Chapter 4GAS FILTRATION MEDIA

T. J Ptak, PhDColumbus Industries, Inc.

VP of Research and Development

2

OVERVIEW

4-1 What is filter media 4-2 Parameters of filter media 4-3 Filter media classification

Depth Surface

4-4 Types of filter media Fibrous filter media Electrostatically enhanced Nano-fibers

4-5 Filter media test methods 4-6 Design process

3

4-1 FILTER MEDIA

Model Characteristics

pressure drop, efficiency,

filter life, cost

fiber or granule diameter

basis weight, thickness, permeability, pore size

4

FILTER MEDIA

Purpose:Cause a separation of particulate solids from a flowing

fluid with a minimum consumption of energy

Very broad classification based on:service lifefiltration principle

5

4-2 PARAMETERS OF FILTER MEDIA

Basis weight: Defined as the weight of media per unit area. Expressed in pounds per 3,000 sq. feet. Metric values are expressed in g/m2 and can be calculated by multiplying basis weight in lb./3000 ft2 by 1.627.

Thickness:Thickness of flat papers is measured with dead weight type caliper gauge having typically a pressure foot of 0.25” diameter and exerting a load of 4 lb./in2.

Permeability:Air permeability is the flow rate of air at 23oC through a sheet of paper under a specified pressure head, usually expressed as CFM per square foot of area at the 0.50 in. water pressure. Often called as Frazier permeability.

Objectives:What parameters describe filter media.

6

PARAMETERS OF FILTER MEDIA

Maximum pore size: Defined as the largest pore. Expressed as a pressure (P) of air in inches of water recorded when the first air bubble appears in the fluid. The size of largest pore (r) can be calculated:

g is the surface tension and is the contact angle.

Burst strength:Is defined as the hydrostatic pressure in psi required to produce rapture of paper sample when pressure is increased at the controlled rate through a rubber diaphragm to a circular area of 1.2 in. diameter.

Stiffness:Ability to resist an applied bending force.

rP

cos2

7

PARAMETERS OF FILTER MEDIA

Tensile strength: The Tensile Strength Test is a measure of the directional strength of the media, measured in pounds per square inch.

Electrostatic chargeFiltration efficiency:

Particulate capture efficiency is a test that results in a filter media performance characteristic. A fractional efficiency characteristic curve is generated, which describes the ability of a filter media to capture particles of differing sizes.

8

PARAMETERS OF FILTER MEDIA

Performance parameters Parameters having impact on manufacturing process:

ThicknessStiffnessTensile strengthFiber type

Glass fiber media with synthetic fiber generally pleat betterSynthetic fiber such as PET required some heat during pleating

Good pleat formation, sharp pleatsPTFE media required different pleaters to avoid abrasion

9

TYPICAL PROPERTIES OF FILTER MEDIUM

GRADE: XYZDESCRIPTION: Polypropylene meltblown with wet-laid PET carrier

TEST METHOD UNIT TARGET MIN - MAX

BASIS WEIGHT TAPPI T410 g/m2 105 95 - 115

THICKNESS TAPPI T411 mm 0.74 0.66 - 0.81

FRAZIER PERMEABILITY TAPPI T251 cfm/ft2 20 17 – 23

STIFFNESS TAPPI T543 mg TBD X 20%

TENSILE MD TAPPI T494 lb/in 7.0 6.3 – 8.0

PRESSURE DROP @ 10.5 fpm TSI 8130 mm H20 6.2 5.7 – 6.7

PENETRATION @ 10.5 fpm TSI 8130 (NaCl) % 0.03 <0.03

10

PRESSURE DROP VARIABILITY

• Pressure drop data from 108 rolls of the XYZ

0

10

20

30

40

50

60

70

80

5.2 5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 7

Pressure drop

Freq

uenc

y

11

4-3 FILTER MEDIA - CLASSIFICATION

Categories based on filter life:Disposable

Single useautomotive cellulose paperfiberglass media

Extended lifeMuch longer life than disposableDriven by end-users and environmental requirementsGenerally nonwoven type

ReusableCan be regenerated by cleaning - dirt cup

Objectives:How industry classified filter media.

