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?