A     Go to back to index

Abrasion:

Flex:   

Abrasion typically caused due to excess movement in part of a filterbag.

In shaker bags, this is often seen in the lower 4” to 24” region of the bag from loosely installed or stretched bags. The wear marks usually develop on the outside of the bag. In Particular, it is well defined if a shaker needled felt fabric is used.

In pulse bags, oversized dimensions in the bag to cage fit will cause the bags to fail along the cage wires. It can often noted by “cut” marks starting on the inside of the bag as the bags “flex” across the wires.

In reverse air style bags, this wear is a result of loose bags or excess cleaning air “flexing” the bag body often against a cell plate thimble.

Inlet:  

Whenever the dirty inlet air stream is directed at the filterbags in a system without any major decrease in velocity, dust abrasion will occur.

Bags will simply keep failing directly in the path of the high velocity air stream. Often this can be mapped on a cell plate diagram.

There are usually 2 events that happen in Pulse Jet Systems:

In shaker systems, the cloth around the bag snap band or cuff will fail first. It will become ragged and then simply tear during the shake cleaning cycle.

Surface:

Where the cloth surface has been abraded by:

Usually due to:

Agglomeration:

The joining of dust particles to form long chains (at least 10 particles).

This process is important in order to facilitate the cleaning down of filterbags. Dust that travels onto the bags is often the lightest. As a result, what goes up may not necessarily come down unless the physical size of the particle can be changed. Agglomeration facilitates this process.

Severe agglomeration will plug a baghouse.

Airflow:

Actual (ACFM):

Actual Cubic Feet of Air per Minute airflow readings at actual temperature and humidity readings with field values always being clearly marked as ACFM or SCFM.

Standard (SCFM):

Air with a density of 0.075 lb. per cubic foot.  Which is generally equivalent to dry air at 70 degrees F and 29.92 in. (Hg) barometer.

Anytime airflow measurements are taken in the field, the temperature and humidity factors have to be considered. Often the ACFM field numbers are converted into their equivalent standard measurements.

Just as a side note, 30,000 CFM (Cubic Feet of Air per Minute) has an equivalent weight of 1 ton!

System:

The total volume of air handled by a ventilation system at its furthest exit point is the system airflow and is usually measured at the system stack.

In a vacuum system, what comes out must come back in. In other words, the total system airflow is the sum of all the air available to the fan regardless of its source. This includes system air leakage.

Air-to-Cloth Ratio:       

This number, also known as the A/C ratio, indicates the velocity of the airflow per square foot of cloth (Face Velocity).

Generally, the smaller a particle is in diameter, the more difficult it is to filter and thereby requires a lower A/C value.

Gross A/C:

The total system airflow divided by the entire sum of the cloth area of all available operating filterbags in a system when all compartments are on-line

Net A/C:

The total system airflow divided by the entire sum of the cloth area of all available operating filterbags in a system when one compartment is off-line for cleaning.

This is considered a design number, as it is possible to have more than one compartment down at one time. Some systems always have one down for rotational maintenance and for cleaning purposes. In this case, the true net number will be higher as the total number of bags will be decreased

Industry standards for a pulse jet baghouse range from a conservative number of 2:1 to an aggressive number of 8:1 and upward.

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Baffle:

A plate, diffuser, or other engineered obstruction designed to deflect the incoming baghouse dirty air stream away from the baghouse filterbags.

This redirection should allow the air stream to fully expand out into the hopper drop out area and slow down. In addition, turbulence created during the redirection also allows for dust particles to fall out of the dust carrying air stream.

Baghouse:

Evaluation:    

The performance of any dust collector is based upon the management of the dust cake, which gathers on the filtering surface of the filter media. This dust cake determines how well airflow is going to pass through to the clean side of the filter media. Also, this surface cake also determines how efficiently dust or particle capture will occur on the dirty side of the baghouse.

We try to analyze a system by observing how effective the dust cake management is. Items that impact this surface cake include:

Hopper:         

The section of the baghouse below the filterbags that acts as a settling chamber for incoming dust particulate. It can also act to some degree like a plenum if it is large enough.

Air generally travels to the baghouse at velocities of 3,000 to 4,500 feet per minute. This velocity is required to maintain the dust in the air stream within the ducting. It is desirable to slow this down to 3,000 feet per minute (or less) as it enters the hopper. By using the open area of the hopper, the air slows down through expansion and the heavier dust particles and/or clumps fall out of the air stream.

This separation guarantees a reduction in the impact of the initial dust loading on the media.

