Safety Issues with Venting Dust Producing Operations

Typical manufacturing, mining and material handling operations produce various types of dust and other contaminants. These contaminants may be quite toxic when they enter the workers’ lungs. Protection systems involve either suppressing the mostly solid particulate contaminants generated or the venting and collection of the contaminants.

Dust Suppression

The first approach is to prevent the generation of these toxic contaminants before they enter the work environment. This can be accomplished by dust suppression systems. An example of this would be a rock quarry or coal mine. As the gravel or coal is processed into smaller usable sizes by crushing and placed on belt conveyors to be delivered to trucks or railroad cars, and then delivered to further processing. All of these operations produce large quantities of dust if they are not controlled in these operations. Dust suppression is achieved by spraying liquid over the rocks and coal so that the operations do not produce dust. In dust suppression technology, compounds are added to the water that eliminates the surface tension of the water so that the liquid coating is spread over a much larger surface area. The dust then stays attached to the pieces and to adjoining particles to prevent dust generation. The spreading of presumably water based solution increases the rate of evaporation. In colder climates the dust and product does not allow the material to freeze so that loading and unloading are facilitated.

Venting of Dust Producing Operations
The most common contaminant in industrial manufacturing operations is solid particulate. First the contaminant is contained by putting enclosures or hoods around the dust generating machines that will allow access for the workers. The hood is then ventilated and the contaminated gas stream is cleaned by a dust collector. The dust is separated from the gas stream and the gas stream is vented to the work environment or outdoors. The process of collecting this dust, disposing of it, maintaining and servicing the dust collector equipment can expose the workers to serious hazards
a) The first hazard often present is accumulation of dust in the bottom portion of the horizontal duct runs. Most well designed vent ducts have cleanout doors every several feet. These ducts are near the ceiling usually 12 or more feet above the floor. The cleanout doors are located on the lowest underside of the duct and when the doors is opened dust will pour out towards the floor exposing the worker and environment to dust that might be inhaled. Special man hoists are recommended and breathing masks are indicated. A better approach is to install pneumatically actuated Duct Cleaner-Boosters in the system. These will momentarily increase the velocity in the ducts pushing the dust accumulation toward the dust collector. It makes the duct cleaning operation automatic and safe, with minimum exposure to the dust.
b) The next hazard is when flammable dusts are produced in the machine being vented to the dust collector. If dry dust is collected, sparks can be entrained from the hood(s), and carried into the collector. There is a coating of flammable dust on the filter elements. The velocities through the filters are much lower than in the duct and if a spark reaches the filter elements, the dust may reach the ignition temperature and start a fire. In well designed ductwork the flow is designed to be laminar. Sparks may be transported for more than a hundred feet. To guard against this occurrence, an in-line spark suppressor with a duct cleaner – booster should be installed. The suppressor device will induce extreme turbulent flow which cools the spark below the ignition temperature and protects against fires.
c) Explosions are another operating hazard. To have an explosion the concentration of dust in the housing or duct must be between the lower and upper explosive concentrations and a spark must be present. In mechanical cleaning (shaker) collectors, the flow is stopped in the filter compartment and the filter elements are agitated all at the same time. A potential for an explosion occurs since the concentration will likely pass through the explosive limits during this action. Protection consists of grounding the filter elements to prevent sparks generated during cleaning. Additional explosion rupture panels are installed and vented outdoors. In continuous cleaning pulse jet collectors, only small sections of the collector are reverse flushed. Around each bag in the cleaned section is a very small volume of air which can pass through the explosive limits. Even if a spark is present, an explosion would be dissipated without danger. If the collector filters are to be replaced the first procedure is to remove as much flammable or explosive dusts from the filters as possible. The exhaust fan’s direction is reversed to maintain a low flow and prevent dust from returning to the hood. The collector is cleaned one section at a time allowing time for the dust to settle into the collection hopper. After several complete cleaning cycles a large portion of the dust will be ejected. This lowers the exposure of the worker in handling the filter elements.
There are two general types of filter elements; those with smooth surfaces usually cylindrical or oval with smooth surfaces, and, pleated filter elements. There is a potential for pleated filter elements to bridge and have dust collected in the valleys of the pleats. Even if a reverse pulse collector is cleaned slowly with the fan reversed, considerable dust may be present in the valleys. Recent new technology provides for wider pleat spacing and stiffer filter media which allows off line cleaning as described above to be effective.
d) There are some contaminants either liquid or solids which are not suited to fabric media collectors. They will not form a filter cake or the dust is very unstable. Gunpowder and the propellant for inflating air bags in an automobile are two common examples. The most common approaches are gas washers that scrub the contaminant from the vent stream. Another approach is to mix inert dust into the vent system so the dry powder mixture is no longer flammable or explosive. Some operations will produce dust that is so wet that it will quickly turn the filter cake into a mud which will blind the filter elements.
e) Wet Dust Collectors have a variety of designs and must deal with the problem of surface tension of the water which is used to clean the gas. To get adequate collection efficiency, historically designers have resorted to higher pressure drop designs so that the solid and liquid contaminants might penetrate through the scrubbing surfaces. The same types of compounds, as in dust suppression systems, allow operating at very high efficiencies with minimal power consumption. It was necessary to design special multi-pass mist eliminators to collect the overspray from the scrubbing surfaces. Therefore gas leaving the collector is often below saturation from the heat regain as it passes through the exhaust fan.

