This is the second part of a series of “anatomy” lessons within Machinery Lubrication. In this issue, the oil filter will be examined to uncover its functional and performance characteristics. Several other related topics will also be discussed, including best practices for oil filter usage, possible filter failure modes, factors for proper filter selection and how to maintain an installed filter.
By definition, an oil filter’s main role is to cleanse oil from destructive contaminants within a machine such as an engine, transmission, hydraulic system and other oil-dependent systems. In the case of automotive oil filters, canister-type filters are the most common. This filter configuration was most likely responsible for the advanced performance of oil filtration technology.
In 1922, Ernest Sweetland invented the first oil filter device for automobiles. It was named the “Purolator,” which was short for “pure oil later.” The spin-on filters common in today’s automotive industry were introduced in the 1950s and were virtually a standard by the early 1970s.
Aside from the automotive industry, oil filtration is an integral part of equipment within a wide variety of industries, including aerospace, power generation, oil refining, manufacturing, mining, etc. Although most current oil filter designs come in canister or cartridge types, several variations in size, filter media, dirt-holding capacities, and flow arrangements are available. For this reason, it is important that filters and filtration systems are selected to meet the needs of the application and with cost, performance, ease of use and environmental conditions in mind.
Oil Filter Types
Oil filters can be characterized by the method in which the contaminants are filtered or the method in which the oil flows through the housing. One technique used to control contamination in filters is through surface-type media. This is the type of filter used in automobiles. In depth-type filters, the filter media are designed to hold much higher levels of contamination and provide a more circuitous path for lubricant contaminants to become trapped.
Other possible contamination control methods include magnetic and centrifugal filtration. Magnetic filtration utilizes rare-earth magnets or electromagnets to attract and collect ferrous particles as the oil passes through a magnetic flux region. Centrifugal filtration works by integrating a rapidly rotating cylinder to produce a centrifugal force for contamination separation from the oil.
Oil filters can also be categorized by the oil flow design. As its name implies, a full-flow filter will draw all of the oil through the filter media. On the other hand, a bypass filter only requires a fraction of the oil flow for sufficient flow rates within the system. The application’s oil flow and contamination control requirements will determine which design is the best option. Another alternative is the duplex filter system, which contains two side-by-side filters in parallel to allow one of the filters to be replaced during uninterrupted operation.
With typical canister-type filters, it is standard for oil to flow from the outside in. This means that the oil travels through the cylindrical filter media from the outward-facing surface into the inner core. However, in some cases, the flow direction is reversed, with the oil coming into the filter through the core and pushed outward through a unique pleat design. This is intended to improve flow handling and distribution as well as reduce filter element size.
Filtration Mechanisms and Filter Media
A filter’s primary function is to remove and retain contaminants as oil flows through the porous component called the media. The media operate under several types of filtration mechanisms, including:
Direct Interception and Depth Entrapment – Particle blockage on the media due to the particles being larger than the taken passages within the media.
Adsorption – The electrostatic or molecular attraction of particles between the particles and the media.
Inertial Impaction – Particles are impacted onto the filter media by inertia and held there by adsorption as the oil flows around.
Brownian Movement – This causes particles smaller than 1 micron to move irrespectively of the fluid flow and results in the particles being adsorbed by media in close proximity. It is much less prevalent, especially in viscous fluids.
Gravitation Effects – These allow much larger particles to settle away from fluid flow regions when there is low flow.
In addition, filter media can be designed to capture particles through two distinct methods:
Surface Retention – Contaminants are held at the surface of the media. This provides an opportunity for the contaminant to become trapped as it comes in contact with the media surface.
Depth Retention – Contaminants are held either at the surface of the media or within the labyrinth of passages within the “depth” of the filter media. This creates several opportunities for contaminants to become trapped.
The graph below shows how depth-type filtration is more efficient in capturing smaller particles when compared to surface-type filters. This can be attributed to the deeper media providing more chances for the particles to be trapped along with the adsorptive and Brownian movement effects being more predominant in depth-type filters. While these characteristics are beneficial, depth-type filters tend to have higher differential pressure across the media as a result of the increased flow restriction from the deeper filter media.
Filter Media Types and Dirt-Holding Capacity
In the September-October 2012 issue of Machinery Lubrication, Wes Cash explained how the porosity of the filter media plays a role in how well the filter can retain captured particles. This is known as the dirt-holding capacity. As pore size goes down, to maintain a low differential pressure across the media, the pore density must go up to account for the oil volume in contact with the surface. The filter depth and size also influence the dirt-holding capacity. Another factor is the filter media material. There are three primary types of filter media:
Cellulose - Comprised of wood pulp with large fibers and inconsistent pore size.
Fiberglass (Synthetic) - Comprised of smaller, man-made glass fibers with more consistent pore size.
Composite - Comprised of a combination of cellulose and fiberglass material.
Cellulose media are advantageous because they can absorb some water contamination. However, these types of media tend to fail more rapidly than synthetic media in acidic and harsh oil conditions. Nevertheless, the primary reason synthetic filter media are preferred is their more consistent porosity and smaller fiber size, which contributes to higher dirt-holding capacity and longevity of the filter.