provide easily opened hoppers near the inlets to retain heavy particles and to arrange for air inlets which will open automatically to admit sufficient air to carry the material along the ducts in case the normal supply should be curtailed, as by shutting down certain branches.
The importance, with industrial exhaust ventilation systems, of providing air inlets to the rooms containing the exhaust systems with means for heating the air, should not be overlooked. In some industries there are generated noxious gases which are heavier than air and which therefore will not rise into hoods placed above the apparatuses unless these are very close to the point of genera tion and have unusually rapid air currents. Table III. lists the more common of these, giving the specific gravity in comparison with that of air taken as 1.