Thread: Want your DC checked for invisible dust?

Perhaps the people that wish to take this more seriously could hire one of these.
One unit could be shared among a group easily in the course of a day...

I think I will, once I have moved into my new workshop.

regards
Andrew

Originally Posted by Andrew Burchill

Perhaps the people that wish to take this more seriously could hire one of these.
One unit could be shared among a group easily in the course of a day...

I think I will, once I have moved into my new workshop.

regards
Andrew

That is a very good idea.

I realize this might be stating the obvious but before you hire one, you may want to think carefully about what you plan to do because sure as eggs you will forget to do something.
Some important measurements, such as how long does it take for a DC to clear a shed, can take a long time (~hour or so) and during this time nothing else can be done with the counter or shed which limits the number of people that could do this in one day. Nevertheless I think 4 - 5 people could do this in a 24 hour period.

It also helps to have an understanding about what represents a low, medium and high count, so you can chase the worthwhile measurements as they come up.

Also how to convert counts into exposure (simple spreadsheet calc).

If you would like I can provide a typical testing procedure?

Today I checked out 2 , 2HP dust extractors.
The shed the DCs operate in, are used ~90% metal work (mainly welding and grinding) and ~10% for woodwork, but the DCs are only turned off when wood working operations take place.
The shed is very large (~20 x 60 m) and also used to store a number of large vehicles (eg Trucks).
Neither DC had been used for several months.

One DC (DC1) had a cloth bag (top and bottom) and the other (DC2) had a single needle felt top and plastic bag bottom.
DC 1 was about 20 years old and about 10% full of shavings.
DC2 is near new and was only about 1% full

There was visible sand and soil, and wood and metal dust on the floor, plus grey welding dust covering all the upper surfaces in the shed, including the DC bags and I could tell this was going to be a very difficult measurement.
The initial still air dust levels in the middle of the shed were not that high, ~50% higher than my shed at home, but just walking around sent the levels up to 2-3 times greater than that.

I pulled DC1 one away from the wall where it was normally stored and fired it up and a great puff of dust emanated from the bags and the dust levels in the shed shot up to over 600,000,000 particles per cubic metre.

I waited ~4 hours for the dust to settle and by then it was down to about double my shed levels at home, and dropping very slowly.
It doesn't matter if the dust levels change slowly over time as the efficiency is measured relative to the shed air by alternately measuring shed air and DC filtered air.

I tested DC2 first and the first thing that I realized is that the particle size distribution was somewhat different from what I usually see in wood working sheds


The blue line is the still air shed dust and compared to woodworking shed dust shows an almost flat profile with substantial 2-5 and >5 micron dust levels.
The red line shows the DC2 (needle felt bag) output.
Note that for all particle sizes, except the >5 micron, this DC puts out considerably more of these particles than it is drawing in.

The I tested the second (cloth bag) DC.
In this graph I show the efficiency in % of the two DCs.



A negative efficiency means the DC is outputting more dust for that particle size range than it is picking up.
How is this possible?
At first I though these DCs were leaking and I did find a very small leak on DC1 so I went around to the other side of the DC and measured air directly emanating from the bag but got more or less the same result.
in the 1 - 5 micron range the cloth bag DC is putting out nearly 100 times more dust than is being drawn in by the DC.
For the needlefelt bag the peak inefficiency is as expected down at smaller particles but not the smallest particles.

I think this is an example of the DC making fine dust from the larger dust particles that are drawn into and already inside the bags and being unable to contain the fine dust they just come pouring out through the bag. When I looked at the efficiency of both bags as a function of time there was a slow drift to the finer but not the finest particles. This could also be a property of motor vehicle exhaust and metal working dust and fumes. They contain large amounts of metal oxides which likely to be more friable than wood dust.

What I will do is ask the owner to clean the bags and we will take the DCs outside in a less challenging environment and repeat the test.
I will not use the above results as part of my study as the environment is too different from typical woodworking sheds.
Aside from that, what started out looking like a waste of time appears to have revealed some interesting results.

