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 - 2014 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 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.” SP is independent of the velocity of the air and can be either negative or positive Air Pressure – TP=SP + VP Velocity Pressure (VP) is only measured in the direction of flow and is always positive. VP Both Static and Velocity Pressure can be 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. TP = SP + VP Because filters add resistance to flow into the system, increasing filter pressure drop decreases air flow velocity 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 3 Categories 1. Mechanical air filters - remove dust by capturing it on the filter media, the material that comprises the filter element. © 2004 Kimberly-Clark Worldwide, Roswell, GA 30076. All Rights Reserved. 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. Air Filtration Principles There are four different principles 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 other 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. Graph of Principles 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 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 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 (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. Two-Stage Electronic Air Cleaner Collector sections are removable and can be washed for reuse Discussed more In Chapter 6 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?
