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"This is a relevant text which shows how things have changed today and everything is interdisciplinary wherein just specific knowledge is incomplete. We all need to focus on diversification in all perspectives. It is extremely fascinating and interesting to realize that how the extreme seeming ends are actually interconnected." -Ashish Kapoor
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FILTERS IN AIR CONDITIONING
ASHISH KAPOOR
2013TTE2756 DEPARTMENT OF TEXTILE TECHNOLOGY
INDIAN INSTITUTE OF TECHNOLOGY DELHI
CONTENTS
Introduction
Nonwoven air filters
Influences of fibre geometry
Charging characteristics
Nanofibrous filter media
Introduction
Air Conditioning
Temperature Humidity
Nonwoven Air Filters EDANA, (The European Disposables and Nonwovens
Association) defines a nonwoven as ‘a manufactured sheet,
web or batt of directionally or randomly orientated fibres,
bonded by friction, and/or cohesion and/or adhesion.’
Nonwoven
Dry laid
Carded Air laid
Wet laid
Carding
Parallel laid
Preferentially oriented
Typical parallel-laid carded webs result in good tensile strength, low
elongation and low tear strength in the machine direction and the
reverse in the cross direction.
Air laying
Isotropic
MD:CD ratio approaches 1
Random laid
Air laying forms randomly oriented web. Compared with carded webs, airlaid
webs have a lower density, a greater softness and an absence of laminar
structure.
Wet laid
A material shall be regarded as a nonwoven ‘if more than
50% by mass of its fibrous content is made up of
fibres with a length to diameter ratio of greater than
300.’
Homogenity
Production
Influences of fibre geometry In nonwoven air filters
Single fibre –Filtering element
Mechanisms of fibre capture
Material
32 Polyester fibre samples Latex bonded Fly ash (2.5-40µ) Linear density, cross-sectional shape, surface roughness, crimp, staple
length. Cross laid card web
Relationship between air permeability and fabric density Relationship between air permeability and latex content
Conclusions At 95% confidence level
1. Cross-sectional shape: Use of trilobal rather than round
fibers improves efficiency with no detrimental effect on drag.
2. Surface roughness: No effect at the levels examined.
3. Linear density: Use of 3-denier rather than 6-denier fiber
improves the efficiency but at the cost of increased drag.
4. Crimp level: Use of crimped rather than un-crimped fibers
improves drag characteristics.
5. Fiber length: No effect at the levels examined.
Physical interpretation
Linear Density With fibres of lower linear density the probability of impact is increased. The
second effect of decreasing linear density at constant fibre mass is an increase in the
number of fibres. This in turn reduces the interfiber distances and facilitates bridging.
Similarly, the increased projected area and decreased pore size would be expected to
produce higher drag characteristics.
Cross -Sectional shape The 3-denier trilobal fibers used in this study have a 25% greater projected area than the
3-denier round fibers. The probability impact increases proportionally. It is difficult to
explain why the greater projected area of trilobal fibers does not cause increased drag. The
increased projected area alone appears not to cause as much of an increase in drag as
would an increase in the number of fibers due to a decrease in linear density, which also
decreases the average interfiber spacing.There appears to be a trapping mechanism
peculiar to trilobal fibers, where particles lodge in the concave region of the fiber.
SEM of filter(6 den ,round, uncrimped)
Crimp This parameter improves both efficiency and drag characteristics. It can be seen that
straight fibres seem to form groups of two or more where the fibers run close together
for a considerable length. The space between them becomes clogged with filtered
particles, and the group then acts as a single, wide, flat fiber with a higher resistance to
air flow. Efficiency decreases because of the larger spaces between these groups.
SEM of filter(3 den, trilobal crimped)
Conclusion
The changes in geometric properties of constituent fibres can modify
the filtration performance of nonwoven fabrics.
Charging characteristics(Electret Filter) Oda and Ochiai
Very effective to remove submicron dust particles from air.
Minimum pressure loss.
Human mask.
The corona charging characteristics were studied
Electret is a dielectric material that has a quasi-permanent electric charge or
dipole polarisation.
Electret filter (or electrostatically charged fibrous filter) is defined as
a filter made of fibers which are electrostatically charged or
polarised. The purpose of fiber charging or polarisation is to
enhance the removal efficiency of the filter material.
Material
Nonwoven polypropylene sheet (Thickness=140µm; fibre diameter=1.6µm)
Experimental
Sheet electrified by corona ions produced by needles.
The charging is controlled by the Screen grid (grid voltage:Vg) located 20mm
above the sheet.
The typical charging time is 30 min. at room temperature.
Photograph of filter sheet
Observations
The surface potential of the filter media is limited to -600V due to local
discharges occurring inside the porous sheet.
The surface potential profile becomes rougher with increasing voltage.
The surface potential decay was accelerated by the relative humidity of
ambient air.
Filtration Efficiency= 99.8% (Electrified sheet) and original 99.4%(CNC).
Conclusion
The electrification effect is not significant.
Corona charging and charge decay
characteristics
Objective Increasing the efficiency of the corona charging of nonwoven filter media for HVAC applications.
