By Mike Fitzpatrick and Ken Goldstein, Ph.D.
Filters are omnipotent in the practice of contamination control. We filter many things—cleanroom air, ultrapure water, process chemicals and high-purity gasses. We install filters at the outlets of our vacuum cleaners and we filter our process effluents.
Sometimes, we even put our people in helmets and filter the air they are exhaling.
Our processes often require that we filter particles from fluids. While end users need to “filter” airborne molecular contamination (AMC), this mechanism is not truly “filtration” but instead relies on the removal processes of chemisorption or physisorption (adsorption and absorption). But we'll leave the topic of AMC removal for another column.
For now, let's define a fluid as anything that “flows”—gases and liquids. We will also deal solely with fiber filters, where the filter media is composed of numerous separate fibers, as opposed to membrane filters. The capability of a filter to remove particles from a fluid is expressed by describing its Efficency, or equivalently, and its Penetration:
- Efficiency (E)—Fraction (or percentage) of particles captured by a filter, for a given particle size and velocity;
- Penetration (P)—Fraction (or percentage) of particles that pass through the filter, for a given particle size and velocity.
And since particles have only two possible fates, to either penetrate the filter or be captured by it, we see that:
- E + P = 1, or
- Efficiency = 1 – Penetration (E = 1 – P), or
- Penetration = 1 – Efficiency (P = 1 – E)
Both Efficiency and Penetration will vary depending upon the particle size of interest and the flow velocity. If we wish to compare the performance characteristics of two different filters, we must specify a common velocity and particle size.
Filters remove particles from fluids using the following mechanisms:
- Electrostatic filtration—Electrical charge differentials attract particles;
- Thermophoresis—Temperature differentials attract particles;
- Mechanical filtration—Interception, impaction and diffusion.
The mechanical filtration mechanisms are interception, impaction and diffusion. Which filtration mechanism dominates the filtration process depends solely on the mechanical properties of the system, including particle diameter, particle velocity, filter fiber diameter, fiber spacing and fluid viscosity.
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In filtering outside make-up air that contains a significant number of large particles, interception and impaction are usually the dominant mechanisms. In process gases, where extensive filtration has already occurred, diffusion is the primary particle removal mechanism. In liquids, interception alone governs and impaction and diffusion have little effect.
In any filter, although a particular filtration mechanism may dominate, all three mechanisms are simultaneously at work, hopefully, contributing to the filter's performance.
Let's take a very simplified look at how these various filtration mechanisms function by examining a particle traversing through the filter in the vicinity of filter fibers (shown in cross-section) along with typical fluid streamlines (Figure 1). While simplified, these diagrams show the essential elements at work.
Sieving
Sieving (Figure 2) is actually a special case of interception (see below). In sieving, a particle attempts to follow the fluid streamline around the filter fiber and is just too big to fit between two adjacent filter fibers.
This is similar to the fly on the window screen phenomenon and helps to explain why our homes are not inundated with insects. While most people will assume that sieving is the most important filtration mechanism, it is usually the least important.
Interception
In the case of interception (Figure 3), a section of the particle surface strikes the filter fiber and sticks to it. Filtration through interception depends on the size and mass of the particle—all other things being equal.
Larger particles are more likely to be removed by interception because their size puts them in closer proximity to the filter fibers that capture them. And since size and mass are usually related, interception also works well on high-mass particles. As particle mass increases, however, impaction plays a significant role.
Impaction
With impaction (Figure 4), a particle follows the streamline where it's straight as we expect. But where the streamline curves around the fiber, the particle is unable to make the turn due to the inertial effects operating on the greater mass.
The particle impacts and sticks to the fiber while the fluid molecules follow the streamline. Filtration through impaction is affected by both the mass and the velocity of the particle. In the case of impaction:
- higher mass => higher removal efficiency (lower penetration);
- .higher velocity => higher removal efficiency (lower penetration).
Consequently, large (heavy) particles, or those with high velocities, are more likely to be removed by impaction. Small (light) particles, or those with low velocities, are less likely to be trapped by impaction. As velocities and mass decrease, diffusion effects come into play.
Diffusion
Diffusion (Figure 5) is usually the dominant mechanism when filtering small (low mass) particles with low velocities. Being small and lightweight, these particles tend to follow the fluid streamlines.
But at the same time, collisions with the fluid molecules tend to move them out of the streamline and, ideally, onto the filter fiber. The particle path is similar to Brownian Motion.
Since the smaller (lighter) particles are more likely to be moved by molecular collisions they are more likely to be captured through diffusion. In the case of diffusion:
- lower mass => higher removal efficiency (lower penetration);
- lower velocity => higher removal efficiency (lower penetration).
While there is a considerable amount of science behind each of these concepts, they really boil down to these three capture mechanisms. As stated earlier, both efficiency and penetration will vary, and will depend upon the size and velocity of the particle.
When discussing filters, the usual relevant question is, “will this filter capture our particles?” And the “universal answer” is: It depends.
We can't provide an absolute answer but can only give a statistical response: X-% of all particles of a certain size will be captured, implying that (100-X)% of our particles will show up on the downstream side of our filter.
And even in that situation, we must factor in the velocity effect on filter performance.
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Michael A. Fitzpatrick has participated in the design and construction of semiconductor facilities for more than 24 years and is a Senior Member of the Institute of Environmental Sciences and Technology (IEST). Mike can be reached at: [email protected].
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Ken Goldstein is principal of Cleanroom Consultants Inc. in Phoenix, Ariz., and is a member of the CleanRooms Editorial Advisory Board. He can be reached at: [email protected]