Welding Laser Plasma Arc Processes Causes Major Problems

Welding laser plasma arc processes include welding fume capture, plasma and laser cutting tables, thermal spray operations. Each of these present unique and difficult issues that will cause a dust collection system to fail.

Welding

Two Stage Electrostatic Collectors; Venting welding fume operations poses some difficult application decisions. Years ago, the preferred method of collecting weld fume was with two stage electrostatic precipitator dust collectors. These had several advantages; they were relatively compact and were generally very effective on general ventilation applications. They could handle relatively large gas volumes through the collectors and generally were located near the roofs of buildings.

Efficiencies of general ventilation; The collection efficiency was variable depending on the velocity going through the collection plates. The lower the velocity through the collectors the higher was the collection efficiency. The same collector might have an 80% collection efficiency at 6,000 CFM and a 98.5 % efficiency at 1,000 CFM The same collector could be applied to different exhaust volumes that would vary as much as a ratio of 6:1. The higher the volume would produce the lowest efficiencies. But the same air would be re-circulated and an acceptable level could be maintained in a particular room or building. The cleaning of the precipitators were accomplished by a detergent wash system.
Loading for general ventilation; The loading for general ventilation units were from 0.1 to 0.5 grain per thousand cubic feet of volume. The washing frequency was typically once or twice a week. The presence of condensed hydrocarbons along with the fume was not a problem. Generally these would be oxidized into solids by the time the filter was washed. These collectors were generally the same ones that were applied as air filters in HVAC systems. The washing systems were designed for 1000 cycle life. This would translate to over ten years of life under these low loading conditions.

Hooded Systems; The trend was to hood the welding operations. The venting of hoods had some pronounced effects on the application of these precipitators. The load would vary from 5 to 20 grains per1000 CFM.

Effects of hooded systems; Usually the washing requirements were to wash the filter every shift or twice per shift because the load was so much higher. On a two shift operation, and washing twice per shift, the washing system had a life expectancy of less than 52 weeks.

Plating; It was necessary to operate particular precipitators at lower volumes with their associated higher efficiency, because of a phenomenon called “plating”. Referring to figure 1, the precipitator will ionize the gas and the welding laser plasma arc particles. As the dust passes through the precipitator it forms a bubble type shape, containing charged particles, which were not collected on the collection plates. The gas quickly loses it’s charge. However the dust that was not collected keeps it’s charge a little while longer and loses its charge as it leaves the boundary of thebubble marked “A” in figure 1. If the precipitator has a low efficiency the bubble is much bigger as marked by “B”. This low efficiency bubble is 2 to 20X as bigger in volume than the high efficiency bubble.
Under certain atmospheric ambient conditions, this low efficiency bubble starts to grow rapidly until the whole room atmosphere is ionized and the room and all the contents become collection plates for the dust. The dust s attracted to the walls, windows, machines, eyeglasses and every object that is grounded. All the surfaces turn blackened within seconds. There have been cases where this happened after the walls were painted white. After the atmospheric conditions go back to normal the plating stops.

Bad Inlet conditions. All precipitators either single or two stage need even velocity distribution across the plates. If we had a gas stream averaging 100 fpm that would be designed to operate at 95% collection efficiency and the real velocities entering the plates varied from 50 to 150 fpm, the section at 50 fpm might have a collection efficiency of 98% and the section at 150 fpm would be running at 80%. This would mean that the overall efficiency might operate at close to 85%. This condition would cause phenomenon described above

Lower Efficiencies, caused by running at lower average velocities were common. In the example above the collector might be selected to run at 125 fpm and an average efficiency of 85%. Plating may be produced because of these lower velocities and lower collection efficiency. As a result, many collectors were purchased based on volume and the supplier’s guaranteed higher collection efficiencies. There were practically no way to specify the collectors except based on supplier claims. Velocity was not a good criterion. Some collectors at relatively higher velocity and longer sets of collection plates would achieve the same result as a collector with short plates and a slower velocity.