What you are finding is far from unusual.
We start with a shed sufficiently dirty that simply walking arround sends the dust figures sky high....and I dare say this woul be far from the dirtiest sheds around, and illustrates my analogy of standing up to your armpits in $##T.

Then we move on to the older dust extractors, 10 years ago almost all dust extractors came with a simple woven cloth bag, the filtration efficiency of which was only half jokingly claimed marginally better than flyscreen.

It has been postulated and proven that in many cases the introduction of a dust extractor such as these actually made the air quality worse than better.
Without the extractor, dust and chips of all sizes ( including invisable dust) would fall to ground or be carried out of the workshop on air currents in fairly short order.
When one of these, low filtration efficiency extractors was introduced the amount of dust ( I wont say fine dust) simply skyrocketed and remained high or continued to increase as long as the extractor was running.

Dust that never even got airborne, but would have in the past fallen straight to ground, now got sucked into the extractor and very efficiently atomised and the dust extractor provided an endless resovour of dust.
So even with the extractor sucking clean air it spews dust by the cloud.

These days all but the cheapest dust extractors come standard with needlefelt bags......but at 5 microns the only thing that has changed is that the dust comming out is a little finer.

When it comes to the sharp point of the argument, in an enclosed workshop are we better off with an extractor fitted with a pleated cartridge filter than no extractor at all?

Consider that a $600 to $800 industrial pleated cartridge filter, made in Australia from properly tested and certified paper is rated at only 3 microns, (I've spoken to the company that makes these and they recon that is about as good as it gets). Are we realy any better off if the filter is still in the room with us.
Yes I know there are cheap chinese pleated cartridge filters in the $150 to $200 bracket that manufacturer claims 1 micron, but many have doubts on the reliability of this claim.

As we all know bay far the best option is putting the extractor outside.

cheers

Hi Bob,

Please continue to post your empirical studies on dust collection. At the end of the day some of it may be academic, and I think a small amount, and some of it may be reinventing the wheel - but many of the wheels are presented without supporting data (either because of legal reasons, proprietary data or lost in the mists of time). For example, it is worth having it explained why bell-shaped inlet ports are more efficient that a pipe end and presenting some empirical studies on it.

As Stephen W. Hawking says, the greatest enemy of science is not ignorance, it is the ILLUSION OF KNOWLEDGE.

Cheers
Michael

Originally Posted by soundman

. . . . . Yes I know there are cheap chinese pleated cartridge filters in the $150 to $200 bracket that manufacturer claims 1 micron, but many have doubts on the reliability of this claim. . . . .

I have several of these in my test plans - then we might both learn something useful

Originally Posted by mic-d

Hi Bob,

Please continue to post your empirical studies on dust collection. At the end of the day some of it may be academic, and I think a small amount, and some of it may be reinventing the wheel - but many of the wheels are presented without supporting data (either because of legal reasons, proprietary data or lost in the mists of time). . . .

Cheers Michael, I agree there are many claims being made on this forum with little or no quantitative supportive evidence which is why I am doing these tests.

I'm the kind of person that when I can, has to "pee on the electric fence" to help build up a picture of a phenomenon. Having worked on dust removal from ultra-clean laboratories for about 30 years I'm beginning to see some of the differences from the way dust behaves in ultra-dirty work environments.

Originally Posted by BobL

I have several of these in my test plans - then we might both learn something useful

Now that is most definitely something worth testing.

There have been some tests on variuos filter media in the past, but those tests are not with current product available in our market and most of the information we have on this particular issue is not independently varified.
AND it is an issue that makes a difference.

Ya might even sell the story to a magazine and get some sort of $$$ return.

I see no need to put anybodies nose out of joint by mentioning brands, because almost without exception the same items will be offeered under several brands and the actual manufacturers are obscured in the commercial process.
In addition many of the usual suspects offer several of the competing options.

the usefull comparison would be.