Styles:

Today we see 3 major types of cleaning systems:

Reverse Air:

There are several basic methods of utilizing the principles of Reverse Air System cleaning. Two of the most common methods are:

Balanced System:

A system that has all the correct pick-up velocities, airflows and carrying velocities is considered to be “in balance”.  A balanced system usually has ducting that is sized to suit a set number of pick-ups. There is often very little design room to add new vents.

If we change the system by closing dampers, adding or subtracting ducts, we will influence how the air will flow into the existing pick-ups. The flows may increase or decrease; as a result, we may not get the expected desired performance: the system is now “out of balance”.

Blast Gate:

A sliding plate installed in a supply or exhaust dust at right angles to the duct for the purpose of regulating airflow.

Blow Tube/Blowpipe:

Blow tubes are used in Pulse Jet baghouses to direct high pressure air traveling from the diaphragm valve across the top of each row of bags.

Each blow tube will have a series of holes (often ¼” in diameter) that direct the airflow out of the tube down into each individual filterbag.

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Cloth Finishes:

The base fabric of many low temperature laminated and composite fabrics, is Polyester Needled Felt. This fabric is cheap, available, and quite durable. By varying the surface characteristics of the polyester felt, the fabric application capabilities can be further enhanced.

The “unfinished” (off the loom) surface of a needled felt is quite “hairy” or fibrous. The hairs will capture any sticky dusts and the surface dust cake will build rapidly. Fine dusts will also be trapped to a good degree by these same fibers. Either way, the extended fiber surface will “trap” dust particles fairly easily causing them to Agglomerate.

To vary the standard surface of a felt in order to deal with different degrees of particle agglomeration, various finishes or coatings are available depending upon the application. This is all part of dust cake management.

A finish is considered to be a mechanical process performed on the fabric.

A coating or lamination is viewed as an “additional” external treatment to the fibers or fabric.

Acrylic:

Several companies have introduced a surface coating manufactured from an acrylic foam style base that enhances the surfaces of various fabrics.

(http://www.lantorinc.com/dust.htm)

Polyester felt is a main base for this coating.

Frost Emission has used this fabric in Lead Battery applications where there is not only a sticky dust present, but fine and dry dust particles are present as well. Another application for this coated fabric is seen in Powdered Milk Spray Dryers. 

Eggshell:

This is a style of mechanical surface finish.

By passing the finished needled felt fabric through two hot rollers (Calendar Machine) and allowing the top roller to “skid”, the surface of a polyester or Polypropylene fabric can be smoothed out to produce a very slick finish.

This finish is not generally used because it is not understood too well. The surface often ends up operating like “fish scales”. Sticky dusts will not stay on the surface as the surface fibers have now virtually disappeared. Low micron dusts, however, will get under these “scales” and plug the open pores.

The filtration Velocity through the available openings in an eggshell finish is often quite high. On one occasion, a fabric used in a Cement Finish Mill was sent to us for analysis: only the eggshell surface was in tact. The base (back) fibers had been completely eroded off!

Fire Retardant

Fabrics that will self-support a flame are called Flammable; fabrics that will not self- support a flame are called Non-flammable. However, both will still burn, the non-flammable fabric simply chars.      

Most fire retardant or flame retardant coatings are usually only effective once the source of flame or ignition has been removed from the process.

Generally, a mixture of sodium bromide and boric acid applied to the fabric through a dipping process creates a protective wrap around the fibers.

Polyester fabrics are flammable with self-supporting burning properties. Cotton or Nomex™ fabrics will not burn once the flame has been removed. However, with air passing over a burnt filterbag, the Nomex™ or cotton may still continue to char.

The fire retardant finishes will not protect a fabric if the dust particles burn on the surface. It is best used as “barrier” to sparking or known system fire issues where the fire can be extinguished quickly.

It can also be used as a “false” dust cake in order to enhance the filterability of the cloth (usually cotton) in very fine dust applications.

Heat Setting

Removes the tension within the fabric yarns by exposing the fabric to heat.

This heat setting stabilizes a cloth in order to reduce stretching or shrinking when in an operational process.

The cloth is passed through a Calendar Machine that uses hot rollers to heat the fabric.

We have seen 14’ long needled felt pulse jet bags stretch to 17’ because the fabric was not stabilized through heat setting.

Membrane

An expanded Teflon coating applied to the surface of virtually any fabric.

The micro pores in a membrane are small enough to stop water from penetrating but are still large enough to allow water vapor to pass though.

From a filtration point of view, membranes are the most efficient fabric available. Mechanical deterioration is the most common cause of their failure.

In order to get a quick idea of what the membrane looks like, envision white “plumber’s tape” applied to a fabric surface.

The most widely known of the membrane products is Gore-Tex®.

