Manufacturers need a safe and reliable solution to vent pressure and control emissions associated with filling storage containers such as silos. A properly designed bin vent incorporating a dust collector can achieve the necessary process control and meet state and federal EPA particulate matter (PM) emission standards. This article outlines the recommended practices for successfully applying and designing bin vent solutions.
As products or materials are loaded into storage containers, the air inside the enclosed vessel is displaced by the product entering the vessel which, in turn, entrains and carries particulate from within the vessel to the surrounding area. If left uncontrolled, the process emissions present hazards to worker health, as well as maintenance problems and even plant safety risks when combustible or explosive materials are involved. Improper venting can also result in productivity limitations when the material flow is restricted due to increased vessel pressure. Depending upon the location and configuration of the storage container and the process material involved agencies such as the Occupational Safety and Health Administration (OSHA), the Environmental Protection Agency (EPA) or the National Fire Protection Agency (NFPA) may regulate the associated emission levels, exposure limits and safety precautions that must be taken when containing the material.
Relieving the pressure developed during the filling process is commonly referred to as bin venting. Dust collection equipment is often integrated with the bin vent design to control the associated particulate emissions. As the material is loaded into the container, the vessel acts as a buffer absorbing the product surge (pressure), which protects the point of material consumption. The surge or pressure is then released through the bin vent and the air volume displaced by the additional material passes through the dust collector where entrained emissions can be removed from the airstream. The result is clean air being discharged from the storage container once the entrained emissions have been filtered through the dust collector. Figure 1 illustrates a typical bin venting arrangement.
Figure 1 – Typical bin venting arrangement
Numerous means exist for transporting materials from the unloading point or station into a storage container, including mechanical systems (screw or belt conveyors, bucket elevators), gravity feeds (telescopic or spiral chutes), or pneumatic conveying devices (dense or dilute phase), which is most commonly used for bulk material processing. Proper bin vent design begins with a complete understanding of the unloading system, the storage container construction, and the material characteristics. It is also essential to be aware of and understand any related regulatory requirements associated with the process.
Understanding the System
To determine the proper design for a bin vent, several important system parameters need to be collected and reviewed including:
• Type of conveying system
• If a pneumatic conveying system is being used, is it a dense- or dilute-phase type? What is the air pressure and volumetric flow rate?
• Volume of material being conveyed
• Length of time to fill the storage container
• Storage container volume
• Size (diam) of the fill line to the storage container
• Maximum freeboard distance
• Design of the inlet and outlet flange including the location of both the inlet fill line and the outlet bin vent.
Material and Dust Properties
Understanding the properties and characteristics of the material(s) being conveyed and unloaded into the storage container is crucial in the design of a successful bin venting system. The type of dust will indicate what the associated hazards are and what technology will be best suited to collect these materials in a safe and effective manner. Begin with a review of the Material Safety Data Sheet (MSDS), which will outline many of the material properties such as bulk density, flammability, and handling requirements. Next, determine if the material dust is combustible or explosive. Other material characteristics must be determined as well. Is the material sticky or hydroscopic? Is there a risk of condensation or moisture being present within the dust?
Gathering and understanding both the process parameters and the material and dust properties will aid in selecting and sizing the proper dust collection equipment to integrate with the bin vent.
Understanding the Regulations
These are several important safety issues and regulations to be considered and reviewed as part of implementing a successful bin vent. Generally speaking, emission regulations for outdoor installations will be covered by the EPA, and OSHA will cover indoor installations. If dealing with combustible or explosive materials, NFPA regulations will be applicable. To ensure full compliance, operators need to consult local authorities having jurisdiction (AHJ) in order to determine all applicable regulations and steps necessary to achieve compliance.
EPA regulations require manufacturers to provide particulate matter (PM) air pollution control devices on potential emission sources. The EPA regulations limit PM emissions to not exceed specific concentrations and limit the opacity of emissions discharges. Other EPA requirements include proper maintenance functions including filter change outs. Manufacturing owners/operators need to contact their local state or federal EPA officials to determine the permitting requirements and current emission standards.
