Impingement and Extended Surface Filters Jason Henderly, CAFS, NCT, Quality Filters, Inc.

Chapter 3: Impingement Filters Impingement- The process in which particles are removed from an air stream because of their inertia. As air containing a particle flows toward a filter fiber or other collecting surface, the particle does not follow the air streamlines because of it’s inertia. Instead it moves in a straight line colliding with the filter fiber or surface to which it may become attached. Mechanical filters whose principal mechanism of dust capture is impingement.

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

Impingement Filter Categories Panel Filters Roll Filters Lint Filters Washable Metal-Panel Filters

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

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

Panel Filters: Materials of Construction Fiberglass Synthetic Fibers Foam

Fiberglass Used extensively in Impingement Filters Usually adhesive coated to improve retention Media density varies Can be used in pads or cardboard frames

Synthetic Fibers Polyester- Most popular material Polypropylene Nylon Modacrylic

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 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

Filter Holding Systems Metal Holding Frames Ring Panels Side Service Housings

Roll Filters 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: Media Advance and Control Automatic roll filters use a motor driven system. Roll filters use a manually operated chain drive. Automatic roll filter systems are usually controlled by an adjustable timer.

Roll Filter Media Fiberglass Synthetic fibers

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. Aircon Filter Mfg. Co.

Chapter 3 Questions

Chapter 4: Extended Surface Filters The most common method of increasing media area is to pleat the filter media to extend the filter depth. Extended surface filters have a media area that is greater than the filter face area. Media velocity is less than face velocity.

Formula for Figuring Media Velocity Q = VA V = Q/A A = Q/V Where: Q = cubic feet per minute (cfm) V = velocity in feet per minute (fpm) A = Area in square feet

Example If our system is a 24” x 24” bank with one filter that sees 2,000 (Q) and has media area (A) equal to 30 square feet, what is our Velocity through any one square foot?

Example V = Q/A = 2000 divided by 30 = 66.66 fpm Know this formula for the exam!

Extended Surface Filters Are Desirable Because: Use Interception and Diffusion Principles Media may perform more efficiently at lower media velocities. Media resistance may be too high when the material is used in a flat sheet. Media resistance is, as a general rule, directly proportional to the velocity of air through it. Filter life will be extended.

Types of Extended Surface Filters Extended surface filters are characterized by the type of media used and the configuration into which it is formed.

Filters Using Mat Type Media Non Supported pocket (bag) Supported Cartridge Rigid Box Pleated panel

Glass Mat Type Media Glass mat is composed of very fine glass microfibers – see page 4.2 for fiber thickness Requires a backing material to provide physical support. The efficiencies of mats of different thicknesses are essentially the same. Thinner mats will have lower pressure drops than thicker mats.

Synthetic Fiber Mat Type Media Available in efficiencies comparable to glass media. Made from various synthetic materials. Have greater physical strength than glass mats. May be heat sealed.

Standard Filter Mat Colors and Efficiencies (Glass Media) Yellow 90-95% Pink 80-85% Orange 55-65% Tan 50-55% Synthetics have no standard color

Non –Supported Pocket Filters (Bags) Pockets expand, exposing all of the media in the bag to the air stream A “veed” configuration is created by sewing stitching between the sides of the pockets to prevent ballooning. Results in a lower pressure drop more effective loading.

Rigid Box Filters Used where in line space is restricted Consist of high efficiency lofted media with a welded wire or expanded metal backing. Pleat spacing and support are maintained by stabilizers on the upstream and down stream sides of the pleats.

Pleated Panel Filters Most use cotton/polyester blend or all synthetic media. Media is supported with a expanded metal or welded wire grid bonded to the downstream side. Have higher efficiencies than glass or plastic mat panel filters while maintaining acceptable pressure drops.

Rigid Style Filters Most “wet laid” media used in air filtration are made by mixing a slurry of water, filter microglass fibers, and suitable binders. Wet laid media usually has higher ΔP than lofted. This can be overcome by adding more media for example the mini-pleated “V” style. Are used when in-line space is restricted Filter fibers used in Rigid-style can be glass, synthetic, or natural cellulose materials. Efficiencies can range from 40 to 98%.

Cell Style Filters Cell-Style Filters-Fabricated by pleating the filter media over corrugated aluminum separators to form a filter pack that is sealed into a light-gauge metal frame. Mini-pleats-Fabricated by pleating the filter media into a filter pack using string, ribbons, adhesive beads, or other methods to maintain pleat spacing. The media pack is sealed into a frame to form rigid panels.

Filter Installations The higher the efficiency the better the seal must be to prevent by-pass. Holding frames must be properly gasketed. Clamps must provide a tight seal between the filter face and the gasket. Filter banks must be suitable reinforced to provide rigidity.

Filter Performance Determined from independent filter testing ASHRAE standard 52.1 ASHRAE standard 52.2 (MERV)

Chapter 4 Questions