It’s not just about the bag

July 27, 2016 John B Johnson

A correctly selected air pollution control system will not only control air emissions successfully, but also contribute to the cement plant’s overall efficiency.

Filter bags are a critical component in assuring that the fabric filter will be able to meet environmental regulations and plant process demand. Though filter bags are usually the first place plant personnel look when problems occur, there are other areas to consider. The bag house equipment design, such as how the bags, are cleaned and how the dust is removed from the bags are equally important. The maintenance practices such as filter bag installation, start-up and shutdown procedures are important to attain optimal performance. FLSmidth Airtech fabric filters are designed to support plant personnel who deal with environmental, process and mechanical changes day to day and longer term.

One of the biggest issues that cement plants face with fabric filters is the ability to distribute gas equally to all of the compartments without causing high velocity problems, which can quickly cause filter bags to break. FLSmidth Airtech has engineered the best design to meet these challenges head on by utilising an exclusive design of gas distribution screens and side inlet that reduces velocity and allows dust to pass directly into the hopper. This contributes directly towards longer filter bag life.

The feature of equalized and low velocity gas streams provides extremely important benefits for the cement plant such as cost reductions in the compressed air energy used for filter bag cleaning.

Expert knowledge
To ensure the fabric filter meets operational goals and complies with environmental regulations, it needs the support of associated services, including qualified technical support, filter training, lab testing, spare parts, make and hold agreements, and ongoing customer service. This requires an experienced solution provider who has both operational know-how and technological manufacturing competencies. Filter bag engineers should be able to evaluate all process conditions, including gas temperature, air flow, volume, and more.

Based on more than 8,000 air pollution control installations and 130 years of experience in the cement industry, FLSmidth's AFT® filter bag specialists and engineers have extensive knowledge and field experience. They offer practical application expertise and can help select the best media according to specific requirements, applications and conditions.

FLSmidth Airtech and AFT (Advanced Filtration Technologies) operations provide filter bags to cement operations across the globe.

“As a part of the FLSmidth Group, we work closely with experienced, qualified process specialists and we support plants in meeting ever-stricter emission norms using the optimal filtration technology.” - John B Johnson, Sales Manager, FLSmidth Airtech.

Global manufacturing and support
FLSmidth’s AFT operations span three major regions around the world. From the US headquarters in Evans, Georgia that supports customers throughout North and South America as well as develop and manufacture products. Covering Europe, Middle East and Africa, sales and customer support work in close collaboration with FLSmidth Airtech in Denmark. And, from the base in India where high-quality filter bags are manufactured in addition to providing sales and customer support throughout in India and South East Asia.

Advanced research
FLSmidth Airtech invests heavily in research and development and focus on determining the best available technologies related to fabric filters and filter bags.

The R&D Centre Dania in Denmark is the largest of its kind in the industry. Its objective is to continuously develop the next generation of solutions for cement producers as it relates to process and environmental challenges. The R&D facility contains a 10 meter pilot fabric filter that can simulate cement plant conditions and provide cleaning efficiency and internal bag pressure data for all filter media types. The pilot fabric filter is also used to confirm best available efficiency media and filter bag life. These R&D capabilities puts FLSmidth in the best position to advise the customer about the best filter bag design, the best filter media choice and the best control system for specific plant conditions.

Helping ever-larger cement plants to reduce emissions
FLSmidth AFT works with cement operations to develop a total emissions strategy while improving plant efficiency.

“Today, many plants need to reduce dust emissions while increasing capacity, and for most, this means investing in new, higher performing equipment. We have the competencies, the experience and high quality products available to deliver long-lasting emission compliance". - Jørgen Iversen, General Manager, AFT Europe.

The strategy includes a preventive programme, personnel training, and supply of filter bags and spare parts. It is efforts such as these, involving a complete filter bag refit and optimising filter media, materials and construction that are often required by cement plants as they prepare for new environmental regulations.

Turnkey and replacement applications
FLSmidth AFT manufactures filter bags for new projects and offers turnkey solutions for filter bag replacements. Thousands of filter bags are currently in service at cement plants around the world. Many AFT bags installed in kiln filters from FLSmidth Airtech have exceeded six years of service life due to their industry leading quality and durability.

AFT also manufactures millions of replacement filter bags for all Original Equipment Manufacturer (OEM) filters with pulse-jet, reverse-air and shaker designs.

New filter membrane
AFTm™ membrane is the latest in ePTFE membrane design. It serves as a primary filter surface allowing excellent dust release and particle retention, keeping the filter bags as clean as possible for a longer life. AFTm membrane is available on fiberglass, aramid, P84, acrylic, polypropylene and polyester filter medias.

Together, FLSmidth's AFT operations and FLSmidth Airtech provide the exact process know-how to run efficient APC solutions.

Find out more about how our filter bag specialists can help you run a long-lasting emissions solution>

How to choose the right filter bag for your application

The kiln
Fiberglass is the most common media for filter bags in kiln filters for good reason. Its temperature resistance of up to 260˚C (500˚F) can withstand the hot gases of the kiln, providing flexibility to plant operations as higher temperature ratings allow for improved throughput and production.

However, fiberglass bags are relatively fragile and must be handled very carefully, especially during installation. Consequently, it is recommended that installation of fiberglass bags is left to experienced, qualified personnel.

A potential drawback of fiberglass bags is their relative inability to withstand ‘overpulsing’ or excessive cleaning. Here, efficient and careful cleaning of the bags is an important factor in preventing failure. An excessive amount of cleaning pulses will often lead to premature failure, so it is necessary to find the optimal frequency to ensure the longest possible service. Qualified fabric filter technicians can provide best-practice recommendations.

Clinker cooler
The typical media for clinker cooler filters is aramid. This sets a maximum temperature limit of 204˚C (400˚F) and a constant of 190˚C (375˚F) since above this, aramid bags will fail. Bag failures can lead to unnecessary stoppages of the clinker cooler, because the defective bags need to be located and exchanged. A particularly useful device is a broken bag detector that can alarm the plant operators in the event of a broken bag as well as pinpoint the location. This minimises production loss and provides better conditions for finding and replacing the bags efficiently and safely. In addition, if fabric filter performance is not cost effective, several filter media upgrades are available.

Finish mill
In finish mill filters, polyester is one of the most widely used media because of its high availability and low cost. However, hydrolysis is a common problem for this process. Polyester becomes brittle when exposed to moisture and temperatures around dew point, approximately 100˚C (215˚F). A better option is an acrylic filter bag because it operates well in high moisture applications. Other benefits of acrylic filter media are good resistance to moist mineral and most acids as well as an excellent resistance to organic solvents.

Coal mill
Typical filter media for coal mill dust collectors include polyester, acrylic and aramid. With a stainless steel scrim, these new and improved media replace the traditional, blended fabric with carbon fibres, also known as epitropic filters. The conductive scrims dissipate static consistently throughout the filter bag at a lower cost.

Static electricity in the coal mill filter could ignite coal dust and cause a fire or explosion. Safety measures preventing static electricity discharge therefore need to be in place to reduce the potential for explosions and fires. These measures include using stainless steel or copper grounding wires sewn to the filter bag or semiconductor filter bags.

Secondary dedusting
It is not uncommon to see undersized dust collectors in applications such as silos, pack houses, belt transport and conveying systems. High differential pressure issues in these units, designed for a specific grain loading, are usually caused by system overloading as production expands. The issue can be solved by retrofitting the design, increasing the size of the collector or using a pleated filter bag design instead. This last option can be the most cost effective because the same housing and other components are used.

John B Johnson 

Jørgen Chr. Iversen

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