NAFA Guide To Air Filtration

NAFA Guide To Air Filtration Chapter 3 Impingement Filters presented by the National Air Filtration Association Fifth Edition

Impingement Filters • Impingement- The process in which larger particles are removed from an air stream because of their (weight or size) inertia. • Mechanical filters whose principal mechanism of dust capture is impingement.

Impingement Filters • Most impingement filters are characterized by relatively high media velocities and by low pressure drops.

Impingement Filter Categories • Panel Filters – fiberglass & synthetic • Washable Metal-Panel Filters • RollFilters • Lint Filters

Panel Filters • Fixed-media air filters in which the face velocity and the media velocity are essentially the same. Filter

Panel Filters • Face velocity-the speed of the air as it approaches the filter face. • Media velocity-the speed of the air as it moves through the filter media.

Panel Filters: Materials of Construction • Metals • Spun-glass fiber media • Synthetic Fibers • Foam

Metal Washable Filters

Spun-glass media • Used extensively in Impingement Filters • Usually adhesive coated to improve retention • Media density varies ¼” to 5” • Can be used in pads or cardboard frames

Synthetic Fibers • Polyester- Most popular material – ring and link panels, pocket filters • Also Polypropylene, Nylon or other synthetic (usually meant to be washed)

Synthetic Fibers • Fiber weight is measured in denier. • As a rule smaller denier fibers will do a better job capturing small dust particles • Dual denier media is commonly used. It incorporates a larger fiber on the air entry side and a smaller fiber on the exit side.

Characteristics of Synthetic Fiber Media • Fiber distribution consistency is the most important consideration when making synthetic media. • Tackifiers are commonly applied to the air exit side of some synthetic medias to prevent dust migration. • Electrostatic charge is present in some types of synthetic media to aid in particle capture.

Foam • Usually polyurethane • Classified by pore size • The more pores per inch (PPI) the higher the “efficiency” • Can be washed and reused

Impingement Filter Media Performance • Determined from independent filter testing • ASHRAE standard 52.1 (now retired) • ANSI/ASHRAE Standard 52.2 (MERV)

Method of Renewal • Filter banks using panel filters are renewed by washing or replacing dirty filter media. • The use of disposable materials has become the predominant method of renewal.

Filter Holding Systems

Devices in which clean filter media is unrolled at one end,exposed to a dirty air stream, and advanced at intervals to keep the pressure drop through the exposed air stream within a desired operating range. Roll Filters

Roll Filters: Media Advance and Control • A motor driven system. • Manually operated chain drive. • Can be controlled by an adjustable timer.

Roll Filter Media • Fiberglass roll (infrequent) • Synthetic fiber rolls

Lint Filters • Use the automatic roll filter principle. • Filter material • Glass media with spun-bond backing • Thin 1,2, or 3 ply cellulose media

Grease Filters • Devices installed in kitchen range hoods to prevent the accumulation of grease in hood exhaust ducts. • Usually made of steel or aluminum • Grease filters do not aid in smoke removal. • They can be washed. • Must meet NFPA Std. 96

Sample Questions Impingement filters are characterized by ________________ and _____________ Relatively high velocities and low pressure drop The velocity of the air through any one square foot of media in an extended surface filter is called? Media Velocity

Sample Questions Having larger fibers on the air entering side and smaller fibers on the air exit side is called? Graduated density Baffle grease filters must meet the requirements of NFPA Standard? 96

Thank you. Other Questions

NAFA Guide To Air Filtration