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Planning Drop Downs
Plan the drop downs for each machine, using
blast gates wherever possible to control airflow.
Rigid Duct
(Main Line)
Y Branch
Clamp
Flexible Duct
To Planer
Figure 49. Drop down setup.
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) duct
and gradual curves, as opposed to flexible duct
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 duct) and has a lot of sharp turns
(90˚ elbows), it will take a lot more effort to travel
from one end to the other.
The purpose of calculating the resistance is to
determine if it is low enough from the machine to
the dust collector to meet the given CFM require-
ment for the machine. Use the following tables to
calculate the resistance of duct work.
Model G0777/G0777HEP (Mfd. Since 04/19)
Elbow
Rigid Duct
Blast Gate
Approximate
Static Pressure
Duct
Loss Per Foot of
Dia.
Rigid Duct
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˚
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 50. Static pressure loss charts.
In most small/medium shops it is only necessary
to calculate the line 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).
To calculate the static pressure 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 value given in the
previous table.
2.
List each type of elbow or branch and multiply
the quantity (if more than one) by the static
pressure loss given in the previous table.
Approximate
Static Pressure
Loss Per Foot
of Flexible Duct
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˚
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
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