Replies

1. 
   romano | Jul 20, 2010 09:39pm | #1

   Tim,

   I suppose you could say that the larger particles fall to the bottom while the finer ones 'float up', but the reality is that the lower bag fills up with wood chips and becomes less and less of a filter. In fact, I and many others use a plastic bag inside the bottom bag for ease of emptying. The upper bag is the one that is really the filter because it doesn't fill up. Having said that, now that you are getting into dust collection, you will hear alot of different advice from people. Alot can be learned from Bill Pentz's site, though it's pretty overwhelming and you you may come away wondering how to put to use what you have learned.  I use a canister and a baffle designed by Phil Thien. Do a search on the Thien Baffle. Aside from a cyclone, I think this is the only way to go. Canisters are better than bags, but are useless when clogged and the ones that have a flapper inside don't have enough square footage of filter area. There's alot to learn out there!

2. 
   timvn | Jul 21, 2010 03:33am | #2

   Thanks, Romano, that's helpful.  I'll see how my unit behaves.  It is probably more powerful than it needs to be for my small workshop -- I bought something that would work when I get a bigger place -- so I might be able to just go with a plastic bottom bag and accept any lost of suction.  That way I could sidestep the filtration question on the bottom bag, at least for the mean time.

   Thanks for the other names -- I've had a brief look and will follow up later.

3. 
   ring | Jul 21, 2010 09:40am | #3

   there's only one way to be sure

   Put a plastic bag on the bottom and try it.  Odds are you won't experience any drop in efficiency, because the surface area of the upper bag filter alone is huge compared to the fan's capacity.  But in any case, it doesn't cost a thing to try and you can always go back. 

   I don't know of any commercial shop that has cloth bags on the bottom.  With plastic you can see when it's full, and you can dispose of it without creating a major dust storm.

4. 
   swenson | Jul 21, 2010 09:52am | #4

   Do you have a 30 micron filter or did you mean 3 micron?  I'm no expert, so take this for what it's worth.  The way I understand it, the dust that stays in your lungs is the one and two micron size.  A thirty micron filter will keep the dust off your furniture but not protect your lungs.

1. 
   romano | Jul 21, 2010 07:36pm | #5

   This is correct, a 30 micron bag is also referred to as a chip collector, or even "dust pump". It will only keep you from using a broom and dust pan. The shop will still have dust all over the surface (or so I've been told). The fine stuff gets through and that's what is bad to breath in. It's worst when the bag is new, or washed. It gets better when it is used a bit. The more clogged a filter becomes, the better the filtration, but at the cost of airflow. You could supplement it with an air cleaner and a dust mask, but at some point it might make sense to get a canister. Go to Wynn Environmental, don't buy the overpriced ones that carry the same name as your DC unit.

5. 
   timvn | Jul 22, 2010 10:05am | #6

   Filtering options

   Thanks for the additional comments.  I'm still in the process of rigging up the ducting but, once I'm done, I'll try switching to a plastic bottom bag and checking the effect on air flow.  Good suggestion and easy enough to try.

   The bags that came with the DC are rated at 30 microns.  When I bought the unit I was aware that this isn't good enough for respiratory safety (the dangerous particles are 0.3-30 microns), but to get a DC with internal filter going down to 2 microns (the best I could get in my area) would have more than doubled the cost, and given me a weaker blower into the bargain.

   I'm considering two options for dealing with the filtering issue.  First, upgrade the top bag with one that filters to 1 micron.  (This is actually what prompted  my query about the bottom bag, since I wondered whether I would need to upgrade two bags, not one, at around $60 a pop excl shipping.)

   In the mean time, would there be any sense in putting the bottom cloth bag over my existing top bag if I used a plastic bottom one?  The bottom and top cloth bags are identical. Since both are the same, will I get 30-micron particles through two layers just as easily as through one?

   Second, I intend to build a booth in which to house the collector.  Partly this will be to reduce noise (my workshop shares a wall with neighbours), but this will also address the filtering issue because I'll put a 1-micron filter on the booth's air vent to filter the air coming back to the shop.

   Even with the additional costs I still expect to be saving on costs.

