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Filtration 101Principles of Filtration


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Definition of Filtration Filtration describes the process of

separating the components of a fluidstream. Components may be contaminantsor ingredients. They may include particles,

gases, tastes, and may even involveseparating suspensions from a fluid.

Two broad categories of filtration:

Particulate filtration Gas phase filtration


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Key Terms Efficiency

The key measure of a filters ability to remove

components of a specific size or type from a fluid

stream. It is easy to assume that higher efficiency

is always better, but the efficiency of a filter shouldbe determined by the purity requirement. Higher

efficiency can come at the cost of higher pressure

drop (discussed later) and increase the energyoperating cost of a filter.


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Measuring Efficiency

Filtration engineers use particle counters upstream

and downstream of filters to count particles and

measure fractional efficiency. Different filtration

applications will employ variations of this test, but

the basic principles are the same.


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Calculating Efficiency Efficiency is typically reported as a percentage and

calculated Efficiency% = 1 - (upstream particlesx/downstream particlesx), where x is the particle size

In liquid filtration, efficiency is often reported in

Beta Ratio. Betax (read Beta at particle size x) isalso based on ratio of upstream to downstream

particles and is calculated as shown below:

Beta25 = 10,000/2,000 = 5Efficiency % = (1-1/Betax) 100

Beta25 = (1 1/5) 100 = 80% Efficiency


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Key Terms Pressure Drop

The resistance to fluid flow imposed by a filter in the processof in the process of separating the components of a fluid

stream. Pressure drop relates to the energy required to push

a fluid through the filter. Filter life is usually based on

Terminal pressure

The maximum pressure drop the filter is designed to handle.

Filter designers seek to reduce pressure drop whendesigning filter systems.


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Measuring Pressure DropPressure upstream minus

pressure downstreamequals Pressure Drop.

Also commonly referred

to as Delta P, orP.

This value is typically

expressed in inches ofwater, or Pascals.

Pressure Sensors Pressure Gaugeor Magnahelic

Some systems use pressure drop as the indicator for changing filters


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Key Terms

Dust Capacity

The mass of dust held by a filter at terminal pressure drop.

Adsorption

The adhesion of thin molecular layers of gases or solutes on

a solid surface. Adsorption is the primary filtration

mechanism in odor and taste removal. Filter Media

A fabric, paper or fibrous bat that is designed to capture and

retain particles from the fluid stream. Media is held in place

in a filter by the frame and determines the performance of

the filter.


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Filtration MechanismsThere are four (4) primary methods that

particles are captured and retained by afilter:

1. Straining

2. Inertial Impaction

3. Interception

4. Diffusion


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Straining

Straining occurs when the particles to be captured

are larger than the pores in the filter media.

Straining is the primary filtration mechanism inliquid filtration due to high viscosity and flow

forces.

In air filtration, straining occurs most often withlarge particles such as hair, fuzz, etc.

Contaminant

particlesFilter

Fiber

Filter

Fiber


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Inertial Impaction

Inertial impaction occurs when larger particlesentrained in the air stream separate from the flow

due to their inertia and impact on the filter fibers Particles captured by inertial impaction are

relatively large and have higher mass (thus theinertia), but still may be much smaller than the

pores in the filter media

Particle separates from

flow stream due to inertiaand impacts filter fiber

FilterFiber


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Interception

Interception occurs when particles in the fluidstream come into close proximity with the filter

fibers and are trapped in the stagnant boundarylayer fluid flow near the fibers.

Interception occurs with smaller particles that donthave adequate inertia to separate from the flow

stream.

Small particles intercept

the filter fiber as the fluid

stream passes aroundit. Particles usually collect

on the sides of these fibers.

FilterFiber


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Diffusion

Diffusion is the dominant mechanism of capture forthe smallest particles. In diffusion, particles are

influenced by molecular interaction known asBrownian motion. These particles move in randompatterns, colliding with gas molecules somewhatindependent of the flow stream. When they comeinto close proximity with a filter fiber they arecaptured.

Very small particles

move by Brownian motionrather than the airstream

and contact filter fibers

FilterFibers


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Particle Attachment

Q: What forces holds a particle in place once it has come into

contact with a filter fiber?

A: The primary force that holds particles to a filters fibers is vander Waals force. This is a molecular level attraction and is

independent of fiber or particle composition.In addition to van der Waals force, particles can be captured

and held by Coulombic forces such as electret charges in the

fibers, or by adhesive applied to the filter fibers.


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Filter Efficiency and Life

It is generally agreed that filters

increase in efficiency over their

service life, as shown in this

graph. This filter began at 25%efficiency, rose to just over

50%

As the filters efficiency

increases, so does its pressuredrop (resistance to flow). Filter

makers typically quote a filters

terminal pressure drop in their

literature.

Filters can lose efficiency if run

beyond their rated terminal

pressureEnergy costs will rise over the life of a filter


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Most Penetrating Particle Size (MPPS)

Air filters capture particles through the

mechanisms described before. Smaller particles

are captured mostly by diffusion, as shown by

the magenta line. Larger particles are captured

mostly by interception, as shown by the blue

line. Summing these lines yields the yellow line

which reflects a filters total capability to

capture particles of various sizes. This graph is

specific to the size of a filter fibers. One veryinteresting phenomenon is the most penetrating

particle size (MPPS). This is the size of

particles most difficult for the filter to capture.

Many factors influence the MPPS for a filter, but

it generally runs in the 0.1 to 0.3 micrometerrange. Some test standards specify testing filters

at the MPPS to reflect their true capability.


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Want to Learn More?

Filtration is a facinating process. To learnmore, visit our Web site at

www.kcfiltration.com and select Resource

Center. There are many papers,presentations and articles on different

aspects of Filtration. These resources are

there for you!
http://www.kcfiltration.com/http://www.kcfiltration.com/

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