System effect factors for various duct outlet lengths

From ASHRAE (1988), with permission

A11-4.gif (26555 bytes) Notes:
  • The effective duct length is the length of duct needed for a fully developed velocity profile to exist. This length is approximately 2.5 duct diameters for 2500 ft/min (13 m/s) or less. Add one duct diameter for each additional 1000 ft/min (5 m/s). For example, 5000 ft/min (25 m/s) flow requires five duct diameters. 
  • The blast area is the smallest flow area in the fan. i.e.. at the scroll cutoff.

KSEF values for various outlet duct lengths

Ratio of blast area to outlet area

No duct

12% effective duct length

25% effective duct length

50% effective duct length

100% effective duct length

0%
pressure recovery

50% pressure recovery

80% pressure recovery

90% pressure recovery

100% pressure recovery

0.4

2.0

1.00

0.40

0.18

no system effect factor

0.5

2.0

1.00

0.40

0.18

0.6

1.00

0.66

0.33

0.14

0.7

0.80

0.40

0.14

-

0.8

0.47

0.22

0.10

-

0.9

0.22

0.14

-

-

1.0

-

-

-

-