Electrical Controls; To further aggravate the problem improper electrical controls were offered. Some operators interpreted SIC controls to mean arcing was not allowed. To eliminate the arcing across the plates, they lowered the voltage controls to halt this sparking. Unfortunately the voltage was lowered so much that the particles were not charged. There were cases where the metal pre-filters were more efficient than the precipitators.

Insulator Coating; The main collection plates are at ground in a two stage electrostatic precipitators. The charged particles will be attracted to the lower voltage intermediate voltage level plates and to the grounded collection plates. The insulators were also at ground level and some of the dust (a very small percentage) stuck to the surface of the insulators. This was a very strong bond and the spray cleaning systems could not keep these insulators clean. Eventually they were coated badly enough that the power supply could not keep the charging electrodes to ionize the gas and the particles. To correct this problem required a major overhaul of the precipitator. The charging electrodes were made of very fine wires and would eventually break and require replacement. Most electrical maintenance men were not familiar with high voltage supplies and maintenance was neglected.

Innovations in Design; In the late 70’s, two stage precipitators with pressurized insulators and more rugged washing systems were introduced. The insulators were subject to a gas stream that entered the collection compartment at a higher velocity than the collection velocity across the plates. This protected the insulators from charged particles. The charging electrodes were made heavier to give much longer charging electrode life.

These innovations increased the collection costs with electrostatic collectors to the point where cartridge dust collectors introduced at the same time were more economical to purchase and operate.

The advantages of the electrostatic collectors were:

1. The pressure drop was constant and usually low.
2. They could collect liquid droplets.
3. They had the potential of long periods of service without maintenance.

Cartridge Dust Collectors

With the development of cartridge collectors, another method of collecting fume dusts became available. The standard design pulse jet fabric collectors with cylindrical bags did not work because the cleaning systems propelled dust through the cake of adjoining rows of bags during the cleaning cycles. In the late 70’s and early 80’s thousands of cartridge collectors were applied to both hooded and non-hooded ventilation systems.

Problems developed in many systems after the mid eighties. High-pressure drops and short cartridge life developed in many systems. The causes were one or more of the following:

The presence of thin films of oil on the surface of the parts that were welded. When electrostatic powder coating finishing systems were widely applied to reduce or eliminate hydrocarbon generation from paint systems, the faces of steel parts required protection from oxides on the surfaces. During the welding process condensing hydrocarbons were liberated and swept up into the ventilating systems and their associated collectors. One of several results followed:

a) The solids to liquid ratio was so high that the dust blotted the liquids and the collection system was not affected by the liquid droplets.

b) The solids to liquids ratio was in range where the powders and hydrocarbon mixture formed a paint and the collection media was gradually plugged. This could take days, weeks or months, but the net effect was the cartridges had to be replaced or laundered pre-maturely.

c) The solids to liquid ratio was so low that liquid wetted the cartridges and they were plugged as in (b) above. Even in cases where the coating was barely discernible, this could occur.

A case in point was in a plant making stainless steel mufflers. The metal was washed after forming and the load in solids was 0.02 grains per 1000 CFM, the pressure drop rose in a six month time period. The re-enforced cellulose media would be air-dried and the pressure drop would be reduced from six inches to 0.3 inches after the elements were installed. After 4 months the pressure drop went up to six inches. After washing the pressure drop went down to 0.8 inches. The next washing cycle came two months later and the pressure drop returned to 1.2 inches. It was less expensive to replace the cartridges than to wash them in such a short interval.

Washing cellulose media cartridge elements: After each washing, the media is wetted, the permeability of the media diminishes, even if no dust remains at or below the surface of the media. The wetting causes the media to matt. If oil wets the media it is a good blotter and the fibers may grow. This causes the pressure drop and base permeability to decrease.

Other media are available that can be washed and are not wetted by oils. These are referred to as oleophobic media. This is a coating on the fibers that does not change the permeability. Otherwise they will be called washable. Often they can collect a mixture of fumes and hydrocarbons because the fibers do not swell.

Treated Spun bond medias are widely applied. Some of these are excellent choices but have limitations. For instance, with tight pleats, the top of the pleat may squeeze so the media in that portion of the pleat may make contact on the clean side when the pressure drop rises. On some applications, over 80% of the pleat of the media may not be effective. The remedy is one of the following.

