Bag filter
Bag Filter
WHAT IS BAG FILTER?
Recovery of particulate matter from exhaust gas is vital for any industry from two aspects
To avoid pollution problems Recovery of finished product-increasing the plant yield For recovering particulate matter different options available are – Cyclones, Venturi, scrubbers, bag filters, electrostatic precipitators etc. Out of these bag filter becomes the best choice if following are dominant selection criteria’s:
Material is expensive and hence high collection efficiency is desired. Particulate size is very fine. Material recovery in powder form is desirable & not in solution form. A conventional bag filter normally has following problems:
Frequent bag choking - hence high plant shut down time. Higher pressure drops across bag filter – hence higher blower electrical power. Bag choking resulting in reduced airflow rate & therefore reduction in plant capacity. Due to the above the bag filter becomes bottleneck for plant operation.
Realizing above we have designed our bag filter using scientific procedure as elaborated in DESIGN METHODOLOGY. We manufacture bag filter as per "Quality Assurance Plan." Hence consistency in quality is guaranteed.
WORKING PRINCIPLE OF BAG FILTER
Figure 1 depicts the working principle of bag filter.
See Figure No.1
The dust-laden air enters the bag filter bustle. The air is uniformly distributed avoiding channeling. Initially a coat of material forms on the bags. Subsequently, the coat acts as the filtering medium. The dust is accumulated on filter elements while the air passes thorough the filter bags from outside to inside. The accumulated powder is dislodged from the bags by reverse pulse-jet air intermittently. The dislodged powder falls on bottom cone and is discharged through powder discharge valves. The dust free air is sucked by induced draft fan and is exhausted to atmosphere. Knockers are provided on conical portion especially for sticky/hygroscopic materials.
MAIN COMPONENTS OF BAG FILTER
Bag filter housing Filter bags Bag cages Filter bag–Venturi assembly Pulse-jet air cleaning assembly Air bleeding assembly 1. Bag filter housing:
Bigger housing size is required for sufficient filtering area and low CAN velocity. It ensures minimum pressure drop across bag filter. This reduces the blower power consumption. Bustle type construction at air inlet assures uniform air distribution. This avoids channeling of air and hence gives consistent bag filter performance. Bottom cone angle of bag house cone is 45?so that the powder slides freely. Higher area is provided for air passage to keep the CAN VELOCITY low. This avoids powder "re-entrainment" and resultant bag choking. Full SS construction is provided even on exhaust side - longer bag filter life. Bag removal from top - maintenance in dust free atmosphere. Figure 2 explains these points.
See Figure No.2
2. Filter bags:
We careful select filter bag fabric considering temperature, particulate size, air-to- cloth ratio. The Felt media bag minimizes pressure fluctuations giving uniform and high collection efficiency. Specially selected Anti-stick coating on bags minimizes bag choking. 3. Bag cages
Figure 3 shows our cage construction.
See Figure No.3
Cages are made out of larger diameter bars to make the assembly rigid.
The pitch between bars is kept less to adequately support bags by cages. This avoids flex line failure between the vertical wires. Careful selection of fit relationship between cage and bag avoid premature bag failure. If bag is oversized then bags may have internal abrasion along vertical lines causing bag failure. Sharp edges on Pan are avoided as they cut / damage the bag where it comes in contact with the edge of Pan. 4. Filter bag-Venturi assembly
Figure 4 explains our Venturi assembly.
See Figure No.4
Higher thickness Venturi mounting plate – uniform pressure is applied - seals better. Venturi are cast –rigidity in clamping reducing leakages. Special Silicone gasket ensures Leak-proof joint between Venturi and Venturi mounting plate. Clamps are provided to hold the bags on cages and seal for leakages. 5. Reverse PulseJet cleaning system
Figure 5 explains working of reverse jet system.
See Figure No.5
6. Air bleeding arrangement
A temperature monitored automatic " Air bleeding ’’ arrangement avoids exposure of bags to higher temperatures and ensures longer bag life. Figure 6 explains the construction.
See Figure No.6
DESIGN METHODOLOGY
In our every Bag filter design following scientific procedure is adopted.
For calculating filtering area selection of filtering velocity is done from reference tables based on material properties. This filtering velocity needs to be corrected to suit the actual operating conditions. Following four factors are considered: 1. Application Factor: It varies for oily, moist, agglomerating material and depending on continuous/intermittent operation. e. g. 0.9 for heavy dust loads and continuous operation.
2. Temperature Factor: It varies as per gas stream temperature. e. g. 1.0 for temperatures upto 45 deg.
3. Particle size factor: It depends on particle size. e. g. 0.8 for below 3 micron.
4. Dust load factor: It varies as per dust loading per volume of gas stream.
e. g. 1.2 for 10 or less grains/cu.ft.
In addition to the filtration velocity, consideration of "CAN VELOCITY" is critical while sizing bag filter. CAN VELOCITY is the velocity in the flow passages between the filter bags. With high CAN VELOCITY, the dust particles, which have been cleaned off the bags, get RE-ENTRAINED in the gas stream. This re-entrained dust gets RE-DEPOSITED on bags. This results in higher pressure drop and consequential deterioration in bag filter performance. Following fig. Shows this effect. The CAN VELOCITY is selected based on the material, bag length. This decides the bag filter house sizing.
EXCLUSIVE FEATURES OF BAG FILTER
Better performance -
The pressure drop across bag filter is very low (40 - 100 mm WC) even for continuous operation. This results in reduced power consumption of fan. Movement of air from top to bottom aids in removing powder from bags eliminating the possibilities of dust re-entrainment. This results in low-pressure drop and precious energy saving. Anti-stick coat on bags facilitates removal of powder avoiding chocking of filter bags. Automatic air pulse jet system with adjustable sequential timer to clean filter bags on-line. Bottom cone angle of housing is 450 hence powder slides freely down.
Ease in maintenance and long life -
Bag removal from top facilitates maintenance in dust free atmosphere. The bag cage has 16 wire rods for better support to bag that improve bag life. Aluminum di-cast heavy wall thickness machined venturies for distortion free and leak free performance. Heavy wall thickness renders long life.
Safety Instrumentation and Alarm -
Fully automatic temperature controlled Cold Air Bleeding system to limit air inlet temperature to protect the filter bag fabric. Differential pressure switch across bag filter is provided to know if bags are chocked. Vacuum pressure switch is provided to protect the housing and bags from collapsing under vacuum. Compressed air pressure switch is provided on pulse jet cleaning system to ensure sufficient pressure is available for proper cleaning of filter bags online.
The bag filter is compact. This results in low civil and erection cost.
