Handbook of air conditioning and refrigeration / Shan K

15.20 CHAPTER FIFTEEN
the performance curves, it can be seen that the greater the blade pitch or blade angle, the higher the
fan total pressure �p t and the volume flow rate.
Reverse Operation. If the rotation of an axial fan is reversed, the direction of airflow is reversed. Propeller
and tube-axial fans without guide vanes deliver about 60 to 70 percent of the volume flow rate in
the original forward direction when their rotations are reversed. Vane-axial fans with guide vanes either
downstream or upstream from the impeller do not perform efficiently when they rotate in reverse.
Tip Clearance and Number of Blades. For an axial fan, the size of the gap between the blade tip
and the casing �, as shown in Fig. 15.12, or the ratio of this gap to the outside diameter of the impeller
�/D imp, has a definite effect on fan efficiency and sound power level. Here D imp indicates the
diameter of the impeller. Clearance � and diameter D imp must be expressed in the same units. Generally,
�/D imp should be equal to or less than 0.0025. When the gap becomes greater, the amount of
air leakage through the gap increases. Various tests have shown that fan total efficiency may drop
from 0.7 to 0.68 if �/D imp increases from 0.0025 to 0.01.
Theoretically, the more blades in an axial fan, the better the guidance of air. At the same time,
the friction between the fluid elements and the surface of the blade increases. An axial fan with a lot
of blades always produces a higher fan total pressure than a fan with fewer blades at the same volume
flow rate, if other conditions are the same.
15.4 FAN CAPACITY MODULATION
In a variable-air-volume (VAV) air system, more than 90 percent of its running hours operate at an
airflow less than the design volume flow rate. Modulating the fan capacity by providing a new
�pt-V˙ curve with lower volume flow and fan total pressure not only corresponds with load reduction
in the conditioned space, but also allows significant energy savings at part-load operation.
Four types of fan capacity modulation are widely used in air systems: fan speed, inlet vanes, fan
inlet cones, and blade pitch modulation.
Fan Speed Modulation with Adjustable-Frequency, Variable-Speed Drives
The peripheral velocity at blade tip u2 affects both the tangential component vu2 and radial component
vm2 of the airflow and, therefore, the V˙ and �pt of the fan. The variation of fan speed produces
a family of similar �pt-V˙ curves, as shown in Fig. 15.17a.
Most widely used, variable-speed drives (VSDs) for fans and pumps are adjustable-frequency
VSDs. These VSDs modulate the speed of the ac motor by supplying a variable-frequency and
variable-voltage power source. The synchronous speed of the motor nm, in rpm, can be calculated as
where f � frequency of applied power, Hz
N p � number of poles
n m � 120f
N p
(15.20)
If the frequency varies, the motor speed changes accordingly. The voltage must be reduced with the
frequency, to maintain a specific voltage-to-frequency ratio, in order to follow the decreasing inductive
reactance of the motor.
The incoming three-phase ac power is rectified into dc supply through a rectifier. Then the dc
power supply is shaped into a pseudo-sine wave of predetermined frequency by an inverter, as shown
in Fig. 15.17b. The advantage of an ac inverter is that its speed can be reduced to one-tenth (or even
less) of its original speed. Through the change of frequency, the speed-torque curve of the motor can