Vaughn,
Better quality filters are typically described in terms of a beta ratio(s) at specific particle size(s). The micron size may also be described in terms of an absolute rating, which represents the diameter of the largest hard spherical particle that will pass through the filter under controlled test conditions. It is also indicative of the largest opening in the filter element.
Lower quality filters may be described in terms of a nominal rating. This is a somewhat arbitrary micron value that the manufacturer assigns. Sometimes filters will be specified only by micron size with no indication of the size being absolute or nominal. In general, if a filter has a micron size and beta ratio or efficiency it can be considered an absolute filter. If it only has a micron size with no beta ratio or efficiency, it should be assumed to be a nominal rating. Unless expressly specified, many equipment manufacturers that supply lubricating systems utilize nominally rated filters due to their lower cost.
Various aspects of filter element construction can play a significant role in performance and should be considered when choosing a filter.
Better filters use filter media that is synthetic in nature. This is most commonly a glass fiber material, although some manufacturers use one or more types of polymers. Synthetic media offer superior performance in a variety of fluids and in the presence of water. Other media, such as cotton fibers and cellulose (paper), will absorb water, causing swelling or breakdown. Some filter elements are specifically constructed or treated to absorb or coalesce limited amounts of free water. Others utilize materials such as Fuller's Earth, activated alumina, and clays for specified purposes. Various media types are dependent on the particular application, so working with a filter supplier is advised in all applications.
Filter pore size is another key feature and has a direct affect on the size of particles that will pass through the filter. Filters with finer micron ratings utilize smaller pore sizes; e. g. , a 3-micron filter will have smaller pores than a 10-micron filter. A related aspect is the fiber size. Two filters rated at 5 microns absolute may appear to be equal, having equal pore sizes, but if one utilizes smaller diameter fibers, it will have greater dirt holding capacity and thus, longer life. Smaller fibers result in increased number of pores, which in turn leads to lower pressure drop and more dirt holding capacity.
Filters can vary significantly in their approach to media support, which typically comes in two forms: internal and external support. In some media, the fibers are bonded together using a resin, while for polymer type media the fibers may be wound while still somewhat molten. As the polymer cools to a more solid state, it becomes self-bonding. The benefits of a bonded media include resistance to deterioration from age, pressure and flow variations, temperature, and heavy dirt loading. Filters with little or no bonding will not perform as efficiently because particles can push through the relatively loose fibers. Unbonded filters can also break under high flow and pressure conditions and actually release captured particles back into the oil. This type of support is classified as internal support.
External support can include a metal core that the media is attached to and may also include support material on pleated type filters and possibly an outer wrap. Polymer-type media is self-supporting and does not require additional support on the inside or outside diameter. By whatever mechanism employed, the goal is to maintain uniform pore size and fiber integrity for maximum performance and life. Filter media that have little or no internal or external support will not provide long, reliable service.
Better filters will also employ depth media, which has a significant cross-section and filtration perpendicular to the media's cross-section. Depth media allows for increased dirt-holding capacity. Some filters utilize surface filtration. Here, filtration occurs only at the outer layer of the filter media. While depth media is good, some filters maintain a constant pore size across the entire cross-section of the filter media. Larger particles get captured in the outer layers of the filter element resulting in a lower effective filter life. This is due to the increasing pressure drop across the filter, falsely indicating the need for a filter change to avoid further pressure drop and potential loss of lubricant flow.
The best filters take the depth media concept one step further by employing a pore-size gradient across the depth of the media. Larger pore sizes will be located at the outer edge of the filter with progressively smaller pore sizes as the oil flows to the inner part of the filter. This method traps larger particles on the outer edge of the filter and progressively smaller particles are captured towards the interior. The pore-size gradient design provides the maximum filter efficiency and life for a filter.
Additionally, many filters employ a pleated element design that provides significantly more filtration surface area for the relatively same element diameter. The design and arrangement of the filter housing(s) is also important. Any filter element can handle only a specific flow rate to insure proper residence time through the media for effective cleaning and minimal pressure drop. Depending on the specifics of the application, the housing may be designed to handle one or numerous filter elements. These elements can be stacked and arranged to maximize the number in the housing. Some of the factors affecting housing design include:
• Lubricant viscosity
• Lubricant flow rate
• Particulate ingression/generation rate
• Desired cleanliness
• Desired clean and dirty pressure drop
• Filter housing cost
Filter housings may be installed as simplex, duplex, or multiplex systems. A simplex system is one filter housing in the lubricating system and is typically installed on less-critical systems where the opportunity for element change is abundant. More critical systems utilize duplex systems where the two housings are installed in parallel. Coordinated valve switching allows filter elements to be replaced without any downtime or loss of lubricant flow. Multiplex systems are employed where flow rate, particulate load, and viscosity are very high.
Stainless steel should be used instead of carbon steel in all filter components that will be in contact with the lubricant. Water contamination is always a possibility and the filtration effort should not be complicated by the addition of rust to the system. For cases where a high degree of cleanliness is required and contaminant load is high, it may be necessary to employ one set of coarse filters and another set of fine filters. The upstream coarse set removes the bulk of large particles, allowing the smaller particles to pass through. Downstream, the fine set "polishes" the oil without undue load from larger particles. Another option in high contaminant applications is to use non-cartridge, automatic cleaning filter units. These typically have a stainless steel filter mesh with a mechanized wiper mechanism and quick opening valve. The wiper sweeps the metal screen of particles and the quick opening valve automatically purges the sludge into a suitable container along with a small amount of oil. This approach may not provide fluid that is as clean as with cartridges, but it is a good method of keeping high-dirt ingression systems a little cleaner.
In summary, better cartridge filter designs will be those that have the following characteristics:
• Synthetic media
• Optimum pore size
• Minimal fiber size
• Internally supported fibers
• Externally supported fibers
• Depth media
• Decreasing pore size gradient
• Pleated element structure
• Appropriate housing design for the application