A) Provide pleats with wider spacing and make them shorter in depth. This will allow full use of the media available in the filter element.

B) Provide a media that has stiffness and will not collapse on itself.

C) Provide a laminated media with the clean side backing very open so that if the pleat squeezes there will be flow through the media.

De-agglomerating dust; Normally we would run a properly designed dust collector at 1 to 1.5 inch water column pressure drop. Sometimes a system will only stabilize at a higher reading (E.G. 3 to 4 inches). One possibility is that it takes 3 to 4 inches to cause the dust to agglomerate and fall to the hopper. It may be de-agglomerating when you pulse at lower pressure drops. In that case off-line cleaning should drop out the de-agglomerating dusts. Some dusts are more susceptible to this phenomenon than others. Often, they put an anti-rust wipe on the material being cut. If it contains ceramics then we will have this problem.

Fume Generating Processes Similar to Arc Welding and Gas cutting

Thermal Deposition Processes

Spray Coating The first type was a flame spray coating machine. These fed a material into a high heat gas torch. The temperature achieved was so high that feed material would produce a material in gaseous/liquid form that started to condense into molten droplets. Though the process is not understood, it is presumed that some of the adhesion was from a nuclear bonding, in addition to the cooling of the molten droplets on the piece to be coated. There were some materials that were too porous and there was limits to the thickness of the coating. The over-spray that did not adhere varied from about 5 - 20% of the material fed into the coating generating gun. The over spray was generally collected by medium pressure air washer scrubbers at a 99% collection efficiency.

Plasma Arc Spray To get smoother surfaces and better adhesion to the target surfaces, an electric arc was added to gas flame. This produced much higher temperatures in the gun at the point where the powder or wire feed entered. It generally produced more over spray (10%-40%). This over spray was much finer and would lose its ability to stick and adhere to surfaces. This over spray was too fine to be collected efficiently with air washer wet scrubbers. Fabric or pleated cartridge collectors were necessary.

One serious problem was encountered. This involved residence time of the dust between the gun and the media collection surfaces. In a system installed in 1975, on a plasma arc spraying machine for coating electrical capacitors. The process was coating plastic surfaces with metal. The cartridge collector filter elements, venting the over spray, plugged up in less than ten minutes. The six cartridges each with 50 square feet of filter, (300 sq. ft. total) received less than 250 grains of dust. The dust collector was connected within 20 inches of the gun. The over spray dust adhered to the media surface and blocked the pores.

Through experimentation and field experience it was determined that if the dust stayed in the gas stream for relatively long periods of time, it would lose its ability to coat the media. Depending on various factors such as the feed rates of gas, solids and the arc current, this time varied. It varied from 0.5 to 1.0 seconds. Referring to the next figure, the residence time will be analyzed.

The part to be coated is placed in a hood with the gun at the front of the hood. The hood is 6 foot long and is rectangular with a 4 x 4 opening. The face velocity of the hood is 350 feet per minute. The duct is sized at a 2500 feet per minute duct velocity and the duct is 15 feet long. We will assume the back of the hood has a transition 2 foot long, designed like an evasé to have uniform velocity distribution.

1) Time to travel through the hood 6 ft / 350 FPM = 0.017 seconds

2) Time to traverse duct to the collector 15 ft / 2500 FPM = 0.006 seconds

Residence time = 0.017 + 0.006 = 0.023 seconds.

The flow through the system is 350FPM x 16 sq. ft. = 5600 CFM

To re-design collection for longer residence time the length of travel in components are altered and the velocity can be modified. The hood is the first to be looked at.

3) The hood would be made 10 foot wide with the same 4 foot by 4 foot opening for the gun and part. The velocity in the wide part of the hood would be 5600 CFM / 100 sq ft = 56 FPM

The residence time in this portion of the hood would be 18 feet divided by 56 FPM = 0.32 seconds.

