BAG FILTER OR BAG HOUSE
1.0 GENERAL INFORMATION
A) INTRODUCTION
i) Bag Filters are designed for filtering contaminated air (air + solids) with fully automatic on-line cleaning system by means of compressed air impulses.
ii) Bag Filters come in various models viz. square, cylindrical,
modular type with side access or top removal arrangement.
iii) Bag Filters can be used as,
1) Dust collector for Dust Extraction Systems.
2) Product collector for Grinding Mills and Pneumatic Conveying Systems.
3) Vent filter for silo venting where Bag Filter is directly mounted on silo top without its hopper.
B) DESCRIPTION
A bag filter is divided in three portions i.e. top portion called as Plenum (clean air chamber), middle portion called as Housing (dirty air chamber) and bottom portion called as Hopper (Refer fig.1.1). The plenum is separated from the housing by the bag plate where the bags i.e. the filtering elements are attached. The bag filter unit is a continuous self cleaning dust filter capable of removing dust particles as small as sub micron size from gaseous streams. High dust collection efficiency is maintained during continuous operating conditions without use of internal moving parts or rapping mechanisms. Versatile application and
simplicity of operation are inherent design features of bag filter.
2.0 - WORKING PRINCIPLE
Dust laden air under suction or pressure enters the lower section of the Bag Filter (BF). (Refer fig.1.1) The air travels through the filter bags (5), which retains the dust particles on the surface of the bag and the clean air passes through venturies (6) and Plenum (clean chamber) (7) to the outlet of BF.
Dust collected on the outside of the filter bags causes an increase in the pressure differential between the dirty and clean air sides of the BF. To control the pressure differential across the BF, a sequential timer (1) actuates a series of normally closed pulse valves (2) at preset intervals causing them to open. A momentary rush of high-pressure air (6-7 bar g) flows from the compressed air header (3) to the blow tube (4) and is expelled from the blow tube through venturies at a high velocity (primary air flow). Air from each ventury induces a secondary airflow. The combined effect of the primary and induced secondary air causes an instantaneous pressure rise on the clean side of the filter bags, causing a reverse flow of air through the filter bags, thus dislodging the dust particles held on the outer surface of the bags sufficient for cleaning. Through this mechanism, the collected dust is released from the bags and falls into the hopper. Dust collected in the hopper gets discharged through suitable discharge device (8) i.e. Rotary Valve, Screw Conveyor etc. Since only a fraction of the total filter area of the BF is cleaned at any given time, continuous flow through the BF at rated capacities is assured.
3.0 - INSTRUMENTATION
A) SEQUENTIAL TIMER
An electronic sequential timer is provided for controlling the bag cleaning mechanism in conjunction with the pulse valves. The timer controls both, the ‘ON' time and the ‘OFF' time i.e. the duration for which the pulse valve is opened or pulse valve is closed. The ‘ON' time or ‘OFF' time are adjustable within a fixed range.
The recommended ‘ON' time is 50 msec and ‘OFF' time is 10 sec.,
however actual setting maybe varied depending on actual dust load,
pressure drop, nature of dust etc. Various combinations of input supply voltage and output voltage to pulse valves are available. Please refer the instrument / wiring diagram of the unit supplied with the bag filter prior to taking up any installation / electrical wiring activity. Wrong electrical connection or reversal of polarity could result in damage to the electronic circuitry.
B) PULSE VALVE
The valves are 2 way normally closed diaphragm valves operated by a pilot Solenoid Valve. These valves are designed for extremely fast opening / closing and high flow rates.
OPERATION:
The diaphragm divides the valve into an upper chamber and lower chamber. The air inlet leads into the lower chamber from where it passes into the outlet when the diaphragm lifts.
