The most important features of a product are often below the surface. Would a race car enthusiast even think of buying a car without getting a good look at the engine? Of course not! Believe it or not, many people attempt to solve their contamination control problems without knowing much about the inside of the filter - the heart of the filter - the media.
Media is the term used to describe any material used to filter particles out of a fluid flow stream. The filtering process takes place within the media, which is made up of an intricate maze of tiny fibers.
The filter media should remove as much dirt as possible, while also allowing the liquid to flow through the filter efficiently with minimum resistance. Long filter life depends on a low resistance to flow and low pressure drop.
There are two main components of filter elements. The first is the design of the filter element itself, and the second is the type of media that is used in such elements.
Filter elements have some attributes that are immediately obvious to the casual observer, such as height, inside diameter, outside diameter, media concentration, type of liner, seal design and the way media and components are glued or potted together.
Liners must be structurally sturdy to with stand pressure variance, yet open enough to allow good flow.
Top seal design must be leak-free, with a gasket or sealing device that ensures a good seal throughout the life of the filter. Standard seals are made of BunaN material, which is fine for most applications. However, if filtered fluid is either diester, phosphate ester fluids, water glycol, water/oil emulsions, or high water content fluids (HWCF) over 150°F, a seal made of a fluoroelastomer such as Viton (from DuPont Dow Elastomers) or Fluorel (from 3M Company) will be required.
Media potting is key because it holds the media in place at each end. Not only should the potting be fully applied around the ends of the media to prevent leaks, but it should also be made of a material that can withstand the application. For instance, epoxy potting should be used in elements that must perform in higher temperature environments.
Inside the element, the filter media can vary in thickness, pleat depth and pleat concentration (Figure 1).
Figure 1. Filter Media are Designed with Specific
Pleat Concentrations, Number of Pleats per
Inch in the Filter Media, for Maximum Performance.
Another obvious trait is the media color, which can indicate the type of fibers used in the media. For example, Donaldson hydraulic filters are generally equipped with either white (synthetic material) or natural brown (paper or cellulose material) media (Figure 2).
Figure 2. Media Color Inside Filter Canister
Varies According to Manufacturer.
Note: Media colors vary according to each manufacturer. It should not be assumed, for example, that any white-colored media is made of synthetic material.
Some of the most important characteristics of filter media can be detected only under a microscope. These characteristics include structure, fiber diameter, volume solidity, basis weight, thickness and layering.
The media must have holes or channels to direct the fluid flow and allow fluid passage. Essentially, the media is a porous mat of fibers that alters the fluid flow stream by causing fluid to twist, turn and accelerate during passage. The fluid changes direction as it comes into contact with the media fibers (Figure 3). As the fluid flows through the media, winding its way through the depths of the layers of fibers, the contaminant in the fluid is captured by the media fibers and the fluid becomes cleaner and cleaner.
Figure 3. Filter Media Must Have Holes or Channels to Direct
Fluid Flow. As Fluid Winds Through Layers of Fibers,
the Contaminant is Captured and the Fluid Becomes Cleaner.
The differences between synthetic and cellulose (paper-based) media can be seen in the close-up photos from the scanning electron microscope in which the media mat is magnified hundreds of times (Figures 4 and 5).
Figure 4. Synthetic Filter Media has
Smoother, Finer, Rounded Fibers
that Create Less Resistance to Flow.
Natural fiber cellulose media has larger, rougher fibers that provide effective filtration for a variety of petroleum-based fluids.
Synthetic filter media has smooth, rounded fibers that are much finer in construction than cellulose. They are superior to cellulose in many applications because the fibers are finer and round. Synthetic fibers offer the filter designer more pore spaces per surface area unit, which means more channels to capture dirt, or let the fluid pass through.
Synthetic fibers have other benefits as well: they are more chemically stable than cellulose in most fluids, have an almost indefinite shelf life, and can perform at higher operating temperatures. These attributes can extend the life of the filter and improve filtration performance.
Figure 5. Natural Fiber Cellulose Media
has Larger, Rougher Fibers that Provide
Effective Filtration for a Variety of
There are four basic ways media captures particles: inertial impaction, diffusion, interception and sieving (Figure 6). Inertial impaction works on large, heavy particles suspended in the flow stream. These particles are heavier than the surrounding fluid; as the fluid changes direction to enter the fiber space, the particle continues in a straight line and collides with the media fibers where it is trapped and held.
Figure 6. Filter Media Captures Particles Using
Methods such as Inertia, Interception,
Diffusion and Sieving.
