Powder Spray Booths

4 Service Reports; inefficiency, filter clogging/blinding, filter cake destruction, oil on filters.

Project in Michigan The system involved coating parts with a PVC Powder. The complaint was that the dust was bleeding through the filters even after 89 hours of operation. We diagnosed the problem that during the pulse cleaning action the cake was also being blown off the media. Our solution was to build a filter cake below the surface that would not be blown off. We fed 1 ounce of baking soda per 100 square feet of filter media which required two hours of hand feeding. This stabilized the pressure drop at 2-3inches and five years later the filters were still operating perfectly with no further bleeding
Project in Northern Ohio Customer has an electrostatic powder paint coating. The booth supplier built his own collector by copying existing designs. The pressure drop was 7 inches water gauge and the volume was insufficient to keep dust from entering the room. We modified the cleaning system to increase flow through the cartridge filters. The pressure drop went to only 2 inches water gauge. Then they installed a shut off damper at the exhaust fan to lower flow and reduce horsepower requirements at sufficient flow.
Project in Antwerp Holland They were using electrostatic booths to coat grating and a bag dust collector. There was serious leakage through the filter-bags. We stopped the leakage by using duct tape on the bottom of the bag cages. The pulse-jet must clean the whole bag evenly. If a solid impermeable stop is not placed at the bottom of the cage the filter cake will be destroyed and dust will bleed through the bottom after each pulse. We remedied the situation in less than an afternoon. The manufacturer changed his design to prevent the leakage.
Project in Cleveland Ohio Machine was designed to remove rust and add coating to wire hangers provided to dry cleaners to hang cleaned clothes. Dust went right through the filter bags even though the pressure drop was less than 3 inches water column. Much of the dust consisted of pieces of scale that were bigger than fifty microns. This was much bigger than the biggest holes in the media.  After investigating we found that the trouble was because of an improper installation of the rotary screw compressor. The installer left off an oil trap that recirculated the oil through the screw. This oil coated the bags and lubricated the fibers so that dust larger than the openings in the bags was able to slide through. We had them install the oil trap and install new bags. They paid for the service call including installing new filter bags. The collector ran with the new bags for several years.

For more information on system design and troubleshooting

Most Advanced Technologies

The original design of most contemporary reverse pulse jet collectors was developed in the early 1970's. This design was a breakthrough in dust collector technology. In 1978, several technologies were developed to remedy operating problems of fabric pulse jet collectors also referred to as a bag house. The cause of these operating problems was a major flaw in the design.

Cause of Operating Problems

The main flaw in the contemporary design of fabric pulse jet collectors was that during the cleaning cycle dust was driven from the cleaning bags at very high velocities, then driven through the filter cake and filter media the adjoining bags. The bags became partially blinded. These velocities were typically 20,000 to 40,000 feet per minute. The permeability of the filter cake and dust imbedded in the media increased. The bag developed a coating to resist further penetration until the system stabilized. These high cleaning jet velocities resulted in short filter element life, high pressure drops and high compressed air usage.

It is and was common for fabric pulse jet collectors to run at pressures between 4 and 6.5 inches, and air consumption of over 1.4 SCFM of compressed air per 1000 CFM of filtered air at 80 psig. As might be expected the heavier density dust and powders ran at the highest pressure drops.

New Technologies

Four new technologies were developed to combat these operating problems:

1. The cartridge collector with pleated filter elements.
2. The high ratio fabric pulse jet dust collectors.
3. PTFE laminated fabric filter bags
4. Bag diffusers

Cartridge Collector

The cartridge collector was immediately widely accepted because it solved a problem with venting electrostatic powder paint systems. Previously, fabric pulse jet collectors on this application gave unsatisfactory service. Within months of their introduction, hundreds of collectors were sold and installed with spectacular results. The cartridge collector remedied some operating problems in the contemporary fabric pulse jet collectors. An example of this was the experience of Nordson Corporation of Ohio, who supplied powder pigment spray systems that sprayed powder on metal surfaces that were then cured to a hard coating. They vented the spray booths into fabric pulse jet collectors. On some pigments, they had bag lives of less than eight weeks and pressure drops of 6 to 8 inches w. g. Even at filter ratios as low as 3:1. They were desperate for a new technology and were the first to embrace it.

