FILTRATION
PART I.
1 Definition
Filtration is a process of separating dispersed particles from a dispersing
fluid by means of porous media. The dispersing medium can be a gas (or
gas mixture) or a liquid.
Upstream
Face of the filter with „filter cake“ of
deposited particles
Downstream
Particles deposited
inside the filter
Filter
Dispersing
fluid
Dispersed
particles
Filter
thickness
Channel
wall
2 Types of filtration
Concerning to filtration surrouding:
Air filtration / Liquid filtration
Concerning to size of filtered particles:
Macrofiltration
for particle size dp: 10-6 m < dp
Microfiltration
10-7 < dp < 10-6
Ultrafiltration
10-8 < dp < 10-7
Nanofiltration
10-9 < dp < 10-8
Reverse osmosis dp < 10-9
Concerning to filtration mechanism:
Surface filtration / Depth filtration
2.1 Relative size of common filtered particles
2.2 List of airborne pollutants and the American Industrial Hygiene
Association, 1993, approved safe levels
Asbestos
0.2 fiber/cc
confirmed carcinogen
Isopropyl alcohol
980 mg/m3
Benzene
0.3 mg/m3
confirmed carcinogen
Lead (fume)
0.2 mg/m3
Bromine
0.66 mg/m3
Manganese
0.2 mg/m3
Cadmium
0.002 mg/m3
Mercury
0.01 mg/m3
Carbon dioxide
9,000 mg/m3
Methanol
260 mg/m3
Carbon disulfide
31 mg/m3
Nitric acid
30 mg/m3
Carbon monoxide
29 mg/m3
Nitrogen dioxide
5.6 mg/m3
Carbon tetrachloride
31 mg/m3
Propane
asphyxiant
Chlorine
1.5 mg/m3
Selenium
0.2 mg/m3
Chloroform
49 mg/m3
Sulfur dioxide
5.2 mg/m3
Cresol
22 mg/m3
Sulfuric acid
1 mg/m3
Ethanol
1,880 mg/m3
Tellurium
0.1 mg/m3
Fluorine
1.6 mg/m3
Tetraethyl lead
0.1 mg/m3
Formaldehyde
0.37 mg/m3
Toluene
188 mg/m3
Hydrogen cyanide
11 mg/m3
Turpentine
560 mg/m3
Iodine
1 mg/m3
Vinyl chloride
13 mg/m3
Iron oxide (fume)
5 mg/m3
Zinc oxide (fume)
5 mg/m3
suspected carcinogen
animal carcinogen
suspected carcinogen
animal carcinogen
confirmed carcinogen
2.3 Surface filtration
All particles which are bigger than pores are captured on the flat filter
surface and create „filter cake“. It is typical for example for woven fabric or
spunbond filters. Thus for these filters the pores distribution and permeability are
important properties. Flat filtration is more common for liquid filtration.
Advantage:
-It is very important for the cleanable filters – filter cake is possible to release
from the filter surface.
-It is possible to capture all particles, which are bigger than pore size.
Disadvantage:
-Filter properties are very instable in time (filter is clogged).
-Pressure drop is higher
Direction
of flow
Captured
particles
Textile filter expressed as a set
of cylinders placed in parallel
2.4 Deep filtration
Depth filter are able to capture particles that are too small to be sieved out as in
flat filtration. Particles, which can be a lot of smaller than the distances between
the fibers penetrate into the fiber structure. Filtered particles are captured in
terms of the filtration mechanisms. This type of the filtration process is
important for the most of filter applications. Next chapters about filtration
variables, properties and mechanisms refer first of all to the deep filtration.
Advantages:
-Longer lifetime
-Better relation between pressure drop and filtr efficiency
Disadvantages:
- Usually it is not possible to clean clodged filter
Direction
of flow
Captured particles
Textile filter expressed as a set
of cylinders placed in parallel
4 Principle of filtration - relation between filtration
variables and filter properties.
It´s simple to say “what is filtration” but difficult to describe relations
between filter properties and the main filtration variables which influence
the filtration process
Filtration
properties
Filtration variables
• Filter variables
• Flowing medium
variables
• Captured particles
variables
•Efficiency
Filtration mechanisms
• Diffusion deposition
• Direct interception
• Inertial deposition
• Electrostatic
deposition
• Sieve effect
•Pressure drop
•Lifetime
•Resistivity against
surrounding
conditions
•Others
(permeability,
porosity...)
