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| FAQ |
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| Q1) What
is the significance of air-to-cloth ratio(a/c ratio) or
filtration velocity in baghouse selection? |
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| This is
the the most important parameter in selection of baghouse
. Air-to -cloth ratio determine the sizing of the baghouse
and functioning of the bafhouse and also its pre and post
connected performance depends on this parameter so also
the life of fabric and other components. |
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| Q2) How
the air-to cloth is selected? |
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| Though
hands on experience is essential to select this parameter,following
parameters plays important role in determining the air-to
cloth ratio: |
| a)
Dust load or quantity of dust present in the air: Higher
the dust load leaser is the air to
cloth ratio. |
| b)
Temperature of the air: Higher the temp,more difficult
to separate the dust from air. |
c)
Particle size of dust to be separated:The more
the fineness of dust,the lesser the air to
cloth ratio,though for good filtation is necessary to
have wide spectrum of particle size
distribution for the dust. |
| d)
Application: There are three types of application where
bagfilters are used- |
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a) Nuisance Venting :
Where fugitive dust emmiting from any source is captured. |
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b) Product Collection : Various
processes when carried out emits product from process
viz.hammermills, dryers, cementmills, etc, where dust
emmited is a product to be used. |
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c) Product collection
from dryers/ combustion processes: These processes contain
either water vapour/combustion products or gases /or bye
products apart from dust and air. |
| Normally product
collection baghouses require indepth analysis of process
and gas composition to arrive at air to cloth ratio, and
practical experience plays a very important role in determining
the air-cloth ratio. |
| There are more
than 5000 types of organic and inorganic solid products
which can be filtered. Understanding product behaviour
and selcting an optimum air-to-cloth ratio for a given
application is the most important decision for baghouse
selection.As the norms of emission levels are getting
stringent and as losses occurring during the various processes
are coming under more scrutiny , the proper baghouse selection
is gaining more importance. |
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| e) Product
characteristics/ properties :The charactestics of the
product to filtered/separated from air plays an important
role in selecting the air -to-cloth ratio.The solid show
different chacteristics during the various processes they
pass thru and predicting their behaviour is really a matter
of experience and judgement.The various properties which
should be critically evaluated are bulk density ,particle
density,flow characteristics,tendancy to absorb moisture,hardness
of the product, abrasive index,corrosive properties,coating
properties, static charge developing tendancy, explosion
characteristics, etc. |
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| Q3) What are the various types
of bagfilter designs available? |
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| Broadly categorizing,
there are four types of baghouses : |
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| a) Shaker
type baghouse: It is the oldest design available
for solid filtration.The filter bags are shaken manually
or by a motor thru eccentric cam,to dislodge the dust
from the filter bags.Very rarely used option in view of
its bigger size compared to other designs,high cost of
maintainance and batch operation. |
However ,some
dust like radioactive dust,and explosive dust where product
contact with can cause explosions requires such shaker
type bagfilder design.
For exremely light dust loading and small amount airflows
like vaccum cleaners .portable dust collectors,the shaker
bagfilter becomes an economical operation and thus is
widely used. |
| In this type
of bagfiler,the filtered dust gets deposited on the inside
surface of filter bag.The filter cloth used should be
preferably nonwoven type. |
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| b)
Pulse Jet type baghouse:
The most commonly used design covering major application
areas.The advances in non-woven type filter media has
made this type of baghouse replace shaker type. |
| A metered burst
of high pressure compressed air is pulsed in the filter
bags for a predermined short period controlled by remote
timer,making bag shake and expand ,helping the dust fall
down in the hopper.This type has gained popularity since
it can accept higher a/c ratios because of advancements
in woven filter bags hence small size ,the operation is
continuous. |
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c)
Reverse air baghouse: Here the
filtered air coming out is again purged back in the filterbag
with the help of blowers instead of compressor.Some dusts
like carbon black,dust emitting from furnaces are extremely
fine, in submicron range .When high pressure compressed
is purged during the cleaning cycle,filter bag expands
and such fine dust penetrates the filtering layer of cloth
and blinds the scrim of fabric causing blinding phenomenon
leading to high pressure drops and shut down of baghouse.
