Impellers can be assembled on a standard hub by omitting some of the blades. Mechanical balance must, of course, be preserved, but there is no need for the blades to be evenly spaced. Peak pressure is reduced and the best efficiency point (b. e.p.) moves to a lower pressure and volume so that the speed must be increased for a given duty.
This can be an advantage when the impellers are directly driven by electric induction motors. Such motors have better efficiency and lower cost at higher speeds — a point which can be particularly significant with large low speed fans. Figure 1.88 shows the performance range of fans with 12 left-or-right-handed adjustable pitch blades, which could be assembled with 10,9,8,6, 4, 3 or 2 blades, and multi-staged.
Posted in Fans Ventilation A Practical Guide