New Engineering Insights of Pleated Cartridge Dust Collectors

Cartridge collectors have been available for over 35 years and have revolutionized the continuous cleaning pulse jet collector technology. They have reduced the emissions coming through dust collectors from typical processing operations like material handling, weld fume venting or an abrasive blast operation.

Pleated cartridge dust collectors reduced emissions coming through the collector outlets from 20-30 milligrams per cubic meter to 0.030-0.040 milligrams per cubic meter with inlet dust loadings from 900 to 1200 milligrams per cubic meter.

To recognize the new insights, it is useful to understand how this breakthrough was achieved.

Background

Pulse jet cleaning collectors with envelop or cylindrical bags, designed prior to 1978 were similar in that they used so called venturies in the bag cages.  The venturi squeezed the reverse flow cleaning jets to accelerate the cleaning jet entering the filter elements of felted media. Referring to figure 1 below, the result was that this high velocity jet ejected the dust from the filter cake on the media and propelled it towards adjoining rows of bags, which were in the filtering mode. These velocities ranged from 28,000 to 40,000 feet/ minute. As long as the pressure drop is below 2”WC across the media the collector runs with very high efficiencies, similar to those for a cartridge or mechanical shaker collector. This low pressure drop usually can be achieved with low density dusts such as fine paper dust. For other dusts, the pressure drop will rise to 4-6”WC which indicates a velocity at which it can travel. At  6”WC this velocity can be traveling up to 28,000 feet per minute with 50 lb/ cu.ft. dust density.

Figure 2 shows the action on a pleated cartridge. The air and dust is ejected from the media cake surface perpendicular to the surface. The dust is directed toward a surface that is in the cleaning mode so the dust will not penetrate through the cake. This is the reason why a pleated filter element can reduce the dust penetration by over 98% compared to a standard felted pleated bag. Figure 3 illustrates the effect of the pleated element when the filter is not designed to clean effectively. The dust collects in the valley of the pleats. When the reverse jet is activated the cleaning air takes the path of least resistance. The portion of the pleat below the bridge is not cleaned and the dust remains on the cartridge.

It has been believed erroneously for over 25 years that operating a pulse jet cartridge filter at a low filtering speed increases the collection efficiency of the filter element. There is an element of truth in this belief. The truth is only the filter area above the pleat is able to be cleaned. If a pleated filter element bridges 80% of the depth of the pleat, it is actually operating at a filtering velocity 500 % higher than the engineered filtering velocity. Not only is the filter media under the bridge not cleaned it raises the operating weight radically. Many pulsed collectors have filter operating weight of 65-80 lbs higher than the virgin filter. The operating weight for a cartridge filter with 360 square feet and a 1/64 inch thick filter cake, with density of 50 pounds per cu.ft, is 25 pounds. In a typical collector rated at 7000 ACFM with 20 cartridges, and ten valves, the excess weight of the dust is 800 pounds or so. Replacing the cartridges exposes personnel to health hazards during handling.

The correct insight is that the pleat spacing should be wide enough so that all of the media could be cleaned by the reverse air cleaning jet. Extensive lab and field tests have uncovered that 220 square feet of media can be cleaned by a 1-inch diaphragm valve at two inch pressure drop and a permeability of the media of 20 CFM per sq.ft. This means that for the tandem cartridge set, described above, where the cartridge set has 360 sq. ft. with 16 pleats per inch, only 75 percent of the media can be cleaned.

Figure 4 illustrates the results of applying this design into a typical application over time. These depictions were drawn from photos taken a week apart through an access door. The white portions of the sketches are the cleanable media.

For best operation these cartridges should have 25 % of the pleat spacing or 3.5 pleats per inch. Figure 5 shows cartridge designs with good pleat spacing of 1/4 and 1/2 inch.

Media Selection
There are two types of media that are usually applied to pleated cartridge filters.  These are cellulose based media, often reinforced with synthetic threads, and spun bond polyester media.

