Efficient Plant October 2021 - 20

feature | lubrication solutions
Fig. 1: Oil fi lter beta rating
Fig. 2: Three levels of beta effi ciency
Micron size
The beta rating of a fi lter is measured by comparing the number of
particles upstream and downstream of a fi lter at a given micron size.
Oil-fi lter manufacturers report the beta effi ciency of an oil fi lter as the size of particle
at which the fi lter achieves a capture effi ciency of 98.7%, 99.5%, and 99.9%.
cles. Unfortunately, nominally rated fi lters are oſt en used because
they are typically the least expensive.
Absolute effi ciency: Th e term absolute might suggest that a
specifi c fi lter " absolutely " removes all particles greater than a certain
size rating. Since the pore sizes of any oil fi lter are not uniform
throughout the fi lter, in practice, absolute oil fi lters rarely if ever
perform at 100% capture effi ciency for their absolute micron rating.
While there are engineering standards that are used to defi ne a
fi lter's absolute rating, in practice most absolute-rated oil fi lters have
a capture effi ciency of 97% to 98% at the stated micron rating.
Beta effi ciency: Th e beta rating of a fi lter is a measure of the
expected capture effi ciency at a given micron size. Th e test, which
is covered under the ISO 16889 standard, uses a multi-pass test
stand to empirically determine the capture effi ciency of a fi lter at
specifi c micron sizes (Figure 1). Th e beta ratio at a certain micron
size is simply the ratio of the number of particles upstream of the
fi lter at a given size range versus downstream. In the example in Fig.
1, for every 100, 5-micron particles upstream of the fi lter, only 10
are measured downstream, giving the fi lter a beta rating of 10 (100
divided by 10) at 5 microns. From the beta rating, the fi lter's capture
effi ciency can then be calculated as a percentage.
While the fi lter beta rating is a good indicator of the expected performance,
do not interpret the result too literally. Th e multi-pass test
is a lab-based test that is conducted under steady-state conditions.
In the real world, pressure surges, changes in fl ow rates, mechanical
vibration, and temperature swings can all aff ect actual fi lter performance,
based on the structural and mechanical design of the fi lter.
20 | EFFICIENTPLANTMAG.COM
For this reason, several manufacturers have developed variations
of the multi-pass test to measure the beta effi ciency of fi lters under
variable (stressing) conditions.
Whenever possible, fi lters should be selected based on the appropriate
beta rating to achieve the desired (required) ISO 4406 fl uid
cleanliness. In most cases, manufacturers report the beta effi ciency
as the size of particle at which the fi lter achieves a beta rating of 75,
200, and 1,000, equivalent to a capture effi ciency of 98.7%, 99.5%,
and 99.9%, respectively (Fig. 2).
Th e impact of beta rating and fi lter performance can be illustrated
by the oil samples shown in Fig. 3, which shows the sharp cut-off in
particles larger and smaller than the fi lter's beta 1,000 rating when
oil is fi ltered through a high-quality fi lter element.
FILTER CAPACITY
In addition to measuring the beta effi ciency, the ISO 16889 standard
determines the dirt-holding capacity of a fi lter. Th is is measured
as the number of milligrams of contaminants captured per square
centimeter of media before the fi lter reaches a terminal diff erential
pressure due to particle loading. Dirt-holding capacity for a specifi c
fi lter is oſt en stated simply as the total amount (weight) of particles a
given fi lter holds, without reference to the total surface area.
Th is is an important consideration and the crux of why some
fi lters with the same part number and micron rating oſt en perform
so diff erently. Most oil fi lters, whether they are elements that are
inserted into a housing or a simple spin-on fi lter, are made from
pleated fi lter media. It is the diff erence in the type of media as well as
OCTOBER 2021
Capture effi ciency
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Efficient Plant October 2021

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