Air Cleaner and

Filtration
Anthony Law

Visiting Lecturer

Department of Mechanical and Aerospace Engineering

The Hong Kong University of Science and Technology

 HEPA Filters

This is a ceiling
mounted HEPA
filter.

High efficiency
particle air filters,
range from
99.97% to 99.995%
efficiency.
 ULPA Filters

These high efficiency particle air

filters are 99.9995% efficient.
Mounted on ceiling.
 Differences between
HEPA and ULPA filters
HEPA filters and ULPA filters are air filters
designed to trap the vast majority of very
small particulate contaminants from an air
stream.
HEPA filters are capable of removing 99.97%
of contaminant particles 0.3 in diameter.
HEPA filters must feature minimal pressure drop
and maximum airflow when in operation.
ULPA filters are closely related to HEPA filters but are even more efficient. ULPA filters
are specified to remove 99.999% of contaminants 0.12 or larger in diameter.
Filter fibers trap contaminants using three primary methods - interception, inertial
impaction and diffusion. Understanding these three methods makes it clear why
particles around 0.3 micrometers are most difficult to filter. Particles less than 0.1
micrometers are easily trapped due to diffusion while particles larger than 0.4
micrometers are trapped by inertial impaction. Particles between 0.1 and 0.4 are
therefore too large for effective diffusion and too small for inertial impaction and
efficient interception, so that the filter's efficiency drops within this range.
 Why is Air Filtration Important?
Homes are being built tighter than ever before to
save on energy costs, so particulates like dust
mites, pet dander, bacteria and others are
trapped inside the home.
On average, we spend 90% of our time indoors
and breathe roughly 3,400 gallons of air each day.
½ of all families have someone directly affected
by allergies.
85% of homeowners think the air outside is worse
than the quality of air inside their home
But, in reality, indoor air can be 3-5 times more
polluted than outdoor air . .

Dust Mites Pets Pollen Bacteria VOCs Mold Smoke Dust

Spores
Introduction of air cleaner

High ambient particle levels in Hong Kong

Complex indoor human activities in residences
Use of air cleaners may be an alternative method to
alleviate the indoor particle levels in some cases
Introduction of air cleaner
Major types of air cleaners:
Mechanical air cleaner
Electronic air cleaner
Hybrid air cleaner

Efficiency is particle-size dependent

 Introduction of air cleaner
An air cleaner = a blower + air
cleaning component

Existing forms of air cleaner

- In-duct air cleaner & Portable air cleaner
- For mechanically and/or naturally ventilated venues
Introduction of air cleaner

Mechanical air cleaner:

Filtration mechanism: particles are trapped directly on filters

1. For small particles: Brownian

Diffusion
2. For large particles: Interception
and Impaction
Introduction of air cleaner

Electronic air cleaner

Removal mechanism: particles are
charged and deposited on
surfaces

1. For small particles: Diffusion

charging
2. For large particles: Field charging
ESP (Electrostatic
Precipitator
 Introduction of air cleaner

Hybrid air cleaner: combined use of the filtration

and electrostatic techniques
To achieve higher removal efficiency
To lower the cost of filters (electret filters)
 Standard: Air Cleaner Tests
Air cleaner tests Chamber test, removal rate
1. American National Standards Institute/Association of
Home Appliance Manufacturers, ANSI/AHAM AC-1-2006.
Testing dusts: Cigarette smoke, Arizona road dust and Pollen

This Standard provides a uniform method to compare and evaluate different

brands of portable air cleaners

2. The Building Services Research and Information

Association, BSRIA Specification S22/99 Particle
Removal Rate (PRR) - This British Standard is similar to
American Standard ANSI/AHAM AC-1-2006.
Testing dust: Cigarette smoke
This evaluates the performance of the air cleaners under different dust
loadings. In addition, it also emphasizes the electrical power
consumption and the relative acoustical performance of the air
cleaners.
 Clean Air Delivery Rate (CADR)
Clean Air Delivery Rate (CADR) (in cubic feet per minute: cfm)
[1 cfm = 1.7 m3/hr] a concept in AHAM standard (Association of Home
Appliance Manufacturers)
CADR is a figure of merit that is the cfm of air that has had all the particles of a given size distribution
removed.

A relationship between the CADR and the filtration efficiency, , is

CADR = removal efficiency x airflow rate through the air cleaner
= x Qa

When a chamber is dosed with particles, the particle concentration will drop with
time due to particle deposition on the floor and other surfaces.
The rate of particle concentration drop can be characterized by a decay rate, k,
which can be obtained experimentally through the correlation between the
initial and final particle concentrations in the test chamber.
Ct=Ci e-kt
where Ct is the particle concentration at time t, Ci is the initial
particle concentration.
 CADR
The CADR indicates the volume of filtered air delivered by an air
cleaner. The higher the tobacco smoke, pollen and dust numbers,
the faster the unit filters the air. The AHAM Label (found on the
packaging of the air cleaner) lists three CADR numbers one for
tobacco smoke, one for pollen and one for dust.

First, find the AHAM suggested room size noted

prominently on the label. This suggested sizing
should match the size of the room you are trying to
clean.
Next, compare CADR numbers from manufacturer
to manufacturer and from air cleaner to air cleaner.
Higher ratings for the dust, tobacco smoke and
pollen CADR numbers indicate that the unit will filter
the air faster than a unit with lower ratings.
If the ratings are same from one unit to the other,
then the air filtering performance is similar.
 AHAM Calculation for CADR

C
Background Test

Ci

Air Cleaner Test

kn and ke have unit of 1/t

t
 AHAM Calculation for CADR
How to get kn and ke experimentally?

Plot vs t

The slope = – kn

Same procedure for getting ke

 AHAM Calculation for CADR

CADR = ηQa

ηQa = V(ke – kn)

Compare to CADR = V(ke – kn)

CADR = Volume of Test Chamber x

(total decay rate – natural decay rate)
 ANSI/AHAM AC-1-2006, Method for Measuring the
Performance of Portable Household Electric Room Air
Cleaners

Particulate Matter Particle Size Limits of

Used Range Measured Measurability (cfm)

Cigarette smoke 0.10 μm to 1.0 μm CADR: 10 450

Test dust (fine) 0.5 μm to 3.0 μm CADR: 10 400

Pollen (paper 5 μm to 11 μm CADR: 25 450

mulberry)
 Methodology Simplified Indoor
Particle Model (Natural Ventilation)

V, C

Exfiltration
Infiltration QC
PQCo G
Indoor source Air cleaner Surface deposition
emission G ηQaC QdC
where
P = penetration coefficient [dimensionless]
Q = volume flow rate [m3/s]
Co = outdoor particle concentration [mg/m3]
G = particle generation rate [mg/s]
η = removal efficiency of air cleaner [dimensionless]
Qa = volume flow rate of air cleaner [m3/s]
C = indoor particle concentration [mg/m3]
Qd = removal rate due to deposition onto surfaces [m3/s]
 Methodology Simplified Indoor
Particle Model (Natural Ventilation)

Mass balance between major indoor

particle generation and removal rates

dC
V PQC o QC Qd C Qa C G
dt

Unknown parameters: P, Qd,

Methodology Simplified Indoor Particle
Model (Natural Ventilation)

Using equation (1) and solving the equation for

steady-state condition (i.e. dC/dt = 0), it gives

PQC o G
C ss …(2)
Q Qd Qa
Methodology Simplified Indoor Particle
Model (Natural Ventilation)
For the case with no air cleaner
operating, Qa=0, equation (2)
becomes
PQCo G
Css ,without air cleaner
Q Qd
For the case with an air cleaner
operating and using the CADR
definition, equation (2) becomes
PQCo G
Css ,with air cleaner
Q Qd CADR
Results and Discussion Air
Cleaning Effectiveness
Air cleaning effectiveness, is
defined as
Css , without air cleaner Css , with air cleaner
Css , without air cleaner

CADR f CADR
CADR Q Qd f CADR 1

CADR
where f CADR removal ratio
Q Qd
 Results and Discussion Air
Cleaning Effectiveness

It has been used to examine acceptable values for

CADR!
The closer the effectiveness to 1, the more ideal the
performance of the air cleaner is in removal of the
particle!
 Results and Discussion Air
Cleaning Effectiveness
The S-curve of air cleaning effectiveness
1.20
ir cleaning effectiveness,

1.00

0.80

0.60

0.40

0.20

-
0.01 0.1 1 10 100

Removal ratio, fCADR

Results and Discussion Air
Cleaning Effectiveness

To the left:
The CADR performed by the air
cleaner is almost useless in that
environment

To the right:
CADR increase Cost increase
Hard to achieve further improvement
in removing particles
 Results and Discussion Air
Cleaning Effectiveness

The S-curve of air cleaning effectiveness

1.20
Optimum range
ir cleaning effectiveness,

1.00

Model II
Air conditioner

Model IV
0.80

Model III
Model I

Model V
0.60

0.40

0.20

-
0.01 0.1 1 10 100

Removal ratio, fCADR

Models on the market
 Simplified Indoor Particle Model
(Mechanical Ventilation)
QfCo

ηf = 0.9

QfC
(1-η)(QfCo+QrC) QrC

G ηQaC QdC

Air cleaner Surface deposition

where
G = particle generation rate [mg/s]
Co = outdoor particle concentration [mg/m3]
C = indoor particle concentration [mg/m3]
Qf = fresh air supply rate [m3/s]
Qr = recirculation air flow rate [m3/s]
ηf = filter efficiency of the ventilation system [dimensionless]
η = removal efficiency of air cleaner [dimensionless]
Qa = volume flow rate of air cleaner [m3/s]
Qd = removal rate due to deposition onto surfaces [m3/s]
 Simplified Indoor Particle Model
(Mechanical Ventilation)

