This article takes an in depth look at baghouses.
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A baghouse is a pollution control device that uses tubes, envelopes, or cartridges to remove, capture, and separate dirt, particulate matter, and dust from the air of a manufacturing or processing facility. The main components of a baghouse are the media or bags used to filter the particles from the air as it passes through the system.
As the air in a baghouse is filtered, the collected matter forms a solid cake like layer on the sides of the filtering material. This is commonly referred to as a dust cake, and it continues to grow until its thickness and size restricts airflow, requiring the bag filters to be cleaned. The three cleaning methods for baghouses are reverse air, shaker, and pulse jet.
The types of baghouses are classified by their cleaning process. The most common cleaning methods are shaker, reverse air, and pulse jet, which are also referred to as gas, compressed air, and mechanical cleaning. Industrial baghouses use these three forms of filtration; each has its advantages.
A filter bag system uses filter bags to remove contaminants and can be easily modified. Filter bags are very durable but are not suitable for wet or moist filtration. Cartridge collectors are used for fine materials while cyclone collectors are capable of operating at extreme temperatures.
A shaker baghouse uses a mechanical process to clean the dust cakes out of the filtration system. The bags hang from the top of the unit and are fastened to a tube sheet at the bottom of the enclosure. Unfiltered air enters from the bottom and is pulled up through the system into the filters. The cleaned air exits at the top, while the collected contaminants remain in the filter bags.
The cleaning process begins by a stoppage of the airflow, which inhibits cleaning. Mechanical shaking is similar to shaking the dirt out of a rug. The hangers at the top of the enclosure shake the filter bags and the caked dust and dirt falls out through the bottom.
Shaker baghouse cleaning is the simplest cleaning process and is not the most efficient method. The hanging bags have to be checked frequently and require a great deal of maintenance and upkeep.
In a reverse air baghouse, the bags are attached to a cell plate at the bottom of the enclosure and hung from a frame at the top for support such that the air pressure will not collapse them. As the air is pulled in, dust and dirt collect on the outside of the filter bags. A fan or medium pressure blower on a rotating arm over the bags directs air into the bags removing the dust.
The two types of reverse air baghouses are round and rectangular with multi-compartments. The round version has a fan or medium pressure blower at the top and can be cleaned while in operation. The rectangular version is divided into compartments that are shut down one at a time during cleaning.
To clean a rectangular reverse air baghouse, a compartment is pressurized by a reverse fan, which causes the filter bags to slightly collapse. The bags are kept from pancaking by rings sewn into them.
The pulse jet baghouse design follows the same parameters as the reverse air and shaker types with bags hanging from a tube at the top of the enclosure. As with the reverse air filters, dust, dirt, particulates, and contaminants collect on the outside of the bag filter and are removed by bursts of compressed air that moves along the length of the bag. The airstream enters below the bag filters and is pulled up through the filtering system.
The process of dirt removal for a pulse jet baghouse is very quick and efficient. The bag filters are cleaned without having to be taken off line. A solid state timer controls the pulses of the compressor that sends pulses of air down the length of the bag, causing a rippling effect that dislodges the dirt from the filter.
A cyclone dust collector is a preprocessing unit in the dust collecting process where dust particles that are heavy or coarse are removed before entering the baghouse. Dirty air enters the cyclone and is quickly spun, forcing heavy particulate matter to the sides of the unit. As the large particles strike the sides of the cyclone, they fall into a collection or discharge container located below the cyclone.
The right baghouse filter can maximize the performance of the filtering system, increase filter life, and reduce downtime. Choosing the correct baghouse filter is difficult since there are so many fabrics and treatment types available. The many types of filter bags are adaptable to fit the needs of any system and are exceptionally durable.
The various types of bag house filters are made from woven or nonwoven fabrics with nonwoven fabrics divided into felted or membrane. All bags are completely or partially woven since nonwoven bags have a woven base.
Woven filters have a repeating pattern and are used for shaker and reverse air baghouses. The tight weave and small gaps between the fibers determine the size of the particles the filter can process. Some woven filters will have a Polytetrafluoroethylene (PTFE) membrane to prevent dust particulates from getting caught in the filter's fibers.
Nonwoven fabrics are bonded together by a chemical or mechanical process and may have a woven backing that is known as a scrim. The nonwoven part of a filter can be felted or have some form of membrane. Nonwoven filters are used with specific types of baghouses with pulse jet being the most common.
Nonwoven filters are designed to remove extremely fine dust, aerosols, and contaminants in a wide variety of industrial applications.
Felted filter fabric is composed of randomly arranged fibers that are punched into a scrim fabric. The construction of a felted filter forces the particulate matter to stick to the surface of the filter and form a dust cake. The irregular nature of the felt fabrics allows it to capture any type of matter that attempts to pass through it.
