Handbook of air conditioning and refrigeration / Shan K

15.30 CHAPTER FIFTEEN
An undersized fan results in an uncontrolled indoor environment. An oversized fan wastes energy
and money.
● Fan capacity modulation. A variable-air-volume system operates at a reduced volume flow rate
during part-load operation. Effective and economical fan capacity modulation is an important factor
that affects the operation of an air system.
● Fan efficiency. Fan efficiency is closely related to the energy consumption of an air system. Fans
should be selected so that they can operate at high efficiency during as much of their operation
time as possible.
● Sound power level. Most commercial and public buildings and many industrial applications need
a quiet indoor environment. Fans are the major source of noise in an air system. Usually, the
higher the fan total efficiency, the lower the sound power level of the selected fan. A fan with a
low sound power level and sound power level at high frequencies is preferable. High-frequency
sound is more easily attenuated than low-frequency sound.
● Airflow direction. In many applications, a straight-through or in-line flow occupies less space and
simplifies layout.
● Initial cost. The initial cost of the fan modulation device, sound attenuator(s), and space occupied
by a particular type of fan, in addition to the cost of the fan itself, should be considered.
Another important factor in fan selection is the fan inlet and outlet connections. Since the effects
of these connections are closely related to the characteristics of the air system, they are covered in
Chap. 21.
In an air system, a fan is usually operated with a duct system, which may also include other
equipment and system components. A comprehensive and detailed analysis of fan operation must
consider the characteristics of the duct system.
Estimated Fan Sound Power Level
The estimated sound power level of a fan Lw, in dB, can be calculated as
Lw � Kw � 10 log (15.21)
where Kw � specific sound power level of a certain type of fan, dB
V˙ � fan volume flow rate or capacity, cfm
V˙ 1 � 1 cfm
�pt � fan total pressure, in. WC
�pt1 � 1 in. WC
In Eq. (15.21), C represents the correction for off-peak operation, i.e., fan operation in the
region of the performance curve where fan efficiency is not maximum. For a fan operating at
90 to 100 percent of its maximum static efficiency, C � 0; between 85 and 89 percent, C � 3.
For each decrease of 10 percent of fan maximum static efficiency up to 55 percent, the C value
increases by 3 dB.
Table 15.1 lists the specific sound power level Kw and blade frequency increments BFI, in dB, of
various types of fans. Here, BFI should be added to the sound power level at blade frequency fblade, in Hz. Blade frequency can be calculated as
fan rpm � number of blades
fblade � (15.22)
60
A comparison of Kw values shows that the backward-curved centrifugal fan with airfoil blades
has the lowest Kw value at various octave bands. The differences between Kw values at various
V˙
� 20 log
V˙
1
�pt � C
�pt1