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This setup is an impossibility as you will see in a moment. In order for the system to move 800 CFM through the 4" pipe, elbows etc the air would have to move twice as fast. The simple formula that shows this is:
CFM = Speed x Cross Sectional Area
We want the same CFM, we have half as much cross sectional area in a 4" pipe as a 6" so the speed has to be twice as high. So just make the air go twice as fast right? You can't. Well, you could but not with the 2hp fan we're using in this system. If you had a 10Hp high pressure blower then maybe...The reason is that as airspeed in a duct increases, friction losses get exponentially higher. The following are pressure losses per 100' of 4" diameter pipe at various CFM. Notice how the pressure loss increases sharply as CFM goes up.
|CFM (airflow)||Pressure Loss|
We'll look at fan curves again but realize that most dust collection fans only operate at a maximum of 8-12" total pressure. You can see that trying to move 800CFM very far through a 4" pipe will eat up all of your available pressure...and that's before you hook up any hose, elbows, filters, etc!
What really will happen with the 4" pipe setup is that the airspeed will try to increase in the pipe but very quickly friction losses will eat up all of the pressure the fan can supply and the air will only move a little faster if any than it did in the 6" pipe. Since the air is moving at roughly the same speed through a pipe that has half the cross section....you get half the total CFM.
This is the pressure loss chart for the 4" system at 400CFM.
Notice how each section of the system uses up more pressure than before and that our total required pressure to move the 400CFM is more than when we were working with 6". This system will need the fan to be able to move 400CFM @ 10.4" Static Pressure.
Let's look at the fan curve again
This situation will also work since 400CFM @ 10.4" is a valid point on the curve, but look what has happened. We've used the same exact system and by simply running the correct diameter pipe we were able to get twice the amount of CFM to our tool. Had one actually set the system up with 4" pipe they would have very poor pickup at the planer and they would be wasting most of the systems energy to friction loss.
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