Thread: How long does it take to "condition" a DC needle felt filter

During my DC testing I never really took into consideration about how old the bag was, or whether it was "conditioned" or not, although checking back on my notes, none of the DCs were newer than 2 months and all had some sawdust in them. Last week I went to measure Jonno2000's new modified 2HP DC flow and while I was at it I thought I would measure his filter efficiencies and was surprised at the amount of dust that was coming through his virtually new pleated filter and his new needle felt bag. The amounts of sawdust in the plastic bag was maybe a couple of litres but what was coming out of the filters was between 8 and 40 times what was going into the DC. I measured the efficiencies several times and notice they were dropping slowly but I didn't have time to hang around and measure what would happen over a longer period.

Back at home I have this never used 2HP DC from the milling yard that I modified to improved the flow rate and this morning I realized that it would make a good candidate for testing how long it would take for the needlefelt bag to become conditioned. That DC has been run mainly up till now without the bags but I have run it with the bags (without any sawdust in it) for about 15 minutes so it is effectively a new bag.

In the first test I tested the filter efficiencies without any sawdust in the bottom bag and measured the filter efficiencies over time just using background dust. The filter efficiency is the ratio of the amount of dust coming out of the filters compared to that going into the DC. The different coloured lines represent different particle size ranges in microns and negative values represent the filter releasing more dust than it is collecting.


Nothing surprising here - the needle felt bag takes a couple of minutes before it starts filtering all particle ranges sizes but even after 12 minutes the smaller particles are still not been filtered to better than 50%. By way of comparison a conditioned needlefelt bag will filter better than 85% for all particle size ranges, and >95% for 0.7 microns or greater.

Then I added a couple of litres of sawdust to the bottom bag and once again measured the amount of dust coming out of the bag and compared this to what was going into the DC..



Now even though the DC has been run for approximately half an hour without any dust inside the bottom bag, and the filter was showing positive efficiency for all particle sizes, significant amounts of dust still initially came out of the needle felt bag.
By significant I mean around 2 times more dust was coming through the bag compared to what was going in.
Initially the efficiency jumps around a bit probably reflecting the composition of the dust and the initial effectiveness of the filter at various size ranges and the ability of the swirling mincing action of the DC to create particles of different sizes.
What the graph does show is that it takes around one hour for a generic DC with sawdust in it to stop emitting more dust than it takes in. If your DC starts out life collecting a lot of sawdust then it may make a real mess of your shed until the bag is conditioned and it is still not really conditioned in terms of the finer particles after running for 3 hours.
The filter still has some way to go before it is conditioned because the larger particles should be able to be filtered to >95%

Unfortunately I could not let it run any longer as it was getting late and the noise from the (un-sound proofed) DC was getting a bit loud for the neighbours - I had the 6" inlet poking out of the door and the sucking noise was very noticeable at the fence line. I could not run the DC for that long inside a closed shed as it self scrubs the air so the filtered air has to be vented and it needs to continually sample fresh "dusty" air. I did this by continually venting the shed with my 3HP unit.

So whats the bottom line here?
Firstly it is yet another reason to put the DC outside the shed.
Secondly if you cannot put your new DC outside on a permanent basis but could take it outside for a few hours I would recommend loading it with a few litres of sawdust and taking the DC outside and letting it run for a long as you can to condition the bag.

Next experiment is what happens if a conditioned bag is washed?

It would be interesting to see how long a PF takes - anyone care to loan me a PF?