What is an element?

The filter element is the device used to remove suspended particles from a fluid. Filter elements are typically associated with cartridges and bags but can also be discs pads, panels, spin-ons, or strainers. These are available in a broad array of materials and retention ratings.

Adsorbent
Absorbent and adsorbent filter elements have an affinity for certain chemicals or chemical families and are used to remove these targeted chemicals from a fluid. The most effective adsorbent filters are depth type and act as a particulate filter. It is important to pre-filter the adsorbent filter to prevent particulate from blinding the adsorbent media. If the adsorbent media becomes fouled with particulate its ability to remove the unwanted chemicals will be reduced. Activated Carbon is the most common adsorbent filter and has a great affinity for hydrocarbons and organics. It is typically used to remove tastes and odors from water and air. Polypropylene has an affinity for hydrocarbons. Melt blown polypropylene bags are often used to remove tramp oils from coolants and water systems.

Melt Blown
Molten polymer is extruded through a die then attenuated and broken to form short filaments which are collected on a moving screen. They bond to form a filter media. The resulting filter media can form a sheet that can be pleated or rolled or spun bonded to form a depth filter cartridge or bag. Melt blown filters can be made with a number of materials of construction but by far the most popular is polypropylene. It offers excellent chemical resistance and good solids holding capabilities. The media can be configured to produce high efficiency filters rated up to 99.9% efficient. Economical melt blown filters are available with decreasing efficiencies. The major limiting factor with melt blown cartridges is temperature. Melt blown elements are rarely rated above 180° F.

Membrane
Membranes are flexible fixed pore size barriers that provide a high level of filtration efficiency. Filters utilizing membrane media are rated at or above 99.97% efficient and can pass several challenge tests including bubble point, bacteria challenge, and pressure hold. Membranes can be constructed from a number of materials including Nylon, PTFE, Polypropylene, and Polysulfone. Filter membranes are used in critical filtration applications such as pharmaceutical and microelectronic manufacturing.

Pleated
Flat stock filter media is often pleated to provide more useable surface area. Increased surface area produces two major benefits: increased dirt holding capacity and lower pressure drop. Most pleats are vertical, however horizontal pleats are available. The number and density of the pleat pack is important. As solids load on the pleated surface pressure drop increases. As the pressure drop increases the pleats have a tendency to "roll over" on themselves, blinding the filter media prematurely. As with any filter, the slower the flow the more solids the filter can accommodate. Pleated filters are not recommended for removing gelatinous or deformable contaminant. Gels have a tendency to either flatten on the surface blinding the media prematurely or extrude through the media and emerge on the clean side.

Porous Metal
Various types of metal can be used as filter media. Porous Metal filters are available in a pleated format as well as cylindrical. The metal is available in various stainless steels; i.e. 316L, 304L, 310, 347, and 430, as well as more exotic metals such as Hastelloy, Inconel Nickel 200, Monel and Titanium. Porous metal filters offer distinct advantages when compared to fabric or polymer filter elements. Advantages of porous metal filters include:

• Cleanable
• High pressure drop capability
• Consistent filtration efficiency compared to most disposable filters
• High temperature and aggressive chemical compatibility
• backwashable

The disadvantages include: the initial cost, cleaning can be difficult, and low solids holding capability. Porous metal filters work best when the application has low solids and hard particles. They are also recommended in high viscosity applications.

Resin Bonded
Acrylic and other fibers can be combined with phenolic or melamine resin to form resin bonded depth filter cartridges. Nominally rated, these filters are used in high temperature or high viscosity applications. Graded density resin bonded cartridges progressively filter finer as the fluid passes through the media. They are designed to trap the largest particles in the outermost layers. They are very effective at removing deformable particles and gels. Typical applications for resin bonded cartridges include:

• Paints
• Inks
• Coatings
• Emulsions
• Oils
• Waxes
• Resins

Porous metal filters are frequently used downstream of resin bonded cartridges to trap any remaining particles or manufacturing debris shed by the filter. If possible resin bonded cartridges should be flushed before use to remove extraneous manufacturing debris.

