Fabric filters

Fabric filters

Commonly known as bag houses, fabric collectors use filtration to separate dust particulates from dusty gases. They are one of the most efficient and cost-effective types of dust collectors available, and can achieve a collection efficiency of more than 99% for very fine particulates.

Dust-laden gases enter the bag house and pass through fabric filter bags that act as filters. The filter bags can be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope shape.

Pre-coating

To ensure the filter bags have a long usage life they are commonly coated with a filter enhancer (pre-coat). The use of chemically inert limestone (calcium carbonate) is most common as it maximizes efficiency of dust collection (including fly ash) via formation of what is called a dust cake or coating on the surface of the filter media. This not only traps fine particulates but also provides protection for the filter bag itself from moisture, and oily or sticky particulates which can bind the filter media. Without a pre-coat the filter bag allows fine particulates to bleed through the bag filter system, especially during start-up, as the bag can only do part of the filtration leaving the finer parts to the filter enhancer dust cake.

Parts

Fabric filters generally have the parts of clean plenum, dusty plenum, filter bag, filter bag cage, venturi assembly, tube-plate, RAV/SCREW, compressed air header, blow pipe, housing and hoppe.

Types of bag cleaning

Bag houses are characterized by their cleaning method: shaking, reverse air, pulse jet and sonic bag filters.

Shaking bag filter (bag house)

A rod connecting to the filter bag is powered by a motor. This provides motion to remove caked-on particles. The speed and motion of the shaking depends on the design of the bag and composition of the particulate matter. Generally shaking is horizontal. The top of the filter bag is closed and the bottom is open. When shaken, the dust collected on the inside of the filter bag is freed. During the cleaning process, no dirty gas flows through a bag while the bag is being cleaned. This redirection of air flow illustrates why bag houses must be compartmentalized.

Reverse air bag filter (bag house)

Air flow gives the filter bag structure. Dirty air flows through the bag from the inside, allowing dust to collect on the interior surface. During cleaning, gas flow is restricted from a specific compartment. Without the flowing air, the bags relax. The cylindrical filter bag contains rings that prevent it from completely collapsing under the pressure of the air. A fan blows clean air in the reverse direction. The relaxation and reverse air flow cause the dust cake to crumble and release into the hopper. Upon the completion of the cleaning process, dirty air flow continues and the filter bag regains its shape.

Pulse jet bag filter (bag house)

This type of bag house cleaning (also known as pressure-jet cleaning) is the most common. A high pressure blast of air is used to remove dust from the filter bag. The blast enters the top of the filter bag tube, temporarily ceasing the flow of dirty air. The shock of air causes a wave of expansion to travel down the fabric. The flexing of the filter bag shatters and discharges the dust cake. The air burst is about 0.1 second and it takes about 0.5 seconds for the shock wave to travel down the length of the filter bag. Due to its rapid release, the blast of air does not interfere with contaminated gas flow. Therefore, pulse-jet bag houses can operate continuously and are not usually compartmentalized. The blast of compressed air must be powerful enough to ensure that the shock wave will travel the entire length of the filter bag and fracture the dust cake. The efficiency of the cleaning system allows the unit to have a much higher gas to cloth ratio (or volumetric throughput of gas per unit area of filter) than shaking and reverse air bag filters. This kind of filter thus requires a smaller area to admit the same volume of air.

Sonic bag filter (bag house)

The least common type of cleaning method is sonic. Shaking is achieved by sonic vibration. A sound generator produces a low frequency sound that causes the filter bags to vibrate. Sonic cleaning is commonly combined with another method of cleaning to ensure thorough cleaning.