Dust collectors have hoppers under the collection chambers to channel the dust out of the system for use or disposal. The hoppers are not designed to be used to collect and store the material, but rather as a place for the material to temporarily collect before passing through the airlock and out of the system. 

Dust Build-Up in Hopper

Depending on the material being collected and the characteristics of it, dust may temporarily build up in the hopper until it can pass through the airlock. Applications that have periodic high loading or applications that have materials with irregular shapes, are sticky, or hygroscopic could all build up in the hopper and form a bridge or rat hole by the dust discharge opening. When this happens, one of the most common solutions is to install a vibrator on the hopper.   The vibrator works by hitting the side of the walls which can cause the material to break up and fall through the dust discharge flange and leave the system.

The Purpose of Vibrator on Hopper

Vibrators can be driven pneumatically or electronically. They also can be used to continuously agitate the hopper preventing any bridging from developing, or they can be periodically turned on to break up any bridging that has developed. When you are installing a vibrator, it is important that you let the manufacturer know how you are planning on using the vibrator so they can recommend the correct vibrator for your application.

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When you are designing your ductwork, there are two ways you can do it. 1) You can use dampers to control the airflow at each pickup and balance the system or 2) You can balance by design which means the ductwork is designed so that the proper airflow goes through each pickup without the need to install dampers. There are positives and negatives with both. The following will go over them.

Balance by Dampers

Using dampers to balance your system provides much more flexibility but is much harder to achieve. This design has dampers at each pickup point which allows you to dial in the design airflow. It will allow you to add and remove pickup point (within reason). Of course, the duct needs to be designed so the minimum air velocity is maintained to keep any dust in suspension.   The major issue with using dampers is that you have to balance the whole system, and every change affects every other damper. This means it takes skill and a lot of patience to balance the system. And any change, whether it is adding or removing a pickup point, the accidental (or deliberate) adjustment of the damper will affect the rest of the system. This could mean a total rebalancing of the whole system.

Basically, what this means is that if you require a versatile system which allows you to bring areas online or take offline while keeping the system as small as possible then using dampers is best for you. Just remember that every time you change the damper location you will need to rebalance the system. If you are not experienced this can be a very frustrating and time consuming job.

Balance by Design

The balance by design ductwork is designed so that the resistance throughout the system is such that the correct amount of air is being picked up at each pickup point. This has a few benefits, the first being it is an install and forget design. (Please note we don’t actually mean forget). Once the ductwork is designed and installed, it will naturally pick up the designed amount of air until something changes. A properly designed system will have enough air velocity in each area to keep dust from falling out. So this means you won’t need to adjust anything. We would recommend periodic checking of airflows to be sure the system is operating at the right airflow. If you find that it isn’t then the ductwork has been damaged, changed, or material has built up in it.

The negative of this design is that once it is installed you can’t change it easily. If you are adding or removing any pickup points then you may need to modify the ductwork to be sure you keep the design airflows. You will have to pay special attention that your airflow remains high enough to keep any dust in suspension. The other negative is that it will pull the airflow from all pickup points all the time. And if you are looking to use only one pickup point at a time, then the system will be bigger than might otherwise be needed.

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Hygroscopic dust can really grow on your nerves! I know the marketing department might take away my writing duties after that line. But seriously, hygroscopic dust can cause major maintenance issues in your dust collectors. Hygroscopic dust captures water vapor and droplets. This causes the particles to grow in size and weight, thereby making them easier to capture and remove from the airstream. However, if you are using filters, that’s where the problems start.

Filters capture dust in between the filter fibers and on the filter cake. When the dust is hygroscopic this can lead to issues. Dust particles in the filter fibers can grow when exposed to water vapor. They can then not release when cleaned, blocking the air pathway, or if they do release during cleaning the filters could be left deformed, allowing particles through the filter.

When hygroscopic filter cake is exposed to water vapor, it can plug up the filter, preventing air from passing through. This prevents any dust collection from the needed areas because there is no airflow to pull the dust into the system. And during cleaning cycles, the filter cake is strongly adhering to the filter and won’t fall off.