Other Hazards and Considerations
Rotary Feeders located in the bottom of a collection hopper can pose a danger to maintenance personnel. There are instances where the feeder fails and dust builds up in the hopper. This dust can ignite and burn. The main approach is to shutdown the section of the collector and let it burn depriving it from getting oxygen. Normally, it will self extinguish. Spraying in water can create an explosion as the water displaces the dust with steam and will go through the explosive limits. On a power plant boiler, a maintenance man decided to pour water into the hopper. The dust was agitated and an explosion occurred.
A) It is preferable to install collectors where the filter elements can be changed from outside the collector housing. Collectors with removable roof doors are widely available. When a filter element extends 8 feet above the doors, even a moderate wind is a problem and workers must plan to protect themselves from these forces.
B) Another type of design is walk-in-plenums. There is a housing tall enough to change the filters, out of the weather elements. The entrance is usually a hinged access door. Rarely do these chambers have lighting. A particular hazard is when in the housing the door slams shut. It is hard to find the door in the dark and a wind can keep it closed. It is advisable to clamp the door with a cable and lock it. Remember the bags and cages must be lowered to the ground and hauled up again and fed to the collector through the access door. The pulse pipes must be removed and temporary storage space provided. These pulse pipes can fall through the holes in the filter mounting plates, forcing a man to enter the collector. Toxic gases can seep into the clean air plenum, so, the breathing zone should be monitored. When a collector is initially started, dust will seep into the plenum until a filter cake is formed. Personnel should not enter the plenum until this conditioning of the filters is completed.

For more information:
Gary Berwick, P.Eng.
Phone: (519) 746-2424
e-mail: gary@qamanage.com
www.qamanage.com

Moisture and Freezing in Pulse Jet Dust Collectors

The root of the problem comes from the fact that as you compress air the moisture holding capacity decreases. Compressors have after coolers in which most of the condensed water is removed before the compressed air enters the distribution system. The compressed air is usually a bit higher temperature than the ambient temperature. As it flows from the compressor to the machines some additional water will condense. Usually any large droplets will be collected in the air line filters before they reach the machines. There is still some moisture but it does not affect the operation of most machines. On other machines where this remaining moisture is undesirable or harmful, dryers are installed between the machinery and the compressor. On compressed air powered pulse jet collectors, the presence of liquid moisture in the pneumatic lines can have serious effects:

1. The water can collect in the compressed air manifold. When sufficient water is collected, it may “squirt” into the filter elements during a cleaning cycle. The drenching of the filter elements is intermittent, but the long term effect is higher pressure drop, more frequent cleaning and premature filter element replacement. Often the filter will dry itself from the exhaust flow through the collector. But residual effects from this wet dry cycling are cumulative. Cellulose cartridge filter elements are especially vulnerable as each wet cycle causes the permeability to increase and harmful effects are much faster.
2. If a collector is installed outdoors in below freezing conditions, even very small amounts of moisture droplets can condense on the diaphragms of air valves. The diaphragms will then stick to the seats of the valves and will not close. This will discharge all the air from the system, since typically 150 to 400 SCFM can be discharged through the valve. Since these valves operate with internal pilot ports, the valve will not close until the supply pressure reaches 25 psig. There needs to be an external shutoff to get the collector (and sometimes associated compressed air supply) back to an operating mode.

