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Fig. 1

Speed increase Impeller blade tipping

It is the easiest way but the most uncommon for This is a very easy mechanical solution but it pro-
big fans. The fan performance changes according vides limited possibility for performance increase.
to the fan laws: The principle of the modifi cation is to increase the
diameter of the impeller by adding “extensions” to
Q = Q * RPM /RPM capacity the trailing edge of each fan blade. The blade tips
r
r
P = P * (RPM /RPM) 2 pressure protrude outside of the back plate (centreplate for
r
sr

s
W = W * (RPM /RPM) 3 shaft power double suction wheels) and side cones of the im-
r
r
Eff. =Eff. effi ciency peller. The tip may follow the blade angle, as shown
r
in the second sketch of fi g. 3 (A) or increase the
Where the suffi x “r” is relevant to the modifi ed fan. blade angle as shown if the fi rst sketch of the same
fi gure.
The effect on the performance curves is shown in The performance curve of the fan varies like a vir-
the figure 2. tual increase of the rotational speed, but with a cor-

rection factor.
The limits of this simple solution, for big fans, are
mainly mechanical and related to the fi xed polarity Q = Q * D /D * K P = P * (D /D*K)
2
r
s
r
sr

r
and installed power of asynchronous E-motors. If W = W * (D /D*K) * K
3
r
r
1
a VFD is installed this solution may be reliable. Fig. 2
FLOW RATE - STATIC PRESSURE FLOW RATE - SHAFT POWER
800
1400
700
1200 0 1480
600
1000
Pressure [mmH2O] 800 Power [kW] 400 1300
500
600
300
400
200
200
0 0 0 100
0
0 50000 100000 150000 200000 250000 300000 350000 400000 450000 0 0 0 0 0 0
Flow Rate [m^3/h] 0 50000 100000 150000 200000 250000 300000 350000 400000 450000
Flow Rate [m^3/h]
Impiantistica Italiana - Gennaio-Febbraio 2023 79