12

FILTER MEDIA - CLASSIFICATION

Categories based on filtration principle:Surface filtration

membranecellulose paper, fiberglass paperscreens

Depth filtrationnonwovens

Classification depends on particle size relative to the size of opening

13

FILTER MEDIA

Membrane ePTFE

14

SURFACE FILTRATION

Complete blocking mechanism:very early stage of filtration in cake filtration

FILTER MEDIUM

FILTRATE

SURFACE STRAINING(COMPLETE BLOCKING)

15

DEPTH FILTRATION

Standard blocking mechanismParticle build up at the surface o pore walls resulting in

diminishing pore sizeplugging

FILTER MEDIUM

FILTRATE

DEPTH STRAINING(COMPLETE BLOCKING)

16

CAKE FORMATION

Cake formation:Bridging mechanism over the surface pores within a filter

mediumCake provides an additional filtration layer

FILTER MEDIUM

FILTRATE

FILTER CAKE

17

4-4 TYPES OF FILTER MEDIA

Media type MaterialsFibrous cellulose, glass, polymeric, metal,

ceramic, carbonMembrane

polymeric PTFE, PFA, nylon, polycarbonate, cellulose, etc.

Sintered metal 316L SS, nickel, etc.Ceramic aluminum oxide, silicon carbide

Fabric cotton, glass, polymericFoam polymeric, metalGranular bed sand, activated carbon

Objectives:How industry classified filter media

18

FIBROUS FILTER MEDIA

Fibrous media:Random web of natural and man-made fibers, which may

or may not be bounded together

Typical parameters:Fiber size 1 - 100 mmThickness 0.5 - 5 (25) mmVoid volume 75 - 99%

19

FIBER TYPES

Natural: “Carbohydrate based (polymers of glucose sugar)”CottonLinen, jute, bamboo, etc.Wood pulp

softwoodhardwood

Manufactured: “Manufactured from polymeric materials”

20

CLASSIFICATION OF NONWOVENS

Classification based on:Web consolidationWeb structureWeb formationFiber type

natural:cotton, wool, wood pulp

syntheticpolymeric: polyester, polypropylene, nylon,...

blended

21

ELECTROSTATICALLY CHARGED MEDIA

Applications:Automotive cabin filtrationResidential and commercial HVACVacuum and air cleanersRespiratory protection

Advantages:Higher efficiencySame pressure drop

Disadvantage:Charge deterioration

22

HISTORY OF ELECTRETS

“Electret”:O. Heaviside - electret produces a static electric field

First electrets:M. Eguchi - waxes of carnauba type or mixtures

First filtration application:1929 patent, filter made of waxes

First recognized electrostatic filter:Hansen resin-wool filter

23

CHARGING PROCCESES

Corona charging:Charging a film - split fibersCharging a fibrous web

Triboelectric:Mixture of different synthetic fibers

Charging by induction:From liquid state

24

SPLIT FIBERS

High level of microscopic charge Rectangular shape (ribbon), coarse fibers, 10-30

25

TRIBOELECTRIC CHARGING

Triboelectric seriesTwo different fibers, coarse fibers: 18-20m

polypropyleneacrylic

polypropylenenomex

26

TRIBOELECTRIC CHARGING

Triboelectric series Positive Wool

NylonSilkCottonAcrylicPolyethylenePolypropyleneModacrylic

Negative Chlorofibers

Challenges:Stability of chargeAmount of charge

27

CHARGING BY INDUCTION

Spun fibers - sprayed electrostaticallyFine fibers; 2-10 microns

28

NANOFIBERS

Electrospinning is a process which involves the drawing of nanofiber in presence of high voltageFrom polymer solution or molten liquid

Conventional drawing of fibers from dies under external pressure

History of electrospinnigIn 1500s Gilbert observed electrospraying processWhen charge piece of amber was brought near to a

droplet of water it formed cone shape and small droplet ejected from the tip of the cone

29

NANOFIBERS

Electrospinning process High Voltage Ground

30

NANOFIBERS

Polymers and solventsNylon 6,6 Formic acidPolycarbonate, PC Dimethyl formamide:tetrahydrofuranPolyvinyl Alcohol, PVA Distilled waterPolylactic acid, PLA Dimethyl formamidePolyacrylonitrile, PAN Dimethyl formamidePolyethylene terephtalate, PET DichloromethanePolystyrene, PS TetrahydrofuranCellulose acetate, CA Acetone Residue of solvent Fiber Diameter, [µm] Surface area, [m2/g]Nanofiber 0.05 80Meltblown 2.0 2Spunbond 20 0.2

31

NANOFIBERS

Pictures of nanofibers

32

NANOFIBERS

Filter media based on nanofibersRequire support layer (carrier, backer)Often non-uniform fiber distribution

Comparison between glass fiber, charged and nanofiber media

Material/Parameter Glass media Nanofiber Charged

Single Layer

Dual Layer

PA-6 PAN # 1 # 2 # 3

Thickness, [inch] 0.013 0.015 0.021 0.020 0.021 0.029 0.030

Pressure drop, [mm H2O]

