Bag Filter :
A bag filter or bag house or fabric filter is an air pollution control device that removes particles ( dust ) out of the air or gas released from pharmaceutical and other commercial processes .
Pharm industry involved a broad range of processes with the potential to generate harmful dust .
Example : - drying , blending , screening , tableting , encapsulating , coating , milling , weighing etc .
Principle :
The use of filter bags for dust collection is based on the principle of filtration , which is a reliable , efficient and economic methods to remove particulate matter from gases .
The dust adheres on the surface of the filter bag and the interior of the filter cloth filter and collect the dust contained in the processed gas .
The high efficiency of these collectors is due to the dust cake formed on the surface of the bags .
The fabric primarily provides a surface on which dust particulate collects through the following four mechanism :
- Inertial collection : dust particles strike the fiber surface placed perpendicular to the gas flow directions instead of changing direction with the gas steam .
- Interception : particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size .
- Electrostatic Forces : the presence of an electrostatic charge on the particles and the filter can increase dust capture .
- Brownian movement : submicron particles are diffused , increasing probability of contact between the particles and collecting surface .
Construction:
The inside of the bag house is divided into several compartments . Each Compartment contain several filter bags in which filter cloth is stitched into cylindrical shape is installed .
Filter bags are upto 40 cm in diameter and 2 - 10 m long .
The number of bags can very from 100 to 1000 or more depending upon the size of the bag house .
A hopper is provided in the bottom of the compartment to collect dust removed from the filter bags .
The Filter bags are made from :
- woven fabric : interlaced yarn type .
- felted fabric : randomly oriented fibers
- membranes : Polytetrafluoroethylene ( PTFE ) laminated to a woven fabric or felt
- sintered metal fibers : small metal fibers randomly oriented on a cylinder
- Ceramic cartridges : fabricate on honeycomb or cylinder shape .
It is important to select the filter material of a filter bag to suit the gas temperature , humidity , gas composition , dust characteristics .
Working :
1. the gas entering the inlet pipe strikes a baffle plate which causes larger particles to fall into a hopper due gravity .
2. the carrier gas then flows upward into the tubes and outward through the fabric leaving the particulate matter as a cake on the insides of the bags .
3. filter must be cleaned periodically .
4. The following methods are used for cleaning the filter bag house :
- mechanical shakers .
- reverse air flow.
- pulse jet .
Mechanical Shakers : = cleaning is done by shaking the top horizontal bar from which the bags are suspended , A motor driven shaft can create vibration to shake off the dust cake .
Reverse Air Flow : = bags are cleaned by injecting clean air into the dust collector in a a reverse direction which presurrises the compartment causing the dust to crack and fall into hopper below .
Pulse jet := bags are cleaned by short burst of compressed air injected , causing the bag surface to flex . this flexing lead to break the dust cake .
Advantages :
1. high collection efficiency , especially particle smaller then 10 micrometer .
2. simple construction and operation .
Disadvantages :
1. high maintainence and fabric replacement cost .
2. problem in handling dust which may corrode the cloth .
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