Dust Collectors Information

Dust collectors are used in many processes to either recover valuable granular solid or powder from process streams or to remove granular solid pollutants from exhaust gases and work environments.

Dust Collection Methods

There are a number of different methods for dust collection that buyers have to choose from. Each has its own strengths and is best suited for certain applications.

Cyclone Separation

Cyclone separators use gravity and a vortex to separate particles from gas streams. Air is directed through a cylinder casing and is spun rapidly, creating centrifugal force which draws particles to the wall of the cyclone. When PM hits the wall, it loses velocity and lift, causing it to fall into a collection hopper at the bottom.

Cyclones are inexpensive, rugged devices that have essentially no moving parts and require very little maintenance. They are not very effective at capturing very fine particulate, but have moderate efficiencies (+90%) for larger particles (>5µm). This makes them ideal as first stage collection devices (precleaners) to lighten heavy dust loads before the use of more expensive pollution control devices (e.g. baghouses, electrostatic precipitators) which have higher collection efficiencies. Cyclones can also handle a range of temperatures and media types, although "sticky" materials are incompatible because they will tend to adhere to the cyclone walls. Cyclones have application ranges from large industrial gas streams to small workshop environments. 

For more on cyclone separators, visit the Cyclone Separator Specification Guide on Engineering360.

Electrostatic Precipitation

Electrostatic precipitators (ESPs) or electrostatic air cleaners use electrostatic forces to separate particulate matter (PM) from exhaust gases. Essentially, particles move through a region where electrodes induce an electric field to charge dust particles, which consequently attach themselves to a collection surface. Once collected, the surface is washed, rapped, or vibrated to dislodge the particulate and collect it in a hopper.

ESPs are highly-efficient and can remove dust and smoke from large volumes airstreams with minimal interruption of gas flow. They are typically quite expensive, but cost much less to operate and maintain. The collection efficiency of electrostatic precipitators is largely dependent on the electrical properties of the particulate being collected. Modern, properly designed ESPs can reach collection efficiencies above 99.9%.

For more on electrostatic precipitators, visit the Electrostatic Precipitators Specification Guide on Engineering360.

Media Filtration

Media filtration devices use screens or filters to physically separate particles from gas streams. The advantages and disadvantages of media filters are described below:

Baghouses, also called fabric dust collectors or fabric filters, are devices which use multiple fabric filter tubes (bags) to capture or separate dust and other particulates. As gas streams are directed through these bags, a layer of particulate called the dust cake builds on the fabric and is responsible for most of the filtering. Bags are intermittently or continually cleaned using either shaking, reverse airflow, or pulses of air.

Compared to other types of air pollution control (APC) equipment, baghouses are incredibly versatile and can be engineered for almost any dust producing application by varying size and bag types. They are very efficient when properly maintained and are also rugged enough to handle rough applications. However, they typically require a lot of maintenance and a relatively dry environment to operate effectively. Their use is also limited to certain operating temperatures and chemical conditions because of the nature of the filter fabrics and operating environment.

For more on baghouses, visit the Baghouses and Baghouse Filters Specification Guide on Engineering360.

Cartridge dust collectors use perforated, cylindrically shaped metal cartridges lined with a pleated nonwoven filtering media. Once installed, one end of the cartridge is sealed off and the open end is used for the clean exhaust. Similar to a baghouse, filtering is accomplished by forcing the gas stream through the cartridge. Cartridge collectors are compatible with reverse-air or pulse-jet cleaning.

While no long as widely employed, cartridge dust collectors are uniquely suited for certain applications involving special shaped and sized dusts (granular shaped & less than 50 microns) and low grain loading levels (less than 5 grains per cubic foot). Cartridge collectors are also satisfactory for collecting dusts which require minimal flexing of the media during the cleaning process. The pleated design provides a greater amount of filtering area while occupying lesser space. Consequently, the air-to-cloth ratio, pressure drop, and filter cartridge size are reduced considerably. Uses include but are not limited to: grinding or sandblast applications, welding fumes, laser and plasma cutter fumes, graphite, pharmaceuticals, and fine chemical powders. 

Industrial vacuums are dust collection devices which use vacuum suction to pull air in for filtering. During operation, air streams are forced through a media filter inside the casing and pushed out through a discharge outlet. Vacuums can incorporate fabric bags, envelopes, or cartridges as filters depending on the media being collected and the design of the device.

In contrast to baghouses and cartridge filters, vacuums are typically portable, single-filter devices which can be operated manually. They are used frequently for spill cleaning, general cleaning, and the removal of fine powders and toxic or hazardous media.

Other filter-based dust collectors can come in a number of other different forms:

Collector booths are filter units which require no ductwork, and allow the worker greater freedom of movement. They are often portable.

Downdraft tables are self-contained portable filtration systems that remove harmful particulates and return filtered air back into the facility, requiring no external ventilation.