Quality Air Management has a two different products to address these problems:

• Manifold Tank Automatic Drain Valve System
• Thermostatically control compressed air manifold heater. This system will allow the collector to operate even when the compressed air dryer is malfunctioning by turning any liquid moisture to water vapor.

QAM also provides dust collector retrofit and consulting services to help resolve common problems that no one else seems to have found solutions.

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

Dust Collectors From China

In recent years there has been a marked increase of dust collectors, built in China or India, coming on the market. Some manufacturers, who claim and/or imply to be USA or Canadian built, are actually branding Chinese built dust collectors. At Quality Air Management, we were approached to enter into such a branding arrangement with a Chinese firm. We turned it down flat.

You can purchase NEW Advanced Technology dust collectors, such as the Ultra-Flow, on the North American market that run with a fraction of the maintenance, operating cost and power consumption. China is producing a copy of OLD conventional technology dust collectors. That technology was obsoleted over 30 years ago in North America.

Read more about... Advanced Technology dust collectors

No doubt, that due to cheap labor, a Chinese built dust collector will have an attractive price. However, price isn't everything! Who do you turn to for warranty problems, and, there will be plenty of them? The North American company will fight it out with the Chinese fabricator, and you will be left holding the bag. Should you buy directly from China, your goose is completely cooked.

The news is full of reports on cheap, poor quality products and a total disregard for quality control, poor labor standards, and environmental issues coming from China. Do you want to take the risk? Then again, when will you receive the dust collector? Delivery is an issue also.

Read more about... Quality Air Management

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

Spark Arresters Prevent Fires

Transport of sparks through ducts; Referring to the sketch below, there is a glowing ember (red particle) surrounded by some hot air (yellow envelop)) which gives the spark buoyancy. This spark (at approx 1400degF) and the hot gas (at approx 800degF) associated with it can travel hundreds of feet in a duct. The ductwork is designed to give laminar (smooth ) flow. This is illustrated on the left of the QUENCHER spark arrestor. Spark suppressors are placed in the duct to change the flow to turbulent (coarse) flow, as shown on the right of the QUENCHER spark arrestor. This agitation or turbulence strips the air from around the ember thereby removing the fuel (oxygen) and breaking up the envelop of hot air, therefore extinguishing and cooling the spark below ignition temperature (pink particle).
Prevention depends on eliminating the causes of ignition. Spark traps can change laminar to turbulent flow and extinguish any sparks in a duct. Design duct systems for dust transport velocities. Install a pneumatic actuated duct booster to flush dust into the dust collector. Use air jets to remove electrostatic charges on duct surfaces. 
Read more about... QUENCHER Spark Arrestors
Read more about... Booster - Duct Cleaner

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Dust Collectors; Old vs New Technology

OLD TECHNOLOGY

•85-90% of the market, sold by all the big conventional suppliers.

•Handle most dust loadings, high temperature.

•Circa 1963; compressed air powered cleaning by rows of bags, venturi accelerated the jet to project to bottom of bag. Filter ratio 10:1 or less, dependent on application. Dust penetration (puffing) unacceptable for re-circulation to the work area.

•Circa 1971; “generic” design, modified to use 10-foot bags. Major design flaws led to selection strictly by filter ratio. Most operated at 4-6:1 ratio. Pressure drop is 6-8”wc. High compressed air consumption with higher cleaning frequency.

•High velocity dust impinges on adjacent bags which are too close together.

•The entire industry copied the same design and very little has changed to this day.

NEW ADVANCED TECHNOLOGY

•Ultra-Flow  by  QAM, circa 2003, are the 6th evolution of the advanced technology.

•Circa 1979; “Advanced Technology” first appeared. Proven technology but little known.

•95% less dust emissions, allows for re-circulation to work area.

•25-40% lower power consumption.

•50-80% lower operating and maintenance cost.

•30-40% smaller footprint.

•No venturi to restrict flow, low velocity - high volume jet = gentle but powerful cleaning pulses = no penetration & complete cleaning. 200% increased bag filter life & uses half as many bags.

•High, side inlet eliminates “can velocity”.

•Supersonic nozzles; very high energy cleaning pulse and only 1/4 compressed air consumption.

•Runs at 18-24:1 filter ratio, independent of process & dust loading.

•Runs at 1.5- 3”wc(max) pressure drop. 