The diaphragm has a bleed hole through which the high-pressure inlet air bleeds into and fills up the upper chamber, creating the same pressure on both sides of the diaphragm. The pressure in the upper chamber acts on the full surface of the diaphragm whereas the pressure in the lower chamber acts on a lesser area of the diaphragm. This keeps the diaphragm pressed down on the outlet port, closing it. The upper chamber is connected to the atmosphere through air passage in the pilot valve base. The plunger in "normal" condition keeps the air passage closed, maintains the pressure in the upper chamber and hence the diaphragm valve remains closed. When the electric supply is given to the solenoid coil, the plunger lifts within the core tube and the compressed air in the upper chamber exhausts quickly to the atmosphere. Because of the exhaust air passage has a higher flow rate than the bleed hole on diaphragm, the pressure in the upper chamber falls suddenly. The pressure in the lower chamber then lifts the diaphragm, causing the compressed air to flow from inlet through outlet When the solenoid is de-energized, the plunger closes the air passage resulting in the diaphragm closing the valve outlet. Please ensure the power supply to the pulse valve is compatible with the coil rating before wiring up. Internal construction / components may vary from manufacturer to manufacturers.
4.0 TROUBLE SHOOTING
| SN | TROUBLE | POSSIBLE CAUSES | SOLUTION |
| a | Excessive Emission from filter outlet |
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| b | Bag filtering action rapidly impaired resulting in reduced air/gas flow through unit |
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| c | High pressure drop (above 150 mmWG) |
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| d | Presure of cleaning air keep reducing |
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Note:
1. It is recommended to use oil and moisture free air for the bag cleaning station.
2. avoid operating the unit at or close to dew point of the gas stream handled. special precaution needs to be taken in such cases.
3. For high temperature application take necessary care when starting-up system from cold, to avoid condensation.
5.0 MAINTENANCE
A. GENERAL
A sound preventive maintenance program will eliminate most breakdown situations and ensure long and trouble free operation of the filter unit.
B. PERIODIC MAINTENANCE PROCEDURE
| PERIOD | COMPONENT | PROCEDURE |
| Daily |
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| Weekly | Hopper | Check for material hanf-up on walls. clean the hopper wall from inside. |
| Monthly | Filter Bags | Check the bags for tears, holes, proper fastening. Repair or replace as necessary |
C. MAINTENANCE PROCEDURE
1. Bag Replacement
In the event of high emission from the filter unit due to damaged or torn bags. it will be necessary to remove all bags from the unit for inspection. The following procedure should be followed after isolating the unit.
A. For Side Access Units
a. Open the access door by standing on external platform.
b. Loose the bag clamps of the first nearest row.
c. Pull down the bag and retainer assembly with bag clamps.
d. Similarly remove bag and retainer assemblies accessible from the access door.
e. Place the portable internal platform on the two support provided in the bag filter housing.
f. Bet into the bag filter's housing with the help of safety cables.
g. Remove the remaining bag and retainer assemblies by standing on the internal platform as described in (b) and (c) above.
h. Inspect all bags minutely for pinhole, tears, punctures and excessive wear and tear especially at bottom of bags.
i. Remove damaged filter bags from retainer.
j. Refit all bags retainer assemblies as per instructions.
B. For Top Removal LTR/HTR Units
a. Open the access door on roof of bag filter and enter u=inot plenum. DO not stand / Step on the blow tubes. Remove the blow tubes by loosing coupling.
b. Lift the bag retainer and ventury assembly by the handle provided on the top Collar.
c. Remove all bag cage and ventury assemblies.
d. Remove filter bags by folding the top snap ring of the bag.
e. inspect all bags minutely for pinholes, tears, punctures and excessive wear and tear especially at bottom end of bags.
f. Replace the damaged bag
g. Carry out assembly of bags
h. Carry out installation of all bag retainer, clamp and ventury.
2. BAG WASHING
Inspect bags for hard caking/scaling and correct process parameters/operating parctice. Bags with hard scales can not be cleaned using the normal cleaning mechanism. Such bags need to be removed from the unit and be washed thoroughly as mentioned below:
Procedure for bag washing
a. Vacuum clean each bag prior to washing
b. Soak in cool or warm water at 60 Deg. C maximum temperature, using mild soap or detergent.
c. Rinse thoroughly to ensure all sealing have been removed.
d. Hand the washed bag and allow to dry completely before re-fitting.
Notes:
1. The duration of soaking will depend on the nature of seals and needs to be established by the user.
2. Do not use any sharp instruments/knives to scrape off the scaling on the bag.