4) The duct could be extended to 200 ft by putting in ductwork in a “serpentine fashion” and enlarged to drop duct velocity to 1,000 FPM. The residence time in duct would be 200 feet/ 1000 FPM = .0.20 seconds

The residence time of the system would be 0.32 + 0.20 = 0.52 seconds.

High Temperature Cutting Processes

This high temperature flame coming from the gas gun proved to be an excellent improvement in flame cutting. Instead of jagged edges near the cut, it became much smoother and for most applications it did not require smoothing the edge or the operation was very quick. With digital cutting machines the precision rivaled other cutting processes.

Plasma cutting and laser-enhanced cutting are in common use. The type of dust produced runs the gamut from arc welding to that of metalizing operations. Most dust is more similar to venting systems for arc welding operations, but to get some cutting characteristics the temperature and flow in the gun are adjusted. This may produce a dust that is prone to coat surfaces and media. When this happens the residence time requirements may be in the same range as the electro deposition processes. Laser cutters work well with 1 second residence time. Some flame cutters have been applied to non-metallic pieces such as wood and plastics. These dusts can contain tars, and oils from non-metallic parts and the collector media can get plugged easily, within a few seconds. With metallic parts, the oils can be an imperceptible film on the metal or originate from the compressed air compressor. In that case, a low-pressure scrubber may be a good choice. Roll filters with replaceable media or a self-feeding pre-coat material system have been employed.

It is crucial to have the correct airflow at initial start-up. Too much airflow will reduce residence time and cause the painting effect. Install a control damper in the main duct and use an approved method to accurately measure the exact airflow. Use the damper to choke the system if needed.

Recently, it has come to our attention that some plasma cutting processes are throwing out the 1 second residence time rule of thumb. Either the process temperature is being cranked up so high that the molten metal atoms still don’t have enough time to form molecules or the dust concentrations are so low that the atoms never get a chance to collide with one another in the laminar flow of the duct system. In these cases, finding the correct residence time

is almost a trial and error process. A new product has come on the market, called a Quencher, which is inserted in the ductwork as close to the source of dust as possible and no less than 10 duct diameters upstream from the collector. This device imparts a high energy multi-directional swirl to the air stream which cools the metallic atoms, accelerate their oxidization, and forces them to collide together and form molecules which can safely be collected without the painting effect.

Use the links below to obtain more information on welding laser plasma arc applications.

More... Quencher spark arrestor

Get help... Consulting and troubleshooting

Spark Arresters and Coolers

Important Factors in Spark Arrester Selection
(1) Pressure drop across QUENCHER style of unit is a function of the Reynolds number which is proportional to the density for air. This means that a unit can be sized smaller if operating at a higher temperature. For instance a suppressor operating at 440 degrees F is 2/3 the size of the typical unit applied at 70 degrees F and the pressure drop will be designed the same. This lowers the cost of the suppressor. The density is also affected by the water vapor in the gas stream. It has little effect at temperatures below 125 degrees F but can be a major factor when operating at higher temperatures.
(2) If the gas steam has dust that might drop out in the duct at the velocities in the blender style or QUENCHER suppressor, a booster must be provided to periodically remove this accumulation. If this unit is not kept clean it might pose a threat by putting an extra load on the duct-work. Without an Automatic Booster System, the suppressor might require periodic manual cleaning.
(3) The booster design is also temperature sensitive and must be altered to accommodate changing gas steam conditions. Most suppliers do not have the capability to modify these booster designs.

There are several approaches to the issue of extinguishing sparks in a gas stream.

Cyclone Dust Collectors
Contrary to common belief Cyclones are not an effective spark arrestor. For a spark arrestor/cooler to work, there must be turbulence to be effective. If you have turbulence in a cyclone pressure drop is very high. They are designed to avoid turbulence. Many bag house fires occur in systems with cyclone pre-cleaners. Amazingly the inlet baffles on the bag-house are more effective as spark arrestors, however they are not foolproof.

