DUST COLLECTION DUCT WORK: WHY SIZE MATTERS

Feb 14

I get a lot of questions about dust collection ductwork—some from people who think the size of the ducts doesn’t matter, and others who believe bigger ducts are always better. Both of these points of view can lead to wasted money and loads of regret.

In this article, I want to offer a simple, easy-to-understand explanation of this small part of a larger subject—specifically why the size of your dust collection hoses and ducts does matter, and why what's right for your shop may not be what you think.

I think this is best broken down into four key factors: volume, velocity, resistance, and pressure. You need to balance these four factors between your ductwork, your tool dust ports, and the capabilities of your vacuum or dust collector. It's crucial that you understand these principles before spending any money on equipment or ductwork. So, let’s dive into these factors one at a time, starting with volume.

Volume

Air may be invisible, but it occupies space. Think about a balloon—it has to stretch to create more space as the volume of air inside it increases. A dust collection hose or pipe can’t expand. It can only hold so much air within its walls. While air can be compressed, it takes a lot more pressure to compress air significantly than a wood dust collector can generate. The only way to increase the volume limitations of your ducts and hoses is to use larger ducts and hoses.

Air volume or airflow is measured in cubic feet per minute (CFM). Generally, a 4-inch duct can accommodate about 4 times the air volume as a 2-inch duct, and a 6-inch duct can accommodate about twice the volume of a 4-inch duct.

But don’t forget, air isn’t the only thing inside your ducts and hoses. Sawdust and wood chips also compete for space with the air.

Before you rush out to buy the biggest PVC pipe and hoses you can find, though, let’s take a look at velocity.

Velocity

While a certain volume of airflow is necessary to gather the dust and chips from your tools, it also takes a certain amount of air velocity to keep those chips suspended and moving through the airstream on their way to the collection bin. If the air velocity isn’t sufficient, dust will drop out of the airstream and pile up in the duct.

You can test this principle by blowing sawdust off your bench through a drinking straw, then trying the same thing by blowing through a cardboard tube. The smaller diameter of the straw restricts the volume of airflow compared to the much larger tube. But the velocity of the air coming out of the straw is much greater, and it moves a lot more dust.

To compensate for the larger tube, you would need to increase the volume of air from your lungs—basically, blow a lot harder to move the same amount of dust.

Here’s the key point: if the volume of air is the same, you’ll get more velocity from a smaller duct. Likewise, if you increase the size of the duct, you’ll need more air volume to maintain that same velocity.

Resistance

If you check the specs on your vacuum or dust collector, you’ll likely see a CFM rating. That indicates the volume it can move when the blower is completely unresisted. But in the real world, we add all sorts of resistance to our dust collection systems. When it comes to just the ducts and hoses, every bend, turn, rib in the hose, and even the walls of the ducts themselves, all add resistance. Resistance slows velocity, and as I mentioned, velocity is what keeps the chips moving.

So, volume is closely related to velocity, velocity is closely related to resistance, and resistance is closely related to our final factor: pressure.

Pressure

It takes more pressure to push an equal amount of air through a smaller space than a larger one. You can test this by drinking water through a regular straw as compared to a little cocktail straw. You have to suck much harder on the cocktail straw, and even then, you won’t drink as much liquid because your lungs can’t generate a lot of negative pressure.

Blower-style dust collectors generate very little static pressure. So, like your lungs, they struggle more when connected to smaller ductwork and hoses. On the other hand, vacuum cleaners generate vastly more static pressure and work better with smaller hoses and ducts.

Clearly, you have to find the right balance of these four factors based on your system.

Real-World Examples

In my main shop, I have a powerful system with a big blower that can move a lot of volume. I used the largest ducts I could to accommodate that volume. Some of my tools, however, have 4-inch ports. I’m obviously not getting my maximum airflow when I reduce from 6 inches to 4 inches. But I have to do it. Still, I use 6-inch main ducts because I have other larger connections in the shop and some tools with dual 4-inch connections. I need that larger main duct to handle the combined air volume from those. In fact, I always leave two 4-inch lines open because I need enough air volume passing through the 6-inch mains to keep the velocity up and the dust moving.

In the corner, I have a less powerful dust collector. It has two 4-inch ports, but they are not meant to be open at the same time. It’s a smaller system that moves significantly less air—so not enough to maintain sufficient velocity through 6-inch ducts. If I were to connect it to ductwork, it would have to be 4-inch, and I could not add many bends or much flex hose that would further resist its modest airflow.

Because both of these blower-style systems generate low static pressure, I avoid reducing them down to smaller hoses and dust ports if at all possible. The only exceptions are for the dual hookups on the router tables, and that's for convenience. Frankly, a shop vacuum would be much better for those small fence ports—and certainly better for small power tools—because its high static pressure can overcome those volume restrictions more effectively than the dust collectors can.

Conclusion

Of course, this just scratches the surface of shop dust collection. There are way more things to consider, and I’ve made more videos on the subject. I’ll link to some of them below.

The bottom line is: big ducts go with big dust collectors, medium ducts go with medium dust collectors, and shop vacuums go with small hoses. And all of them work better with as little added resistance as possible from excessive duct length, bends, or corrugated hoses.

Understanding the balance of volume, velocity, resistance, and pressure is key to setting up an effective dust collection system in your shop.

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