140
3 Fans
level and are suitable for air conditioning and hot air heating systems in homes.
However, they have a low degree of reaction. The total pressure rise at the rotor
level is mainly available as kinetic energy. Efficiency is impaired if a large part of
the kinetic energy at rotor outlet must be converted into pressure. Such a fan is not
suitable to deliver air through a large pipe system with high energy dissipation, if a
lower efficiency is less acceptable.
Figure 3.26 shows rotors with backward and forward curved blades, and a motor
incorporated into the hub. This is an external rotor motor: the rotor runs externally
around a stator coil instead of running within a stator coil, as is more usual. Motors
that are incorporated into the hub are commonly used in fans of small size. The rotors are very similar to the rotors of Fig. 3.1, but they are right-turning.
3.7 Axial and Mixed-Flow Fans
3.7.1 Degree of Reaction with Axial Fans
Figure 3.27 sketches two customary axial fan types and Fig. 3.28 sketches the corresponding velocity triangles. The first type has an outlet guide vane ring with decelerating flow, downstream of the rotor. The second type has an inlet guide vane
ring with accelerating flow, upstream of the rotor. Both vane rings aim at axial flow
at the machine inlet and outlet. The degree of reaction is very high, near to unity,
with both types, being
The meaning of w mu is the tangential component of the mean relative velocity.
With the machine with downstream vane ring R < 1 (80–90 %); with upstream vane
ring R > 1 (110–120 %). Most fan applications aim at pressure increase and not at
velocity increase. The velocities downstream and upstream of the fan are about
equal (Fig. 3.27). A high degree of reaction is desired, in principle.
R
w
w
u w
w
w
u w
w
w
u
u
u
u
u
u
mu
=
−
=
−
−
= −
1
2
2
2
1
2
2
2
2
1
2
2
2
2
∆
(
)
.
Fig. 3.27 Axial fans with
high degree of reaction
3 Fans
level and are suitable for air conditioning and hot air heating systems in homes.
However, they have a low degree of reaction. The total pressure rise at the rotor
level is mainly available as kinetic energy. Efficiency is impaired if a large part of
the kinetic energy at rotor outlet must be converted into pressure. Such a fan is not
suitable to deliver air through a large pipe system with high energy dissipation, if a
lower efficiency is less acceptable.
Figure 3.26 shows rotors with backward and forward curved blades, and a motor
incorporated into the hub. This is an external rotor motor: the rotor runs externally
around a stator coil instead of running within a stator coil, as is more usual. Motors
that are incorporated into the hub are commonly used in fans of small size. The rotors are very similar to the rotors of Fig. 3.1, but they are right-turning.
3.7 Axial and Mixed-Flow Fans
3.7.1 Degree of Reaction with Axial Fans
Figure 3.27 sketches two customary axial fan types and Fig. 3.28 sketches the corresponding velocity triangles. The first type has an outlet guide vane ring with decelerating flow, downstream of the rotor. The second type has an inlet guide vane
ring with accelerating flow, upstream of the rotor. Both vane rings aim at axial flow
at the machine inlet and outlet. The degree of reaction is very high, near to unity,
with both types, being
The meaning of w mu is the tangential component of the mean relative velocity.
With the machine with downstream vane ring R < 1 (80–90 %); with upstream vane
ring R > 1 (110–120 %). Most fan applications aim at pressure increase and not at
velocity increase. The velocities downstream and upstream of the fan are about
equal (Fig. 3.27). A high degree of reaction is desired, in principle.
R
w
w
u w
w
w
u w
w
w
u
u
u
u
u
u
mu
=
−
=
−
−
= −
1
2
2
2
1
2
2
2
2
1
2
2
2
2
∆
(
)
.
Fig. 3.27 Axial fans with
high degree of reaction