NAFA Guide To Air Filtration Chapter 2 Principles of Air Flow, Air Pressure And Air Filtration presented by the National Air Filtration Association Fourth Edition

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 = 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” 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 Airflo w FILTERFILTER HighLow Highest Pressure Lowest Pressure High Low

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. 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 Fiber Airstream Larger particles do not move around the fiber with the airstream and are carried into the fiber due to their speed, weight and size. Particle Airflow

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 Particle Fiber Airflow 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.

Diffusion Airstream Particle Fiber Airflow –Explains the capture of very small particles. Brownian Motion – the erratic path taken by minute particles because they are bombarded they are bombarded by air (gas) molecules. This erratic path increases the probability that particles will come in contact with fibers.

Straining Airstream Fiber Particle Airflow Straining occurs when the smallest dimension of a dust particle is of a dust particle is greater than the distance between adjoining media fibers. adjoining media fibers.

 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. Adhesives – also known as Tackifiers

 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? ImpingementInterceptionDiffusionStraining

Sample Questions If Static Pressure goes up, Velocity Pressure goes? Down – SP+VP=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 VPSP TP

Thank you. Other Questions?

Electrostatically Charged Filter Media (active and passive)  2.Electrostatically charged filter media (passive) utilizes the flow of air across synthetic fibers to create an electrostatic charge. Electrostatically charge filter media (active) involves imparting a charge during manufacture using triboelectric, corona discharge or induction charging. Electrostatically charge filter media (active) involves imparting a charge during manufacture using triboelectric, corona discharge or induction charging. The advantage of charged filtration materials is that the charge on the fibers increases the filtration efficiency without affecting resistance to airflow.

Electronic Air Cleaners also called Electrostatic Precipitators 3.Electronic Air Cleaners are devices that utilize a first stage (ionizer section) to impose a positive charge on incoming dust particles and then collects them in a second stage (collector section) by attracting the charged particles to oppositely charged collector plates.

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.

 Because Interception and diffusion effects are complementary, it is an accepted practice to combine both under “interception”.  Interception poses a stronger influence with larger particles, whereas diffusion effects explain the capture of smaller particles. Interception Forces

Electrostatic Attraction Airstream Fiber Particles are pulled to the fiber due to electrostatic attraction (charge) of the fiber. That is opposite of the particle charge. This is called Dipole Moment. Particle Airflow - +

Van der Waals forces:  The forces of Molecular Attraction help keep a dust particle attached to a fiber.

Dipole Moment:  A particle tends to align itself so that the more negatively charged side of the particle is adjacent to the more positively charged sectors of the fiber.

Electronic Air Cleaners  Dr. Gaylord Penny of the Carnegie Institute of Technology determined that the generation of ozone could be nearly eliminated if electrostatic precipitation was done in a two-stage operation.  Electronic Air Cleaners also use lower voltage than the earlier stack-gas cleaning electrostatic precipitators.

Electronic Air Cleaners  Found in Chapter 6 of the NAFA Guide to Air Filtration (Fourth Edition).  Will be covered in more detail in a later session.