The cellulose media usually have sufficient stiffness to keep their shape without pinching in the tops or in the valleys of the pleat so that filtered air can flow unimpeded into the cartridge during flow reversals. They are usually banded or wrapped with an outer expanded metal or perforated core to prevent the pleats from inverting during the changes from filtering flow to cleaning flow. One limitation of cellulose based media is that humidity affects the dimensional stability of the media. As the humidity changes the length of the pleat changes enough that the pleats get curved and sometimes even crease. These are stress points that can cause premature failure from the cycling of the cleaning system. While cellulose media can be laundered, each laundering cycle changes the permeability of the media. It is only effective to launder these cartridges twice before 60% of the permeability is lost and the cartridge reaches the end of its useful life.

The first spun bond medias, as applied to pleated cartridges, were very flexible. When first started and operated at pressure drops below 1”WC, they were very effective even at filtering velocities of 12 to 14 feet per minute. Unfortunately, as the pressure rose, the tops of the pleats collapsed and had the same effect as bridging except in reverse. The tops of the pleats were rendered useless. The spun bond media could be laundered indefinitely with no loss of permeability. Recently a new spun bond media has been available. This new spun bond is constructed of a spun bond which is stiff and does not deflect. This allows the collector to operate at a low pressure drop with unlimited cartridge life and can be returned to “like new” condition by cleaning off-line. Spun Bond cartridges can also be laundered an indefinite number of times making them permanent filter elements.

Pressure drop                         Compressed air usage at 85 psig
(across the cartridges)             (needed for cleaning)           

0.90 inches w. c                        0.4 SCFM per 1000 CFM of filtered air
1.5 inches w. c.                         0.45 SCFM per 1000 CFM of filtered air
2.5 inches w. c.                         0.90 SCFM per 1000 CFM of filtered air
3.5 inches w. c.                         1.20 SCFM per 1000 CFM of filtered air

Conclusion
The designer can select and specify pleated cartridge elements that are smaller in size and more efficient by applying the engineering insights listed above to the collectors he operates. Even existing collectors can be modified by changing the cleaning systems and installing the optimum configuration of pleated cartridge design.

For more on ... Advanced Technology Dust Collectors

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

Combustible Dusts

(Aluminum, Magnesium, Niobium, Tantalum, Titanium, Zirconium)

These dusts are highly combustible and present a very significant explosion hazard. There are some stringent fire codes dealing with these dusts which draw their regulations, for the most part, from NFPA 484, Standard for Combustible Metals.

Unfortunately, most end-users are not aware of these standards or safe methods of dust collection for these dusts. Worse, the dust collection industry is very negligent in guiding these people with proper and safe applications engineering. This document is an attempt to provide some of this valuable information.

First of all, we strongly encourage readers to obtain a copy of NFPA 484 and comply with it. There are far too many stipulations which go beyond the scope of this document. A copy of excerpts pertaining to each type of dust can be requested from Quality Air Management.

We will analyze the use of different dust collection methods to these dusts:

1. Dry Dust Collectors; include baghouse (both mechanical cleaning and pulse-jet self-cleaning), cartridge or pleated filter collectors, disposable media filters, electrostatic precipitators, cyclones.
2. Wet Dust Collectors; there are many styles of wet collectors available. The pro’s and con’s of each type is beyond the scope of this document.

In all cases the blower for drawing the dust-laden air into the collector shall be located on the clean air side of the collector. The dust producing equipment and dust collector must be

Mixing of Metals is not permitted, unless the entire system is disassembled and thoroughly cleaned prior to and after its use. A placard must indicate, for example, “Aluminum Metal Only - fire or explosion can result with other metals”.

Wet collectors are designed specifically to be used for all these dusts. These collectors are designed for collection of metal dust only, not for powder, smoke or fumes. The use of additional dry filter medium either downstream or combined with a wet collector is not permitted. Contact QAM technical support for a safe method to handle these contaminants which the wet collector can not handle. The cleaned air can be recycled to the work area if the collector is efficient enough to ensure safety of personnel. A provision for an unimpeded vent, when the machine is shut down, must be provided. Magnesium dust requires a powered positive venting of the sludge tank at all times during shutdown of the collector.

Dry collectors are allowable for aluminum, niobium  dusts, but are prohibited for all the other dusts. They must be located outside buildings. Filter media must dissipate static electric charge (be aware that grounded conductive media gives a false sense of security). You must avoid accumulation or condensation of water at all costs which could cause a hydrogen gas explosion. Explosion vents must be provided. Recycling of air into the building is prohibited.