Read more about... different dust collectors

Laminated Filters

In 1975 the Gore Corporation introduced PTFE membrane laminated bags. This prevented the dust driven from the bag in the cleaning mode from penetrating below the surface of the media through the filter cake. Hundreds of thousands of bags were successfully installed that eliminated many operating problems with contemporary designs. The laminated bags lowered the bag permeability, which sometimes limited the filter ratio. Typically the bags cost 5-6 times more than conventional bags. Since then patents for this construction have expired.

High Ratio Pulse Jet Fabric Collectors

In 1978, Scientific Dust Collectors introduced another breakthrough in pulse jet dust collector technology. The venturies were removed from the bags and pulse jet was changed so that velocity of the jet decreased by 60 to 80%. The velocity of the dust leaving the bag was also decreased by 60 to 80%. At the same time the volume of the pulse jet was increased by more than 3 times. As an adjunct of these changes, these collectors were operated at filter ratios between 12 and 20:1, with pressure drops under 2 inches WG. In 1983 Carter Day Corp. introduced the same design with oval bags. Since then over 4,000 of these collectors operating at high filter ratios have been installed and are operating with those parameters. These collectors must have special inlets which eliminate any upward velocity of dust laden air entering the filtration section of the collector. The low velocity cleaning jets increase the collection efficiency on fine particulates. Fine particulates may have difficulty falling into the hopper if the collector has a hopper inlet. Thus the high side inlets of modern technologies.

Read more about; High-ratio pulse jet bag house or fabric dust collectors

Bag Diffusers

These diffusers consist of light gauge perforated cylinders inserted into the bags below the venturies of existing collectors. They operate to improve dust collector operation by slowing the velocity of the jet as the cleaning air exits the filter bag toward adjoining bags. Because they too increase collection efficiency on finer dust, collectors with bottom/hopper inlets can encounter limited effectiveness with this modification.

American Foundry Society Tests 1978

A test run by AFS on foundry dust showed that the penetration of the dust compared to a standard pulse jet was astounding:
STD Fabric Pulse jet at 4:1 filter ratio = 800x10-5 grains per cubic foot with 10 grain inlet
Cartridge Collector at 2:1 filter ratio = 3x10-5 grains per cubic foot with 10 grain inlet

Gortex laminated bags = 10x10-5 grains per cu.ft. with 10 grain inlet
Low Velocity Cleaning Jet Fabric Collectors* = 10x10-5 grains per cu. ft.
*These are New Technology collectors as described above. This test was run in 1978 prior to the introduction of the new technology low velocity jet high ratio dust collectors but other independent tests showed the performance shown above.

Later tests of new technology cartridge dust collectors, such as ULTRA-FLOW, revealed that they operate at 2x10-5 grains per cu.ft. but at 8:1 filter ratio (instead of 2:1 for conventional designs).

Determination of Flow ratings of New Technology versus Contemporary Designs

The flow capacity of a bag or cartridge in a reverse jet self cleaning collector is limited by the volume of the reverse cleaning jet. It is obvious that if I have 50 CFM flow in the filter element and the reverse jet volume is 40 cfm, the flow in the bag will not be reversed and no cleaning will take place on line. Whether the bag has 10 sq. ft. of media or 100 sq.ft. of media it will not clean on line. However, if I take this reverse jet and increase the flow to 200 CFM (50 CFM x 4) it will clean on line easily. If we temporarily accept the premise that you need four times the cleaning flow to maintain equilibrium, adding more square feet to the filter element will not increase filtering flow capacity unless you also increase the reverse flow volume. Another limitation is the operating permeability of the filter media and dust cake as covered in the first part of this article.

An important limiting factor in filtering volume through collectors is the well- known “can velocity” parameter. This effect is most pronounced on very light density dusts such as paper dust, feathers, grain, dried organic fertilizers, dust from recycling etc. Dried manure is similar to grain and is susceptible to “can velocities”. New technology collectors have special inlets where there is no upward vertical component to the dust laden gas as it enters the filter compartment of the dust collector.

