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Do you need to select a bag filter fabric for the dust collectors in your process? If so, you're probably aware that fabrics for bag filters come in a wide range of fibers, constructions, and treatments and are suited to filtering products with various characteristics. BPS provides information to help you choose a filter fabric that will reliably filter dust in your process while handling your product's characteristics and giving you the most for your filtering dollar. Sections cover filter fibers, fabric construction, fabric treatments, dust collector types, and which filter fabrics are used in various industries.

Reliably processing your product means reliably filtering the process air. This prevents air contamination and loss of valuable product. The best way to ensure reliable filtering is to choose the right filter fabric for your process and product. The following information reviews some filter fabric basics that can help you match a filter fabric to your requirements.

 

 

Baghouse Related Articles

 Cement Americas, May 1, 1999
 

With increased local and global attention being given to the control of air pollution, containment of nuisance and process dust in all industrial applications is becoming increasingly important.

This calls for the proper design, installation, operation, and maintenance of dust collection equipment. Since its inception, the fabric-style dust collector (baghouse) has offered companies the ability to effectively capture airborne particulate from an air stream. Whether toxic or not, containment of particulate is necessary to provide a healthy and clean work environment.

To ensure that bag house equipment functions as designed, an inspection schedule is recommended, as well as the timely repair and replacement of damaged or malfunctioning equipment. A routine inspection and maintenance program will positively impact the equipment's performance and life. The following is an overview of procedures that can be used to tailor such a program.

41. Inspection/Maintenance Program A typical program consists of a schedule for periodic inspections that are performed on a daily, weekly, monthly, semi-annual, and annual basis. When a baghouse is not periodically inspected, the effectiveness of its operation can be adversely affected. Subsequently, the bag house may not meet the outlet emissions specified on an EPA operating permit.

Figure 1 provides a sample inspection log for baghouses. Logs such as this may be used as is or modified to fit a specific installation. In either case, they cover the most important items that should be monitored to maintain an effective and efficient dust collection system. An added benefit is the development of an accurate history of operation, should questionable performance be experienced at a later date.

42. Pressure Drop Pressure drop, or differential pressure, is the amount of static resistance experienced when operating a positive or negative pressure bag house. The pressure drop is typically measured across the filter bags in inches of water column (in. w.c.). Pressure drop is a good indicator of the amount of dust that has collected on the filter media, and, if continually monitored and logged, the condition of the bags themselves.

New filter bags have the lowest pressure drop because of the inherent permeability of the media. As the bags develop a dust cake, some particulate embed themselves into the interstices of the filter media, and the pressure drop will increase accordingly. It is the filtering of the airstream through this accumulated dust cake that provides high efficiency collection of fine particulate. In fact, the highest efficiency a dust collector can offer is just before the cleaning mechanism is initiated. However, high differential pressures can cause bleed thru or blinding of the filter media. Therefore, it is suggested not to exceed the manufacturer's recommended operating pressure drop.

Keeping a daily log of a baghouse's differential pressure from the time the filter media is new will provide the opportunity to diagnose problems that may occur (i.e. an increase in dust emissions, reduced ventilation air at the dust source, shortened bag life, etc.). Following an initial seasoning or conditioning period of the filter bags, the pressure drop should stabilize into a consistent operating range relative to the cleaning cycle, application, and style of equipment. Therefore, at subsequent bag changes, this operating range can be predicted. Deviation from this historical level will alert an operator to investigate the cause of such an occurrence.

43. Cleaning System Any method used by the equipment to dislodge accumulated dust cake from the filter media is its cleaning system. These systems may be reverse air, shaker, or pulse clean. Regardless of the style of cleaning, it is imperative that this system function properly at all times. Without an effective cleaning system, dust will continue to build on the bags. The result will be an increased pressure drop and reduced volume of ventilation air at the pick-up points. Further, airstream velocities within the ductwork will decrease and cause drop-out of dust in the ducts. This may choke the entire system and render it ineffective.

As indicated on the sample inspection logs, cleaning systems require more than just periodic monitoring. All components of the system should be regularly inspected and corrections be made in a timely manner. Besides the items noted on the logs, refer to the original equipment manufacturer's installation & maintenance manual to include other items specific to their equipment.

44. Hopper Discharge The hopper on a baghouse is not to be used for storage of the collected product, unless originally designed to do so. Storing material in a hopper can lead to bridging of the dust, or it may set up as a solid mass, thus requiring considerable labor and down time to correct the problem. Material buildup, if not discovered in time, can fill a hopper to its inlet and plug the unit. The hopper should be emptied on a routine basis. Further, with low-density materials, the airstream may sweep the dust into the bag section and ruin filter bags and clog the dust collector Bag-house.

It is strongly recommended that, whatever method is used for material discharge (rotary valve, screw conveyor/pneumatic conveyor, etc.), it should be inspected frequently. This inspection also should occur at shutdown and bag changes.

45. Visible Emissions Any particulate that can be seen discharging from the exhaust stack is considered visible emissions. These emissions are an indication that there is a breach in a seal or a broken (torn) filter bag. In either case, the leak must be found and corrected immediately. Not only will the emission cause a health concern and damage the property outside the plant, monetary fines imposed by the local, state, and federal environmental agencies also may result. In addition, a fan located downstream of the collector can be damaged from abrasion or become imbalanced if this condition is not corrected quickly.

The exhaust from the dust collector should be continually monitored and checked off in the inspection log. Besides visual inspections, one may consider incorporating a broken bag detector into the clean air ductwork. Should a bag begin to fail, or there be a leak in the bag seal, the particles that bypass the media will be detected. Typically, these detectors use triboelectric or scattered light technologies. These devices can be wired to an alarm horn, siren, or flashing light bag-house.

