Dust collection systems pick up dust generated by process equipment and move it away for disposal or to be recycled. They use hoods to gather the air around the dust generation equipment. The dusty air is then sent to the dust collection equipment through ductwork. The dust collection equipment often consists of a pre-filter to help collect the dust followed by the main dust collector. An exhaust fan is often at the end and it powers the system by pulling the air through each of these components.

Cyclones

Cyclone dust collectors are very dependent on the airflow that is going through them. Cyclone pressure drops are based on many factors, but the two most important are the geometry of the cyclone and the volumetric airflow through the cyclone. The pressure drop formula of a cyclone has the square of the volumetric airflow through the cyclone. So, doubling the airflow will quadruple the pressure drop (2x ACFM leads to 4x dp). If your airflow changes through the system, the pressure drop through your cyclone can fluctuate. 

Since dust collection system static requirements are all related, the exhaust fan will find its operating condition naturally. However, if the airflow decreases, then the removal efficiency of cyclone will decrease too. This means the cyclone will remove less of the finer dust particles, thereby sending them on to the filters. That will cause greater dust loading for the filters, which could increase the frequency of cleaning (more plant air usage, greater filter wear, etc.) and decrease the lifespan of the filters.

Filter Collectors

Filter collectors would actually benefit from lower airflow. The lower airflow would decrease the air to cloth ratio, which can extend the life of the filters. However, as described in the rest of this paper, the overall performance of the dust collector system will decrease. And if the system has a cyclone dust collector, the benefits gained by a lower a/c ratio could be counteracted by higher dust loading coming through the cyclone.

Higher airflow will increase the air to cloth ratio and could put more strain on the filters. The velocity of the air going through the filter could increase enough that holes in the filters could develop, thereby causing dust to bypass the filters.

However, filter collectors have a pretty good range of airflows they can handle without significant effect on the system. Unlike cyclones, the pressure drop in a filter is based less on airflow and more on the filter pack on them.

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We all know that if we don’t regularly maintain our cars, we will end up having issues in the future. If we forget to change the oil, change the brakes or get a tune up, it will eventually catch up to us. Why do we think our dust collection system is any different? Dust collectors are made of various pieces, all of which have to be operating correctly for the system to operate as desired. Usually, a dust collection system consists of the dust collector, exhaust fan, ductwork, and hoods/pickup points. Additional accessories such as airlocks, controls, instrumentation, and dampers all can affect the operation of the system.

The following questions with quick explanations will help you evaluate your dust collector. If after going through this guide you think your dust collector needs a tune-up, contact a local expert or Aerodyne Environmental and have them come in to help you get your system working properly.

Pickup Points

1. Are the areas near the pickup points dusty? Is there any dust in the air, is it hazy, or is breathing in the area a bit difficult?

2. Are there dust piles around the area? Dust piles and layers of dust can become airborne and if the dust is explosive, it can lead to an explosion causing significant damage and injury.

3. Can you feel airflow being pulled into the hood or pickup point?Often times, dust collection systems will be modified and this could cause a loss of airflow at other areas in the system.

4. Did the dust collection system ever work as expected? Sometimes a dust collection system has never fulfilled expectations. This doesn’t mean that changes can’t be made to get it working better.

Ductwork

1. Does the ductwork have holes? Holes allow air to leak into the system, this will cause less air to be picked up at your hoods and pickup points.

2. Are you plugging up your ductwork? Plugged up ductwork is caused by low air velocities through the ductwork. This will cause low airflow at your hoods and is an explosion danger.

3. Have you added or de-commissioned pickup points on your system? Adding or removing pickup points can cause the system to be out of balance. If you have done this without re-evaluating the whole system, your system may not be operating correctly.

Dust Collector

There are a variety of different dust collectors available—baghouse and cartridge collectors, cyclones, and wet scrubbers. Each one of them has different things to look at. We will only address a few of them here. Contacting a dust collection expert such as Aerodyne Environmental can help you diagnose issues.