Old style unspecified woven filter bag
New style generic needle felt filter bag
Needle felt bag of reputable and tracable manufacture from specified material
Generic pleated cartrige filter
Pleated cartrige filter of reputable and tracable manufacture from specified material

I think I have an unused, clean, unspecified woven bag I could loan for testing.

cheers

Originally Posted by soundman

the usefull comparison would be.

Old style unspecified woven filter bag
New style generic needle felt filter bag
Needle felt bag of reputable and tracable manufacture from specified material
Generic pleated cartrige filter
Pleated cartrige filter of reputable and tracable manufacture from specified material

I think I have an unused, clean, unspecified woven bag I could loan for testing.

cheers

Thanks for the offer but already have several I can test.
I will hopefully be testing all of those filters.

Cheers

This evening another candidate.
Large very organized moderately clean shed with professional grade well looked after machinery.
Operator said he rigorously used his DCs and VCs but commented that after only a few days of full time work a fine patina of dust would settle all over the shed.
Owner not in a position to place DCs outside or vent outside shed.

Outside air levels slightly below average for inner suburban location especially since a major highway was less than 150 m away.
Large open starting roller door ensured shed levels were more or less the same as outside air level

DC wise
DC1) Brand name 2HP DC 1 with brand name pleated filter but empty cloth bag bottom? There's a problem already,
DC2) Another Brand name 3HP DC with twin plastic bag bottoms and 3rd party pleated filter tops, 15% full
VC1) 5 year old high quality brand name VC - bag was about 1/2 full
VC2) 10 year old high quality brand name VC- bag was about 1/4 full.

Firstly the DCs

DC1:
PF1 appears to be working well and one can see the effect of twirling the handle on the pleated filter 1.
I was interested to see that cleaning the filter improves the efficiency - I always though the opposite would happen - anyone have any ideas on this?
The cloth bag is working about as poorly as I would expect a cloth bag to work at for fine particles but very poorly and even negatively for the medium and terrible for the largest (dashed green line) particles. I recommended the owner replace the cloth bag with plastic

PF2 has some issues at the medium to large end of the scale.
These are third party filters but DC2 also had several serious leaks around the plastic bag to DC housing seal.
These were almost always associated with folds in the plastic bags underneath the spring steel clips.
This highlights the issues of sealing the DCs.

Both of these DCs would be contributing to that fine dust patina that the shed owner was experiencing.




Now for the VCs.
Just a reminder that these are high quality brand name units and they were both spewing out invisibles like there was no tomorrow
The newer of the two units (VC1) was generating 80 times more fine dust than it was capturing.
The mg/m^3 dust levels being produced by this unit are a potential health hazard and would also explain the fine patina of dust being observed in the shed.
VC2 was at least not generating the bigger particles but it was still far from satisfactory on the smaller ones.
We tried to see if there was anything obvious like a damaged seal but there was nothing obvious.
The owner said that the VC was sucking up visible sawdust and I was planning to test it in action but I did not want to potentially damage the particle counter.
I have recommended to the shed owner he gets these units checked out professionally.

Regarding the pleated cartrige filter that performs better after it has been flogged rather then before.

This is a known issue and far from uncommon, the issue revolves around air pressure across the filter.
When the filter is clean and the air pressure is low the filtration works at its optimum, as the filter loads up the air pressure across the filter increases and the air flow is able to force more particles thru the filter.

This is known to happen in paper car air filters.

It stands to reason that the thinner the filter membrane the more inclined it will be to this behaviour.

It also occurs to me that a filter made of paper that performs just fine, may exhibit this problem if it is damaged in the folding process or it does not tolerate folding well.

If you look at some of the better pleated cartrige filters you will see the paper is folded around a curved edge mandrel, some chepaer items I have seen have very sharp folds, this must compromise the paper.

I would be very interested to see how cheap chinese pleated cartrige filters perform compared to locally made industrial grade units made from specified paper.

