Cyclones - Push or Pull?

[Anthony Whitesell]

Two part question:

1) What are the advantages of the push versus pull cyclone designs?

2) Can you convert any cyclone from one to the other?

[Jim O'Dell]

I see no advantage to a push design. The biggest disadvantage would be that everything hits the blower. Makes it's life really hard. Pieces of metal could spark, causing more possibility of dust to ignite. I just wouldn't do it.
Convert one over? Shouldn't be hard to do, but I wouldn't. Jim.

[Dick Thomas]

+1 on Jim's comments; And another disadvantage: In push mode you will quickly learn about every minute air leak in your system downstream of the blower. :(

[Alan Schaffter]

Whoa!! Not so fast!!

Background- I configured both of my DCs as "push-through" cyclones and have used them in that mode for over 10 years with no problems. The first one was a modified Wood Magazine design and for the last 6 years I have used a "push-through" configured 3D Pentz design (see photos below).

First let me address "The biggest disadvantage" mentioned. Every single stage DC ever made is essentially configured as a "push-through"- the collected debris (dust, wood chips, chunks of wood, nuts, bolts, pieces of metal, etc.) enters the blower first where it can strike the impeller! There are thousands (millions?) of these units out there, and it is very rare to hear about a damaged impeller. I can't remember the last one I heard about, and that was probably a plastic impeller! My DC has sucked up all sorts of things including a DP chuck key, tape measure, and recently a 1" X 3" piece of 10 ga. steel with no damage.

Secondly, the air flow immediately cools any spark that might be created and the velocity and volume of air are so high it makes the fuel to air ration too low to ignite or sustain a fire. Like the static myth, I have never seen one confirmed report of a home shop DC related fire.

As to "another disadvantage" it is actually an advantage. A down-stream leak in a "push-through" cyclone system will show up as a puff of dust or dust trail on the duct so is easily located and fixed. The hose connection at the bottom of the cone and at the lid, and the entire lid on the dust bin of a "pull-through" cyclone must be absolutely air tight. If it is not, dust can be drawn out of the bin and cyclonic separation can be disturbed, both of which can allow dust to pass through to and clog the filter. On a "push-through" system, a leak is a leak and has no effect on the cyclone separation.

As to other advantages- both configurations allow remote mounting of the blower.

The biggest advantage of a "push-through" cyclone- not only don't you need an airtight dust bin, you don't need a bin at all!!!! I use inexpensive grocery store 39 gallon lawn and leaf bags clamped directly to the bottom of the cone. When they are full (I have a high dust alarm) I undo the band clamp, remove and tie off the bag, and carry it to the curb. No messy job trying to empty a bin! There is just no comparison- try it and you will be convinced!

Here is a link to a rather long (sorry) video (http://videos.americanwoodworker.com/video/Automatic-Dust-Collection) tour of my system.

Can any cyclone be configured as a "push-through"? probably. I can't say for sure about a ClearVue, but I think the PETG it is made with would be tough
enough.

I would never use a blower with a plastic impeller- for that matter I wouldn't have any DC with a plastic impeller!!!

Old Wood Magazine cyclone mounted outside:

http://www.ncwoodworker.net/pp/data/500/medium/P2240015.JPG

New cyclone and same 3 hp blower. Filter mounts to the top (shop level).

http://www.ncwoodworker.net/pp/data/1329/medium/P10100382.JPG

Dust "bag". Though not needed I usually have the bag sitting in an open trash barrel which makes it easier to maneuver around.

http://www.ncwoodworker.net/pp/data/1329/medium/P10100531.JPG

[John McClanahan]

I would think a pull system would be more desirable, but often big outdoor units that I see are the push type.

John

[Anthony Whitesell]

Alan,

Actually Alan, your cyclone video is what prompted me to ask the questions. Thank you for the time and the video. Specifically regarding your systems, where does the top "port" of the cyclone body go on each of your DCs. In the Wood mag version it appears that it is piped back to the shop. Yes/no? And what about your Pentz Cyclone? Are either routed back to a filter?

[Alan Schaffter]

Alan,

Actually Alan, your cyclone video is what prompted me to ask the questions. Thank you for the time and the video. Specifically regarding your systems, where does the top "port" of the cyclone body go on each of your DCs. In the Wood mag version it appears that it is piped back to the shop. Yes/no? And what about your Pentz Cyclone? Are either routed back to a filter?

Yes, both top ports are routed to filters in the shops. The newer unit also has a diverter so I can discharge outside in mild weather.

Filter shoehorned into my tiny old shop:

http://www.ncwoodworker.net/pp/data/500/medium/DC_piping.JPG

Setup in my new shop has two filters in parallel in a plenum that is vented to the shop:

http://www.ncwoodworker.net/pp/data/1329/medium/P10100471.JPG

http://www.ncwoodworker.net/pp/data/1329/medium/P10100513.JPG

[Anthony Whitesell]

Alan,

Since you built your own cyclone, how did you pick which filter to use? (ie., which specifications did you consider)

[Alan Schaffter]

Alan,

Since you built your own cyclone, how did you pick which filter to use? (ie., which specifications did you consider)

I went with one that was efficient down to .5 microns, with the most area I could find for the best price. I started with one Farr style 9L300BL (http://www.wynnenv.com/cartridge_filters.htm) then added a second one when I built the new cyclone.

I'm sure there is better engineering info available, but one rule of thumb says you should have a minimum of 1 sq. ft. of filter per 10 CFM of flow meaning a 1000 CFM DC should have a 100 sq. ft. filter. Wynn Environmental recommends one of their 9L300BL filters (300 sq. ft.) for systems of 700 or 800 CFM or less and a minimum of two filters for anything larger. But all filters aren't the same, they all don't flow the same with the same SP resistance. Filter area is like CFM- more is ALWAYS better.

That being said, I discharge outside, whenever I can.