Material 100 mm × 85 mm samples of nonwoven sheets of PP (sheet thickness of 300 μm and average fiber diameter of 20 μm
Photograph of nonwoven filter media
“Nonwoven fabrics are nonhomogenous dielectrics”
Method
Charging using the positive corona discharge generated by a high-
voltage
wire-type dual electrode
Triode type electrode arrangement
V-I characteristics were recorded with
and without filter media.
Charge time=10 s
Measurement of surface potential Measurement of charge
Result
V-I characteristics SPD Curves
Conclusion
Different electrode arrangements for uniformity of corona charging.
Threshold voltage
Local discharges inside fabric limit surface charge and potential.
Nanofibrous Filtering Media
100nm-1000nm
Electrospinning
High permeability
Low basis weight
Small pore size
High specific area
Good interconnectivity of pores
SEM of nanofibrous filtering media
Tensile stress and strain curves of electrospun nanofibrous substrates
Challenges in Fabrication of Nanofibre mat
homogeneity in size distribution of fibres
uniformity in deposition and orientation of fibres
durability of fibre layers in nanofiber mat.
Operative particle size in various interaction mechanisms
Applications
Penetrating aerosol particulate filtering media Triple layer design of fibrous filters dedicated to remove the
nanoparticles along with other polydispersed aerosol particles (the back
support layer of densely packed microfibers, the middle nanofibrous
layer for collection of most penetrating aerosol particles and front
porous layer of fibers of a few micrometers diameter for collection of
micrometer sizedd particles).
Hospitals, healthcare facilities, research labs, electronic component
manufacturers, military and government agencies, food,
pharmaceutical and biotechnology companies.
High efficiency air filtering media High efficiency particulate air (HEPA) filters have minimum removal
efficiency of 99.97% of particles greater than or equal to 0.3m in
diameter
High flux ultrafiltration membrane
Porous polymeric ultrafiltration membrane manufactured by the conventional
method (phase immersion method) has its intrinsic limitations, e.g. low flux
and high fouling tendency due to geometric structure of pores and the
corresponding pore size distribution.
Hollow fibre module
Coalescence filter The coalescence filter is economical and effective for separation of
secondary dispersions . Coalescence filter performance depends on flow rate
of feed, drop sizes in the feed, filter bed depth and surface properties of filter
material/s.
Catalytic filter
Affinity filter for highly selective separations
Ion exchange filtering media
Other Application Areas
Carbon fibre ionizer assisted medium air filter in HVAC system.
Indoor air quality enhancement
Ozone removal
References
1. Sabit Adanur, Wellington Sears Handbook of Industrial Textiles. Technomic
Publishing, Lancaster. pp 288-289. R.S. Barhate, Seeram Ramakrishna. (2007,
June). Nanofibrous filtering media: Filtration problems and solutions from tiny
materials. Journal of Membrane Science. Volume 296 Issue 1-2. pp 1-8.
2. Ned Galka, Abhishek Saxena. (2009, July- August).High efficiency air
filtration: The growing impact of membranes. Filtration and Separation.
Volume 46 Issue 4. pp 22-25.
3. R.S. Barhate, S.Sundarrajan, D.Pliszka, S.Ramakrishna. (2008, May). Fine
Chemical Processing: The potential of nanofibres in filtration. Filtration and
Separation . Volume45 Issue 4. pp 32-35.
4. Jae Hong Park, Ki Young Yoon, Jungho Hwang. (2011, August). Removal of
sub micron particles using a carbon fibre ionizer- assisted medium air filter in a
Heating, Ventilation and Air Conditioning (HVAC) System. Buildings and
Environment. Volume 46 Issue 8. pp 1699-1708.
5. Belaid Tabti, Mohamed Rachid Mekideche, Marius- Cristian Plopeanu,
Laurentiu Marius Dumitran, Lazhar Herous, Lucian Dascalescu. (2010,
March/April). Corona –Charging and Charge Decay Characteristics of
Nonwoven Filter Media. IEEE Transactions on Industry Applications.
Volume 46 Issue 2. pp 634-640.
6. Tetsuji Oda, Jun Ochias. (1988). Charging Characteristics of
aNonwoven Sheet Air Filter. Electrets, Issue 6. Proceedings, 6th
nternational Symposium on (IEEE Cat. No. 88CH2593-2).
7. P.Zhao, J.A. Siegel, R.L. Corsi. (2007). Ozone Removal by HVAC
Filters. Atmospheric Environment. Volume 41 Issue 15. pp 3151-3160.
8. George E.R. Lamb, Peter Costanza, Bernard Miller. (1975, June).
Influences of fibre geometry on the performance of nonwoven air
filters. Textile Research Journal. Volume 45 Issue 6. pp 452-463.
9. George E.R. Lamb, Peter A. Costanza. (1979, February). Influence of
fibre geometry on the performance of nonwoven air filters Part II: Fibre
diameter and crimp frequency. Textile Research Journal. Volume 49
Issue2. pp 79-87.
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