Thus in reverse air baghouse,high volume of air at low
pressure is injected to avoid fine dust penetrating filter
bags. |
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d)
Off-line cleaning baghouse: Here the baghouse is
normally split in no.of compartment i.e.4,6 or 8,out of
which one compartment is always under cleaning cycle and
others are under filtration cycle.The air flow to the
compatment under cleaning cycle is stopped,when cleaning
is taking place.The chances of fine dust penetrating the
filterbag reduces greatly,insuch design.
This type of design is most commonly used in electric
arc furnace exhaust ventilation . |
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| Q4) What are the
other important factors which should be looked into for
baghouse performance? |
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| Apart from air-to
-cloth ratio,following important factors must be considered |
| a)
Type of inlet: Depending on the dust load and dust
characteristics,the correct inlet must be chosen. A correctly
chosen inlet with appropriate inlet velocity reduces dust
load on the filterbags,by separating part of the in the
hopper itself. |
There are various
types of inlet options available,as follows
i) Baffle type inlet ii) Tangential inlet iii) Top entry
inlet iv) Extended hopper inlet v)housing inlet vi)Combination
inlet etc. |
| b)
Hopper Valley Angle: Important factor ,since the
separated dust must flow down and discharged out of the
baghouse.A correctly chosen hopper with proper valley
angle ensures smooth discharge. |
| c)
Filterbags: Another important area for good baghouse
performance.The quality of filter bags should be chosen
based on the dust load,fineness of product,temp.of air,type
of heat,moisture content ,type of application, desired
outlet emission. |
| d)
Properly sized discharge valve: The collected dust
dislodged from filerbags must be removed quickly,since
accumulation of dust can cause carry-over problems.The
density of such airborne dust changes,thus it is essential
to provide bigger size airlock.If dust removal is manual,then
the container must be emptied at regular intervals. |
| f)
Compressed air quality/quantity: This is the most
neglected area in baghouse ,though not properly taken
care,can cause severe problems.The compressed air must
be free from moisture,and oil. The compressed air pipelines
must be adequately sized. |
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| Q5) What is importance
of cake built-up during baghouse operation?Why it is essential
to maintain a particular pressure drop in the baghouse? |
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| This is the most
important aspect to be looked into while operating any
baghouse,yet it is the most neglected area.For any baghouse
to operate properly,it is essential that initial layer
of dust must be allowed to deposit on the nonwoven filter
bags.This dust layer performs two important functions
: |
| a)
It acts as a filter media and enhances the filtration, |
| b)
It protects the filter fabric from dust coming in the
baghouse. |
| Because of such dust
cake build-up,the pressure drop increase across the
clean and dirty chamber of baghouse.The cleaning mechanism
should be adjusted in such a manner that,a constant
pressure drop range must be maintained,across the baghouse.
The sign of constant pressure drop in the baghouse is
an indication of good filtration. |
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| Top |
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| Q6) Is it advisable
to install cyclone as pre-collector prior to baghouse
to reduce load? |
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No,unless
and until the load on baghouse is excessive. For good
filtration it is essential that dust contain wide specrum
of partical size range.If a cyclone is added as a precollector,it
separated the coarse particles from the gas/air stream,leaving
bafhouse to filter fine particles.The filtration performance
deteriotes if only a specific sized particles are filtered.While
designing such baghouses, where there is precollector,special
care should be taken while chosing a/c ratio and quality
of filter bags.However,if the dust load is too high,it
is recommended to add a properly sized pre-collector,
prior to baghouse. |
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| Q7) How the choice
of filter bag should be done? |
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| Though
every filterbag manufacturer has their propriatory specs
and treatments to every filter fabric,following guidelines
will help to choose a correct type of fabric for any application. |
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| GENERAL
GUIDELINES FOR FILTER SELECTION |
| SR NO |
|
Polyproplene |
Polyester |
Acraylic |
Glass |
Names |
RYTON |
TEFLON |
| 1. |
Maximum
Operating Temperature |
85
- 900C |
140
- 150 0C |
120
- 1300 C |
250
-260 0C |
180
-2000 C |
190
0C |
2600 C |
| 2. |
Humid Heat
Moisture |
Excellent |
Poor |
Excellent |
Excellent |
Good |
Good |
Excellent |
| 3. |
Resistance
to alkalinity |
Excellent |
Fair |
Fair |
Fair |
Excellent |
Excellent |
Excellent |
| 4. |
Resistance
to acidic condition |
Excellent |
Fair |
Good |
Poor |
Fair |
Excellent
|
Excellent |
| 5. |
Abrasive
Resistance Excellent |
Excellent |
Excellent |
Good |
Fair |
Excellent |
Good |
Good |
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| GENERAL
GUIDELINES FOR SURFACE TREATMENT |
| SR NO |
Treatment / Finish |
Purpose |
Can be applied
to |
| 1 |
Singed
Finish |
For
improved cake release |
Major type of fabric |
| 2. |
Glaze finish |
Cake release
proportion of fabric improves , may be for
a short duration |
Polyester
Polyproplene |
| 3. |
Egg Shell
Finish |
Cake release
properties improves may be for short period |
Can restrict air flow Polyester Polyproplene |
| 4. |
Silicon
Treatment |
Helps is
formation of initial dust layer on surface.Provides
water repellent properties |
Polyester |
| 5. |
Flame Reluctant
Finish |
Reduces
combustive properties of fabric (But it can
not be classified on flame proof) |
Polyester
Polyproplene |
| 6. |
Teflon Treatment |
Improves
water repellent an oil repellent properties
& fabric considerably .Facilitating cake
finish . (May restrict air flow) |
|
| 7. |
Antistatic
Treatment |
Helps to
dissipate static charge developed by products |
Polyester |
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| Top |
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| Q8) What is a cartridge
filter?What are the areas of application? |
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| Cartridge
filters are new types of filters where pleats are formed
of filter bags around a diameter. These type of filters
offer a very large filtration area compared to a cylindrical
filterbag.Thus the height of filter bag reduces thereby
reducing the height of the baghouse.A typical 1 mtr long
cartridge offers area of 4-8 m2,which is equivalent to
3 to 6 bags of 125 mm.dia having 3.6 mts length. |
| Another
major difference is cartridge filter works on surface
filtration against depth filtration that of non-woven
filter bags,which results in less pressure drop across
baghouse having catridge filters. |
| The
typical pressure drop across cartridge filter is 50-75
mm WG against 150-175 mm that of non-woven filter bagfilter.Due
to this lower pressure drop,the centrifugal fan absorbed
power reduces. |
| The
typical pressure drop across cartridge filter is 50-75
mm WG against 150-175 mm that of non-woven filter bagfilter.Due
to this lower pressure drop,the centrifugal fan absorbed
power reduces. |
| The
typical pressure drop across cartridge filter is 50-75
mm WG against 150-175 mm that of non-woven filter bagfilter.Due
to this lower pressure drop,the centrifugal fan absorbed
power reduces. |
| However,cartridge
filters has many limitations i.e. it cannot take higher
dust loads,leading to choking of cartridges.For process
applications the performance of cartridge filter is not
proven, though there are instances of use of same.The
another disadvantage is cartridges are susceptible to
higher temp. |
| However,cartridge
filters has many limitations i.e. it cannot take higher
dust loads,leading to choking of cartridges.For process
applications the performance of cartridge filter is not
proven, though there are instances of use of same.The
another disadvantage is cartridges are susceptible to
higher temp. |
| Thus
cartridge filters are good option where height constraints
exists,dust loads are nominal like nuisance venting and
duty conditions are not severe.However a great care needs
to be exercised while designing a cartridge baghouse. |
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| Q9) How to arrive
at product loss at outlet of baghouse? |
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To
carryout product loss at outlet of bahouse,it is essential
to carry out emission tests.After carrying out atleast
4 to5 emission tests,average emission level in mg/Nm3
is arrived at,which is nothing but amount of product
lost in milligrams per normal metre cube of air flow.
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| Example
- The emission level at the outlet of drier was observed
at 140 mg/Nm3.The airflow is 20000 m3/hr at 110 deg.centigrate.The
product loss to be calculated as follows: |
| Convert
volume of air to Nm3/hr.i.e. at 20 deg.centi. : The vol.at
20 deg will be 15,300 Nm3/hr. |
| Multiply
emission level with airflow to arrive at product lost
per hour.The loss of product will be 2.142 kg/hr,which
works out to 51.40 kg/day on 24 hrs operation,and 1.5
tons/month.Assuming cost of product at Rs.10/- per kg.,Rs.15,000/-
of product is lost per month. |
| The emission
levels increase as filter bags reach replacement.During
such time emission levels go higher,leading to higher
product losses. |
| Top |
| ROTARY - AIRLOCK |
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| Q. 1) What is function of a
rotary-airlock? How does it differ from rotary feeder/
discharger ? |
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| Rotary
feeders/ dischargers are used for either feeding or discharging
solids against no pressure differential i.e. there is
no pressure exerted at the inlet of airlock valve e.g.
discharging solids from a bin /silo in to any hopper/bin
will require a rotary discharge valve, similarly for feeding
solids to a hopper will require a rotary feeder. |
| When
rotary feeder/ discharger are subjected to differential
pressure/ vaccum, it is termed as rotary airlock or rotary
airseal, e.g. solids withdrawal from a cyclone or baghouse
operating under negative pressure would call for a rotary
-air lock valve or feeding solids to a pneumatic conveying
pipeline calls for a rotary-airlock valve. |
| Since the
rotary air-lock valve are subjected to differential pressure,
the valve essentially calls for closer tolerances compared
to rotary feeders/ dischargers. There are many other factors
which differentiate air-lock, such as no.of pockets, vent
arrangement, bearing arrangement etc. |
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| Q. 2) What are different types
of rotary airlock valves? Where are they used? |
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| There are
3 major types of airlock valves : a) Drop-thru type b)
Offset type c) Blow-thru type. |
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| a) Drop-thru airlock |
| They
are essetially used for freeflowing solids and powders,
which do not create problem in free rotation of rotor
and donot tick to the rotor pockets. Major solids and
powders are handled by drop-thru valves, either for discharge/
feeding application or for pneumatic conveying applications. |
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| b) Blow-thru airlock |
| They
are used in pneumatic conveying systems. |
| The material which
tends to stick to the rotor pockets such as milk powder,
dairy whitener, pigments etc., which often jam the pockets
resulting in reduced capacity or no feed condition. In
blow-thru valves, the conveying air sweeps the valve pockets
keeping them clean. The sticky material enters pneumatic
conveying pipeline and the clean pockets are free to accept
new material. |
| Such type of valves
are also used for conveying materials which donot fluidise
easily and whose particle density is very high such as
grains. |
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| c) Offset airlock |
| Some
materials are very tough to break and they tend to elongate
when trapped between rotor tips and casing leaving either
scring marks on the casing and tips or in worst case jamming
the airlock. Material like PP granules, gelatin, soft
bones etc. fall in this category. As the name suggests
the inlet of such valve is positioned offcentre. Due to
this the pocket gets filled correctly without overflowing
of material and possibility of material getting trapped
between close tolerance reduces drastically ensuring smooth
flow of material. |
| Conversly
material which tends to shear/ break easily or are very
fragile in nature can also be handled by offset type airlock,
with little changes in design. |
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| Top |
| Q. 3) Are rotary airlocks completely
leakproof? |
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| Being used
for feeding materials against high pressure differential,
rotary airlock have a very close tolerance depending on
application, to avoid leakage of air when airlock rotate.
However, it is practically not possible to manufacture
a rotary airlock which is totally leakproof. The leakage
through airlock depends on pressure differential, axial
and radial tolerances, type of rotor and also on nature
of material fed through airlock. |
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| Q. 4) What are the major factors
which are essential to consider while designing rotary
airlock/ feeder/ discharger? |
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| a)
Volumetric capacity of material. |
| b)
Material properties i.e. bulk density, particle density,
aerated bulk density, particle size,
temper. |
| c)
Material characteristics e.g. whether it is sticky,
hygroscopic, free flowing, abrasive, packy,
tough etc. are also important in choosing a rotary airlock
valve. |
| d)
Application area/ location of rotary airlock needs to
be critically reviewed as same rotary
airlock under different locations tends to give different
capacity. |
| e)
Pressure differential undr which airlock is functioning
is an important parameter. |
| f)
The load under which rotary airlock is going to operate
is instantaneous or steady also
needs to be specified. |
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| Q5) What are different types
of rotor configuration ? Where are they used? |
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| The following
types of rotor configurations are available,though some
applications call for special rotor configurations: |
|
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| a) Closed end rotor: |
| Here
the blades are covered by endplates on both sides.This
leaves a very close tolerance between rotor and casing
endplates,axially,reducing leakage of air. |
Normally used in rotary air-lock
valves subjected to differential pressure.Also when the
material is very fine,the closed end rotor helps to minimise/avoid
ingrace of particles into bearing housing.However,powders
which are sticky,special care needs to be taken.
|
| b)Open end rotor: |
| Here the
blades are not covered by endplates,thus these rotors
are used for low pressure differential duty or in feeders/dischargers,since
leakage of air ia more. |
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| c) Scalloped rotor: |
| In
this case,the capacity of rotor is reduced by joining
the two blades with a curved shape plate. The pocket which
looks "V" shape gets a shape of cup. |
| This is done because
many powders tend to stick to the rotor at the area where
two blades join together near shaft. The cup or scalloped
shape of rotor ensure that,the material does not sick
to rotor. |
| For blow-thru
valve,scalloped rotor is normally preferred. |
| Scalloped rotors
reduces the dischage capacity of valve,which somewhat
gets compensated by high filling efficiency. |
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| d) Replaceable tip rotor: |
| Here the
blades are provided with tips.These tips can be replaced
after tolerance between rotor and casing increases due
to wear and tear. |
| Most commonly
used for abrasive materials. |
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| e)Adjustable/replaceable
rotor: |
| Here
the tips can adjusted to retain the tolerance between
casing and rotor.After the tips gets worn out the same
can be replaced. |
Again these rotor are used
for abrasive product duty condition. However, frequent,
adjustment of tolerances can be a tedious job and needs
skills.Thus it is preferred to replace the blades that
keep on adjusting it.
|
| f) Reduced capacity
rotor: |
| Here
the capacity of rotor is reduced intentionally,by providing
a horizontal plate between the rotor blades.Somematerials
are very light or fluffy or fibrous in nature,due to which
the pocket filling efficiency gets reduced.In such cases,providing
the plate helps in obtaining more filling efficiency than
a standard rotor configuration. |
There are many
other types of rotor configurations available apart from
most commonly
mentioned above.However,choosing them needs great care
and application knowedge is
must backed with practical experience. |
| Top |
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| Q6) What is filling efficiency
of a rotary air-lock valve? |
| |
Depending
upon its size,type of rotor configuration used,type of
rotary airlock,and its
geometry each valve has a specific discharge volumetric
capacity. |
| For
every rotation the valve discharges specific volume of
solids/powders.This is called as the theorotical volumetric
discharge of that airlock. |
| However,due
to the process thru which product passes,the actual bulk
density of the product reduces .Moreover,when the rotor
rotates, accepting the material from process,the pockets
donot get filled completely. |
| Because
of this the actual volumetric discharge is often found
less than the theorotical discharge. |
| The
ratio of actual volumetric discharge to theorotical volumetric
discharge is called as filling efficiency of that rotary
airlock. |
For
every product the same airlock has different filling efficiency
.Even if the product is same the filling efficiency varies
depending upon location where the airlock/feeder/discharger
is installed. |
| Even
for the airlock the filling efficiency varies with the
increase or decrease in the speed. |
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| Q7) How to arrive
at the size of the valve? |
| |
| Find
out the volumetric thruput (m3/hr) reqd. This is done
by dividing capacity in kg/hr by bulk density of the product.Care
to be exercised in selecting right bulk density,since
it tends to vary with the process. |
| Based
on the capacity chart of the valve, match the reqd. capacity
with that of rotary valve. Ensure that the valve is selected
at right speed(recommended by manufacturer) to achieve
maximum filling efficiency. |
Selecting
airlocks for pressure differential applications,requires
a thorough knowledge of
process and product. Always rely on manufacturers experience
to choose airlocks on such applications. |
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