Mechanical shaker style collectors; are highly susceptible to static electricity charges, and explosions.

Baghouse collectors; there are conventional designs, sold by 95% of dust collector suppliers, and new advanced technology designs.
Conventional; Due to the inefficient clean systems, only 10-20% of the filter media ever gets clean. This allows dust to accumulate in the collector beyond what is permissible by NFPA 484.
Advanced technology (i.e. Ultra-Flow); These are designed to clean 100% of the media on a regular basis. The cleaning frequency can be set to maintain a cleanliness that meets NFPA standards.

Electrostatic Precipitators are prohibited because they filter the air by applying an electrostatic charge across the air stream. That is a source of ignition and the dust will accumulate in the unit and coat the collection plates. This is a prescription of a very large and very load BOOM (explosion).

Cyclones; high efficiency models can be used for these dusts but must be located outside the building. Explosion vents are permitted. Recycling of air into the building is prohibited.

Read more on ... Wet dust collectors

Valve Problems; Pulse-Jet Dust Collectors

The collector was shipped with the pilot solenoid boxes being shipped separately. The service engineer came to start up the collector. When the compressed air was piped up and the main valve opened, the pressure would not build up in the manifold. Examination of the piping revealed that the wrong ports in the solenoid valves at the solenoid enclosures were connected to the valves. This kept all the solenoid pilot valves open to atmosphere causing all eight diaphragm valves to open at the same time. The compressed air supply was not sufficient to allow even one diaphragm valve to be opened continuously. The connection is sized to provide air to supply a single valve to run no more than a 10% duty cycle.
The service engineer connected tubing to the correct ports from the solenoid valve enclosures.
After that was complete, the compressed air supply was again turned on. Again the pressure in the manifold would not increase enough to pressurize the manifold. From the sound of it, at least one diaphragm valve was open. To determine which one(s) was the culprit, He checked the ports that were open to atmosphere and found the solenoid valve that was open. He squeezed the flexible tubing leading to that valve and the valve de-energized and pressure built up in the manifold to 85 psig. He replaced the tubing and the collector started pulsing.
He next listened to the pulse. It had a hissing sound when the diaphragm valve was opened. The gauge on the manifold dropped to below 15 psig. Both of these symptoms indicate that a valve is open too long. (On the control panels it is usually a pot adjustment and labeled “on time”). This “on time” should be adjusted to the minimum time that shifts the diaphragm valve. All cleaning of the bags takes place in 5 milliseconds after the valve completely opens. It takes a valve 10 to 70 milliseconds to fully open depending on the valve used. The sound should be more like a thumping noise. The pressure in the manifold after each pulse should be no less than 50 psig. This proper adjustment of the “on time” often reduces air consumption by 50 to 90%.

Other General Considerations

Another cause of these symptoms may be debris in the compressed air line. In connecting the compressed air supply to a pulse-jet collector often new piping is installed. In the process of threading the pipes and installing fitting shavings may accumulate in the piping. Sometimes the air line filter is installed far away from the dust collector. If these shavings get into the collector valves they may clog up the internal vent port in the diaphragm valve and get into the actuator on the solenoid valves causing them to stay open. Before the piping is hooked up to the manifold on the collector, the pipes should be blown clean. The pulsing may cause the shavings to gradually move down the pipes and not show up until a few days after start up.

Another possibility occurs when it is noticed that diaphragm valves require frequent replacements of the diaphragms and the spools in the pilot solenoid valves also require unusually high replacement. It is possible the air line lubricant has the incorrect fluid and will attack the sealing materials. These valves are good for at least 100,000 cycles which is 4-5 years for a single shift operation, 5 days per week.

Read more ... Consulting and troubleshooting dust collector problems

Quencher (spark arrestor) with Plasma/Laser Cutting

Some people have used Quenchers, and other style spark arrestors in plasma and laser cutting applications but still experienced fires in their dust collectors. In the majority of cases, the Quencher alone is sufficient to control dust collector fires. However, in some isolated cases, sparks are only one issue to deal with these applications. A good spark arrestor is definitely needed to stop sparks and embers, but, it is no guarantee against fires in the dust collector.

