Technical Reference
Basic Filtration Principles How Filter Media Functions In a Filtration System The job of the media is to capture particles and allow the fluid to flow through. For fluid to pass through, the media must have holes or channels to direct the fluid flow and allow it to pass.That’s why filter media is a porous mat of fibers that alters the fluid flow stream by causing fluid to twist, turn and accelerate during passage.
How Filter Media Collects Particles There are four basic ways media captures particles. The first, called inertia , works on large, heavy particles suspended in the flow stream.These particles are heavier than the fluid surrounding them. 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.
The second way media can capture particles is by diffusion . Diffusion works on the smallest particles. Small particles 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 are collected.
The fluid changes direction as it comes into contact with the media fibers, as illustrated above. As the fluid flows through the media, it changes direction continuously as it works its way through the maze of media fibers. As it works its way through the depths of the layers of fibers, the fluid becomes cleaner and cleaner. Generally, the thicker the media, the greater the dirt-holding capacity it has.
The third method of particle entrapment is call interception . 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 intercepted or captured when they touch a fiber.
Looking at a cross section view of the fibers, we can see how the flow stream is accelerated as it flows into the spaces between the fibers.
The fourth method of capture is called sieving and is the most common mechanism in liquid filtration. As shown below, this is when the particle is too large to fit between the fiber spaces.
Engine Liquid Filtration • 53
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