To prevent this from being an issue you will need to identify where the water vapor is coming from.

Airlock

If humidity is leaking in from the airlock then a low leaking airlock would be needed. Rotary valves have a small space surrounding the rotor allowing it to rotate without locking up. This area constantly allows air to leak around. A double dump valve which uses two flaps in series to isolate the hopper or a rotary valve with wipers can cut down on air/humidity leakage into the hopper.

Housing holes

If there are holes in the housing, high humidity outside air can leak in. Replacing or patching the holes will prevent air leakage. If corrosion is an issue, changing materials of construction or coating the hopper might help prevent future issues.

High water content in the compressed air used for cleaning

Install water traps and filters to keep the compressed air used for filter cleaning with low humidity. Often times this can cause greater issues than leaking airlocks or hoppers because the high pressure air goes through the filter, expanding the filter to shake off the dust. This delivers the humidity to the dust collected in the filter, which could cause greater pluggage.

High humidity air coming in with the dust

Installing a pre-filter will remove the majority of the larger heavier particles before they can see the filters. This minimizes the amount of material the filters contact, extending their life. Pre-filters such as cyclones and dropout boxes can often provide additional help on issues above and even extend the life of filters caused by high dust loading.

So when you have hygroscopic dust and your filters are plugging up too fast, try to isolate the area where humidity is coming.

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This is one of the most important questions you can ask. The answer to this question can require you to invest in expensive protection equipment or not, but that is much better than the loss of thousands of dollars in equipment, production time, and most importantly, employee safety. There are a few ways to answer this question. Some of which are safer than others. The main question you will need to ask yourself, is the dust combustible?

Is My Dust Explosive?

If the material can react with oxygen then there is a chance it can be explosive. However, just because it is combustible, doesn’t mean it is explosive. Generally if a dust is greater than 500 micron in size, it is not explosive. However, what if the dust is smaller than 500 microns?

The safest course is to send your sample into a test facility for a simple Explosibility Screening (Go/ No Go) test. This test will take a small sample and try to ignite it to see if the sample will explode. If it explodes then an explosion severity test should be performed, which provides data on the speed of the explosion (K_st) and strength of the explosion (P_max). This data is very important to designing equipment that will prevent, contain, or divert explosions safely.

Some dust might be generated from material you got from a vendor, and they might have already done explosion testing on the material. You might be able to use this data for the design of your equipment depending on the process. The main thing being, what are you doing with your process? If you are sorting, grinding, drying, etc. the material then you can be changing the explosibility of the dust. If there has been a change, then the material should be tested again or you can assume it to be explosive and use the suitable published data.

The Explosibility of Dust

The explosibility of dust is highly dependent on the material size and humidity. A few examples would be aluminum. Spirals of aluminum from trimming wouldn’t be explosive, but a fine aluminum powder would be. Another example is wood dust. Dry wood dust is very explosive, while wood with a high water content is not.

You should not depend on your history working with the dust, because dust explosions are fickle. You might be working with dust for 20 years and have no issues and then suddenly there is an explosion. Or it might be the 1^st time you work with a dust that an explosion happens. One thing is for sure, if you have had flash fires with the dust, you definitely should have it test for explosibility. A flash fire is a sign that you have an issue with your process and should do further investigation to make sure you won’t have an explosion and to prevent more flash fires.

Dust Hazard Analysis (DHA)

If your dust is explosive, you will then need to have a Dust Hazard Analysis (DHA) done on your process/ building, etc. The DHA reviews your process, operational and maintenance procedures, equipment, etc.   It then provides procedures, equipment changes, etc. to address the issues and provide for a safe process. OSHA will request a copy of your DHA during any inspection. So it is important that you have an answer to the question “Is my dust explosive?” and have something to back up the answer in case you are asked by OSHA, your insurance company, local fire inspector, etc.

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What are the benefits of using a cyclone as pre-filter for a baghouse / cartridge collector?

