MODULE Skills THEORY AND Workshop FUNDAMENTALS · 2019-03-20 · FUNDAMENTALS Particle capture...
Transcript of MODULE Skills THEORY AND Workshop FUNDAMENTALS · 2019-03-20 · FUNDAMENTALS Particle capture...
March 2019 | www.airah.org.au/nation | HVAC&R Nation | 15
The mechanisms for the separation of particles
and gaseous contaminants from the air are
fundamentally different. Particles are separated
by mechanical or electromechanical effects,
whereas gases are generally separated by
adsorption or absorption.
PARTICLE CAPTURE FUNDAMENTALSParticle capture theoryTheories of filtration are generally based on
particulate trajectories around an isolated cylinder
(a fibre) and particulate deposits due to inertia,
interception and diffusion onto fibrous strands.
Particle capture by a particulate air filter
fibre involves two different mechanisms:
• The probability that one of the dust particles
will collide with one of the media fibres,
(deposition)
• The probability that the particle, once
contacting the filter fibre, will continue
to adhere to it (adherence).
Fibrous filters consist of a maze of fine fibres, with
a large bed depth in comparison to the dimensions
of the particles to be filtered. The diameters of
the fibres must be small relative to the inter-fibre
distances or the filter will clog with a consequent
rapid rise in air resistance. Filter construction
depends on the duty for which it is designed.
The media of a filter designed for sub-micron
particles is usually composed of fine fibres,
many of which will be less than one micron
in diameter. Air velocity is restricted to a few
centimetres per second allowing diffusion
to take place. Initial resistances for these types
of air filters are usually high. Pleating of the filter
media is used to increase the surface area and
hence lower the resistance. Initial resistances
of 180Pa or higher are experienced when
such air filters are operating in their normal
air velocity range.
For removal of relatively low concentrations
of particles, larger than a few microns in diameter,
filters composed of coarser fibres are used in
the form of a loose mat with larger inter-fibre
distances. Air velocities are high to make use
of inertial effects and initial resistances are usually
lower than 150Pa.
Initial resistance, for both sub-micron and larger
than 1 micron filters, at normal air velocity range,
varies from 10pa to 300pa and more, due to a wide
range of factors.
Industrial filters are typically made from woven
fabrics and, when new, operate inefficiently
against low particulate concentrations. With high
particulate concentrations, deposits will build
to form a cake on the fabric, causing the filter
to act like a sieve with a consequential rapid
rise in resistance. When the resistance increases
to a pre-determined value, the cake is removed
by mechanical action, after which the filter is
ready for a further cycle starting at low efficiency
followed by deposition of a fresh cake of dirt.
Theories relating to particulate trajectories
around an isolated cylinder lying transverse
to the airflow are difficult to correlate with
actual filter performance due to the following:
• Aerodynamic interference between
neighbouring fibres
• Fibres do not lie transverse to airflow
in the media material
• Fibres are not all of the same diameter
• Finer fibres often twist around larger fibres
• Not all fibres are circular in cross-section.
Despite obvious limitations to theories, some
consideration has been given to assessing single
fibre capture ability and calculation of mean fibre
diameter from pressure drop measurements.
From this information an assessment of efficiency
is possible.
Particle capture methodsThere are five ways in which particles can be
deposited on, or be captured by, the fibres of
an air filter media. As shown in Figure 1, these are:
• Straining (sieved by fibres)
• Inertia (impingement)
• Interception (being caught)
• Diffusion (dispersal)
• Electrostatic attraction (attracted to fibre)
These particle-capture processes all occur
simultaneously within the filter media and each
contributes, to some extent, to the overall filter
efficiency.
Straining
The particle is larger than the gap between
the fibres. It cannot pass through and is captured.
Straining effects are typically only applicable
to relatively large particles or very fine media.
Inertia
Larger particles do not move around the fibres
(with the airstream); they are carried straight into
the fibre by their own inertia and are captured
by this impact.
The effectiveness of particle collection by inertia
forces increases with increasing particle mass
(i.e., increasing particle size) and increasing particle
(air) velocity. For the typical air velocity used in
HVAC systems the impingement effect becomes
dominant for particles with diameters greater
than 1.0μm.
AIR FILTRATION
THEORY AND
FUNDAMENTALS
March 2019 | www.airah.org.au/nation | HVAC&R Nation | 15
HVAC&R
SkillsWorkshop
MODULE
119
Cleaning indoor air and removing pollutants from the airstream
is a recognised indoor air quality strategy. This is one of the significant
advantages that mechanical ventilation can provide over natural
ventilation systems, where air cleaning is more difficult. Cleaning the
air also maintains clean internal HVAC surfaces, retaining system operating
efficiency and avoiding HVAC hygiene issues.
This Skills Workshop provides an overview of the theory and fundamentals
of the science of particulate air filtration.
PU
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OU
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PROUDLY SPONSORED BY
Skills summary What?
A guide to the concepts and principals of air filtration.
Who?Relevant for equipment manufacturers/suppliers, designers and installers and maintenance contractors.
Interception
Midsize particles do move along the airstream lines and these also
contact fibres and are captured by the fibre. The probability of a particle
hitting a fibre due to interception increases with increasing particle size
and fibre size. Interception dominates arrestance for particles with diameters
between 0.5 and 1.0μm.
Diffusion
The smaller particles move randomly across the airstream lines and contact
fibres by Brownian motion. This random movement, outside of airstream lines,
causes the particle to contact the fibre and be captured.
The effectiveness of particle collection by diffusion increases with decreasing
particle size and decreasing air velocity. Where there is no predominant
electrostatic interaction, particles with a diameter of less than 100nm
are deposited almost exclusively by diffusion.