Unlike a woven filter, the fibers are inconsistently placed with each fiber acting as a target to capture particulate matter by impact and interception. Felted filters are two to three times thicker than woven ones.
The original fibers for baghouse filters were natural fibers such as wool or cotton, which are inexpensive but have temperature limitations. Both fabrics are still used for low temperature applications. Wool is the better of the two since it can withstand moist or humid applications and can be shaped into thick felt filters.
The majority of modern baghouse filters use some form of synthetic fibers, which can operate at extreme temperatures and are resistant to chemicals. Fiberglass is the most used due to its ability to withstand extreme temperatures.
Polyester felt is resistant to chemicals, abrasion, and dry heat. Filters made of polyester are the first choice for dry heat applications as they are superior to all synthetics for this type of application. Polyester is not used in moist or damp heat since it degrades in those conditions.
Polypropylene is used in applications that involve chemicals and moisture. It is resistant to most acids and alkalis. Polypropylene has a smooth surface that offers good cake release, resistance to blinding, and zero moisture absorption capabilities.
Nylon is used for abrasive conditions and applications and has good resistance to the effects of alkalis but will break down in the presence of mineral oxides and high temperatures.
Fiberglass has excellent resistance to acids with the exception of hydrofluoric acid. It cannot be used in applications where chlorides, bromides, or cyanides are present. Fiberglass requires extra support when used in tubular bag form.
Teflon® is ideal for chemical environments with high temperatures. It has excellent chemical and abrasion resistance. Of the available filter fabrics, Teflon® is the most expensive and is only used when all other fabrics fail. It comes in woven and felt finishes.
PTFE Tetratex® is a membrane material that is exceptionally resistant to acids and alkalis. It is applicable in environments at 135° C or 275° F and comes in sizes and dimensions to fit any type of baghouse.
The major benefit of pleated baghouse filters is a significant increase in the filtering surface; this allows for a lower air to cloth ratio. Deep pockets of woven fabric provide greater space for dust retention. Since pleated filters are shorter than average filters, they are above the inlet stream; this design reduces possible abrasion from the incoming dust.
The majority of baghouses can be refitted to accept pleated filters without the need for extensive redesigning or restructuring. Pleated filters can easily be mounted on cell plates, and they have an exceptional efficiency rating.
For the best possible performance of a baghouse filter, it is normally treated to improve its stability, durability, strength, and endurance. Certain types of treatments improve a filter's cake release properties. Natural fabrics require pretreatment such as being preshrunk to eliminate shrinkage when in use.
Calendering is a high pressure treatment used to press fabrics with rollers to flatten and smooth them. The process of calendering increases the surface life of the filter, gives the fabric more stability, and creates a uniform bag surface.
Napping is a scraping process that raises the surface fibers using pointed metal or burrs on a cylinder. The process increases the ability of the fiber to collect particulate matter.
Singeing is passing the material over a flame to burn off flimsy or disconnected fibers from the surface to make the surface more uniform.
Glazing heats the fabric near the melting point to fuse and flatten the fibers. The process leaves a sheen on the material and reduces any possible shrinkage.
Coating is applying a substance to the surface of the filter to add protection and durability. The types of coatings include polyvinyl, cellulose acetate, and urea-phenol to name a few. The coating enhances the filter's resistance to temperature changes and assists in chemical resistance.
In many cases, new filter bags are porous and exceptionally small particles can filter through. To avoid this, the filters are installed and have a powdered precoating applied that prevents blinding and clogging.
Fire retardant is not a fire proofing medium, but it is sprayed on filter material to provide protection against sparking. It is a surface treatment that reduces the probability of sparks igniting the filter material.
Baghouse filter cages offer support for a baghouse filter bag. They are made of vertical wires in groups of ten, twelve, or more. Though there are a wide variety of materials used to make baghouse filter cages, steel and stainless steel are the most commonly used. For top load units, the cage may have a rolled flange or venturi top, while split collars are used for bottom loads.
Venturi tube cages guide the air flow from the blowing tube to the filter to prevent airflow deflection. They promote the full mixing of ejected air with secondary air. Venturi cages are made of aluminum and carbon, galvanized, or stainless steels.
The efficiency of a baghouse depends on the method used to dispose of the collected dust and particulate matter. Baghouses are exceptionally efficient at cleaning dirty air and releasing clean, filtered air. Part of the process is collecting dirt, dust, particles, and other materials that are produced from manufacturing operations.
The final step in air filtration is to safely, efficiently, and ecologically dispose of the collected pollutants. This has to be completed in the most economical way, so none of the collected material is released into the air.
The part of the baghouse that collects the pollutants is called a hopper; this comes in many varied forms. Failure to empty and clean the hopper directly and irrevocably affects the operation of the baghouse system.
All recovery hoppers are located underneath the baghouse filter system since the material leaves the baghouse enclosure via gravity and falls into the hopper or collector.