Baghouse and Cartridge Collectors

1. When was the last time you changed filters? Some customers have to change filters twice a year, others haven’t changed them since they were installed. Both examples could have issues.

2. What is the pressure drop across your filters? Pressure drops over 6” W.C. usually tells you that you need new filters soon. A low pressure drop can mean that you have holes in your filters or haven’t properly developed a dust cake on the filter.

3. When was the last time you did a maintenance check on your dust collector? If you haven’t done it in the past year or two, it might be time. Filters, tube sheets, diffusers, air valves, manifolds and a variety of other items can all develop issues.

Cyclone

1. Have you checked your cyclone for holes? Cyclones often wear faster than other dust collectors, so a yearly check for pin holes or wearing is always a good idea.

2. Do you have a filter after your cyclone? Since cyclones have lower removal efficiencies than most other dust collectors, if your cyclone doesn’t have a filter afterward, you might want to monitor the emissions regularly to make sure you’re not emitting too much dust.

Wet Scrubbers

1. What is the pressure drop through the wet scrubber? Higher than normal or lower than normal pressure drops always tell you something is happening. Higher pressure drops usually means that dust is building up in the system. Lower pressure drop usually means that less airflow is going through the system.

2. Do you monitor your water usage and the particulate loading of the overflow? The overflow stream of the wet scrubber removes the particulate and the dissolved solids from the system.

3. Are you taxing your water treatment plant? Water treatment is expensive so if you can lower the amount of water going to it, you could save money.

Exhaust Fan

The exhaust fan usually provides all the motive force in a dust collection system; therefore, if the fan isn’t working properly the whole system will be having issues.

1. Is the fan vibrating excessively?
2. Is the fan making more noise than usual?
3. Are the bearings in the fan running hot?

Excessive vibrating and unusual noises can mean that something is wrong with the fan. Damage to the impeller or dust build up on the impeller could cause this. Some fans do get unbalanced over time and could cause the fan to stop working. A bearing running hot could be caused by an issue with the fan or an issue with the bearing. Hot bearings, while unlikely, have been known to cause fires or explosions if all the conditions are right.

Miscellaneous

Other items to check on your dust collection system include dampers, airlocks, controllers and instrumentation. Dampers not operating correctly, or accidently opened/closed can cause operating issues. Airlocks under dust collectors can cause air leakage into the system or prevent dust from leaving the system. Controllers and instrumentation can cause the system to shut down, or provide wrong reading, thereby affecting the operation of the system.

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Using bags, cartridges, and filters for dust collection is one of the best ways of capturing dust particles in the airstream. The filter allows the air to pass by while preventing the particulate from passing through. The dust particles then begin to build up on the filter. This build up helps prevent smaller particles from passing through, but also makes it harder for air to pass through. Over time this dust layer will prevent the air from passing though, thereby plugging up the filters. 

To prevent plugging of the filters, they are often cleaned by blowing air through them in the opposite direction of airflow. Baghouses and cartridge collectors use compressed air which expands the filter slightly so that it drops off material. The dust then falls into the hopper below. The periodic expansion of the filters will over time cause holes to form.

Dangers in Cleaning Out Filters

For HEPA filters and other inline filters, it is not as easy to clean filters during operation. Usually the filter needs to be pulled out of service and then air is blown through the filter (in the opposite way) to remove the dust from the filter. However, using high pressure air to blow out the filters can also cause holes to develop in the filter.

When these holes develop in filters, air will rush through the holes, taking dust with them. Since the holes in the media have lower resistance than the filters, air will find it easier to go through the hole than the filters. This will cause the hole to grow in size over time. And as more and more air passes through the hole, more dust will bypass the filter and contaminate the cleaned air.

Pressure Gage Is Recommended

When using filters, it is always recommended to install a differential pressure gage across the filter. As dust builds up on the filter, the differential pressure will increase. And as the pressure drop across the filter increases, less air will be flowing through the filter. So when you notice a high pressure drop across the filter, this means it is time to remove and replace or clean the filters. If you don’t replace the filter and the pressure drop decreases then it probably means that you have a hole (or holes) in your filter.

