An underperforming pulse jet dust collector or baghouses is frequently the result of several issues but most often is the result of either very high dust loading, inefficient cleaning, or some combination of the two. Left unchecked, these problems can result in process bottlenecks and increased operating costs. And, unlike the paper jam in the office printer, you can’t just ignore the problem and hope someone else fixes it.
So, while this is not necessarily an exhaustive compendium of potential issues, listed below you’ll find 10 common and easy-to-handle action items that you and your staff can take to avoid costly shutdowns and frequent filter changes.
1. Red-Lining: Like keeping your car’s tachometer under the red line, make sure your collector is pulsing correctly at about 90-100 psi, but not above that level. The pulse duration should not be above 150 milliseconds, and the off time should not be less than 20 seconds (timing may vary based on application, but 20 seconds or more of off time is a good rule of thumb, and saves energy.) Pulsing more frequently or with more pressure doesn’t address the underlying issues, and will only serve to beat up your filters causing premature wear. Plus, it puts greater stress on your compressor, pulse valves, and your header tank (air reservoir) may not be able to refill in time before the next pulse, creating quite a downward performance spiral.
2. Air Flow: Air-to-cloth ratios (total ACFM divided by total square footage of filter media) do not really tell the whole dust loading story. Even with a relatively tame air-to-cloth ratio of say 3:1, if the incoming air is overloaded with dust or the dust is particularly nasty (carbon black, anyone?) then a lower air to cloth ratio may not help much. However, your filter supplier should be able to recommend the best filter media and proper air to cloth ratio to best handle your particular dust. If you have a fabric filter baghouse, a good way to reduce air to cloth ratios is to add more filter media with pleated filter bags. Your filter supplier can make recommendations on sizing calculations. Also, check your inlet air distributor/baffling and be sure it is intact and reasonably clean to be sure that airflow is being distributed equally to all filters, not just the ones closest to the inlet.
3. Cleaning Air: Poor cleaning performance is easy to recognize, and typically is the result of an undersized or restricted cleaning air system. Ensure the size of the header tank matches the cleaning requirements (the header tank size is determined by the number of valves, valve and inlet line size, and OEM of the collector; your filter supplier can help with this). There should be a compressed air filter/dryer ahead of the reservoir, and no restrictions from the compressor to the reservoir. An auxiliary pre-regulator or receiver tank of air can be added in cases where the cleaning air pressure is less than adequate. If all those items look good, the next item to check is the pulsing as controlled by your timer board or control system.
4. Pulsing: The pulsing frequency can’t be faster than the ability of the compressor to refill the reservoir tank to full pulsing pressure of 90-100 psi, so be sure to test this by checking pressure throughout the day. Also, be sure you hear a sharp crisp blast of pulse air. If there is a long pause between pulsing rows, and/or one valve that doesn’t sound like the others (a low and slow sound rather than the sharp and fast sound of a properly operating valve) then you may have a bad diaphragm or solenoid valve, or both. Fortunately these valves are not expensive and are fairly easy to replace. Again, a proper pulse sound will be well under a second in length. If it’s longer than that, check the valve and the timing setting.
The next issue to try to minimize is dust re-entrainment, so it’s time to look at your pulse sequencing and dust evacuation methods.
5. Pulse Sequence: Stagger the pulse sequence so that newly cleaned filters don’t take in dust from the neighboring row of filters being pulsed. A filter row cleaning sequence of say 1-3-5-2-4 is preferable to 1-2-3-4-5.
6. Hopper: Never use the hopper for storage. Evacuation equipment (rotary airlocks, screw conveyors, etc.) should be sized to unload the hopper before any accumulation occurs. Units with slide- gates should be equipped with sealed drum adaptors at all times, with the slide gate open to the drum. This issue pops up frequently and it’s usually an easy fix.
7. Emissions: If you are seeing dust emissions, a number of causes could be at fault. Some common causes include a production change that is causing a higher concentration of submicron particles in your inlet dust, improperly installed filters, a cracked tubesheet, etc. You can find a leak path by conducting a fluorescent powder test. If there are no leaks and the process dust is getting through the filter media, then upgrading to a more efficient and stronger filter media like one with ePTFE membrane will help. Your filter supplier will have details on the best media option for you, along with multiple colors of leak detection powder.
8. Media Types: All dust types have specific characteristics and require different handling. Some of the factors that may influence the right choice of media include humidity, temperature, dust conductivity, acidity, stickiness, etc. If you are reclaiming the dust, utilize the surface loading properties of a spun bond polyester. Sometimes you will need an enhanced media with a surface treatment like an ePTFE membrane, nano fibers, oil- and water-resistant coating, or a conductive surface treatment among others. Again, a review of your system and needs with a qualified filter vendor will get you pointed in the right direction.
9. Drop Out Box: The drop out box is the distance between the bottom of the filters and the hopper. The greater the distance, the better the conditions are for the heavier dust particles to be dropped from the airflow into the hopper instead of contacting the filter surface. To optimize this, consider replacing long fabric filter bags with pleated bags which are shorter, as well as having more surface area. This increased drop out box area will also remove the bottom of the filter from the abrasion zone. See your filter supplier for correct sizing information. (See Fig. 1)
10. Evaluate Your System Fan. Be sure to measure the fan’s airflow volume and velocity from time to time, and adjust as needed to be sure that the cleaning air can overcome the inlet air. “Nesting” can also be an issue, where dust accumulates and nests at the top of the filters on the underside of the tubesheet. Nesting is frequently the result of a combination of light dust and high interstitial velocity.
While you are at it, this is a good time to check all duct work, slide gates, and damper openings to be sure there are no slipping belts or clogged ducts along the way. Dust-laden air doesn’t flow to the collector easily; it flows kicking and screaming all the way, so reducing the number of duct bends and turns along with keeping ductwork clean will go a long way toward overall system performance.
As mentioned earlier, the above checklist is not an exhaustive list but a routine periodic check your team can perform monthly (or quarterly for smaller systems) to ensure the system is optimized for your operation. A more efficiently tuned system will result in reduced energy consumption, fewer emissions, and less frequent filter changes. You will also see more efficient dust pick up, longer lasting equipment and filters, and better overall company bottom line.
So while you may be tempted to ignore the paper jam in the printer and hope someone else fixes it, don’t ignore little dust collection issues that can turn into big issues in a hurry. Besides, nobody else at your company will walk by and fix the dust collector for you.
Matt Gorecki is North American director of sales, and Paul Rzepka is product and technical manager, Nordic Air Filtration - North America. For more information, visit www.nordic-air-filtration.com.
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