Which Baghouse Hopper Discharge Method Is Best?

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6 baghouse hopper dust discharge styles

Many problems arise over how to properly dispose of dust in the baghouse once it is collected. Improper dust disposal can directly impact the operation of your baghouse. Storing dust in your baghouse hopper is a terrible idea. Dust collector hoppers are designed for temporary storage only. If collected dust builds up in the hopper it can cause several problems.

  1. It directly causes filter bag abrasion, the wearing holes near the bottoms of the bags. This happens because the rising dust levels disrupts the carefully engineered airflow mechanics within the baghouse. When high speed air is pulled across the surface of a pile of dust in the hopper it picks the dust back up (i.e. dust reentrainment) and essentially throws it back at the filter. The effect is much like sandblasting your filter bags, something no bags will ever be able to withstand.
  2. Large amounts of dust can provide ample fuel for fires or even combustible dust explosions. Sparks and embers can make their way into the unit from cutting, grinding or other friction generating processes as well venting of furnaces and other heat sources. These ignition sources can ignite bags upon reaching the baghouse or pass through to the hopper. In either case, large amounts of dust in the hopper provide ample fuel for continuing a fire or making it much worse.
  3. Excess hopper buildup will block off the baghouse and cause a loss of suction throughout the system. Loss of suction at pickup points can shutdown entire plant processes, damage equipment and even cause environmental safety hazards and increase emissions levels past permissible limits.

Clearly, failure to keep the discharge working efficiency can have serious consequences. So how can you make sure your discharge system meets the needs of your baghouse?

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6 baghouse hopper dust discharge stylesDifferent Baghouse Discharge Systems

Let’s review a few common dust discharge methods and some guidelines for choosing the best option for your application.
#1, 3 – Covered Box or Drum with venting
An enclosure (usually a box or container) directly underneath the discharge holds the dust. To prevent dusting and back pressure issues the enclosure is vented by (a) a small vent with filter attached to it or (b) with a duct vent piped back to the collector or the inlet duct. Simple system, but requires maintenance to remove collected dust or else it can backup into the discharge (blocking the system) and overflow the container. Good for systems with light dust loads and nonhazardous materials.

#2, 5, 6 – Removable Storage Containers
Uses drums or bags to collect dust from discharge. When filled, technicians remove them by hand or using a forklift for disposal and then replace them with a new container. Good choice for easily handled, non-toxic dusts. Can also be useful for products that then get shipped by truck from plant (e.g. fly ash sold to cement plants, etc.) Requires technicians to monitor fill levels and replace as needed.

A dust transport method for baghouse discharges is by screw conveyor

The most common automated dust transport method for baghouse discharges is by screw conveyor

#4 – Discharge to conveying system
Preferred where possible, this method ensures the prompt removal of discharged dust. This proves the best solution for large units with heavy dust loads and applications requiring dust to be transported far from the collector after disposal such as hazardous material disposal, or for reuse in process. On the downside, it is more expensive than other methods and requires additional maintenance costs to maintain system.

Conclusion

We have seen that the best method of hopper discharge varies from application to application and from unit to unit. However, this does not mean that all discharge methods work for all baghouses. As outlined above, serious problems arise when the baghouse hopper discharge system is not adequate to the dust loads passing through the unit. Additionally, the disposal methods may require more man power than available at the plant and lead to spillage and other issues.

These issues can be avoided by not leaving the dust discharge method to chance. Review the operating parameters, dust loading rates, dust characteristics and eventual use of the dust (including disposal in landfill) before selecting a discharge method.

Baghouse.com has helped many plants retrofit their existing dust collectors with new hopper discharges, dust transportation and removal. Contact us today and let us advise you on how to improve your discharge system and thereby improve your dust collector efficiency today!

Need Help With Your Hopper Discharge?

Baghouse.com has helped many plants retrofit their existing dust collectors with new hopper discharges, dust transportation and removal. Contact us today and let us advise you on how to improve your discharge system and thereby improve your dust collector efficiency today!

FAQs – Dust Collection System Design and Operation

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Dust build up inside a duct connected to a dust collector

Question: What is “normal” differential pressure in a baghouse?

