Filtration 101

Toolbox Articles: Filtration 101

Filtration and Separation, August, 1999

Filtration 101

"To filter or not to filter?" that is the question - or is it? The real question we need to ask ourselves is, what type of filtration do we need? In today’s engineering climate, most of us will have to learn something about filtration. Most engineers do not spend a great deal of time learning about a subject unless there is an immediate application. Therefore, the fundamentals of filtration technology is here introduced in a quick and simple manner.

The proceeding discussion very basic. Filtration theory, terminology, test standards, classification and selection are outlined and explained. This information provides a solid basis of knowledge from which an engineer can make sound decisions regarding filter selection and application in most engineering projects.

Filtration applications
The reasons for using filtration technology are many. In some cases, filters are used to purify the product. In others, they are used to clean auxiliary fluids. Cleaner fluids can also extend equipment life by reducing erosion. Below, is a short list of examples.

• Increase pump, bearing, and tool life (cutting and grinding coolants)
• Keep valves from sticking
• Prevent nozzles from plugging
• Increase product yields (semiconductor IC manufacturing)
• Provide clearer cleaner products (high purity solvents, RO pre-filtration, potable liquids, edible Oil)
• Remove agglomerates for smoother coatings (inks, paints, and varnishes)
• Remove bacteria to prevent spoilage (wine, beer)
• Protect strata from wells (secondary oil recovery)
• Remove harmful by-products ( heavy metals as hydroxides, pyrogen removal)
• Recover precious metals
• Recycle fluids as much as possible to reduce disposal costs.

Filtration theory and terminology
Filtration is defined as the physical separation of constituents from a fluid by means of flow through a permeable or a porous medium. A common example is the coffee maker. The coffee grounds are removed from the brewed coffee by a filter. The coffee filter (porous medium) provides the physical separation of the grounds from the water (constituents).

Filters are rated by the size of particles for which they are designed to remove. The size is defined in terms of "microns". A micrometer is actually the correct term. One micrometer is equal to 10 -6 meter.

To place the micrometer into physical perspective, the unaided eye can see a 40 micron object unaided. This is approximately the diameter of a human hair.

Filter classification
Filters are classified according to the size of the particles for which they are intended to remove. Different sized particles require different types of filters. Table 1 gives a broad overview of the classification system.

Table 1
System for filter classification

For a filtration system to work, there are a couple of basics requirements. First, there must exist a porous media that allows flow. Second, there must exist a pressure difference across the medium, such as gravity, a vacuum, or a positive pressure source. The differential pressure across the filter is defined as the pressure from the feed minus the pressure downstream or after the filter.

A typical filtration assembly consists of a housing and the filter medium. The housing retains the fluid, positions the medium, provides the fluid a pathway through the medium, and contains the pressure of the system.

Filter rating
Most filter ratings are based on filtration efficiency and dirt holding capacity. The filter’s efficiency is a measurement of its ability to remove particles. In other words, how effectively and consistently can a filter remove particles of a given size.

For example, say that a filter is rated at 90% efficiency for 5 micron particles. This means that the filter will remove 90% of the particles flowing through it that are 5 microns in size and larger.

Another way to denote particle removal efficiency is to use Beta Ratios:

• Beta Ratio. The ratio of the number of particles of a given size and larger in the flow upstream of the filter, to the number of particles of the same size and larger downstream of the filter.

The Figure 1 can be used as a quick reference for comparing percent removal efficiency and Beta Ratio. There are two type of efficiency ratings: nominal and absolute.

• Nominal. The size of particles removed at a set efficiency under established conditions. Manufacturers can vary nominal ratings anywhere from 50-98% removal efficiency, depending on product and company.

• Absolute. This implies 100% removal of particles at a set rating. Filter manufacturers vary their definition of absolute to mean anywhere from 98.7 to 99.99%, also depending on product and company.

Filtration efficiencies and performance can vary with actual "real world" conditions. Filter manufacturers rate their filters under laboratory conditions. The field performance of a filter can be affected by flow rate, viscosity of the fluid being filtered, concentration of contaminant, and measurement techniques.

Filter life
Filter life is determined by the filter’s Dirt Holding Capacity (DHC). DHC is defined as the amount of contaminant (weight basis) fed to a filter that attains its terminal differential pressure (i.e. the end of its service life, typically 30-50 psi). This sounds like a misnomer, but it is not. The dirt retention capacity is the actual amount of dirt that a filter retains.

Filter media
There are two basic types of filtration media: Depth and Surface. Each type has its own advantages and disadvantages. Please refer to Table 2 for an overview.

Table 2
Depth media versus surface media

Conclusion
Filtration is utilized for the removal of a wide range of contaminants, from the filtering of boulders to the separation of ions. Though the science of filtration is vast and complex, the selection of a filtration system can be simplified by remembering a few basic points:

Filter micron ratings may not be comparable among manufacturers. For example, you are asked to replace a 50 micron filter from "Company A" with a filter from "Company B." The first question that should be asked is "what efficiency of particle removal is needed, 50% 90% 99.98%, nominal, absolute?" One must know how specific filter manufacturers rate their own filters. A filter’s micron rating should only be used as a guide to narrow down initial selections. Remember, filter companies rate their filters under laboratory conditions, not actual application testing.

There is no substitute for cartridge filter testing in actual "real world" use. Just because a filter has a long service life in a laboratory does not necessarily mean it will in real applications.

Each specific process will dictate whether surface or depth filtration media is needed.

Bio: By Ron D. Masters and William J. Campagna, Jr., Parker Hannifin, Lebanon, IN.