What is Element Type?

Element type refers to the basic configuration of the filter element. Filter element types are designed to fit into specific filter vessels or housings. Most filter element types can be identified as, bag, cartridge, coalescer, disc, pad, panel, spin on, or strainer.

In many instances, an existing filter housing installed within a manufacturing plant or facility dictates the choice of element type. In the absence of an existing installation, choice of element type is generally governed by convention to industry standards or common practice. In such instances the filter element type is matched to a vessel and both are purchased together.

Examples of element type conventions follow:

When the option exists for choosing amongst the various element types available, a number of factors must be considered. Considerations include:

• Cost
• Both initial and ongoing element replacement cost
• Filtration efficiency
• Dirt capacity or expected filter life
• Flow capacity
• Maintenance frequency and ease of service
• Inventory requirements
• Disposal cost and handling
• Familiarity by plant personnel with format

Typically, there is little crossover in employing one format over another. For example, you would not use a panel filter for filtering a hydraulic fluid, nor would a coalescer be used in a sterile fluid final filtration application. Nevertheless, there are instances where, in the absences of compatibility and efficiency concerns, multiple element types can be used. Choice is then determined by flow capacity and the nature of the contaminant being removed. Bag filters for example provide great flow capacity and significant surface area for long life, while cartridge filters can provide comparable flow (assuming sized properly) with finer particle removal capability.

Bag
Bag filters are single open end (SOE) filter elements in which the fluid enters the bag though the opening at the top and is filtered by passing through the sides and bottom of the bag. Filter bags are available in a wide range of filter media including polypropylene, polyester, nylon, PTFE, and more in micron ratings from .5 through 800.

Bags have traditionally been chosen because they offer low-pressure drop and can accommodate high flow rates. Melt blown pleated and depth bags developed over the last ten years offer higher dirt holding capacity and higher efficiency than standard felt bags. Bags are an excellent choice for batch applications, like paints, inks and coatings, where speed of change out, easy disposal and economy are important factors.

Cartridge
Most filter cartridges are cylindrical with an outside diameter and an inside diameter. Typically, the fluid flows from the outside of the filter cartridge through the filter media and into the core. The filtered fluid then exits the core through the housing outlet.

Cartridges are available in a wide range of filter media, formats, sealing mechanisms and micron ratings.

In most process industries double open end (DOE) filter cartridges are used for particle removal of 1 micron and above. Single open end (SOE) cartridges are typically used in critical applications calling for filtration below 1 micron.

Solid cartridges in which the fluid passes through a dense tortuous path are called depth cartridges. Surface filter cartridges use a thin layer of filter media to trap contaminant. Surface filters can be pleated to increase the amount of surface area available to hold contaminant. Depth cartridges are considered more effective on gelatinous contaminants while pleated surface filters hold a greater amount of hard particles.

Coalescer
Coalescers separate one fluid from another fluid by intercepting and coalescing droplets of the suspended fluid while allowing the other fluid to pass through the media. For example separating water from oil. The drops of intercepted fluid typically coalesce into large enough droplets to allow gravity separation.

Coalescers are typically used in fuels to remove moisture and the refining industry to separate moisture from gases.

Coalescers are typically cartridges and have particle removal capability; however, a filter should be installed ahead of the coalescer to prevent particulate from fouling the coalescer media and reducing its separation capabilities.

Disc
Filter discs are typically used in laboratory applications. They are small surface filters designed to separate small amounts of solids from liquids. Some larger discs (293 mm) are used in pilot scale processing applications for clarifying purposes.

Cellulose, Nylon, polysulfone, polypropylene and PTFE are the most popular disc materials. Discs are available absolute rated and nominally rated.

The largest users are in the pharmaceutical and biotech industries and in quality control laboratories. Discs are small enough to be inserted under a microscope so contaminant can be examined.

