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FILTRATION. Filtration is probably the most widely utilized technique for aerosol measurement, owning to its flexibility , simplicity, and economy .

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filtration
FILTRATION
  • Filtration is probably the most widely utilized technique for aerosol measurement, owning to its flexibility, simplicity, and economy.
  • The central concept in aerosol filtration is the collection, through removal from the gas phase, of a representative sample of the aerosol on a suitable porous medium or filter.
filtration2
FILTRATION
  • The essential components of a filter sampling system for aerosol measurement are shown in this figure.
filtration3
FILTRATION

Reading: Chap 9

Removing particulates from an air stream

Trapping aerosol in its pathway through tracheal media

Porous

membrane

filter

Fiber

filter

Capillary

pore

membrane

filter

Granular

bed

filter

Q: Does a filter work like a microscopic sieve?

fibrous filter
Fibrous Filter
  • Cylindrical shape fibers
  • Most perpendicular to the direction of air flow
  • Fiber size: submicron to 100 um
  • Material: cellulose, glass, plastic
  • Porosity: 70% to > 99%
  • Velocity: low ~ 10 cm/s, laminar

Glassfiber filter

TiO2 Nanofiber

hepa filter

High-Efficiency ParticulateAir (HEPA)

HEPA filter

HEPA filters are composed of a mat of randomly arranged fibres.

The fibres are typically composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers.

HEPA filters are critical in the prevention of the spread of airborne bacterial and viral organisms.

porous membrane filter
Porous Membrane Filter
  • Irregular path through complex pore structure
  • Porosity: 50% - 90%
  • High efficiency & high pressure drop
  • Material: cellulose ester, sintered metals, PVC (polyvinyl chloride), Teflon

Cellulose ester porous membrane filter

Silver porous membrane filter

http://www.2spi.com/catalog/spec_prep/images/silver.gif

capillary pore membrane filter
Capillary Pore Membrane Filter
  • An array of microscopic cylindrical holes of uniform diameter
  • Pores are perpendicular to the surface of the filter
  • The straight path results in lower efficiency compared to other types of filter
  • The smooth surface is particularly useful for collecting particles for observation in microscope

Nuclepore capillary pore membrane filter

Anopore® inorganic pore membrane filter

http://www.2spi.com/catalog/spec_prep/images/pg82_1.gif

granular bed filter
Granular Bed Filter
  • A bed of fine granules
  • Good for corrosive aerosols and at high temperature
  • Good for both air and aerosol
  • Materials: activated charcoal, glass, quartz, metal beads
  • Aerosols are removed by washing, volatilization or using solvents
single fiber efficiency

Cross section

of fiber

projected area stream

Air flow

Single Fiber Efficiency
  • E: the fraction of particles approaching a fiber in the region defined by the projected area of the fiber that are ultimately collected on the fiber

Packing density/solidity

Fiber: 0.01  0.3

Membrane: 0.1  0.5

u0

Q: Theoretical max efficiency?

(face velocity)

slide12
Total length of fiber in a unit volume
  • # of particles collected when a unit volume of aerosol passes through an element of a unit cross section and thickness dh
  • Filter Efficiency

df: fiber diameter

n: aerosol

concentration

A: cross-sectional area of the unit volume

Fraction of effective collection volume in a unit time

Total # in the unit volume

Q: Impact of df? a ?

filtration mechanisms

Valid for

0.005 < a < 0.2

0.1 < U0 < 200 cm/s

0.1 < df < 50 mm

Ref < 1

Filtration Mechanisms
  • Diffusion (Lee & Liu, 1982)

Q: Physical meaning?

Q: Impact of U0? a ?

http://aerosol.ees.ufl.edu/respiratory/section04.html

Lee, K. W. and Liu, B. Y. H.,

Aerosol Sci. Technol., 1:47-61, 1982

filtration mechanisms15
Filtration Mechanisms

Q: Calculate the single fiber efficiencies for a 0.5 um diameter particle at 20OC and 1 atm due to Browniam Diffusion for fibrous filter having a fiber diameter of 5 um and a solidity of 0.2 and operating at an air flow velocity of 15 cm/s.

Assume the particle density is 1 g/cm3, diffusion coefficient is 6.2 X 10-7 cm2/s.

slide16
Interception (Krish & Stechkina, 1978)

Q: Effect of increasing velocity? Increasing R?