If we consider the conventional designs where they have basically a reverse air volume of 250-350 CFM, and 20,000 to 40,000 FPM cleaning jet velocities, specifying a low filter ratio is excellent engineering. However, specifying the new advanced technology results in better filtration, lower operating costs and longer filter life. The better engineering approach is to specify dust collectors by considering the capacity and design of the cleaning system. This can be accomplished by taking the compressed air flow capacity of various sizes of diaphragm valves and multiplying by the number of valves, if orifices are installed in the pulse pipes. If the pulse pipes have converging-diverging nozzles installed, the cleaning jet volume is increased by 70% with the same increase of filtering flow in the collector. (This assumes that the total area orifice opening are maximum at 85% of throat area)
¾ “ valve orifices = 688 CFM;  nozzles = 1032 CFM
1 “ valve orifices = 1224 CFM;  nozzles = 1836 CFM
1 ½ “ valve orifices = 2754 CFM; nozzles = 4406 CFM

 Example of comparing two collectors:

Standard Design Collector with 81 bags, (9) ¾ inch valves, 10 sq. ft. per bag
9 x 688 CFM = 6,192 CFM, 6192/1000 sq. ft. = 6.192 (nominal filter ratio) based on cleaning volume with no regard to can velocity. For low density dusts, the ratio must be lowered for lower density dust.

Advanced Technology Collector with 81 bags, (9) 1 inch valves with nozzles, and special inlet to eliminate can velocity considerations.
9x 1836 CFM = 16,524 CFM, 16524/1000 sq. ft = 16.524 (nominal filter ratio)

The net cleaning jet velocity should be specified as no more than 9,000 fpm and gross cleaning velocity no more than 15,000 fpm for dense dusts.

Upward component flow of air entering the filter compartment should be limited to 150 fpm for lower density dusts and 350 FPM for higher density dusts.

Specifying collectors based on filter ratio alone penalizes the supplier who provides more cleaning capacity with the larger more expensive cleaning components, and better engineered cleaning systems, depriving the client of superior performance at lower cost.

When to specify Fabric or Pleated filter elements

The other decision is between pleated filters and unpleated filters. The selection for pleated filters is confined to dust that has a thin filter cake. If the cake is deep, bridging will take place in the valleys of the pleats rendering the media below the bridges uncleanable. A new advanced technology bag house/fabric dust collector can be specified on virtually any dry particulate dust application, even those traditionally reserved for cartridge dust collectors.

Read more about; Dust Collection

Bag House Dust Collector Technology

History

The first pulse jet collector was developed by Pulverizing Machinery of Summit New Jersey in the early 1960’s, to collect dust from their Pulverizers. They had tried to use the Blow Ring design but it could not handle the dust (powder) loads as Pulverizers became bigger. The typical load to the collectors were between 50 and 300 grains per cubic foot. The collector design was based on the same blow ring velocity and the cages were based on available designs from hipping pulverizer shafts. The pulse valves selected were diaphragm valves that were fast and the lowest cost valve available. This happened to be a ¾ inch valve. They decided to use several valves in a collector and pulse them with an electronic timer. It was found that the hole sizes and venturi were an air ejector design that had the same jet velocity that the blow ring collector was using. But the big breakthrough came with the realization that the dust was ejected from the bag during the first 4 or 5 milliseconds of the valve opening. It became apparent that the frequency of cleaning was a function of the load to the collector. For instance for loadings of 300 grains the collectors would operate at a filtering velocity of between 7 and 9 feet per minute. At material handling facilities such as a quarry would operate at a filtering velocity of 14 to16 feet per minute. The typical pressure drop in these collector designs were about 2 to 3.5 inches WG pressure. The typical compressed air usage on the high loads were 1 to 2 SCFM at 80 psig per 1000 CFM of filtered air. For loads under 10 grains per cubic foot, the air usage was 0.2 to 0.8 SCFM per 1000 CFM of filtered air. Determining the filter velocity (then referred to as filter ratio) became a rather complicated procedure. The ratio presumably was determined by dust load, fineness of the dust, temperature of process gas stream, and other factors.

The hopper inlet was a carry over design from both the blow ring collector and the previous mechanical shaker collectors.