Pad
Filter pads are available in a huge variety of shapes and designed to fit into filter presses and vertical stacked disc filter housings. Pads are nominally rated and designed to be inexpensive enough to be used to remove large amounts of contaminants. The food industry and metal working industry where the solids levels are very high are good applications for filter pads.

By layering the pads, a depth effect is created which builds a tortuous path good for removing gelatinous contaminants. A pre-coat can be added to improve the filtration characteristics of the pad.

Filter presses that utilize pads tend to be very operator intensive and require extensive cleanup and set up.

Panel
Panel filters are usually associated with HVAC air filtration, but they can range from inexpensive 1" deep air conditioning filters to pleated HEPA filters to computer hard disc drive filters. Panel filters are used anywhere air quality needs improvement. Applications include:

• Electronic cabinet cooling fans
• Blowers and miniature cooling and ventilating units
• Clean rooms and laboratories
• Intake Air
• General facility HVAC
• And more

Panel filters can be flat (pad) or pleated.

When comparing a 24" x 24"x 2" pleated filter with a pad of the same size, note that the pad contains 4 square feet of media, and the typical pleated filter contains 12 square feet of media. This increase in surface area allows the pleat to trap more particles than the pad.

As for efficiency, this rating is based on the ASHRAE test method # 52-76, and uses "atmospheric dust" as the control contaminant. This dust ranges in size from .001 to 10.0 microns in size. The average efficiency of a pad is rated below 20%, and the average efficiency of a standard pleat is above 30%. The pleat is 50 times more efficient than the pad.

At a particle size of 1 micron, the standard pleat is 40 times more efficient, and at 10 microns the pleat is in excess of 97%-more efficient. The pad will be more efficient on larger micron size particles as well. However, the majority of the particles that must be removed to help ensure good Indoor Air Quality are smaller than 10 microns, and some are smaller than 1 micron.

Panel filters are available in polypropylene, polyester, and foam and can be impregnated with activated carbon for odor removal. Some filter media can be electrostatically charged to improve filtration efficiency. The electrostatic charge provides a coarse filter the ability to remove sub-micronic particles while maintaining a low pressure drop.

Arrestance ratings are used for comparing only low efficiency filters. Since large particles account for most of the weight of the "dust", this rating will not indicate how effective a filter is in removing the smaller particles. Only seven percent of the total particles in the air can be measured by the Arrestance test.

Spin-on elements
A spin-on element is a combination of a filter element and a pressure vessel. The pressure vessel is usually a deep drawn steel shell that is lock seam connected to a stamped steel tap plate. The filter element is bonded to the tap plate. The tap plate is threaded with a standard SAE or BSP connection thread size. Sealing is accomplished by the use of a gasket that is either mounted in the tap plate, or supplied loose, depending on the manufacturer or the spin-on element size.

Strainers
Strainers are cleanable perforated metal filters designed for low solids high flow applications. Strainers are generally rated over 75 micron but are more typically measured in perforation sizes up to ½ inch. The most common perforation size is 1/32 inch. Particle retention below 1/64 inch is obtained by welding a mesh screen inside the strainer basket.

Strainers come in two basic families "Y" strainers and basket strainers. Basket strainers fit into a housing and the basket hangs perpendicular to the line. "Y" strainers are also in-line but the basket is at an angle, generally 45 degrees, and must be below the pipe in order to trap contaminant.

Since the "Y" strainer basket is located beneath the pipe at angle removing and cleaning the basket can be difficult. "Y" strainers are not recommended when straining hazardous or environmentally unsafe liquids. The "Y" strainer is also smaller than a basket strainer so it holds less contaminant and requires more frequent cleaning.

Basket strainers can be manifolded together with a crossover valve for uninterrupted flow.

Strainers are usually sized based on flow rate and a clean pressure drop of less than 2 psid and often have an element filter located downstream to remove particulate to fine for the strainer to capture. Strainers work well in tandem with filters because they remove large particulate that can blind and reduce a filter's service life.