4.8 5.5 2.7 3.7 1.7 1.0 3.3

Penetration, [%] 37 (DOP)

30 (DOP)

7.8 8.0 6.2 5.0 1.0

33

NANOFIBERS

Comparison of V- cell filter performanceSupplier/Parameter Supplier 1

NanofiberSupplier 2

GlassSupplier 3 Charged

Supplier 3 Charged

Supplier 4Glass

Supplier 4Glass

Media grade 3 layer nanofiber

Glass fiber Charged synthetic # 2

Charged synthetic # 3

Glass fiber Glass fiber

Filter type V-cell – 4V V-cell – 3V V-cell – 3V V-cell – 3V V-cell - 4V V-cell - 4V

Pressure drop, [inch H2O]

0.33 0.32 0.27 0.42 0.60 0.31

MERV 15A 14 15 16A 16 14

Dust holding capacity, [g]

318 220 226 126 N/A N/A

Media area, [Ft2] 172 157 (8PPI) 78.6 (4 PPI) 100 200 200

34

4-5 FLAT SHEET MEDIA TESTS

Broad range of standards for testing flat sheet media propertiesPhysical properties, optical, electrical and others

Flat sheet tests - filtration performance:Design tool for media manufacturersDesign tool for filter manufacturers

predict filter performance

Quality assurances:Filter media and filter manufacturers

Objectives:Test methods for flat sheet media.

35

FLAT SHEET MEDIA TESTERS AUTOMATED TESTERS

Fully automated testers: Measure efficiency up to 99.999% Challenge aerosols - oil or NaCl particles “Monodisperse” challenge aerosols

Automated testers to determine most penetrating particle size (MPPS) Measure efficiency up to 99.9999999% Particle size range – 15 to 800 nm

Limitations Velocity range Particle size range

36

FLAT SHEET MEDIA TESTERS AUTOMATED TESTERS

Fully automated tester, TSI 8130:Good instrument for quality testsCannot be calibrated:

Performance checked against reference glass fiber materialBroad range of 95% confidence intervals

Correlation between different instrumentsRound Robin test:

5 different instruments Reference material

Parameter #1 #2 #3 #4 #5

ΔP at 32 lpm, [mm H2O] 26.2 28.8 30.7 30.3 28.8

P at 32 lpm, [%] 0.180 0.229 0.180 0.154 0.191

37

FLAT SHEET MEDIA TESTS AUTOMATED AND EFFICIENCY

Flat sheet tests with TSI 8130 tester Challenge aerosol NaCl or oil Flow rate 32 lpm (10.5 fpm)

85 lpm Flat sheet fractional efficiency test:

Challenge aerosol KCl Particle size range 0.3 to 10 µm Sample size 1 or 2 ft2

Air velocity corresponding to filter Particle detection optical particle counter

38

4-6 FILTER MEDIA DESIGN PROCESS

Customer requirementsSelection of filter medium

Calculate media velocityCalculate filter size

Prediction of filter performance from flat sheet media tests

Construction of prototypeValidate filter performance

Objectives:Correlation between filter media and filters.

39

FILTER DESIGN PROCESS

Filter dimensions:Often given by customer

Media selection:To meet performance To meet durability requirementsTo meet cost requirementsTo satisfy manufacturing processUnderstand supplier production capacity, quality and line width

Filter design:Component selection

Frame material, adhesives and other materialsPleat optimization

40

FILTER DESIGN PROCESSMEDIA VELOCITY

Face Velocity Face Velocity Media Velocity

Filter performance is determined at media velocity

41

FILTER DESIGN PROCESSPRESSURE DROP

Experimental results for various media High velocity application Linear function of velocity ΔP = AV

0 25 50 75 100 1250

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Coarse fibers 1

Coarse fibers 2

Fine fibers 1

Fine fibers 2

Velocity, [fpm]

Pre

ss

ure

Dro

p, [

in. H

2O

]

42

FILTER DESIGN PROCESSPRESSURE DROP

Experimental results for various media Low velocity application, MERV 10-14 Linear function of velocity ΔP = AV

0 5 10 15 20 250

0.1

0.2

0.3

0.4

0.5

Glass fiber 1

Glass fiber 2

Synthetic fine fibers

Velocity, [fpm]

Pre

ss

ure

Drp

, [in

. H2

O]

43

FILTER DESIGN PROCESSPRESSURE DROP

Impact of media area – pleat density Filters with 36 ft2 of media area; minipleat Filter media – glass and synthetic fine fibers

