DEPTH VERSUS SURFACE FILTERS – THE CONTAMINANT IS THE KEY

Introduction

Disposable liquid filter cartridges are typically designed for either “Depth” or “Surface” filtration.  These terms relate to the construction of the filter media and the mechanisms used to trap and retain particles.  Each is suited for contaminant characteristics.  These characteristics should be defined and considered in the selection of the filter media.

Categories of Solid Particles

Rigid Solids:

A rigid particle is a solid material that does not deform or readily breakdown into smaller particles.  During filtration a rigid particle maintains its size and shape.  Examples include sand, mol sieve dust and activated carbon fines.

Deformable Solids:

A deformable particle is a semi-solid material that can change its shape when a force such as pressure is applied to it.  During filtration deformable solids have the potential to change shape.  Examples include gels, waxes, and biofilms.

Shear-Sensitive Solids:

A shear-sensitive particle is a solid material that readily breaks apart into smaller particles when a shearing force is applied.  During filtration shear-sensitive particles have the potential to change shape and size, as well as generate additional smaller particles.  Examples include agglomerated solids, iron sulfide and asphaltenes.

Three Filtration Mechanisms

There are three key mechanisms used for filtration of particles.  Filter media is constructed to exploit one or more of these.  They are direct interception, inertial impaction, and diffusional interception.

Direct Interception:

A sieve is a structure with openings, voids, pores or holes of a specified size and quantity.  Sieves stop particles by direct interception.  This refers intercepting particles on the surface of the structure because they are too large to fit through the openings.  A pump suction strainer is a good example of a sieve.

Inertial Impaction:

Inertial impaction exploits Newton’s First Law, which states that a body in motion at a constant velocity will remain in motion in a straight line unless acted upon by an outside force.  If a contaminant particle is suspended in a fluid stream and is denser than the fluid, it will have higher inertia than the surrounding fluid.  Inertia is related to a body’s resistance to changes in direction of its movement.  When a fluid enters a fiber matrix such as a filter media it encounters a tortuous path created by the fiber matrix.  As the flowing liquid weaves around fibers on its way through the media, the contaminant particles with greater inertia will depart from the flow path of the fluid and continue a straight path, colliding with the fibers.  After collision, the particles may adhere to the fiber or reenter the fluid stream and continue these collisions until coming to rest or exiting the filter.  When properly applied, this mechanism allows particles that are much smaller than the openings in a filter media to be captured.  A good example is a fiber mat-type HVAC panel filter.  Although the open area is so large that you can see through the filter, it still captures small particles that have impacted the fibers.

Diffusional Interception:

Diffusional interception refers to the capture of particles as they diffuse according the principle of Brownian Motion.  Particles diffusing through a filter media come to rest after randomly impacting one or more fibers.  This mechanism works on particles that are small enough that they move randomly through a fluid by bouncing around between molecules.  This is applicable to submicronic particle filtration and primarily considered in the design of gas filters.  However, diffusional interception is at work to some degree in all filters.  Media can be engineered to increase its contribution to the overall filtration performance.  It is referred to as engineered depth media.  The engineering challenge is to increase the probability of fiber impacts within the media as much as possible with minimal performance degradation pertaining to other metrics like pressure drop and service life.

Depth Filtration

Depth filters trap and retain particles within the cross sectional area of the media and exploit all three filtration mechanisms.  Increasing this area provides more filtration capacity; deeper (thicker) filters have the potential to trap and retain more particles.  Well-designed depth filters are constructed with graded porosity.  This means the pores are more open on the upstream surface layers and become tighter toward the downstream layers.  This allows contaminant particles to penetrate deep into the media and be trapped throughout it with smaller particles accumulating within the inner layers and progressively larger particles building toward the outer layers.  By design this maximizes the dirt holding capacity, while leaving space for fluid to traverse the media without prematurely building up pressure drop.  Examples include fiber mats, sintered metal filters, melt blown and string wound cartridges.

Depth filters are ideal for applications where the contaminant particles have a broad range of sizes, allowing all layers of the media to be fully utilized.  Deformable and shear-sensitive particles are also filtered more efficiently with depth filters.

Surface Filtration

As its name suggests, a surface filter traps and accumulates contaminant particles on its surface.  The media is generally a thin layer of woven or non-woven fibers creating a mesh.  The area between the fibers creates pores that sieve out particles from the flowing fluid like a screen.  While some of these filters have very thin layers of graded porosity that enhance performance, direct interception is the primary filtration mechanism at work.  In addition to the direct interception occurring at the surface, contaminant that cannot penetrate accumulates and forms a filter cake.  As the pores plug and the cake builds, the filter becomes more efficient and is eventually completely obstructed.  Plainly, the service life is directly related to the surface area.  So many surface filters are designed with pleated media and other creative ways to pack as much useful media surface area into the filter geometry as possible.  Examples of surface filters include strainers, screens, and panels or cartridges with very thin synthetic and paper media.

Surface filters are well suited for applications containing particles with relatively predictable profiles, such as rigid particles that form a porous cake on the filter media surface.  They are also useful where very specific sizes or types of contaminants are targeted and work well for applications where a high flux rate or a cleanable filter is advantageous.  They should not be expected perform well when the contaminants are deformable or shear-sensitive.

Summary

In summary there two fundamental categories of disposable cartridges.  These are surface filters and depth filters.  Each exploits fundamental filtration mechanisms in different ways to target contaminants.  Surface filters use direct interception to capture rigid solid particles on their surfaces and build a filter cake that helps improve filtration over time.  Depth filters use all three filtration mechanisms to trap and retain rigid and semi-solids particles when there is a broad particle size distribution that maximizes utilization of its many layers.  Understanding this basic concept and the contaminants to be targeted prior to selecting a filter cartridge will improve the odds that the filter will produce the desired results efficiently.  Know filters and let the contaminant dictate the type you use.