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darryl
10-29-2013, 11:59 PM
I'm almost finished building a down-draft sanding table at work. All that's left is to make a filter to prevent the fine dust from coming through. Couple ideas we've had- get a furnace filter sheet that you cut to fit, or stick a bedsheet to some chicken wire and fold that up into pleats to maximize surface area. Either way, I will be building the framework to hold the filter.

The main question is what makes a good fine dust filter? The blankets that are made for furnaces, and the pre-made filters seem to be fairly open- they don't collect as much dust as it seems they should, given the amount in the air in the shop at times.

This project is using the old dual squirrel cage fan that came from our old furnace. Whatever filter we use, it should flow easily since there isn't a lot of pressure here, but we're thinking we need to preserve a high flow rate. Pretty sure we can get a decent result from a sheet, probably a flannel sheet would be good. I'm leaning in this direction- I have 30 inches of height to use, and by pleating the chicken wire with the sheet attached, I can easily get the full length of a bedsheet into the 32 inches of width available. With that much surface area it should flow decently while filtering out a high percentage of the dust.

That's where I'm at right now- any better ideas?

dp
10-30-2013, 12:05 AM
There is a lot of information here on dust and chip collection.

http://www.jpthien.com/smf/

lakeside53
10-30-2013, 12:50 AM
I'd just buy a real pleated filter ( the type that has pleats 2-4 inches deep, 4 sq foot or so outside physical size) rated for sheetrock, wood dust etc. Look at the filters used in commercial wood shops...

Any single layer "sheet" will be marginal and quickly block. I've had many "filter bags" on the output of sawdust extractors, and they only stop the lumps then block up. A real vacuum cleaner bag would be better, but you don't have the static pressure to use one.


I'd just suck the dust though the table and pump it somewhere else for filtering (or even "outside").

darryl
10-30-2013, 02:25 AM
Yes, I find that the filter bags block up fairly quickly, and those must be at least marginally acceptable or they couldn't be usable. I'm trying to go with larger surface area to make things better- it makes sense that a real pleated filter would be a good idea. I'll look into that.

boslab
10-30-2013, 06:21 AM
What about the filter cartridges i see being changed in air con units in shops, they are square with a green wire reinforced membrane inside the cardboard, or is that the pleated filter you guys refered to, we don't get much air con in the UK lol
Mark

Baz
10-30-2013, 09:30 AM
A filter that works by having holes will always, a) let smaller bits through, and b) get blocked by the bigger bits.
Hi-flow filters work by making the air change direction going through a maze of fiblres so that the dust particles can't turn the corner and hit a (sticky) fibre. As in the old fashioned oily wire mesh engine filters.
The alternative for hi-flow is a vortex perhaps with 'sticky' cloth on the outer wall.
If it is for work the discharge either inside or outside probably has to meet regulations. A commercial filter pad would show you tried but a home made jobie would be a lawyers dream ticket for anyone with lung problems.

Duffy
10-30-2013, 10:39 AM
Since you are using a furnace fan, you dont have much static pressure to spare. 2" deep peated 30% air filters will pretty much stop smoke. A couple of 2X2 foot units, side-by-side should work, and they are fairly cheap . I use them in my furnace, and a case of 10 or 12 is about $50.00.
I have a 1/2 hp 2-speed fan from a furnace, mounted in a roll-around cabinet with a filter in each end. It will keep the dust out of my shop, or clear it in a few minutes, if I forget to turn it on until AFTER I started sanding! The bonus is that my "portable" planer sits on it. My fan moves close to 1000 cfm, but I think that yours, with the friction loss through the table, will be quite a bit less.
FWIW, a chicken wire/bed sheet unit MAY work, but it IS a bit Heath/Robinson, and a royal pain to wash, iron, and repleat:o! Especially when asbuilt filters are so cheap and available.

cameron
10-30-2013, 01:47 PM
The dust you really need to worry about is the really fine stuff. If you have a filter to stop the fine stuff, you need a pre-filter to catch the coarser stuff that would quickly plug up the fine filter.

If you're young enough and intend to do much woodworking, you shouldn't be guessing about the probable efficiency of makeshift filters.

Boucher
10-30-2013, 11:00 PM
Look at these; http://www.oneida-air.com/ Someone here recommended using the cyclone seperator and removing the fine over the pleated filter. With the fine filter removed the actual air volume is increased and works much better.

darryl
10-31-2013, 01:11 AM
We did discuss the various filter types today, and I will be looking to see what I can get for a reasonable cost. This is a small shop where there is no money available for improvements. Most of the upgrading I've done myself, and without the imperative of being paid for my time. This one has cost me a few dollars out of pocket, but that's of no consequence. What is of consequence is that previous to this the sanding has been done virtually open-air, with the little filter on the sander being of little effectiveness- any type of material that would impede the passage of dust to some degree is going to improve the situation.

A cyclone separator is not going to happen here, but I have seen dust collectors using several furnace filters arranged in a sort of pleated pattern- I think that will be the answer here. If I can get a box of 10 or so, I'll build a frame so they can all be used at the same time.

What's ironic is that every friday, we clean our furnace filter- by vacuuming it with the shop vac- problem transferred to the vac filter.

EddyCurr
10-31-2013, 04:26 PM
The message that there is no budget for a commercial cyclone is clear, but
don't discount this approach without considering the various examples of
home shop cyclones that come up in an online search.

Taking advantage of a venturi to speed and slow air and allow particles to
separate from the flow prior to reaching the fine filter media is key to extending
filter life and minimizing maintenance frequency/effort.

Quick & cheap? Take a Rubbermaid Roughneck 20gal/76L garbage can. Get some
dryer vent & hose, cut two holes in the lid, extend a length of vent through one
hole to within a few inches of the bottom (inlet), extend a short length through
the other hole (outlet). Use your imagination to attach and seal the lengths of
vent pipe to the lid in a manner that will stand up to repeated handling. The end
result will be a pre-filter that will make a noticeable difference in reducing the
material that passes on to the filter media downstream.

The following example uses components available from BusyBee, but illustrates
what could be done with less expensive dryer vent material from a hardware store.
The grey hose is the inlet side, a vacuum cleaner hose attaches to the outlet side.

http://www.slalom4me.com/imaged_a01/jpg/other/tools/DustCollector/Cyclone/2013.10.31_CyclonePreFilter_01.jpg

Downstream, consider using gravity and velocity again ahead of whatever filter
elements decided on for the main stage. The following images show a Tepco air
cleaner that came from a welding shop.

It has three stages:

a tray in the bottom cabinet (below the inlet hose)
an ionizing section
an activated charcoal filter element (missing from empty slot above ionizer)

The motor, blower and ionizer transformer occupy the upper-most section
and the scrubbed air vents out the top.

http://www.slalom4me.com/imaged_a01/jpg/other/tools/DustCollector/Tepco/2011.04.27_Tepco_Chassis_01.jpg

http://www.slalom4me.com/imaged_a01/jpg/other/tools/DustCollector/Tepco/2011.04.27_Tepco_Chassis_02.jpg

Another version of this air cleaner substituted canvas-like pleated filter media
in place of the ionizing components. If you arranged your filter & blower cabinet
in a similar fashion (downflow inlet, tray, updraft to filter(s), blower, exhaust outlet,
this may be beneficial to the cause.

About filter media and frames for same. Camfil Farr is one company that offers
a wide selection of filter types and shapes. I use their 24x12x12 media mounted
on stretcher frames for my home furnaces. Circumstances may preclude you from
using their products, but their website is free to browse for inspiration.


Camfil Farr (http://www.camfil.ca/Products/)

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