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"What is the difference between an oil filter and a strainer? Do they have different applications? When should each be utilized?"
Generally, an oil filter and a strainer seek to perform a very similar task — to remove contaminants as oil passes through — but at a completely different scale.
Strainers are employed to capture much larger contaminants. By larger, this refers to objects of approximately 40 microns or greater. A 60-mesh strainer captures contaminants at around a 240-micron level, while a 100-mesh strainer captures at a 150-micron level. The mesh-to-micron particle size correlation is determined by the number of open square-shaped gaps across one linear inch.
Forty microns also happens to be the lower limitation of what is visible to the unaided eye. In reality, if particles can easily be seen, they should be trapped by a strainer. The typical application of a strainer allows it to be cleaned relatively easily and reused.
Filters usually are much more complex in regard to the pathways through which oil must pass. This provides more opportunities for contaminants to be captured more efficiently, as well as for other types of contaminants such as moisture to be trapped.
One major operating parameter that is distinctly different in each application is flow restriction. Strainers are much less flow restrictive than most filters, especially compared to depth-type filters. This raises the question of the best application for a strainer versus a filter. In most cases, strainers are seen in suction lines into a pump. If suction pressure (toward a vacuum) is increased due to flow restriction, it can result in pump damage via cavitation.
Unclogged strainers can be quite effective in this application. However, as soon as the strainer surface area becomes compromised with high levels of larger contaminants (resulting in higher flow restriction), pump problems may occur.
With their higher capture ability, filters are much better applied where positive pressure exists and where constant flow exists (not pulsating). This means the best application would be on a return line or offline kidney-loop filter location.
In application, the choice of a filter must be balanced with the system’s operating and environmental requirements. If the requirements call for a filter at a lower micron level (and/or better beta ratio), then the size of the filter (fluid flow cross-sectional area) must be increased or a higher differential pressure will result.