• Publication Date: 05/01/2021
  • Author(s):
    Constance, John A.
  • Organization(s):
    Engineers Collaborative, The
  • Article Type: Technical Articles
  • Subjects: Dust collection and dust control

John A. Constance | The Engineers Collaborative

Many manufacturing processes in the powder and bulk solids industry work with material that creates dust, which needs to be removed from the work area. But dust collection systems aren’t one size fits all, so the system needs to be tailored to your specific plant and application needs. Before you have a dust collection system designed and built for your plant, there are some questions you should ask. If you don’t have the correct answers suitable for your application before the system is designed, fabricated, and installed, the resulting problems can be expensive to correct afterwards.

What do you want your dust collection system to do?

The first question you’ll want to consider is what you want your dust collection system to do. If you don’t specify the results you want the new or upgraded dust collection system to accomplish, you likely won’t get the outcome you want. For example, let’s say that a blender charging operation is generating fugitive dust. If the dust is considered a nuisance dust according to OSHA standards, the dust’s allowable airborne concentration is 15 mg/m3 of total dust in the plant area and 5 mg/m3 of respirable fraction (amount that can be safely inhaled) within the plant personnel’s breathing zone.1 If your operation is generating nuisance dust above those levels, your dust collection system’s capture hood or hoods will need to be effective enough to reduce the dust concentration to acceptable levels.

So, how do you determine the amount of airborne dust the dust capture hood(s) will need to capture? First, measure the dust concentration that’s generated at the operator’s breathing zone and in the operation area. Your industrial hygienist or an outside one can take these measurements for you.

Then, with the air measurement results, you can specify the nuisance dust’s airborne concentration that the system’s dust capture hoods need to achieve. A properly designed dust capture hood can reduce the dust’s airborne concentration by an average of 90 percent without taking the process’ solids away or interfering with the production operation. For instance, if your air measurements show the plant area’s total dust concentration at 50 mg/m3, a properly designed dust capture hood can bring this level down to somewhere close to 5 mg/m3. If that level is satisfactory to you (and meets regulation standards), you can specify to the dust collection system manufacturer that each dust capture hood achieve a total airborne dust concentration of less than 15 mg/m3 without material loss and production interference. Also, as previously mentioned, you’ll need to specify that the dust collection system at each dust capture hood achieve a respirable dust concentration in plant personnel breathing zones of less than 5 mg/m3 without material loss and production interference.

An actual case at a manufacturing company involved a toxic dust where the respirable exposure without dust collection was measured at 0.82 μg/m3. The properly designed dust capture hood’s installation reduced the exposure level to 0.054 μg/m3 — a 93 percent decrease that satisfied the desired reduction target.

If you don’t achieve your targeted dust concentration reduction at dust collection system startup (your industrial hygienist performs air sampling with the dust capture hoods in place), the design engineer and contractor must correct the problem. In addition, if you don’t specify your required dust collection system performance results in your purchase orders to the design engineer and involved contractors, you won’t get what you want, and correcting the problem will cost you more.

Why do you need shop drawings?

Having accurate dust collection system drawings is important to ensure you get the system you requested. Shop drawings prepared by the fabricating and installation contractor show the system details before the components are fabricated and installed. The contractor makes the shop drawings from field measurements and the design drawings and specifications. The contractor will invariably do some things differently than the design drawings show, and these contractor changes may or may not be acceptable. If you don’t find out about these changes until system startup, you may not be happy with the results. You need shop drawings to show what you’re going to get with your system before construction starts. These shop drawings should also be used during periodic construction inspections to ensure that what’s being installed is what you want. The shop drawings will also be your as-built drawings for your records, which will be beneficial for regulation compliance purposes to satisfy OSHA safe workplace requirements and help you make effective changes to the dust collection system down the road.

Why should you consider exhaust system noise?

One issue to get ahead of before it becomes a problem is a loudly operating dust collection system. Some process operators might complain about noise at the dust collection hoods, but what exactly is the source of the noise? Most noise comes from the system’s exhaust fan. Fan noise exits the fan outlet and inlet, and noise radiates from the fan casing. However, outlet and inlet fan noise can be attenuated with silencers, while noise from the fan casing can be attenuated with ventilated enclosures or sound-deadening fan housing jackets. These noise-­reducing or -canceling means should be considered during the dust collection system’s design phase. Otherwise, making these noise-reducers an afterthought for post-system installation will result in costly system changes.

Noise will also be generated by high air velocities and turbulence within the ductwork. Dust transport velocities shouldn’t exceed those needed to minimize dust settling and plugging the ductwork. For example, dust velocities can be as low as 3,000 fpm for light-density dry dust without the fear of duct plugging. Running dust transport velocities at lower speeds will reduce noise as well as energy costs.

What information do you put in the dust collector specification?

