09 air filtration ix

5/20/2018 09 Air Filtration IX

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International MSc Programme Sustainable Energy EngineeringInternational MSc Programme Sustainable Energy Engineering

THERMAL COMFORT AND INDOOR CLIMATE

Lecture:

- AIR FILTRATION

Assist. Prof. Igor BALEN


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Why air cleaning?

Comfort

Health

Preservation of mechanical equipment

Creating a sustainable environment

Improving the quality of production processes

e.g. microelectronics, pharmaceutical and food processing

Sources of contamination:

Atmospheric smoke (industry, traffic...), dust (sand, soil, road dust...),

organic particles (bacteria, pollen...)

Internally generated - skin and hair, fibers, equipment, condensate...


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Environmental particle size distribution

Empirical considerations:

- typically, 0.005% of particles have a diameter larger than 10 m

- only 50 particles out of every million are visible to the naked eye


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Particle size distribution in a normal urban air

Empirical considerations:

- 0.5% of particles in a typical sample represent 91% of total weight

- 92% of particles in a typical sample have a diameter less than 0.5 m


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Air filtration mechanisms

- removal or collection of dust particles in air filtration is performed by

various combinations of the following basic mechanisms:

Straining. If the filter spaces are smaller than the size of the dust particles,

the particles are trapped.

Inertial impaction. A sudden change in direction causes a collision

between the dust particles, and fibrous media.

Diffusion. For very fine dust particles, Brownian movement causes theparticles to settle.

Interception. Dust particles may follow the airstream, contact the fibrous

media, and remain there.

Electrostatic effects. Particles and the filter medium are charged to

collect the dust in the airstream.


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Particle bounce


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Face veloci ty

2.5 m/s

Coarse filter

media velocity

2.5 m/s

Medium-efficiency filter

media velocity

0.11 m/s

HEPA fil ter

media velocity

0.02 m/s

Inertial impaction &straining

Interception &Diffusion

Diffusion

2.5 m/s 2.5 m/s 1.3 m/s 1.3 m/s


Effective particle removal


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Difference between surface-loading and depth-loading filtration


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Increasing Filtration Area

Mat Rolls Bags Rigid Cells

Pockets

Basic dry fiber filter construction


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Air filters and corresponding filtration classes


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Filtration classes according to ASHRAE St. 55


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Filtration classes according to EN779

DIN EN 779 1) EUROVENT 4/9

Av erage Arrestance [%] Coarse filters Average Arrestance [%]

G 1 Aa 65 EU 1 Aa 65G 2 65 Aa 80 EU 2 65 Aa 80

G 3 80 Aa 90 EU 3 80 Aa 90

G 4 90 Aa EU 4 90 Aa

Av erage Efficiency [%] Fine f ilters Average FCE 2) [%]

F 5 40 Ea 60 EU 5 40 Ea 60

F 6 60 Ea 80 EU 6 60 Ea 80

F 7 80 Ea 90 EU 7 80 Ea 90

F 8 90 Ea 95 EU 8 90 Ea 95

F 9 95 Ea EU 9 95 Ea

1)Arrestance and Eff iciency values to DIN EN 779 2) Fractional Collection Efficiency at

correspond with the values to ASHRAE 52.1 0.4 m (test aerosol: DEHS)


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Filtration classes according to EN1822


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Filtration classes and clean room classes


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- Coarse filters. Used to remove coarse dusts (5 to 80 m) such as

standing dust on surfaces, pollen, and textile fibers. These filters are

characterized by low pressure drop, low cost, and good efficiency on lint

and larger particles (5 m and larger), but low efficiency on normalatmospheric dust. They are generally panel or pleated filters of EN779

G 1,2 (MERV 1,2,3,4) with weight arrestance efficiencies between 65 and

75 percent. Usually, they are dry and disposable. Synthetic and cellulose

fibers, and bonded glass fiber mats with most of the glass fibers greater

than 10 m in diameter belong to this category. Design air velocity through

the media usually ranges from 1.5 to 4 m/s. The air filter is discarded as

soon as the final pressure drop is reached.