The problem:
1.    The operator may have to reset the heat setting of the plasma head. It could be generating too much atomic static particles. This causes a "painting" effect on the cartridge media, eventually clogging it.
2.    Large heavy particles of molten metal can be generated in the process.
3.    You should use spun bond wide pleat cartridges, to ensure proper clean out of the cartridges. That way the dust will spread over a large surface of media, instead of on the outer surface only.
4.    Current cartridges that are clogging over time (can vary from hours to weeks, depending on loading). When clogging occurs, the air flow drops and sparks can slip through any spark arrestor (not just the Quencher). This sets fire to the combustible dust accumulated on the surface of the cartridges.

Normally, plasma cutters have different characteristics depending on the settings of the cutter torch. The quantity of dust produced is relatively small. At some torch settings the dust is reactive by initiating an atomic bond between the dust and the surface of the cartridge, forming a hard durable impervious coating which totally or partially plugs the filter media. This mechanism is an inherent part of the plasma coating process to put wear resistant coatings on shafts, turbine blades etc. that allow the parts to receive very long lives. In the plasma coating machinery, the key to collecting the overspray in cartridge or fabric collectors is to allow the atomic bond to dissipate. This is accomplished by extending the time that particles travel from the torch to the filter media elements. In plasma coating systems at this time, depending on torch settings will vary from 0.5 to 0.8 seconds depending on the metals being sprayed.

In plasma cutting applications often the dust being emitted from the torch does not require any special considerations. In fact, collectors can operate for many months quite well with moderate pressure drops. Then the torch settings are changed because of various factors such as the composition or thickness of the pieces that are cut. As the settings of the gun or the speed of the cut is changed, the dust can act as a plasma coating torch and the cartridges start plugging. Sparks are often produced. If the dust is combustible the sparks may ignite the coating on the cartridges. Normally the fuel on the cartridge surface is not very heavy so the fires do not damage the housing of the collector. The cartridges are then usually replaced. The QUENCHER spark arresters are sometimes applied to limit the risk of fires and extend cartridge life. In the tandem horizontal type collectors, the cartridges are usually tight spaced, so, as the pressure drop rises, the pleats are pinched in the valleys so the pressure drop goes up. Combustible dusts can put pounds of dust to be stored in the cartridges to fuel a fire in the collectors. However, the squeezing of the pleats also causes pressure drop to increase and slow the flow through the dust collector. This often allows dust to be released into the work area.

Although spark arrestors will protect the system from sparks, pieces of molten metal go through the spark arrestor unaffected. These heavy, hot particles lodge on the surface of the cartridge and ignite the combustible dust coating. The heavy molten particles need to be dropped out of the system prior to the spark arrestor and collected safely, so as not to cause a fire in that collection device. Cyclones and drop out boxes are sometimes used for this. However, be aware that these devices have little effect on sparks / embers which are light buoyant particles and slip through to the dust collector.

An excellent example of these effects was the experience of the Day division of Donaldson who supplies this design. In cutting the filter mounting plates for their design they plasma cut holes in a 1/4 inch thick plate. They found that the filters plugged quickly in the after filters. They added distance in the filters venting the operations. This experience occurred 20 years ago and we do not know how this operation is now performing.

Our recommendation is to replace the current cartridges with a wide spaced stiffened spun bond media carried and pre-coat the cartridges with a 1/64 inch thick coating of inert pre-coat material.

We suggest you send each job application data (layouts & pictures) to QAM technical support at gary@qamanage.com and/or call 800-267-5585. We’ve dealt with plasma cutting applications for decades and feel that yours would be a common problem. If you contact us, we'll be happy to work with you on this.

