Fan aerodynamics
It is not the intention of this book to give detailed data for the aerodynamic design of fans. As has been said elsewhere, it rather seeks to inform both manufacturers and users of the information necessary at their common interface, so that correct choices are made to their mutual advantage.
Nevertheless, it is of value to cover the basics of the theory, to show what is and is not possible, It will also show the back
Figure 3.1 Theoretical flow pattern in a centrifugal fan impeller with backward inclined bladed impeller
Energy in air at impeller exit = torque x angular displacement
= rate of change of (tangential momentum x radius x angular
Displacement)
= tangential momentum x radius x angular displacement
— m vw2 r2 to
In like manner the energy in air at impeller inlet
= m vw1 r, co
Now r, co = u., and r2 co = u2
Energy given to the air by the impeller
= m (vw2 U2 — vw1u.,)
U2 and Q = nd2 b2 
Cot Я2 2U2 J 
Now, as: P=PVW2 P=PVw2 
7td, b 
Q 
MgH =m (v H^(«. 
U, — V» 
U0 
U, — V„ 
U0 
Or 
The theoretical or Euler head H developed by the impeller is defined as the height to which the same weight of gas could be raised by an equal amount of work. Thus: 
P = pu2 
U, + ■ 
cot (180°Я2) 
7td, b. 
Q 
























Figure 3.8 Axial flow blade velocity triangles 
Figure 3.5 Deviation of actual fan characteristics for impeller having backward inclined vanes 
Figure 3.6 Characteristics for radial blade fan 
Figure 3.7 Characteristics for forward curved fan 
Characteristics for radial and forward curved fans are shown in Figure 3.6 and Figure 3.7 respectively.
Important Note
It must be emphasised that all the above assumes straight flow into the impeller eye and consideration of the equations will show that if this is not the case then the pressure developed will be reduced.
Variable inlet vanes purposely use this fact to impart swirl in the direction of rotation. This can be progressively increased by closure of the vanes with a corresponding reduction in the pressure developed. There will of course be some additional friction losses. Further information is given in Chapter 6, Section 6.5.
More importantly, from the system designer’s viewpoint, it will be seen that if straight flow into the fan inlet is not achieved due to poor inlet connections, then the fan will not develop its test pressure. Insufficient straight ducting on the fan inlet side, sagging flexible connections, absence of straighteners in bends, and too tight bends can all be responsible. Where fans are mounted in plenum chambers there must be a sufficient distance from the fan inlet(s) to the chamber walls for the same reason.
Often the system designer is himself short of space. He may then have to provide less than ideal connections. A section on system effect factors (Chapter 5, Section 5.4) has therefore been included and this will enable the designer to make such allowances as are necessary in specifying the fan duty so that the required flow may be achieved.
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