Answer: In most applications a baghouse dust collector should run between between 3″ to 6″ w.g. under normal use. Once levels rise above 6″ (roughly) and the cleaning system cannot return it any lower (even when turned to continuous cleaning or “Test” mode) it is a sign that the filters are beginning to be blinded and likely need to be changed. It is not advisable to run a baghouse with a DP higher than 6″ for any length of time as this will have an impact on the function of the entire system. Running at such a high DP will lead to a number of problems including vacuum loss at the pickup points of the system (loss of suction), lower air speeds in the ductwork, higher emissions, and higher energy usage.

If you are seeing levels below 3″ after having run the baghouse for sometime you liking are getting false DP readings. When brand new bags are installed in a dust collector they should provide approximately 1″ of resistance alone. Once they begin to load dust that number will rise to between 2″ – 3″ no matter how much you clean them.

A clean on demand baghouse controller (i.e. clean on pressure) is the best way to keep a dust collector running in the recommended DP range. (see article: 3 Cheap Ways to Increase Efficiency in Dust Collection Systems)

Dust build up inside a duct connected to a dust collector

Maintaining the minimum conveying velocity in the dust collection system prevents dust drop out and build up inside the ductwork

Question: What is minimum conveying velocity in my baghouse and why is it important

Answer: The minimum airspeed required to keep dust particles suspended in the conveying system (i.e. dust collection system). When the air in any part of the dust collection system slows below the minimum conveying velocity the dust will begin to drop out of airstream and settle to the bottom of the ductwork (known as product drop out).

Maintaining the airspeed throughout the system above the minimum conveying velocity is required to prevent the accumulation of dust in the ductwork. Overtime, dust can accumulate into large piles, eventually blocking off part of the ductwork and reducing suction downstream in the system, further compounding the problem. Blockages can also cause the passing airstream to accelerate (forcing same air through a smaller space) that can lead to abrasion issues and eventually wear holes into the ductwork. Large accumulations of dust can eventually collapse sections of the ductwork due to the added weight.

Preventing product drop out is even more serious in applications involving combustible dust. Any accumulations of dust within the ductwork provide a potential fuel source for any ignition source that may find its way into the ductwork such as sparks. Additionally, if a fire starts in one part of the system it could continue to propagate throughout the rest of the system being fed by the accumulations in the ductwork. Further, if the system is operating below capacity due to blockages, dust may accumulate elsewhere in the facility including on elevates spaces that can then become fuel for both primary and secondary dust fires and explosions.

FAQs – Dust Collector Filters

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Question: Why are my baghouse filters so expensive to replace?

Answer: Many simply buy their replacement filters from the OEM or sales rep that supplied their baghouse. Often times, manufacturers and sales rep organizations deliberately sell their units cheaper and then make convince their customers that they are locked into using a proprietary filter design that only they can supply and thus they charge outrageously high prices for them. Other times, they convince their customers to use an outdated or rarely-used technology so hard to find form other manufacturers that it nearly guarantees them your repeat business for replacement parts. This is common with many cartridge collector OEMs, whereby they win the initial unit sale by undercutting other manufacturers (often by recommending a undersized dust collection system) and then plan on making their profit on the expensive replacement filters later on. This marketing technique is commonly called the “razor blade” system, for its well-known use by makers of disposable razors and cartridges.

Need New Filters?

We offer replacement baghouse filters, cartridges, and pleated filters for all makes and models of dust collectors, including the most popular brands Farr, Donaldson / Torrit, Wheelabrator, and more. Often we can offer significantly better prices than buying from the original dust collector manufacture, sometimes as much as 50% less! If you do not believe us, give us a try and let us give you a quote for your next set of replacement baghouse filters.

Question: How long will my baghouse filters last?

Answer: Baghouse filters have an average service life of 1-3 years in most applications. Some can go beyond that without major increases emissions, while others may last less than a year in more difficult applications. The main reason to replace baghouse filters is because when old they begin to leak and thus the system is no longer collecting particulates as its designed to do. Filters can also be damaged prematurely by sparks/embers that can cause fires or even explosions. Upset conditions in the process may cause a spike in temperature (beyond the maximum for the filter fabric) or may create an acid flash or similar chemical attack on the bags. Finally, bags may be damaged during maintenance or by other external forces.

The main signs that your filters need to be replaced are that they are can no longer be cleaned effectively by the baghouse and/or they start leaking.

Baghouse filters that are blinded

Blinded filters must be replaced.

Question: What does it mean when my baghouse filters are “blinded”?

Answer: Blinded filters means the filters are so loaded with dust that they can no longer be cleaned by the baghouse cleaning system and must be replaced.

