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All installations and services must be performed by qualified service personnel.
However, there are two, 90 degree turns in the branch that will cause a large
pressure drop, not accounted for in our calculations. To compensate for this
drop, we should increase the effective length of the branch. For a 90 degree,
mitered elbow, the equivalent straight duct length is given by the following
formula,
For a 6-inch diameter 90-degree elbow, the equivalent straight duct length is,
So, the total equivalent straight duct length is,
Now, the required duct diameter should be recalculated,
Thus, for section "A", the required duct diameter is 7 inches.
Now, it is necessary to determine the allowable pressure drops for all other
segments in the branch. The equation for duct diameter can be rearranged to
give the pressure drop for any flowrate, providing the duct length, the flowrate,
and the duct diameter are known. The equation looks like this,
For section "B", if the flowrate is 120 CFM, the length is 11 ft., and the duct
diameter 7-in., the pressure drop will be,
( )
=
L
ft
.
65
90
elbow
=
L
65
*
90
elbow
=
+
Le
71
ft
.
2
⎡
. 0
00123174
*
136
=
D
⎢
A
0
1 .
⎣
. 0
00123174
(
)
∆
=
P
in
.
W
.
G
.
. 0
00123174
*
11
∆
=
P
B
. 4
86
7
( )
D
in
.
*
.
12
6
=
32
5 .
ft
.
12
=
*
32
5 .
ft
.
136
ft
. .
. 0
2058
⎤
. 1
82
*
120
=
. 6
⎥
⎦
( ) (
*
Le
ft
.
*
V
CFM
( )
. 4
86
D
in
.
. 1
82
*
120
=
. 0
00644
in
68
≅
67
in
.
7
in
.
)
. 1
82
.
.
W
.
G
. .
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