Dust collection ducting: why a bigger pipe can collect less
The counterintuitive fact about duct runs is that diameter is not a free upgrade. Go up a size without the airflow to fill it and the air slows below the speed that carries chips — and the pipe you installed to improve collection becomes the place the material settles.
Most ducting advice is a shopping list. The part that decides whether a layout works is one relationship, and it is arithmetic you can do before buying anything.
Transport velocity is the whole constraint
Wood chips do not flow like air. They stay airborne in a duct only while the air is moving fast enough to keep them suspended. Below that speed they fall out, collect at the bottom of horizontal runs, and gradually choke the pipe.
The commonly used planning figures are roughly 3,500 FPM (feet per minute) in a main and 4,000 FPM in branches, where the heavier material enters. Treat these as planning numbers; the exact value depends on the material, and fine dust alone needs less than the chip stream from a planer.
Velocity and volume are tied by area, which is where diameter stops being free:
4 in. duct — area 0.087 sq ft — ~350 CFM at 4,000 FPM
5 in. duct — area 0.136 sq ft — ~545 CFM at 4,000 FPM
6 in. duct — area 0.196 sq ft — ~690 CFM at 3,500 FPM
area = π × (diameter ÷ 2)², in feet · CFM = area × velocity
Read that as a requirement, not a capacity. A 6 in. main needs about 690 CFM arriving at it to keep material moving. Put a 6 in. main on a collector delivering 400 CFM through a long run and the air in that pipe is crawling at roughly 2,000 FPM. The chips drop.
This is why a small collector is often better served by 4 in. duct than by the 6 in. everyone recommends. The recommendation is correct for a machine that can fill 6 in. It is wrong for one that cannot.
Adding up what the run costs
Every fitting and every foot of pipe takes pressure away from the fan, and small collectors do not have much to give. The losses are usually counted as equivalent feet of straight pipe, which lets you total a layout on paper.
- Straight rigid pipe is the cheap part. It is the baseline everything else is measured against.
- Corrugated flex hose costs several times what the same length of rigid pipe costs, because the ribs tumble the airflow. This is the single most common avoidable loss in a home shop — people run ten feet of flex because it was easier than cutting and fitting rigid.
- Tight elbows are expensive turns. Two 45° elbows behave much better than one tight 90°, and let you fine-tune the angle. The radius ratios are in the reference note on 4-inch elbow fittings.
- Wyes rather than tees. A branch entering at a shallow angle merges with the flow. A branch entering at 90° collides with it.
The rule that follows: flex hose only for the final connection where movement is genuinely required, and keep that stretch as short as the machine allows.
One gate open
Blast gates exist so that the collector's entire airflow goes to one machine rather than being divided among all of them. A run with four open gates has the air split four ways and nothing reaching transport velocity anywhere.
Which gives a design rule that simplifies the arithmetic considerably: size the system for one machine running at a time. You are not building for simultaneous use, and designing as though you were produces a main too large to keep velocity.
If you forget gates, you find out the same way everyone does — collection gets gradually worse over months as the horizontal runs silt up, and performance returns the day you take the pipe apart.
Laying it out
Three things in order, before you buy pipe:
- Find the collector's real airflow at the end of your run, not its badge rating. The badge is measured at the inlet with nothing attached. What matters is what survives twenty feet of pipe, two elbows and a machine port.
- Pick the diameter that your actual CFM can keep at velocity, using the table above. This frequently means smaller than the internet advises.
- Put the collector so the longest run is short. Moving the machine is usually cheaper than paying for the pipe to reach it, and the layout that minimises total duct also minimises the losses you spent step two calculating.
Keep horizontal runs to a slight fall back toward the collector if the building allows. It will not save a system that is under velocity, but it keeps a marginal one from accumulating.
What ducting does not solve
All of the above is about material the collector can capture. The fraction that escapes the hood and stays airborne is a different problem with a different machine, and a perfect duct run does nothing about it — see do you actually need air filtration in your shop?
And the standard trap: a collector with a coarse filter bag returns the fine fraction straight to the room. Spending on ducting while running a 30-micron bag improves the part you can see and leaves the part that matters untouched.