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April 17, 2019
12 Min Read
The performance of a dust collection system is dependent upon numerous factors that vary from one system to the next. Dust particle size, duct diameter, the number and location of collection points are a few of these components, but there is so much more to consider when designing a system. Slight miscalculations in the initial design phase can cause costly performance issues after the system has been installed and commissioned for operation. With larger systems typically requiring large capital investments, ensuring that a system that fits the collection needs of a facility before installation is critical because the cost of corrections after the fact will likely be driven by the size of the system. Regardless of the size of the system, it is important to know that it will perform as required due to the potential safety and environmental issues that can arise with poor system design or failure to understand the factors that can affect the system’s ability to capture and collect dust.
Whether designing a new system or evaluating your current system, being equipped with some basic knowledge about the key factors that affect performance is imperative. Let’s take a look at 10 questions that should be considered when evaluating the performance of your dust collection system.
1.) What is loading and how is it calculated?
Dust collectors such as baghouses and cartridge collectors use bags or filters to separate dust from air. Since both of these collectors use fabric filters, excessive dust loading over a short period of time can negatively impact performance. Dust concentrations in baghouses that are greater than 5-7 grains/cu ft will likely cause operational and maintenance issues. For cartridge collectors, dust loadings should be kept below 2 grains/cu ft.
To calculate loading, take the dust being sent into the collector (lb/hr) and multiply it by 7000 grains/lb. You then calculate the total system air volume per hour. Finally, divide the grains per hour by the total airflow in an hour to get grains/cu ft.
Here is a loading calculation example for a collector that is capturing 2 lb of dust/hr. This collection system provides 1000 CFM of air volume:
2 lb/hr * 7000 grains/lb = 14,000 grains/hr
1000 CFM * 60 min/hr = 60,000 cu ft/hr
(14,000 grains/hr) / 60,000 cu ft/hr = 0.23 grains/cu ft
Overloading filters decreases system efficiency due to reduced airflow through the collector. This reduction in flow means that dust will not be collected at collection points as intended. This can be costly in many respects, beyond the fact that the system is not collecting dust as designed. Filters will have to be replaced more often, which may mean that the entire system has to be taken offline. Additionally, safety and environmental concerns must be considered, with the potential for dust to enter the atmosphere, both within the facility or into the outside air.
2.) Why does particle size distribution matter?
Dust particles come in many different shapes and sizes, depending on the composition of the parent material and where applicable, the nature of the processing operation that creates the dust. For instance, wood dust created by a sander will have a much different particle size than ground limestone. Within each of these types of dust, there is a variance in the particle size. This is known as the particle size distribution. This is an important value to know for a few different reasons.
First and foremost, there are potential safety and environmental concerns. Dusts can be inhalable or respirable at smaller sizes and may be regulated by OSHA or industrial hygiene standards to maintain worker safety. Of further concern is the combustibility of a dust and its ability to be dispersed in an explosion, which is also subject to regulatory oversight.
From an operational standpoint, dust collector filters are designed to be permeable to air. This is why filter selection is critical. While most collectors can be fitted with filters that have very high removal efficiencies down to 1-2 micron dust sizes, it is not prudent to use filters designed for smaller particles when the dust being collected has a larger particle size. This will lead to decreased air flow through the collector and reduced efficiency.
Dust collector designers and manufacturers have removal efficiency curves that provide the expected removal of dust at a certain particle size. To accurately predict the removal efficiency of the dust collector, the particle size distribution must be compared to the removal efficiency curve and calculated. Most baghouses and filters are highly efficient down to very small particle size. Cyclones on the other hand, are less efficient and may require a more in-depth investigation of particle size distribution on most applications
3.) Could the system benefit from a pre-filter cyclone?
Using a cyclone as a pre-filter before the collector is an effective method of reducing the load on the collector. The cyclone is designed to remove larger, heavier dust, while allowing smaller particles to pass through it and into the collector filters. In many applications, 80-90% of the particulate matter entering the cyclone is removed, which is a significant reduction in overall system loading. With this decreased volume of dust reaching the filters, filter life is extended, with cleaning and replacement needed less frequently. This means reduced maintenance cost while sustaining optimal operating efficiency.
If the material being collected by the system is a high value material or product, the cyclone allows collection of this material without it being contaminated with other dusts that may be presents in the baghouse or cartridge collector.
An additional benefit of using a cyclone is its ability to act as a spark arrestor and lower the temperature of the air going into the collector. Hot air temperatures exhausted from foundries, glass making plants, and power plants can burn the filter media used by most baghouses, and it is not uncommon for sparks to enter dust collection systems that are used for sanding, sawing, or grinding operations. Cyclones are highly effective at mitigating both of these risks.
4.) How does the fan wheel-style affect performance?
The exhaust fan could be the most important component in the dust collection system because it provides the motive force for the whole system. If it isn’t performing as required, the system will fail. It’s all about pressure and volume and being able to generate and maintain enough of both to get the dust to the collector. That’s why choosing the proper type of fan impeller (wheel) is crucial.
There are three types of wheels/impellers:
Radial wheel: These wheels should be used when the fan is on the dirty side of a dust collector or after a dust collector where a large amount of dust remains in the airstream. Material handling applications typically use radial wheel-type fans, as their wheels are designed to handle dust in the air in which they operate. The open-type wheel is used when there is a high dust loading, and/or the dust is fibrous. Its design helps prevent the dust from wrapping around the wheel.
Air handling wheel: The air handling wheel is designed for clean airstreams or extremely light dust loading. These wheels should always be used on the clean side of and dust collector and never on the dirty side. They are usually more efficient in air movement than the radial wheels.
