3. The Myth
The filter bags filter the
dust
The fact is the cake formed on the filter bag surface
does the filtering job. Permanent cake on the filter
Protects the filter from impingement and erosion
Of the dust laden gases.
4. General principles of dedusting
To prevent equipment from generating the dust and to
achieve trouble Free operation of dedusting plant certain
conditions must be met. Some of these conditions are
described in the following sections
Dedusting plants may be divided into the following
components
A. Equipments to be dedusted
B. Dedusting pipes and valves
C. Filter
D. Fan and motor
E. Insulation
5. General theoretical design guidelines
Filtration velocity
The maximum allowed filtration velocity depends on the flow
resistance of the bag pulse filter cake and the cleaning ability
of the filter bag and of course on the cleaning method.
the pressure difference (∆p) across the filter bags increases:
the higher the filtration velocity
the thicker the filter cake (residual dust layer after cleaning) and
the quicker it builds up (dust load time/cleaning interval)
the higher the Blaine value of the dust (the finer the dust)
the more compressible or sticky the dust (important for AFR usage)
the older the bags (due to penetration of fine dust particles
into the fabric)
false air ingress via the worn out rotary air lock , worn out filter body (
this causes dust entrainment also)
6. Mode of operation
1. Identify the equipment to be dedusted .max.6 dedusting points
2. should be connected to one common filter
3. Calculate the dedusting airflow for each dedusting point and duct
4. Calculate the air flow for the complete dedustiong pipe system
5. Determine the pipe sizes from the air flows
6. Choose the auxiliary equipment- throttle valves,ventilation flaps ,
filter, fan and insulation-,based on the dedusting air volumes
and other data such as Barometric pressure , temperature
and humidity
7. Dust collecting system design guidelines
Amount of dust sources to vent
No more than six (6) to eight (8) dust sources to vent should be
connected to one dust collector.
Venting air volume
The true required venting air volume that the ventilation system has
to handle must be determined first.
Therefore, it has to be determined how much vent air is required
at each dust point. The recommended standard volumetric
requirements for the venting of typical applications in ambient air
are shown in table.
The air flow for the remaining venting pipes before the filter
should be the sum of the presented values as shown in mentioned
appendix.
8. Deviations from the above guide lines may be possible
depending on the practical Experience ,esp depending on the
characteristics of the material like Abrasion, moisture, size and
explosive nature etc;also on the equipment type like open or closed.
At least the following equipments are considered ‘open’
- Apron conveyor or feeder
- bucket conveyor
- crushers
- Weigh feeders
- Belt conveyors
The following equipments are closed
- Drag Chain
- Pneumatic transport system
- silos
- screw conveyor
- air lift
- bucket conveyor
9. Can velocity
The can velocity is the theoretically calculated raw gas velocity between
the filter bags at the bag bottom area. This is valid independently of raw
gas inlet design. The maximum allowed velocity depends on the gas flow
direction between the bags and the terminal velocity of particle
collectives/agglomerates that have to fall into the hopper by gravity.
From experience, we know that the vertical upward vector of the gas
flow velocity should be lower than 1.0 or maximum 1.3 m/s
10. Insulation
In cases where the relative humidity of the dedusting air is so high that
condensation may arise as aresult of heat emitted by the dedusting pipe
and the filter bottomh hopper should in rare situations be equipped with
heating elements
Concentration of dust
The type dust concentration in the dedusting air from the transport
equipment, silos, stores etc is between 20 and 50 g/ cuM of the
dedusting air.This concentration is valid if the equipment and the pipe
Connections are designed in conformity with the design of the supplier.
12. 1.3 m/s
10 to
20
m/s
10 to 20 m/s
10 m/s for non
explosive abrasive
dust Like clinker , slag
and fly ash
20m/s for explosive
dust Like coal. For
lignite bends must
strictly be avoided
Dust concentration
20- 50 g/cum
< 45 microns
2 M
At material discharge
chute, drop height
must be
not more than 2M.
If it is more than
2 M, Baffle plates
are provided
Discharge chute
13.
14. 2 M
Drop height must be
Not more than 2M.
If it is more than 2 M
Baffle plates are
provided
Discharge chute
62. Dedusting of Q20 m3/min from the top plate of the
Screen or Q m3/min through the subsequent equipment.
Q20 = Q m3/min rounded up to the air flow for an air velocity
Of 20 m/s in the dedusting pipe line
Q= necessary dedusting air
flow m3/min =16 m3/min* screen area in m2
79. Selection criteria
The filter medium is the all-important central feature
of any dust collector operating on the filtration principle.
With the correct or incorrect choice of the filter material,
the whole dust collection operation will stand or fall in
actual practice.
Important selection criteria are:
filter type, particularly cleaning principle
moisture level,gas temperature (average and peaks)
composition and chemical properties of the gas
raw gas dust load, particle size particulate abrasiveness
allowed dust load in the clean gas physical and chemical
properties of the dust
80. Air-to-cloth ratio (A/C ratio)
The air-to-cloth ratio (A/C ratio) is simply a mathematical
expression used to measure the amount of filtering cloth
area available to filter a given volume of air at a given
flow rate. There are standard air-to-cloth
ratios based on cleaning mechanism styles, and this
ratio is used to determine the operating limits of the
bag house.
The guide ratios for collectors with pulse-jet cleaning
are as a function of the bulk material to handle for auxiliary
Equipment venting as follows:
1.2 m3/m2 x min for slag, coal and clinker dust
1.5 m3/m2 x min for limestone and cement dust
82. a, b:manual or
mechanical, by
rapping or shaking
c: mechanical, by
vibrating
d:pneumatic, by
reverse air flow
(often combined
with shaking or
vibrating)
e:pneumatic, by
compressed air
(pulse-jet)
Vibrator
(a) (b) (c) (d) (e)
83. Selection criteria
The filter medium is all –important central feature of any dust collector
operating n the filtration principle. With correct or incorrect choice of
the filter material,The whole dust collection operation will stand fall in
actual practice
Important selection criteria are
• Filter type, particular cleaning principle
• Moisture
• Gas temperature ( average and peak)
• Compostion and chemical properties of the gas
• Raw gas dust load, particle size
• Particle abrasiveness
• Allowed dust load in the clean gas, emission level , mg/M3
• Physical and chemical properties of the dust
Further more, the filter medium must satisfy the following conditions
• High air permeability ( low pressure loss)
• Good mechanical strength
• Good thermal stability at operational temperature
• Good dimensional stability at operational temperature
84. Furthermore, the filter medium must satisfy the
following conditions:
high air permeability (low pressure losses)
good mechanical strength
good thermal stability at operational temperature
good dimensional stability at operational temperature
Without membrane
85.
86.
87.
88.
89.
90.
91.
92.
93.
94.
95.
96.
97.
98.
99.
100.
101.
102.
103.
104.
105.
106.
107.
108.
109. Thank you for your kind attention
K.P.PRADEEP KUMAR
Sr G M Operation
Karikkali Works
Chettinad cements LTD
