Thread: Busting some cyclone myths

Originally Posted by Chris Parks

No and hasn't been for many years.

Really? Still advertising both left-hand and right-hand models on the Clearvue US website.

Originally Posted by DomAU

Really? Still advertising both left-hand and right-hand models on the Clearvue US website.

Only to a specific one off order as needed, you can conform that with Stephen if you want.

Originally Posted by NeilS

+1

Tim, your pressure/flow graph indicates little or no difference at the higher flow rates, which is reassuring for anyone considering going with the Clearview, as I am. If I have any misgivings about the Clearview cyclone (not the impeller and its controls which are spot on) is that the inlet is on the opposite side of the cyclone to where I would expect it to be, given the direction of fan rotation.

Some stats with a larger impeller on both your left and right hand inlets models would be even more reassuring for both your cyclone and the Clearview options.

Hi Neil,

Testing with a larger fan is definately on the agenda at some point, the problem is finding something suitable at a reasonable price. In my small shed, I can get around 600cfm through my machines with my modified 2HP fan which is good enough for me so this testing would be for academic porposes only.

One thing I have just done is to produce a system curve for the cyclone. I measured the pressure drop between cyclone entry and outlet at various flow rates. These are shown as the individual points on the graph below. Also plotted in blue is a theoretical curve to indicate how the figure quoted for a Bill Pentz cyclone (2.25" at 1000cfm) would look at various flow rates. This is based on theory that for turbulent flows, pressure loss is proportional to the square of air velocity.

I think this shows that my humble offering performs pretty much as predicted by BP and also demonstrates the error involved with taking the qouted 2.25" loss figure as a constant. If you have something like a Clearvue running at 1000cfm through a heap of ducting, you will be giving up that amount to the cyclone, when I'm running with my 2HP fan at 600cfm, the cyclone pressure loss is only about 1".

Regards Tim

Cyclone system curves.JPG

Originally Posted by Muchacho

Testing with a larger fan is definitely on the agenda at some point, the problem is finding something suitable at a reasonable price.

Thanks Tim

Yes, if it is like it is here in Australia, it's a problem, particularly if you only have single phase, which is the case for most hobby woodworkers.

It's the main reason I'm looking at the Clearview fan and control unit (and probably their total setup) for my next implementation where I only have single phase available.

I note that you have a much larger chute diameter at the bottom of your cyclone than the Pentz spec, well at least for his cone ratio of 3. Is there a reason for this?

Originally Posted by NeilS

Thanks Tim

Yes, if it is like it is here in Australia, it's a problem, particularly if you only have single phase, which is the case for most hobby woodworkers.

It's the main reason I'm looking at the Clearview fan and control unit (and probably their total setup) for my next implementation where I only have single phase available.

I note that you have a much larger chute diameter at the bottom of your cyclone than the Pentz spec, well at least for his cone ratio of 3. Is there a reason for this?

Hi Neil,

The chip outlet is around 200 mm diameter. I can't recall how or why I came up with that size or how it compares to the BP design, I think i just settled on that for my prototype because it looked right. When it came to making the MK2 version, having that size sure paid off as it now allows me to ship a 1350mm tall cyclone in an 800mm tall carton.

As far as getting hold of a decent fan, I reckon the best performer at a reasonable price on this side of the ditch is the OAV one below which is a rarity for a 3 HP fan as it has a 14" impellor. Its still a bit pricey at about NZ$1200.

Do you have something similar in Oz? I reckon this would be good for about 800cfm at 6" SP and quite adequate for a small shop without too much ducting or fittings.

Cheers, Tim


OAV dusty.JPGOAV dusty 2.JPG

Originally Posted by Muchacho

The chip outlet is around 200 mm diameter. I can't recall how or why I came up with that size or how it compares to the BP design, I think i just settled on that for my prototype because it looked right.

The BP spec for a cone ratio of 3 is 167mm, which I have used on the ones I have built. That is quite a small outlet on a 500mm diameter cyclone and at one stage early on I was having trouble with the chute plugging up with long wet shavings when I was green turning large diameter bowls. The long shavings come off at a rapid rate and were in effect plaiting themselves together into thick long ropes as they spiraled down the cone. I corresponded with Bill Pentz about cutting back the end of the cone to open up the chute size to overcome this problem and he advised me strongly against doing this. His explanation was that the point at which the air stream in the cone stalls (that is when it lets go of its suspended load) before reversing direction and coming back up the centre of the cyclone is right at the bottom of the cone if it is proportioned correctly according to his cyclone specifications. So, I just stopped collecting that type of shaving via the cyclone when wet turning; there is little fine dust produced when wood is wet like that anyway.