Static Baffle-Box Spark Arrestor
Many dust collector suppliers offer this type. It consists of air entering at one end of a baffle box running over a baffle plate which drops out the sparks and much of the dust collected. The air exits at the other end, and then travels to the dust collector. The big drawback is that a hopper and flexible or solid hose connection to a collection barrel is required. Also, these devices do not eliminate all of the sparks. There is not enough turbulence generated to ensure hundred percent spark arrestance. Sparks may ignite the contents of the collection bin

Mesh Filters
This is a common stop-gap measure where the filter is placed at the exhaust duct of hoods or installed in the duct-work. When clean, the mesh filter will stop at best 80% of sparks. These filters do not produce enough pressure drop to be fully effective. It only takes one spark to ignite dust in the duct or set a dust collector on fire. The only thing these filters do is clog up and add to your maintenance.

Blender Type Air Mixers
A number of these air blender/mixers have been applied successfully as spark coolers and suppressors. Over the last 5-6 years standard air mixers have been adapted and applied between the spark generating process and dust collector. They were applied in processes where fires in the dust collectors had previously occurred. One supplier hired a consultant to develop a market for these air blender/mixers as a spark arrestor/cooler. This blender design was an outgrowth of mixing two gas streams of different temperatures to insure a uniform temperature after the static mixer. It was deduced that the gas stream produced turbulent flow as it passed through the blades and this was the reason it could be adapted to spark cooling. However, these are air mixers first and spark arrestors second. There are performance limitations because not enough turbulence is imparted to the spark ember.

Improved Spark Arrestors
QAM developed the QUENCHER, which is a variation of the blender/mixer design. Employing a 60 year old spin vane mist eliminator technology developed by Sly Manufacturing in the early 1960’s, led QAM to vary the blade designs to have the most effective performance, inducing maximum turbulence to the gas stream, and lowering the cost. Maximum turbulence is the key to spark arrestance. After several tests it was found that the air blender/mixer design did not impart enough turbulence and some sparks got through, especially at low gas stream velocities. Eventually, there was a specific design which imparted the most effective swirling and turbulence thereby extinguishing the sparks quickly and most effectively. In fact, during testing of the QUENCHER, the arrestor cell would light up as a ball of fire, however, one inch past the cell nothing was left in the gas stream. These designs were incorporated into the QUENCHER. QAM has developed special application data in which the blade angles are adjusted to produce minimum pressure drop for different temperatures and gas densities. To our knowledge, no one else accounts for the gas density effects on spark arrestors. In truth, due to the advanced design, even applying the incorrect parameters to a QUENCHER may not result in a failure to put out sparks. Since the pressure drop across the blender and mixer are a function of the velocity through the device, the development of a pneumatically operated booster was introduced to prevent dust dropout accumulating in the static blender/mixer. It also blows out accumulations on the blades.
Read more: Quencher Spark Arrestor


Liquid Spray Systems.
For many years these systems were the only available systems to prevent fires caused by sparks. The system consists of electronic detectors that detect sparks and react to their presence. When a spark is detected liquid sprays are actuated and water sprayed into the duct. The sprays actually cool the gas stream below the dew point. However, in dust collection systems, the water then wets the filter bags or cartridges. This prevents fires but the gas flow is interrupted and the bags must be either replaced or dried out before the process can resume. The detector sensitivity can be lowered to prevent excessive actuations, but, this reduces the reliability of the systems. The detector missing a spark is an ever present danger and a fire may occur. Bag or cartridge replacement is definitely required.

Static Blade Spark Suppressor (Tri Pass)
These were developed in Japan to replace multiple cyclones in Coal fired boilers. They found that the multiple cyclones did not stop sparks from entering the dust collectors. The first ones were installed in the early 70’s. They ran at 1.5 inches of pressure drop and were fabricated from structure angles to resist the wear of the abrasive ashes in the coal that they fired. There are several of these applications installed in the USA designed by one of our colleagues.

We trust that the above information will enable you to evaluate and select the most suitable method and supplier for your application. Buying our QUENCHER/BOOSTER combination will give you a risk free unit, fine tuned for each application. 

More on... Dust Collection and Spark Arrestors

Welding Fumes

Venting welding fume operations poses some difficult application decisions. Welding is different now than it was even five years ago. Welding is welding, the difference primarily is the coating or lack of it on the metal to be deposited. Anti-spatter is putting the coating on the welding rod from the bottom.