Axial wheel: These wheels are usually never used on dust collection systems. They will move a lot of air, but without much force behind them. If possible, stay away from axial fans on your dust collector system. Consideration should also be given to the material that is used for the fan blades and housing. For example, abrasive dusts will quickly erode fan components that are made of soft metal. Incompatibilities that could lead to corrosion should also be considered.
5.) Why are differential pressure gauges used in dust collection systems?
Using a differential pressure gauge to measure pressure drop through a collector is an effective method of monitoring the health of a dust collection system. Air that flows through a newly installed system with minimal leakage, a properly functioning fan, and filters that are clean will experience a pressure drop as it travels through the collection unit. This drop is normal. System manufacturers typically provide normal pressure drop figures for their units.
Regular collection of pressure drop data is recommended to track gradual or sudden changes. Gradual changes are likely caused by filters or ductwork becoming clogged, whereas more sudden changes could be the result of system or fan damage, torn or missing filters, or leakage that is allowing water or air into the system. In either case, pressure drop data can be used to see trends and prevent full blown problems before they result in the system having to be shut down.
6.) What is a DHA?
DHA is an abbreviation for Dust Hazard Analysis. Any industrial facility that creates dusts or uses powders is mandated by NFPA 652 to complete a DHA to identify the presence of combustible dusts and establish a plan for eliminating or mitigating potential risks associated with these dusts in their facilities. The deadline for completion of the DHA is September 2020. OSHA has begun issuing citations and fines using the NFPA under the general duty clause.
The DHA should be as simple or as complex as the process and needs to be formally documented and needs to be updated as collection points are added, or new raw materials are brought into the facility. The main purpose of the DHA is to educate the owner and operators on the true hazards and dangers they are facing with their dust, and to make sure they take the proper precautions with it. The DHA is a tool to prevent loss of life, equipment, production time, and capital.
7.) What are dampers?
Dampers are used in dust collection systems to control the airflow to specific branches of the ductwork. This is done by opening or closing the damper because air, like water, travels the path of least resistance. By using a damper, you are changing the path of least resistance for the air travelling in the system. Every dust collection system should have a manual damper near each hood or pickup point to allow the system to be balanced. This flexibility allows airflows to be varied between collection points, depending on the changing operational needs of the entire system.
Without this type of control, an unbalanced system could cause too much air to be pulled from one area causing loss of viable product while in another area not enough airflow is available to capture dust. Soft connects, or spaces between flanges, are occasionally used to control airflow at a pickup point. However, this isn’t very efficient as it keeps the airflow in that area constant, even at times when it is not required. Dampers are a much better solution, as they allow the ability to close off branches that are not being used.
8) How does ductwork design affect a system?
Ductwork is usually the largest component of a dust collection system 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 essentially a transportation network that is used to move dusty air from one place to another.
As with transportation system design, a good ductwork layout will utilize straight lines when possible and attempt to limit the overall size of the system. Larger systems requiring many elbows and transitions will experience inefficiencies due to flow and velocity losses caused by friction.
Ductwork diameter is dependent upon the material being conveyed and the distance between the collector and pickup points. The correct diameter helps ensure that the required balance between velocity and flow to move the dust is maintained.
Care should be taken whenever adding or removing drops to a system after its installation, as any modification has the potential to negatively affect the flow and velocity in the system. It is possible to retrofit a system by adding dampers or changing fan speed to restore system balance after it has been modified, but it is always prudent to evaluate the potential impact to the system prior to making any changes.
9) What is the right type of hood?
A good dust collection system is reliant upon properly designed hoods to capture dust. The key word here is capture, as the hood’s purpose is to maintain a capture velocity that will allow dust to be drawn into the system. Capture velocity is the required air speed that is necessary to overcome any surrounding air currents that would otherwise prevent the flow of dust into the hood or enclosure. An improperly designed hood will fail to effectively collect dust or require more airflow than should be needed.
Hoods usually should be placed as close to the source as possible for the best results. The best size and shape are dependent on many factors, but hoods usually incorporate tapered or conical designs improve capture velocity and reduce friction in the system.
A great resource for hood design is the ACGIH Industrial Ventilation Handbook. It provides recommended hood designs, airflow requirements, and sizes. Dust collection manufacturers and designers, HVAC consultants, and manufacturer representatives can also be consulted.
10) Can used dust collectors be used for certain applications?
A sensible way to limit the capital expenditure required when installing a dust collection system is to use a used collection unit. Initial system design can proceed as it normally would, laying out collection points, calculating the required airflow, determining duct sizes, and calculating static pressure. From this initial design work, a shopping list of requirements can be generated.
If a used collector is available that can meet or exceed the requirements of the system, then it should be fully inspected and evaluated for any mechanical, electrical, or structural problems. It may be helpful to consult the manufacturer of the used unit to help assess its condition. The availability of and cost of replacement filters and other parts should also be considered.
A dust collector that is failing to collect dust can directly impact a company’s bottom line, as well as the health and safety of its workers. That is why periodic system performance evaluations are vital. This list of 10 questions is certainly not an exhaustive list of questions that should be asked when evaluating your dust collection system. Each system will likely have requirements or characteristics that will necessitate specific design considerations that will differentiate it from other systems. But, having a basis of general knowledge about system design and operation will allow you make more informed judgments of system performance.
Cole Solberg is business development manager, Aerodyne Environmental, Chagrin Falls, OH. For more information, call 440-543-7400 or visit www.dustcollectorhq.com.
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