Determining Main Line Duct Size
The general rule of thumb for a main line duct is that the velocity of the airflow must not fall below 3500 FPM.
For small/medium sized shops, using the inlet size of the dust collector as the main line duct size will usually keep the air velocity above 3500 FPM and, depending on your system, will allow you to keep multiple branches open at one time.
Mark your drawing as in Figure 38, but using the inlet size for your dust collector as the main line.
) | ) | ) |
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Figure 38. Main line size labeled on sketch.
Determining Branch Line Duct Size
The general rule of thumb for a branch line duct is that the velocity of the airflow must not fall below 4000 FPM.
For small/medium sized shops, using the dust port size from the machine as the branch line duct size will achieve the correct velocity in most appli- cations. However, if the dust port on the machine is smaller than 4", make the branch line 4" and neck the line down right before the dust port.
Note: Systems with powerful dust collectors work better if multiple blast gates are left open. This also allows you to run two machines at once. Experiment with different combinations of blast gates open/closed to find the best results for your system.
Write your determined branch line sizes on your drawing, as shown in Figure 39.
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| 5'' | 6'' |
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| 4'' | 4'' |
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Figure 39. Branch line sizes labeled on sketch.
Here are some frequently asked questions when determining branch line sizes:
How do I figure which size of branch line to use if the machine has two dust ports?
Simply add the total CFM given for each size from Figure 35 and refer that CFM number to Figure 40. Then, split the branch line just before the dust ports with matching duct sizes.
What if two machines share the same branch line? You have two options:
1.If both machines will be running at the same time, add the total CFM given for each size from Figure 35 and match the branch line given in
Figure 40.
2.If both the machines will never be run at the same time, reference the machine with biggest dust port to Figure 40 and add blast gates after the
Total CFM | Branch Line Size |
400 | 4" |
500 | 4" |
600 | 5" |
700 | 5" |
800 | 6" |
900 | 6" |
1000 | 6" |