Thread: Cyclones - pros & cons

When referring to the efficiency of a cyclone there are two things that are being referred to. One is how efficiently it separates out the heavier fractions of particles, which is its main function. The second measure of efficiency is how much loss in pressure the cyclone ‘consumes’ to achieve that degree of separation. The higher the pressure loss from the cyclone the higher the overall pressure required for the DE system to do its job. This in turn translates into higher power requirements.

A cyclone that might not be a necessary part of a DE system to do the job will cost more than required to run over time.

A few comments.

Originally Posted by NeilS

.Emptying the bin below a cyclone is a lot less messy without the fine dust fraction.

Bins below cyclones still contain very significant amounts of fine dust.
Cyclones are not black and white separators so loads of fine dust still ends up in the bin(S). A Clearvue cyclone will extract 99.9% of 5µm dust. Some of this is because much fine dust is entrapped on the larger volumes of coarse dust particles. Also see below.

Even cyclones that are highly efficient at dust separation do not remove the particles below 5µm, which is the size that is most damaging to our health. If exhaust air is to be vented back into workshops then the costly filters that can remove sub-5µm particles are still required.

Again cyclones are not either/or. They will remove some very fine dust even down to <1µm. If a cyclone like a Cleavue can remove 99.9% of 5µm dust it can probably still remove 99% @ 2.5µm and possibly 90% @ 1µm, etc The bin does endup holding most of the fine dust.

Cyclones increase the pressure drop (loss) in a DE system. A cyclone of Penz design adds approx 2” to 3” wg drop to the system. This can be a significant loss in an underpowered/specced DE setup. Note: That 2” to 3” wg pressure loss across the cyclone is not a fixed figure as pressure drop increases with volume throughput (square of cfm increase, whereas pressure drop through filters is more linear), but is near enough for this exercise.

Only well designed cyclones like a Clearvue drops ~2-3"WC. Most cyclones will drop more than this and Bill Pentz has measured drops of up to 4.5" - given that most Hobby level DCs only generate 7.5 - 8.5" WC to start with this represents a major loss of flow.

Flow loss is a really important issue because it means less fine dust is being collected at the source of the dust. Remember while a cyclone might be 99.9% efficient @5µm if the cyclone has only picked up 50% of the dust at source then that leaves 50% floating around in your shed. Cyclones do become more effective on larger systems because they have much larger flows to start with so can afford to lose some loss in flow without impacting on overall fine dust collection.

A filters-only DE will need its expensive filters replaced much sooner than filters after a cyclone, which will eventually make that a more expensive option. The Return on Investment for a cyclone-only DE setup will probably require more intense use over a longer time period of time to justify that option over a filters-only DE.

The issue with most "filters only DCs" is they leak - even when they are brand new. Poor seals (especially the collection bags but also the filters) and damaged filters are usually to blame.
Just about every badly leaking filter I have seen have been the old calico type bags which have pin size holes cause by abrasions or punctures from large chips coming up from the collection bags because the collection bags were let get too full. The next leakiest filters were the thin type needle felt filters like the ones used on teh Hare and Forbes DCs. The thicker type needfelt bags (like those on the Carbatech machines) are much harder to abrade and puncture. Pleated filters are also more liable to be punctured. However, if DCs are vented or located outside a shed then this doesn't really matter - and of they leak a little more later in their life then the same applies. Only when a constant visible stream of dust starts emanating from the filters do they probably need replacing

Any further pros and cons, ifs and buts?

What seems to be largely ignored in DIY dust generation is the value of forced ventilation. Its cheap, relatively easy to install, and goes a long way to taking care of the fine dust generated by activities which regular (especially budget level) dust extraction finds difficult to deal with, eg hand and power tools and dust that escapes from machinery. For small weekend warrior sheds, something to collect chips (even small cyclone) and some forced ventilation will usually be sufficient.

