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BUILDING DESIGNAND ENGINEERING APPROACHES

TO AIRBORNE INFECTION CONTROL

AIR FILTRATION

Steve Rudnick

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WHAT IS AN AIR FILTER?

A porous structure — usually made of fibers — that removes particles suspended in air as the air flows through the porous structure.

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DEFINITIONS• Collection Efficiency (E):

Fraction or percentage of entering particles collected in filter

• Penetration (Pt): Fraction or percentage of entering particles not collected in filter

• E + Pt = 1.0

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TYPES OF FIBROUS FILTERS• Heating, Ventilating, and Air Conditioning

(HVAC) Filters– Rated by ASHRAE tests– Lowest rating: MERV 1– Highest rating: MERV 16– Used as pre-filters to protect HEPA filters

• High Efficiciency Particulate Air (HEPA) Filters

• Ultra-Low Particulate Air (ULPA) Filter (

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• High-Efficiency Particulate Air Filter:

– Referred to as a “HEPA” filter

– Throwaway, pleated, fibrous filter in a rigid frame

– Collection Efficiency ≥ 99.97%(for 0.3 μm particles)

– Pressure drop ≤ 1 in. w.at rated airflow (when new)

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HEPA FILTERFrame

Pleated Filterpaper

Corrugatedaluminumseparators

Adhesive

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HANDLING OF HEPA FILTERS

• Never touch filter paper or aluminum spacers because paper will be damaged.

• Remove new HEPA filter from its box by turning the box upside down and lifting the box off the filter.

• Use a bag-in, bag-out system for changing filters if they cannot be decontaminated.

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HEPA FILTER PAPER

• Paper-like mat of glass fibers perpendicular to airflow

• Fibers randomly oriented in the mat

• Organic binder is added for strength.

• Most of the mat is air (> 90%)

• When particles contact fibers they attach and adhere firmly.

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HEPA FILTER PAPER

1 μ1 μ

1μm1μm

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MANUFACTURE OF HEPA FILTERS• Standard paper-making methods, but porosity of

paper is much higher.

• Well-mixed slurry of glass fibers and binder flows on a porous conveyer belt.

• Liquid is removed by suction.

• Resulting paper is rolled onto a spindle.

• One piece of paper is pleated with aluminum spacers between the pleats.

• Edges of paper are glued to a rigid frame.

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DESIGN PARAMETERSFOR FIBROUS FILTER

• Thickness

• Porosity (fraction that is air)

• Distribution of Fiber Diameters

• Air Velocity

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PERFORMANCE PARAMETERSFOR FIBROUS FILTER

• Efficiency (or Penetration )

• Pressure Drop

• Dust-Holding Capacity

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PRESSURE DROP ACROSS A HEPA FILTER

Pressure drop (Δp) is directly proportional to filter paper thickness (t) and air velocity (V):

k1 depends on filter porosity and fiber diameter.

tVkpΔ 1=0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Filter Thickness(or Air Velocity)

Pres

sure

Dro

p

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PENETRATION THROUGH A HEPA FILTER

Penetration (Pt) decreases exponentially with filter-paper thickness (t):

k2 is a complicated function of many parameters including fiber diameter, filter porosity, particle properties, and velocity.

)-(exp= 2tkPt0

50

100

0

Filter Thickness% P

enet

ratio

n

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PRIMARY COLLECTION MECHANISMS• Inertial impaction

• Diffusional deposition(due to Brownian motion of particles)

• Interception

• Gravity (minor except for very large particles)

• Electrostatic attraction

Note: particles are not removed by sieving

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INERTIAL IMPACTION

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DIFFUSIONAL DEPOSITION

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INTERCEPTION

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ELECTROSTATIC ATTRACTION

• Electrically charged filters can give very high collection efficiency with relatively low pressure drop.

• They should not, however, be used for critical applications because their charge can be neutralized.

– By heat and humidity– By ionizing radiation

– By solvents (e.g., paints)

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ELECTROSTATIC ATTRACTION: IMAGE FORCES

Neutral

Fiber

+

+

–

Charged Particle

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EFFECT OF INERTIA, DIFFUSION, AND INTERCEPTIONON THE PENETRATION VS. PARTICLE-SIZE CURVE

Most penetratingparticle size

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EFFECT OF VELOCITY ON MOST PENETRATING SIZE PARTICLE

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PARTICLE ADHESION TO FIBERS

• On contact, particles attach firmly to surfaces

• Adhesive Forces

– Van der Waals

– Electrostatic

– Capillary (due to moisture)

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PERIODIC FIELD TESTING OF HEPA FILTERS

• Generate polydisperse dioctylphthalate (DOP) droplets upstream of filter using a Laskin nozzle.

• Measure upstream concentration with a light-scattering photometer.

• Measure downstream concentration.

• Passes if penetration is