By 1969, there were over 10,000 collectors in operation. Almost all of them were installed on process equipment or in Foundries. Pulverzing Machinery changed their name to Mikropul and licensed FlexKleen to build and Market collectors. The collectors for MikroPul had 4 ½ inch diameter bags and the FlexKleen units had 5 inch bags. The Mikro units had six foot and on occasion a collector with 8 ft long bags (to compete with FlexKleen on some projects) and the flex units had nominally eight foot long bags. Bag life was usually 4 to 7 years.

Engineering Disaster 1971

In 1971, the patent was challenged and the Pulverizing Machinery patent was declared invalid. The market changed radically because Air Pollution Control Regulations became effective. Many new suppliers entered the market. In order to compete Mikropul changed their design. They went from 6 foot to 10 foot bags. They increased their pulse pipe holes by the same ratio. The whole industry followed and copied the new design for hole size and venturi throat diameter. At the time, Mikropul had 40,000 venturies in stock and kept the same venturi sizes. This increased the jet velocity of the cleaning jet by 66 per cent.

This was when the dust collector market was growing at a 20% annual rate. With the new designs pressure drop increased to 4 ½ - 6 ½ inches WC. Compressed air consumption increased by over 50% for similar applications. Bag life was reduced by over 50%. In reaction to these problems the filter ratios were reduced to between 4-6 on almost all applications.

Reasons for Disaster

What happened was no one at that time realized a rather obvious truth, that the velocity with which the dust is ejected from the bag during cleaning is proportional to the velocity of the cleaning jet. At the new velocities, dust is driven toward adjacent rows of bags in the filter mode. Depending on the dust density, the dust will be driven through the adjoining cake into the clean side of the bags. The cake becomes more dense and the pressure drop increases until the process stabilizes which takes 16-100 hours. Even after the equilibrium, the dust still penetrates and bag wear is high. With low filter ratios it takes longer for the bag to wear out and require replacement.

Low Pressure pulsed air cleaning systems

Reverse Air Fan induced pulsed air collectors

In the mid 70’s, it was discovered that the compressed air cleaning pulse jet collectors were encountering high pressure drops when applied to grains and to a lesser extent on woodworking applications. The reason was that the compressed air as it left the pulse pipes was subject to refrigeration cycle as the compressed air expanded. The first approach was to apply reverse air blowers to the cleaning system. The blowers were mounted on the roof of the collectors and the reverse flow was pulsed with mechanical dampers. The reverse air jet actually was higher in temperature than the process gas stream because of heat regain from the cleaning air as it passed through the fan. The downside of these collectors was that the fan on top of the roof of the collectors were difficult to service and as collector systems expanded the weight of the fan was significant. These collectors pioneered some arrangements that allowed them to operate with grain dust with densities under10 pounds per cubic foot. They introduced the high cyclonic inlets of cylindrical shaped collectors. They also featured a rotating reverse air manifold. There are thousands of these collectors, some we serviced for over 20 years.

Air Pump Pulse Jet collectors with 8-10 psi operation

This was an outgrowth of the reverse air induced fan pulse jet designs. They used the technique of a cylindrical housing and a rotating pulsing arm but reduced the effects of the refrigerant cycle and loss of energy when the compressed air expanded. Also, in the advanced technology concepts, they reduced the velocity and the effect of the air leaving the bags propelled to adjoining bags during cleaning. The oval shaped bags reduced the re-entrainment effect. The collectors usually had the effect of a high inlet as the air entered the bags mostly through the hollow cylinder in the middle of the cylindrical housing. Some of the advanced technology designs used a high cylindrical inlet similar to the F4 fan cleaning units. These collectors with bottom inlets were also applied to boilers at conservative issues.

Today’s Conditions

The disastrous design, mentioned above, continues to be employed by most of the pulse jet collector suppliers in the world, especially for boilers. The market has become one in which it is a commodity and the equipment is built by the lowest cost suppliers.

New Technology

Early 1980s, one of our associates worked for a company called Scientific Dust Collectors and developed a new pulse jet collector that basically changed the cleaning system design. The key to this design was that he changed the jet velocity to a fraction of the existing designs. This eliminated the penetration of dusts from the row of cleaning bags to the adjoining row in a filtering mode.