If you are working with a high value material or product, have high maintenance costs to the baghouse/cartridge collector, or want to avoid cross-contamination, then you should investigate using a cyclone ahead of your baghouse or cartridge collector. The cyclone will allow you to recover the dust without contamination from the filters. The filters in a baghouse or cartridge collector use a dust layer built up on the filters to collect the dust from the airstream. During the process of filter cleaning, dust from this layer is dislodged and falls into the hopper. Any particulate from prior batches can cause contamination of the dust.

A cyclone, on the other hand, collects the dust and deposits it in the hopper continuously. There is very little dust buildup inside of a cyclone. This minimizes the chances of contamination from prior batches. Since there aren’t any filters, a cyclone can be washed or wiped out to remove the prior batch particles.

How does ductwork affect my system?

Ductwork is usually by far the largest component and often the most overlooked. Depending on the size of your system, the ductwork can span hundreds of feet and have dozens of side streams. The ductwork is like railroad tracks, it moves the dusty air from one place to another.

Often times, additional lines will be added to a dust collector system after installation. Without proper evaluation of the system, this could negatively affect the performance of the whole system.

This means that just because the dust collector system was operating correctly before, it might not after an additional pickup point or hood is added. What happens is that when you add additional pickup points, you change the balancing of the system. This could change the airflow to each and every hood and pickup point in the system, so while the system was originally adequately venting an area, it might not after a change.

A few things can be done to address this, such as changing fan speed, adding dampers, modifying ductwork, etc. The main thing you need to keep in mind is that if you slow the airflow through ductwork too much, you begin to build up dust within the line. This will further restrict your airflow and it can become a fire / explosion hazard.

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Airlocks FAQ Volume 1

Airlock valves are important components in many dust collection and process systems, yet they are often overlooked. We hope this helps you choose your next airlock.

 

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Designing a dust collection system can be quite a daunting task. With so many collector options and so many application variables to consider, it is difficult to know where to begin. Cyclones are among the oldest and still most reliable methods of dust collection available. Because they require very little maintenance, have low up-front cost, and offer unmatched versatility, cyclone collectors remain a viable solution to many air-handling challenges. Although heightened environmental regulations and collection efficiency needs have shifted industry toward the use of filter-media collectors, cyclonic dust collection still plays a vital role in many air-handling systems. These five questions will help determine if a cyclone dust collector is right for your application.

1. How big is my dust?
   

   Cyclonic dust collection relies on inertial forces to separate dust particles from an air stream. The larger and denser the particulate is, the greater its inertia. This is the reason cyclones have such high collection efficiencies when handling relatively large dust particles.

2. How much dust is too much?
   

   Grain loading or dust loading refers to the amount of dust particulate that is suspended in a gas stream. This is typically measured in the number of grains per cubic foot of gas. This is an important number to consider when designing a pollution control system. Not only will this factor into the size requirement of a dust collector, but it will also determine the appropriate type of dust collector. The strict air pollution control standards in the United States often necessitate a “filter-media” dust collector, such as a bag house, for the final collection stage.

3. Can I reuse the dust I am collecting? 
   

   Dust generated by handling dry bulk materials can be hazardous but also valuable. Unfortunately, most filter-type dust collection systems are designed for disposal rather than product reclamation. Filter media collectors such as cartridge filters and bag houses often do not allow collected particulates to be recovered for reuse due to contamination or particulate size issues.

4. Do I have heat or humidity concerns?
   
   Air handling in manufacturing processes is often a delicate balance with a number of variables to contend with. Process heat and humidity in the air stream create a difficult challenge when it comes to dust collection. Collection of red-hot dust particulate is simply not possible with many bag houses because cotton filters are flammable and flame retardant filters can be costly.
5. How much should I spend?
   

   Perhaps the most important and most difficult question asked when designing a dust collection system is how much to spend. The simple answer is, it depends. It depends on what the overall goal of the system should achieve. The best dust collection systems are those that were designed with several functions in mind: capacity, operation costs, maintenance costs, and product/ material value.

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Double dump valves are available in both motorized and pneumatic actuation. To figure out which would be better for you we need to consider your application. A double dump valve is really two (2) valves with flapper plates in series controlled by the same mechanism. At least one of the flapper plates is closed at all time. The main things we need to know are the pressure, temperature, and flow rates along with the area classification the double dump valve will go in.

