Handbook of air conditioning and refrigeration / Shan K

For a SWSI centrifugal fan with A blast/A out � 0.7, a multiplier K blast � 1.4 should be applied to the
above values. If A blast/A out � 0.6, then K blast � 2; and if A blast/A out � 0.9, then K blast � 0.75.
For double-width double-inlet (DWDI) centrifugal fans, an additional multiplier K DI should be
applied to the C s,0.8 values. For a 90° elbow connected to the duct at horizontal turning orientation,
K DI � 1.25. The outlet system effect coefficient C s,o can then be determined as
(20.23)
For details, refer to AMCA, Fan and Systems (1973).
For fans installed in air-handling units and packaged units and rated as complete units, the ratings
are based on a straight duct length of 2 duct diameters from the unit outlet. The outlet system
effect loss is not included in the laboratory-rated fan performance curve.
Selecting Fans Considering System Effect Losses
AIR SYSTEMS: BASICS AND CONSTANT-VOLUME SYSTEMS 20.23
C s,o � K DIK blastC s,0.8
L/L eff
0.12 0.25 0.5 1.0
90° turn right or down 0.64 0.44 0.20 0
90° turn left or up, parallel with inlet 1.16 0.76 0.36 0
90° turn left or up, opposite to inlet 0.80 0.52 0.24 0
If the calculated volume flow rate of the design operating point Pd (as shown in Fig. 20.8) is V˙ d, in
cfm (L/s), and the calculated system total pressure loss is �psy, in in. WC (Pa), the selected fan that
FIGURE 20.8 Selecting fan with performance including system effect loss.