4.1 Filtration properties – output of the filtration process I.
Filter efficiency
It is the ratio of particles captured by a filter over the total number
of particles found in the air upstream of the filter. Filter efficiency
can either be based on specific particle size ranges or based on the
total number of particles of all sizes.
Efficiency can be defined by formula 1,
where G1 is an amount of penetrated
particles (which haven´t been captured) and
G2 is total amount of particles upstream
 G 
E  1  1 .100
 G2 
formula 1.
Expression G1/G2 is named „Penetration“ of filter
Efficieny is changing during the filtration process (see chapter 6.3.4
Nonstationary filtration)
Filtration properties – output of the filtration process II.
Pressure drop
Pressure drop indicates the restance to flow. It is defined as a difference between the pressure
of flowing media upstream and downstream of the filter. For expression of pressure drop is
necessary to assign air flow or air velocity (linear relation).
p = p1 - p2,
where p1 is pressure drop upstream and p2 downstream
of the filter. Pressure drop is changed during the filtration
proces (see chapter 6.3.4 Nonstationary filtration).
Filter lifetime
Filter lifetime determines the time when the filter must be removed. It is defined as a time or
as an amount of the filtered particles, which are loaded into the filter until the filter is full.
According to EN 779 standard the filter lifetime is defined as a „Dust holding capacity“:
J = Es.mp
where Es is mean filter efficiency and mp is the amount of the
particles loaded into the filter until the final pressure drop (250
or 400 Pa) was reached
Filtration properties – output of the filtration process III.
Other properties I.
Permeability
It is the ability of a material to allow the passage of a liquid or gas through porous material. It
is possible to find more defininitions, whic depend on the level of simplification:
1) According to EDANA 140.1 standard it is defined by formula:
Q
MS 
A
where Ms is permeability (l/dm2/min), Q is the flow
(l/min)and A is the filter surface. Permeability is tested with the
pressure drop 196 Pa (98,1 Pa for some standards)
2) According to the Darcy´s law the permeability is defined by formula:
K 
Q
A.p
where K is permeability (m/Pa/sec) and p is the pressure
drop (Pa).
3) According to the Darcy,s law is possible to define permeability as a „permeability
coefficient“ defined by formula:
Q .h.
k1 
A.p
where k1 is the permeability coefficient (m2),  is the
dynamic viskosity (Pa.sec), and h (m) is the thickness of the
filter.
4. According Hagen-Dupuit-D´Arcy´s model is permeability defined as:
p 
 .h
K 3 .A
.Q 
 .C.h
A
.Q 2
where K3 is permeability coefficient and C is form coefficient.
This model is suitable for higher flow of viscose liquid (such as water etc…). When we
compare HDD model with D´Arcy´s law, the main difference is nonlinear relation between
flow and pressure drop.
Permeabilityof laminated textiles
For simple D´Arcy´s law it is possible to deduce relation between the permeability of one
layer and more layers. For most of the applications we can assume that the flow through
the laminated textile is the some as flow through one layer. Than the total pressure drop
and total permeability are defined:
pt   pi
i
1
and
K1total

i
1
K1i
,
where pi and K1i are pressure drop and peremability coefficients of each layers
Filtration properties – output of the filtration process IV.
Other properties II.
Porosity and pore size
Porosity of porous medium is defined as a percentage of the porous material volume not
occupied by fibers.
Very important is size or size distribution of pores, which depends on the pore definition and
on the used test method.
Testing methods:
1.
Image analysis of 2D microscopic wiew – direct method
2.
Sifting of defined particles through the textile
3.
Penetration of liquid agent into the textile – relation between pore size and surface
tension of liquid.
a) Wetting agent is pushed away from textile due to pressured gas – Bubble point
method
b) Non-wetting agent is pushed into the textile – Mercury porosimetry
For more informations see subject „High functional textiles“.
Description of simple Bubble point method:
We assume circular pores. Wetting liquid (wetting angle = 0) try to go through the
pores due to wetting force F= .D. . Against this force we can act by pressured
gas (Fp = p.Apore). D is pore diameter,  is liquid surface tension, p is gas pressure
and Apore is pore cross section surface. When the first bubble of gas is going
through the pore – both forces are in equilibrium.
At first bubbles are going through the maximum pore. When we can measure flow
rate of gas is possible to measure the distribution of pore sizes.