Plain

This is the “off the loom” state of a needled punched fabric. The surface is untreated and is not mechanically altered.

The surface has a “hairy” or “fibrous” profile.

Singeing

The process of removing protruding hairs from the surface of a plain fabric through exposure to an open flame.

Singeing is useful for most applications and helps promote dust release.

Spark Proofing

A similar finish to flame retardant. Spark proofing is supposed to cause a chemical reaction if a spark hits the fabric surface that “envelopes” the spark and smothers it. Phosphates are often used in this connection.

Tri-Coat (Teflon, Graphite, Silicone)

A coating almost exclusively used for coating fiberglass fabrics.

Fiberglass fibers are brittle and tend to break easily. The Tri-coat or Tri-Temp coat prevents mechanical damage to the bags and also provides a dust release agent.

The coating color is dark gray or almost black. A simple teflon coating is a beige color

Other

European felt manufacturers have always produced “engineered” fabrics. Their philosophy is building a cloth to suit the application or process. This has been accomplished by working with:

This technology produces a final engineered filtration product. BWF is one main producer of these technical filtration fabrics.

Cloth:

Each of the following subjects is a large area to cover in a small amount of space. These have been added to the Subscription Site where extensive articles are available.

Construction:

Please refer to the Subscription site for further explanation.

Efficiency:

The measure of filtering capability of a fabric against stated dust particle sizes and quantities at “usual” air-to-cloth ratios.

Most standard clothes have a stated general guideline efficiency of 99.99%. But what does this mean?

We can probably state that the efficiency will be 99.99%

This number is then plugged in to the formula for calculating the baghouse efficiency.

Example of 99.90% efficiency calculation (Use Ctrl-Click if needed)

Example of 99.98% efficiency calculation (Use Ctrl-Click if needed)

Failure:

Please refer to the Subscription site for further explanation.

Scrim:

The internal support or base of a needled felt fabric.

Looks like a woven grid similar to cheesecloth. The scrim fibers are usually of the same fiber make-up as the fibers being needled to it.

The scrim is supposed to add additional strength support to the felt.

Specifications:

Please refer to the Subscription site for further explanation.

Testing:

Please see the Subscription site for this discussion

Condensation:

The process of changing a vapor into liquid by the extraction of heat.

Condensation can cause:

Condensation is usually averted or minimized through:

Containment:

If a dust source cannot be controlled reasonably through a ventilation collection system, then the source should be isolated from the rest of the work area; this is containment.

The source may be boxed in with curtains, walls, or even sheet metal panels.

Please see: General Discussion of Dust Source Sites for Mines Web site and refer to the booklet list

Control:

If a dust source can be ventilated and the emissions captured at the immediate point of release, then the dust is being “controlled” through ventilation.

The use of a “local” exhaust hood is often referred to.

Controlled dust sources usually require less housekeeping hours than Contained sources

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Damper:

An adjustable plate installed in a duct for the purpose of regulating airflow. This can be, for example:

Dust Build-up:

Bridging

When dust packs in the body or hopper of a baghouse, it can cause the dust to stop flowing out of the system. We refer to this packing as bridging.

Dusts with strong agglomeration qualities such as detergent dusts, will quickly build on the surface of a fabric. As this agglomeration grows, the dust will bridge over between the bags and trap new incoming dust into the mass.

Dusting on the clean side of the baghouse

When inspecting a baghouse, a judgment call has to be made on the potential severity of any dust leakage from the filterbags.

Due to the Plenum effect of a baghouse clean side chamber, dust that passes through a filterbag may not make it any further than the cell plate. Build-up results on the cell plate and this is what we find during an inspection.

In many instances, only when the dust is disturbed during the cleaning cycle will we see emissions. Settled dust has no inertia to become airborne otherwise.

We judge this build-up based on previous experience. It is not always accurate; however, the cost of running a stack analysis is very expensive.

Layer Description          Interpretation

Often we will recommend a filterbag replacement if the dust is spread throughout the baghouse clean side.

Dust Loading:

The amount of dust that enters into a baghouse is referred to as dust loading.

A grain is 1/7000 of a pound. It is a simple unit of weight and is not a count of the number of particles. Neither measure of weight gives any indication of the size or quantity of particles present.

Typical dust loadings into a baghouse are in the range of 2 to 30 grains per cubic foot:

A 25,000 cfm baghouse with a loading of 2 gr./cfm handles 428 lbs./hr

A 25,000 cfm baghouse with a loading of 30 gr./cfm handles 6,400 lbs./hr

A process that uses a cyclone or drop out box ahead of the baghouse can report a high process dust loading; however, this pre-separation chamber will minimize the dust loading into the baghouse. The “pre-cleaner” is always noted in the system efficiency calculation.