So when you are cleaning your filters, be sure to check that no holes develop. The holes can be caused by high airflow when blowing out or after repeated expansions by compressed air. The use of differential pressure gages will help you identify if a hole develops before noticing dust in the clean air. If you find that you are cleaning or replacing filters too often, installing a pre-filter such as a cyclone can lower the dust loading on the filters, thereby extending their life.

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When we think about dust particles being picked up by dust collectors we think of the particles as small spheres. And often for theoretical removal efficiency of a dust collector the dust is assumed to be a sphere. This is done to simplify the calculations as spheres act predictably compared to other shapes. However in real life dust isn’t normally spherical in shape. Dust is often created by material rubbing against each other and breaking off. This material often breaks along weak points in the material structure, which often isn’t spherical in shape. The material can have a wide range of shapes.

The heavier the material the less the material shape affects how the particle acts in the airstreams. While the lighter the particle the more subjective the particle is to the forces being applied by the airstream. For example, two pieces of dust that weigh the same will act differently if the volumes of the two particles are different. The lighter unit (larger volume) will be harder to capture in a dust collector than the heavier one.

Spherical Particles

Spherical particles are easier to predict when flowing through an airstream. A spherical dust particle looks exactly the same no matter what angle you are looking at it. So basically the particle will react the same no matter which way a force acts on it. However as the particle becomes less spherical, the geometry of the particle offers more area for forces to affect it from certain angles and less from others. For example, a cylindrical particle will have smaller surface area if the force hits the round ends (let’s call these top and bottom) of the cylinder rather than hitting the longer straight walls (let’s call these the sides) of the cylinder. And the more surface area available the more force will be applied to the particle, which can cause the particle to move in the airstream, ex. spin, wobble, etc.

Particle vs. Filter

To understand how the shape of a particle can affect its removal in a dust collector let’s look at the cylindrical dust above heading to a fabric filter. If the particle reaches the filter with its side facing the filter, the chances of it getting through the small openings in the filter are very small. While if the particle reaches the filter with the top/bottom facing the filter, it could possibly slip through the opening and get past the filter. So as you see, the orientation of the particle could affect whether it is being collected or not. And since there are thousands of particles moving through the dust collector, there will be a small percentage that will hit the filter just right and pass through, thereby lowering the removal efficiency of the dust collector.

So if your dust collector isn’t getting the removal efficiency that you expected from theoretical calculations, the particle shapes could be the cause of the lower removal efficiency. Special particle size tests can be done to show the different shapes so that the particle shapes can be taken into account.

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One common mistake operators make in dust collectors is to use the dust collector hopper to store material. The hopper in a dust collector isn’t designed for this. Instead it is a temporary home for the dust collected while the material is being removed. This period should be as short as possible for the following reasons:

• The dust collector isn’t designed to support a hopper fully filled with material. The added weight could cause structural issues with the vessel and the supports.
• As the hopper gets fuller, there is a greater chance that material will be re-entrained into the airstream, thereby causing a lower removal efficiency, increased wearing on the housing, filters, etc.
• Large amounts of dust in the hopper could become airborne during an incident, which could fuel an explosion in the dust collector. Removing the material from the hopper isolates the material storage from the dust collector.
• Storage in the hopper could cause bridging or rat holing of the material. This could cause the material to backup into the separation zone (example: begin covering filters). This will cause major operational issues in the dust collector and decrease removal efficiency and airflow through the system. 

So when you are operating your dust collector, be sure to remove the dust collected in the hopper as soon as you can. Airlocks such as rotary valves, double dump valves and trickle valves (ex. Aerodyne’s Vacu-Valve) are ideal for keeping the process isolated from the outside while also allowing the collected material to leave the collection hopper.

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The airflow in dust collection systems is crucial to the proper operation of the system. The dust collection system consists of the hood/pickup points, ductwork, dust collector(s) and exhaust fan. The hood/pickup points are designed to capture the dust. This design requires a range of airflows to properly work. If too little airflow is going through the hood, dust will escape from the hood. If too much air is going through the hood, the system can capture material it isn’t supposed to (example picking up product from the conveyor belt, not just dust lingering over the belt in the air).

