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

LL

OU

T

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

tio

n e

ffici

en

cy

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

rce

nta

ge

pe

ne

tra

tio

n (

%)

1.0

0.8

0.6

0.4

0.2

00.01 0.1 1.0F

rac

tio

na

l co

lle

cti

on

effi

cie

nc

y

Particle diameter (microns)

Diffusionregime

Diffusionand

interceptionregime

Inertialimpaction

andinterceptionregime

MPPS

18 | HVAC&R Nation | www.airah.org.au/nation | March 2019

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