nafa guide to air filtration chapter 2 principles of air flow, air pressure and air filtration...
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NAFA Guide To Air FiltrationNAFA Guide To Air Filtration
Chapter 2Chapter 2
Principles of Air Flow, Air PressurePrinciples of Air Flow, Air Pressure
And Air FiltrationAnd Air Filtrationpresented by thepresented by the
National Air Filtration AssociationNational Air Filtration Association
Fourth Edition
Principles of Airflow, Air Pressure Principles of Airflow, Air Pressure and Air Filtrationand Air Filtration
Cleaning the air is a combination Cleaning the air is a combination of understanding airflow, air of understanding airflow, air pressure and air filtrationpressure and air filtration
Principles of Air FlowPrinciples of Air Flow Quantity of air is expressed in cubic feet per Quantity of air is expressed in cubic feet per
minute (CFM) and is represented by the minute (CFM) and is represented by the letter Qletter Q
The velocity of air is expressed in feet per The velocity of air is expressed in feet per minute and is represented by the letter Vminute and is represented by the letter V
The area of a filter is expressed in square The area of a filter is expressed in square feet and is represented by the letter Afeet and is represented by the letter A
Formula for Airflow through a filter is Formula for Airflow through a filter is Q=VAQ=VA
You need to know any two to find the other You need to know any two to find the other factor, therefore:factor, therefore:
V= Q/AV= Q/A
A=Q/VA=Q/V
Example: a 24 x 24 x 12 filter with 60 sq. ft. of Example: a 24 x 24 x 12 filter with 60 sq. ft. of media in it placed in a duct with 2000 cfm media in it placed in a duct with 2000 cfm will have ??? fpm through any 1 sq. ft.?will have ??? fpm through any 1 sq. ft.?
Answer = 33.33 fpmAnswer = 33.33 fpm
Air Pressure – TP=SP + VPAir Pressure – TP=SP + VP
Static Pressure (SP) in a duct exerts itself in Static Pressure (SP) in a duct exerts itself in all directions at once. Think of itall directions at once. Think of it
as “bursting pressure” as “bursting pressure”
Velocity Pressure (VP) Velocity Pressure (VP)
is only measured in the is only measured in the
direction of flow and is direction of flow and is always positivealways positive..
VP
Both are measured with an instrument called a Pitot tube
Filter Pressure DropFilter Pressure Drop
Airflow
FILTER
High Low
HighestPressure
Lowest Pressure
High Low
Affect of Increased Filter Pressure Affect of Increased Filter Pressure DropDrop
Velocity Pressure plus Static Velocity Pressure plus Static Pressure combine to give Pressure combine to give total pressure (TP) in a total pressure (TP) in a system.system.
Because filters add pressure Because filters add pressure into the system, increasing into the system, increasing in filter pressure drop in filter pressure drop decreases air flow velocitydecreases air flow velocity
Three types of air filter Three types of air filter CategoriesCategories
1.1. Mechanical air filtersMechanical air filters - remove dust by - remove dust by capturing it on the filter media, the material that capturing it on the filter media, the material that comprises the filter element.comprises the filter element.
This capture involves two different considerations.This capture involves two different considerations.– The probability that one of the dust particles will The probability that one of the dust particles will
collide with one of the media fibers.collide with one of the media fibers.– The probability that the particle, once contacting the The probability that the particle, once contacting the
filter fiber, will continue to adhere to it.filter fiber, will continue to adhere to it.
Mechanical Air Filters.Mechanical Air Filters.
There are four different process responsible There are four different process responsible for the capture of dust in a mechanical filter.for the capture of dust in a mechanical filter.– ImpingementImpingement
– Interception Interception
– DiffusionDiffusion
– StrainingStraining
ImpingementImpingement
FiberAirstream
Larger particlesdo not move around the fiber with theairstream and arecarried into thefiber due to theirspeed, weightand size.
Particle
Airflow
Airflow
The effectiveness of the impingement process The effectiveness of the impingement process depends on the following:depends on the following:
Dust particle sizeDust particle size Density of the dustDensity of the dust Depth of the filterDepth of the filter
Velocity of the airflowVelocity of the airflow
InterceptionInterception
Particle
Fiber
Airflow
Airflow
Interception occurs when a dust particle follows the air Interception occurs when a dust particle follows the air
streamlines, but still comes in contact with the fiber as it passes streamlines, but still comes in contact with the fiber as it passes
around it. Interception is fiber-size dependent and is enhanced around it. Interception is fiber-size dependent and is enhanced
when the size of the fiber is closest to the size of the particle.when the size of the fiber is closest to the size of the particle.
