Open paddle blades
This open paddle blade design is represented diagrammatically in Figure 1.66.
Figure 1.66 Open paddle impellers
This is the impeller for heavy dust burdens in excess of those possible with the shrouded radial. Its efficiency is only moderate (up to 60% static) but it is suitable for high temperatures. As there are no shrouds or backplates, the blades are free to expand. Standard units may therefore be used with gases up to 350°C, but special alloy wheels can be designed for the very highest temperatures.
It will be seen (Figure 1.67) that the pressure characteristic is stable over the whole range of flows but that the power rises continuously with flow. Open paddle fans are manufactured in various widths, where casing inlet and outlet areas are virtually equal. The narrower units are also suitable for high pressure applications such as direct injection pneumatic conveying.
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