Cyclone size vs. CFM?

[krharwood]

Hi all,

I've searched the forum for some advice on this subject but haven't seen much...  Maybe it's here and I simply haven't set my search criteria correctly.

Here's my question: Is there anecdotal knowledge relating diameter, height and inlet/outlet sizes of the cyclone to the CFM of the air moving through it.

I'm sure there must be differences between the requirements for a shop vac powered system, a small (500 CFM) single stage dust collector and a large (1000+ CFM) collector.  I also think that the side inlet modification must be much more efficient than the original version with two tubes entering through the top.

Any thoughts on the above would be greatly appreciated, as I'm hoping to start building my separator right after New Years.

Thanks,
Kevin

[RonS]

Kevin, Check out this web site. there is a lot of information about Ducts, Dust collection, flow rates and  etc. Look in the Engineering tab (Left side bottom) and right above in the Dust Collection tab. There are a lot of specs on this web site.

Ron 

http://www.spiralmfg.com/prod.htm

[krharwood]

I had an opportunity to read that material over the holidays...  Some I understood, some I think you'd need an engineering degree to really be able to process.

So, back to the original question.  Some folks have built separators powered by low cfm shop vacs.  Some have built separators powered by high cfm, 1.5 HP (or larger) dust collectors.  Due to power and space considerations, I'd like to use my 600 cfm, wall mounted dust collector to pull through an appropriately sized top hat separator.  Is there any anecdotal knowledge in the group that could give a starting point for the construction of an "off sized" top hat separator?  For instance, given a stated cfm, could we approximate an acceptable volume inside the top hat where the air would maintain sufficient speed to throw the dust to the outside wall, but not so fast as to generate more than a minimum amount of bypass?  Could we perhaps get a guideline that identifies the optimal space (if any) below the input duct (as a percentage of input duct diameter) to allow the air/dust to pass below the duct and maintain the cyclone?

Would anyone be willing to contribute these these or any other ratios that seem meaningful along with how satisfied they are with the performance of their build?

Thanks,
Kevin

[retired2]

I had an opportunity to read that material over the holidays...  Some I understood, some I think you'd need an engineering degree to really be able to process.

So, back to the original question.  Some folks have built separators powered by low cfm shop vacs.  Some have built separators powered by high cfm, 1.5 HP (or larger) dust collectors.  Due to power and space considerations, I'd like to use my 600 cfm, wall mounted dust collector to pull through an appropriately sized top hat separator.  Is there any anecdotal knowledge in the group that could give a starting point for the construction of an "off sized" top hat separator?  For instance, given a stated cfm, could we approximate an acceptable volume inside the top hat where the air would maintain sufficient speed to throw the dust to the outside wall, but not so fast as to generate more than a minimum amount of bypass?  Could we perhaps get a guideline that identifies the optimal space (if any) below the input duct (as a percentage of input duct diameter) to allow the air/dust to pass below the duct and maintain the cyclone?

Would anyone be willing to contribute these these or any other ratios that seem meaningful along with how satisfied they are with the performance of their build?

Thanks,
Kevin

Kevin,

It sounds like you are planning to build a centralized DC system that serves multiple tools, one at a time, with a 600 CFM wall mounted unit.  If that is right, I have some bad news for you.  At best, a 600 CFM unit is marginally powered for this task, and at worst it is woefully underpowered.  If you add a separator to the system is will surely be the latter, and you will be very disappointed with the results.

A 1.5HP 1200CFM is not "high-powered".  It is pretty much the minimum requirement for a centralized system in a small shop.  And depending on the manufacturer those specs are very much over-stated.  More importantly, you need to know something about the DC's SP rating, and better yet, its fan curve.  On 600 CFM systems, you probably won't get either of these performance specs from the manufacturer.

Here' s a link to some DC reviews that shows how CFM and SP are related for the tested units.

http://www.portercable.com/uploads/PCD/Documents/News/182DustCollectors.pdf 

This is an oversimplification, but to design a DC system you need to start by determining the SP losses for your worst case, usually the longest run.  Then you need to choose a DC that is able to overcome those losses while maintaining sufficient CFM at the tool.  For most tools you need 350-450 CFM for adequate pick-up.  Then you need to size the piping to insure the FPM is adequate to keep the dirt suspended.  For woodworking that is usually 3500-4000 FPM.  Determine the largest size pipe that will provide that number of FPM.  However, don't exceed the DC's inlet size.

Hope this helps.

[phil (admin)]

I agree w/ retired2, I think a 1-HP unit, coupled with a separator, will provide an unsatisfactory result.

[krharwood]

I was planning to place the separator and DC under the outfeed table behind my table saw.  It would consist of a couple of feet of duct feeding directly into the separator and another foot or so of duct to get to the collector.  If the design is successful, the dust collector will only be pulling air thru the separator and filtering fines from the output air flow.  This unit would be dedicated to capturing dust from the saw. 

I was also thinking about attaching a smaller unit (shop vacuum powered) to pull from the blade guard.  This unit would be mobile and could be attached to the miter saw, drill press, router or sander as needed.  Might not be the best solution, but given my budget and the the equipment that I already have, it seems reasonable.

Any residual fine dust escaping into the air is scrubbed by a ceiling mounted JDS 1200 cfm airfilter.

So back to the earlier questions....  Does anyone have any thoughts on the size that would be appropriate for a top hat DS in front of a 500 cfm DC?  Would anyone be willing to contribute to a "database" identifying measurements of successful separators?

Thanks,
Kevin

[retired2]

Even though your planned set up is not quite as taxing as I first understood, I still think it is a marginal proposition, and you won't know for sure if it will work until you've tried it.

However, here's some more to think about.  In the thread describing my top hat build, I posted the measured CFM's my DC was pulling at an inlet port upstream of the separator.  My DC is a 1.5HP, 1200 CFM Delta.  Without the separator installed the measured air flow at the port was 705 CFM.  With the separator installed the flow dropped to 434 CFM.  That is a 38% drop. 

Now let's assume for the sake of discussion that you might see a similar drop, and let's say that your 600 CFM rated system is going to pull 500 CFM without any separator.  It could even be less than 500 since the manufacturers rating is probably free flow, i.e. no piping or filter bag attached.  And no matter how short your connection, there will be line losses from the entry port itself, the connection piping or flex hose, and the filter bag.  But let's say you are starting with 500 CFM without the separator, you could see a drop of 38% with the separator installed.  That leaves you with only 300 CFM (approx.), and that is very marginal for the machines you plan to use it with.

If, in spite of this uncertainty you still wish to proceed, here's what I would suggest you do for size.  The height will be determined by the size and configuration of your inlet pipe.  Most likely your plumbing is 4", so that means your separator will have to be around 5" high.  With your low CFM, you will want to keep the diameter small, but not so small that the inlet port intersects the outlet port or you will have serious by-pass problems.   So, that means your separator will need to be about 15" or slightly larger.  The diameter of your may waste container might influence the final size.

Regarding a database of separator build sizes, I have no energy for it.  If you read many of the threads in this forum, which I suggest you do, then you will quickly realize that there are so many build variables that a database would not be of much value.  Now, having said that, nearly every build described in this forum follows the basics that Phil has developed, and those that don't have not worked very well, if at all.