Uploaded by agnbandara

NAFA Guide to Air Filtration 2

advertisement
NAFA Guide To Air Filtration
Fifth Edition
Chapter 2
Principles of Air Flow, Air Pressure
And Air Filtration
presented by the
National Air Filtration Association - 2014
Principles of Airflow, Air
Pressure and Air Filtration
Cleaning the air is a combination
of understanding airflow, air
pressure and air filtration
Principles of Air Flow
 Quantity of air is expressed in cubic feet per
minute (CFM) and is represented by the
letter Q
 The velocity of air is expressed in feet per
minute and is represented by the letter V
 The area of a filter is expressed in square
feet and is represented by the letter A
Formula for Airflow through a
filter is Q=VA
You need to know any two to find the other
factor, therefore:
V= Q/A
A=Q/V
Example: a 24 x 24 x 12 filter with 60 sq. ft. of
media placed in a duct with 2000 cfm will
have ??? fpm through any 1 sq. ft.?
Answer = 33.33 fpm
Air Pressure – TP=SP + VP
Static Pressure (SP) in a duct exerts itself in
all directions at once. Think of it
as “bursting pressure.” SP is independent of
the velocity of the air and can be either
negative or positive
Air Pressure – TP=SP + VP
Velocity Pressure (VP) is only measured in
the direction of flow and is always positive.
VP
Both Static and Velocity Pressure can be measured with
an instrument called a Pitot tube
Filter Pressure Drop
Low
High
High
Airflow
F
I
L
T
E
R
Lowest
Pressure
Highest
Pressure
Affect of Increased Filter Pressure
Drop
Velocity Pressure plus Static
Pressure combine to give
total pressure (TP) in a
system.
TP = SP + VP
Because filters add resistance
to flow into the system,
increasing filter pressure drop
decreases air flow velocity
Principles of Particulate Air
Filtration
 Mechanical - remove dust by capturing it
on the filter media, the material that
comprises the filter element.
 Electrostatically Charged Filter
Media –(passive and active) increases
efficiency by positive/negative particle
charge attraction of fibers
 Electronic Air Cleaners - (two-stage)
external power source imposes charge on
particle
3 Categories
1. Mechanical air filters - remove dust by
capturing it on the filter media, the material that
comprises the filter element.
© 2004 Kimberly-Clark Worldwide, Roswell,
GA 30076. All Rights Reserved.
Mechanical Air Filters.
Capture involves two different
considerations.
–The probability that one of the dust
particles will collide with one of the
media fibers.
–The probability that the particle, once
contacting the filter fiber, will
continue to adhere to it.
Air Filtration Principles
 There
are
four
different
principles
responsible for the capture of dust in a
mechanical filter.
– Impingement
– Interception
– Diffusion
– Straining
Impingement
Airflow
Particle
Airstream
Fiber
Airflow
Larger particles
do not move around
the fiber with the
airstream and are
carried into the
fiber due to their
speed, weight
and size.
The effectiveness of the impingement
process depends on the following:
Dust particle size
Density of the dust
Depth of the filter
Velocity of the airflow
Interception
Interception occurs when a dust particle follows the air
streamlines, but still comes in contact with the fiber as it passes
around it. Interception is fiber-size dependent and is enhanced
when the size of the fiber is closest to the size of the particle.
Particle
Airflow
Fiber
Airflow
Diffusion
–Explains the capture of very small particles.
Airflow
Airstream
Particle
Fiber
Airflow
Brownian Motion –
the erratic path
taken by minute
particles because
they are bombarded
by other air (gas)
molecules.
This erratic path increases the probability that particles
will come in contact with fibers.
Straining
Airflow
Airstream
Particle
Fiber
Airflow
Straining occurs when
the smallest dimension
of a dust particle is
greater than the
distance between
adjoining media fibers.
Graph of Principles
Adhesives – also known as
Tackifiers
Impingement filters are frequently
treated with adhesives that coat the
fibers and create a bond between
them and any dust particles that
may impinge upon the fiber.
This helps prevent the particle from
being dislodged due to velocity or
vibration
 The strength of the bond between the fiber
and the particle depends on several forces
of attraction:
– van der Waals forces
weak electrical forces
– Dipole moment
greater concentration of
Electrons at one point or
end of a molecule
– Electrostatic attraction
Naturally occurring or induced
during manufacture
3 Factors Affecting Mechanical
Filter Selection
 Efficiency – number and size of challenge
particles removed from airstream
(Discussed completely in Chapter 7)
 Pressure Drop – resistance to airflow
created by an air filter
 Capacity – amount of air specified for a filter
– usually expressed in cfm – affects both
pressure drop and efficiency
Electrostatically Charged
Media
 Electrostatic attraction:
– It is also possible for filter media to become
electrostatically charged by a variety of
methods, one of which is the flow of air
(especially dry air) through it.
– “Passive electrostatic filters”
Charged Fiber Technology –
(Active) also called Electret Media
 Triboelectric charging – results from the rubbing
together of dissimilar polymers.
 Corona Charging – involves exposing fibers to
an electrode designed to create high voltage
either positive or negative.
 Charging by induction – charging the material
while extruding it from a molten state.
Electronic Air Cleaners
(two-stage)
Electrostatic Precipitation
 The principle of electrostatic precipitation as
a means of stack gas cleaning goes back to
the earliest part of the twentieth century.
This type of ESP was a single stage type
and could not be used in air conditioning
systems because of the concentration of
ozone formed by the electrostatic field.
Two-Stage Electronic Air
Cleaner
Collector sections
are removable
and can be
washed for
reuse
Discussed more
In Chapter 6
Sample Questions
Rated flow of air through a filter at specified
pressure drop is called it’s?
Capacity
The four principle of air filtration are?
Impingement
Interception
Diffusion
Straining
Sample Questions
If Static Pressure goes up, Velocity Pressure
goes?
VP
SP
Down – SP+VP=TP
TP
In the filter industry, a manometer is used to
measure?
Pressure drop across a filter
An Electronic Air Cleaner collects particles
by?
Electrostatically charging them and collecting
them on oppositely charged plates
Thank you.
Other Questions?
Download