The covered box container has small vents with filters attached to prevent back pressure. It is a very simple system that requires constant monitoring and maintenance. The covered box method is ideal for light dust loads of nonhazardous materials.
Much like the box collector method, in bag collection, a bag collects the dust. Once a bag is filled, it is removed by hand or mechanical device, and a new bag is attached. This method is an easy way to handle non-toxic dust. The bag has to be closely monitored and regularly replaced.
A screw conveying system removes the need for constant monitoring of the discharge unit. The filtered material falls directly into the screw conveyor that removes it and sends it to a collection hopper. It is an ideal method for heavy dust loads and the disposal of hazardous materials. A screw conveying system is more expensive than other methods and requires regular maintenance.
A dense phase pneumatic conveying system is connected to the baghouse discharge hopper, and it removes the collected material through a series of pneumatic pressurized pipes. Pneumatic pipes are installed below rotary valves, and they collect the dust from the hopper. Pressure is supplied by a blower that transports the dust to a silo at the end of the pipeline.
If a system has several baghouses, each baghouse can be connected to the system. Pneumatic systems can operate using pressurized air or vacuum air.
Baghouse systems are the main method of filtration for a wide variety of dust producing industries that are required to meet air quality standards. As concern grows regarding the number of pollutants in the air, various filtering methods have become an essential part of industries such as grain production, feed production, and silica manufacturing.
Though the process of filtering dust is somewhat similar from industry to industry, baghouses are designed to meet the specific requirements of the dust material that each industry produces. Adjustments to enclosures, filters, discharging units, and other factors are made to ensure the most success for the baghouse system.
Baghouse units are used as a part of the operation of an asphalt mixing plant. The design for asphalt mixing filtration includes several filtration bags in an enclosed chamber. The air from the mixing plant is fed into the baghouse system and passes through the filtering bags. Filter fabrics are woven or felted.
The initial filtering process begins with a cyclone separator that traps and separates the heavier dust. The lighter dust is then pushed to the baghouse system. The filtering system for an asphalt plant must be capable of withstanding extreme temperatures and corrosive gases.
Grain production requires dust filtration at several points of production such as milling operations, sifting, grain elevators, and bag filling. At every step of the process, dust and debris are being produced that have to be controlled to meet the requirements put in place for the environment and the safety of workers.
As the demand to produce and process grain increases, each aspect of the operation is working faster and producing more air polluting materials. With the ever increasing demands on dust control and emissions, more technologically advanced dust control systems are needed, which include the use of multiple baghouse systems.
Cement production is an industry that has strict and rigorous standards for the control of air pollution. Pulse jet baghouses are extensively used by the cement industry. In the production process, an assortment of waste gases with high concentrations of dust are created; they are required to be contained and controlled.
In the cement industry, baghouse systems serve two purposes, the first of which is the filtering of the gases and dust. Their second function is recovery of cement materials that have been released in the cement production process.
In addition to the cement production line, stone crushing, grinding operations, and fly ash processing are aspects of the process that require air filtration.
The chemical industry has the highest and most challenging demands for baghouse operations. Chemical productivity depends on highly reliable and durable equipment to meet the harsh and stressful conditions of the industry. Filtering equipment must be able to withstand exposure to an extremely aggressive environment and high temperatures as well as high humidity levels.
Dust loading in the chemical industry is fast and constant and requires frequent cleaning cycles to ensure smooth operation. A baghouse system must be able to capture and separate all of the chemical compounds from chemical products.
The filtration of silica dust is difficult due to the abrasive nature of the dust. Baghouse filters have to be abrasion resistant and able to withstand the rapid rate of the flow of the dust as well as the quantity of dust.
Sand and gravel are extracted from pits using earth moving equipment. The collected material is taken to a processing plant where it is crushed, screened, and sorted by size. For collection of the dust produced by the process, specially designed pulse baghouses are used to remove particulate matter from a huge volume of dust, debris, and assorted fragments produced by the process.
Baghouses fall under the auspices of the Environmental Protection Agency (EPA), the Occupational Safety and Health Administration (OSHA), and the National Fire Protection Association (NFPA) due to the possible impact an improperly performing baghouse can have on the environment or workers and the possibility of combustible dust. Each year, the EPA has been steadily revising air pollution standards to make them more stringent and increase the protection for the public.
The main concern of the EPA, regarding baghouses, is the production of particulate matter and its possible release into the environment. Particulate matter (PM) is microscopic solids or liquid droplets that are small enough to be inhaled and cause serious health problems. The size of the dangerous PMs is measured in micrometers, and they are not visible to the naked eye.
The NFPA develops standards for the prevention of fires and dust explosions.
The focus of OHSA is on the health and safety of workers. The organization has developed several standards regarding baghouses. A few of OHSA‘s concerns include:
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