DiffusionDiffusion
Airstream Particle Fiber
Airflow
Airflow
–Explains the capture of very small particles.Explains the capture of very small particles.
Brownian MotionBrownian Motion – –
the erratic path the erratic path
taken by minute taken by minute
particles becauseparticles because
they are bombarded they are bombarded
by air (gas) molecules.by air (gas) molecules.
This erratic path increases the probability that particles This erratic path increases the probability that particles will come in contact with fibers.will come in contact with fibers.
StrainingStraining
Airstream FiberParticle
Airflow
AirflowStraining occurs when Straining occurs when
the smallest dimensionthe smallest dimension
of a dust particle is of a dust particle is
greater than the greater than the
distance betweendistance between
adjoining media fibers.adjoining media fibers.
Impingement filters are frequently Impingement filters are frequently treated with adhesives that coat the treated with adhesives that coat the fibers and create a bond between fibers and create a bond between them and any dust particles which them and any dust particles which may hit them.may hit them.
Adhesives – also known as Adhesives – also known as TackifiersTackifiers
The strength of the bond between the fiber The strength of the bond between the fiber and the particle depends on several forces and the particle depends on several forces of attraction:of attraction:– van der Waals forcesvan der Waals forces
– Dipole momentDipole moment
– Electrostatic attractionElectrostatic attraction
3 Factors Affecting Mechanical Filter 3 Factors Affecting Mechanical Filter SelectionSelection
EfficiencyEfficiency – number and size of challenge – number and size of challenge particles removed from airstream particles removed from airstream (Discussed completely in Chapter 7)(Discussed completely in Chapter 7)
Pressure DropPressure Drop – resistance to airflow – resistance to airflow created by an air filtercreated by an air filter
CapacityCapacity – amount of air specified for a filter – amount of air specified for a filter – usually expressed in cfm – affects both – usually expressed in cfm – affects both pressure drop and efficiencypressure drop and efficiency
Electrostatically Charged MediaElectrostatically Charged Media
Electrostatic attraction:Electrostatic attraction:– It is also possible for filter media to become It is also possible for filter media to become
electrostatically charged by a variety of electrostatically charged by a variety of methods, one of which is the flow of air methods, one of which is the flow of air (especially dry air) through it.(especially dry air) through it.
– ““Passive electrostatic filters”Passive electrostatic filters”
Charged Fiber Technology – (Active) Charged Fiber Technology – (Active) also called Electret Mediaalso called Electret Media
Triboelectric charging – results from the rubbing Triboelectric charging – results from the rubbing together of dissimilar polymers.together of dissimilar polymers.
Corona Charging – involves exposing fibers to Corona Charging – involves exposing fibers to an electrode designed to create high voltage an electrode designed to create high voltage either positive or negative.either positive or negative.
Charging by induction – charging the material Charging by induction – charging the material while extruding it from a molten state.while extruding it from a molten state.
Electronic Air CleanersElectronic Air CleanersElectrostatic PrecipitationElectrostatic Precipitation
The principle of electrostatic precipitation as The principle of electrostatic precipitation as a means of stack gas cleaning goes back to a means of stack gas cleaning goes back to the earliest part of the twentieth century.the earliest part of the twentieth century.
This type of equipment could not be used in This type of equipment could not be used in air conditioning systems because of the air conditioning systems because of the concentration of ozone formed by the concentration of ozone formed by the electrostatic field.electrostatic field.
Electronic Air CleanerElectronic Air Cleaner
Collector sectionsCollector sections
are removableare removable
and can be and can be
washed for washed for
reusereuse
Sample QuestionsSample Questions
Rated flow of air through a filter at specified Rated flow of air through a filter at specified pressure drop is called it’s?pressure drop is called it’s?
CapacityCapacityThe four principle of air filtration are?The four principle of air filtration are?ImpingementImpingementInterceptionInterceptionDiffusionDiffusionStrainingStraining
Sample QuestionsSample QuestionsIf Static Pressure goes up, Velocity Pressure If Static Pressure goes up, Velocity Pressure
goes?goes?Down – SP+VP=TP Down – SP+VP=TP
In the filter industry, a manometer is used to In the filter industry, a manometer is used to measure?measure?
Pressure drop across a filterPressure drop across a filterAn Electronic Air Cleaner collects particles An Electronic Air Cleaner collects particles
by?by?Electrostatically charging them and collecting Electrostatically charging them and collecting
them on oppositely charged platesthem on oppositely charged plates
VP SP
TP
Thank you.Thank you.
Other Questions?Other Questions?