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 36 to calculate the resistance of duct work.
Duct | Approximate | Approximate | ||
Dia. | Static Pressure | Static Pressure | ||
| Loss Per Foot of | Loss Per Foot | ||
| Rigid Pipe | of Flex Pipe | ||
| Main | Branch | Main | Branch |
| Lines | Lines | Lines | Lines |
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 34) 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 36.
2.List each type of elbow or branch and multiply the quantity (if more than one) by the static pressure loss given in Figure 36.
3.Add the additional factors from Figure 37 to your list.
Additional Factors Static Pressure Loss
Seasoned (well used)
at 3500 | at 4000 | at 3500 | at 4000 |
FPM | FPM | FPM | FPM |
Dust Collection Filter
Entry Loss at Large
1
2" | 0.091 | 0.122 | 0.35 | 0.453 |
2.5" | 0.08 | 0.107 | 0.306 | 0.397 |
Machine Hood
2
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 |
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Fitting | 90˚ | 45˚ | 45˚ | 90˚ |
Dia. | Elbow | Elbow | Wye(Y) | Wye(Y) |
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3" | 0.47 | 0.235 | 0.282 | 0.188 |
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4" | 0.45 | 0.225 | 0.375 | 0.225 |
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5" | 0.531 | 0.266 | 0.354 | 0.236 |
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6" | 0.564 | 0.282 | 0.329 | 0.235 |
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7" | 0.468 | 0.234 | 0.324 | 0.216 |
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8" | 0.405 | 0.203 | 0.297 | 0.189 |
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