F =  .  . D
bubble
Wetting agent
D
Fp = p . Apore
textile
4.2 Statinary and nonstacionary filtration
It is important that the filtration properties are changing during the filtration
process. A captured particle, since it occupies a finite space, becomes part of
the filter structure, able to contribute both to pressure drop and to filtration
efficiency. When we neglect this assumption the filtration process is named
„stationary“. It is possible in the beginning of the filtration process. When we
assume that the deposited particle influences filter properties the filtration
process is named „nonstationary“ [Pich, 1964]. Secondary proceses of
nonstationary filtration are:
1. Filter clogging – particles fill the filter structure
•
increase of pressure drop
•
increase of filter efficiency
2. Particle disengagement
•
decrease of filter efficiency
3. Capillary phenomena
•
flushing of drops
•
formation of fluid layers in placed where the fibers are spiced
•
condensation of water
4. Loss of electric charge
•
decrease of filter efficiency
5. Filter destruction
4.3 Test method of filtration properties:
Tested properties are efficiency, fractional efficiency, pressure drop, pressure drop vs. air flow,
filter lifetime etc... Properties are tested as initial or during filtration process. Methods are
differ in the particle substance (electrical properties, adhesion etc...), particle size (coarse/fine),
particle size range (monodisperse, polydisperse), particle concentration etc...
1) Synthetic dust
The dust is blend prepared from melted anorganic (and organic) particles. The most known is
ASHRAE dust that has the some parameters as the dust around Arizona roads [ASHRAE 52,2,
1999]. It is used for coarse filters (particles are coarse and polydisperse). It is possible to test
change of properties during the filtration process and filter lifetime. Dust is measured by
weighting method. This method is very popular and easy to use. However, it is open to
criticism because weight measurements give predominantly the weight of the largest particles
in the sample. Used standards are: EN 779 [EN 779, 200], ASHRAE 52,2 etc...
2) Athmospheric dust spot efficiency
In the Atmospheric Dust Spot Efficiency ambient outdoor atmospheric air is passed through
the unit being tested and samples are taken at the inlet and outlet of the unit to evaluate its
collection efficiency on the dust particles suspended in the atmosphere. This test is replaced
with DEHS aerosol method because athmosperic air composition is changing. Used standard
was older version of EN 779 [Gustavsson, 1999] .
3) Oil aerosols (DEHS, DOP, paraffin oil)
As the test matter is used aerosol from liquid oily substances. The most known are:
dioctylphtalate (DOP), diethylhexylsebacate (DEHS) and paraffin oil. Two types of oil
aerosol are known: Cold and hot. If the oil is dispersed and dryed in cold ambient conditions
(Laskin nozzle) then the size range of particles is wider (polydiperse aerosol). If the oil is
dispersed and dryed in hot ambient conditions then is possible to obtain monodisperse
particles (0,1-0,3 m). Particles are analyzed by laser particle counter or by spectrofotometric
method. It is possible to detect efficiency of selected particle size (except paraffin oil).
Particles are insenzitive to electrostatic field. Initial values of This method is used for fine
and high efficient filters – HEPA (high efficiency particulate air filter) and ULPA (ultra low
penetration air filter) filters.
4) NaCl aerosol
Sodium Chloride aquelous solution is dispersed and dryed. These polydisperse particles have
mean size 0, 65 m and their penetration through the filter is analysed by spectrofotometer.
This method is suitable for quick test of high efficient filters (respirators especially). Used
standards are: BS 4400 [BS 4400, 1969], EN 143 [EN 143, 2000], etc...
5) Methylen blue test
The solution of methylen blue is dispersed and dryed. Particles are analysed by comparing of
the blue colour intensity upstream and downstream the filter. It is suitable to high efficient
filters. By reason of narow gauge usage is replaced by sodium chloride aerosol test.
Summary of test methods:
method
synthetic
dust
Test standard particle substance
name
ANSI/AHAM Arizona roads dust
particle
particle
diameter preparation
(m)
0,5 - 3
aerosol
generator
injector
particle
detection
aerodynamic
sorter
weighting
method
ASHRAE
72% fine dust
EN
23% molocco black
CAN
5% cotton linters
ISO
Testing dust
2 – 125
SAE
10 - 40
athmospheric ASHRAE
Athmospheric dust Cca. 0,3 straight from opacitometer
dust
CAN
air
(light opacity)
oil aerosol
ASTM
DOP test;
0,3
evaporation, optical particle
ASME/ANSI
di-octylphtalate
0,2 – 0,3 condensation
counter,
IES
spectrofotometer
MIL-STD
0,3 – 2
Laskin
UL
nozzle
EN
EN
BS
aerosol NaCl
Methylene
Blue test
BS
EUROVENT
EN
NF
BS
DEHS aerosol
0,1 – 0,3 evaporation,
diethylhexylsebacate
condensation
0,2 – 3
Laskin
nozzle
Paraffin oil;
0,40,26 evaporation,
CP27 DAB7
condensation
NaCl particles
0,02-2
median
0,6
dispersion,
drying
Methylen blue
particles
-
dispersion of
water
solution
photometer of
the light
diffusion
spectrofotometer
blue spot size
4.4 Types of filters based on filter efficiency:
Filters are classified according to international standards:
a) European standards EN 1822 (1998) and EN 779 (2002):
Type of filter
Filter Test standard,
class test method
Efficiency
(%)
Type of filter
Filter Test standard,
class test method
Efficiency (%)
Coarse filter
G1
EN 779,
up to 65
HEPA filters
H10
over 85
G2
synthetic dust
65 - 80
H11
G3
80 – 90
H12
G4
over 90
H13
H14
Fine filter
F5
EN 779,
40 – 60
F6
DEHS aerosol
(0,4 m)
60 – 80
ULPA filters
EN 18221:1998,
over 95
Liquid particle
aerosol with
defined size
distribution
(DEHS, DOP,
Paraffin oil)
U15
over 99,5
over 99,95
over 99,995
over 99,9995
Overall value
F7
80 – 90
U16
over 99,99995
F8
90 – 95
U17
over 99,999995
F9
over 95
Applications of filters according to EN 779 and EN 1822 standards.
Filter class
Captured particles
End use
G1
Insects, fibers, coarse ash, particles with
diameter around 1 mm
Coars pre-filters
Pollen, fibers, dust, fog...
Pre-filters for heavy poluted air, painting boxes, home climatisation,
kitchen digesters....
Pollen, fine dust, spores, partly bacteria
Air ventilation of restaurants, workshops, storages, home, pre-filters
for F9 class filters, heat exchangers
F7
Carbon black, bacteria, partly smoke
and soot
End filters for shops, workrooms, homes etc... Pre-filters for H11 –
H12class
F8
Oil smoke, partly metal oxide smoke
and tobacco smoke
End filters for laboratories, workshops etc... Pre filters for active
carbon filters and H13 – H14 class (surgery, pharmacy
Every aerosols, partly tobacco smoke
End filters for nuclear power plants, optical laboratories, light
industry, hospitals etc...
Every viruses, radioactive aerosol
End filters for hospitals with higher requirements,for public
protection equipments, for food, electronic, pharmacy and foil
industry
Aerosol microparticles
End filters for hospitals and biotechnology laboratories with higher
requirements and strong rules for leakage test
Filters for clean areas ISO 1 -3
G2
G3
G4
F5
F6
F9
H10
H11
H12
H13
H14
Aerosol microparticles
U15 - U17
b) American standard ASHRAE 52.2 (1999)
Coarse filters (MERV 1 – 4) are tested by synthetic dust, other filters are tested by
pottasium chloride particles with defined size (0,3 - 10 m) divided onto three ranges.
MERV
(Minimum
Efficiency
Reporting
value)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Composite Average Particle Size
Efficiency in size range (%)
Range 1
Range 2
Range 3
0,30 – 1 m 1 – 3 m
3 – 10 m
Average
Arrestance
(%)
E3 < 20
Aavg< 65
E3 < 20 65< Aavg< 70
E3 < 20 70< Aavg< 75
E3 < 20 75< Aavg
20  E3 < 35
35  E3 < 50
50  E3 < 70
70  E3
E2 < 50 85  E3
50  E2 < 65 85  E3
65  E2 < 80 85  E3
80  E2
90  E3
E1 < 75 90  E2
90  E3
75  E1 < 85 90  E2
90  E3
85  E1 < 95 90  E2
90  E3
95  E1
95  E2
95  E3
Minimum
Final
Resistance
(Pa)
75
75
75
75
150
150
150
150
250
250
250
250
350
350
350
350
Applications of filters according to ASHRAE 52.2 standard.
Minimum Efficiency
Reporting Value
(MERV)
Captured particles
End use
1–4
Pollen, dust mites, sanding dust,
spray paint dust, textile fibers...
Pre-filters, window air conditioners...
5–8
Mold, spores, cement dust,
snuff,...
Commercial buildings, residential,
industrial workplaces, spray paint
boxes...
9 – 12
Legionella, humidifier dust, lead
dust, milled flour, coal dust,
nebulizer drops, auto emissions...
Superior residental, better commercial
buildings, hospital laboratories...
13 – 16
All bacteria, most tobacco smoke,
most paint pigments, sneeze...
Hospital inpatient care, general
surgery, superior commercial
buildings...
17 - 20
Viruses, carbon dust, sea salt,
smoke...
Cleanrooms, pharmaceutical
manufacturing, carciogenic and
radioactive materials...
3. Types of filters based on its shape:
A) Flat filters:
Description and examples:
Flat filters are used without frame or (for bigger size) holded by rigid
frame or supporting grid. They would be divided onto two variants.
Bulk filters are: thermal or chemical bonded nonwovens, needle
punch etc... Thin filters are:woven and knitted fabrics, spunbond,
meltblown etc....
Filter
Polluted
air
Supporting
grid
Clean
air
a) bulk filter
b) thin filter
End use:
Cheap filters for common applications (vacuum cleaners, kitchen
digestor, paint boxes, cabine filters in cars...) , pre-filters for most of
air ventilation systems.
B) Pleated filters
Description :
It is suitable fo high efficient filters. Pleating process leads to bigger filter surface and
consequently to smaller pressure drop. It is possible to pleat flat materials, which stiffness
and elongation is similar to paper (for example wet-laid nonvoven from glassfibers). It is
necessary to hold textile pleats by rigid frame. Filter thickness is usually from 1 to 5 cm.
Polluted
air
Rigid
frame
Air flow
direction
Filter
thickness
Clean
air
Filter
End use:
Pre-filters, HEPA filters (High Efficiency Particulate Arrestance) used in air ventilation and
air condition systems, Auto cabin air filters, industrial applications, respirators for halfmasks
etc...
Examples of pleated filters:
C) Pocket filters
Description and examples:
Principle is similar to pleated filters, only filter thickness is similar to other filter
dimensions. Generally it is possible to use nearly all textile fabrics („paper properties“ are
not necessary). At first are stitched or bonded each pockets and then it is embed onto the
frame. Big dimension of this filter would be disadvantage.
Polluted air
Filter
Clean
air
End use:
Pre-filters for pleated HEPA filters or final filters for less superior industrial applications.
D) Cartridge filters
Description and examples:
Flat (bulky) filter or pleated filter is wrapped around the perfored tube. The advantage is
smaler dimension of filter with regard to acting surface.
Clean air
Variants of cartridge filter
cross-section
Polluted air
Filter
Flat (bulky) filter
Pleated filter
Perforated
tube
Container
End use:
Most of filters inside the car, industrial applications etc... Very ofter used for liquid
filtration.
Examples of cartridge filters
E) Bag filters, pulse-jet filters
Description and examples:
Principle is similar as cartridge filters however bag length is much bigger than diameter and
usually filter is cleanable by reverse pressure pulse. Commonly many bag filters are used
for one application (hundreds). Most of the dust is collected on the surface of filters. When
the increasing pressure drop reached a set value, the filters are cleaned by a short burst of
compressed air moving in reverse direction. Typical maximum pressure drop is 1000 – 2000
Pa, typical pressure pulse is in range 0,5 – 1 MPa and cleanig time 0,1 - 100 sec.
Filters
Outlet of clean air
Inlet of
polluted air
Back pulse of
pressed air
Output of captured
particles
End use:
Industrial applications: chemical processings, cement fabric, incineration, power generation
etc...
Examples of pulse-jet filters
Steel
frames
Drum filters
1.The rotating drum is actually a frame with
mounted filter panels. The filter panels use a
polyester or stainless steel filter cloth that is
of a specific micron porosity.
2. Waterborne solid waste particles enter the
Drum Filter from the inside of the rotating drum.
3. Solid waste particles (larger than the micron
porosity rating of the filter screens) are
ocaptured n the filter screens.
Water passes through the filter screens.
5. A float level switch or timer initiates filter
screen rinse.
6. The screen is rinsed and solid waste particles
wash down into the waste
trough where they collect and exit the filter.
Disc filters: