Appendix E: Effect of Undersized Straight Duct and Elbows on
Forward Curved Fans
Notes:
1. Single width single inlet (SWSI) fans shown.
2. Fan inlet and elbow positions (rotation) must be
oriented as shown for proper application of the sys-
tems effect factor.
Notes:
1. For double width double inlet (DWDI) fans use the
same system effect factor curve for SWSI fans.
Notes:
1. For double width double inlet fans use the same sys-
tem effect factor – pressure drop curve as for SWSI
fans but if elbows are installed within the 100% ef-
fective duct length multiply the pressure drop value
by the following multiplier based on elbow position
from Table G1.
a. Elbow position B = pressure drop x 1.25
b. Elbow position D = pressure drop x 0.85
c. Elbow position A and C = pressure drop x 1.00
Position D
Position C
Position B
Position A
Inlet
% ef
fective
duct length
Figure E1: Fan inlet and outlet duct elbow positions
A
B
C
D E
F G H I J K L
M
N O
P
Q
R
S
T
U
V
W
X
5 6 7 8 9 10
15
20 25 30 40 50 60
5.00
4.00
3.00
2.50
2.00
1.50
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.25
0.20
0.15
0.10
Air Velocity, FPM in Hundreds
System Ef
fect Factor
– Pr
essur
e, Inches w
.g.
Air Density = 0.075 lb per cu. ft.
Figure E2: System effect factor – pressure drop
curves for SWSI fans
Table E1: System Effect Factor Curves for SWSI Fans
Model
Outlet
Elbow
Position
No
Outlet
Duct
12%
Outlet
Duct
25%
Outlet
Duct
50%
Outlet
Duct
100%
Outlet
Duct
VHC-36,
-42 &
-50
A
Q
Q–R
R–S
U
B
P
Q
R
T
C
N–O
Q–P
P–Q
S
D
O
P
Q–R
S–T
VCES-VHC-IOM-1F – VHC-36, -42 & -50
37