During normal operation dust particles accumulate on the surface of the filters and form a dust cake, which is then cleaned by the pulses of compressed air during the cleaning cycle. Overtime, some dust particles pass through the surface layer and become embedded deep within the fibers of the filter fabric where it cannot be removed by the cleaning pulses. Eventually, the fabric becomes so filled with dust particles that it severely restricts the movement of air through the filter. When this occurs the filter is said to be “blinded”.

When a baghouse can no longer clean itself down to a lower DP range (e.g. below 6” of DP) even with constant cleaning (i.e. continuous cleaning mode or test mode) it is likely that the filters are blinded and must be replaced.

FAQs – Baghouse Controls and Cleaning Settings

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a baghouse dust collector control board
a baghouse dust collector control board

With Clean on Demand Controllers  the “On-Time” setting  must be set correctly or else bag cleaning will suffer. On on Turbo baghouse controllers (and on many others) you need to first find the preset that corresponds to the on-time setting (here shown as F1), push select, and then enter the proper value (.1-.15 ms)

Question: What does the “On-Time” setting on my baghouse control board do?

Answer: On-Time determines how long the pulse valve is open for during a cleaning cycle. This setting is VERY important for proper cleaning of the baghouse filters on a pulse jet dust collector. If set too long then the air pulses will be weak and waste compressed air, lower pressure in the air header (causing delays while it builds back up or weakening the following pulses) or even damage the filters. If set too short not enough air volume will be released to clean the entire bag. It will also cause uneven dust loading on the bags, which in turn can cause a long list of problems in the collector.

Question: What should I set as the “On-Time” on my baghouse control board?

Answer: As a general rule, this should be set to between .01 – .015 ms (milliseconds). For certain specific applications (such as pleated filters, or certain “sticky” dusts) your dust collector OEM might provide you with a slightly different setting.

Question: What is clean-on-demand and why should I use it instead of just setting a timer?

Answer: Clean on demand (or clean on pressure) is a means of controlling a pulse jet baghouse cleaning system. It is the most effective way to operate your dust collector and it can lead to considerable savings in several areas.

Using a clean on demand baghouse controller, operators set high and low differential pressure points (usually 5.5″ and 3″ respectively). When the DP reading hits the high point the control board begins firing the pulse valve(s). It will continue firing them in order until the DP drops below the low point.

In contrast to a simple timer board, a clean on demand controller only cleans the bags only when necessary to maintain stable operation. This prevents over cleaning (which increases wear and produces higher emissions), reduces compressed air use (costly in most plants), and reduces wear on the diaphragm valves. Additionally, clean on demand controllers are able to adapt to changes in dust loads (common in most applications) something timers cannot do.

PTFE Membrane Vs PTFE Finish – Is There a Difference?

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PTFE membrane on a baghouse filter under a microscope

Yes, there is a difference between the two. One is used to increase collection efficiency and one is used primarily for protecting the filter bags from chemical attack.

What Are PTFE Membrane Filter Bags?

PTFE membrane is semi-porous layer of PTFE bonded to the surface of a filter. This membrane acts at a permanent dustcake, capturing incoming dust particles (i.e. particulate matter or PM 2.5) on the surface of the filter (surface filtration) as opposed to normal filters that require a thick layer of dust buildup (dustcake) to actually reach peak efficiency. This means that PTFE membrane bags can operate at peak collection efficiency from the moment they are installed, and do not need to be precoated. Overtime, the membrane also works against the dust penetrating deep into the depth of the filter fabric, which is the cause of filter blinding. For this reason, PTFE membrane bags often last considerably longer than standard filter bags and have a more consistent differential pressure over time.

PTFE membrane on a baghouse filter under a microscope

PTFE membrane is a thin layer of PTFE laminated to the surface of a filter bag. It captures dust on the surface of the filter and easily releases the dust when pulsed.

PTFE Baghouse filters with PTFE membrane have the highest collection efficiency of all filters in production today. Bags using membrane technology can collect particulate matter down to 2.5 microns in size at over 99.99% efficiency. (In laboratory testing one OEM’s PTFE filters had 0.00% detectable emissions in the test rig). For this reason, in the applications with very tight emissions requirements, PTFE membrane filters are the standard.

Membrane bags are not recommended for a few limited applications, usually involving oils and hydrocarbons are these can close off the pores of the membrane and cause the filter to plug up.

