South Bend Tools
For Machines Mfd. Since 11/20
Model SB1099
-27-
S Y S T E M D E S I G N
Planning Drop Downs
Calculating Duct Resistance
Figure 46. Drop-down setup.
Figure 47. Static pressure loss charts.
To Planer
Blast Gate
Flexible Duct
Rigid Duct
Rigid Duct
(Main Line)
Elbow
Clamp
Y Branch
Plan the drop downs for each machine, using
blast gates wherever possible to control airflow.
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
requirement for the machine. Use the following
tables to calculate the resistance of duct work.
Duct
Dia.
Approximate
Static Pressure
Loss Per Foot of
Rigid Duct
Approximate
Static Pressure
Loss Per Foot of
Flexible Duct
Main
Lines
at 3500
FPM
Branch
Lines
at 4000
FPM
Main
Lines
at 3500
FPM
Branch
Lines
at 4000
FPM
2"
0.091
0.122
0.35
0.453
2.5"
0.08
0.107
0.306
0.397
3"
0.071
0.094
0.271
0.352
4"
0.057
0.075
0.215
0.28
5"
0.046
0.059
0.172
0.225
6"
0.037
0.047
0.136
0.18
7"
0.029
0.036
0.106
0.141
8"
0.023
0.027
0.08
0.108
9"
0.017
0.019
0.057
0.079
Fitting
Dia.
90˚
Elbow
45˚
Elbow
45˚
Wye(Y)
90˚
Wye(Y)
3"
0.47
0.235
0.282
0.188
4"
0.45
0.225
0.375
0.225
5"
0.531
0.266
0.354
0.236
6"
0.564
0.282
0.329
0.235
7"
0.468
0.234
0.324
0.216
8"
0.405
0.203
0.297
0.189
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
numbers 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.
Содержание SB1099
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