Read more about... Advanced Technology dust collectors

Check out our... Dust Collector Selection Guide and Correspondence Course 

Blinded Filter Bags

This is an Ultra-Flow 60 bag advanced technology baghouse dust collector, collecting rubber, fiberglass fiber and metal slivers from two hammer mills, in a tire recycling plant.

Problem: Dust bags started out at 1-1.5”wg pressure drop, then, rose to 7-7.5” in a short space of time. Cleaning off-line only reduces the pressure drop to 5-5.5”wc. Then a third set of bags, installed a year later, started at 4”wc and clogged (7-8”wc) in 2-3 hours.

Observation (first set of bags, June 2, 2009):

        I.      There was no indication of leakage from the dirty side to the clean side of the collector.

      II.      A bag was sent to our testing lab for analysis. The findings were;

a.       Confirmed no leakage of the bags.

b.     The bag was totally blinded on the dirty side. It was a paste-like dust cake. It is also an indication of moisture in the process getting onto the bags.

c.        The bag was cleaned well but permeability was still low at 2-4 CFM. This indicates chemical attack of the media, likely from some kind of solvent. That renders the bags not recoverable.

   III.      Further very deep laundering of the bag with a surfactant started recovering the bag and increased the permeability to almost the “as new” state. This revealed that instead of chemical attack, the pores of the media were being “painted” making the bag coat with a sticky substance. No dust collector cleaning system can handle paint, blinding the bag media. There is likely rain, snow, road tar and solvent coming in with the tires. This moisture, when heated by the action of the hammer mill, would create latex and/or solvent based paint that got onto the bags. This situation would be intermittent. Once the moisture got through and the process dried up, the paste/paint on the bags would dry out also, leading to the false conclusion that the dust collected was dry.

  IV.  A subsequent squeeze test of the dust collected, alone, revealed no presence of oily substance, leading us to conclude that the painting is caused by moisture (water or snow remaining in the tires when put through the hammer mill.

Solution:

      II.      Teflon bags would resist solvents. We would advise Teflon impregnated bags, not membrane (i.e. Gortex), to keep the permeability at a manageable level. The use of Teflon membrane would require de-rating the dust collector performance by 50%.

   III.      Too much heat may be generated at the hammer mill which can be corrected by using a current controller (instead of a voltage limiting speed limiting controller) on the drive, producing a powder instead of paint. Also an inert dust could be fed in as pre-coat to make it a powder cake instead of a gooey coating.

    IV.      Same as III.

Observation (third set of bags, April 2010):

3. These bags no longer displayed the wetting and hydrocarbons of the first set, due to corrections in the process. The polypropylene bags, from a new supplier, appeared heavier and glazed on both sides of the bag material.
4. The snap band collar was uniformly dirty, not just the part below the tube-sheet groove, and there was dirt on the inside of the bag. This was an indication that the bags were installed wrong. Instead of snapping the band’s groove into the tube-sheet edge, they laid the snap band on top of the tube-sheet. That created a loose fit and a large leak of air from the dirty chamber to the clean air chamber. Each pulse would blow dirt back down into the bag, coating the clean side with dirt. This negated the cleaning system by blinding the bags.
5. The hopper was being discharged into a porous bag instead of the original sealed bin. This results in the air being drawn up through the discharge into the collector. The upward “can velocity” hangs up the dust into the collector and causes the dust to be re-entrained onto the bags during a cleaning pulse and will not drop to the hopper and bag below. When we opened the access door on the hopper a large amount of dust would drop to the bag when the suction was released at the discharge.
6. A replacement set of polyester bags had a high 4-5”wc initial pressure drop when installed.

Solution:

1. Our testing laboratory confirmed that the bags, when clean, were 5-10 cfm permeability instead of 25-35 as specified for those bags. That accounted for the high initial pressure drop. The test also confirmed that we now have normal dry rubber dust and none of the sticky stuff from before. A new set of bags was supplied, under warranty, switching back to the standard singed polyester material since moisture was no longer an issue. Polypropylene was supplied for the second set to account for the moisture getting in the process, at the time.
2. The new set of bags was supplied with our standard o-ring seal. In this way, the bags could not be installed improperly, creating a reliable seal at the tube-sheet.
3. The hopper discharged was sealed by a rotary air-lock which prevented dust hanging up in the hopper and filter bags.
4. The new set worked well. The initial high pressure drop reading was the result of a clogged air line filter to the magnehelic gauge. After cleaning out the filter and air line, the false reading came back to the normal 1.5”wc.