In fact, many suppliers refuse to bid fabric or cartridge collectors for this application.

Electrostatic Precipitators:

Years ago, the preferred method of collecting weld fume was with two stage electrostatic precipitator dust collectors. These had several advantages, they were relatively compact and were generally very effective on general ventilation applications. They could handle relatively large gas volumes through the collectors and generally were located near the roofs of buildings. The collection efficiency was variable depending on the velocity going through the collection plates. The same unit could run at 98%, 90% or 80% collection efficiency at volumes that would vary as much as a ratio of 3:1. The higher the volume would produce the lowest efficiency. On general ventilation lower the efficiency did not cause any problems. The cleaning of the precipitators was accomplished by a detergent wash system. The loading for general ventilation units were from 0.5 to 1 grain per thousand cubic feet per minute. The washing frequency was typically once a week or once every two weeks. The presence of condensed hydrocarbons along with the fume was not a problem. Generally these would be oxidized into solids by time that they were collected. These collectors were generally the same ones that were applied in HVAC systems. The washing mechanisms were designed for 1000 cycle life which would translate to over ten years of life under these general ventilation loading conditions. As the welding operations generated heavier loads, especially in high production facilities, the trend was to hood the welding operations. This venting of the hoods had some pronounced effects on the application of these precipitators:

Effects of hooded systems

1. The load to the precipitators was increased to levels that varied from 5 to 20 grains per 1000 cfm. This was 10 to 40 times as high as the general ventilation requirements.
2. The distribution of air across the intake of the precipitators became critical.
3. Lower efficiencies; The precipitators, because of competitive pressures, were sized for the higher volumes and lower efficiencies

The increased loading meant that the life of the washer systems became 1/10 to 1/40 of that of the general ventilation units. The translated into washer units that had less than six months service life. These HVAC service designs also had problems with coatings building up on the high voltage insulators so they required replacing at about the same frequency as the washers.

The gas distribution across the collection plates radically affected the collection efficiency. If we had a gas stream that averaged 100 fpm that would be designed to operate at 95% collection efficiency and the real velocities entering the plates varied from 50 to 150 fpm, the section at 50 fpm might have a collection efficiency of 98% and the section at 150 fpm would be running at 80%. This would mean that the overall efficiency might operate at close to 85%. Lower efficiency caused by running at higher average velocities were common. In the example above, the collector might be selected to run at 125 fpm and an average efficiency of 85%.

The electrostatic precipitator ionizes the gas molecules and in the process places a charge on all of the dust particles. The charge attracts the dust to the grounded plates. Later these collecting plates are cleaned to remove the dust from the unit.

Running the collectors at these higher loads and decreased efficiency allowed some dust to pass through the precipitator while the dust still retains its charge. As the exhaust gas mixes with the ambient air the dust usually loses its charge and the gas stream loses it’s ionization. The length of time required to lose its charge depends on the dust loading in the exit gas stream as well as the humidity of the ambient air. Dry air will allow the charge to be retained longer. Under certain conditions of low humidity and higher dust concentrations, the dust will hold the charge indefinitely. Suddenly the whole room will be ionized and the charged dust will be attracted to all the neutral surfaces in the room including walls, eyeglasses worn by personnel, windows and even light bulbs. This phenomenon is known as “plating”. There have been occasions where a newly painted plant was coated with black soot within minutes. It is because of these circumstances that welding fume collection was changed to pulse cleaned dust collectors.

