Flowering Elbow

Cyclonic Dust Separation – Overview

How does a cyclone work?

Essentially the function of a cyclone separator is to remove the majority of the dust and debris that you suck up, and separate it out from the air stream that carries it. It does this by introducing the dust filled air to the cyclone chamber via an inlet mounted tangentially to the cylinder at the top. This causes the air and dust to circulate around the outside of the container, and as such centrifugal forces keeps the particulate mater to the outside edges. The ‘air ramp’ forces the air stream and dust particles down into the conical shaped container where the cone shape creates a cyclone with higher centrifugal air speed.

If the cyclone works well, the higher air speeds in the cone will throw the dust particles, both large and small, off to the side. Finally, near the bottom of the cone the air reaches a ‘turn-around point’. This is the crucial point when the clean air changes direction, hopefully leaving behind the particulates that it was carrying, and goes up the centre of the upper cylinder and off to the vacuum cleaner. The dust, by virtue of its inertia (and to some extent gravity) continues on down, falling into a large settling chamber where the air is almost completely still. Thus, in a helter-skelter stylee, dust goes down the chute, while the clean air moves away up the central inlet to the vacuum.

In short, the cyclone prevents your filters from clogging with dust particles, preventing the loss of suction that most casual DIYers experience when they hook up their houshold vacuum, or even their dedicated shop vac to a woodworking tool.  For a quick look at how cyclones work, play this video of my first prototype, having its test run.

What cyclones are not:

So called ‘cyclonic dustbin lid’ separators, like this, work ok for what they are, but would actually be more accurately marketed as drop boxes.  As with many upright vacuum cleaners that call themselves things like ‘cyclone power daemon’ they suffer the same few major problems: the ‘cyclone’ is not actually a cyclonic separator at all, just a drop box that happens to swirl a bit, and/or that the cyclone assembly is not separate from the collection point, so while air swirls, it is not necessarily a good thing because it disturbs any collected dust, which is thus likely to be sucked up into the extractor where it clogs the filter or, if small enough is thrown back out into the atmosphere – cough cough!

This double cyclone build:

A real cyclonic separator, does just that, it separates off the dust into a separate holding chamber, where the air is relatively still and the dust can settle.  In these pages we consider the cyclone in a number of parts:

• The upper body – the part that is simply cylindrical in shape and provides a mounting for the air inlet and outlet.
• The cone – the section that is cone shaped (surprise surprise). It channels the dust down through the dust chute (on the end of the cone) and into the collection chamber.
• The air inlet and ramp – the inlet brings in the dust filled air, the air ramp reduces turbulence by directing that air downwards into the desired spiral.
• The air outlet – is another cylinder that sits in the centre of the upper body. The clean air is sucked out the air outlet.

I wanted to make the cyclone clear, so we could see what was going on inside. This was important to me, because despite the amount of reading I have done around the subject, cyclonic filtration still seems a bit like black magic to me and I wanted to watch it actually happening in detail. Having it transparent also means I can observe the performance differences when I tweak the design, change the airflow, or vary the waste matter being separated. Plus we can instantly see if the collection chamber is full or if there is a blockage or a similar problem.  Basically having it clear is just a lot cooler!

The cyclone is based on an ‘upper:cone ratio’ of 1.64 (the length of the cone is 1.64 * the diameter of the upper section), which has been shown to work very well. If you are making a cyclone out of sheet steel Bill Pentz’s fantastic excel spreadsheet can scale the design and calculates all the dimensions for you. I experimented with different materials so the actual construction varied substantially, because I wanted to see through to the inner workings. Still, the spreadsheet provides an excellent starting guide if you want to use steel- you can download and checkout the spreadsheet here (it is near the end of the page). If you want to replicate the DS cyclones, you don’t actually need it because for each step I have included all the measurements in SketchUp diagrams (if you want to download the actual SketchUp files they are included in the Resources and References step at the end). Either way, if you are considering any kind of dust related antics I recommend you check out his site, there is a real trove of information there.
The two good features of Pentz’s design, other than the upper:cone ratio, are its tall and thin rectangular section sloping inlet, and air ramp.  Both of which help to direct the air stream in a turbulent free way into that characteristic desirable helter skelter shape.  Pentz says that using such a cyclonic filtration system:

“Even shops that make multiple 55-gallon drums of MDF dust daily find they can go six months and still see almost no fine dust in their filters. The same is also true in terms of scaling this design to much larger. I designed one of these for a huge cement processing facility and they went from having to replace filters monthly to every five to six months, plus were able to use a much smaller horsepower motor.” (Bill Pentz)

For the dust sniper, we need to consider how this will integrate with the rest of the system. I made two cyclones. Why? Because that way we can keep the airflows of the two vacuums we use separate. Separate airflows gives us more flexibility, it means we can store different waste materials in separate containers – useful if you plan to re-use the sawdust or wood chips for anything (like composting or PVA-wood dust casting). It also means we can use one or both vacuums at once depending on the extraction requirements at hand. I have had good results, for example, using both at once to extract dust from a bandsaw (from modified collection points), where as a lot of of my tools are quite happy with one vacuum going (no sense having two going if we can do a good job with one right?).

This is Mk1 in action almost ready to be integrated into the Dust Sniper project.