More about... Spark Arrestor applications with plasma/laser cutters (PDF)

Air Density Considerations

Foundries in Mexico City
It was a nice trip for our service engineer in Mexico City with a stay at the Camino Real, freely translated as the royal road. It was one of many sites visited that week all with the same complaint. The pressure drops of the systems were much lower than expected. The result was that power was wasted and in the case of an arc furnace the electrodes burned up at an alarming rate.
The problem boiled down to the fact that using the Industrial Ventilation manual produced higher pressure drops than those calculated in their procedures. The pressure drop across the collectors was lower than for historical numbers. After a couple of days in the office we came to a startling conclusion. The pressure drop was related to the Reynolds number.
We came up with a procedure that was quite simple. To figure pressure drops all you had to do was convert air flows to SCFM instead of ACFM. It worked like a charm. We were then able to modify fan speeds to suit each application. The fan speeds were selected based on actual density at the proper temperature and altitude. This procedure can be used to figure pressure drop at any density due to altitude or gas temperature.

Venturi Scrubber Modifications for fan exhausters.
The normal procedure is to use two factors one for the pressure at 30-49 inches of water for a typical venturi scrubber system and the other for the temperature. If the temperature is calculated to be 160oF a common value exhausting a melt furnace the density will be too high and the required pressure will not be developed. The pressure developed by the fan would be more than 40% less than using the multipliers found in the fan catalogs. The fan belt drive must be modified to compensate for these radical differences in density.

Vacuum Conveying System for GM Brake Shoe Plant in Ohio
The collector was designed to run eight inches of vacuum. Inspection revealed that the pressure drop across the bags was 5.5 inches water gauge of pressure drop. It was a 40 bag collector cylindrical in shape with a high inlet. The load was 30 grains per standard cubic foot and the compressed air pressure was 85 psig. The flow was 200 ACFM. It had seven valves 3/4 inch diaphragm size. The complaint was the collector required pulsing every 4 seconds to maintain the pressure drop. Since the gas density was approximately 50% of atmospheric the pressure drop was equivalent of running at 11 inches w.g. at atmospheric with a filtering velocity of 1:1  when based on SCFM instead of ACFM. The collector on this basis should have run at about 0.2 to 0.3 inches of water pressure drop across the filter mounting plate. After an investigation it was discovered that six of the blow pipes failed because of a manufacturing error. When they were replaced the pressure drop dropped below 0.4 inches and maintained that pressure when the collector was cleaned every four to five minutes. It was an excellent demonstration of the pressure drop across a pulse jet collector when the density drops.

General
In designing several pneumatic conveying systems based on actual CFM on positive displacement systems pressure drop and capacities, the pressure drop is higher than predicted. The collector should be designed on the basis of SCFM as outlined above.
Read more... System design assistance and consulting

Coal Dust and Quenchers

First let us review the facts on explosions.

Explosive dusts have lower and upper explosive concentrations. An explosion can only occur if the following is true. Assuming we have outside air with 20% oxygen being used to ventilate the mill, it is probable that the outlet from the hammer-mill will be between the upper and lower explosive limit concentrations. Between these limits if there is enough energy in the spark that ignites the dust, an explosive flame front will be triggered and move along the duct toward the ventilation outlet and presumably toward a dust collector, and, we are assuming this is to be a fabric (baghouse) collector. Determining the upper concentration limit is very difficult. The lower explosion limit is around 30 grains per SCFM for combustible dusts like coal. Once an explosive flame front is ignited the normal procedure is to put explosion vents in the ducts and collectors.

The action of Quencher spark arrestor.

Our Quencher spark suppressor works by changing the flow in the duct to turbulent flow. During laminar flow the spark can be carried for extremely long distance as the spark travels with a layer of air which insulates it. When turbulent flow is induced, the spark is immediately cooled so it lacks sufficient energy to start a fire or trigger an explosive front. Most insurance underwriters require explosion venting of the duct systems and dust collectors. The way of achieving this conversion from laminar to turbulent flow is by slowing the air down to lower the power consumption of the conversion device. The conversion device thoroughly mixes the dust laden gas stream with propeller type fixed vanes to extinguish the spark(s). To rid the potential low velocity build up of coal dust in the spark arrestor, it is periodically boosted in air stream speed to sweep the dust into the collector. This uses a compressed air propelled stream identical in design to that of a reverse pulse jet collector with time durations similar to that in those dust collectors. This the “Booster - Duct Cleaner” option is offered with the Quencher spark arrester.

Read more about ... Quencher Spark Arrestor