Types Of Dust Extractors

Types Of Dust Extractors

Last Updated on July 14, 2021 by Gabriel Goddy

Wouldn’t it be amazing if you could clean up all the dust in your home without going through much stress? It would, definitely. You could benefit a lot from dust extractors. If you read through this guide, you’ll see the types of dust extractors you could choose from.

What Is A Dust Extractor?

A dust extractor is a system used to enhance the quality of air released from industrial and commercial processes by collecting dust and other impurities from air or gas. Designed to handle high-volume dust loads, a dust extractor system consists of a blower, dust filter, a filter-cleaning system, and a dust receptacle or dust removal system. It is distinguished from air purifiers, which use disposable filters to remove dust.

Uses of Dust Extractors

Dust extractors 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 before venting to the atmosphere.

Dust collection is an online process for collecting any process-generated dust from the source point continuously. Dust extractors may be of single unit construction, or a collection of devices used to separate particulate matter from the process air. They are often used as an air pollution control device to maintain or improve air quality.

Mist extractors remove particulate matter in the form of fine liquid droplets from the air. They are often used for the collection of metalworking fluids, and coolant or oil mists. Mist extractors are often used to improve or maintain the quality of air in the workplace environment.

Fume and smoke extractors are used to remove sub-micrometer-size particulates from the air. They effectively reduce or eliminate particulate matter and gas streams from many industrial processes such as welding, rubber and plastic processing, high-speed machining with coolants, tempering, and quenching.

Types of dust extractors

Five main types of industrial dust extractors are:

  • Inertial separators
  • Fabric filters
  • Wet scrubbers
  • Unit extractors
  • Electrostatic precipitators

Inertial separators

Inertial separators separate dust from gas streams using a combination of forces, such as centrifugal, gravitational, and inertial. These forces move the dust to an area where the forces exerted by the gas stream are minimal. The separated dust is moved by gravity into a hopper, where it is temporarily stored.

 

The three primary types of inertial separators are:

  • Settling chambers
  • Baffle chambers
  • Centrifugal extractors

Neither settling chambers nor baffle chambers are commonly used in the minerals processing industry. However, their principles of operation are often incorporated into the design of more efficient dust extractors.

  • Settling chamber

A settling chamber consists of a large box installed in the ductwork. The increase of cross-section area at the chamber reduces the speed of the dust-filled airstream and heavier particles settle out. Settling chambers are simple in design and can be manufactured from almost any material. However, they are seldom used as primary dust extractors because of their large space requirements and low efficiency. The practical use is as pre-cleaners for more efficient collection.

  • Advantages:

simple construction and low cost

maintenance-free

collects particles without the need for water.

  • Disadvantages:

low efficiency

large space required.

  • Baffle chamber

Baffle chambers use a fixed baffle plate that causes the conveying gas stream to make a sudden change of direction. Large-diameter particles do not follow the gas stream but continue into a dead air space and settle. Baffle chambers are used as cleaners

  • Centrifugal extractors

Centrifugal extractors use cyclonic action to separate dust particles from the gas stream. In a typical cyclone, the dust gas stream enters at an angle and is spun rapidly. The centrifugal force created by the circular flow throws the dust particles toward the wall of the cyclone. After striking the wall, these particles fall into a hopper located underneath.

The most common types of centrifugal, or inertial, extractors in use today are:

  • Single-cyclone separators

Single-cyclone separators create a dual vortex to separate coarse from fine dust. The main vortex spirals downward and carries most of the coarser dust particles. The inner vortex, created near the bottom of the cyclone, spirals upward and carries finer dust particles.

  • Multiple-cyclone separators

Multiple-cyclone separators consist of several small-diameter cyclones, operating in parallel and having a common gas inlet and outlet, as shown in the figure, and operate on the same principle as single cyclone separators—creating an outer downward vortex and an ascending inner vortex.

Multiple-cyclone separators remove more dust than single cyclone separators because the individual cyclones have a greater length and smaller diameter. The longer length provides longer residence time while the smaller diameter creates greater centrifugal force. These two factors result in better separation of dust particulates.

The pressure drop of multiple-cyclone separator extractors is higher than that of single-cyclone separators, requiring more energy to clean the same amount of air. A single-chamber cyclone separator of the same volume is more economical but doesn’t remove as much dust.

Cyclone separators are found in all types of power and industrial applications, including pulp and paper plants, cement plants, steel mills, petroleum coke plants, metallurgical plants, sawmills, and other kinds of facilities that process dust.

  • Secondary-air-flow separators

This type of cyclone uses a secondary airflow, injected into the cyclone to accomplish several things. The secondary airflow increases the speed of the cyclonic action making the separator more efficient; it intercepts the particulate before it reaches the interior walls of the unit, and it forces the separated particulate toward the collection area.

The secondary airflow protects the separator from particulate abrasion and allows the separator to be installed horizontally because gravity is not depended upon to move the separated particulate downward.

  • Fabric filters

Commonly known as baghouses, fabric extractors use filtration to separate dust particulates from dusty gases. They are one of the most efficient and cost-effective types of dust extractors available and can achieve a collection efficiency of more than 99% for very fine particulates.[6]

Dust-laden gases enter the baghouse and pass through fabric bags that act as filters. The bags can be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope shape.

  • Pre-coating

To ensure the filter bags have a long usage life they are commonly coated with a filter enhancer (pre-coat). The use of chemically inert limestone (calcium carbonate) is most common as it maximizes the efficiency of dust collection (including fly ash) via the formation of what is called a dust cake or coating on the surface of the filter media.

This not only traps fine particulates but also protects the bag itself from moisture and oily or sticky particulates which can bind the filter media. Without a pre-coat the filter bag allows fine particulates to bleed through the bag filter system, especially during start-up, as the bag can only do part of the filtration leaving the finer parts to the filter enhancer dust cake.

Wet scrubbers

Dust extractors that use liquid are known as wet scrubbers. In these systems, the scrubbing liquid (usually water) comes into contact with a gas stream containing dust particles. Greater contact between the gas and liquid streams yields higher dust removal efficiency.

There is a large variety of wet scrubbers; however, all have one of three basic configurations:

  1. Gas-humidification – The gas-humidification process agglomerates fine particles, increasing the bulk, making collection easier.
  2. Gas-liquid contact – This is one of the most important factors affecting collection efficiency. The particle and droplet come into contact by four primary mechanisms:
  • Inertial impaction – When water droplets are placed in the path of a dust-laden gas stream, the stream separates and flows around them. Due to inertia, the larger dust particles will continue in a straight path, hit the droplets, and become encapsulated.
  • Interception – Finer particles moving within a gas stream do not hit droplets directly but brush against and adhere to them.
  • Diffusion – When liquid droplets are scattered among dust particles, the particles are deposited on the droplet surfaces by Brownian movement or diffusion. This is the principal mechanism in the collection of submicrometre dust particles.
  • Condensation nucleation – If a gas passing through a scrubber is cooled below the dewpoint, condensation of moisture occurs on the dust particles. This increase in particle size makes collection easier.
  1. Gas-liquid separation – Regardless of the contact mechanism used, as much liquid and dust as possible must be removed. Once contact is made, dust particulates and water droplets combine to form agglomerates. As the agglomerates grow larger, they settle into an extractor.

Electrostatic precipitators (ESP)

Electrostatic precipitators use electrostatic forces to separate dust particles from exhaust gases. Several high-voltage, direct-current discharge electrodes are placed between grounded collecting electrodes. The contaminated gases flow through the passage formed by the discharge and collecting electrodes. Electrostatic precipitators operate on the same principle as home “Ionic” air purifiers.

The airborne particles receive a negative charge as they pass through the ionized field between the electrodes. These charged particles are then attracted to a grounded or positively charged electrode and adhere to it.

The collected material on the electrodes is removed by rapping or vibrating the collecting electrodes either continuously or at a predetermined interval. Cleaning a precipitator can usually be done without interrupting the airflow.

The four main components of all electrostatic precipitators are:

  • Power supply unit, to provide high-voltage DC power
  • Ionizing section, to impart a charge to particulates in the gas stream
  • A means of removing the collected particulates
  • A housing to enclose the precipitator zone

The following factors affect the efficiency of electrostatic precipitators:

  • Larger collection surface areas and lower gas-flow rates increase efficiency because of the increased time available for an electrical activity to treat the dust particles.
  • An increase in the dust-particle migration velocity to the collecting electrodes increases efficiency.

The migration velocity can be increased by:

  • Decreasing the gas viscosity
  • Increasing the gas temperature
  • Increasing the voltage field

Fan and motor

The fan and motor system supplies mechanical energy to move contaminated air from the dust-producing source to a dust collector.

Types of fans

There are two main kinds of industrial fans:

Centrifugal fans

Axial-flow fans

  • Centrifugal fans

Centrifugal fans consist of a wheel or a rotor mounted on a shaft that rotates in a scroll-shaped housing. Air enters at the eye of the rotor, makes a right-angle turn, and is forced through the blades of the rotor by centrifugal force into the scroll-shaped housing. The centrifugal force imparts static pressure to the air. The diverging shape of the scroll also converts a portion of the velocity pressure into static pressure.

There are three main types of centrifugal fans:

Radial-blade fans – Radial-blade fans are used for heavy dust loads. Their straight, radial blades do not get clogged with material, and they withstand considerable abrasion. These fans have medium tip speeds and medium noise factors.

Backward-blade fans – Backward-blade fans operate at higher tip speeds and thus are more efficient. Since material may build up on the blades, these fans should be used after a dust collector. Although they are noisier than radial-blade fans, backward-blade fans are commonly used for large-volume dust collection systems because of their higher efficiency.

Forward-curved-blade fans – These fans have curved blades that are tipped in the direction of rotation. They have low space requirements, low tip speeds, and a low noise factor. They are usually used against low to moderate static pressures.

  • Axial-flow fans

Axial-flow fans are used in systems that have low resistance levels. These fans move the air parallel to the fan’s axis of rotation. The screw-like action of the propellers moves the air in a straight-through parallel path, causing a helical flow pattern.

The three main kinds of axial fans are:

  • Propeller fans – These fans are used to move large quantities of air against very low static pressures. They are usually used for general ventilation or dilution ventilation and are good in developing up to 0.5 in. wg (124.4 Pa).
  • Tube-axial fans – Tube-axial fans are similar to propeller fans except they are mounted in a tube or cylinder. Therefore, they are more efficient than propeller fans and can develop up to 3 to 4 in. wg (743.3 to 995 Pa). They are best suited for moving air-containing substances such as condensible fumes or pigments.
  • Vane-axial fans – Vane-axial fans are similar to tube-axial fans except air-straightening vanes are installed on the suction or discharge side of the rotor. They are easily adapted to multistage and can develop static pressures as high as 14 to 16 in. wg (3.483 to 3.98 kPa). They are normally used for clean air only.

CONCLUSION

Dust extractors as you have read above come in various types. The type you decide to choose should depend on the need you have for it.