Transcript Principles of Airflow, Air Pressure and Air Filtration

NAFA Guide To Air Filtration
Fourth Edition
Chapter 2
Principles of Air Flow, Air Pressure
And Air Filtration
presented by the
National Air Filtration Association
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 in it 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”
Velocity Pressure (VP)
is only measured in the
direction of flow and is always positive.
VP
Both are 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.
Because filters add pressure
into the system, increasing
in filter pressure drop
decreases air flow velocity
Three types of air filter
Categories
1. Mechanical air filters - remove dust by
capturing it on the filter media, the material that
comprises the filter element.
This 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.
Mechanical Air Filters.
 There are four different process 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 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.
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 which
may hit them.
 The strength of the bond between the fiber
and the particle depends on several forces
of attraction:
– van der Waals forces
– Dipole moment
– Electrostatic attraction
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
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 equipment could not be used in
air conditioning systems because of the
concentration of ozone formed by the
electrostatic field.
Electronic Air Cleaner
Collector sections
are removable
and can be
washed for
reuse
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?