Motorized Double Dump Valve

A motorized double dump valve uses a cam rotated by a motor and gearbox to open and close each of the dump valves in series. The cam is designed so that at least one valve is closed at all time, thereby providing an airlock. The cam also provides a certain amount of cycles each minute, which controls the amount of material that is available to pass through the valve.

Pneumatic Double Dump Valve

A pneumatic double dump valve uses one pneumatic cylinder on each dump valve to control the operation of it. A timer is used to control the cylinders. The timer cycles cylinders so that one valve is closed at all times.

Differences Between Pneumatic and Motorized Double Dump Valve

Pressure - Motorized double dump valves are limited in the amount of pressure they can handle. The cam drive doesn’t provide direct force on the flap plates. This means that higher pressures and weights will push the flap plate open. The pneumatic valve, however, provides direct pressure on the flap plate so it can handle higher pressures and weights.

Temperature – A motorized double dump valve has a gearbox and motor located next to the housing of the valve. At higher temperatures, this can cause issues with the grease in the gearbox and motor. Also, the cam drive uses wear plates that might degrade faster at higher temperatures. The pneumatic valve, however, can be supplied with high temperature seals on the pneumatic cylinder.

Material Flow Rates – Unlike a rotary valve, which is continuously passing material through, double dump valves fill one chamber while emptying the other. This significantly cuts down on the flow rate through the valve. A motorized double dump valve size is limited due to the cam design. A spool piece cannot easily be installed to increase the volumetric flow through each chamber. While with a pneumatic design, each flap plate is controlled by its own pneumatic cylinder, a spool piece can easily be installed to allow additional material to flow through.

Area Classifications – The electrical classification of the area the valve is being installed can also effect the valve selection. A pneumatic valve has very few electric parts and can be designed to be intrinsically safe. A motorized unit has an electric motor, so the motor must be selected to for the area classification.

When selecting the best double dump valve for your application, look at the pressure, temperature, material flow rate, and area classification.

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Dust collection filters such as the bags in baghouses, cartridges in cartridge collectors, and HEPA filters all filter out dust particles by collecting them in between their fibers. Dust builds up on the bags and cartridges to form a filter cake over the filter further restricting the ability of dust particles and airflow to pass through the filter. This restricting of airflow causes a pressure drop (resistance) which is directly related to the amount of dust built up on the filters. As the pressure drop increases, the airflow through the collector will decrease. Most baghouses and cartridge collectors have cleaning cycles which remove some of the dust from the filters keeping the pressure drop low.

The cleaning process on the filters usually consists of blowing higher pressure air through the filters, thereby causing it to expand slightly. This knocks off some of the top layer(s) of the filter cake which lowers the pressure drop through the filters. Other units have the filters connected to a mechanical shaking system. The airflow through the filter will be stopped during the cleaning process. The mechanical shaker system will gently shake the filters dislodging the dust to fall into the hopper.

Even with these cleaning functions, the filters will eventually plug up and significantly decrease the amount of air they allow to pass. This will starve the pickup points of airflow, allowing dust to escape and never enter the collection system. Depending on the application, this can happen within months or years of putting in new filters. The process of replacing the filters usually requires the system to be turned off. Cartridge collectors are usually easier to replace than bag, but either way the time and labor it takes to change are significant.

Ways to Extend Filter Life

It is therefore very advantageous to extend the time between filter replacements. The following are a few ways to extend filter life.