This allowed pulse jet collectors to operate at lower pressure drops (2-3 inches w.c.), lower air consumption (50-75% less), 3 to 4 times more bag life and filter ratios of over 12 : 1 on any application while decreasing dust penetration by up to 90%.

There are many different considerations of using this new technology more effectively, which we can teach the client to apply. These techniques were developed over last 30 years for Ultra-Flow, Carter Day (now Donaldson), Dustex and with several smaller companies. These include air distribution baffles and special inlet and outlet configurations. Now our has joined our group as a consultant. With the help of this system designer, having over 60 years of experience, QAM has brought this advanced technology to the Canadian market and carry on the legacy during the last 10 years. We do not provide a design which is relatively simple but provide a technology which will enable our clients to develop new radical designs and systems. Specifically, there are many little details such as protecting the inside of pulse pipes from corrosion, techniques for reducing corrosion when burning coals containing sulphur, special techniques to start up and shutdown when faced with unusual conditions, techniques for adjusting cleaning systems to operate at higher elevations, inspection techniques for qualifying critical components etc. This technology is an on going process. We have seen radical changes in periods of less than six months as new components and new manufacturing procedures are developed.

This technology allows the client to adapt to different field conditions and to burn different fuels. These collectors have been applied to incinerators which operate under the most difficult conditions. By comparison a coal fired boiler is relatively simple. Sometimes the operating techniques may change as the source of fuel is changed. There are some advanced innovations that are especially significant to operating costs. When air expands from 90-100 psi absolute, in an orifice blowpipe it accelerates the air to sonic velocity. Then the pressure in the blowpipe is 100 psia, the pressure in the throat is 52.8 psia absolute or 38 psia. The energy below that down to atmospheric is dissipated. There are techniques to design nozzles to install on pulse pipes which will increase the velocity to around 1690 feet per second and lower air consumption by 30%.

Read more on ... New Technology Bag House Dust Collectors

Spark Arrestors and Booster - Duct Cleaners

Service Reports:

Chambly, QC: The installation was at a microbrewery. This is a problem common to all breweries that recycle their boxes. In the process, they have an automated machine including a band saw which cuts up the boxes and then bundles the cardboard for recycling. A good deal of paper dust is generated by the process and needs to be collected through an extraction duct from the saw to a dust collector.
The problem is that these boxes may still have staples and bottle caps in them. When the saw hits the staple or bottle cap a spark is generated which is drawn into the extraction system and produces a fire in the duct and in the dust collector itself by igniting the cardboard dust.
Our proposal was to install an LC series high ratio dust collector providing the maximum filtration efficiency and making the system the most compact possible. Floor space was an issue. However, to protect both the duct system and dust collector from ignition, a QUENCHER in-line spark arrestor/cooler was installed in the ductwork at the outlet of the extraction hood for the band saw.

Read more about ... Quencher Spark Arrestor

The installation has been running since June 2005. The maintenance supervisor indicated that the system should have caught fire within a maximum of two months of operation, which was the experience they've had until then on the other systems in the plant. There have been no incidents to date. He also stated that there was no light dust accumulation around the dust collector outlet which was typical in the other systems that they had. This is an indication that the dust collector is operating at a high level of efficiency and that there has been no sparks which would have burned holes in the filter material.

London, ON: This is a metalworking shop which does a good deal of welding and grinding. The dust and fumes are extracted to a central dust collector through source capture articulated arms and downdraft tables. These processes produce a lot of sparks which get drawn into the dust collection system.
In one system, the main duct line is oversized and the dust is dropping out into the duct work prior to reaching the dust collector. As a result, the sparks that are transported through the duct will ignite this volatile dust in the duct causing a fire to be drawn into the dust collector.
We recommended that they. install a "booster duct cleaner" which would blow this settled dust down the duct and to the dust collector before it could be ignited by the sparks We also recommended that they install a QUENCHER in-line spark arrestor/cooler to quench the sparks and prevent them from going down the duct work. The client has yet to install the booster which means that he has not solved the dust settling problem but they did install the QUENCHER. Since the installation of the QUENCHER, there has been no further ignition of the dust in the ductwork or in the dust collector.

Read more about ... Booster-Duct Cleaner

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.

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