As for the vacuum cleaners......no surprises there......
Several of the name brand up market shop vacs, use a pleated cartrige that looks to be about the size & shape of that out of a toyota landcruser......but when you put the two beside each other, it is obvious that the filter from the vac has less pleats and thus less filter area and the filter material is thinner and coarser than the factory or generic after market toyota.

I recently saw a "workshop dust extractor" that was powered by a single stage vac motor that came with one of these filters......BUT..it also came with a paper bag ( like a vac bag) fitted over the pleated cartrige......I looked at it scratched my head and wondered which was finer the paperbag or the pleated cartrige......the whole thing was a joke....commercially availabe brand name item.

As for the sealing of plastic dust bags to the rim......most of the DC where designed to seal cloth bags and even do that badly.......one quick and effective improvment is to get several meters of 25mm velcro adhesive loop tape and apply that around where the dust bags seal...it improved my machines no end.

As for getting "Professional help" with his Vacuum machines.......UM, considering that they are probaly working as well as they where designed.......from whom is he going to get this help and will it cost more than the machine is worth?

cheers

Hi Bob
I have just seen your thread for the first time. I am a volunter
Willy
Jarrahland (in Port Kennedy, south of Rockingham
0427 739 178

Originally Posted by soundman

Regarding the pleated cartrige filter that performs better after it has been flogged rather then before. This is a known issue and far from uncommon, the issue revolves around air pressure across the filter.
When the filter is clean and the air pressure is low the filtration works at its optimum, as the filter loads up the air pressure across the filter increases and the air flow is able to force more particles thru the filter. This is known to happen in paper car air filters.

While your explanation makes some sense, I have measured a number of paper filters and my experience has been the opposite, the more the filter is blocked the better it filters especially the finer particles (of course it becomes more restrictive). Maybe there is a sweet spot I have passed over. I have my own theory which I will test next time I test one of these filters

As for getting "Professional help" with his Vacuum machines.......UM, considering that they are probaly working as well as they where designed.......from whom is he going to get this help and will it cost more than the machine is worth?

Humm, . . . I would think at more than $1000 each for these vacuum cleaners one would expect some sort of service support from the supplier would be in order. I could not believe how bad VC1 was - remember this was not sucking any sort of significant dust at the time. If this is the best these top of the line European vacuum cleaners can do then a great many people around the world are been sold a pup. I will reserve judgement until further testing.

I have no doubt about the variance you have seen with these filters.

While there may be some sort of sweet spot thing going on..that is hardly a good thing.
More likely you are observing the variance in the quality of the filters.

As for the vac.......Bob, Bob, Bob, you are an optomistic bugger.

We live in an overpriced and underserviced country where price is not a reliable indicator of quality.

If ya walked back into the shop where the vac was baught and said that the measured particile size was unsatisfacory.....all you would get is either a blank stare or some sort of vacuous fob off....or if the "customer service officer" was on the go....they would sell you a new bag or set of filters...$$$

If you speak to a vacuum cleaner enthusiast (a mate of mine owns 12 at last count), or a good vacuum cleaner salesman they will tell you there is only one brand that has a hope of satisfying a fussy industrial customer and you need to buy the model with the optional post filter.

The other problem as I have mentioned before with vacuum cleaners especially industrial ones is finding one that is being operated to spec.

Most of these guys, "buy and use" with out reading specifications or instructions (this guy had a cloth bag in use with a pleated cartrige filter ) and simply don't want to know.

They don't get emptied frequently enough, the filter eliments certainly don't get cleaned and replaced often enough and a very large part of the time the various pre and post filter eliments are not even in place.

A great many industrial users will not replace the ( poor) pleated cartrige filter in their industrial vac until it is torn and spewing dust...AND.....some one has complained or the site safety officer has banned it till fixed.

Even at their best, most people are perfectly happy with an industrial vak that does not spew visable dust when it is turned on......talk to them about...."Invisable Dust"......"sorry mate cant see it"

cheers