Mass balance between major indoor

particle generation and removal rates

dC
V (1 f )(Q f C o Qr C ) Q f C Qr C Qa C Qd C G
dt

when steady state reached ,

(1 f )(Q f C o ) G
Css
Qf Qr Qa Qd (1 f )Qr
 Simplified Indoor Particle Model
(Mechanical Ventilation)
When there is no deposition and air
cleaner performance
Mass balance between major indoor
particle generation and removal rates
dC
V (1 f )(Q f C o Qr C ) Q f C Qr C G
dt

when steady state reached ,

(1 f )(Q f C o ) G
Css
Q f Qr (1 f )Qr
 The Various Deposition Terms
Difference between k (kn or ke), Qd and vd
k – particle decay rate (1/s)
Qd – removal rate due to deposition onto surfaces (m3/s)
vd – deposition velocity (m/s)
Recall equation (1) in the previous slide:

= PQC0 – QC – QdC – ηQaC + G

= PQC0 – QC – VkC – ηQaC + G

= PQC0 – QC – vdAC – ηQaC + G

Qd = Vk = vdA
where A = total surface area for particle deposition in the
room (m2)
 Example

Q: An office building is located in an area where the ambient

PM10 particulate level is 100 µg/m3. Fresh air supply rate to the
building is 10 L/s/person. Recirculation air is 10 times the fresh air
supply. The recirculation air is well mixed with the fresh air before
passing through the filter in the HVAC system. Filter efficiency of
the ventilation system is 85%. The indoor PM10 particulate
generation rate is 18 µg/sec/person. The office has a volume of
600 m3 and there are 10 occupants inside the office.

a) What is the steady state indoor PM10 level?

b) An air cleaner with a Clean Air Delivery Rate (CADR) of 600 m3/hr is
now used in the office. What is the steady state PM10 level after the air
cleaner is operated? After that, please also estimate the air cleaning
effectiveness?
c) Instead of using the air cleaner, what efficiency level of the filter
should we utilize in the ventilation system in order to reach the same
steady-state PM10 level as found in (b)?
a)What is the steady state indoor
PM10 level?
Solution:
Vol. = V = 600 m3;
Outdoor PM10 = Co = 100 µg/m3;
Fresh air supply = Qf = 10 L/s/person = 10 10 L/s = 0.1 m3/s = 360 m3/hr;
Recirculation air = Qr = 10 Qf = 1 m3/s = 3600 m3/hr;
Air supply after the filter = Qs = Qf + Qr = 1.1 m3/s = 3960 m3/hr;
Filter efficiency of the ventilation system = = 0.85;
Indoor PM10 generation rate = G = 18 10 = 180 /s
The following figure represents the control volume of the question. Assume the ai
is perfectly mixed in the room. By material balance equation,
dC
Qf V G (1 )(Q f C o Qr C ) Q f C Qr C
dt
dC
V G (1 )(Q f C o 10Q f C ) Q f C 10Q f C
dt
= 0.85
when steady state achieves,
0 G (1 )(Q f C o ) (1 )(10Q f C ss ) 11Q f C ss
Qs Qr G (1 )(Q f C o )
C ss
(1 )(10Q f ) 11Q f
Qf
G 180 (1 0.85)(0.1 100)
(1 0.85)(10 0.1) 11 0.1
191 g / m 3
b) An air cleaner with a Clean Air Delivery Rate (CADR) of 600 m3/hr
is now used in the office. What is the steady state PM10 level after
the air cleaner is operated?
Solution:
The removal of PM10 by the cleaner = CADR = c Qc , where the subscript c means the
cleaner
Cleaner CADR = 600 m3/hr = 0.167 m3/s
Qf
By material balance equation,
dC
V G (1 )(Q f C o Qr C ) Q f C Qr C c Qc C
dt = 0.85
dC
V G (1 )(Q f C o 10Q f C ) Q f C 10Q f C c Qc C
dt Qs Qr
Cleaner
when steady state achieves, Qf
G
0 G (1 )(Q f C o ) (1 )(10Q f C ss ) 11Q f Css c Qc C ss

