Good Info on Dust Collection

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Forum topic by Camper posted 10-25-2010 01:17 AM 1711 views 2 times favorited 2 replies Add to Favorites Watch
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232 posts in 2857 days

10-25-2010 01:17 AM

Topic tags/keywords: dust collection

As I was browsing the web trying to learn more about a DC set up, I came across a post in another forum which I thought was pretty factual and useful info so I decided to pass it along.

”....The fact is that almost any dust collector can be connected to a hard-pipe run, provided you understand the engineering science behind dust collection. Yes, you can take a machine that moves a buh-zillion cubic feet per minute (CFM), hook it to a fat pipe and pick up sawdust. In a small shop, however, you may not have room for 6” pipes and humongous air-moving machines. You may also want to be able to hear yourself think—there is nothing so stressfull as spending a few hours in a small room, soaked to the gills in the decibels of a high-volume dust sucker. To use something smaller and quieter for dust removal, you need to do a little math.

At the heart of dust collection is this simple fact: To suspend sawdust and wood chips in an air stream and take them somewhere, that stream must be moving at at least 3500 feet per minute (FPM). And as Dan The Man Bernoulli found just a few short centuries ago, the speed of your air stream is linked to the diameter of your pipe. The smaller the pipe, the faster the air moves. So you can move saw dust with a small dust sucker, you just have to use a small pipe.

How small do you make the pipe? Well, here’s where we lose the math-o-phobics and continue with a smaller, heartier crew. The equation to figure the largest useful pipe diameter for a given dust collector is this: (CFM/FPM) x 144 = Area of the pipe (cross-section, in square inches). The Shopsmith Dust Sucker moves air at 330 CFM. We want the resulting air stream to move at least 3500 FPM. So, (330/3500) x 144 = 13.58. The cross section of a 4” pipe is about 12.5” (pi x 2 squared = 12.56). So the largest useful pipe the Shopsmith machine can be hooked to and still pick up sawdust is 4”. Or is it?

Something else enters the picture. Dynamic pressure (or total pressure), the kinetic energy with which the dust collector moves the air, decreases over the length of the run due to friction and turbulence. It also decreases with bends of pipes and ridges of flexible hoses. Remember, the largest possible cross section for the Shopsmith is 13.58 square inches. The cross section of a 4”-diameter pipe is 12.56 square inches. The two measurements are way too close. The pressure will drop too much over the length of the run, the FPM will suffer, and the sawdust will drop out of the air stream. This is where I have to be honest and say that you are all fortunate that the 4” hook-up for the Shopsmith Dust Collector is no longer available. It was useless for anything but the shortest of runs.

However, you can make a very serviceable run for a small shop using 3” PVC pipe. This pipe has a cross section of about 7” and is more than large enough to move the type of waste you will create when woodworking. With the Shopsmith machine attached , it will sustain a robust air stream at or above the magic FPM for a good twenty to thirty feet, depending on the number of bends. We’ve built two such runs in the National Woodworking Academy, each run servicing up to five woodworking tools. And they work very well.”

Any thoughts?

-- Tampa-FL

2 replies so far

View crank49's profile


4030 posts in 2972 days

#1 posted 10-25-2010 03:09 AM

Yes and no. This is only half correct.
Making the pipe smaller will only increase the velocity if there is additional available pressure.
Increasing the pressure requires more horsepower if the CFM remains the same.
In a horsepower starved system, reducing the pipe size will only reduce the flow.

Most of the ratings advertized on small shop collectors are worthless.
They tend to give maximum CFM and/or maximum static pressure.
These numbers mean nothing unless related to each other.
You have to know CFM volume at a specific pressure to design a system.

View Camper's profile


232 posts in 2857 days

#2 posted 10-25-2010 02:54 PM

Crank, thanks for your comment. I thought that the article did address somewhat your comment. If I understand correctly, the author indicates that by calculating the pipe diameter based on the advertised CFM is erroneous due to the drop in pressure caused by piping/bends etc which will cause a decrease in CFM. Therefore the design must consider this drop in determining the pipe diameter and hence the recommendation of 3” instead of 4” where the 4” was calculated based on the max CFM provided by the manufacturer. Elsewhere, I have seen the impact of piping length and bends on CFM so combined with this info, one can approximate the resultant CFM and calculate the functional pipe diameter. Let me know if I am missing anything.

-- Tampa-FL

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