String Wound
For over 50 years string wound elements have been a common and economical filter. Made by wrapping yarn around a core, these nominally rated elements are available in a wide range of materials and retention ratings. String wounds are made by winding a single strand of yarn around a perforated center core in a controlled diamond pattern. String wound elements are manufactured with a graded density structure that decreases in size from the outside surface to the center core. String wound cartridges are available with retentions of .5 through 100 micron. Caution should be used when using string wound cartridges in systems sensitive to foaming. The lubricating oils used in the string wound winding process can leach into fluids.

Wrapped elements
Wrapped elements are constructed by "jelly-roll" wrapping various media layers on a core to form a cylindrical element. Coalescers are commonly wrapped cartridges, containing fiberglass media and various support mesh and media flow distribution layers.

Felt
By combining pressure, moisture, heat, mechanical and chemical actions fibers are built up to form a felted filter media used in filter bags. Felt construction is generally chosen when finer than 100 micron particle retention is required but a nominal rating is sufficient. Felt filter bags offer economical filtration, high flow rates and low pressure drop. These characteristics make felt an excellent choice for batch applications. The felt media is thick enough to provide some depth filtration, but felt bags are primarily surface filters.

Felt bags work best on hard particles but can be constructed from adsorbent materials like polypropylene to help remove tramp oils from coolants and paints. A felt bag is made by sewing the seam along the vertical side of the bag. Various methods are used to plug the holes left by the needles. Most manufacturers use a barbed needle that pulls the thread and fibers back into the hole thus plugging it. It is always an area where by-pass occurs and one of the reasons felt bags have a nominal rating. Recent innovations in seam welding have vastly improved the efficiency of felt bags.

Multifilament
Multifilament mesh is a woven fabric. Each thread of the fabric consists of many smaller diameter threads twisted together to form the filter media. Inexpensive and disposable multifilament bags are nominally rated and available in retention ratings from 75 through 800 microns. Polypropylene and polyester are the most common materials of construction. The bags offer high flow rate capability and low pressure drops. They do not have high dirt holding capability. Multifilament bags often act as pre-filters removing large debris from the process stream.

Monofilament
Monofilament mesh is a woven fabric with evenly spaced holes. Each thread is a single filament. Monofilament mesh media has excellent:

• Strength
• Flow rate
• Pressure drop
• Media migration characteristics

Monofilament bags are typically used as insurance filters preventing debris from damaging pumps and instruments. Nylon is the most common material of construction and micron ratings from 5 through 800 are available. Monofilament bags hold very little solids but can be cleaned and reused.

It is very important to choose the correct filter media for each application. Temperature, chemical compatibility, the nature, size and quantity of the contaminant, flow rate, viscosity, cost, and criticality all affect filter performance. Each application should be researched to ensure the correct media is chosen.

Some "rules of thumb" to consider when choosing a filter are as follows:

Soft deformable particles are best removed by depth filters. Gelatinous soft particles will tend to flatten out on surface filters blinding them prematurely. As the pressure drop increases, the particles will be forced through the surface and emerge on the clean side.

In applications where the contaminant is hard, pleated filters tend to last longer than depth filters. The surface of the pleated filter will load and form a filter cake. This will increase the life of the filter and the efficiency. Ideally, start with as low a clean pressure drop as possible to avoid compressing the filter cake and blinding the filter.

The slower the flow the more solids a filter is able to accommodate before coming to terminal pressure. Often by halving the flow, the life of the filter is tripled.

Melt Blown bags and cartridges tend to "shut down" when terminal pressure drop is reached. Other filters can partially unload or release trapped contaminant.

Resin bonded cartridges tend to have higher amounts of media migration initially than other types of cartridges.

Gelatinous particles and elongated particles are difficult to back pulse off reusable filters.

When possible, over size the filter vessel. The savings in replacement elements will make up for the expense of the larger filter vessel.

Fiberglass is the filter media of choice for critical hydraulic applications.