What is Oil Filtration
By Dennis Morgan
Full Flow versus Depth Filtration
Although there are many different options in the marketplace for use as secondary filtration, the most common methods are either an add on full flow filter system or an add on depth filter system. These systems will be installed outside the equipment lubrication circuit as an off-line, side stream or kidney loop configuration. Both full flow and depth filtration flow oil through a filter media to remove the contamination, but how the oil flows is very different.
In Full Flow (or Pass Through) filtration, the oil flows directly through the filter media. This type of filter will be pleated and is the type of filter element most people are familiar with. Full flow filters act to stop contamination by only allowing particles smaller than the pore size (gaps in the media) to pass through. Because of the pass through nature of this type of filter, pleated full flow filters have a very short flow path through the media.
There are two common types of media used for full flow filters, pleated cellulose and pleated microfiberglass. Cellulose is a natural fiber media that is inexpensive and provides a moderate level of filtration efficiency. Micro-fiberglass is a synthetic media that provides a much higher level of filtration efficiency than pleated cellulose. Both types of media can handle high flow rates, are useful for high viscosity oils (ISO680) and are usually safe for special oil additive packages that may be diminished by use of depth filtration.
Depth filtration however, has a long flow path, as much as 9 inches. There are two types of depth filtration, direct flow or axial flow. Direct flow is similar to pass through full flow except the path of flow is much longer (usually several inches) and the flow rate is restricted by the longer flow path.
Axial Flow Depth Filtration uses rolls of media (see photo to the left) and the oil does not flow through the media, but rather along the layers of the roll of media. The media is generally high strength cellulose paper that has been creped before rolling. This type of media allows for cleaning the oil at a low ISO Cleanliness level, removal of free and dissolved water and is relatively inexpensive. There is far more media in Axial Flow Depth elements than most other types and thus are able to hold large amounts of solid contamination and water. The element to the left has 8 pounds of paper media in the rolls and will hold up to 10 pounds of solid contamination and up to 1 gallon of water.
While the elements are being constructed and the paper is being rolled, it is stretched slightly to allow the creping to open wider than its normal amount. As the paper gets rolled layer upon layer, the creping goes back to its relaxed state creating an interlocking of the layers in the roll. This is important because as the oil flows between the layers of paper, the interlocked creping form a jagged flow path to help dislodge the contamination from the oil.
Axial flow depth elements do not have a bypass valve so NO unfiltered oil can ever leave the element. Because of this, they must be installed in systems that have a relief valve before the filter, such as an internal relief valve in a gear pump.
Both full flow and depth filtration have their advantages and disadvantages. The chart below illustrates some of the pros and cons of each type of media.
Ultimately, the decision about secondary filtration will be made by looking at all the factors involved and choosing a method that best fits situation. Filtration, is like most other industry, does not have a one size fits all solution. There are many factors to be considered when choosing and sometimes tradeoffs have to be taken into consideration. For example, Lubrication Engineers Duolec™ oils should not be filtered with depth media filtration (due to the medias' effectiveness in removing some forms of additive chemistry) so Micro-Glass is the obvious choice. However, if water contamination is a problem in a Duolec™ application, the Micro-Glass will not remove the water. Depending on the severity of water contamination, another oil purification option may need to be considered in addition to Micro-Glass filtration to get the Duolec™ as clean as desired.