Electrostatic interaction
Under the electrostatic interaction mechanism the particles are pulled
toward the fibre due to an electrostatic charge that is imposed on the fibre.
The effectiveness of particle collection by electrostatic interaction increases
with decreasing air velocity and particle size and increases with charge
on fibre or particle.
Electrostatically charged fibres may lose their charge over
time resulting in rapid and permanent reduction in arrestance.
Capture efficiency and air velocity
The relationship between air velocity and the efficiency of the interception
impingement and diffusion processes are shown in Figure 2. Although this
is a simplistic view, a relationship between efficiency and velocity has been
demonstrated experimentally.
The diffusion effect decreases with increased air velocity while the
impingement effect increases with increased air velocity (up to a point) and
the interception effect is relatively constant, i.e., it is independent of air velocity.
Capture efficiency and particle size
The relationship between particle size and the efficiency of the interception
and diffusion processes are shown in Figure 3.
Diffusion is the dominant effect for the smaller particle sizes (≤0.1µm), and
interception is the dominant effect for the larger particle sizes (≥ 10.0µm).
Diffusion and interception effects equally influence particle sizes between
0.1µm and 1.0µm, see Figure 4.
Penetration of sub-micron particles
The particle size that most greatly penetrates a filter is a function of filter media
construction, particulate challenge density, and air velocity. This size is called
the most penetrating particle size (MPPS) for that filter. Figure 5 shows a typical
particle size-penetration distribution curve for a hypothetical filter media,
indicating the MPPS.
The MPPS for this filter lies between 0.2 and 0.3 micron and this performance
characteristic can be modified by changing the media filter fibre diameter
and packing density and by adding electrostatic charges onto fibres.
MPPS can be any particle size in the range (not necessarily 0.3 micron),
dependent on flow, velocity and fibre packing/diameter.
HVAC&R Skills Workshop
Airstream
Dust particle
Interception
Fibre
Airstream
Dust particle
Inertia
Fibre
Airstream
Dust particle
Straining
Fibre
Fibre
Airstream
Dust particle
Diffusion
Fibre
Airstream
Dust particle
Electrostatic attraction
ChargedFibre
Figure 1: Straining, inertia, interception, diffusion and electrostatic effects.
16 | HVAC&R Nation | www.airah.org.au/nation | March 2019
Figure 2: Air velocity and the impingement, interception and diffusion processes.
Figure 3: Particle size and the interception and diffusion processes.
Velocity
Effi
cie
nc
y
Impingement
Diffusion
Interception
0.010 0.10 1.0
Fil
tra
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Particle diameter (μm)
Diffusion
Interception
Total
March 2019 | www.airah.org.au/nation | HVAC&R Nation | 17
Filters are tested in the MPPS range (e.g. HEPA filters tested at 0.3 micron)
in the knowledge that if an efficiency rating is established at this particle
size range then the efficiency at all other size ranges would be better.
AdhesionThe second mechanism needed for particle filtration theory is the adhesion
of the particle to the fibre.
For the filter to be effective, particles must adhere (stick) to the fibre
surfaces they come into contact with. See Figure 6. In general, it is
assumed that particles adhere to surfaces when they come into contact.
Particles can fail to adhere when there is high relative velocities between
the particle and filter fibre surface, resulting in re-entrainment of dust in
the airstream. All air filtration devices depend on adhesive forces. Factors
contributing to adhesion include:
• Electrical forces between particle and filter surface. The force between
two molecules is inversely proportional to the seventh power of their
distance apart. These forces, called Van der Waals forces, are of greater
importance in adherence of small particles than for large particles.
• Electrostatic forces. The attraction of particles because of their
electric charge.
• Surface tension due to films of moisture on particles or nearby
surfaces. Relative humidity is significant in the retention of particles larger
than one micron and in the cohesion of aggregates. Capture efficiency
usually decreases as relative humidity of the air decreases. This, however,
is not true for high-efficiency filters removing sub-micron particles.
• Surface contaminants such as soluble materials or very small particles
that affect the closeness of contact between the filter surface and particle.
• Size of particles, shape and filter surface roughness, which influence areas
of contact.
• Duration of contact. The forces of adhesion between a filter surface
and particles larger than 10 microns increase with time.
• Temperature. By altering surface tension forces, temperature can influence
adhesion.
FIBRE COATING ENHANCEMENT EFFECTSImpingement filters can be treated with adhesives or gel coatings that coat
the fibres and create a bond between them and any dust particles that may
impinge upon the fibre, increasing the adhesion.
This coating helps prevent the particle from being dislodged, due to air
velocity or fibre vibration by increasing the adhesion between the particles
and the filter fibres.
Next month: Safety with A2L, A2 and A3 refrigerantsPROUDLY SPONSORED BY
March 2019 | www.airah.org.au/nation | HVAC&R Nation | 17
PU
LL
OU
THVAC&R Skills Workshop
This month’s Skills Workshop has been
taken from the newly updated
DA15 – Air Filters and Cleaning
Devices – Selection
and Application.
APPLICATION MANUAL
DA15
THE AUSTRALIAN INSTITUTE OF REFRIGERATION, AIR CONDITIONING AND HEATING
AIR FILTERS
Figure 6: Particles adhere to fibres.
Figure 5: Typical filter relationship between penetration and particle size.
Figure 4: Relationship between particle size and capture method.
Particle size (μm)
Air velocity 0.04m/s
MPPS
0
2
4
6
8
10
0.2 0.4 0.6 0.8 1.0
Pe
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nta
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pe
ne
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%)
1.0
0.8
0.6
0.4
0.2
00.01 0.1 1.0F
rac
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effi
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nc
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Particle diameter (microns)
Diffusionregime
Diffusionand
interceptionregime
Inertialimpaction
andinterceptionregime
MPPS
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