For indoor applications, worker exposure to emissions associated with bin venting applications is a primary concern. In particular, there are concerns for emissions within the worker’s respirable range, meaning that it is small enough to penetrate the nose, upper respiratory system, and lungs. The small particulate size along with the emission’s chemical composition or potential toxicity poses a serious threat to worker health. As a result, organizations such as OSHA, the National Institute of Occupational Safety and Health (NIOSH) and the Centers for Disease Control and Prevention (CDC) have developed permissible exposure limits (PELs) for hazardous materials.
Accumulated dust presents a potential risk of combustion or explosion under the right conditions. The NFPA publishes standards associated with specific materials and processes where combustible or explosive dusts are present. NFPA standards provide both practical steps to reduce the likelihood of a fire or explosion happening and the necessary safety provisions should an explosion occur. Safe handling of combustible dusts is also currently an OSHA national point of emphasis with additional regulation expected.
Designing a Successful Bin Vent Dust Collection Solution
Designing a dust collection system involves several key steps, including the dust collection equipment selection, properly sizing the dust collector, and selecting the appropriate filter media. If any one of these is done incorrectly, the system will not perform as expected and can result in higher emissions and operational costs.
There are several dust collector models that can be applied to bin venting applications, with the most common being cartridge filter collectors, pleated baghouses, and shaker (envelope) baghouses. To help a manufacturer choose the appropriate model for their specific application and facility, several factors including performance requirements, installation location, size of dust collector, utility availability, and maintenance capabilities, must be balanced and the trade-offs reviewed to make the appropriate selection.
Performance requirements that influence the dust collector model selection include airflow capacity, dust characteristics, efficiency requirements, filter life, and operational costs. A cartridge collector has larger airflow capacity and offers a great deal of flexibility in filter media solutions and configurations. Pleated baghouses have a lower airflow capacity than cartridge collectors but are a better selection for agglomerative dusts or where moisture is present. Shaker units are further limited in airflow capacity and would not be recommended for agglomerative or moisture applications.
The installation location most commonly used for bin vent applications is mounting the dust collector directly on top of the storage container. Choosing this location reduces the equipment and installation cost by eliminating ductwork. When mounting the dust collector directly on top of the container, the collector is normally installed onto a flanged opening in the container located as far away from the fill line as possible. In this case, the dust collector will have an open-bottom configuration with a matching flange allowing the bin vent collector to be secured to the container. With the open-bottom construction, the collected particulate will fall directly back into the container during filter cleaning cycles, allowing for the stored material to be reclaimed.
Overall size or footprint is an important consideration as available space at the top of a storage container is at a premium. The additional airflow capacity available to cartridge collectors translates into a smaller installed footprint and more compact package than the other collector models mentioned above.
Figure 2 – Cartridge-style bin vent dust collector installed on pneumatically conveyed material storage container
Cartridge collectors can be supplied with or without an air mover or fan (commonly referred to as powered or unpowered). With mechanical and gravity conveying feed systems, the use of a bin vent fan is required to maintain a negative pressure in the vessel to prevent dust leakage from seams and other opening in the bin. With pneumatic conveying systems, the use of an air mover on the bin vent unit is normally not required.
Each of the dust collector models can be configured to accommodate top mounting or fan options as described, but availability of utilities at the storage container’s location must be considered in collector selection. For example, cartridge collectors and pleated baghouses require compressed air to operate the filter cleaning mechanism. If compressed air is not available, a shaker collector can be used since it requires only electricity to operate the filter-cleaning mechanism. The disadvantages of the shaker dust collector verses the other dust collector options are the shaker’s larger footprint and less effective filter cleaning mechanism.
When compressed air is available, a cartridge collector or a small pleated baghouse is the preferred model as both can be configured to offer high filtration efficiencies, small overall footprint, superior in-place filter cleaning, and ease of operation and maintenance.
Sizing the dust collection system requires determining the air volume that will be exhausted through the bin vent when material is loaded into the container. In the most basic sense, the air volume required for effectively venting is equal to the volumetric flow rate of material flowing into the container. However, in actual practice there are additional considerations that must be taken into account. For example, when loading utilizes pneumatic conveying, the volume of compressed air, not just the material volume, also must be included within the airflow calculations. Furthermore, the expansion of the compressed air must be calculated and also included as part of the total volume entering the container.