Read more about: Troubleshooting dust collector problems,

Furnace and Dust Collector Fire Hazards

Fires in brass furnaces have always been a danger.

First let us review the process; as the material is fed into the furnace it has many metals including zinc, tin etc. Some of these actually go to vapor and then condense and turn into solids.

The key is that these metals are very fine with very large area to weight ratios. The exhaust is generally cooled by mixing with ambient air so the metals are not appreciably oxidized. The dusts collect, with the other dusts, in the dust filter cake.

When the collector is shut down the metals start to oxidize and the effect is like catalytic combustion. The oxidation produces heat. The dust is usually a good heat insulator and "hot spots" occur. Sometimes the temperature is high enough to start a fire when the flow was stopped. More often, when the collector is turned on, the initial flow fans the sparks and when conditions are optimum for combustion, a fire will start. Most of the time small holes or scorching can be noticed on the bags before a fire.

Sparks may occur as scrap is added to the molten metal. This is common when the scrap is oily.  The usual time to add scrap into the furnace is at the end of the shift when the collector is especially vulnerable to fires.

The approach to prevent fires is to extinguish sparks if they are present and to cool the hot spots when air is not flowing through the system.

To extinguish sparks the flow before the collector must be changed from laminar to turbulent flow. This is accomplished by installing a QUENCHER spark arrestor in the air conduit to the dust collector.

To keep the "hot spots" cool, my suggestion is to pulse the collector off line every thirty minutes or so for one complete cycle to cool the "hot spots". If the off-line cleaning is too frequent, the cake will be destroyed or damaged, so, the cleaning must be controlled.

When selecting a fabric pulse jet collector, high-ratio technology designs can operate at filter ratios of 16:1.  Cartridge collectors are not a good selection as the pleats may promote formation of the hotspots described above.

We first used this technique at St Joe Mineral, which was near Pittsburgh, 30 years ago, on their zinc oxide furnaces. We were informed that they were venting through an AAF pulse jet collectors. AAF has managed to put out some of the worst pulse jet collector designs in the Industry. From the description it sounds like a AAF FabriPulse. That collector if it is top access design has these venturies that wedge in the top of the bag. Using the American vernacular, it sucks. The purpose of the venturi is to seal the top of the bag with the cleaning jet. There are openings around the top of the bag below the wide part of the venturi. This, in effect, allows the jet to grow until the growth is stopped by the walls of the bag. That is an over simplification of the process, but it is a fact that it sucks. The net result is that the collector cleans poorly and there is a lot of dust that is forced into the surface and subsurface filter cake.

On any kind of brass furnace it is best to keep the dust cake porous and thin. As I explained previously, in a brass and other process, the zinc goes from vapor to liquid to solid and forms zinc fume. This zinc fume because of its large surface area to weight ratio can burn or explode quite easily.

We were involved in a legal action where the customer hired a man to change bags on a MikroPul collector venting a zinc dipping operation where they were coating pipes. The young man, after he was half finished (inside removal) sat on the temporary grate and decided to light up a cigarette. The collector exploded and then burned down. He was blown out the access door with the explosion and the sprinkler heads went on after the fire started and water poured over him as he was lying on the ground.

Since we were told that the fires started when the process flow continued we need to look at the source of ignition. If the ignition is caused by sparks, the best way to suppress sparks is by going from laminar to turbulent flow in the dust before reaching the collector, with a good in-line spark arrestor. The next source of ignition might be through the cleaning jet. The cleaning jet can supply oxygen from the compressed air and when it reaches the cake maybe sufficient to cause some sparks similar to small explosions to occur in the cake. This may ignite the rest of the fine fume fuel to start a fire. This would be very pronounced, if the collector was running at a high pressure drop with a dense thick cake and frequent pulsing.

We can attack the symptoms or the causes. One way to attack the symptoms is to limit the thickness of the cake. This can be accomplished by installing PTFE membrane laminated bags. Another way to attack the symptoms is to clean the collector with compressed nitrogen instead of compressed air.

One cause may be because of the atrocious design of the cleaning system. The way to remedy the poor design is to modify the cleaning system design. To implement the change we need to throw away the venturies and modify the pulse pipes so they can run without venturies. We can get the pulse pipes modified so they will induce more cleaning air per unit of compressed air, possibly lowering the formation of sparks on the bag surface. It would allow the collector to run at a lower pressure drop with less frequent pulsing.