The Important of Ductwork

Ductwork is like a highway for the dust in the dust collection system. It allows the airflow to be directed to the dust collector from the pickup points/hoods. The ductwork should be sized so that the airflow velocity is fast enough to keep the dust in suspension, but taking into account that the faster the airflow the higher the pressure drop is through the ductwork. The minimum velocity required to keep the dust in suspension is dependent on the dust. It can vary, but usually 4500 FPM is a safe velocity. Elbows can also increase the pressure drop of ductwork so try to minimize the elbows when designing the ductwork. Make sure you know the pressure drop in your ductwork so you can have enough static pressure in your fan to keep the design airflow.

Pressure Drop in Dust Collectors

Dust collectors have a pressure drop associated with them. Usually the higher the pressure drop the greater your removal efficiency will be, however different technologies will have different pressure drop and removal efficiencies. For example 10”WC pressure drop on a cyclone will have a lower removal efficiency than 10”WC on a filter. Be sure you have enough static pressure to operate your dust collector throughout the normal life of it. A fabric filter will build up dust on it, thereby increasing its pressure drop over time. Be sure to have enough static pressure to handle the required airflow at the point the filters are dirtiest, or else your airflow will suffer.

Exhaust Fan vs. Airflow

The exhaust fan should be designed to provide the required airflow with enough static pressure throughout the operational life of the system. This means having enough static available at the higher airflow (pressure drop increased in hood, ductwork, and dust collector) and to handle dirty filters until they are replaced (or cleaned). It is often good to slightly oversize the fan and to use a variable frequency drive (VFD) to adjust the fan as required.

Another good device is a digital airflow meter (such as Aerodyne’s GPC airflow meter) which lets you monitor the airflow through the system to be sure it is operating as it was designed.

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Material of construction is extremely important to the durability of a piece of equipment. The suitability of the materials of construction is based on the process, which includes the different components, phases, temperature, and pressure. For example: a cyclone’s material of construction would be dependent on the following:

Material (dust) Being Collected

Properties of the dust being might dictate the materials of construction. Food or pharmaceuticals will usually require stainless steel to prevent / minimize contamination. Carbon steel is often acceptable for wood applications. Other times, the material properties require a special material. For example, abrasive material might require AR steel or a coating to help prevent erosion of the cyclone.

Chemical Composition

The chemical composition of the process can dictate the materials of construction. If a component will react with the materials of construction, it could cause premature failure. For example, water or high humidity can cause rusting of carbon steel so stainless steel might be better suited. Or if an acid is a component of the gas stream, then a high alloy metal or fiberglass construction might be better suited.

Temperature and Pressure

High or low temperature can cause materials to change their properties. Material can become brittle or they may react more with components. For example fiberglass can’t handle higher temperatures whereas metals usually can. And some material might have good compatibility at lower temperatures but very poor compatibility at higher ones. Sometimes pressure can affect the material properties, but usually it will affect the thickness of the walls.

Aesthetics

Some customers have plant requirements for their equipment. For example, a food or pharmaceutical facility may require stainless steel construction even though the equipment is on the waste process and everything will be disposed of.   In a similar vein, the finishing of the equipment might be determined because all the other equipment has a high polished finish and the customer wants it to fit in.

So, when determining the materials of construction, there are many factors that contribute. It is best if the end user, with help from the vendor(s), determine the materials of construction. The end user usually knows the process better than the equipment manufacturer and therefore is in the best position to determine the materials of construction.

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Rotary valves will occasionally jam. This can happen when material gets between the rotor and the housing or when oversized material cannot fit into the rotary valve pockets.   Depending on the type of jamming that is happening you handle the situation differently.

Space Between Rotor and Housing

Rotary valves have a space between the rotor and the housing. This space allows the rotors to rotate freely but it can also allow air to leak across the valve. When materials falls on the rotor edge it can buildup and jam the rotary valve. So for existing rotary valves, you might have to replace or modify the existing rotor. Beveling or chamfering the rotor edges help the material to fall off the edge and into the valve pockets.

