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

Shot Peener and Blasting Operations

The abrasive generated dusts have some unique properties that need to be addressed to accomplish the following by application of advanced technology dust collector designs:

1)    Lowering the dust penetration through the filter cake filtering mechanism will protect the environment and employees.

2)    Re-circulation of the vented air can reduce energy consumption.

3)    Further economic savings, by lowering energy consumption, for operating the vent system at lower pressure drops and reducing pulsing air consumption.

4)    Filter life can be increased by 3 to 4 times.

 

For many years, abrasive blast operations were vented from the blast cabinets through mechanical shaker dust collectors. In these units the initial collection efficiency was relatively low. As the cake became thicker, the pressure drop increased. It would eventually increase so high that the flow through the system would be choked. 

Several mechanical arrangements to remove excess dust were development. During the process of removing the dust, a portion of the dust remained on the media. This residual dust had formed a filter cake and even after cleaning the collection efficiency was high enough to allow re-circulation. In the early 1960’s continuous cleaning pulse jet collectors were introduced. These collectors were much smaller than the mechanical cleaning collectors and reduced the complexity of the venting duct-work. The typical collector was 15-25% of the size of the compartment-ed mechanical cleaning collector. However, the average particulate load penetrating through the collector was about 2.5 milligrams per cubic meter, which could not be re-circulated into the working environment.

In 1973, pleated filter elements were introduced into the dust collection market. It was believed that pulse jet collectors would be more efficient if they were pleated because of low velocities through the filter element. Most dust collector manufacturers and designers packed too many pleats in the cartridge filter element. Also, the cleaning system was undersized for the amount of media at that must be cleaned.  The result is dust bridging on the inner pleat.  In many cases, we can lose up to 80% of the effective workable area of filter media.  This led to short filter life and high maintenance cost.

Advanced technology modifications have improved the operations of each of these types of collectors, reducing the foot print, lowering pressure drop (with lower power consumption) and reducing penetration of dust through the collectors. The next breakthrough in the advanced technology reverse pulse jet designs came in the realization that the capacity of a filter element depended on the reverse air volume of the cleaning jet. More volume could be applied per filter element and the footprint reduced drastically.

 

Cleaning System Designs must be sufficiently sized to develop the required volume to vent the process. The most volume that can be filtered depends on the number and size (or equivalent size) and number of valves. The advanced technology uses a proprietary supersonic nozzle design which increases the reverse jet flow by 70 per cent over a conventional orifice by getting a more efficient pressure to velocity conversion.

There are several considerations for safety and reliability that must be taken into account.

View and print ... Shot Peeners (PDF)
Read more about ... Advanced Technology pulse jet dust collector

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