3.6 Rotor Shape Choices with Radial Fans
139
with respect to noise. The rotors with backward curved blades of Figs. 3.1 and
3.25 are manufactured very carefully with regard to noise generation. Among other
things (e.g. well rounded leading edges), they feature many blades, namely 13. This
is much more than strictly necessary (the forward curved blade rotor has 42 blades).
Due to the high number of blades, pressure differences between pressure and suction sides are small. This strongly contributes to silent operation. A practical difference between both types is a much lower vibration level with forward curved
blades, due to the lower rotational speed. The lower efficiency is less important for
small power applications. Fans with forward curved blades are therefore preferred
in homes. A typical type in homes is a fan incorporated into a box. The box is more
or less cubic and contains a fan with forward curved blades (sometimes 2 fans) and
is divided in two sections. The suction pipe of the box leads to the suction eye of
the fan. The fan delivers air to the second section of the box, where dump diffusion
occurs. The delivery pipe of the box is connected to this second section.
The degree of reaction is a main parameter determined by the application. It is,
as discussed in Sect. 3.2, related to the blade shape. From Fig. 3.6 it follows that
the total pressure rise of a pure reaction machine (R = 1) equals zero. No work is
done by the blades on the fluid, which moves purely in radial direction in the absolute frame, if no pre-swirl is applied. The shape of the workless blade follows
from v r
u = constant, i.e. no change of angular momentum. If the blade is still more
backward curved, the machine becomes a turbine. Even if centrifugal hydraulic
turbines have been constructed in the past, a centrifugal form of a radial turbine is
not advisable, since the centrifugal force causes pressure rise in the flow sense. A
turbine is intended to utilise a pressure drop. Thus, as a principle, a radial turbine
should be a centripetal machine. With β 2 between 0 and the value for R = 1, the rotor
has backward curved blades. Rotor work is rather small, but a significant part of it
is used for pressure increase in the rotor. This type is appropriate if mainly intended
to build up pressure. Conversion of kinetic energy into pressure by a downstream
diffuser generates large losses. Large fans, with which efficiency is important,
therefore exclusively have backward curved blades. Moderately backswept blade
fans feature a lower relative velocity in the rotor channels for the same flow rate.
Solid particles in the gas flow cause then less blade erosion. This fan type is thus
appropriate for gases containing solid particles, as e.g. flue gases. It is also applied
for pneumatic transport. Fans with forward curved blades enable the generation of a
target total pressure rise at a low peripheral velocity. Such fans have a low vibration
Fig. 3.26 Rotors with motor
incorporated into the hub.
(Courtesy ebm-papst)
139
with respect to noise. The rotors with backward curved blades of Figs. 3.1 and
3.25 are manufactured very carefully with regard to noise generation. Among other
things (e.g. well rounded leading edges), they feature many blades, namely 13. This
is much more than strictly necessary (the forward curved blade rotor has 42 blades).
Due to the high number of blades, pressure differences between pressure and suction sides are small. This strongly contributes to silent operation. A practical difference between both types is a much lower vibration level with forward curved
blades, due to the lower rotational speed. The lower efficiency is less important for
small power applications. Fans with forward curved blades are therefore preferred
in homes. A typical type in homes is a fan incorporated into a box. The box is more
or less cubic and contains a fan with forward curved blades (sometimes 2 fans) and
is divided in two sections. The suction pipe of the box leads to the suction eye of
the fan. The fan delivers air to the second section of the box, where dump diffusion
occurs. The delivery pipe of the box is connected to this second section.
The degree of reaction is a main parameter determined by the application. It is,
as discussed in Sect. 3.2, related to the blade shape. From Fig. 3.6 it follows that
the total pressure rise of a pure reaction machine (R = 1) equals zero. No work is
done by the blades on the fluid, which moves purely in radial direction in the absolute frame, if no pre-swirl is applied. The shape of the workless blade follows
from v r
u = constant, i.e. no change of angular momentum. If the blade is still more
backward curved, the machine becomes a turbine. Even if centrifugal hydraulic
turbines have been constructed in the past, a centrifugal form of a radial turbine is
not advisable, since the centrifugal force causes pressure rise in the flow sense. A
turbine is intended to utilise a pressure drop. Thus, as a principle, a radial turbine
should be a centripetal machine. With β 2 between 0 and the value for R = 1, the rotor
has backward curved blades. Rotor work is rather small, but a significant part of it
is used for pressure increase in the rotor. This type is appropriate if mainly intended
to build up pressure. Conversion of kinetic energy into pressure by a downstream
diffuser generates large losses. Large fans, with which efficiency is important,
therefore exclusively have backward curved blades. Moderately backswept blade
fans feature a lower relative velocity in the rotor channels for the same flow rate.
Solid particles in the gas flow cause then less blade erosion. This fan type is thus
appropriate for gases containing solid particles, as e.g. flue gases. It is also applied
for pneumatic transport. Fans with forward curved blades enable the generation of a
target total pressure rise at a low peripheral velocity. Such fans have a low vibration
Fig. 3.26 Rotors with motor
incorporated into the hub.
(Courtesy ebm-papst)