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

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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

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5. Belaid Tabti, Mohamed Rachid Mekideche, Marius- Cristian Plopeanu,

Laurentiu Marius Dumitran, Lazhar Herous, Lucian Dascalescu. (2010,

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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.

QUERIES?