Handbook of air conditioning and refrigeration / Shan K

AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.17
of the impeller, air leaves the blade at its outlet with a velocity v 2. For a blade section whose diameter
at blade inlet equals the diameter at blade outlet, the peripheral velocities u 1 � u 2, and the axial
velocities v m1 � v m2. The total pressure rise �p t, in in. WG, as air flows through the impeller is
given by the same equation as for a centrifugal fan when velocities are expressed in fpm:
�p t � 1.66 � 10 �6 �v u2u 2
To design a blade such that the total pressure developed at various radii of the blade is constant, the
blade angle near the hub of the same blade must be increased in order to provide a higher v u2 when
the radius becomes smaller. A greater hub ratio D hub/D bt results in a smaller variation of the blade
angles of the same blade.
After air leaves the blade and before it enters the downstream fixed guide vanes, there is a rotational
component v u2 in the direction of rotation. The function of the downstream guide vanes is to
convert this dynamic pressure to static pressure with minimum energy loss.
The static pressure developed in an axial fan is the combined effect of the following:
● The corresponding drop in relative velocity pressure
C 2�(w 1 2 � w2 2 )
● The conversion of rotating dynamic pressure to static pressure
2 2 C2�(v 2 � vm 2)
● The conversion of the difference in velocity pressures
C 2�(v 3 2 � v4 2 )
where v 3, v 4 � mean axial velocities at cross-sectional areas with and without hub, respectively,
fpm (m/s)
C 2 � constant equal to 1/(60 2 � 2 � 32.2 � 5.192) � 8.3 � 10 �7
FIGURE 15.13 Pressure-volume �pt-V˙ for axial fans with the same impeller
diameter.