What Are Filter Bags with PTFE Finish or Treatment?

In this use of PTFE, rather than creating a surface layer or membrane on top of the filter, the filter fabric is coated in a bath or spray of liquid polytetrafluoroethylene (PTFE) resin. This is done to protect the filter. The treatment improves the flex life, heat and chemical resistance and dust release from the fabric. This increases the service life and efficiency of the filters. PTFE finish is commonly used in applications with corrosive chemical compounds, sticky dusts, or high moisture contents.

Would you like to learn how PTFE membrane filter bags can improve your dust collector operation and save you money? Contact us today to find out more!

A Baghouse filter with PTFE (Teflon) Membrane

PTFE membrane or PTFE finish can be applied to a baghouse filter made from any fabric, such as fiberglass, polyester, or aramid (Nomex). It can even be used with pleated filters or cartridges.

Can I Reuse My Baghouse Cages When I Change my Filters?

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Bent and rusted dust collector filter cages

The answer is that it depends…In many applications it is possible to reuse the cages once or possibly more. But in many other applications it may not be wise to reuse the cages.

Damaged Baghouse Cages From Corrosion and Mishandling

The main two types of cage damage we see are physical damage (bending, breaking, warping, etc.) and corrosion. Cages frequently get bent or broken by maintenance personnel over time by handling during changeouts and maintenance work. Other times, cages are damaged when stacked on top of each other for storage.  Additionally, fires or temperature excursions can warp cages.

Rip in a baghouse filter caused by a broken support cageCorrosion can occur from chemical attacks (e.g. acid flashes,) or upset conditions within the system. In some applications, high moisture levels can promote corrosion of the metal cages very quickly. And sometimes cages are stored outside uncovered  and left exposed to the elements.

Reusing Damaged Cages Will Cause Your Baghouse Filters To Fail Sooner!

There are three ways damaged or corroded cages cause filters to fail:

A bent baghouse cage has worn holes in the filter.

Bent or warped cages can eventually wear holes through your bags and cause massive leaks.Using damaged or corroded cages along with new baghouse filters will damage your filters and lead them to fail. This means you will experience leaks or even total filter failure much quicker than normal. Such failures can have high costs in terms of maintenance, possible system shutdowns, lost production as well as the costs of buying replacement filters sooner than normal.

  • When pulsed bent cages can cause the filters to rub against each other or against the sides of the collector causing the bags to tear open
  • Bent, twisted, or broken cages will have many sharp edges that can cut holes in the fabric very quickly when pulsed
  • Rusted or corroded cages will eat through the bag fabric and very quickly cause holes to appear.

In applications with very corrosive environment
(acid/alkali compounds, high humidity, condensation, etc.) such as foundries, dryers, chemical processing, etc., it is essential to avoid reusing cages that are damaged or corroded.

Even in applications with milder environments, the frequent handling of cages during repeated changeouts will eventually result in damage to the cages. Using cages in good condition extends filter life and improves efficiency by preventing:

  • Tearing or cutting filter on sharp edges
  • Damage from abrasion from other bags or unit
  • Corrosion eating through the fabric on the bags (Consider investing in galvanized, epoxy-coated or stainless steel cages to improve resistance to corrosion
  • Ineffective cleaning by ensuring the bags are properly fitted to the cages (Bags need to fit correctly
    across entire cage in order to get the required snap back when pulsed with air…not possible with bent cages

Reusing Damaged Cages Can Cost You Dearly

Bent and rusted dust collector filter cages

Corroded (rusted) cages will lead to early bag failure. Avoid reusing cages if they are damaged or corroded in any way.

Even minor rips, holes, and tears in the bags can lead to exceeding emissions limits. In many plants, such a problem will cause an emissions spike, becoming a “reportable event”, with possible fines and sanctions from regulators as a consequence. Additionally, any baghouse malfunction has the potential for causing plant-wide problems, that may lead to shutdowns, production losses and other costs far in excess of replacing the cages.
You can avoid all these problems and more by not using damaged or corroded cages in your baghouse. Additionally, if you keep spare cages on site at your facility make sure to store them where they are protected from corrosion and avoid stacking to many rows on top of another to prevent bending.

Baghouse Filter Cage Styles

Do you have corroded or bent cages in your baghouse? Let us give you a quote today for new cages along with your next set of replacement filters and receive a discount on both! Contact us now for a free quote.