   I just hope I don't lose too much suction with these measures.  I turned the DC on last night with the ducting mostly rigged up and the suction seemed pretty powerful.  (The unit is rated 1050 cfm and my workshop is a single-car garage.)  I became all excited hearing the rush of air and scrabbled around for dust and chips to feed into the furthest port...

   Tim

1. 
   zolton | Jul 23, 2010 08:42am | #7

   Dust pump..

   Tim,

   Like some of the other posters you've already heard from, I use a plastic bag on the bottom of my two-bag Jet dust collector. The plastic bag sits inside the original cloth bag that came with the unit. That helps to contain it and keeps it from blowing off the bottom of the blower.

   However, I replaced the top bag with a Shaker felt bag. According to Highland Hardware, where I purchased the Shaker felt bag, there is less air resistance from air blowing through this bag than there is through the original bag. So, supposedly, I get superior small-particle dust retention in the Shaker felt bag, and easier collection with the plastic bag.

   In practice this seems to be true, though I have no scientific evidence to back it up. All I know is that it works, and works well. I've only got this dust collector hooked up to my sliding miter saw these days, as I invested in a Penn State cyclone a few years ago, so it doesn't get the use it once did. But for what I need it for it is performing admirably.

   I would recommend getting one of the Shaker felt bags. The filtration is much finer than what you can yield with the original bags, and they aren't very expensive. And it allows you to use a plastic bag on the bottom with little loss in performance.

   Zolton 

2. 
   Don01 | Jul 23, 2010 03:49pm | #8

   Dust Collection

   It is to bad you did ask before buying. 

   Basics. Dust and air are sucked into the blower and exhausted through a pipe into a separation chamber. Hopefully there is a metal ramp that spirals slightly. This ramp induces a bit of centrifugal force to the particle stream. This helps drive the heavier particles down into the waste bag. The exhaust flow is up and out through the upper bag. When the particle stream enters the separation chamber it experiences a change in velocity (slows down) and direction. Like fine river silt, the particles will settle out in areas of low turbulence. Think of a industrial stack or even tobacco smoke being exhaled. Once the stream exits the control structure - exhaust stack or mouth, it just floats aimlessly and dissipates. 

   A dust collection cyclone has a large almost continuous ramp which results in excellent centrifugal separation and huge exhaust filter area.

   Suction. The CFM ratings on single stage dust collectors is a mystery number. There is no standard as to how this number is calculated. What is fact is the amount of air that can be pulled or pushed through a pipe of a given diameter to achieve a certain volume (CFM) and velocity (FPM). A piece of 4 inch duct will handle about 350 CFM @ 4000 FPM.    

   Bags. The lower bag is for waste collection only. Check to see if it has a rubber or vinyl coating on the inside. It should not pass air. If it does, someone gave you the wrong lower bag. The upper bag should be a heavy felt. This provides filtration for the air flowing back into the shop. The larger this bag is, the better.  Every time you start your DC, the bag gets hit with the full force of the exhaust stream. Until a good cake is built up, all kinds of dust is blown right through the bag back into the work space. 1 micron bags are bad, this 30 micron thing should be tossed in the trash as soon as you can. If you isolate your DC in a separate room with the 30 micron bag in place, you will need to monitor that room very carefully as the amount of fine dust that will accumulate will be considerable. The motor ventilation could become compromised and if this room is connected in any way to your living space, the health of anyone in the area will be compromised. Wood dust is a known carcinogen.

   Filtration. The rating of filter media used in dust collection is rarely absolute ie: 1, 5 or 30 micron. In a wood shop environment, the media would become plugged very quickly. Dust collector media relies on successive layers of dust to be built up over time to form a cake. This cake is what really provides the filtration. However, with felt bags that are not supported by a rigid frame, a significant amount of cake will fall of into the waste bag when the DC is turned off.  The next time you power it up, the cake has to be re-established before you see any reasonable level of filtration. So in this time, a lot of particulate will pass through the bag and returned to your work area.

   Canister filter cartridges offer a rigid form and more importantly considerably more surface area for the exhaust stream to flow through. Lower resistance in the exhaust filter will benefit separation and reduce losses.  

   If you haven't done so go to Bill Pentz site and read: 

   http://billpentz.com/woodworking/cyclone/index.cfm

   Until you understand all of this, buy yourself some very good respirators and wear them always when in your work area. 

   Don