Handbook of air conditioning and refrigeration / Shan K

20.22 CHAPTER TWENTY
FIGURE 20.7 (Continued)
Outlet system effect loss at the design volume flow should be added to the calculated total system
pressure loss.
Outlet System Effect Loss Coefficient
Outlet system effect loss �p s,o and outlet system effect loss coefficient C s,o both depend on the effective
duct length ratio L/L eff, the air velocity at the fan outlet v fo, and the configuration of the duct
fitting with respect to the fan outlet. Here L indicates the length of duct section between fan outlet
and the duct fitting in duct diameters.
The effective duct length L eff is the length of duct between the fan outlet and a cross-sectional
plane at which the uniform velocity profile has been completely recovered, in duct diameters or
equivalent diameters D and L and L eff must be expressed in the same units. Effective duct length can
be evaluated as
L eff � 2.5 � v fo � 2500
1000
(20.22)
For a single-width single-inlet (SWSI) centrifugal fan, the configurations of a 90° elbow with respect
to the fan outlet at the end of the connecting duct can be classified as 90° turn right or 90°
turn down, 90° turn left or 90° turn up, parallel with inlet, and opposite to inlet, as shown in
Fig. 20.7c.
According to AMCA Fan and Systems, Publication 201 (1973), the magnitudes of C s,0.8 at various
L/L eff values and elbow orientations with a blast area to fan outlet area ratio A blast/A out � 0.8 are
as follows: