Dust Collector Equipment and uses

A dust collector is a system used to improve the quality of air released from industrial and commercial processes by collecting dust and other impurities from the air or gas.A dust collector system designed to handle large dust loads consists of a blower, dust filter, filter cleaning system, and dust collector or collection system.It differs from an air purifier, which uses a disposable filter to remove dust.

Uses

Dust collectors are used in many processes to recover valuable particulate solids or powders from process streams, or to remove particulate solid pollutants from exhaust gases prior to discharge to the atmosphere.Dedusting is an in-line process used to continuously collect dust generated by any process from the point of source.A dust collector can be a single unit structure or a collection of devices used to separate particulate matter from process air.They are commonly used as air pollution control devices to maintain or improve air quality.Mist collectors remove particulate matter from the air in the form of fine liquid droplets.They are commonly used to collect metalworking fluids, coolants or oil mist.Oil mist collectors are commonly used to improve or maintain the air quality of workplace environments.Fume collectors are used to remove submicron particles from the air.They effectively reduce or eliminate particulate matter and airflow in many industrial processes such as welding, rubber and plastic processing, high speed machining with coolants, tempering and quenching.

Type:

The five main types of industrial dust collectors are:

Inertial separator

Fabric filter

Wet scrubber

Unit collector

Electrostatic precipitators

Selecting a dust collector

Dust collectors vary widely in design, operation,efficiency, space requirements, construction, and capital, operating, and maintenance costs.Each type has advantages and disadvantages. However,dust collector selection should be based on the following general factors:Dust Concentration and Particle Size For mineral processing operations, dust concentrations range from 0.1 to 5.0  grains (0.32 grams) per cubic foot of air.Dust (0.23 to 11.44 grams per cubic meter) can range in particle size from 0.5 to 100 Micrometer (µm).Required degree of dust collection.The required degree of dust collection depends on its potential to be a health hazard or nuisance, plant location, allowable emission rates, nature of the dust, its residual value, etc.Precipitator selection should be based on desired efficiency and should consider the need for high-efficiency,high-cost equipment such as electrostatic precipitators; high-efficiency,moderately cost equipment such as baghouses or wet scrubbers; or lower cost Primary equipment, such as dry centrifugal collectors.Airflow characteristics Airflow characteristics have a significant impact on collector selection.For example, cotton fabric filters should not be used in temperatures above 180 °F (82 °C).Also, condensation from steam or water vapor can darken the bag.Various chemicals can attack fabric or metal and cause wet scrubber corrosion.Dust characteristics-Moderate to severe concentrations of many dusts, such as those from silica sand or metal ores, can be abrasive to a dry centrifugal collector.Hygroscopic materials can blind the bag collector.Sticky material will adhere to the collector element and plug channel.Certain particle sizes and shapes may preclude certain types of fabric collectors.The flammability of many fine materials precludes the use of electrostatic precipitators.

Disposal Methods:

Dust collection and disposal methods will vary depending on the material, plant process, capacity, and type of dust collector used. Collectors can be unloaded continuously or in batches.Dry materials can create secondary dust problems during unloading and disposal, which is not the case with wet collectors.Disposing of wet slurry or sludge can be another material disposal issue; if wastewater is not properly disposed of, it can cause sewer or water contamination issues.Selecting the proper size collector depends on the airflow and air-to-cloth ratio which determine the efficiency of the system.Optimal dust collection equipment increases employee retention and protects equipment that helps reduce maintenance and replacement costs.Selecting an oversized, undersized or underperforming collector can lead to many problems that affect performance and maintenance costs.Therefore a dust collector suitable for the company's specific workplace should be selected.Employees must be provided with a safe and healthy work environment. Also, employee efficiency and production cannot be overlooked.

Fan and motor

• A fan and motor system provides the mechanical energy to move the contaminated air from the dust generation source to the      dust collector.

Types of fans

• There are two main types of industrial fans:

• Centrifugal fans

• Axial fans

• Centrifugal fan

• Centrifugal fans consist of a wheel or rotor mounted on a shaft that rotates in a scroll-shaped housing.Air enters through the eye of the rotor, turns at a right angle, and is forced by centrifugal force through the rotor's vanes and into the volute housing.  Centrifugal force exerts static pressure on the air.The diverging shape of the vortex also converts some of the dynamic pressure into static pressure.

• There are three main types of centrifugal fans:

• Radial Blade Fans-Radial blade fans are used for high dust loads.Their diametrical blades do not become clogged with  material and can withstand considerable wear.These fans have medium tip speeds and a medium noise figure.

• Reverse Blade Fans-Reverse blade fans operate at higher tip speeds and are therefore more efficient.These fans should be used after dust collectors as material may build up on the blades.Although they are noisier than radial blade fans, backward facing blade fans are often used in high volume dust collection systems due to their higher efficiency.

• Forward Curved Blade Fans-These fans have curved blades that are angled in the direction of rotation.They have low space  requirements, low tip speed, and low noise figure.They are typically used against low to moderate static pressures.

• Axial fan

• Axial fans are used in low drag systems.These fans move air parallel to the fan's axis of rotation.The helical action of the  propeller moves the air along straight through parallel paths, creating a helical flow pattern.

• The three main types of axial fans are:

• Propeller Fans-These fans are used to move large volumes of air at very low static pressure.They are typically used for general or dilution ventilation and perform well up to 0.5 in. wg (124.4      Pa).

• Axial Fans -Axial fans are similar to propeller fans except that they are mounted in a tube or cylinder.As a result, they are more efficient than propeller fans and can generate winds of up to 3 to 4 inches of water (743.3 to 995 Pa).They are best for moving air that contains substances such as condensable smoke or paint.

• Vane Axial Fan -A vane axial fan is similar to a tube axial fan except that the air straightening vanes are mounted on the suction or discharge side of the rotor.They easily accommodate multiple stages and can generate static pressures up to 14 to 16 inches of water (3.483 to 3.98 kPa). they are usually only used to clean the air.

Electric motors

Electric motors are used to supply the necessary energy to drive the fan.Motors are selected to provide sufficient power to operate fans over the full range of process conditions (temperature and flow rate).

Configurations

Dust collectors can be configured as one of five common types:

1. Ambient Fixtures -Ambient fixtures are free hanging systems for applications where the use of source capture arms or ductwork is limited.

2. Collection Stations-Collection stations do not require plumbing, allowing workers greater freedom of movement.They are usually portable.

3. Downdraft Filter Table-A downdraft filter table is a self-contained, portable filtration system that removes harmful particles and returns filtered air to the facility without the need for external ventilation.

4. Source Collector or Portable Unit-A portable unit is used to collect dust, mist, smoke or smoke at source.

5. Stationary installations-An example of a stationary collector is a baghouse.

Parameters involved in specifying dust collectors

Important parameters for specifying a dust collector include airflow, which is the velocity of airflow produced by the vacuum generator; system power, the power of the system motor, usually measured in horsepower; dust and particle storage capacity, and the minimum particle size that the unit filters.Other considerations when selecting a dust collection system include temperature, moisture content, and the potential for combustion of the collected dust.Fine removal systems may consist of only one filtration system (such as filter bags or cartridges).However, most installations use primary and secondary separation/filtration systems.In many cases, the heat or moisture content of the dust can negatively affect the filter media of a baghouse or cartridge collector.These units can be preceded by a cyclone or dryer to reduce heat or moisture content before reaching the filter.Additionally, some units may have tertiary and quaternary filtration.All separation and filtration systems used within the unit shall be specified.A baghouse filter is an air pollution abatement device used to trap particulate matter by filtering the airflow through large fabric bags.They are usually made of fiberglass or fabric.A cyclone separator is a device that separates fine particles suspended in air or gas by means of a centrifugal device.An electrostatic precipitator is an air cleaner that charges dust particles by passing dust-laden air through a strong (50-100 kV) electrostatic field.This causes particles to be attracted to the oppositely charged plate so that they can be removed from the airflow.An impingement system is a device that removes particles by impinging aerosol particles into a liquid. Modular media type units combine a variety of specific filter modules in one unit.These systems can provide solutions to many air pollutant problems. A typical system consists of a series of disposable or cleanable pre-filters, disposable V-bags or cartridge filters.A HEPA or carbon final filter module can also be added.A variety of models are available, including free hanging or pipe mounting, vertical or horizontal mounting, and fixed or portable configurations.Filter elements are made from a variety of synthetic fibers capable of collecting sub-micron particles without creating excessive pressure drop in the system.Filter cartridges require regular cleaning.

A wet scrubber, or venturi scrubber, is similar to a cyclone, but has an orifice unit that sprays water into a vortex in the cyclone section, trapping any dust in the slurry system.The water medium can be recycled to continue to filter the air.Ultimately the solids must be removed from the water stream and disposed of.

Filter cleaning methods

In-Line Cleaning-Automatically timed filter cleaning allows for continuous, uninterrupted collector operation for high dust operations.

Offline Cleaning-Filter cleaning is done while the collector is off.Practical as long as the dust load on each collector cycle does not exceed filter capacity. Dust can be removed and disposed of to the greatest extent.On-Demand Cleaning-Automatically initiates filter cleaning when the filter is full, based on a specific pressure drop across the media surface.Reverse Pulse/Reverse Jet Cleaning-A filter cleaning method that delivers compressed air from the clean side of the filter to remove accumulated dust cake.Shock/Rapping Cleaning-Filter cleaning method in which high velocity compressed air is forced through a hose causing the filter to be randomly rapped to remove the dust cake.Especially effective when the dust is very fine or viscous.