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.
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
(x = particale size in micron µm)
|ßx = 2
|ßx = 4
|ßx = 10
|ßx = 20
|ßx = 40
|ßx = 60
|ßx = 75
|ßx = 100
|ßx = 125
|ßx = 150
|ßx = 200
|ßx = 300
|ßx = 500
|ßx = 1,000
|ßx = 2,000
|ßx = 4,000
|ßx = 5,000
|ßx = 10,000
|ßx = 20,000
|ßx = 50,000
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.