The second way media captures particles is by diffusion. This works on the smallest particles because they are not held in place by the viscous fluid and diffuse within the flow stream. As the particles traverse the flow stream, they collide with the fiber and get caught.
Direct interception works on particles in the mid-range size that are not quite large enough to have inertia and not small enough to diffuse within the flow stream. These mid-sized particles follow the flow stream as it bends through the fiber spaces. Particles are captured (intercepted) when they touch a fiber.
The fourth method of capture, sieving, is the most common mechanism in hydraulic filtration. This occurs when the particle is too large to fit between the fiber spaces.
All four methods of capture are used in hydraulic filtration in varying degrees. Inertia and interception are less effective in applications where surging or interrupted flow is a factor; sieving and diffusion are more often used in designing filter media for machine lube applications.
Figure 7. Filters Mounted in Parallel
Can Filter up to 200 gpm of Fluid.
When purchasing a new filter or selecting a replacement element, it is important to first answer some basic questions about the application. For example, where will the filter be used? What is the required cleanliness level (ISO code) of the system? What type of oil is being filtered?
It’s also important to think about the fluid’s viscosity. In some machinery lubrication applications, for example, the oil is thick and resists passage through the layer of media fibers. Heating techniques and the addition of polymers can make the liquid less viscous and therefore easier to filter. An easier option is to mount several filters in parallel, with media that is designed for more viscous fluids.
Next, think about duty-cycle and flow issues. Working components such as cylinders often create wide variations in flow, also called pulsating flow, which is problematic for filters. On the other hand, dedicated off-line filtration (also called kidney loop) can be designed to produce a consistent flow which often leads to much better cleanliness levels with the same efficiency filtration. It should also be noted that filters used in applications with steady, continuous flow can last longer than filters that must endure cycles of pulsating flow at higher pressures. Generally, the lower the micron size rating of a filter, the more often it needs to be changed because it is trapping more particles.
Figure 8. Materials for New Filter Media
are Measured and Blended. Mixture is then
Formed into Hand Sheets, the Standard Size
for Media Performance Testing.
Finally, it’s wise for the equipment user to evaluate the relative worth of discrete components in a system, to calculate how much it would cost to replace the equipment in the event of component failure and to make sure those areas are well-protected with proper filtration. For example, high-performance servovalves are sensitive and costly components that need to be protected with finer filtration (media with lower micron size ratings).
Many different types of contamination are present in hydraulic fluid, causing various problems if they are not removed. These include foreign matter such as wear particles, water, air and sludge. Particulate filtration deals primarily with foreign matter and wear particles.
Foreign matter comes in a variety of forms including both solid liquid and gaseous, typically dirt, sand and fibers are the most common dealt with by filtration. Built-in contaminants are caused during manufacture, assembly and testing of hydraulic components. High levels of foreign matter are often present in new hydraulic fluid; therefore, it is best to filter these fluids before putting them into the system. Foreign matter can also enter the system through reservoir vents if air is not filtered properly.
Wear particles are generated by a variety of methods including abrasion, erosion, adhesion and fatigue. These are all made worse by the other noted contaminants, causing an avalanche of wear with more contamination. This can be catastrophic. Therefore, it is critical to keep the system clean.
Figure 9. After New Media Formulas
have been Designed Using Computer-Aided
Engineering and Fluid Mechanics Software,
Actual Samples or Hand Sheets of Media are
Created in the Laboratory for Testing Purposes.
There may be factors unique to the application that must be considered when selecting the right filter media. Following are a few rules of thumb to get things started. Consulting a fluid power specialist or filter manufacturer to get the best filter element for the specific application is suggested.
For bulk hydraulic oil and turbine oils, it is recommended to use filtration with a nominal rating of two microns, and for bulk gear oils it is recommended to use a filter with a nominal rating around 6 to 7 microns.
Figure 10. Elements Undergo Further Lab Testing,
ISO Standards Testing and Real-World Trial
Runs Before Released into the Product Line.
For hydraulic system recommendations, the filter media should be selected to protect the most sensitive components in the system. If the system is powered with a gear pump, then a porous (10-micron) filter may be acceptable. If there are more sophisticated pumps, such ]as vane or piston pumps, then the filter should target smaller particles. If there are servo control valves in the system, then the target particle size should be no higher than three micron at a high efficiency rating (Beta 5 = 200).
Information was collected from many sources, both public and private, including Donaldson Company, Inc. Engineering Departments, Eaton Corporation, the Lightning® Reference Handbook from Berendsen Fluid Power, Hydraulics & Pneumatics Magazine, National Fluid Power Association (NFPA), and various industry authorities.