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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All dry dust collectors have a hopper which temporarily collects the dust while it is moved out of the system. The hopper usually has a flanged outlet on bottom which allows the dust to fall outside of the dust collector vessel. When you are deciding what to put under the hopper it is important to make sure that air doesn’t flow out of or into the opening while still allowing the dust to empty from the hopper. An airlock (discharge valve) is used to prevent this.

An airlock comes in a variety of designs. The most common is the rotary airlock while the simplest design is a trickle valve (Aerodyne Vacu-Valve). What these valves do is prevent the higher pressure air from going to the lower pressure air. Generally dust collectors are designed to be under vacuum. This prevents dust from escaping the vessel and helps protect the exhaust fan from being damaged by dust loading. However, some systems do have pressured dust collectors for operational reasons. Either way it is important that there be some kind of airlock under the hopper.

Dust Collector Under Pressure

A pressurized dust collector will blow air out of the hopper if no airlock is installed on the hopper. This will create a dust cloud around the dust collector. If indoors this will coat the surrounding equipment and become a nuisance to employees in the area. Plus it’s pretty unseemly having a dust collector spewing dust out in a facility.

Dust Collectors Under Vacuum

A system under vacuum, however, is much more impacted by not having an airlock. The airlock prevents outside air from entering the system. So if you don’t have an airlock the exhaust fan will begin pulling air into the system through the hopper. This does two main things. For one, it lowers the airflow at the pickup points that are collecting the dust. Air will flow the easiest path (much like water). So if you have an opening allowing air to enter through the hopper then air will take advantage of this. And since your exhaust fan doesn’t care where the air comes from it will pull much of the air through the hopper. This airflow means that there will be less air coming from the pickup points. This could cause insufficient dust pickup at those points and even dust buildup in the ductwork. The second issue is that any dust collected in the hopper could get re-entrained by the air entering through the hopper outlet and then leave the dust collector. This will decrease the system removal efficiency and could cause violations of permits, increased fan maintenance, etc.

So when you are operating a dust collection system, make sure that the dust discharge flange is installed with an airlock, that way you will have the system operating at its peak efficiency.

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How Do Vacu-Valve Dust Valves Work?

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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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Dust collectors can be used for a variety of reasons. Some dust collectors are placed around applications that create dust such as saws, grinders, lathes, etc. The dust collector system is used to pull air from the area producing the dust via a hood and through some ductwork until it gets to the dust collector. The dust collector then separates the dust from the air. The cleaned air is then vented out of the dust collector through an exhaust fan and either outside or back to the facility. Other dust collectors are used to clean the air in a facility. The dust collector collects air away from the equipment and processes generating the dust. Many times this air is collected up near the ceiling of the facility. The air is then sent through the dust collector and vented outside or back into the building.

Whenever possible, you should try to capture the dust as close to the process generating the dust as you can. The reason is it is easier to capture the dust closer to where it is generated. If you are collecting the air near the dust generation equipment you will not have to collect as much air, which allows you to size a much smaller system, thereby saving money on capital expenses and operational expenses. Whereas if you are trying to capture the dust away from where it is generated, you will have to process more airflow to capture the dust.

However, there are times that you cannot place hoods right by the dust generating equipment. In those situations you will need to design your dust collection system so that it is big enough to capture the dust while keeping it as small as possible. This might mean building special hoods, using curtains, etc. all to help minimize the airflow required while maximizing the dust entrainment.

Dust collectors can also be used in vacuum systems. A vacuum system allows you to vacuum up dust around the facility without using portable vacuums. This might be advantageous when dealing with explosive applications, since most portable vacuums aren’t rated for explosive applications. And with the NFPA specifications requiring housekeeping of the dust facilities as a main action item, having vacuum systems for explosive dust is desirable.

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What information do I need to size my dust collector?

The attached Questionnaire will provide most of the questions that would be asked for a dust collector. The information that absolutely must be provided for a dust collector to be sized and quoted are:

1. What is the airflow through the dust collector?
2. What is the temperature and pressure the dust collector will experience?
3. What is the dust being collected?
4. Is it explosive?
5. What removal efficiency do you require?

The five above will allow a supplier to provide a quote. However, with only the above information, the dust collector performance cannot be guaranteed.

See dust collector questionnaire.

Is my dust explosive?

To help answer this question, NFPA has released NFPA-652 and 654. A combustible dust is defined as a finely divided combustible particulate solid that presents a flash fire or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations.

Basically, what this means is when the dust is in the air and its concentration is enough to cause a flash fire or propagate a deflagration or explosion if exposed to a spark or heat source, then it’s considered combustible. Continue Reading.

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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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5 Common Mistakes When Selecting a Dust Collector

The video presents a quick, do-it-yourself examination that helps identify symptoms of possible inefficient dust collection.

 

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A prominent coffee bean roaster is planning on building a new roasting facility in New Jersey. The head engineer contacted Aerodyne about trying to find equipment that could separate the coffee beans as they are pneumatically conveyed from one portion of the facility to the other. The engineer specified that the problem the facility was facing was space.

How Aerodyne GPC operates Differently in Coffee Industry

After sending over the specs of the application, Aerodyne concluded that the roasting facility could easily house a GPC-20 horizontal dust collector for the operation. The Aerodyne GPC Dust Collector operates differently than other dust collectors. A sloped spiral inlet directs the dirty gas stream toward a fixed ground plate and hopper of the dust collector.

The ground plate forces vortex reversal to occur in a much shorter space, eliminating the need for a long, tapered body. As the dirty gas stream strikes the convex ground plate, fine particulate that has not completely made it to the dust collector walls is deflected into the hopper. The ground plate also shields collected particulate from the forces of the vortex reversal, acting as a barrier between the separation chamber and the collection hopper. This innovative design enables a compact dust collector to operate at high efficiency, even when installed horizontally.

The GPC offers a unique dust collection solution. With the compact size and excellent removal efficiencies, it is an economical and low maintenance solution to removing dust and particulate from an air stream.

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Aerodyne GPC Used in Coffee Roaster's Process

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