As the name suggests, a dust collector specification document is what you give the dust collector supplier stating your system’s necessary requirements such as air quantity, explosion protection, and required filter life, among others. Normally, you provide information on such items as air quantity, filtration efficiency, operating pressure, electrical bonding, and explosion venting or suppression to the dust collection system manufacturer. However, there are some specifications you shouldn’t recommend on your own. Unless you have specific on-the-job experience, you shouldn’t select the dust collector filter media yourself, as you might not pick the correct filter media for your application. There are several different types of dust collector filters on the market today, such as bag filters and pleated filters. In addition, there are numerous filter materials to choose from, for instance, polyester or fiberglass. Filter type and material selection should be left up to the dust collector manufacturer based on your dust’s characteristics. Dust samples, and not process solids samples, should be sent to the dust collector manufacturer for analysis. You should, however, specify how long you wish the filter media to perform without an unacceptable reduction in dust collection system airflow. Your filter-life requirement must be spelled out clearly in your purchase order. For example, a 12-month filter life (operating at 24 hours a day, 7 days a week) causing no reduction in system exhaust airflow over that time period can be achieved with the proper filter media type and quantity.

Why do you need balancing dampers in your ductwork?

No matter how well an industrial dust collection system is designed and installed, there’s a very good chance that the system won’t work as expected if it’s not properly balanced at dust collection system startup. You must balance the airflow through all the branch ducts to maintain the design airflows through the exhaust system hoods and pickup points. If your system is designed using the static pressure balance method, there wouldn’t be any air-balancing dampers in the system to adjust. Using this method, the dust collection system’s duct sizes and system layout would’ve been engineered in the design phase to balance the airflow within the system’s pickup points, hoods, and ducts based on the static pressure loss through each section. However, the system should still be checked at startup to ensure that design exhaust air quantities are flowing through the system.

Most industrial dust collection exhaust systems are designed using the air balance by damper adjustment method. In this method, the duct diameters are sized to satisfy the desired dust transport velocities, and dampers are installed at each pickup point within the system and manually adjusted to balance the airflow. If this applies to your system, the airflow must be balanced by adjusting the dampers at system startup. Adjustable balancing dampers give you some flexibility with airflow changes that might be needed because of process changes that always occur down the road. However, be sure that the damper slides are securely locked in place after balancing.

What protects personnel and property from an explosion inside your dust collector?

If a fire starts inside the hopper of a dust collector handling combustible dust, fire protection sprinklers built into the dust collector may be able to extinguish the fire before an explosion takes place. However, if the dust collector goes through a filter cleaning cycle before the fire is extinguished by the water from the sprinklers, there’s a very good chance that the fire will ignite the resulting dust cloud. Also, if a sufficient amount of dust is dislodged from the filters during the cleaning cycle, resulting in a dust cloud exceeding the minimum explosible concentration (MEC), an explosion will occur. The MEC is the lowest concentration of a combustible dust in air that will propagate a flame. However, sprinklers can’t extinguish an explosion, so an explosion prevention and protection system will be necessary.

Such explosion prevention and protection systems include spark or flame detectors within ductwork leading to a dust collector that are designed to protect the dust collector from a fire caused by sparks or burning embers upstream from the dust collector. These detectors actuate abort gates, dampers, or extinguishing systems to keep sparks or embers from entering the dust collector. Spark or flame detectors that actuate abort gates, dampers, or spark extinguishers are effective as long as the combustible dust concentration within the ductwork is consistently below the MEC.

Having spark or flame detectors in place won’t guarantee 100 percent safety, however. A spark detector coupled with an abort gate or spark extinguishing system won’t stop a propagating flame front (fire and explosion debris) from entering your plant back through the dust collector’s inlet or outlet return air ducts. For more detail on how to prepare for and handle this issue, refer to the National Fire Protection Association’s NFPA Standard 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, Annex C, Section C.1.2.2.2 The dust collector inlet and outlet return air ducts need mechanical or chemical isolation in case of an explosion within the dust collector.

PBE

References

  1. “1910.1000 TABLE Z-1 – TABLE Z-1 Limits for Air Contaminants,” Occupational Safety and Health Administration, United States Department of Labor, accessed March 10, 2021, www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9992.
  2. NFPA 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, Annex C: Informational Primer on Spark Detection and Extinguishing Systems, Section C.1.2.2: Detector Sensitivity and Speed, 2013 Edition, National Fire Protection Association (NFPA), 1 Batterymarch Park, Quincy, MA 02169-7471; 800-344-3555, fax 800-593-6372 (www.nfpa.org).

For further reading

Find more information on this topic in articles listed under “Dust collection and dust control” in our article archive.


John A. Constance, PE (215-300-9563), is a consulting engineer at The Engineers Collaborative. He has more than 30 years of experience designing, engineering, and troubleshooting dust control and industrial air systems for bulk solids processing and handling industries.

The Engineers Collaborative • Canandaigua, NY
215-300-9563 • www.engcollab.com

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