G (1 )(Q f C o )
Css
(1 )(10Q f ) 11Q f Qc
c
Css , without air cleaner Css , with air cleaner
for the cleaner, Css , without air cleaner
180 (1 0.85)(0.1 100) 191 162
Css
(1 0.85)(10 0.1) 11 0.1 0.167 0.15
191
162 g / m 3
 c) Instead of using the air cleaner, what efficiency level of the filter
should we utilize in the ventilation system in order to reach the same
steady-state PM10 level as found in (b)?
By material balance equation,
dC
V G (1 )(Q f C o Qr C ) Q f C Qr C
dt
dC
V G (1 )(Q f C o 10Q f C ) Q f C 10Q f C
dt
when steady state achieves,
0 G (1 )(Q f C o ) (1 )(10Q f C ss ) 11Q f C ss
G 11Q f C ss
(1 )
(Q f C o ) (10Q f C ss )
G 11Q f C ss
1
(Q f C o ) (10Q f C ss )
180 11(0.1 162)
1 101 %
(0.1 100) (10 0.1 162)
It is impossible to upgrade the filter in the ventilation system in order to
reach the same steady-state PM10 level as found in (b).
 What is a Cleanroom?
A specially designed & constructed room in which the air supply,
air distribution, filtration of air supply, materials of construction,
and operating procedures are regulated to control airborne
particle concentrations to meet appropriate cleanliness levels.
Why have a Cleanroom?

First cleanrooms were in hospitals to prevent disease

transmission and infection in operating rooms (over
100 years ago!)
Valuable tool to prevent particulate and bio-
contamination
Most well known use is in semiconductor industry, but
also essential in pharmaceuticals, flat panel displays,
space program, photonics, life sciences, industrial
(painting, assembly), etc.
Essential for LCDs because of coating processes, small
cell gaps
The Cleanroom itself is only part of the solution
What is a Particle?

A particle is very small discrete mass of solid or liquid

matter, usually measured in microns ( m).
A micron is 1/1,000,000 of a meter.
A human hair: 60 90 m.
Visible particle: 50 m
Escherichia coli (a bacteria): 1 m
Sources of Contamination
Smoking Particle Generation
Associated Contamination
 Contamination due to
different activity level
Activity
m)
Sitting or standing with no 100,000
movement
Sitting or standing but with 500,000
minimal head, forearm and
hand movement
Sitting or standing with average 1,000,000
arm and body motion
Changing positons, such as 2,500,000
going from sitting to standing
Slow-speed walking (e.g. 2 5,000,000
miles/hr)
Average-speed walking (e.g. 7,500,000
3.57 miles/hr)
 How are Cleanrooms Classified ?

Maximum of 1000 particles of 0.5 micron.

Maximum of 7 particles of 5 micron perm
Maximum of 100 particles of 0.5 micron. Maximum no. of visible micro-organisms i
No particles of 5 micron permitted.
Maximum no. of visible micro-organisms is 1.
 Cleanroom Classification
The classes are a level of
airborne particulate cleanliness.
A Class 1000 means that less
than 1000 particles (0.5 microns
in size) are present per cubic
foot. The higher the class
number, the more the particles
present.
 Cleanroom Classification

Interested in Number Density, not Mass Concentration.

US Federal Standard 209D (Class 1, 10, 100, 1000, 10000, 100000)
ISO 14644-1-1999 (ISO 1, 2, 3, 4, 5, 6, 7, 8, 9)
Facility Parameters That Need to Be
Controlled
Temperature

Humidity
Air
Cleanliness
Room
Pressure
Air movement

Lighting
 Cleanroom Classification -
US Federal Standard 209D
In US, classes still referred to as defined by Federal Standard 209D

** In 1 cubic foot of volume

Cleanroom Classification - ISO 14644-1

Now superseded by ISO 14644-1; particles

measured per cubic meter
Typical Air Exchange Rates

ISO Class US Fed Std. 209E Air Change Rate (hr-1)

4 10 300 540
5 100 240 480
6 1,000 150 240
7 10,000 60 90
8 100,000 5 48
Methods to Achieve
Cleanliness
Positive Pressure/Airflow
Keeps contamination out of the work area
Filtration
HEPA (High Efficiency Particulate Air) and ULPA (Ultra Low
Particulate Air) Filters
Materials Selection
User Protocols
Two Major Types of Cleanrooms
Turbulently ventilated room (nonunidirectional)
Receiving clean filtered air through air diffuses in the ceiling. This air mixes with the
room air and removes airborne contamination through air extracts at the bottom
of the walls. The air changes are normally equal to, or greater than, 20 per hour, this
being much greater than that used in ordinary rooms, such as in offices. In this style
of cleanroom, the contamination generated by people and machinery is mixed
and diluted with the supply air and then removed.
Unidirectional flow (laminar flow)
High efficiency filters are installed across a whole ceiling (or wall in some systems)
and these supply air. This air sweeps across the room in a unidirectional way at a
speed of around 0.4 m/s and exits through the floor, thus removing the airborne
contamination from the room. This system uses more air than the turbulently
ventilated cleanroom but, because of the directed air movement, it minimises
the spread of contamination about the room and sweeps it out through the floor.
A superior cleanliness
Both have clean air on top, moving to the bottom.