Bag Filtration

Advanced filtration solutions for a variety of applications.

A baghouse filter or fabric filter is a kind of air pollution control device which is used to collect particulates out of gasses released by commercial process gases. This air-material separator performs well in a relatively small loading of particles. The use of baghouses to track particles came into widespread use in the late 1970s with the invention of high-temperature fabrics which are capable to withstand at high temperatures like 350 °F (177 °C). While basic concepts remain the same, today bag house filers provide most advanced filtration solutions than ever. Generally, the process takes place from inside to the outside with a speed of 100 m3/m2 h and the particles settle in the bag. A bag filter normally has a high-pressure inlet on the top and filtrates exit at the sides as well as from the bottom. Hence bag filters are normally used with a supporting vessel. In situations where no support like a filter vessel is used, the pressure drop across the filter needs to be at a lower level. Various industries including steel mills, power plants, food manufacturers, pharmaceutical industry, chemical production industry utilize industrial-scale baghouses to control the emission of air pollutants.  Among the other industrial-scale dust collection systems such as electrostatic precipitators, Baghouse filters are widely used as they are cost-effective, and show a high collection efficiency, even when the particle size is too small (sub-micron sized particles). Further, these bag filtration systems successfully provide a lot of surface area into a small space.

How do bag filters work?

Baghouses consist of long, cylindrical bags or pleated filters made of woven or felted fabric as a filter medium. A single baghouse installation can comprise of several compartments of baghouses. These bags or pleated filters are mounted inside a casing or sheet metal housing. Usually non-woven and less pleated cartridges are used in industrial applications where the gas that goes through the baghouse is characterized with low dust concentration and gas temperature is 250 °F (121 °C) or less. Fans on the outside of the baghouse blow dust-laden gas from fugitive or process sources through the filters. Filtration occurs from inside to outside or outside to the inside depending on the cleaning method. In either way capturing the suspended particulate matter and solids on the bags takes place while pushing the clean air through the outlet. The filter cloth in the baghouse allows the formation of a particulate matter layer on its surface. It is known as dust cake. The filer cloth provides a surface for dust to get collected and this happens due to the combination of the following four mechanisms.

  • Inertial collection – Instead of changing the direction of dust particles along with the gas stream, they get strike on the fibers placed perpendicular to the gas flow.
  • Interception – Fraction of particles in the gas flow does not cross the fluid streamlines. They come in contact with fibers because of the fiber size.
  • Brownian movement – Submicron size particles are diffused by increasing the probability of contact between the collecting surfaces and the particles.
  • Electrostatic forces – Dust capture also occurs due to electrostatic forces between electrostatically charged particles and the filter.

With the continual operation of the baghouse, the thickness of the dust cake made out of particulate matter also increases. This happens until the thickness of the dust cake reaches to a level where the dust-laden air can no longer pass through it. At this point when a sufficient amount of pressure drop (ΔP) develops the bags must be cleaned. Cleaning can be done while the baghouse is in operation (online) or is offline (in isolation) depending on the type of baghouse. A single baghouse with several compartments allows some compartments to be cleaned while others remain in service. The normal filtration process resumes when the cleaning process in the compartment is completed.

Baghouse of cleaning systems

With time, when the dust is continuously captured and the filter cake on the filter surface gets thickened, these bags should be cleaned in order to perform better filtration. These bags are cleaned by removing the excess dust on the filter surface. Various types of cleaning mechanisms and technologies are available in the industry to perform this cleaning task. However, the method of dust collection and the method of cleaning depends on the type of baghouse. The three most common baghouse cleaning systems are described below.

Pulse-jet baghouse.

A pulse jet baghouse is a self-cleaning dry filtration system. These are the most common type of bag filters used on an industrial scale in order to meet workplace air quality standards given by the Environmental protection Agency (EPA). As the name suggests the pulse jet baghouse filters utilize pulses or short bursts of compressed air to clean the filter. In these filtration systems, the dust gets collected on the external surface of the filter fabric. As the filtration process goes on dust filter cake develops on the external filter surface. When this dust filter cake thickens with time and when it reaches to a certain thickness by disturbing airflow, a controller gets activates based on a defined time or defined pressure drop. The activated controller gives a sequential pulse of compressed air into the interior of the filter bags by knocking the dust filter cake. The knock-like response given by the pulse is strong enough to fall off the dust cake under gravity to the hopper for removal. As this process takes place quickly, the baghouse can be cleaned with a 99.9% efficiency without taking it offline. This system successfully removes dust and performs filter cloth reconditioning completely than other systems.

