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).  

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.

How Do Vacu-Valve Dust Valves Work?

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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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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.

Aerodyne GPC Used in Coffee Roaster's Process

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In dust collection one of the most important dust characteristics to define is the particle size of the dust. The larger the dust the easier it is to capture. While the smaller the dust particle the harder it is to capture and remove from the airflow. This applies to both heavy and light particles. What this means is that in order to accurately predict your removal efficiency, you need to know the particle size distribution.

Breakdown of Particle Sizes

The most useful particle size distribution includes a breakdown of the particle sizes under 50 microns in size. Most dust collectors will pretty much capture all or nearly 99% of particles greater than 50 microns, so it isn’t as important to know if you have 5% dust at 51 microns and 3% at 55 microns, a simple 8% over 50 microns would be effective.

However, when dealing with dust less than 20 microns, knowing the particle size distribution is very important. For example, some cyclones might get you around 85% removal of 10 microns dust but 60% of 5 microns dust and only 30% of 2 microns dust. So, if you have that 60% of the dust is less than 10 microns, it isn’t known how much of that is in the 2 microns ranger or the 10 microns range. This means that when calculating your removal efficiency, the estimated removal efficiency could range from less than 30% to 85%. The only way to know is to have a breakdown of the particle sizes.

Particle Size with Standard Deviation

Another way of providing a particle size distribution is to provide a mean particle size with standard deviation. This provides a decent approximation of the particle size. Personally, I would prefer the actual test data, because you are providing actual test data and not approximations. Depending on the process of dust generation, the approximation accuracy will vary. It also requires the person estimating the removal efficiency to calculate a particle size distribution. This takes more time and increases the chance of an error being introduced.

So, when you are looking to get your dust collector engineered, remember it is important to provide the equipment manufacturer with a particle size distribution. Often times you can send a sample in to a lab and within a week or so have a full distribution curve for a few hundred dollars. This will allow the manufacturer to better understand what they have to capture and select the best equipment for your application.

How Do Horizontal Dust Collectors Work?

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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.

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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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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When it is time to add a dust collector to your application, you have to find a place for the dust collector.  This isn’t always easy.  New facilities are usually designed around the equipment that is being installed.  However when a system in an existing facility is modified or installed, you will have limited space to install the dust collector system.  There are four factors that you should take into account when finding a place for your dust collector. (Part 4 of 4)

Surrounding areas

As mentioned above, the area surrounding the dust collector significantly affects where the dust collector can be installed.  Location of utilities such as electricity and compressed (plant) air all affect the dust collector location.  Baghouses and cartridge collectors usually require compressed air for cleaning, wet scrubbers often require makeup water and a drain to either local or plant water treatment facilities. 

Dust Collector Interference

The dust collector may interfere with surrounding equipment and operations, such as cranes and vehicle traffic.  Dust collectors are often tall pieces of equipment requiring large head room when indoors.  And the noise and explosion protection could affect personnel space such as break rooms, rally points, foot and vehicle traffic, etc.

So when looking for a space to install your dust collector make sure you take into account all the different factors such as explosion protection, utilities, operations, and workers safety and comfort. And install the dust collector as close as possible when conforming to the above requirements.

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