Cyclones with different cone ratios may have a different chute diameter (eg Bill's short cone ratio spec of 1.64 has a chute D of 150mm). Tim, what ratio have you used on your MkII.

Just a thought; if your cyclone meets one of the ratio specs, other than being truncated at the end of the cone, and you don't want to add that on because of packaging constraints, you could just make that as bolt-on that could be sent inside the other components.

Originally Posted by Muchacho

As far as getting hold of a decent fan, I reckon the best performer at a reasonable price on this side of the ditch is the OAV one below which is a rarity for a 3 HP fan as it has a 14" impeller. Its still a bit pricey at about NZ$1200.

I'm not currently up to speed on what is available here. Others on the forum may know.

When I looked at all of the options available here almost a decade ago when I built mine there were few offerings with an impeller above 12". I went with the best available option at the time, a 3HP, 3ph, with 13" impeller and built the more efficient long cone (ratio of 3) cyclone, which I only just managed to fit below the high end of my workshop, with some modifications. I did investigate having a specialist outfit make a larger impeller for me but the cost was horrendous and I would have still had to construct a matching fan housing and get that right.

Another thought: I negotiated to buy just the motor & fan unit for mine, without all of the other bits that I didn't want, so got it at a reduced cost. It might be worth seeing if you can do the same on the OAV if that is your best option.

Originally Posted by NeilS

Tim, what ratio have you used on your MkII.

Hi Neil,
The cone is around 840 long and 470 diameter so l/d radio is around 1.8. As I mentioned earlier the design is not based on any of BP's magical formulas, only what I thought looked right and what would be easy to set up. For example the connections are all standard pipe sizes to allow simple connections with a length of flex or directly to a pipe spigot.

Perhaps a longer cone is more efficient, but for me I didn't have the extra ceiling height required and as I exhaust outside, the supposed drop in efficiency is no issue. One thing I do know is that the pressure drop for my cyclone is the same as for BP's design and that after 2 years of use the only sign of inefficiency outside my shed is a bit of dust build-up on the louver blades at the exterior exhaust.

Originally Posted by NeilS

Another thought: I negotiated to buy just the motor & fan unit for mine, without all of the other bits that I didn't want, so got it at a reduced cost. It might be worth seeing if you can do the same on the OAV if that is your best option.

Good point. The main reason for mentioning this particular unit was in case any one else was looking for a suitable medium sized fan for their cyclone project.

There's common theme on many woodworking threads about needing to match cyclone and fan rotation, and if this can't be achieved then adding a flow straightening device in the cyclone air outlet tube will provide the best results.

With this in mind, I knocked up an straightener and ran another bunch of measurements to produce a fan curve and cyclone system curve. With all this data on the same graph, I think it's fair to conclude that there is no real loss in performance associated with mis-matching fan and cyclone rotations and similarly, no noticable nett effect of having a flow straightening device in the cyclone outlet tube.

I say no nett effect of the straightening vanes as the cyclone system curves show an increase of about 1" pressure loss from the vanes but only a slight change in overall airflow on the fan curve. The vanes must be doing something to improve arflow but this is offset by the additional frictional losses within the outlet pipe caused by the vanes.

IMG_0185.JPGCyclone fan curves.JPGCyclone system curves 2.JPG

Originally Posted by Muchacho

Testing with a larger fan is definately on the agenda at some point, the problem is finding something suitable at a reasonable price. In my small shed, I can get around 600cfm through my machines with my modified 2HP fan which is good enough for me so this testing would be for academic porposes only.

What is the lowest flow you think we should be looking at and why is 600cfm ok for you?

Would you recommend 600 cfm as a tolerable flow rate for anyone buying your cyclone?

Do you think that 600 cfm is an adequate speed to capture the fine fine sub 5 micron particles that escape to the air?

I am not trying to be confrontational here so let me explain, the problem with anyone recommending flow rates is they have never done any back to cyclone back testing (that I have seen) given how easy the access to the equipment is relative to what it used to be and the use of a VFD to control the fan speed. By back to back testing I would like to see the dust levels data over (for instance) a day in a workshop using different flow rates. BP's research if you could call it that is very old though the physical basics are most probably never going to change and it is about time someone re-visited all his work but that is never going to happen. He was ostracised in the extreme when he tried to initially present it in the US so he withdrew from the debate and left his website up. He did contribute to this forum a few times but never came back which was a shame.