Krish, A. A. & Stechkina, I. B., “The theory of Aerosol Filtration

with Fibrous Filters”,

in Fundamentals of Aerosol Science,

Ed. Shaw, D. T., Wiley, 1978.

http://aerosol.ees.ufl.edu/respiratory/section04.html

filtration mechanisms17
Filtration Mechanisms

Q: Calculate the single fiber efficiencies for a 0.5 um diameter particle at 20OC and 1 atm due to Interception for fibrous filter having a fiber diameter of 5 um and a solidity of 0.2 and operating at an air flow velocity of 15 cm/s.

Assume the particle density is 1 g/cm3

slide18
Impaction (Yeh & Liu, 1974)

Q: How to have a larger Stk?

Yeh. H. C. and Liu, B. Y. H.,

J. Aerosol Sci., 5:191-217, 1974

http://aerosol.ees.ufl.edu/respiratory/section04.html

filtration mechanisms19
Filtration Mechanisms

Q: Calculate the single fiber efficiencies for a 0.5 um diameter particle at 20OC and 1 atm due to Impaction for fibrous filter having a fiber diameter of 5 um and a solidity of 0.2 and operating at an air flow velocity of 15 cm/s.

Assume the particle density is 1 g/cm3,

cunningham slip correction Cc=1.33

slide20
Gravitational Settling
  • Enhanced collection of diffusing particles due to interception

Q: Any other mechanism?

filtration mechanisms21
Filtration Mechanisms

Q: Calculate the single fiber efficiencies for a 0.5 um diameter particle at 20OC and 1 atm due to Gravitational Settling for fibrous filter having a fiber diameter of 5 um and a solidity of 0.2 and operating at an air flow velocity of 15 cm/s.

Assume the particle density is 1 g/cm3,

cunningham slip correction Cc=1.33

slide22
Total Efficiency

Q: How do these efficiencies change wrt dp?

slide23
Single fiber efficiency of different mechanisms (h = 1 mm, a = 0.05, df = 2 mm & U0=10 cm/s); Table 9.2
slide24
Single fiber efficiency of different mechanisms (h = 1 mm, a = 0.05, df = 2 mm & U0=10 cm/s); Table 9.2
slide25

Q: Implication?

Q: Why standard filter test using 0.3 um DOP?

slide26
Total collection efficiency as a function of dp for 2 face velocities (h = 1 mm, a= 0.05, df= 2 mm)
slide27

Total collection efficiency as a function of face velocity for 4 particle sizes (h = 1 mm, a= 0.05, df= 2 mm)

slide28

Filtration characteristics of a fibrous filter; h = 1 mm,  = 0.05 and df = 2 mm

* Important mechanisms included contribute more than 20% of the total ES.

slide29

valid for

At minimum efficiency (assuming only diffusion & interception)

Q: Valid assumption?

slide30

Pressure Drop

  • As air passes through filter media, the filter structure causes a resistance that is a measure of air permeability or the pressure drop
  • The measurement of the pressure drop across filter media plays a central role in the practical estimation of filtration efficiency.
  • Pressure Drop

Q: Dp for filters having df smaller than 1 mm is less than predicted. Any reason? Implication?

filter quality figure of merit
Filter Quality (Figure of Merit)

Q: Is an increase in pressure drop bad?

Q: Does it matter if particles are solid or liquid?

Q: What is “pleated filter”? What’s its advantage over a flatpiece filter?

quality factor
Quality Factor

qF= -ln (P) / p(P: penetration, p: pressure drop )

b measured at 26.0 cm/s face velocity

c based on PRE (physical removal eff)

electret filter
Electret Filter
  • Fibers (of insulating plastic) are corona charged of fibers impregnated with insulating resin particles (~1 µm); collection efficiency enhanced without increasing resistance
    • Coulombic attraction
    • Image charging
  • Lose charges when exposed to:
    • Ionizing radiation
    • High temperature
    • High humidity
    • Organic liquid (aerosol)
    • Accumulated dust can mask the charge

Q: Does it collect neutral particles?

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+

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minimum efficiency reporting value merv for hvac heating ventilating and air conditioning filters
Minimum Efficiency Reporting Value (MERV) for HVAC (Heating, Ventilating and Air Conditioning) Filters

http://www.mechreps.com/PDF/Merv_Rating_Chart.pdf

summary
Summary
  • Types of filters: fiber, membrane, granular bed
  • Single fiber efficiency vs total filter efficiency
  • Filtration Mechanisms: diffusion, interception, impaction, gravitational settling, electrostatic attraction
  • Dominate mechanisms as a function of operating conditions (face velocity, particle size, fiber size, solidity)
  • Pressure drop, filter quality