Am I Wrecking My Cyclone FIlter?

[Alan Schaffter]

Uh, question. Alan. I understand the picture (cool idea by the way), but if the fines collected in the filter in the first place and you hook it back up to the DC, what is stopping them from collecting in the filter again?

Before I installed the diverter, I had to hope the cyclone would get a significant part of the dislodged fines, but I'm sure some would still get through to the other filter. Now I have a diverter so can discharge outside when I clean a filter and during all but the coldest and hottest weather so I don't use or need to clean my filters much any more.

[ian maybury]

Filter cleaning doesn't seem to be the most straightforward task, and th need for it is highly connected to cyclone performance on fine dust separation too.

I've no experience yet, but if I try to repeat from Bill Pentz's pages it goes like this. Blowing filters clean risks damaging them - by opening up gaps that will pass fine dust in the very fine fibres that are supposed to stop the dust getting through. He mentions using max 40psi and taking care.

As a filter blinds up the amount of pressure drop/back pressure it creates increases. Running while dirty is very bad for the filter because it apparently drives the dust hard into the fibres in ways that damage them, and makes them very hard to clean.

Cleaning is also related to the issue of the build up of the cake layer that improves filtration.

Deciding an interval for cleaning is going to depend on how much dust the system is having deal with (none if you are dumping it to exhaust), how good a job the cyclone is doing on dropping the dust out (i.e. how much is getting carried into the filters), what proportion of the available time the system is running for, how much filter area you have (twin cartridge systems have enormous area compared to bags and the like), and how well the filters (a) work (leakage is one way of getting a longer cleaning interval), and (b) control the rate at which the resistance to flow increases in response to whatever dust is building up.

Bill seems to figure that you need the ability to measure pressure at the inlet to the filters - using e.g a Dwyer gauge, or a manometer.

The baseline is the pressure recorded just after installing clean new filters. He seems to say that you can expect this pressure (seen while the filter is still clean) to increase over the first several cleanings, and to plateau when the filter is primed. (it's built up the layer it needs to perform at its best)

I've no idea to what level the pressure should be allowed to increase before cleaning the filter.

Far from being good news, the giveaway signal that the filters are at the end of their life comes when the the pressure seen immediately after cleaning having plateaued for a while starts to fall. This is presumably an indication that the media is starting to develop larger openings than is ideal - that pass the air more easily, but don't do a good job on fine dust.

There's a bit of set up involved in getting to the point of being able to do this - chances are that there's a lot of people out there with leaky filters congratulating themselves on how good a cleaning job thy have just done......

ian

PS I mentioned this before, but one possible theoretical advantage of installing your filters in a cabinet giving outside to inside airflow (IF that is using the blower to blow backwards through the filters provides enough pressure to be useful as for cleaning - it may not) is that it (by exhausting the filtered air through an outlet/duct end back into the shop) makes it a lot easier to connect an arrangement to reverse the airflow through the filters. Just connect a branch from the blower exhaust to the filter cabinet outlet using a flexible hose, and open the blast gate at the cabinet inlet (that normally sends the collected air and dust to exhaust insted of to the filters) so that the air passing backwards through the filters is dumped outside through the exhaust...

I guess the best way to use filters is to ignore them - to if you can exhaust all of your dust.

[Alan Schaffter]

Filter cleaning doesn't seem to be the most straightforward task, and th need for it is highly connected to cyclone performance on fine dust separation too.

I've no experience yet, but if I try to repeat from Bill Pentz's pages it goes like this. Blowing filters clean risks damaging them - by opening up gaps that will pass fine dust in the very fine fibres that are supposed to stop the dust getting through. He mentions using max 40psi and taking care.

As a filter blinds up the amount of pressure drop/back pressure it creates increases. Running while dirty is very bad for the filter because it apparently drives the dust hard into the fibres in ways that damage them, and makes them very hard to clean.

Cleaning is also related to the issue of the build up of the cake layer that improves filtration.

Deciding an interval for cleaning is going to depend on how much dust the system is having deal with (none if you are dumping it to exhaust), how good a job the cyclone is doing on dropping the dust out (i.e. how much is getting carried into the filters), what proportion of the available time the system is running for, how much filter area you have (twin cartridge systems have enormous area compared to bags and the like), and how well the filters (a) work (leakage is one way of getting a longer cleaning interval), and (b) control the rate at which the resistance to flow increases in response to whatever dust is building up.

Bill seems to figure that you need the ability to measure pressure at the inlet to the filters - using e.g a Dwyer gauge, or a manometer.

The baseline is the pressure recorded just after installing clean new filters. He seems to say that you can expect this pressure (seen while the filter is still clean) to increase over the first several cleanings, and to plateau when the filter is primed. (it's built up the layer it needs to perform at its best)

I've no idea to what level the pressure should be allowed to increase before cleaning the filter.