Sheds are dust traps, design it to be ventilated properly and dust extraction needs are hugely reduced as it will self ventilate. Unfortunately in the era of tin sheds that won't happen. Neil, I wish you well for a prolonged discussion here but as there are so few people actively involved in DE as a day to day thing it becomes difficult to support an on gong discussion of this nature. There are commercial operators out there who give bad advice on a daily basis and charge big money for doing it and they would never come here to discuss the problems of DE. I know of at least two projects that I don't want to have any dealings with due to basic bad design.

Neil, thank you for the great writeup. I do feel like dust collection is "too hard" and I'm hopeful that someday there will be easier answers.

One additional thing I considered is that if the dust collector is in an enclosure that vents outside, then with a cyclone the air exits the enclosure immediately, but a single-stage collector will exhaust into the enclosure (and then out). Venting into the enclosure may be good because it provides fresh air to cool the motor. Or it might be bad because it exposes the motor to more fine dust.

I have not setup my cyclone yet, but I did a lot of research into venting outside. My impression is that in cold climates, venting outside is often not that bad because as soon as you turn off the dust collector and close the window, the radiant heat from all the tools in the shop heats the air quickly. However, in hot and humid climates, it takes much long for the humidity level to return to a reasonable level, which really impacts comfort. I decided to vent outside, but can change my mind later if it doesn't work.

Mark

Originally Posted by mwells

Neil, thank you for the great writeup. I do feel like dust collection is "too hard" and I'm hopeful that someday there will be easier answers.

That's I recommend forced ventilation for many DIY operators.
For small sheds with smaller machines, and occasional DIY use cyclone is simple uncessary.

One additional thing I considered is that if the dust collector is in an enclosure that vents outside, then with a cyclone the air exits the enclosure immediately, but a single-stage collector will exhaust into the enclosure (and then out). Venting into the enclosure may be good because it provides fresh air to cool the motor. Or it might be bad because it exposes the motor to more fine dust.

Most DC motors are Enclosed Fan Cooled (TEFC) so some fine dust doesn't worry them. The fan usually blows the dust off the motor. Abrasive dust could could cause long term problems for bearings but wood dust is not that abrasive.

I have not setup my cyclone yet, but I did a lot of research into venting outside. My impression is that in cold climates, venting outside is often not that bad because as soon as you turn off the dust collector and close the window, the radiant heat from all the tools in the shop heats the air quickly.

It's not just the machinery but the heat in the shed walls and materials inside the shed also contributes to reheating the shed IR

However, in hot and humid climates, it takes much long for the humidity level to return to a reasonable level, which really impacts comfort. I decided to vent outside, but can change my mind later if it doesn't work.

Humidity is strongly dependent on temperature but takes time to happen so unless you can inject additional coolth into the shed air (ie via an AC) to counteract the machine heat it will take longer to draw the remaining coolth from the shed walls. floor and other materials to reduce the humidity.

I have a venting DC and 3 temperature/Humidity gauges in my shed and one outside so I can assess changes in shed temperature/humidity whenever dust extraction is used.
At an external temperature of 35-40º if I turn on my (small) AC set at 23º it takes about 60-80 minutes to get the temp down to a steady 26ºC as read by the sensor in teh middle and about 2.1m above the shed floor. Their temp is ~2ºC lower at my head height and cooler still at floor level, but I will stick to the temp at 2.1m height because this is where the permanently turn on temp sensor is located.
Humidity tracks with temp.

Using the DC does not produce anywhere near even air temperature changes inside the shed.

