Backward inclined blades
The impeller of these is represented in Figure 1.70.
Figure 1.70 Backward inclined bladed impeller
Figure 1.71 Backward inclined fan — typical characteristic curves
These may be considered at the “maids of all work”. Due to their simplicity the blades lend themselves to simple methods of construction, at a moderate price, and they can easily be flanged for rivetting and spot welding up to size 900 mm. The design is of the high-speed type making them suitable for direct connection (Arrangement 4 and 8 for many duties).
Fan static efficiencies up to 80% peak have been achieved with the medium widths using the very latest aerodynamic knowledge. The wider fans have the additional advantage of a non-overloading power characteristic so that, with correct motor selection, the fan may operate over its complete constant speed pressure-flow curve. In its working range, the curve is also comparatively steep, so that large variations or errors in system pressure will have a smaller effect on flow rate. (See Figure 1.71).
The blades are self-cleaning to a certain degree and are in any case easy to clean because of their single plate flat form. They are therefore suitable for free-flowing granular dust burdens or moisture-laden air. In the absence of special factors, this impeller is the recommended form for all applications including commercial and industrial ventilation systems, low and high velocity air conditioning, the clean side of collectors in dust extract systems, fume extraction, etc.
Standard fans are available for operation at gas temperatures up to 350°C and special units employing high temperature alloys can be custom-manufactured for gases up to 500°C. In general terms, the narrower the impeller, the fewer the number of blades and the greater the blade outlet angle. Both these factors are conducive to the acceptance of higher dust burdens but counter-balanced to a certain extent by boundary layer effects and higher abrasive velocities.
Posted in Fans Ventilation A Practical Guide