Wet Scrubbing

Wet scrubbers are devices that use a liquid (often water) to capture and remove pollutants. Through agglomeration, adherence, or encapsulation, wet scrubbers merge dust particles with the scrubbing liquid to increase the size and mass of the particles and make collection easier. Wetted particles are then collected through a separation or filtering process based on the type of scrubber. Types of particulate scrubbers include gravity spray towers, cyclones spray chambers, impingement scrubbers, packed bed scrubbers, and venturi scrubbers.

Wet scrubbers, most notably venturi scrubbers, typically have very high particulate collection efficiencies. Because they use liquids like water, wet scrubbers are also able to absorb gaseous pollutants such as acid gases. Unfortunately, the use of water can also be expensive and requires the devices to be built from more costly, corrosion resistant materials. The slurry that is collected is also more expensive to handle than dry solid and may be too contaminated to recycle.

For more on wet scrubbers, visit the Air Scrubbers, Wet Scrubbers, and Gas Scrubbers Specification Guide on Engineering360.

Collection Categories

Particulate collectors can also be categorized based on their mode of operation.

Ambient units are designed to continuously filter airborne particles as air circulates through an environment. They are used in applications where there is no specific source of the dust creation/pollution or where the use of proper ducts and source-capture arms is limited. They also may be desired for applications where a source collector does not provide sufficient capture of small particles.

Source collectors are designed with ducts and arms to capture and filter air at the source. They can be either stationary or portable units.

Process Parameters

There are a number of parameters important to consider for the implementation of any dust collection system. These variables play a role in determining the suitability of a particular collector type and in how it is designed.

Chemistry is the makeup of the particulate matter in the gas flow. Chemistry determines a number of material properties related to the collection and the design of the collector.

Gas flow rate describes how fast the gas stream moves through the system. Depending on the collection system, higher flow rates can decrease collection efficiencies, cause particle re-entrainment, and increase pressure drop; insufficient flow can result in poor gas distribution, particle dropout, or dust buildup in ducts. Manufacturers often provide design firms with airflow and airspeed tables for different materials to indicate the amount of air that needs to be moved for effective operation and collection.

Particle size and size distribution is the average size and size distribution of PM in the gas flow. Larger particles pick up charge more easily, while an abundance of small particles may suppress the generation of the corona.

Resistivity is a measure of the particles' resistance to electrical conductance, which is important for ESPs. Particles with high resistivity have difficulty acquiring charge, while particles with low resistivity may lose their charge too easily and not stick to the collection plate. Resistivity is influenced by the particulate's chemistry and the gas temperature.

Gas temperature is the temperature of the gas flow in the system. Most collection systems have a limited range of temperatures at which they can operate effectively and/or safely. Gas temperatures above the limits may require cooling before entering the collector.

Performance Specifications

Performance specifications are the ratings assigned by a manufacturer that describe a collector's performance. These are the specifications most important for industrial buyers to consider when selecting a suitable dust collector for their application.

• Airflow or gas flow rate defines the acceptable flow rate or range of flow rates of the gas stream through the collector, measured in cubic feet per minute (cfm).
• Minimum particle size filtered indicates the minimum diameter of particulate matter that a dust collector is capable of capturing, measured in micrometers (µm). This rating effectively defines the range of capability of the collection device.
• Efficiency, collection efficiency, or capture rate defines the percentage of particulate matter in the filtered gas stream that is effectively captured by the collector. Efficiencies for most dust collectors are typically very high, some guaranteeing over 99.9% under appropriate conditions.

Features

Dust collection systems may incorporate a number of features which may be important to consider during the selection process.

Continuous duty collectors are rated for 100% duty cycle, meaning they are made for constant use and continuous operation.

Collectors can be designed for quiet operation for applications and environments where low-noise is desired or required. Collectors may be designed to accept or use either HEPA or ULPA filters.

High efficiency particulate air (HEPA) filters are replaceable extended-media dry-type filters in a rigid frame having a minimum particle collective efficiency of 99.97 percent for a 0.3 micron particle, and a maximum clean filter pressure drop of 2.54 cm (1") water gauge when tested at rated air flow capacity.

Ultra low penetration air (ULPA) filters are extended media dry filters in a rigid frame that have a minimum particle collection efficiency of 99.999 percent for particles greater than or equal to 0.12 micron in size. Collection systems may include instrument panels or other control devices for user operation. Panels may be equipped with alphanumeric keypads, gauges, safety switches, on-off switches, or timer controls.

Modular collector systems are assembled in parts. This simplifies construction and repair, and also allows systems to more easily incorporate add-ons and upgrades. Dust collectors are commonly designed to accept optional odor control filters. These filters are placed after the initial particle collection to remove specific gases or contaminants from the air that could cause odors.

Standards

Because of the volatile nature of many dust filled environments, dust collectors often must abide by or meet certain fire safety standards.

• ATEX is a directive of the European Union that consists of standards for both collection systems and their operating environments.
• The National Fire Protection Association (NFPA) has codes and standards for preventing or protecting against dust explosions. These standards include requirements for the collector, ductwork, and exhaust fan(s) in a system.

References

Baghouse.com - What Kind of Dust Collector Is Right For Me?

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Fisher-Klosterman | Baghouse.com | Nederman