Example of 99.90% efficiency calculation (Use Ctrl-Click if needed)

Example of 99.98% efficiency calculation (Use Ctrl-Click if needed)

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Entry Loss:

The loss in total pressure caused by the air (gas) stream flowing into a duct of hood  (usually expressed in inches w.g.).

All movement of air requires and expends energy. This energy loss is reflected in the accumulated systems losses that an exhaust fan and motor have to overcome.

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Fabric:

Knit:               

Generally considered as being composed of intermeshing loops of yarn.

Most knit filterbags are circular knits. In other words, the bag is manufactured directly off a knitting machine rather than being produced from a flat panel of fabric.

Knit fabrics are common is Steel Arc Furnace applications and in woodworking operations.

Knit bags have had several problems over the years:

1. When they get a hole, it will tend to run into a split and enlarge.

2. Certain knits have severe stretching problems. In some cases the bags are manufactured short in length in order to “allow” for the stretch.

3. Knits are not the most efficient filtering media available as bleeding of fine dusts can be an issue.

Needled Felt:

A felt made by the placement of loose fiber in a systematic alignment with barbed needles moving up and down.

Needled felts are considered to be 3 dimensional as they have depth. A woven fabric is generally considered to be a 2 dimensional fabric. As a result, it needs the filtering dust cake to produce the third dimension.

Woven:                      

A two dimensional fabric produced by interlacing lengths of fibers in a length by width pattern.

The woven fabric relies on the dust cake to produce the 3rd dimension needed for baghouse filtration.

The surface of the woven fabric determines how the dust cake will be retained and released. In other words, how it will be managed.

The woven fabric’s surface is adjusted through the varying of the interlacing of the fibers and the type of fiber used.

Fiber:

Denier:                      

The weight in grams of 9000 meters of any linear material. A one-denier fiber would weigh 1 gram if 9000 meters were placed on a scale. A two-denier fiber would weigh 2 grams if 9000 meters of it were weighted.

As the denier number increase, the size of the fiber increases.

A one-denier polyester is not equal in size to a one-denier nylon because of the difference in densities of the material.

Denier is important in terms of needled felts in particular. As the denier decreases, the diameter of the fiber decrease and more fibers can be packed into the cloth. This results in a tighter filtration surface.

Filter:

Bag:               

Generic term for a fabric filter element. The bag is sewn into tubes or envelopes with an assortment of hardware attachments often used.

Bags are made out of vast array of medias. Polyester is the usual low temperature selection. Nomex™ is often the first choice when temperatures run between 275 and 400 degs F. Beyond 425 F, fiberglass is just about the only choice (from a cost point of view).

Cake:             

The layer of dust on a bag dirty side surface, which helps filter the fine dust particulate and keep it on the bag surface.

The dust cake is where the dynamics of baghouse filtration really occur. In essence, if we can manage this dust layer as needed, then the system should perform, as required.

Media:                       

Fibers are produced in continuous lengths or filaments.

Filter media are made up of cut lengths of fibers joined together by a variety of mechanical processes in order to form a filtering surface or mat. Filter media generally fall into one of 3 categories:

Low and medium temperature (425 degs F or less) pulsejet baghouses use needled felts almost exclusively. Felts have a three-dimensional quality that allows for continuous filtration of a dirty air stream. The fiber shape, its fineness (denier) and density, along with the surface finish of the media, determines the filtering qualities of a felt fabric.

High temperature applications (425 degs F or more) tend to use woven fiberglass both in pulsejet and reverse air baghouses. Often a membrane-coated cloth is used to increase the efficiency capture capability of the media.

The chemical and physical properties of a fiber will also determine in which process application a media can be used.

Pressure Drop:                                  

The resistance to airflow as measured from the dirty side of the bags to the clean side and is usually measured in inches of water.

The change in pressure drop will occur with the addition or removal of the dust cake on the media surface.

Filtration:  

Diffusion:                               

The collection of particles as they pass through and contact previously deposited dust cake particles on the bag surface.

It is the management of this dust cake that is the real center of all baghouse filtration.

Impaction:                             

Due to particle inertia (momentum) in the gas stream, rather than flowing with the gas molecules around a fiber obstruction, the particle moves forward and impacts the fiber.

Impaction is more effective with larger particles than smaller ones.

Interception:                          

Small particles in a gas stream can exhibit random motion behavior. If these particles pass within a fiber one-radius distance, there is a possibility of the particle contacting the fiber during its movement. This is called Direct Interception Filtration

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Grain:                           

1/7000 pound or approximately 65 milligrams.

