How to Select the Correct Duct Diameter

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How to Select the Correct Duct Diameter
October 7, 2011 No comments

Many dust collection systems suffer due to poor ductwork designs. There is a large amount of misinformation about what the proper size pipe is to run. In reality, proper pipe sizing can get complicated and depends on many variables; tool size, air requirements, length of pipe run required, number of machines running at one time etc.

A very common and very disastrous problem in duct design is to simply run all 4" diameter pipe. People run 4" pipe for several reasons... usually not the right ones! Most "import" tools and chip collectors are outfitted with 4" diameter ports, 4" pipe is common, 4" pipe is inexpensive, 4" pipe is the most common size of PVC pipe. None of these reasons means that 4" pipe is the "right" size to run for optimal performance.

Tools are sold with 4" ports because the chip collectors have 4" ports. That doesn't mean that a tool wouldn't be much better off with a 5" or 6" port.

Lets look at an example of a small shop that has a large tool in it. In this case a 20" planer. Now I realize that not everyone has a 20" planer but more and more small shops are able to afford "large" tools. Planers, Drum sanders, more powerful table saws all require a lot more airflow than they did a few years ago. This simplified layout only shows the duct run from the system to the planer. We will assume this is the run in question or the worst case for this shop.

Dust collection ductwork diagram for 20 in. Planer
A 20" planer needs to have around 800CFM to clean it properly.

The question stands, what size pipe should I run? Lets assume this is a single operator shop so the planer will not run with any other tool. All we want to do is deliver 800 CFM to the planer, that's the worst case our system will have to support. Lets also assume that the system will be run with a 2HP fan.

Here are two examples. A right and a wrong way to design this ductwork. The first (right) way uses 6" diameter pipe run all the way to the planer from the cyclone. This design will work and be able to deliver the desired 800 CFM. The second (wrong) way uses all 4" pipe run from the cyclone to the planer. This design will not get the desired 800CFM to the planer. In fact, it will deliver about half that amount.


The RIGHT way to do it…

6 inch ducting to 20 inch Planer
This layout shows the 6" pipe run. We've also letter-coded each piece or section of the system (see table below).
A Hood Entry From Planer #DOR050000
B 5' of 6" Hose #DHF060500
C 6" 90 Deg. Large Radius Elbow #DEA900600
D 20' of 6" Straight Pipe #DPT260660
E 2 x 6" 90 Deg. Large Radius Elbows #DEA900600
F 2' x 6" Pipe #DPT260624
G Cyclone Dust Collector #XGK020105H
H Plenum and Filters #FCS133695HF

The graph below shows how pressure is lost through each section of the system assuming we are moving at 800CFM. Each component of the system adds resistance whether its the corrugations of the flexible hose or the fabric of the filter media...it all creates friction losses for the moving air.

Pressure loss in 6 inch ducting layout

Note that the fan adds pressure. Points on the suction side of the fan experience negative pressure (vacuum). Points past the fan in the system experience positive pressure since the air is being pushed from there forward.

The total loss from all the components in the system is calculated to be 8.4". This means that if we are to actually move 800CFM through this system we will have to have a fan that can move 800CFM @ 8.4" Static pressure.

Lets check our 2HP's Fan Curve and see if it will work. We determined we need 800CFM @ 8.4" pressure. Look at the fan curve below. This is a valid point on the curve so we know the setup will work as expected.

2HP Fan Performance Curve

The WRONG way to do it…

4 inch ducting to 20 inch Planer

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 × Cross Sectional Area

We want the same CFM but we only have half as much cross sectional area in a 4" pipe as a 6" pipe, so the speed has to be twice as high. So just make the air go twice as fast right? Well, you can't - Not with any typical 2HP fan blower. The reason is that as airspeed in a duct increases, friction losses increases exponentially. The following chart shows pressure losses per 100' of 4" diameter pipe at various CFM ratings. Notice how the pressure loss increases sharply as CFM goes up.

CFM (airflow) Pressure Loss
100 CFM 0.65"
200 CFM 2.4"
300 CFM 5.1"
400 CFM 9.0"
800 CFM 26"

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 compared to 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.

Pressure loss in 4 inch ducting layout
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" pipe. This system will need the fan to be able to move 400CFM @ 10.4" Static Pressure.

Let's look at the fan curve again…

2HP Fan Performance Curve
This situation will also work since 400CFM @ 10.4" is a valid point on the curve, but look what happened! It delivers 50% of the CFM needed with 25% greater air resistance placed on the system.

By using the same exact dust collector and simply running the correct diameter piping 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.