46. Exhaust Fan In a dust collection system, an exhaust fan is needed to accelerate ventilation air from the point of pick-up, through the ductwork and baghouse filter media, and out the exhaust stack. A fan is selected to accommodate each application with respect to volume (ACFM, or actual cu ft of gas per minute) and pressure drop throughout the system. This pressure drop is calculated by evaluating the static resistance of the bag house, all ductwork, and pick-up points/hoods.

Should an exhaust fan experience loose or worn belts or an imbalanced impeller, it will not exhaust the volume of air it was originally designed to handle. Without adequate ventilation air, a dust collection system will not operate effectively. Therefore, thorough fan inspections are to be performed on a semi-annual basis. However, any time unusual vibration, squealing, or other obvious variances from standard operation is observed, the original manufacturer should be contacted for evaluation and comment.

47. Filter Media The most important item in a baghouse is the filter media, because it allows for the accumulation and support of a dust cake. This dust cake is what provides high filtering efficiencies during operation. Periodic inspections of the filter bags is mandatory. Inspect the clean air side of the bag houses for leaks and the bags for tears. Should pressure drop within a dust collector become extremely high, relative to historical data, the cause may be excessive dust cake or blinding of the filter bags.

Excessive dust cake is evident when visually inspecting the filter bags (when the dust collector is presumed to be clean) and finding them covered with a layer of the collected dust. Should this occur, one could suspect that the cleaning system is not functioning properly. However, if the dust cake has hardened to the bags and will not dislodge easily, the most probable cause is moisture in the baghouse. Moisture in a dust collector may have resulted from dew point excursions, high moisture content in the process gas or in the compressed air supply, or a leak in the collector or ductwork that allowed water to enter the dust collector.

The other obvious cause of high differential pressure may have been caused by the blinding of the filter bags. Blinding can occur from improper start-up conditioning of the filter bags following the previous bag change.

48. Structural Integrity The structural integrity of equipment can not only affect its performance, but also cause health and housekeeping concerns, as well as reduce equipment life. An overall inspection should be done annually. Inspect all welds, joints, and flange seals, as well. Any leaks in the collector must be sealed either mechanically or by using silicone caulking. In a negative pressure system, a breach in a seal or weld will introduce ambient air into the collector. With this air, moisture and contaminants can find their way into the collector. In a positive pressure system, dust will blow out of the collector causing housekeeping problems and a potential health hazard to employees exposed to the dust.

Look for the obvious. Check the structural support members for signs of fatigue and excessive corrosion. Be certain that all fasteners are in place and tightly secured, especially on the ladder and access platform. Replace any missing bolts, clean and reweld any cross bracing or gussets that may have cracked welds. Look closely at the filter's external walls for corrosion or signs of bowing. Clean and repaint where necessary. Repair any holes that may have developed in the dust collector walls or hoppers.

49. Auxiliary Equipment Aside from the baghouse itself, a thorough inspection of any system will include a check of all miscellaneous complimentary equipment. Some of these items may include the exhaust fan, rotary airlock valve, screw conveyor, inlet and/or outlet dampers, etc. It is very important that any ancillary equipment be added to the inspection log.

410. Ductwork Another important component in a ventilation system is the ductwork. If the particulate does not have an opportunity to reach the bag house, the dust collector will not be able to perform its function. Standard practices suggest a minimum airstream velocity within any duct of 3,500 fpm, and between 4,000 and 4,500 fpm for heavier dusts, such as sand. Should the dust travel at lower than adequate velocities, it will tend to settle and accumulate in the ducts and choke the system. This restriction of flow will increase the pressure drop in the system as well as the energy required to induce the air to move. The result will be reduced ventilation air at the pick-up points. Periodically inspect the entire length of the duct work for dust accumulation.

Start-up procedures Proper start-up procedures will help extend the life of new filter media in a dust collector. What is generally accepted as "start-up" procedures is the process designed to intentionally develop a dust cake on the bags. This is referred to as seasoning, or conditioning, the filter media.

Seasoning of a collector's filter bags is one of the most important procedures that a company can perform. In a fabric filter dust collector, the filter media is used to support a dust cake. A dust cake is the porous layer of collected particulate that develops during the conditioning period of new collector bags and following each cleaning cycle. The process can be accelerated in many installations by introducing a precoat material, such as agricultural lime, into the system. Commercial precoats also are available.

Following installation of the filter bags and inspection of the related auxiliary equipment, the exhaust fan can be started. However, it is extremely important that the new filter bags are not exposed to the full volume (ACFM) of the fan.

First, close the fan damper (or inlet dampers) to one-half open until the monitoring gauge reads about 50% to 65% of the manufacturer's recommended maximum flange-to-flange differential drop. At roughly 75% of the manufacturer's recommended differential pressure, the cleaning system can be initiated. Normal operation and periodic cleaning will bring the pressure drop to a calculable and historically stable level.

Depending on the application, development of this differential pressure may take a number of hours or even days. This is necessary to ensure that the new filter media is exposed to low filtering velocities of dust-laden air. Reducing the volume decreases the airstream's velocity (air-to-cloth ratio), thus protecting the virgin bags from a high velocity impingement of dust. Should the bags be exposed to the fan's full volume, fine particles may embed themselves into the inner fibers of the bags and begin blinding condition. This also can damage the fibers of the media, reducing the life of the bags.

With this information as a guide, a maintenance program can be developed for any dust collection system. However, this is not offered as an all-inclusive list. Each piece of equipment and application is different, and each has its own unique components and features. Those unique characteristics should be accounted for in the maintenance program.

 

 

 

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