The first suspicion for pressure drop problems on arc spray is that the spray is coating the cartridge, since the compounds have not lost there coating characteristics before they reach the surface of the media and seal or partially seal the surfaces. This is referred to as “painting effect”.
The symptom of this is that the pressure drop goes up quickly within 20 minutes. It does not recover from on line cleaning. The pressure drop for off-line cleaning quickly ratchets until the you get to the top of the hump in the fan curve.
We ran some tests on an arc spray operation in Atlanta about five years ago where we installed a pilot unit with varying lengths of pipe and found on that operation that it required 0.8 seconds of transit time from the spray gun to the surface of the cartridge filter elements, to prevent “painting”. Other reports that we received from Torit installations reinforced our conclusions. They sprayed aluminum, titanium, nickel, and ceramics. They were coating turbine blades for jet airplanes.
The observation of very fine dust floating in the collector, when the fan was off, can be very significant. It would be important to observe whether the same was evident in the clean air plenum. If it was, I would want to question the efficacy of the filter seals. The next possibility that this observation raises is whether the dust agglomerates on the media surface and/or stays agglomerated during the cleaning cycle. If it does not stay agglomerated during the cleaning cycle the coarser agglomerates will fall into the hopper and the finer fractions will return to the surface of the filter media. This will cause a ratcheting of the pressure drop at each cleaning cycle.
It was also observed a gradual increase in pressure drop during the arc spray operation. This can be caused by the lack of agglomeration as described above. However if this is the cause, the collector should return to initial pressure drop after an extended cycle of off-line cleaning. We would say that you need to put fifteen minutes off time and pulse the collector four times.
Other processes that might have some of the symptoms for arc spray
Oil in the compressed air. If they have a screw compressor there is a filter that comes with the compressor. Sometimes the installer forgets to put it in. With pneumatic cylinders, the air line cleaners are sufficient to allow suitable operation.
Moisture in the compressed air line. Here again they should have a refrigerant or a desiccant dryer.
Condensation on the media surfaces because of the cooling of the cleaning air jet. If the latter is the case, the pressure drop increase will generally be apparent more likely in the morning when the difference between the dry and wet bulb temperatures are most likely to occur.
Replacing a third of the cartridges is generally not a good idea. It is also important which ones you replace. If they are tandem cartridges I would replace one on each tandem set. Personally, if I were to replace cartridges to run a test I would replace the ones furthest from the pulse-jet orifice/nozzle. When you do that, you do not even need to initially clean off-line. I figure you are wasting most of your media in a tandem configuration so when you replace cartridges on a trouble job, it is smart to replace only the outer one.
We need to determine if the cartridge is plugging from the dirty side or the clean side. We assumed it was from the dirty side but the clean side is a possibility. We usually check to see if there is any dust in the clean air plenum. If there is, the problem may be the seals. You may need special seals for this fine submicron dust.
You can always send us a used cartridge for a lab test. Make sure you seal the opening(s) to the clean air side. I usually use good ol’ duct tape.
For assistance with a trouble job… Technical assistance with dust collectors