138
3 Fans
Figure 3.25 shows the characteristics for rotors with backward curved and forward curved blades, fitting into the same volute. Coding 40 indicates that the rotor
diameter is 40 cm. With a 2-pole driving motor, rotational speed is about 2900 rpm
and with a 4-pole one it is about 1450 rpm at 50 Hz. With an equal diameter, the rotor with forward curved blades, with half the rotational speed, produces about three
quarters of the pressure rise of the rotor with backward curved blades ( ψ about 1.2
compared to about 0.4). Flow rates are comparable, with the inflow and outflow
velocities of the fan being about the same. The rotor with forward curved blades is
broader than the rotor with backward curved blades, but the through-flow does not
fill the whole width, as explained before (Fig. 3.19). The outflow velocity of the
rotor in the absolute frame is higher with the forward curved blades than with the
backward curved blades. The ratio is about 1.5. With forward curved blades, a high
velocity reduction occurs within the volute, due to tangential dump diffusion with
rather high energy dissipation. With forward curved blades, the efficiency is significantly lower than with backward curved ones. The maximum efficiency with backward curved blades is about 81 %, compared to about 64 % with forward curved
blades. The characteristics are very different with both rotor forms. The total pressure rise is almost independent of the flow rate with forward curved blades, but it
is strongly diminishing with increasing flow rate with backward curved ones. With
forward curved blades, the minimum in the characteristic may be more pronounced
than in the example, depending on the blade angles chosen. In most applications,
the form of the characteristic is not critical with fans. The fan load mostly consists
of a duct system or a space. The total pressure rise by the fan compensates friction losses, bend losses or diffusion losses, which are all proportional to the flow
rate squared. The intersection of the fan characteristic and the load characteristic
is mostly about perpendicular. There is no risk of unstable operating points (see
Chap. 8: pumps). But there are examples of applications with relevant form of the
characteristic. Fans supplying combustion air to boilers with chain grates (coal or
household waste) should feature a steep characteristic, as flow resistance of the fuel
layer may vary strongly. It is advisable that this does not affect the flow rate much,
which makes a fan with strongly backward curved blades suitable.
The total pressure rise value is not critical for many applications. The peripheral
speed of a welded rotor may easily amount to 50 m/s. The corresponding 1 2
2
2
u
r is
about 1500 Pa. With many applications, the total pressure increase needed is much
lower. Typical air velocities in ducts lie around 10–20 m/s. At 16 m/s, the dynamic
pressure (
/ )
ρv
2
2 is about 150 Pa. The total loss coefficient of the load thus must
amount to about 10 in order to attain 1500 Pa. Note that larger peripheral speed does
not create great problems. A speed of 80 m/s is easily attainable with straight backswept blades (almost no bending stress). Such a fan thus attains ( ψ ≈ 0.6) 4500 Pa total pressure rise. Peripheral speeds until 100 m/s occur, attained by applying thicker
steel plates (until 10 mm thickness) and a large number of blades (until 16).
Fan generated noise strongly increases with the rotational speed. Note that, for the
Fig. 3.25 fans, with similar flow rate and total pressure rise values, the noise generated with backward curved and forward curved blades is comparable, notwithstanding the higher rotational speed with backward curved blades. A rotor with forward
curved blades, with high turning and recirculation zones, is rather unfavourable
3 Fans
Figure 3.25 shows the characteristics for rotors with backward curved and forward curved blades, fitting into the same volute. Coding 40 indicates that the rotor
diameter is 40 cm. With a 2-pole driving motor, rotational speed is about 2900 rpm
and with a 4-pole one it is about 1450 rpm at 50 Hz. With an equal diameter, the rotor with forward curved blades, with half the rotational speed, produces about three
quarters of the pressure rise of the rotor with backward curved blades ( ψ about 1.2
compared to about 0.4). Flow rates are comparable, with the inflow and outflow
velocities of the fan being about the same. The rotor with forward curved blades is
broader than the rotor with backward curved blades, but the through-flow does not
fill the whole width, as explained before (Fig. 3.19). The outflow velocity of the
rotor in the absolute frame is higher with the forward curved blades than with the
backward curved blades. The ratio is about 1.5. With forward curved blades, a high
velocity reduction occurs within the volute, due to tangential dump diffusion with
rather high energy dissipation. With forward curved blades, the efficiency is significantly lower than with backward curved ones. The maximum efficiency with backward curved blades is about 81 %, compared to about 64 % with forward curved
blades. The characteristics are very different with both rotor forms. The total pressure rise is almost independent of the flow rate with forward curved blades, but it
is strongly diminishing with increasing flow rate with backward curved ones. With
forward curved blades, the minimum in the characteristic may be more pronounced
than in the example, depending on the blade angles chosen. In most applications,
the form of the characteristic is not critical with fans. The fan load mostly consists
of a duct system or a space. The total pressure rise by the fan compensates friction losses, bend losses or diffusion losses, which are all proportional to the flow
rate squared. The intersection of the fan characteristic and the load characteristic
is mostly about perpendicular. There is no risk of unstable operating points (see
Chap. 8: pumps). But there are examples of applications with relevant form of the
characteristic. Fans supplying combustion air to boilers with chain grates (coal or
household waste) should feature a steep characteristic, as flow resistance of the fuel
layer may vary strongly. It is advisable that this does not affect the flow rate much,
which makes a fan with strongly backward curved blades suitable.
The total pressure rise value is not critical for many applications. The peripheral
speed of a welded rotor may easily amount to 50 m/s. The corresponding 1 2
2
2
u
r is
about 1500 Pa. With many applications, the total pressure increase needed is much
lower. Typical air velocities in ducts lie around 10–20 m/s. At 16 m/s, the dynamic
pressure (
/ )
ρv
2
2 is about 150 Pa. The total loss coefficient of the load thus must
amount to about 10 in order to attain 1500 Pa. Note that larger peripheral speed does
not create great problems. A speed of 80 m/s is easily attainable with straight backswept blades (almost no bending stress). Such a fan thus attains ( ψ ≈ 0.6) 4500 Pa total pressure rise. Peripheral speeds until 100 m/s occur, attained by applying thicker
steel plates (until 10 mm thickness) and a large number of blades (until 16).
Fan generated noise strongly increases with the rotational speed. Note that, for the
Fig. 3.25 fans, with similar flow rate and total pressure rise values, the noise generated with backward curved and forward curved blades is comparable, notwithstanding the higher rotational speed with backward curved blades. A rotor with forward
curved blades, with high turning and recirculation zones, is rather unfavourable