Sizing the dust collector appropriately is determined by the total airflow requirement based on the hood design and any other sources of air such as bleed in. The target filtration velocity, also known as the air to media ratio (AMR), is the ratio of airflow volume to the available filter area. For bin venting applications, the suggested AMR ranges between 1 and 3 ft/min for cartridge collectors, 4 ft/min for pleated baghouses, and 7 ft/min on shaker collectors. Dust loading for bin venting applications can be very high, especially on pneumatic conveying material handling systems because of the constant flow of material. For heavy loading conditions the air-to-media ratio should be reduced or de-rated. Additional application considerations, including product type, conveying methods, and filter life expectations, will similarly influence unit sizing.
The options and configuration of the dust collector are also important considerations in handling the hazards associated with the process material. When dealing with hazards like toxic dusts, options such as bag-out collars to reduce housekeeping and minimize exposure to dust during filter change outs are recommended. If the materials or the environment is corrosive, stainless steel construction may be required for the dust collector.
When handling combustible dusts, collectors can be equipped with sprinklers or extinguishing systems. It is important to consult a fire expert to ensure that the extinguishing agent matches the dust being collected. Collectors can also be fitted with filters featuring fire retardant filter media. While this media will not prevent a fire, it is designed to deter combustion by slowing the spread of a fire should it occur.
When dealing with an explosive material, consult NFPA regulations for guidance on how to safely design and maintain dust collection systems in combination with the material container. To mitigate a potential explosion, the dust collector should be equipped with a rupture panel or a chemical suppression system. It is important to note that while the collector’s internal volume is common with the storage container, both the collector and container must be evaluated and designed independently in accordance to the respective regulations.
Selecting a filter media technology that aligns the dust characteristics with system performance requirements is a critical step in achieving a reliable and operationally efficient system. From a performance standpoint, the filter technology must have particle removal efficiency capable of collecting the contaminant. ASHRAE Minimum Efficiency Reporting Values (MERV) can be used to review and compare initial filter efficiency and capability of collecting particulate within a given particle size range. If you are dealing with submicron particulate, it is important to utilize filter media with a higher MERV rating that is capable of collecting a significant portion of particulate in this range.
The most commonly available medias include cellulose (cellulose blends), Nanofiber technology, and synthetics such as Spunbond Polyester. For dry, free-flowing dusts commonly used within the bulk powder industry, cellulose based media are generally a good selection. However, if the dust is hygroscopic, agglomerative, or if moisture is present, synthetic media may be a more appropriate choice. To make this decision, there are benefits and trade-offs for each media option that should be considered.
Filter media technology is generally categorized into two types: surface-loading or depth-loading. Depth-loading filters – often referred to as commodity filters - have larger spaces or pores that allow particles to penetrate deep within the media substrate. As the particulate builds up within the filter media depth, the pressure loss across the filters increases. The increase in pressure loss will require more frequent filter cleaning cycles (compressed air) and more energy to maintain the target airflow. Additionally, filter cleaning will be less effective as the particulate is lodged within the depth of the filter media. These depth-loading filter characteristics result in shorter filter life, which translates into higher operational costs and maintenance due to more frequent trips to the top of the storage container to replace the filters for bin venting applications.
Surface-loading filter technology, such as Nanofiber or ePTFE membranes, creates a layer or barrier on the surface of the filter media, inhibiting the particulate from penetrating into the depth of the filter. Keeping the particulate on the surface keeps the pores or the filter media open longer resulting in lower operating pressure loss and energy consumption. Surface-loading enables the cleaning system to more effectively dislodge the collected particulate from the filter requiring less frequent cleaning cycles resulting in less compressed air consumption. Improved cleaning and lower pressure drop leads to extended filter life, a more reliable process, and reduced filter replacement costs.
Bin venting is a common application for dust collection equipment. Taking a systematic approach to ensure the proper sizing and correct bin venting options will lead to both optimal performance of the dust collector and a cleaner environment while helping the manufacturer meet air pollution control standards. Consult a dust collection equipment expert to review and understand the requirements of these systems and to prescribe a successful solution so that the material handling system operates properly and the environment is appropriately protected.
Travis Haynam is director of business development, and Joe Topmiller is senior application engineer at United Air Specialists Inc. (Cincinnati, OH). For more information, call 800-252-4647 or visit www.uasinc.com.
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