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Calculating Duct Resistance
Adding duct work, elbows, branches and any
other components to a duct line increases airflow
resistance (static pressure loss). This resistance
can be minimized by using rigid (smooth) pipe and
gradual curves, as opposed to flexible pipe and
90˚ elbows.
To help you think about this resistance, imagine
riding a bicycle in a tunnel that is an exact replica
of your duct work. If the inside of the tunnel is very
bumpy (flexible pipe) and has many sharp turns
(90˚ elbows), it will take a lot more effort for you to
travel from one end to the other than if your path
is smooth and straight.
The purpose of calculating this resistance is to
determine if it is low enough from the machine to
the dust collector to meet the given requirement
for the machine. Use the charts in Figure 32 to
calculate the resistance of duct work.
Duct
Approximate
Dia.
Static Pressure
Loss Per Foot of
Rigid Pipe
Main
Branch
Lines
Lines
at 3500
at 4000
FPM
FPM
2"
0.091
0.122
2.5"
0.08
0.107
3"
0.071
0.094
4"
0.057
0.075
5"
0.046
0.059
6"
0.037
0.047
7"
0.029
0.036
8"
0.023
0.027
9"
0.017
0.019
Fitting
90˚
45˚
Dia.
Elbow
Elbow
3"
0.47
0.235
4"
0.45
0.225
5"
0.531
0.266
6"
0.564
0.282
7"
0.468
0.234
8"
0.405
0.203
Figure 31. Static pressure loss charts.
-32-
Approximate
Static Pressure
Loss Per Foot
of Flex Pipe
Main
Branch
Lines
Lines
at 3500
at 4000
FPM
FPM
0.35
0.453
0.306
0.397
0.271
0.352
0.215
0.28
0.172
0.225
0.136
0.18
0.106
0.141
0.08
0.108
0.057
0.079
45˚
90˚
Wye(Y)
Wye(Y)
0.282
0.188
0.375
0.225
0.354
0.236
0.329
0.235
0.324
0.216
0.297
0.189
In most small/medium shops it is only necessary
to calculate FPM for the line (from the dust collec-
tor to the machine) with the longest duct length or
the most fittings (operating under the assumption
that if the line with the highest resistance works,
the others will be fine).
The line with the longest duct length and most fit-
tings in our system sketch (see Figure 27) is the
one connected to the router table, which draws
220 CFM.
To calculate the static pressure loss of any
given line in the system, follow these steps:
1.
Make a list of each size duct in the line,
including the length, and multiply those num-
bers by the static pressure loss value given in
Figure 31.
2.
List each type of elbow or branch and multiply
the quantity (if more than one) by the static
pressure loss given in Figure 31.
3.
Add the additional factors from Figure 32 to
your list.
Additional Factors
Seasoned (well used)
Dust Collection Filter
Entry Loss at Large
Machine Hood
Figure 32. Additional factors affecting static
pressure loss.
Model G0671/G0672/G0673 (Mfd. Since 6/17)
Static Pressure Loss
1
2
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