When Jams Caused by ...

For jams caused by oversized material, a valve with larger pockets is required. This could mean a larger rotary valve or replacing the rotor with a rotor containing one less pocket. Please note if the rotary valve is on an explosive application, NFPA requires a minimum of six vanes on the rotor. If these options aren’t possible, looking into a double dump valve might provide larger clearance for oversized material to pass through.

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One of the issues when figuring out if your dust is explosive is the particle size of the dust and the location where you are assessing the dust. Material that consists of very large particles sizes (larger than 500 micron) tend not to be explosive. While larger particles can catch fire, they tend to prevent explosions. However, this might not be as straight forward as you think.

Is the Process Explosive?

Let’s assume that you have a conveyor belt that feeds a grinder which then empties onto another conveyor belt which feeds a bagging station. Let’s assume that the material starts out with a particle size distribution of 10% less than 500 micron and 90% greater than 500 micron of a combustible material. After it is fed into the grinder, its target size is around 200 microns. The question is, is this process explosive? Your 1^st thought would be that before the grinder, the dust is too large to be explosive, and you might want to take the material leaving the grinder and have it tested. So you test the material leaving the grinder on the conveyor and find out it isn’t explosive. Therefore you assume the whole process isn’t explosive. However, this might give you a false reading.

Is the Dust Cloud Explosive?

For example, the material falling on the conveyor belt that feeds the grinder creates a dust cloud. That dust cloud is made up of the finer particles, so it is probably made up mostly of the 10% that’s less than 500 micron. And this material could be explosive. Not only that, but if that 10% of the material particle size distribution has a significant amount of dust less than 100 micron, this could very well be the dust cloud which is much more likely to be explosive. And since the smaller the particle size the better the chance of the material being explosive, your dust cloud could be explosive dust. This would then float in the building and layer the surrounding equipment or rafters with explosive dust.

Now looking at the material leaving the grinder, even though it tested as not explosive this is the material on the conveyer or that was bagged. If the grinder feed creates a dust cloud then this will again be the finer dust. So while the majority of your material is around 200 micron, there is a small portion (now much greater than the initial amount that was less than 100 micron) that is very fine. And again, this portion of the dust is the dust that is creating the dust clouds at the exit of the grinder and in the bagging section.

So when you are collecting dust to see if it is explosive, be sure to collect the material that is pertinent. You should try to collect dust that is actually forming a cloud to make sure it is explosive and not lump it in with material that won’t form the cloud. If you have an existing baghouse or cartridge collector, use the material that is collected in the dust collector for testing.

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Now that it is spring, it’s time to start thinking about summer shut down! If you are planning on doing maintenance on your dust collection system now is the time to order your parts.

If you are changing bags also look into doing the following:

1. Pull your airlock off and perform any needed maintenance on it (replace bearings, change out rotors or plate, replace Vacu-Valve sleeves, etc.)
2. Have someone check your exhaust fan a few weeks before shutdown so that if the fan needs rebalancing or bearings replaced, you can schedule it during the shutdown.
3. Inspection of your baghouse a few months before shutdown so any damage in the housing, bad air valves, tube sheet, etc. can be replaced during the process.
4. Inspect the ductwork and look for holes and dust buildup. Make sure dampers are working.
5. Look into installing a cyclone pre-filter to
    
   1. Increase filter life
   2. Decrease compressed air usage
   3. Collect dust before contamination in the filters
6. Inspect explosion protection equipment and have any chemical isolators/suppression equipment maintained. Be sure to have the clearance of the rotary valve measured to be sure it is still in compliance with NFPA requirements.
7. Schedule a Dust Efficiency Clinic to evaluate the system performance so any modifications can be installed during plant shutdown.

Doing the above will help you revitalize your dust collection system and make sure you get the best possible performance out of it.

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To improve efficiency and safety, there is no substitute for an on-site inspection by an experienced expert. Click below to start with a free 20-minute phone consultation by clicking the button.