Cartridge Dust Collectors:

There are thousands of cartridge collectors in service on these applications

However, there were big differences in power requirements, cartridge life and penetration of dust through the collectors: These were due to the following :

1. Cleaning controls: The much used pressure drop actuated cleaning control has some serious flaws. It is impossible to predict the pressure drop at which the most effective filter cake is formed. It is likely, that this pressure drop may be just slightly higher than the initial pressure drop. If the cartridges are not cleaned early enough the result is that the dust will bridge across the pleats and a large percentage of the filter media in the cartridge (that which is below the bridge) will be unclean able. Initially, the collector should be cleaned by a timer control until the pressure drop stabilizes. The control can be extended until the pressure drop starts to rise and then the timing speeded up slightly. Alternatively, the pressure control can be set slightly above the stabilization point. The best operation occurs when the collector runs at the lowest possible pressure drop. Penetration of dust, cleaning frequency and cartridge replacement time, increase with pressure drop, usually related to the square of the increase in pressure drop.
2. The filter area of the cartridge can be and often is excessive. The media that can be cleaned by the cleaning system is the only media that is useful. Often, suppliers cram more media into a given size cartridge by inserting closely spaced pleats. This cramming of media provides useless media and causes operational and structural defects. If pleats are close together, the adhesive that holds the cartridge together will not wet the media and a strong bond cannot be developed between the end caps and the filter media. A poor bond may allow leakage between the media and the adhesive. If the filter cake is deeper than the open distance between the pleats the cartridge will bridge. Typically cartridge filtering ratios should be no less than 3 FPM, and generally will operate at 4-7 FPM. Most systems operate at lower filter ratios where pressure actuated controls do initiate frequently enough to clean the cartridges effectively. Conventional cleaning systems may not allow for recommended filter ratios of 4-7 FPM. Advanced technology collectors will run at even higher filter ratios and clean the media more effectively, promoting higher efficiency and longer cartridge life.
3. The cartridge shape should be flat and cylindrical. Truncated cone bottoms, formed into the closed end cap, causes damage to the filter cake allowing excessive penetration through the dust collector. The cone develops more cleaning pressure near the bottom than in the rest of the cartridge. Premium construction cartridges are often necessary to develop maximum cartridge life and improved collection efficiency.

Oil-Smoke:

The above applies to welding of clean parts. Recently, there has been a trend to vent from welding operations where the parts are coated with a light film of oil.

A mixture oil and powder can produce oil based paint. This oil/powder mixture can progressively blind the cartridges and produce premature cartridge failure. When you collect oil on the cellulose cartridge, it acts like a blotter and the media swells, permeability and available collection media is reduced. This latter phenomenon is in at edition to the painting action. With some hydro/oleo phobic media, that is specially coated, the oil will not wet the fibers and the media will not swell. Some of these cartridges can be recovered by washing them in detergent, but over a period of time the oil starts to dry and a solid is formed which cannot be washed. Typically after two or three washings the cartridges must be replaced, as there will a creeping higher initial pressure drop.

Usually, the welding fume load is extremely light and with the special fibers, the cartridges can last for several months before they need to be washed or replaced. Actually even cellulose based fibers can be washed successfully two or three times, but the time between washing goes down for each wash as the residual permeability decreases after each washing.

There is some promise in using a precoat (filter aid) of some dust on the media to blot some of the oil so a paint will not form. There are some commercially available coatings that are promising. The choice will be empirical.

Stainless Steel

This is one of the most deceptive applications we can deal with. Most of the time, you will believe that the material has not been pickled with oil or another preservative. The metal looks perfectly dry. Some manufacturing processes will prewash the stainless, thinking they are removing any residue that may be there. When you well donned the material, to everyone's surprise, a residue appears at the dust collector filter elements. A way to determine this is to soak the filter elements in a tank and observe a film of oil form on the surface of the water. Another test would be to take a folded piece of tissue paper and squeeze it between the pleats of the filter element for 24 hours. Automatic pre-coating, or seeding, of the filter elements is a solution to this problem.

Weld-aid (anti-spatter)

Welders often use “weld aid” type of substance to prevent spatter from adhering to the work piece. It is mostly water with a 15% fatty substance. It is usually sprayed on the work piece before welding. This is a very greasy substance that is difficult to remove from the cartridge media and fills in the pores. You should have a washable media, such as polyester spun bond, preferably with a non-stick surface. All paper, cellulose formulations are unacceptable media selections because water gets absorbed and the media swells. Do not use exotic medias that have low permeability. It is recommended to install an automatic filter-aid feeder, such as the one designed and sold by QAM. You also should consult a welding expert to change the welding process, which will eliminate the necessity of using weld-aids or change to a powdered version.