Pulse-jet filtration systems offer several advantages over other types of dust capturing systems.

  • Ability to clean on-line
  • High collection efficiency
  • Can operate under a range of temperatures, pressures, and various gas stream conditions

Disadvantages

  • Requirement of special fabric types suitable for high temperatures
  • Cannot operate at high moisture levels in influent gases
  • Requirement of dry compressed air.
  • Requires special fabrics for higher temperatures.

Shaker Dust Collectors/ Mechanical shakers

Shaker dust collectors or shake-cleaning baghouses are the best solutions for instances where compressed air is not available for filter cleaning purposes. These bags are connected to the bottom with a cell plate and suspended at the top with a horizontal beam. In shaker dust collectors dirty gas enters the system from the bottom of the baghouse and dust gets collected on the inner surface of the filter bag.

Cleaning a mechanical shaker can be achieved by shaking the topmost horizontal bar where the bags are suspended. A motor-driven cam and shaft produce vibrations or waves in the bags in order to shake and disturb the dust cake. The main advantage over other types is the cleaning process can be done continuously without shutting down the whole system. This can be achieved by sectionalizing the bag house where one compartment can be taken out of service for the cleaning purpose while other compartments are allowing to function by diverting the airflow to other compartments.

Advantages

  • Simplicity in designing and operation.
  • Sectionalizing allows to clean without compromising the whole system.

Disadvantages

  • Requires more energy and time than other cleaning methods.
  • Consumes large space and a large number of bags
  • Cannot operate in high temperatures.

Reverse Air Baghouse

There are two types of reverse air baghouses

Modern type- Round reverse air collector where the dust gets collected o the exterior of the filter bag. By directing reverse air generated by a medium pressure blower or a fan into the bag can remove the dust cake. Reverse air bag house requires a lower cleaning air pressure than compressed air pulses utilized in pulse jet nag house cleaning. This baghouse type is famous in the industry as high volume, low-pressure cleaning systems. Cleaning can be done online. Bag life is high as this cleaning method minimizes the wearing and tearing of bags.

Old type- Contains rectangular housing with a number of compartments. In this type, dust gets collected on the interior of the filter bag. Old type reverse air baghouses need offline cleaning. Hence individual bag houses are divided into compartments as cleaning can be performed section by section. Usually, the whole compartment is pressurized by using a blower or system fan in the reverse direction. Once this is done by using pressure from reverse flow filter bags get slightly collapse. Hence these are sewn connected to rings with high rigidity to prevent collapsing while cleaning. As these baghouses need offline cleaning, this type of reverse baghouse filter needs to be oversized to manage a portion of it to be off-line during cleaning.

Advantages

  • Higher bag life
  • Sectionalizing allows to clean without compromising the whole system.
  • The cleaning action is very gentle, which lengthens bag life.
  • Can operate at high temperatures

Disadvantages

  • Cleaning air must be filtered before use.
  • Requires actions to remove residual dust buildup.
  • More maintenance is required than other types

 Tips to consider when selecting a baghouse

  • Minimum particle size that the baghouse is capable of filtering  
  • the volumetric flow rate or airflow
  • Pressure drop
  • Gas temperature
  • Bag material

This is a very important factor where consideration must be given when designing and selecting a baghouse filter. Fabric filter media must be matched with gas stream (chemically and physically) and system conditions. Following are some factors to be considered when selecting the bag material

  • Moisture levels
  • Particle size
  • cost
  • Air-to-cloth ratio
  • Electrostatic nature of the particles
  • Fabric’s resistance to cleaning energy
  • Acidity or alkalinity
  • Operating conditions such as temperature and pressure drop

What we offer

FLTR provide you the best filtrations solutions according to your requirement. With a full range of technical solutions, we offer you the best baghouse filter housings incorporating your specific needs.

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