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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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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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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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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In most applications, cyclones do NOT replace baghouses, cartridge collectors, wet scrubbers, electrostatic precipitators, etc. Modern air pollution regulations require dust collectors to provide greater removal efficiencies in the PM2 and PM10 than most cyclones can provide.

How Cyclones work?

Cyclones collect dust by having the contaminated air enter (usually on a tangent) and spin around the cyclones walls until they get to the bottom. The air then reverses and spins up the middle of the vessel and leaves from the top, while the dust falls into the hopper.

The centrifugal forces on the particles and droplets, force them out towards the walls and away from the air exiting the center of the vessel. Cyclones provide high removal of the larger, heavier dust particles and moisture droplets.

The Benefits of Cyclones

Cyclones remove very high percentages of the larger, heavier particles. This means they often remove as much as 80%-95% of the total particles. However the removal efficiency of the smaller, lighter dust isn’t very high. The PM2 and PM 10 particulate will often get through the cyclone in high enough concentrations that the system won’t comply with state, local, and federal standards.

When installed in front of a dust collector (baghouse, cartridge collector, etc.) as a pre-filter, they significantly reduce the loading on the primary dust collector. This helps extend the operational life of the dust collector, increase removal efficiency, and decrease maintenance. Cyclones are often used to lower the loading on a dust collector (baghouse, cartridge collector, wet scrubber). The cyclone removes the large material, allowing the dust collector to get the high removal efficiency on the PM 2 and PM 10 particulate.

Aerodyne GPC Cyclone Dust Collector

View the animation of the GPC Industrial Dust Collector to see the compact, high efficiency cyclonic design. Unlike typical cyclonic dust collectors, the GPC Industrial Dust Collector uses a ground plate to force vortex reversal in a much shorter space, eliminating the need for a long, tapered body.

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Part 6: Scope of Supply

Cyclones do not stand alone in most applications.  They require additional equipment for them to work.  Depending on your application, equipment can include an airlock, exhaust fan, explosion protection, instrumentation, insulation, vibrator, and even ductwork.  The cyclone can be purchased as a system with all the equipment required to operate or it can be purchased by itself and integrated into an existing system.  The system is then put together by the end user or a 3^rd party.  Many times, the sum of the accessory equipment is greater than the cost of the cyclone.  A brief description of the potential accessories and how they affect the cyclone is below.

1. Airlock
   
   An airlock is essential for the proper operation of a cyclone.  It isolates the cyclone from the outside atmosphere.  Without it, material will fly out the bottom or get re-entrained in the system.  Airlocks could be a simple extended hopper (55 gallon drum), rotary valve, or specialized valves like the double dump valve or trickle valve.  Special designed valves are available for quick cleaning applications and explosive applications.
2. Explosion Protection
   
   Explosion Protection is a very important consideration for dust collectors as about 70% of dusts are explosive. The equipment that protects against explosions can add significant cost the cyclone.  Explosion protection equipment such as explosion vents, isolation valves, chemical suppression, and isolation systems can all quickly increase the price of a cyclone.  This equipment is required for explosive applications. 
3. Exhaust fans 
   
   Exhaust fans are required on every dust collection system. They provide the motive force required to pull air through the system.  When selecting the fan, be sure to take into account all ductwork and equipment.    It is best to install the fan downstream of all dust collectors so that the most efficient impeller designs can be used.  This will save money on operational costs.  However, if the fan needs to be installed in high dust areas, there are impellers designed for these operations.
4. Instrumentation 
   
   Instrumentation for cyclones are rather simple compared to other dust collectors. The main instrumentation used on cyclones are differential pressure gages/transmitters and level switches for the hoppers.  Explosive applications can have special instrumentation and some applications will monitor temperature and humidity for equipment downstream of the cyclone.  But for most standard cyclone applications, differential pressure is the only recommended measurement.
5. Insulation, Heat Tracing, Steam Jacketing
   Insulation, heat tracing, steam jacketing are required for special applications. These can add significant expense to the cyclone.  Insulation and heat tracing can be done at the fabricator but usually are done onsite to prevent damage in shipping and installation.  Steam jacketing must be done in the design and fabrication phase and can add significant cost to the cyclone.
6. Vibrators or Air Jets
   Vibrators or air jets are used for applications where material bridges in the hopper. Vibrators can usually be installed after installation and will help keep the hopper from bridging up.  Air jets blast air into the hopper, breaking up any bridging and letting material flow out of the airlock.  It is best to have the connections for these to be included in the cyclone design and installed once on site.