Once the shed reaches 26ºC, if I turn on the DC there's no detectable change in air temperature in the bulk of the shed for several (as long as 5 minutes) minutes. There is a very distinct change in the temperature of the air path from the inlet vents (under shed door and a couple of other places to the machine port where the dust is being extracted. it forms a sort of ever increasing sized air bubble from the vents to the dust ports with temperature can reach close to outside air temperature. The bubble size and location changes depending on what machines are in use. This makes any feral assessment very difficult but I will just give you my observations for the table saw which is about 2m from the temp sensor

After that it takes about 3 minutes to raise the bulk shed air temp about 1/2ºC If I then turn the DC off ,the temp recovers back to 26 degrees in about 5 minutes, obviously also drawing coolth from the shed walls, floor and materials and the AC. Humidity also recovers but it's a bit slower, it has to go somewhere - I suspect it is absorbed onto cooler surfaces. If I repeat this many times on a hot/humid day I can sometimes see dew on cool metal surfaces.

If I run the DC for longer the temp rises about 1/2ºC/min until the bulk of the shed air temp reaches external air temp and may even go higher than outside temp after that due to machine heat. Again humidity tracks temp. After that, if I turn off the DC, again it takes about an hour to recover the original 26º.

So I can still use the DC with no major changes in shed temp for about 6-10 minutes but then I have to give the DC a rest for a few minutes before using it again. This is not usually a problem. If I have to use the DC for extended periods I do that early in the mornings when it cooler, or just before lunch and then leave the AC on over lunch to recover. Thankfully in daylight hours the Solar PV system takes care of the power costs of this sort of activity.

Originally Posted by mwells

Neil, thank you for the great writeup. I do feel like dust collection is "too hard" and I'm hopeful that someday there will be easier answers.

One additional thing I considered is that if the dust collector is in an enclosure that vents outside, then with a cyclone the air exits the enclosure immediately, but a single-stage collector will exhaust into the enclosure (and then out). Venting into the enclosure may be good because it provides fresh air to cool the motor. Or it might be bad because it exposes the motor to more fine dust.

Mark... modern motors are enclosed and impervious to dust. Switches and any other motor control gear are more susceptible.

If you are venting your enclosure outside you are unlikely to have issues with air cooling the motor, but that may depend on the ambient temperature... I know your neighbouring state, Arizona, can get bit toasty outside in the high summer.

Originally Posted by mwells

... I did a lot of research into venting outside. My impression is that in cold climates, venting outside is often not that bad because as soon as you turn off the dust collector and close the window, the radiant heat from all the tools in the shop heats the air quickly. However, in hot and humid climates, it takes much long for the humidity level to return to a reasonable level, which really impacts comfort.

Interesting observation, Mark.

Cyclones are noisy.
If vented outside in a city environment they will get the neighbours upset and the council on your back.
Mufflers are another very complex design area that won’t necessarily save your ass.
Don’t ask me how I know.
H.

Here is some temp data for my shed showing the variation even when the AC is running

Blue line is at 2pm outside temp is 36.5ºC
Zero height above floor is the temp of the concrete floor itself.

Orange line is after half an hour of AC. It would get even colder than this if I was to let the AC run for longer without using any machinery.
That location is about 3m away from and in front of the AC which has oscillating vents so it does spread the coolth around a bit.
There are some things hanging down from the ceiling that partially block the AC.

Grey line is also after about 35 minutes of AC and about 6m away from AC and over to one side, sort of in the warmer part of the shed.
I should set up a small fan to provide some circulation which would even the air temps out a bit.

Using the DC the shed air temperatures in this sort of situation would vary from the orange line to the blue line depending on how long the DC is run.





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Originally Posted by clear out

Cyclones are noisy.
If vented outside in a city environment they will get the neighbours upset and the council on your back.
Mufflers are another very complex design area that won’t necessarily save your ass.
Don’t ask me how I know.
H.

Thanks, Clear Out.

Yes, I should have added that 'Con' for venting cyclones impellers directly outside. Whereas, filter bags and pleated filters do provide some noise suppression if they are then vented outside.

The noise level from a cyclone impeller that is vented directly outside can be considerable and usually requires some form of muffler (there are threads on those in this forum).

The other factor with cyclones is that they are typically running with larger diameter impellers and higher HP motors, which generate more noise (sound pressure). The cyclone body component itself isn't particularly noisy.