1 BUILDING DESIGN AND ENGINEERING APPROACHES TO AIRBORNE INFECTION CONTROL AIR FILTRATION Steve Rudnick
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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
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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