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

Filter - glass fiber 2

Filter - synthetic fine fiber

Flat sheet - glass fiber 2

Flat sheet - synthetic fine fiber

Flow Rate, [fpm]

Pre

ss

ure

Dro

p, [

in. H

2O

]

44

FILTER DESIGN PROCESSPRESSURE DROP

Impact of media area – pleat density Filters with 55 ft2 of media area; minipleat Filter media – glass and synthetic fine fibers

0 5 10 15 20 25 300

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Flat sheet - glass fiber 2

Filter - glass fiber 2

Filter - synthetic fine fiber

Flat sheet - synthetic fine fiber

Flow Rate, [fpm]

Pre

ss

ure

Dro

p, [

in. H

2O

]

45

FILTER DESIGN PROCESSPRESSURE DROP

Impact of filter depth Filters: 1 inch; 2 inch and 4 inch deep Same filter medium

0 25 50 75 100 125 150 1750.00

0.05

0.10

0.15

0.20

0.25

0.30Filter - 1 in.Filter - 2 in.Filter - 4 in.Flat sheet - synthetic coarse

Air velocity, [fpm]

Pre

ss

ure

Dro

p, [

in. H

2O

]

46

FILTER DESIGN PROCESSPRESSURE DROP

Impact of filter design Filters: V – cell and Rigid box

0 10 20 30 400

0.1

0.2

0.3

0.4

0.5

Flat sheet - glass fiber 2

V-cell- glass fiber 2

Flat sheet synthetic

Rigid box - synthetic

Air velocity, [fpm]

Pre

ss

ure

dro

p, [

in. H

2O

]

47

FILTER PRESSURE DROPFLOW TYPE

Components of filter pressure drop:Inertial flow (Bernoulli flow)

Exchange of potential energy to kinetic energy

ΔP ~ ρ V2

Viscous laminar flowΔP ~ µ V

Slip flow

Complex flow pattern within pleat

48

FILTER PRESSURE DROPFLOW PATTERN

Flow pattern through pleats may not perpendicular to the filter medium Real flow pattern Assumed perpendicular flow

49

FILTER PRESSURE DROPFLOW PATTERN

Pleat collapsing and pleat deformation Collapsing Deformation

50

FILTER PRESSURE DROPMEDIA UTILIZATION

Tested filters MERV 14 - 20 x 20 x 2” minipleat Media area = 109 ft2 Media area = 55 ft2

51

FILTER DESIGN PROCESSEFFICIENCY

Impact of media type Multilayer synthetic, V = 25 fpm

0 2 4 6 885

90

95

100

Flat sheet

24 x 24 x 12 in. V-cell

24 x 24 x 4 in.

TSI 8130

Particle size, [µm]

Eff

icie

nc

y, [

%]

52

FILTER DESIGN PROCESSEFFICIENCY

Impact of media type Glass fiber, V = 15 fpm

0 2 4 6 860

80

100

Flat sheet

24 x 24 x 12 in.V - cell

24 x 24 x 2 in.

TSI 8130

Particle size, [µm]

Eff

icie

nc

y, [

%]

53

FILTER DESIGN PROCESSEFFICIENCY

Impact of media type and filter design Synthetic, V = 30 fpm

0 2 4 6 860

70

80

90

100

Flat sheet 24 x 24 x 12 Rigid box

TSI 8130

Particle size, [µm]

Eff

icie

nc

y, [

%]

54

FILTER DESIGN PROCESSEFFICIENCY

High velocity, low filter media area Synthetic, V = 150 fpm

0 2 4 6 80

20

40

60

80

Flat sheet 24 x 24 x 2 in. Filter

TSI 8130

Particle size, [µm]

Eff

icie

nc

y, [

%]

55

FILTER DESIGN PROCESSEFFICIENCY

Impact of dust loading Synthetic, V = 70 fpm

0 2 4 6 820

40

60

80

100

Flat sheet

24 x 24 x 4 in. Filter initial

24 x 24 x 4 in. Filter after 1st loading

TSI 8130

Particle size, [µm]

Eff

icie

nc

y, [

%]

56

QUESTIONS

Which media is classified as surface type? Nonwoven Membrane Electret Cotton sheet

Which are charging methods? Triboelectric Wet laid process Corona None

Does efficiency of flat sheet media and full filters correlate?

YES Partially NO

Does pressure drop of media and filters correlate? NO YES Somewhat

What is typical size of nano-fibers? 2-5 microns < 0.1 micron 0.1 – 0.8 micron >1.0 micron