In summation, cyclone design and costs are based on the size and construction required in your specific application.  Special applications can require costly equipment and design changes that will increase the cyclone cost significantly.  When selecting a cyclone for your application, be sure to include all the factors in to your cost estimate so you are can accurately determine your budget.

Dust Collection Deathmatch: Baghouse vs. Cartridge

 

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Part 4: Application Requirements

Cyclones also need to be constructed so that they meet the specifications and requirements of the applications they are going in.  Some applications, like woodworking or where the material being collected is waste, don’t need any special constructions.  Applications such as food and pharmaceuticals can be very specific.  Every application, customer, process, and facility has their own set of requirements.  These could include a special quick clean access or a higher weld spec to prevent accumulation in the vessel.

1. Quick Clean
   
   Quick Clean design allows for the cyclone to be easily accessed for cleaning. It is often used in food and pharmaceutical applications.  These applications usually have a strict cleaning protocol and/or require cleaning between batches.  The design changes required for quick clean design can often significantly increase the price of a cyclone.  Often times it will require redesign of the standard cyclone to allow access and minimize material buildup.  Horizontal cyclones are often best for these situations.
2. Special Welds
   
   Special welds are required in most food applications. The welds are often areas where material can build up and contaminate the material being collected in the cyclone.  To prevent this, the welds are often required to be ground down.  This is a time consuming process and is done individually with power tools.  Depending on the specification, special welds can often significantly increase the price of the cyclone, even as much as doubling the cost.  Often times, the welds will require specific welders in a shop to work on the unit, thereby increasing the lead time and cost.  When you are specifying special welds, it’s often a good idea to have a sample or a picture of the weld you are looking for so the fabricator and end user are on the same page.  See the drawing below for an example of a sample.
3. Special Finishing
   
   Special finishing of the materials of construction is another requirement in many stainless steel applications. Some food and pharmaceutical applications require the metal to be finished or polished to a certain specification.  Usually, the internal finishing is the most critical due to it being in contact with product, but often times the facility has a specification on the external too.  This is often for cleaning and uniformity in the facility.  Special finishing of the metals can often significantly increase the price of the cyclone.  Often times, with the higher finishes allowance has to be made in the pricing for discarding material that doesn’t meet to specification after it has been worked on.

How Do Horizontal Dust Collectors Work?

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If you are like many plant engineers who are working within the confines of a limited work space, rest assured that you don’t need to compromise on the efficiency of your industrial dust collection system. A traditional, high-efficiency cyclone is not the best solution for facilities with low ceilings.

Space Limitation

If space limitations prevent you from properly orienting this type of cyclone in an upright position, this will result in a loss of efficiency due to particulate settling on the side of the collector and being carried out with the exhaust gas. Attempting to solve this problem by installing your vertically oriented dust collection cyclone outdoors may encourage condensation that can shorten service life and reduce efficiency.

Horizontal Cyclones

Today, more plant engineers are choosing a horizontal configuration for reliable, high-efficiency dust collection. These options include the Aerodyne GPC Cyclone and counter-cyclonic dust collectors, such as the Aerodyne SplitStream™ Dust Collector.  The SplitStream™ Dust Collector uses a secondary air stream that directs material toward the collection hopper, and may be installed horizontally with virtually no loss of efficiency.

Because this design does not rely on gravity to bring the dust to the hopper like conventional cyclones, its operational efficiency is not affected by horizontal installation. In addition, this type of dust collection system may be suspended from a ceiling, conserving valuable space on the manufacturing floor.

The SplitStream™ Dust Collector the ability to increase/decrease process temperature while collecting dust. The unique counter cyclonic design element of the SplitStream™ Dust Collector ensures that a minimal amount of particulate comes in contact with the interior walls. This eliminates excessive wear and enables abrasive particulate collection.

How Do Horizontal Dust Collectors Work?

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The airflow in a dust collection system is extremely important, since the air collected at the hoods is removing the dust from the affected areas.  This air is then sent to the dust collector to remove the dust from it.  While it is always desirable to use the least amount of air as possible in your dust collection system, you must have enough air so that it is actually doing the job it is meant to.   If not enough air is going through the system, you will not capture all the dust from the pickup areas, thereby allowing dust to escape into the facility, cause health / nuisance issues and/or fire hazards.  If the airflow is too high, you could be picking up product/ raw materials, wasting energy and increasing maintenance issues.

There are many reasons that the airflow through a dust collection system can change.  A few of them include:

• Environmental changes – temperature, humidity, etc.
• Changes in the system – such as opening/ closing dampers, dust buildup in the ductwork, damage to the ductwork and/or hoods.
• Dust collection issues – such a plugged filters, old filters, holes in the filter, etc.
• Damage to the exhaust fan – such as bearings, damaged impellers, etc.

Unlike liquids, there is no easy and inexpensive way to monitor airflow in a dust collection system.  The most common way is to periodically manually measure the airflow by inserting a pitot tube in the ductwork and measuring the air velocity.  The airflow is then calculated.

One option to monitor your airflow is to install a cyclone pre-filter ahead of the filters.  A cyclone’s pressure drop increases as the airflow increases in them.  Usually the manufacturer of the cyclone can provide a curve, showing the relationship between the pressure drop and the airflow.  So if you monitor the pressure drop through the cyclone, you will notice if the airflow suddenly increases or decreases.  And if the pressure drop drifts too far from a certain pressure drop you will know to troubleshoot the system.

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Dry dust collectors and humidity don’t react well together.  Baghouses and cartridge collectors operate by having dirty (dusty) air enter the housing.  Multiple filters (bags or cartridges) are located in the housing.  The air travels through the filter and then exits the dust collector.  The filters are a mesh of fibers that allow air to travel through small holes, while dust is too large to pass.  The dust builds up a layer (dust cake), further limiting the subsequent dust’s ability to pass through while allowing the air to pass.  From time to time the dust collector will clean the filters by shaking them or using compressed air to expand them.  This causes the outer layer of the dust cake to fall off.

Humidity can cause problems in the process described above.  Humidity is a way to express the amount of water in the air, (the higher the humidity, the higher the amount of water in the air).  When high humidity air enters a dust collector with fabric filters, there is a chance that the water vapor will condense and create water droplets or the dust on the filters will absorb the water as the airflow passes through.  Many dusts change their physical properties when the water content increases.  Some become sticky, some become hard like concrete, etc.  When this happens to the dust cake, it affects the ability of the air to pass through and the ability of the dust to fall off during cleaning.

When dust becomes sticky, it will adhere to neighboring dust particles and not want to fall during cleaning.  Dust that becomes hard (like concrete) will prevent air from passing through.  When one of these issues develop in a dust collector the pressure drop across the dust collector will increase.  Over time, an increased pressure drop will lower the airflow being pulled through the system.  This will decrease the airflow being picked up at hoods, therefore lowering dust collection at where operators are located.

Humidity can also be introduced in the dust collector in the compressed air.  When air is compressed, the temperature of the air increases, thereby increasing the water content of the air.  The compressed air is at its saturation point.  As the air moves through the line, it cools, thereby condensing water in the system.  If additional drying technology isn’t installed, the compressed air will have water droplets in it as it is used in the dust collectors.  This will cause the filter cake to wet and cause issues as described above.  This is why it is important to dry compressed air when dealing with dust that is affected by water.

However, sometimes no matter how you treat your compressed air, you continue to have issues.  This could be because the airline travels outside and in winter time it gets very cold. It could be because you are located near a large body of water, and humidity is high.  One way to minimize the effect on your dust collector is to minimize the dust getting to your dust collector.  Cyclone pre-filters are ideal for these applications.  Cyclones collect dust and water droplets using centrifugal motion.  This means they aren’t affected by the changes to the dust as a filter is.  Cyclone pre-filters can often remove up to 80% of the dust before a dust collector with filters.  This means you can lower the number of water droplets going into your dust collector and minimize the cleaning required, thereby not getting as much water from the compressed air system. 

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