Bagfilters – PJBF, RABH

ONLINE PULSE JET BAG FILTERS

In an online bag filter, bags are cleaned row by row, even when the dust laden gas is filtered. The sequence of cleaning is controlled automatically by a sequence controller. This operates the assembly of solenoid and pulse valves which direct the air flow into the manifolds. The holes are jig drilled for perfect alignment with the venturi centre for achieving maximum cleaning efficiency.

In an online bag filter, bags are cleaned row by row, even when the dust laden gas is filtered. The sequence of cleaning is controlled automatically by a sequence controller. This operates the assembly of solenoid and pulse valves which direct the air flow into the manifolds. The holes are jig drilled for perfect alignment with the venturi centre for achieving maximum cleaning efficiency.

REGULAR HOPPER ENTRY

The pulse jet bag filter in its most rudimentary form, consists of filtration elements encased in housing. Below this casing is a hopper with a discharge valve, to remove the dust that is collected on the bags. The entire unit is supported from the ground on structural legs. A caged ladder provides access to the top of the unit for maintenance.

Dust laden air enters through the hopper by suction (normally on positive pressure). The heavier dust particles fall off at the entry itself, while the lighter dust gets carried upward to the bags. The dust gets deposited on the outer surface of the bags and clean air moves out from the center of the bags and escapes out from top air outlet. This is known as filtration. 

The dust collected on the outer surface of bag is removed in a pre-determined cycle by a momentary pulse of high-pressure compressed air. The compressed air moves from an air reservoir or compressed air header, via the particular pulse valve into the compartment manifold and thereon into the bags, in the row beneath it. 

Due to the pressured entry of compressed air into the bags, the bags get inflated. Thus the dust is collected on the bag as flakes falls down. Soon the bag comes back to its original position and the cleaning process is completed.

Dust flakes fall from the sides of the hopper and slide into the rotary air lock valve. As the valve travels clockwise, the dust trapped in between the vanes gets discharged from the opening below.

TWIN TRAIN – EXTENDED HOPPER

In the on-line Twin Train type of unit, the dirty gas enters the common inlet plenum, formed as an aisle between the two filter modules. From there it flows to both the modules; and undergoes filtration as in the regular unit. 

The cleaned gases travel back to the common outlet plenum from the individual clean air plenums, and then flow out through the outlet duct to the ID fan.

CASING ENTRY

Here the dirty gas enters the casing through the inlet-cum-bypass damper, if applicable, into a central chamber formed by perforated sheets. The gas flow towards the bags is partially upwards from below the sheets, and partially through the perforations, ensuring the separation of heavier dust and better gas distribution respectively. 

The lighter dust gets carried upward to the bags. This gets deposited on the outer surface of the bags and is removed by a pre-determined cycle by a momentary pulse of high pressure compressed air as in a typical hopper entry type of bag filter.

PRE-SEPARATOR CASING WITH BAFFLE – TKB SERIES

As the cement industry has started installing bigger size cement plants, conventional bag filters have been found to be inadequate to handle large gas flows in excess of 6,00,000 m3/hr, due to severe limitations like high gas flow per bag, high velocity of gases through bag filter, higher pressure drop across filters, high space requirement due to bigger size filters and high compressed air consumption.

To overcome these limitations, Thermax has designed a new series of long bag type bag filters – TKB series. Some of the unique features incorporated include:

Low pressure air cleaning system

The TKB Pulse Jet Bag Filter (PJBF) design incorporates a specialized low pressure cleaning system that offers following distinct advantages over conventional bag filters, with high pressure cleaning system. 

These include:

• Lower pressure drop across the unit
• Lower consumption of cleaning compressed air
• Longer filter bag life
• Lower noise level during operation

The low pressure cleaning system uses a special immersion type header mounted pulse valve, as shown in picture 1 below. 

This system is designed to achieve:

• Highly efficient & quick pulse of cleaning air
• Lower pressure drop of cleaning air due to elimination of bend in pulsing system
• Effective Flushing of dust

This cleaning system is so effective that it achieves 25% more cleaning pressure on the bag. The impulse is built in half the time it takes the conventional system.