I always like to look at how the operation of the collector interacts in the process of venting the furnace. 

Read More...  About assistance with dust collection applications.

Sound Engineering Basis for New Technology

Design flaw #1 for conventional designs:

Conventional designs with cylindrical bags propel the dust from the rows of bags in process of being cleaned toward the adjoining rows in the filter mode. This high speed jet (between  350 and 400 ft/sec) drives the dust through the filter and filter cake, partially blinding the bags and reducing dust holding capacity by 80-90 percent with dense dusts. To operate at reasonable pressure drops, the potential filtering capacity of the bag is reduced by up to 80%. This high velocity dust also raises outlet loading above 100x10^-4 grains per cubic foot.

The new technology design reduces the exit velocity from the bag to between 190 and 250 ft/sec depending on gas density. This keeps the permeability of the media plus filter cake to a few percentage points higher than a new bag. It typically holds several times more dust between cleanings, even at filter ratios of 15 to 20, compared to conventional designs.

Design flaw #2 for conventional designs:

The filtering capacity of the filter element is limited by the reverse air volume generated by the cleaning system. The reverse air volume is also based on the diameter of the venturi at the entrance of the bag. This, for a four inch by 1.875 diameter throat bag is only 20% of the area of the opening at the top of the bag.

The new technology removes the restrictive venturi used in conventional designs and opens up the opening by 4 to 5 times. This increases the cleaning volume while reducing the pulse jet speed by 3 to 3.5 times. Half of the bags are removed and replaced with new bags and cages with the venturi eliminated. The rest of the bag openings are plugged and no longer used.

Other considerations

When these changes are made, the fine dust which formerly bled to the outlet is collected on the bags and ejected to the hopper. Because it is so fine, the vertical flow entering to the bag compartment, from a hopper inlet, would prevent this dust from falling into the hopper. This is the effect of upward “can” velocity.

The retrofit design removes half the bags from the collector. The dusty air enters from the bottom and also through the opening in the center of the bag compartment. This reduces the upward can velocity coming from a hopper inlet to a level 70 - 80% less than before the modification. Now the fine dust falls into the hopper unimpeded. It is equivalent to putting a high inlet in the center of the collector.

95% of the time, the collector will pass the initial engineering review. A report will be issued for your approval, before any fabrication of components begins.

A normal compressed air requirement, for contemporary designs, is 0.9 to 1.2 SCFM of compressed air per 1000 CFM of filtered air. Predicted for advanced technology designs is only (0.328 x (0.9 to 1.2) = 0.3 to 0.4 SCFM per 1000 CFM of filtered air.

Based on an average system requirement of 10 inches water column, a two inch reduction in pressure drop across the dust collector would reduce power consumption in the exhaust fan by 20%. 

Read More... New Advanced Technology Dust Collectors

Booster - Duct Cleaner

It is very common to come across airflow problems, in industrial ducting systems;

• Dust drop-out in the ductwork, causing high maintenance
• Blockages in pneumatic conveying systems
• Low air conveying speeds in ducting systems
• Fire and explosion hazards caused by debris igniting in the duct
• Dust accumulations in spark arrestors and other devices in the air stream

A very simple solution exists for remedying these common problems. It is called an Auto-Booster / Duct Cleaner.

   

The duct booster is a pneumatically propelled jet generating system using the same jet pump design and components as are found in advanced technology pulse jet dust collectors. It is like having a booster fan in the duct system, with no moving parts. It will increase air speed in ducts by 3000-5000 feet per minute for short bursts of time. This will pick up the dust lying on the bottom of the duct and push it along to the dust collector. The air jets also remove electrostatic charges on the duct surfaces which are a source of ignition. When averaged over a day’s operation the cleaner need not be actuated except once in every one to four hours, and therefore air consumption is negligible. The Booster is usually powered by shop air at 85 PSI. Optional supersonic nozzles can be added to the blow pipes for more efficient pressure-to-velocity conversion.

It can also be designed for various low air pressures from 7-20 PSI, thereby allowing operation where shop air is not readily available.

The duct cleaner can be actuated by a manual push button or using the output from one of the positions on a pulse sequencer controlling the cleaning cycle of a dust collector.

Read more about... the BOOSTER - Duct Cleaner