Home > CleanFuel Program > Filtration Basics > Principles of Fuel Filtration
Principles of Fuel Filtration

Fuel filtration includes the separation and removal of particles, water and organic substances. Filters capture or separate contaminants a number of different ways – some better than others.

  • There are four basic ways a filter media captures particles:

    • Inertial Impaction takes place when large particles flowing in a fluid stream impacts the filter media in a straight line and becomes trapped while the fluid moves around the fiber and flows through.

    • Diffusional Interception works to filter the smallest particles. As particles collide frequently with liquid molecules, they move in a random manner (referred to as Brownian motion) until they collide with the fiber and are collected

    • Direct Interception works on midsized particles that are too large to pass through the fiber openings. They become logged in the filter media and are removed. This happens most commonly during fuel filtration.

    • Sieving occurs when filtering through pores with identical dimension such as a screen.

  • There are three basic ways a filter media separates water from fuel:

    • Absorption filters are often used to capture water in a fuel stream. They are frequently super absorbent polymers or SAP. However they are costly when large amounts of water are present. Other types of absorbents are used for different purposes. Clay filters are used to remove acids, organometallic compounds like copper and surfactants.

    • Coalescing filters are used in water removal often as the first in two stages of filtration. As fuel passes through the coalescer media, small droplets of water coalesce (come together to form large droplets). Once through the media, the large heavy droplets of water fall out for removal. The image to the right shows water coalescing along filter media. Some water still trapped in the flow of the fuel requires a second stage of filtration.

    • Water Separator filters are used as a second stage in water removal along with coalescer filters. The filters are constructed of a Teflon coated material that is hydrophobic (water repellant). The fuel passes through and the water falls out for removal.

Filters can also be identified as either depth or surface type. Depth filters remove particles throughout the medium unlike surface type that only collect particles on the surface. As a result, depth filters have a greater holding capacity than surface type. Fuel filters are generally categorized in three ways:

  • Non-fixed random pore depth type are common, e.g. felt filter bags often used as prefiltration. Because of their construction, they tend to work by means of inertial impaction and diffusional interception to trap particles. Once the fiber passages become blocked, the chance for fiber separation is greater allowing contaminants to escape downstream. As pressure increases, the pore size is often enlarged decreasing filter efficiency.

  • Fixed random pore depth type depend mostly on direct interception for particle removal. Made of single to multiple layers, the medium are constructed so as not to distort thus retaining contaminants under pressure. This type of filter media is often found in the high quality filter cartridges required for critical filtration. Because the pore size does not increase with pressure, this filter type is more efficient than the non-fixed random pore depth type.

  • Surface type media is often found in screen wire mesh and Teflon coated water separators. The holding capacity is limited by the pore size and thickness as shown in the picture of the woven wire media.

Ultimately, the decision on which filter to use will depend on the application. Often a combination of filter types are used. Filtration systems designed with multiple stages of filtration will use different types of filters to achieve both cost efficiency and fuel quality results.

Not all filters are the same. There are various mechanisms for rating a filter. However there is no one accepted method. Generally manufacturers will rate filters as nominal, absolute or Beta ratio. Nominal rated filters are not well defined and definitely not reproducible. Many rate nominal filters by efficiency. So a filter with a 6µm 90% efficiency rating is allowing 10% of the contaminants 6µm and over through. Understanding the efficiency of a filter and the application required is imperative. These filters are most often non-fixed random pore depth type. When tested for rating, results are rarely repeatable.

Absolute rated filters are most often fixed random pore depth type. They are rated based on the largest hard or glass spherical particle that can pass through the media when tested. In theory an absolute rated 3µm filter will not allow anything 3µm or larger to pass through. This is thought to be unrealistic since the pressure and filter loading of real applications will allow contaminants through. Absolute ratings are more dependable than nominal ratings and when tested results are more repeatable. However, the efficiency of absolute rated filters can vary. Understanding the absolute rating and the efficiency of the filter is important.

Filter Beta Ratio and Efficiency
Beta Ratio (x = particale size in micron µm)EfficiencyContaminants UpstreamContaminants Downstream
ßx = 250.0000%100,00050,000
ßx = 475.0000%100,00025,000
ßx = 1090.0000%100,00010,000
ßx = 2095.0000%100,0005,000
ßx = 4097.5000%100,0002,500
ßx = 6098.3333%100,0001,667
ßx = 7598.6670%100,0001,333
ßx = 10099.0000%100,0001,000
ßx = 12599.2000%100,000800
ßx = 15099.3330%100,000667
ßx = 20099.5000%100,000500
ßx = 30099.6670%100,000333
ßx = 50099.8000%100,000200
ßx = 1,00099.9000%100,000100
ßx = 2,00099.9500%100,00050
ßx = 4,00099.9750%100,00025
ßx = 5,00099.9800%100,00020
ßx = 10,00099.9900%100,00010
ßx = 20,00099.9950%100,0005
ßx = 50,00099.9980%100,0002


Beta ratio (symbolized by ß) is a more accurate method for rating a filter since implicit in its rating is an efficiency number. The Beta rating system is based on measuring the total particle counts upstream (influent) and downstream (effluent) the filter. A 3µm, ß1000 filter will be 99.9% efficient. A 3µm, ß100 filter will be 99% efficient. The difference in number of particles filtered is noticeable. The ß1000 allowed only 100 particles per 100,000 to pass, while the ß100 allowed 1,000 particles to pass. The chart on the next page shows the differences in Beta Ratio and efficiency. Knowing the cleanliness requirements of the fuel being filtered will help narrow down the filter needed.