Far from being good news, the giveaway signal that the filters are at the end of their life comes when the the pressure seen immediately after cleaning having plateaued for a while starts to fall. This is presumably an indication that the media is starting to develop larger openings than is ideal - that pass the air more easily, but don't do a good job on fine dust.

There's a bit of set up involved in getting to the point of being able to do this - chances are that there's a lot of people out there with leaky filters congratulating themselves on how good a cleaning job thy have just done......

ian

PS I mentioned this before, but one possible theoretical advantage of installing your filters in a cabinet giving outside to inside airflow (IF that is using the blower to blow backwards through the filters provides enough pressure to be useful as for cleaning - it may not) is that it (by exhausting the filtered air through an outlet/duct end back into the shop) makes it a lot easier to connect an arrangement to reverse the airflow through the filters. Just connect a branch from the blower exhaust to the filter cabinet outlet using a flexible hose, and open the blast gate at the cabinet inlet (that normally sends the collected air and dust to exhaust insted of to the filters) so that the air passing backwards through the filters is dumped outside through the exhaust...

I guess the best way to use filters is to ignore them - to if you can exhaust all of your dust.

I think you are spot on in your analysis.

There are a lot of limited and often contradictory information and recommendations about cartridge filter design, how to clean, expected life, recommended flow direction, and impact if the manufacturer's recommendations are not followed. Frankly, I can't see how a basic spun bonded polyester filter could be fabricated so flow (not filtering) differs in to out vs out to in. In special filters, one side of the media is often coated to facilitate attraction and capture of dust like in most one-time-use HVAC or special filters that have a limited life and are then thrown away. Sometimes the media is coated with or made from a material like Gortex so cake is easier to remove or the filter can be washed with water like Clean Stream shopvac filters. But a .3 micron hole is .3 microns no matter what side you look at it from. The pleats are pretty much the same on both sides, just a little tighter on the inside. I think one of the worst ideas implemented by some DC manufacturers is the beater bar. That has to significantly reduce the useful life of the filter media. Reverse air flow and a vibrator have a lesser chance of damaging the filter. HVAC filters made from a metal mesh and coated with oil were used on Navy ships (still are?). On a regular schedule the filters were removed, cleaned in a tank, sprayed with fresh oil, then reinstalled. The principle was that any dust striking the oil would be captured by it and held there while the rest would pass on through and hopefully be captured later. That wasn't a problem since they weren't dealing with high concentrations of dangerous .5 -10 micron wood dust.

I've said this in other posts, but home shops and possibly some small commercial shops are the only ones who attempt to remove the chips and fine dust so they can return the DC air to the shop. Large commercial installations have big cyclones for the larger stuff and discharge the smaller stuff outside directly or through filter bag houses that are external to the factory. They have shaker mechanisms that run on a schedule or pressure differential. But they don't try to return the air to the factory!

From an operator not a filter-life perspective, if you want an absolute indicator for when to clean your filter(s), the answer is when you no longer have the recommended OSHA or EPA CFM at the machines, assuming you had it in the first place. The problem being, that number will be different for each machine and equate to a different acceptable SP loss at the filter. You either need to continuously monitor the CFM at every machine or find the filter SP drop at the point when the CFM at the most demanding machine no longer meets the recommended level. This effort is a bit beyond most hobby WW shops, especially since I suspect a vast majority probably don't meet minimum CFM requirements to begin with.

[Anthony Whitesell]

Frankly, I can't see how a basic spun bonded polyester filter could be fabricated so flow (not filtering) differs in to out vs out to in. In special filters, one side of the media is often coated to facilitate attraction and capture of dust like in most one-time-use HVAC or special filters that have a limited life and are then thrown away.

I can only comment on how "differs in to out vs out to in" and it's actually pretty easy. It has to do with stretching the material. If you over pressurize the filter in the wrong direction, then it causes the "pores" to stretch and become larger, obviously allowing larger particle to pass through. It's easy to design a filter to work outside in because the material isn't under pressure and the more pressure you apply the smaller it tries to make the filter and avoids streching the material/pores. Working this way also requires an extranal housing to hold the filter element. Building a filter to with inside out, the airflow is trying to stretch the material and pores and must be specifically design to support the filter and minimize the stretching. The upside is that it only requires a plenum of some sort to direct the airflow to the center of the filter (as us woodworkers typically see on our DCs)

[ian maybury]

What's a bit sobering about it all guys (and this is not to take from Bill Pentz's work - more to suggest that there is now more needed in this area) is that in the end the lack of hard data about on shop air quality and cyclone and filter dust retention performance in real world conditions means we're all pretty much flying blind. That we're just going through the motions - that in the end most of us haven't a clue as to how good or bad a job their system is doing, or what it truly needs by way of maintenance.

Ditto in the case of our buying decisions regarding cyclone and/or filter systems - there's almost nothing by way of reliable data out there on their performance. Lots of hype, claims, put downs and innuendo - but not much data.