A grain is 1/7000 of a pound. It is a simple unit of weight and not a count of the number of particles. Neither measure of weight (grain or pound) gives any indication of the size or quantity of particles present.

Grain Loading:                      

See Dust Loading

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Hood:

Design           

Each process we are trying to ventilate will usually have a generally accepted, and previously tried, airflow and capture velocity design for dust control.

Many types of hoods have been tried and tested in the work place and in labs. However, each pick-up is actually unique because the type of dust and how that dust is generated will be unique. The industry accepted standards are an excellent guideline. You should use your own in-house experiences to make the final set-up.

We do know that capturing a dust particle at its point of origin requires less airflow and energy than when it is airborne in the building air streams.

For this reason, the more we can “box” a dust source in with a hood, the less need there is for high capture velocities and airflow. The more open a dust source is, the greater the interference from surrounding drafts and ventilation fans, therefore the greater the need for higher capture flows.

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Isometric Drawing:      

A virtual 3-dimensional line drawing of a ventilation system.

This is very useful for displaying a complete system layout without having to be concerned about dimensions. Always recommended for troubleshooting a system

Isometric template paper is usually available at office supply stores in the drafting supplies department. You can also make your own template using many of the drawing programs available. The angle for the drawing lines is between 30 to 35 degrees

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Micron:

A measurement of length equivalent to 1/1000 of a millimeter.

When used to measure dust particles, a one micron diameter particle is the threshold for generic fabric filtration. Any particle below one micron is difficult to control and requires special consideration.

Human hair has a diameter range of 35-150 microns.

Most particles collected in filtration processes are below useful mesh measurement levels.

Particles larger than 10 micron are visible to the naked eye.

Invisible dust particles that enter the human lung are hazardous to health.

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Needled Felt:      

A felt made by the placement of loose fiber in a systematic alignment, with barbed needles moving up and down.

Needled felts are considered to be three dimensional as they have depth. A woven fabric is generally considered a two dimensional fabric. As a result, it needs the filtering dust cake to produce the 3^rd dimension.

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Particle:

Bleeding:

The passage of fine particles through a filter media from the filtering side to the clean side..

Bleeding generally has 2 causes:

A lack of dust filtering cake on the surface of the bags allowing the fine particles to travel through any open pores.

A very open pore cloth in an application where a dust cake has difficulty building

Blinding

The penetration of dust particles into the depths of a media that results in open pores becoming plugged.

A blinded bag may show very little surface dust cake; however, by slicing the bag open, deep penetration can be seen in the media profile.

Shape

Different processes will produce different dust particle shapes. For example, hot processes will generally produce a spherical shaped particle. A mechanical blasting process will produce jagged particles.

Shape is important because it will determine how the dust filtering cake will be formed on the surface of the bag and, to some degree, how it will act.

Size Distribution

The comparison of dust particle average (mean) diameters taken from the typical dust stream in the baghouse.

It must be remembered that the dust particles captured in the hopper may not truly reflect the dust particles on the filter bags. The filter bags will almost always see the lower diameter particles as only the fine particles tend to get carried through particularly to the top of the bag. This is due to reduced carrying velocities in the baghouse.

Permeability:

Fabric:

Measured on a Frazier porosity meter, or permeometer, etc.

The ability of air (gas) to pass through the fabric, expressed in cubic feet of air per minute per square foot of fabric with an 0.5” H₂O pressure differential.

Operating:

A new fabric has to have a reduction in air flow due to high initial porosity.  At the point where an equilibrium of dust being collected and/or dust being cleaned off of the arrives without causing a further decrease in cloth airflow, this is called the operating or working cloth permeability.

Plenum:     

Often thought of as a product drop out box. Dust enters into the box and settles out due to a reduction in the carrying velocity forced by the large volume of the box. A true plenum can be used as the central point for all ducting to tie into, not just the main duct run.

Due to its large size, the resulting low flow velocities and its low static pressure (Vacuum or Bursting Pressure), ducts can be added or subtracted, within reason, without throwing the system out of balance.  This allows for some flexibility in reworking or adding to a previously designed ductwork system.

Clean air chamber:

The clean side of a baghouse is referred to as the clean air plenum. A plenum is simply an area where the air velocity can slow down. In some systems this can be a small area, but is still called a plenum.

Drop out chamber:

In the effort to minimize overloading the filterbags with dust, the incoming air stream is sometimes directed through a large “box” or “cyclone” before the baghouse. The drop out box will cause the air stream to expand, velocity to slow down, and larger dust particles to settle out of the air stream.

Pressure: