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Media Filtration in Dust Collectors
 Filter media
is used to
filter out dust particles
from the polluted air stream, normally by using a screening material (such as
fabric), placed in the path of the air stream. The dust particles stopped
in the dust collector by the
filter media form a ‘dust cake’ on the incoming surface and as more dust
particles collect on the filter media the thickness of the dust cake increases.
This increases the efficiency of the dust collector filter media (capturing up to 99.99 % of
dust particles in some cases).
The two main functions of media filters
in major types of industrial dust collectors are:
-
to stop dust particles on its surface while allowing
clean air molecules to pass through and
-
to provide easy release of the dust cake during
cleaning.
Media filter cleaning is normally done by a burst of clean
air in
the reverse direction to the air stream with a velocity greater than air stream.
The greater force of the cleaning air burst dislodges the dust cake from the
filter media and drops it into the hopper area. For greater efficiency, a fan is
used to push or pull air through filter media openings.
Media filtration in specific applications – important considerations:
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Temperature: Upper limits of filter media
handling dust filled air stream temperatures are - 200° F for cellulose and
500° F for Fiber glass material
-
Combustion: Filter media is not fireproof.
Combustion can be countered with fire retardant coatings
-
Static removal: Static electrical charges may
build up on the filter media and could discharge, causing an explosion.
Static charge can be defused by,
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carbon impregnation in wet laid media
(cellulose) and
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metalloid finish in polyester media, (spun
bodied) for better dust cake release and superior static dispersion
qualities.
-
Hydro and Oleophobic finish imparts moisture and
oil mist resistance to polyester media. It also improves
dust collection
efficiency and strength
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DUST COLLECTOR EXPERT HINT:
A common term used in dust collectors is pressure drop.
This is a term related to static air pressure. Any aerodynamic
resistance that an air stream must overcome causes a pressure change. For
example, woven filter material standing in the path of the air stream slows down
the air as it tries to pass through. Air pressure in front of the filter
is high, while air pressure beyond the filter is low. The filter causes
increased static pressure in front of it, and a pressure drop (drop in static air pressure)
beyond it.
Mathematical Variables to be considered in design of filter media:
Static pressure is an important variable in design of filter media.
It is the positive or negative pressure that causes surface to
expand or contract (expressed in water gage). In the case of dust
collectors, it is the resistance to be overcome by air in the
dust
collector duct. Velocity pressure (VP) is the pressure needed to
effect a change in velocity of the air molecules and Velocity V, the
rate of speed of matter.
The value of air density is important in efficient design of filter
media. |
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i) Air
density (mass per unit volume of air) at standard conditions
Using the perfect gas equation (relates to pressure, density,
temperature and the gas constant for air), air density is calculated
as 0.075 lbm per cu.ft at a standard conditions (STP) (temperature at
70 F, zero water content and standard atmospheric pressure of 14.7
pounds per square inch absolute). This value is used in fan and air
flow equations.
ρ (greek letter
rho) or density of air (at STP conditions) = P/RT
(where P = atmospheric pressure, T = temperature, R = gas
constant for air)
ρ = 0.075 lbm
per cu.ft |
ii) Relationship between
volumetric flow rate (Q), velocity (V) and cross sectional area (A):
Q = volumetric flow rate expressed as cubic feet per minute (CFM)
V= average velocity expressed in feet per minute (fpm)
A = area (sq. ft)
Q = (V) (A)
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Eg. Determining
flow rate of air (Q), through an 8 inch diameter (A) duct,
at a velocity (V) rate of 4000 fpm
Q = 1396
ft³/min |
This equation is used in flow pipe
applications.
iii) Relationship between velocity and velocity pressure:
This relationship helps to determining critical pressure
requirement to move air stream through air ducts and the fan.
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VP (velocity
pressure) =ρ [ V² / 1096² ] |
ρ = mass density (lbm/cu. ft)
VP = velocity pressure (inches of water gauge)
V = velocity (feet/minute)
When ρ equals 0.075 lbm/cu. ft at standard conditions for air
VP = [ V ft/min / 4005² ]
When air is traveling at 4000 ft/min through any duct size at
standard conditions
VP = [ 4000² ] / [ 4005² ]
By using the equation Q = VA it is
easy to determine actual flow rate Q (CFM) through duct size.
iv) Frasier permeability rating for Filter Media:
It states that volumetric air flow rate number is determined at
½ inch of water gauge pressure through an area of one square foot of
media. The Frasier permeability number is 20-40 CFM for standard
filter bag media and 4-30 CFM for Cartridge Filter Media.
The working status of filter cartridge or bag can be gauged by the
magnehelic differential pressure gauge that measures pressure
between a port in the dirty air chamber and a port in the clean air
chamber. If the value of this velocity pressure differential is low
(1 ½ to 2 wg) it indicates good balance and if high (5 to 7 WG), it
indicates that the system is out of balance.
(The differential pressure reading is not the Frasier Permeability
rating) |
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Main types of Filter media that are used
in various types of dust collection filters. |
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i) Media used in Baghouse filters |
|
Media |
Use/Characteristic |
|
Polyester |
Commonly used material in the industry |
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Singed polyester |
Improves dust cake release |
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PTFE membrane polyester |
Captures fine particles where artificial dust cake is needed |
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Aramid |
Suitable for high temperature applications |
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Polypropelene |
Known for superior chemical resistance |
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ii) Media used in Cartridge filters |
|
Media |
Use/Characteristic |
|
Cellulose |
Commonly used cartridge media material. |
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Cellulose/polyester |
Has
high durability and good abrasion resistance |
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Spun bonded polyester |
Has
good dust cake release and excellent moisture tolerance and abrasion
resistance |
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iii) Media used in Pleated bag filters |
|
Media |
Use/Characteristic |
|
Spun bonded polyester |
Commonly used pleated bad media |
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Properties of various fabric media
for
low-medium temperature, dry filtration: |
|
Fiber Generic name |
Cotton |
Polyamid |
Polypropelene |
Polyester |
|
Fiber Trade name |
|
Nylon 66 |
Herculon |
Dacron® |
|
Recommended continuous operation
temperature (dry heat) |
180 º F
82 º F |
200 º F
94 º F |
200 º F
94 º F |
270 º F
132 ºF |
|
Water vapor saturated condition (moist
heat) |
180 º F
82 º F |
200 º F
94 º F |
200 º F
94 º F |
200 º F
94 º F |
|
Maximum operation temperature (dry
heat) |
200 º F
94 º F |
250 ºF
121 º F |
225 º F
107 º F |
300 º F
150 º F |
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Specific density |
1.50 |
1.14 |
0.9 |
1.38 |
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Relative moisture regain in % (at 68
degree F and 65% relative moisture) |
8.5 |
4.0-4.5 |
0.1 |
0.4 |
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Supports combustion |
Yes |
Yes |
Yes |
Yes |
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Biological resistance (bacteria,
mildew) |
No (on treatment) |
No effect |
Excellent |
No effect |
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Resistance to alkalies* |
Good |
Good |
Excellent |
Fair |
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Resistance to mineral acids* |
Poor |
Poor |
Excellent |
Fair (not recommended) |
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Resistance to organic acids* |
Poor |
Poor |
Excellent |
Fair |
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Resistance to oxidizing agents* |
Fair |
Fair |
Good |
Good |
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Resistance to organic solvents* |
Very good |
Very good |
Excellent |
Good |
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* at operating
temperatures. Based on fiber manufacturers published specifications. |
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Properties of various fabric media
for
high temperature, dry filtration: |
|
Fiber Generic name |
Aramid |
Glass |
PTFE |
Polyethylene
Sulfide |
|
Fiber Trade name |
Nomex® |
Fiberglass® |
Teflon® |
Rylon® |
|
Recommended continuous operation
temperature (dry heat) |
400 º F
204 º F |
500 º F
260 º F |
500 º F**
260 º F |
375 º F
190 ºF |
|
Water vapor saturated condition (moist
heat) |
350 º F
177 º F |
500 º F
260 º F |
500 º F**
260 º F |
375 º F
190 º F |
|
Maximum operation temperature (dry
heat) |
450 º F
232 º F |
550 ºF
290 º F |
550 º F**
290 º F |
450 º F
232 º F |
|
Specific density |
1.38 |
2.54 |
2.3 |
1.38 |
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Relative moisture regain in % (at 68
degree F and 65% relative moisture) |
4.5 |
0 |
0 |
0.6 |
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Supports combustion |
No |
No |
No |
No |
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Biological resistance (bacteria,
mildew) |
No effect |
No effect |
No effect |
No effect |
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Resistance to alkalies* |
Good |
Fair |
Excellent |
Excellent |
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Resistance to mineral acids* |
Fair |
Very Good |
Excellent |
Excellent |
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Resistance to organic acids* |
Fair |
Very Good |
Excellent |
Excellent |
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Resistance to oxidizing agents* |
Poor |
Excellent |
Excellent |
(PPS fiber is attacked by strong
oxidizing agents) |
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Resistance to organic solvents* |
Very good |
Very good |
Excellent |
Excellent |
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* at operating
temperatures. Based on fiber manufacturers published specifications.
** 475 degrees for reverse air shakers |
Methods to Improve Filtering Properties of Fabric
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Weaving of fabric material in any of the
following ways.
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Plain weave – Fabric can be made porous or
tight by number of counts per inch.
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Twill weave – Has fewer weaves and is more
porous and flexible
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Sateen weave – has least number of weaves
and is porous, flexible and smooth
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Use of needled-felt material – With its short
felt fibers pressed together and mechanically fixed by needle punch machine,
this material has high dust collection efficiency and flow rate.
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Use of singed material – It improves the surface
of the bag.
Dust Cake Characteristics on Bag Filters
The high efficiency of fabric collectors depends on the formation of a good
filter cake. For a good filter cake to form, dust particles must have
interlocking characteristics. Porous cakes increase dust collection and retains
more dust and can operate at a lower pressure drop. A good mix of collectors,
inlets and operational procedures can deal with any dust or powder.
The thickness of dust cakes on baghouse collectors varies between 1/16th and
1/8th inches and on cartridge collectors the cake is less than 1/64th inch
thick.
Media
Filtration Aids
A ‘precoat’ of inert material applied on the filter surface helps when the
dust collector contains moisture, oil and small dust particles sizes. Applying
the precoat on the clean surface of the filter media forms a protective dust
cake layer. The use of this type of a filtration aid helps in efficient release
of the dust cake, captures small particles and increases overall efficiency.
However ‘precoats’ have limited use. Frequent applications may be needed to
maintain the protective coating and there could be difficulties in recycling
product dust from hopper if precoat material mixes with dust. Ideally, precoats
must be applied on job specific basis after considering the cost and benefits of
the system.
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