Cleaning Dust Collection Bags
From contributor S:
I have no hard info to back up this statement, but I was under the impression that the dust cake that forms on the interior surface of the bags actually helps the filtration process by acting as a sort of pre-filter. Logically this can make sense, though I suppose the argument could also be made that it restricts the airflow so it might hamper the efficiency of the bags.
In either case, we have never cleaned or washed our bags. We shake them out periodically of course, but that's it. We have 16 bags (4 per collection chamber) and we've never noticed a drop in efficiency over the last 4 years.
From contributor H:
Just shake them or blow them out whenever you empty the containers. Don't wash them. It makes them thinner and less effective. Where do you think all the lint from a drier screen comes from? Not from the drying process; it is produced in the washing process. Drying just releases them from their base.
From contributor B:
What kind of bag-filters are we talking here? Bags from single-stage dc's, bag-house dc? Do you know what cloth is used in those filters? Needle-felt? Woven or knitted polyester? Beane*Bag? What waste were these bags filtering?
Off hand, I'd suggest vacuuming the bags thoroughly if that is practical, or use compressed air to blow out the filters. I'd follow the manufacturer's recommendation for cleaning.
Some filters, such as the Beane*Bag filters, claim to retain up to 90% of their original filtering capability after 6 washings. These filters use a patented loop-pile knit, and offer air-permeability to 55-65 cfm per sq.ft.
Regardless of the fabric type used in your filters, at one point, "blinding" will occur. This is a situation where filtered waste becomes permanently imbedded in the fabric, negatively affecting the performance of your system. At that point, the filters will require replacement.
From contributor R:
I did clean it in the washing machine from time to time and each time I am surprised of the efficiency of vacuum. Manufacturers don't sell dirty bags. Air needs to flow through the material. A clean bag gives me more than twice the vacuum power. I must write that I don't have many.
From contributor E:
I have been going through a similar problem with our collector and did not realize until recently how much the buildup of cake on the inside of the bags was affecting the performance of our DC system.
Most bags supplied with collectors only filter down to 30 microns, which means they work great when you first buy them, but as the dust builds up inside, the collector becomes more inefficient because the impeller can only pull in as much cfm as it can push out through the bags.
A colleague's solution was to switch over to custom-made oversize shaker-felt bags that filter to one micron. With the smaller holes, even after the cake builds up inside the bag, there is less of a falloff in performance over time. The manufacturer specializes designing bags based on the unique specifications of your system.
I did not realize until looking into this issue how undersized (in terms of cfm/square feet of bag) the bags supplied by the manufacturers were. Also, the upper/lower bag designed units lose efficiency as they fill, because of the lessening available area in the lower bag.
The point is that if you are frequently having to clean your bags to maintain performance in the system, this might be a better way, and also reduce the ambient dust in the shop (assuming your collector is inside). There is an in-depth explanation of what I am talking about at the manufacturer's website (americanfabricfilter.com). We are going to be trying this out with a new oversized shaker felt bag on our collector this week. We'll see if there is an improvement.
From contributor B:
You should see a significant improvement by switching from the 30 micron filter-bag to the oversized 1 micron needle-felt bag. By increasing the surface area of the filter, you'll reduce the interstitial velocity of the air (air velocity through the fabric). This will enhance your fine filtration spec. The slower those particles travel, the easier it is for the filter to capture those particles.
Air permeability of the needle-felt bags is much higher than the polyester weave 30 micron filters. Filter-bags such as the Beane*Bag filters offer up to 55-65 cfm per square foot, and also offer you enhanced particle release.
30 micron polyester weaves have a very low air-permeability rating, and are more susceptible to high pressure-drops. Here's why. Those fabrics are usually comprised of a single layer of woven polyester threads. Air can not flow through those threads, so it flows through the gaps between the threads. Back-pressure is higher with this type of filter, and this pressure can easily force smaller particles through the gaps of that single fabric layer.
When you take a very close look at those threads, you'll notice that each thread is comprised of hundreds of very tiny fibres. Well, in a needle-felt filter, there are no threads. Each of those tiny fibres are overlaid one atop another, then run through a needling press. The needle severs some of the fibres as it passes through the dense layer of fibres. This splays the ends of the fibres, like a "Y". Each leg of that "Y" interlocks with adjacent fibres. This eventually results in a thick fabric of specific weight per yard. That might be 8oz, 10oz, 16oz, etc. A 16 oz needle-felt filter is about 1/8" thick.
When you mount this type of filter to your dc, you'll immediately notice that there is lower back-pressure, or "pressure-drop" at the filter. Air can now flow around each of those tiny fibres, over the entire surface of the filter. The thicker the fabric, the more circuitous the route of travel, and the more opportunity to capture that very fine waste.
PS. The Beane*Bag filters use a patented loop-pile knit rather than needling. They claim enhanced particle release characteristics, along with air-permeability ratings of 55-65 cfm per sq. ft. "Tube" construction, means there are no seams to cause potential leaks.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor O:
Cleaning will not harm or reduce the thickness of the filter. This happens by abrasion of the media by the wood shavings.
A cleaned filter should flow at 22-35 CFM. A MAG gage is a very simple and inexpensive way to check your systems health and determine when the filters are in need of cleaning or replacement. Finally, Beane filters do last a long time and are easy to clean, but most shops do not find the cost of these filters justifiable.
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