Handbook of air conditioning and refrigeration / Shan K

30.6 CHAPTER THIRTY
load. If glycol is not blended with the chilled water, the lowest chilled water temperature should not
be lower than 37°F (2.8°C) to protect against freezing.
Influence of Inlet Vanes on Small Centrifugal Fans
Inlet vanes mounted at the inlet of the centrifugal fan block a certain percentage of the air passage.
For a small centrifugal fan, these vanes have a considerable effect on fan performance compared
with similar types and sizes of fans without inlet vanes, even when the inlet vanes are fully open.
● The ratio of the inlet diameter D 1 to the impeller diameter D 2, D 1/D 2, is different for forwardcurved
and backward-curved centrifugal fans. For forward-curved fans, D 1/D 2 varies from 0.8 to
0.9, and for backward fans, it varies from 0.65 to 0.8.
● The percentage of blocked air passage for small centrifugal fans may be between 15 and 25 percent.
● Although the blocked area may be only 15 percent of the total air passage, the eddies and turbulences
after the inlet vanes, even if they are fully opened, result in a greater energy loss.
The following is a comparison of fan performance between backward-curved and forwardcurved
centrifugal fans with inlet vanes when they are fully opened and fans without inlet vanes.
These data are taken from the manufacturer’s catalog of vertical modular AHUs (1990).
Without inlet vanes With inlet vanes
Blade BC BC FC BC BC FC
D2, in.
V˙
, cfm
20
10,400
22.25
13,000
15
6000
20
10,400
22.25
13,000
15
6000
vf, fpm 2063 2063 2143 2063 2063 2143
�ps, in WC 3.75 3.75 3.75 2.25 3.17 3.50
�pt, in. WC 4.01 4.01 4.04 2.51 3.43 3.79
bhp, hp 10.93 12.66 5.79 10.38 14.45 5.95
rpm 1841 1645 1296 1839 1646 1291
pt,i /pt,o 0.63 0.86 0.94
BC � backward-inclined or backward-curved centrifugal fan
FC � forward-curved centrifugal fan
V˙ � volume flow rate, in cfm (L/s)
vf � air velocity at fan outlet, fpm (m/s)
�ps, �pt � fan static pressure and fan total pressure, respectively, in. WC (Pa)
bhp � brake horsepower, hp (kW)
rpm � revolutions per minute
�pt,i, �pt,o � fan total pressure with and without inlet vanes, respectively, in. WC (Pa)
If the ratio D 1/D 2 for a backward-curved centrifugal fan of impeller diameter D 2 � 20 in. (508 mm)
is taken as 0.75 and the percentage of blocked area at the inlet is 20 percent, the velocity at the fan
inlet when the inlet vanes are fully opened is
v inlet � V˙
A free
�
10,400
0.8�[(20 � 0.75)/(12 � 2)] 2
� 10,590 fpm (53 m/s)
For such an extremely high velocity, the ratio of fan total pressure of this BC centrifugal fan of
20-in. (508-mm) impeller diameter when the inlet vanes are fully opened is p t,i/p t,o � 0.63. More
than one-third of the fan energy output is lost as a result of the installation of inlet vanes. Such a decrease
in fan total pressure due to an extremely high velocity v inlet was verified in a field installation