• Install a pre-filter before the dust collector. The greater the concentration of dust getting to the filter, the faster the filter will plug up or fail. So if you can lower the dust concentration, you will extend the life of the filters. Pre-filters include cyclones and dropout boxes. Pre-filters remove most of the larger particles, leaving only the smallest to be handled by the filters.
• Install pulse-on-demand controllers. A pulse-on-demand controller monitors the pressure drop across the filters. And when the pressure drop gets too high, the unit will activate the cleaning process until the pressure drop falls below a shutoff point. This extends life by preventing unnecessary cleaning of filters which can cause holes to develop in the filters. It also can save money on compressed air and keep the airflow within desired range.
• Install different filters. Not all filters are the same. Some filters are designed to minimize issues with wet dust. Others have higher temperature ratings so they won’t degrade. If you are replacing filters too often, contact your filter supplier or system manufacturer to see if there are other filters that are more suitable. These special filters are more expensive, so it might be prudent to look at adding a pre-filter and pulse-on-demand controller to extend life even further at the same time.
• If you are plugging up the filters with hygroscopic dust, check the collector housing for leaks that might allow humidity into it. Also, check the compressed air supply to make sure humidity isn’t being added to the system.
• Install insulation on the dust collector if you have high humidity. Insulation will help prevent water vapor from condensing at night or during winter, thereby preventing water droplets from damaging the filters or causing dust to plug up in the filters.
• Install a spark arrester before the dust collector. Depending on the process being vented, sparks can be pulled into the dust collector which, thereby cause the filters to catch on fire. Installing a spark arrester prevents this. Often times a pre-filter (cyclone or dropout box) provide the same protection.

If you are replacing filters too often then think about making some of the above changes to your system next time you are due to replace filters. That way you can extend your filter life and operate for longer periods of time without maintenance.

 

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This shows some of the particle size ranges of common dust applications along with the range where dust collectors work. As shown cyclones and dropout boxes will not get 100% of the dust.

There can always be some material getting through as the particle size distribution might always include smaller dust than normal. Wet scrubbers and filter collectors (baghouses and cartridge collectors) will collect the larger particles easily. However, installing a pre-filter cyclone to remove the larger particles will lower water usage (wet scrubber) and increase filter life (filter collectors).  

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Five Signs Your Dust Collection System Needs a Pre-Filter

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When you have a dust collector capturing valuable material, you often have to regularly clean the dust collector to remove the material and if it is perishable, remove it before it contaminates the other material. Some dust collectors are easier to clean than others.

Wet Scrubbers

Wet Scrubbers are pretty much self-cleaning. Since they use water to capture the dust, it pretty much washes the walls and internals regularly. The main issue with wet scrubbers are that if you are trying to use the material (whether in the process or as a product) then you will need to remove it from the water stream (unless having it in a water stream is desirable).

Cyclones

Cyclones are very low maintenance. Since they use centrifugal motion to remove the material, there isn’t much buildup inside the cyclone. So there really aren’t many places to build up. Designing the vessel with access doors will help keep the cleaning easy. Aerodyne’s horizontal cyclone has a unique design that allows you access to the whole cyclone through its back plate.

Fabric Filters

Fabric filters collect the material on the filters (bags/ cartridges). This builds up a layer of material which remains on it for the life of the filter. During cleaning, the material falling off might have recently been captured or material that has been on the filter for an extended period of time. This could contaminate the material being collected.

1. Baghouses require the bag to be removed in order to be cleaned or replaced. Often times this is a very time consuming process and can often require workers to enter the baghouse. If contamination is a serious concern, baghouses should not be used.
2. Cartridge collectors are much easier with replacing the cartridges. Often times this can be done in minutes compared to hours of a baghouse. So you can remove the cartridges, clean them and re-install. This will allow you to clean or replace the filters after each batch, thereby minimizing any contamination.

One option is to install a cyclone dust collector as a pre-filter before your baghouse or cartridge collector. The cyclone will capture the majority of the material without contaminating it for use in your process / product. While the smallest fines will then be captured in the baghouse / cartridge collector for disposal or treatment. This allows you to capture as much of your valuable material with minimal cleaning / contamination while also providing the high removal efficiency for the environment.

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5 Signs Your Dust Collection System Needs a Pre-Filter

Watch the video from the Dust Efficiency Clinic discusses how using a pre-filter will optimize your dust collection system. 

 

If you prefer this valuable information in white paper form, get our whitepaper, Top 5 Reasons to Use a Cyclone as a Pre-filter.

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To learn more about which dust collector, please contact our experts at 440-543-7400 or visit our website: www.dustcollectorhq.com.

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