1 / 36

FILTRATION

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

cairbre
Télécharger la présentation

FILTRATION

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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.

  2. FILTRATION • The essential components of a filter sampling system for aerosol measurement are shown in this figure.

  3. 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?

  4. 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

  5. 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.

  6. 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

  7. 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

  8. 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

  9. 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)

  10. Single Fiber Efficiency

  11. 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 ?

  12. 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

  13. 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.

  14. 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

  15. 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

  16. 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

  17. 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

  18. Gravitational Settling • Enhanced collection of diffusing particles due to interception Q: Any other mechanism?

  19. 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

  20. Total Efficiency Q: How do these efficiencies change wrt dp?

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

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

  23. Q: Implication? Q: Why standard filter test using 0.3 um DOP?

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

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

  26. 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.

  27. valid for At minimum efficiency (assuming only diffusion & interception) Q: Valid assumption?

  28. 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?

  29. 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?

  30. 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)

  31. 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? - - - - - - - - - - - - + - - - - - - - - - - -

  32. Minimum Efficiency Reporting Value (MERV) for HVAC (Heating, Ventilating and Air Conditioning) Filters http://www.mechreps.com/PDF/Merv_Rating_Chart.pdf

  33. 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

  34. Reflection

More Related