• Effectively flushes the bag & reduces re-deposition of fine dust
• Can clean bags up to 10 meters length
• Cleans the internal of the filter media to have lower longer term pressure drops
• The filter media is not flexed as extensively as in the conventional cleaning, thus increasing the media life

Unique Gas Flow Path

The TKB series filters are specially designed for high inlet dust loads. The gases enter the filter casing entry plenum behind a baffle & are directed upwards by this specially designed baffle. The 900 turn that the gases take on entry, helps in separating a lot of dust at entry. This reduces the dust load on bags. 

The unique gas flow path makes the gases enter the filter with an upward bias on the velocity. This upward bias reduces the impact of the dust particles on the bag surface, enhancing the life of the bags.

Wider Pitch of Bags

Pitch i.e. centre to centre distance between the bags plays a very important role in the performance of the bag filter. In the TKB series filters that are designed for handling very high gas flows & very high dust loads, we adopt a higher pitch, which in turn increases space between bags by as much as 36%. This additional clear space allows the high amount of dust load to fall by reducing gas velocity between bags. This also reduces the sliding friction at the bags, enhancing the life of the bags.

Specially Reinforced Cages

Conventional cages can not be used in long bag filters as they tend to bend due to the excessive length. We use higher thickness wires for cages when the height of bag or cage is more than 6 m or over. This higher thickness ensures straightness of the cage which is very important to maintain the necessary space between the filter bags when fitted on the tube-sheet of the bag filter.

All the above features that have been incorporated in our TKB PJBF, make our designs unique, easy to operate & maintain. These features make our TKB PJBF the most popular in Indian Cement industry.

CIRCULAR TANGENTIAL ENTRY

Here the gas enters into the bag filter tangentially. This gives the dust laden gas a circular motion, which helps in removing the heavy & coarser particles that are present in the gas stream. These particles are collected directly into the hopper. 

Due to this process, only very fine particles move above and get collected on the bag surface. Thus the total dust load on bags is reduced, which helps in maintaining lower pressure drop across bags, lesser frequency of bag pulsing. This ultimately increases bag life. The gas entry is at casing bottom which also ensures separation of coarser particles from fine, by gravitational force. Due to their circular construction these bag filters can withstand more +/- pressure as compared to normal rectangular bag filters. 

The circular construction of bag filter also ensures they are leak proof. The possibility of dust accumulation inside casing (on edges etc.) is eliminated. Due to the arrangement of bag as shown in the images each and every bag is exposed to the dust laden gas. Due to the swiveling arrangement shown, the top cover is pushed on one side during bag removal and hence there is room for lifting the top cover. Due to the special top cover & outlet plenum construction, the entry of outside air is avoided.

FLUSH MOUNTED – CIRCULAR

Similar to the insert-able bag filter, the casing of the bag filter is circular instead of rectangular. This is normally used where the design pressures are either very high or it is a vacuum.

OFFLINE PULSE JET BAG FILTERS

This mode incorporates the advantages of pulse jet bag filters, as well as that of a reverse air bag house. Each compartment is similar to an online pulse jet bag filter. The cleaning process consists of sequentially isolating each compartment and cleaning it with compressed air. All compartments are then cleaned automatically. Offline cleaning is suitable for light and fine dust, and for larger volumes where the occurrence of fluidisation is a distinct possibility.

The offline bag filter contains 4 to 6 bag filter compartments. Each compartment is similar to online bag filter. Gas flow through a compartment is stopped temporarily when the pulse cleaning of the bags is to be done. The dust dislodged from bags settles quickly in the hoppers because of absence of gas movement in the bag filter compartment. During this time the remaining compartments keep filtering the gases as usual. Each compartment is cleaned one after another in same manner.

The starting and stopping of gas flow through individual compartments is achieved through a compact pneumatic damper system. Generally the offline bag filter is employed for larger gas volumes.