As important as CFM is the rate of dust generation and how it is being made. This depends on things like the type of activity, wood and machinery.

I haven't undertaken dust level measurements in a workshop over a whole day but I have undertaken this in a shed over a few hours for specific WW activities.

On my lathe, 600 CFM is OK for small / short spindle work but as soon as the work is more than about 50 mm in diameter and longer than the width of the BMH used for extraction (~300mm) fine dust starts to leak away from the work. Fine dust also spins off the work as the chisels get blunter and more fine dust is made. (BTW I don't worry about the chips as they fly all over the place).
I found 900 CFM works for most of the work I do which lately has been making BMHs from MDF up to ~200 mm in diameter.

On my 1m long x 150mm wide belt sander I usually use ~400 CFM of suction through a BMH at one end. The dust levels in the shed depends on where on the belt the work is being sanded. If it's a small piece of wood held on the belt close to the BMH then the 400 CFM is sufficient not to increase shed dust levels. BUT, if it's at the other end of the belt, or a long piece of wood , then the levels in the shed rapidly increase. To counteract this I run a 1200CFM ventilation fan about 2m above the sander. Even this is always able to cope and I swing in another 400 CFM connection close to the belt.
BTW this belt has a "belt wipe" on it otherwise the belt drags some dust away from the BMH.

With most machines the shed dust levels depends on the frequency of use. Short period activity with sufficient gaps of no dust making obviously will allows a DC to more easily clear the dust than constant use.

I haven't tested this extensively on a TS but I have found 600 CFM to be insufficient on a table saw when the CFM has to be split between the cabinet and the blade guard. AT least 400 is needed on the guard and 600 on the cabinet alone.

Routers are similar requiring both under and over extraction. ^00 CFM might be enough if the work is constricted enough but what then matters is pressure because constricted air intakes on low pressure systems will rapidly drop flow rates.

The only machine that I have tested that gets away with low flow is a trill press where 200 cfm is sufficient (even the sub 100 CFM from a VC is usually enough) for fine dust control, drilling does not make much fine dust anyway.

Hi Chris,
I think you may have misunderstood my post(s). I don’t believe I have ever suggested people should target a certain flow rate. Everyone’s situation is different and the last thing I want to do is to try and steer anyone down a certain path. One thing I will do, and this thread is an example of this, is to put up a contrasting point of view to some of the opinions made on this forum and others.

Originally Posted by Chris Parks

What is the lowest flow you think we should be looking at and why is 600cfm ok for you?

I think it is a mistake to target a minimum flow rate. It disappoints me when I see people making posts asking for advice about improving their system only to be knocked back with well-intentioned but totally discouraging responses implying that they are wasting their time unless they can get 1000 cfm at every machine.
I encourage people to make step changes to their systems as resources permit. Simple things like taking the dusty outside the work space or boxing it in, increasing the size of machine outlets, replacing long runs of flex with smooth bore pipe,replacing bags with proper filters, etc will make a noticeable difference and not necessarily cost a lot if actioned progressively as time and funds permit.

Why is 600cfm ok for me? That’s simply because I’ve decided it is. Compared to my original 1HP dusty, I now have a much cleaner shed, bag changes are quick and painless, I never have to clean out a filter bag, I don’t have to brush stray chips off my thicknesser and planer tables and most importantly I rarely feel the physical effects from dust after a machinery intensive shed session.

Originally Posted by Chris Parks

Would you recommend 600 cfm as a tolerable flow rate for anyone buying your cyclone?

I recommend people do their research and make their own decisions about what is the best solution for their needs. However, I’ll be the first to acknowledge that my system is borderline in its performance. With the benefit of hindsight, I should have built mine with a 3HP fan, the small cost difference will now be long forgotten but the additional performance will remain.

Originally Posted by Chris Parks

Do you think that 600 cfm is an adequate speed to capture the fine fine sub 5 micron particles that escape to the air?

I have no way of determining what is ‘adequate’ so can’t comment on this. BP says we should have between 800 and 1000cfm to comply with a number of US occupational health standards. By this measure, I am only 60% to 75% adequate. Am I concerned? Not at all. Its good to have a target which I could easily achieve if I wanted, but right now I see no need.


Best regards, Tim