Bill pointed out the issue, and even suggested cost effective means (Dylos particle counter, cyclone testing etc) - but the fact is that nobody has really taken up the baton and run with it. Not in the sense of generating data suitable for publication anyway. Least of all the mags and the business - neither of whom seem whether for fear of cost, liability or treading on toes (of advertisers, or other inhabitants of the glass house) seem prepared to apply scientific method, and be guided by the results.

The recent mag article on what was largely uncontrolled chucking of buckets of dust into a cyclone to see how long it took for the filters to block was a case in point - good at least to see cyclones clearly identified as massively slowing the blocking of fine filters, but in reality a measure of just how far behind the technical realities public perception is.

The prospect of the public system getting involved and possibly legislating/issuing directives for hobby/diy/sub-industrial woodworking air quality safety before we get our house in order is pretty scary - but if it was to happen there wouldn't be much scope to blame anybody else....

I may have posted the photo before, but I went for a filter cabinet and outside to inside air movement for two reasons: (a) my local Donaldson rep seemed to think this was what the filters were designed for, and (b) they were going in a tight space and I didn't want them exposed to damage.

Does it add anything to their performance despite it's having taken days of work to build?? I've not got a clue....

ian

finished filter cabinet.jpgfilter cartridge mounting.jpgfilter cabinet (open).jpg

[Alan Schaffter]

I can only comment on how "differs in to out vs out to in" and it's actually pretty easy. It has to do with stretching the material. If you over pressurize the filter in the wrong direction, then it causes the "pores" to stretch and become larger, obviously allowing larger particle to pass through. It's easy to design a filter to work outside in because the material isn't under pressure and the more pressure you apply the smaller it tries to make the filter and avoids streching the material/pores.

While possible, especially with recent developments in non-technology, a design like that or one which works like a check valve in operation is likely well beyond the complexity of typical cartridge filters at the DC filter cost, and not worth the cost. I bought my Farr style 9L300BL cartridge filters when they cost $62 ea. and thought that was outrageous. Now I see the prices are more than double that!

Simply stated, a filter is designed to allow air to pass while stopping particles. To do that, the filter must have media with holes which are small enough to stop the particles but large enough and in sufficient numbers to allow a large volume of air to pass. It is easy to see, depending on the number of holes and the amount of dust in the air, sooner or later most if not ALL the holes will be blocked and no air will pass. At that time the filter must be cleaned or replaced. Any other technology like electrostatic, just won't work with the volume and velocity of the air and volume of dust volume we deal with. It is the main reason almost all vac type filters have a very limited useful life before they must be thrown away or carefully washed.

The reality is filters are secondary to the main function of a dust collector- collecting dust at the source. They contribute nothing to that function, and more often than not, detract from it. So why pay money for something that causes a DC to be more complex and costly, and reduces its effectiveness?

[Alan Schaffter]

What's a bit sobering about it all guys (and this is not to take from Bill Pentz's work - more to suggest that there is now more needed in this area) is that in the end the lack of hard data about on shop air quality and cyclone and filter dust retention performance in real world conditions means we're all pretty much flying blind. That we're just going through the motions - that in the end most of us haven't a clue as to how good or bad a job their system is doing, or what it truly needs by way of maintenance.

Ditto in the case of our buying decisions regarding cyclone and/or filter systems - there's almost nothing by way of reliable data out there on their performance. Lots of hype, claims, put downs and innuendo - but not much data.

But just think how we have contributed to this segment of woodworking retail sales!!

[Anthony Whitesell]

While possible, especially with recent developments in non-technology, a design like that or one which works like a check valve in operation is likely well beyond the complexity of typical cartridge filters at the DC filter cost, and not worth the cost. I bought my Farr style 9L300BL cartridge filters when they cost $62 ea. and thought that was outrageous. Now I see the prices are more than double that!

Not quite folliwing you there, Alan. I am only speaking to filters in general. All filters in general. Filters are designed to have airflow in one direction. In that direction, the filter material is being compressed to avoid enlarging the pores during pressurization. In addition, they also have a support mechinism in the same direction. Filters are easiest to design for flow from the outside in (opposite of the DC filters) because the filter material is always being compressed. The filter support only has to prevent the filter from collapsing. To design a filter to work with flow inside to out, the material must be laid in a manner that it will be under compressed and the support will keep the filter from expanding. Unfortunately, keeping every square inch of filter under compressed while trying to blow it up loke a ballon is very difficult. Usually a leak will form. We use the latter in our DCs, so we must be wary of those aspects when deisgning, using, and cleaning our system.

Unfortunately I can only speak to the designs and to the compromises. I can't speak to the solutions. I work with many process guys where I have to control the system so as not to wreck their filter. That's how I've become knowledgable about filters. I've had to correct operations that did or potentially damaged filters in the system.