Karl,
You want to use velocities and coinciding CFM for particulate being collected. If you move the air faster than necessary, will require more horse power to overcome the additional static pressure (friction/resistance).
If you use a dedicated dust collector for sander, I would target 1,100 CFM for each 7” port. Since 2,200 CFM in a 11” diameter pipe has a velocity of 3,400 FPM, I will select 10” diameter as we don't want to drop below 3,500 FPM. 2,200 in 10” diameter is moving at 4,000 FPM.
Once CFM is arrived at, now we have to calculate the resistance of piping etc. It is the collectors job to overcome the resistance/static pressure and deliver CFM needed.
Here is an example. Lets say the dust collector is 40' away from sander and will have 3, 90 degree elbows in the 10” diameter run. Also, 5' drop pipe with a wye to 2, 7” diameter and 3' of 7” flex off each leg of y-branch. Now we will calculate resistance. 2,200 CFM in 10” diameter has a static pressure of 2.4” WG per 100' pipe. 1,100 CFM in 7” diameter has a static pressure of 3.8” WG per 100'.
Static pressure/resistance factors
Entry loss – 1”
Seasoned filters – 2”
Note – Have to convert all elbows to straight feet of pipe.
7” diameter – 6' flex = 18' straight pipe.
2, 7” 45 degree elbows = 13' straight pipe.
Total 31' 7” diameter straight pipe.
1, 100 CFM in 7” diameter has a static pressure of 3.8” WG per 100' or 1.18” WG in 31'.
10” diameter – 3, 90 degree elbows = 20' each + 45' straight pipe. Total 105' 10” diameter straight pipe.
2,200 CFM in 10” diameter has a static pressure of 2.4” WG per 100' or 2.52” in 105'.
Collector requirement is 2,200 CFM at 6.7” SPWG.
Now we look for a dust collector. Please see attached performance chart for a 7 ½ HP (Model 750) dust collector. We can see that we are right in the middle of 2,000 CFM at 7.1” SPWG and 2,500 CFM at 6.6” SPWG. Bingo, that's our unit. In addition, 1 micron filter material is good for a sander.