Basic types of filters


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- Low-efficiency filters. Low-efficiency air filters including EN779 G 3,4

(MERV 5,6,7,8) are often used to remove dusts between 3 and 10 m,

such as spores, molds, hairspray, cement, and other solid particles. Low-

efficiency air filters are often pleated filters or sometimes pocket (bag)filters. Natural and synthetic fibers including glass fibers, viscous

corrugated wire mesh, bonded glass fibers, and electrostatically

discharged fibers have been used as filter media. For synthetic fibers and

bonded glass fibers, their diameters may vary from 1 m to several m. As

the efficiency increases, the diameter of the fiber is reduced and they are

spaced comparatively closer together. Most low-efficiency filters have a

face velocity of 2.5 m/s to match the face velocity of the coil in AHUs.

Pleated filters often extend their filter area 2 to 8 times their face area, so

that the air velocity penetrating the filter media is reduced to 0.25 to 0.5m/s. They are widely used in packaged units and air-handling units in

commercial and institutional buildings, industrial workplaces, and more

demanding residential applications.



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- Medium-efficiency fil ters. Medium-efficiency air filters including EN779

F 5,6 (MERV 9,10,11,12) are used to remove dusts of size 1 to 3 m such

as welding fumes, legionella bacteria and coal dusts. They have a 1 to 3

m particle size efficiency from E2 < 50 % up to E2 > 80 %. They are oftenpocket and box filters with pleated mat to extend surface area, and are

made of synthetic fibers including bonded glass fibers of diameters from


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- High-efficiency fi lters. High-efficiency air filters including EN779 F 7,8,9

(MERV 13,14,15,16) are used to remove particles of 0.3 to 1 m such as

bacteria, viruses, cooking oil fumes, smoke. They have a 0.3 to 1m

particle size efficiency E1 < 75 % up to E1 > 95 %. The filter media areoften made of glass fibers of submicrometer and micrometer diameter.

They are often in the form of a pleated mat in a cartridge, or in the form of

a pocket filter. Media velocities range from 0.03 to 0.5 m/s, although face

velocities run to 4 m/s. High-efficiency air filters are often protected by low-

or medium-efficiency prefilters to extend their service life. High-efficiency

filters are widely used in air systems for hospitals, high-demand

commercial buildings, and precision manufacturing workshops.



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- Ultrahigh-efficiency fi lters. HEPA (high efficiency particulate air) filters

have an efficiency of 99.97 % for dust particles 0.3 m using the DOP

test method. ULPA (ultra-low penetration air) filters have an efficiency of

99.999 % for dust particles 0.12 m using the DOP method. The filter

media are made of glass fibers of submicrometer diameter that are formed

into pleated paper mats. Some of the larger fibers act as the carrier of theweb. Surface area of the filter medium may be 50 times the face area of

the ultrahigh-efficiency filter, and the rated face velocity may vary from 1 to

2 m/s at a pressure drop of 160 to 340 Pa for clean filters. The face

velocity of high-capacity HEPA/ULPA filters can be raised to 2.5 m/s.

Sealing of the filter pack within its frame and sealing between the frame

and gasket are critical factors that affect HEPA/ULPA filter penetration and

efficiency. To extend the service life of an HEPA/ULPA filter, it should be

protected by either a medium-efficiency filter or two filters: a low-efficiency

filter and a medium-efficiency filter located upstream from the HEPA/ULPAfilter. HEPA/ULPA filters are used to remove air contaminants such as

unattached viruses, carbon dust, combustion smoke, and radon progeny.

They are widely used in clean rooms and clean spaces for the microelec-

tronics industry, pharmaceutical industry, precision manufactu-

ring, and operating theaters in hospitals.



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Typical application of filters

Prefilter Filter Final filter


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Basic types of surface-loading filters


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Basic types of depth-loading filters


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Air filtration

Filter mat Panel filter

Pocket filter

HEPA filter

p = 20 - 600 Pa

v = 0,2-2,5 m3/m2 s

craw

< 5 mg/m3

Dust collection

Filter bag Cartridge filter

p = 1000 - 3000 Pa

v = 0,005-0,05 m3/m2 s

craw

> 5 mg/m3

Basic types of filter construction


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Panel Filters are made up of coarse fibers with a high porosity. They are

commonly made 15 to 100 mm thick. Unit panels are available in standard

(600 mm x 600 mm) and special sizes. This type of filter is commonly used

in residential air conditioning and is often used as a prefilter for higher-efficiency filters. Usually, they should be serviced when their operating

resistance reaches 120 Pa.


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Moving-curtain roll filters may have the media automatically advanced

by motor drives on command from a pressure switch, timer, or media light-

transmission control. A pressure switch control measures the pressure

drop across the media and switches on and off at chosen upper and lowerset points. This control saves media, but only if the static pressure probes

are located properly and unaffected by modulating outside air and return

air dampers. Most pressure drop controls do not work well in practice.

Timers and media light-transmission controls help to avoid these problems;

their duty cycles can usually be adjusted to provide satisfactory operationwith acceptable media consumption. Resistance of 100 to 125 Pa is typical

of this class.



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Extended-Surface Filters use media of random fiber mats or blankets of

varying thicknesses, fiber sizes, and densities. Bonded glass fiber,

cellulose fibers, synthetics, and other materials have been used

commercially. The media in filters of this class are frequently supported bya wire frame in the form of pockets, or V-shaped or radial pleats. In other

designs, the media may be self-supporting because of inherent rigidity or

because airflow inflates it into extended form such as with bag filters.

Pleating of the media provides a high ratio of media area to face area, thus

allowing reasonable pressure drop and low media velocities. They typicallyhave an initial resistance from 25 to 250 Pa. It is customary to replace the

media when the final resistance of 125 Pa is reached for low resistance

units and 500 Pa for the highest resistance units. Depth in direction of

airflow varies from 50 to 900 mm.



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Ultrahigh-efficiency fi lters. HEPA filters, and ULPA filters are made in an

extended-surface configuration of deep space folds of submicrometre

glass fiber paper. Standard dimensions of this filter are 610 by 610 by 292

mm. Such filters operate at duct velocities near 1.5 m/s, with resistancerising from 120 to more than 500 Pa over their service life. These filters are

the standard for cleanroom, nuclear and toxic-particulate applications.



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Adsorption filters. Activated carbon adsorbers are most widely used to

remove objectionable odors and irritating vapors (including indoor volatile

organic compounds, VOCs) of very small gaseous molecules from the air

stream by adsorption. Adsorption is the physical condensation of a gas orvapor onto an activated substance. Activated substances are highly

porous. When air flows through a typical assembly with a face velocity of

1.9 to 2.5 m/s, the corresponding pressure drops are between 50 and 100

Pa. In general, a higher humidity or higher operating temperature decre-

ases the adsorption capacity of the activated carbon. The maximumoperating temperature is 38C.


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Inefficient filters

Dirty air bypass

- between filters

- between filters and frames

- between holding frames

Damaged filters: torn, ruptured filters

Improperly installed filters

Badly maintained filters

Possible causes of poor air filtration


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Air filter installation

- important requirements of a satisfactory and efficiently operating air filter

installation are as follows:

The filter must be of ample capacity for the amount of air and dust load it

is expected to handle. An overload of 10 to 15% is regarded as themaximum allowable. When air volume is subject to future increase, a

larger filter bank should be installed initially.

The filter must be suited to the operating conditions, such as degree of air

cleanliness required, amount of dust in the entering air, type of duty,allowable pressure drop, operating temperature, and maintenance

facilities.


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Air filter installation

- air cleaners may be installed in the outdoor-air intake ducts of buildings

and residences and in the recirculation and bypass air ducts, but prefilters

and intermediate filters (in a 3-stage system) should be placed upstream of

heating or cooling coils and other air-conditioning equipment in the systemto protect that equipment from dust.

- where high-efficiency filters protect critical areas such as clean rooms, it

is important that the filters be installed as close to the room as possible to

prevent the pickup of particles between the filters and the outlet. The

ultimate is the unidirectional flow room, in which the entire ceiling or oneentire wall becomes the final filter bank.

- filters should be installed so that the face area is perpendicular to the

airflow whenever possible.

- initial, average, and final pressure drops during the operating period,which affect the energy consumption and service life of the filter and the air

system, must be determined. Monitoring the pressure drop of the air filters

have a direct impact on filter performance.


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Nonwoven filter media


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Stabilizing spacers on pocket filters


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Gas turbine air intake


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Maxipleat filter pleat pack


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Filter cartridge


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09 air filtration ix

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