Tough Welding Fume

Service Report; 1201

Location: Cascade Canada, Guelph, Ontario.

Equipment: two Torit model DFT 3-18, tandem cartridge dust collector, self-cleaning pulse jet style.

Application: welding and cutting shop

Description: Client wanted to maximize the capacity of the dust collectors to meet the needs of their shop. These collectors were purchased second hand. A complete survey of the shop revealed that, in one case the collector was slightly undersized, and the other barely made it. It was noted to the client not to go by the catalogue CFM performance for these units. These ratings are always overstated and upon questioning Torit, they will advise the real performance of the collector. In this case, it was 5500-6000 CFM. The fan was sized to provide up to 10” WG of pressure. We judged that we had enough fan to do a level 1 retrofit of the collector to get them to the 7000 CFM they needed and reduce filter maintenance by 67%.

Problem: Shortly after starting up the dust collector, the pressure drop rose quickly to over 10” WG. The start-up pressure drop was 0.5” WG (better than expected). The filter cartridges were heavily loaded and full of dirt.

Investigation and Resolution: We checked that the retrofit was done properly, and it was. However, these dust collectors were purchased second-hand and the cleaning systems were defective. The client had to completely refurbish it. We removed the polyester spun bond pleated cartridge filters for inspection. They were heavily bridged with dust and welding fume. When using a compressed air hose with a good nozzle to manually clean the filters, very little air would blow through to remove the dust. We blew the cartridge from the dirty side and the dust blew off easily and completely. However, there appeared to be a staining on the filter media.
We sent a cartridge out to be tested. What we found in the test of the current filters;
•    The permeability test (ability for air to travel through the filter media) revealed that the filter media (spun bond polyester) was totally blinded.
•    Water cleaning only restored the permeability by 35%. Therefore the blinding dust is not easily water soluble.
•    Solvent cleaning restored it to 60-70 %, indicating it is solvent soluble but not totally. However, if the lab didn't allow enough time (48 hours) for the media to dry, that could explain that the media would be swelled some when they checked it. This tells us that we are possibly dealing with something hydrocarbon based.
•    Dry vacuuming restored it to 90-95%. This could mean a very fine dry dust that squeezes into the larger pores of the polyester media but may not fit in the tighter pores of paper media and would sit on the surface. If that is the case, it would blow out when we try it.
•    Therefore it was decided to test two 80/20 paper cartridges in the dust collector for a week.
•    If the paper filters are no better, then we have a problem with the fume and the solution will be to use a "pre-coat" on clean filters to prevent this difficult material from getting onto the media.

At the end of the week, As a result of the investigation mentioned above, I make the following comments:
•    We pulled the paper filters out and tried the blow test with a good nozzle on the air line. By simulating a pulse (quick short burses), the air seemed to go through adequately to clean the filters.
•    There was nonetheless a residue on the filter. In my opinion, this is attributed to the very fine hydrocarbon like nature of a component of the collected fume. However, there was very much less residue than with the polyester media. We took out one of the polyester filters, for comparison, and ran the same test. That one was completely blinded and no air got through.

•    After evaluating the test done by the lab and the ones we did on-site, our conclusion is that there is an unusually fine fume, although dry, exhibiting hydrocarbon-like properties, which is blinding the larger pores of the polyester filters but not as much the smaller pores of the 80/20 paper filters. This fume seems to imbed itself in the polyester media, making it impossible to dislodge, but for the most part rides on the surface of the paper media. This is a factor that we could not predict at the beginning of this project.
•    To confirm the unusual nature of this fume; when I washed my hands, the dirt on the surface washed out with a normal wash but there was something imbedded in my finger prints. After a second intense scrubbing that material did come out.

Our recommendation is to replace the polyester filters by 80/20 filters. However, it is not sufficient to leave it at that. I expect the filters will still clog in time and they are not washable (no matter what anyone may tell you). Paper expands when wet and does not restore itself, so you see similar characteristics after a wash, as we see with the polyester filters. Therefore, we also recommend using a "pre-coat" inert material. I must re-calculate the filter specifications since paper filters have a lower permeability than the polyester. I still want to keep the wide pleat spacing but can not have them as wide with paper as we had with the polyester filters. I am also investigating the use of "nano-fibre" coated media (somewhat like Torit Ultra-Web). I'm not particularly enamored with this stuff, but if they can convince me of its value in this particular case, it may be an alternative to pre-coating.

Read more about ... Baghouse retrofit service, or, Cartridge retrofit service