1. 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

2. Principles of Airflow, Air Pressure and Air Filtration
Cleaning the air is a combination of understanding airflow, air pressure and air filtration
Air filter professionals have to understand how air flows through a filter, how the pressure determines flow and the principles of air filtration all interact to clean the air.

3. 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
(explain formula)

4. 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 = fpm
The formula for flow through a filter is Q=VA. You need to know any two factors to solve for the third one. (Give Example) This is important because air filters are tested to a specific efficiency at a give cfm. Installing filters in a system where the velocity through any 1 sq. ft. changes over the tested velocity will affect pressure drop and will affect efficiency of the filter. For example, given the same dimensions, a 6-pocket filter will not perform as efficiently as a 8-pocket filter if the system is designed around an 8-pocket filter. Efficiency is based on the most penetrating sized particle (MPSP)

5. 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
Slide is self explanatory…be sure candidates understand total pressure is static pressure (including pressure drop of a filter) plus velocity pressure and that they act as a teeter tooter - when static pressure goes up, velocity pressure goes down and vice versa. Most systems are designed to operate at a specified pressure and changing the parameters changes the operating characteristics of the system.

6. Air Pressure – TP=SP + VP
Velocity Pressure (VP) is only measured in the direction of flow and is always positive. VP
Called a Pitot (pronounced pee-tow) tube
Both Static and Velocity Pressure can be measured with an instrument called a Pitot tube

7. Filter Pressure Drop Low High High Low Lowest Pressure Highest
Airflow F I L T E R
Highest
Pressure
The highest pressure in an HVAC system is at the blower discharge – the lowest pressure is at the blower intake. Because the air downstream of the filter is of lower pressure than that above, we call the resistance the filter adds to the system, “pressure drop.”

8. 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
Chart shows as pressure goes up, air flow goes down…this demands that a filter be correctly installed in a system. The example to keep in mind here is that reduction in airflow results in less circulation of air. In other words, you are not moving the air as frequently in and out of the space hence particle contaminants are not being captured and the fan has to run longer to satisfy the thermostat (increased energy consumption).

9. 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

10. 3 Categories
1. Mechanical air filters - remove dust by capturing it on the filter media, the material that comprises the filter element.
Slide is self explanatory
© 2004 Kimberly-Clark Worldwide, Roswell, GA All Rights Reserved.

11. 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.
Slide is self-explanatory

12. Air Filtration Principles
There are four different principles responsible for the capture of dust in a mechanical filter.
Impingement
Interception
Diffusion
Straining

13. Impingement Larger particles do not move around the fiber with the
airstream and are
carried into the
fiber due to their
speed, weight
and size.
Airflow
Particle
Fiber
Airstream
By a show of hands, how many have an SUV? Let’s put you are in the driver’s seat, the accelerator is strapped at wide open and you are headed toward a forest. You’ll probably miss the first tree, and possibly the second, however the momentum of the car will ultimately hit a tree. Same with big particles traveling at high rates of speed. The airstream will flow around the fiber, but the particle, with more mass, will exit the airstream and impact itself on a fiber.
Airflow 3

14. 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
Dust Particle Size – larger the particle, greater the inertia. Density of the Dust – heavier the particle the greater the mass and more inertia. Depth of the filter – the thicker the media, the greater the number of times a particle will be exposed to the probability of capture.

15. 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.
Fiber
Airflow
Particle
Some particles just pass close enough to the fiber that they are attached and held by van der Waal’s forces. These are same charges that allow you to rub a balloon against your hair and stick it to a wall. These are weak electrical charges from the particle and caused by the air moving across the fibers of the filter. Particles cling to the fiber with van der Waal’s forces until another force that is stronger dislodges them. This fact can be proved when you next change your filter. Knock it against the floor and thousands of particles will fall to the floor. The same is true if the particle is overcome by drag forces of the airstream. 5

16. Diffusion Explains the capture of very small particles.
Fiber
Airflow
Brownian Motion –
the erratic path
taken by minute
particles because
they are bombarded
by other air (gas)
molecules.
Airstream
Particle
Smaller particles moving at slower speeds are captured because they are literally pushed against the fiber by gas molecules in the air. This mechanism is known as Brownian Movement and you may have seen this as you looked through a microscope in high school biology – the “vibration like” movement of small objects as they are pushed upon by other forces.
This erratic path increases the probability that particles
will come in contact with fibers. 4

17. Straining Straining occurs when the smallest dimension
of a dust particle is
greater than the
distance between
adjoining media fibers.
Airflow
Particle
Fiber
Airstream
A basic concept is that the functional part of a filter is the media and that media is made up of fibers, be they glass, synthetic, metallic, ceramic, etc. Straining is indeed a mechanism of filtration, but only acts on the largest particles in the air – for instance, leaves, lint, and droplets.
Airflow

18. Graph of Principles
This graph shows the relationship between Diffusion, Interception and Impingement. Diffusion is higher at lower velocities with smaller particles, impingement is higher at higher velocities with larger particles and Interception is fairly constant with velocities and is particle and fiber size dependant.

19. 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
To increase the ability of a particle to adhere to a fiber, impingement filters are treaded with an adhesive coating sometimes called a “tackifier.” This does not increase the efficiency of a filter, just the ability of the particle to remain on the fiber.

20. 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
Van der Waal’s forces apply to a gas molecules. His experiments compressed a cylinder to ½ its capacity and he thought the pressure would double. It did not because the molecules realigned their charge surfaces the same as they do when attaching to a filter, attaching the positively charged molecules to the negatively charged molecules.
Dipole Moment occurs when there is a greater concentration of electrons a one point or end of a molecule than there is at another. This causes the particle to align based on the positive charge of the particle. Electrostatic attraction occurs when particles or fibers have a charge based on air movement over a type of material (passive) or a charge is imparted to the fiber during manufacture (active) either through triboelectric, corona discharge or induction.

21. 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
Slide is self-explanatory

22. 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”
Most all filters develop some kind of “passive electrostatic” charge due to air blowing across the fiber. This is the same as your feet walking across a rug, picking up a static charge.

23. 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.
Slide explains active charged media types

24. 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.
The difference between single-stage ESP’s and those used in HVAC systems and free-standing models is the large amount of ozone produced by the single stage precipatators.

25. Two-Stage Electronic Air Cleaner
Collector sections
are removable
and can be
washed for
reuse
Discussed more
In Chapter 6
Duel stage units use a separate ionizer section and collector section to capture particles. This type of system will be more fully discussed in the presentation on “Aior Cleaners” in Chapter 6.

26. 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

27. 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 VP SP TP

28. Thank you.
Other Questions?