Hi Matt, i'd not noticed this thread was on the go. Some thoughts:
I can't see the point in using an ambient air filter if there is a choice, at least not until you have done everything possible to catch the dust at the machine before it gets into the room. This because you have to wait for the ambient filter to move all the air through itself, and breathe dust in the meantime - which means you will breathe dust all of the time if you are cutting frequently.
The other little problem is that the air you have cleaned returns to the room and mixes with the dusty air still there (and that's before we talk of dead zones that are not easily drawn to the filter, and the effects of people movements stirring up dust already deposited in the shop - testing with dust meters has shown it takes minimal amounts of the latter to drive dust levels in a shop well over what's permitted by regulatory standards), so it's probably what's called an asymptotic or dilution process. i.e. say you can access/circulate enough air to reduce the amount of dust in the air by half every half hour - then it will take another half hour to reduce the now lower concentration of dust by half again, half an hour to reduce it by half again etc - as the air gets cleaner progress gets slower and slower - it actually would take infinitely long to clean the air completely.
The essential problem (even if the filter does a decent job) is the mixing of the 'cleaned' air back into the dusty air in the room.
On filtration. The reality so far as i can figure based on Bill's information is that sellers of small dust collection systems with very poor filtration seem to rely on the fact that so far as they are concerned they are sold for 'personal' use. i.e. that they will not be used in a regulated workplace - one with employees.
The fact is that it's no problem to buy HEPA (>99.97% efficient at 0.3 microns) or similar European standard efficient filters from industrial suppliers like the UK branch of Donaldson Filtration in Yorkshire and others. Here's a page page on the MERV standards and what they mean:
http://www.air-quality-eng.com/glossary.php HEPA is another standard:
http://www.answers.com/topic/hepa Donaldson do a roughly 320 dia X 700 long pleated HEPA cartridge for about £100 in the UK.
This is well established territory in industry - it's a legal requirement not to expose employees to dust levels above those permitted. (which depends on the type of dust - somebody has already posted the situation on wood dust) Here's the legal situation from a local consultant:
http://www.degrandison.ie/405-dust.htm
The practical difficulty seems to be that if we use filters fine enough to capture the dangerous fine dust they will blind too quickly to be economic. This is why an effective cyclone is necessary to separate out most of the dust if you are going to filter properly.
The other big practical issue as you know from Bill Pentz is to install enough fan power to suck enough air in around the workspace to stop the dust being thrown clear into the room.
These two factors explain the corners that seem to be cut on the lower end DIY bag filter systems - retail is about 4X the manufacturing cost.
As before one way around this is a DIY dust system along the lines of Bill Pentz's. That's where i'm headed. Done that way and ducted it's more trouble, but is not a lot more expensive than one of the larger bag systems. It's cheaper than even the entry level industrial systems done by some of the makers.
Something around £320 will get a blower landed from Clear Vue in the US (but you can build this yourself from MDF buying just a fan impeller from them for about £200 landed), another £220 will buy a new 4kW motor. (used would be cheaper?) The materials to build the blower and the cyclone might be £175. You could hook this up to your machines on one at a time basis using a shortish length of smooth bore PU 150mm flexible and do a very good job of dust collection. (your machine hoods might benefit from some work to get them breathing freely enough)
About another £1,000 will DIY duct a small shop properly if you buy it right, and if your duct runs end up on the long side you may want to consider an inverter (many run without) to bump the motor speed up to 3450 rpm.
I've no connection with Bill Pentz other than having happened upon his stuff on the web, and having struggled with a typical 1HP/4 inch hose bag system knew i had to do better. This is his site, the information is all there if you dig, and it links to Clear Vue:
http://billpentz.com/woodworking/cyclone/index.cfm
This is the Clear Vue user help forum, lots of info there with photos of DIY system builds in the gallery:
http://www.clearvuecyclones.com/Bullentin/index.php
Building a DIY system like this does entail rather a lot more spade work than buying an off the shelf dust circulator :wink:, but i just can't afford to pay to have a proper standards compliant system built by a commercial provider and i'm damned if i'm going to bugger my lungs with wood dust...