Automatic seeding system

With a special design and a seeding system, it can last indefinitely. A layer of inert absorbent dust is fed into the collector. The collectors not pulsed except twice a day. The inert layer blots up the oil fume and the oxides from the welding are also mixed on the surface. There is an airlock, usually the smallest kind available which feeds the dust back into the bags after the collector is restarted or even pulsing is done while the collector is operating. The ULTRA-FLOW bag-house can run at 18:1 ratio and is probably less costly than the cartridge collector for the application.

Read more on ... Ultra-Flow Baghouse dust collectors

SPARKS & FIRES

Sparks are a very common occurrence in the welding process. These sparks are swept up into the collection hood or device and are transported to the collector. Fires can occur in exhaust ducts as well as inside dust collectors. Requirements of fires or any combustion process are: a) Fuel, in gas, liquid or solid form. b) Oxygen (Atmosphere consists of 20 per cent oxygen) c) Fuel must be raised to the ignition temperature to start burning.

Transport of sparks through ducts.

There is a glowing ember surrounded by some hot air which gives the sparks buoyancy. This spark and the hot gas associated with the spark can travel hundreds of feet in a duct. The ductwork is designed to give laminar (smooth) flow. Spark suppressors are placed in the duct to change the flow to turbulent (coarse) flow. This agitation or turbulence strips the air from around the ember removes the fuel (oxygen), therefore extinguishing and cooling the spark below ignition temperature.

Read more about ... Spark Arresters and Coolers

Prevention depends on eliminating the causes of ignition. Spark traps can change laminar to turbulent flow and extinguish any sparks in a duct. Design for proper dust transport velocities. Install pneumatic actuated duct booster to flush dust into dust collector. Use air jets to remove electrostatic charges on the duct surfaces.

View and print... fires and explosions

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

Plasma Cutting & Cartridges

This is the result from using our cartridge and fabric filter element inspection service.

1) It was not the cartridge that was normally supplied to clients.
2) It had an inverted cone reinforcement in the bottom closed end cap. We always recommend a flat closed end cap for maximum life. The pleat spacing was optimum for this kind of application.
3) The seals (gasket) were resilient which insures effective sealing between the clean air and dirty air compartments. The cartridge exhibited no evidence of improper installation or handling.
4) You could expect indefinite life on this filter element on all suitable applications with an advanced technology pulse jet cleaning system like ULTRA-FLOW.

This plasma cutting operation is quite common and we see many cartridges with problems from these operations. The dust generated is extremely fine and problems with seals and installation account for a big majority of problems. The other problem, that we see is that the settings on the cutting head are such that the dust can be prone to plasma coat the filter elements. The solution of the coating problem is to give the dust time to lose its reactivity before it reaches the filter media. None of these usual problems were evident on this filter element.

A) My first observation was the color of the coating on the filter. In cutting ferrous metal with a plasma arc cutter, the dust is black and coated with fine easily removed powder. In handling of these filter elements, we usually wear a mask because of the fine dust generated in the inspection process. In this case there was no dust generated in the procedure. The color of the coating was brown.

B) My second observation was that there was hard inflexible crust covering the dirty side of the pleats as if the element had been painted. It had the same strength as baked on auto finish.

C) The only time I previously observed this kind of coating on a cutting operation was when the plates were covered with some kind of coolant, cutting oil or pickled. On some cutters they use compressed air at the head. I would suggest checking the air line lubricant as a possible source of the binder that is creating this paint like coating.

Finally, a continuous coating of filter aid could be maintained on the surface of the filter media. The dust load is usually quite light and the coating might be about a 64th thick. The surface coating would be painted instead of the media. When the collector pulses the inert filter aid, coated with the paint, would be ejected into the collection hopper. How often to clean would be a judgment call. I would expect the cleaning and re coating every 2- 4 hours would be appropriate.

Read more ... Retrofit Service for Cartridge Dust Collectors
Find out about ... Most Advanced Technology Dust Collectors