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Review of Eurovent 4/10 - 1996
In Situ Determination of Fractional Efficiency in
General Ventilation Filters
The purpose of the EN
4/10 research paper was what many of us in the industry of air
filtration have looked for as a method of validating the
performance of one filter system to another in real time.
Fractional or particle size efficiency of filters in use in
the end users application is difficult to predict with
laboratory style testing where the conditions and parameters
are controlled. At the same time interpreting the results of
testing performed in the actual environment can be very
different than what we expected from the filter.
First lets review what
the EN 4/10 guidelines are.
1) Particle counter
with sampling probes upstream and downstream of the filter
a. Preset distances from the filter in test
b. Valves to switch from up/downstream
2) Computer – to
track multiple counts
3) Diluter – a
device to reduce the concentration down to the particle
counter limits
4) Pump – may be
necessary to achieve isokinetic flow to the sampler
5) Determine whether
ambient aerosol is significantly different from the
calibration aerosol (procedure to make ‘lab’ like corrections)
6) Moisture check to
make sure within the instrument capabilities
EN 4/10 is specifically
designed to target the particle sizes from 0.2 – 1.0 microns.
Although there is a provision for the ‘greater than one micron
size’, this size category is regarded as having unreliable
data reporting. Sampling lines should be of a material
that will not allow the particulates to adhere to the inner
wall surfaces of the hose. The sample probe itself should have
a sharp edge at air entry and be a minimum of 8mm (appx 3/8
inch) in diameter. The valves should be of the same diameter
as the tubing to reduce the possibility of creating a pressure
blockage or particle retention point.
In Situ sampling requires
the ability to install a probe upstream and downstream of the
target filter. This requires installation without leakage from
the insertion of the test lines. The filter system should be
sealed to prevent leakage of the framing members and/or frames
to filters.
Efficiency determination
is made as the result of a minimum of twelve counts, of a
minimum of 20 seconds each, conducted successively upstream
and downstream of the filter. There are some provisions for
purge included for the lines between samples. The reported
efficiency is essentially a computed average of the upstream
vs downstream concentrations. Although the procedure outlined
allows for the averaging of two of the upstream measurements
to one downstream measurement and not conversely.
Potential
for errors
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Humidity may have an
influence on the instruments if operating beyond their
limits. It can also coerce variations in the efficiency and
particle behavior.
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Temperature could have
problems with instruments in cold conditions.
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Aerosol Composition the
refractive index, density, and shape of the particles can
cause variations from test aerosol calibration.
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Uneven Aerosol
Distribution will occur and will show variations in time.
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Turbulent Airflow has
the potential to distort the results with non-directional
airflow patterns.
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Coincidence errors are
the limitations of the particle counter itself.
What we find from the
outline of Eurovent 4/10 – 1996 is an effort to take the
laboratory out into the field and determine fractional
efficiency. Many references are made in correlation to “in the
lab conditions”. This is not surprising as lab folks look for
repeatability in any thing they do as a methodology. What is
difficult to accept from their work is the complete disregard
to the particulate larger than one-micron. This is a large
part of our market segment and to disregard the values found,
In Situ appears as a choice, to ignore the potential for
unloading or releasing of particulates from the media.
Previously published papers have documented for example the
release of fiber from unsupported air-laid microglass media.
The phenomenon of agglomeration of smaller particles can lead
to the inability of a filter to retain the larger created
particle due to the media velocities or turbulence within the
system.
Additionally in most
scientific studies when taking numerous measurements those
that are statistically out of proportion from the rest are
discarded while the samples used for calculation in the EN4/10
are not. This leads to some calculations that are not really
descriptive of the actual performance. It is important that
each number or set of values makes sense with the sampling
techniques and that human errors of statistical significance
are eliminated.
It is not possible to get
good representative values when inserting a hose into the side
of a duct without sealing the penetration, as there will be
turbulence caused by the air entering the duct through the
remaining orifice space. Additionally the size of the plenum
area upstream and downstream must be the same otherwise there
will be different values in the face velocity, which will
alter the particle capture ability.
While there are
definitive issues with the degree of parameters set in this
document it remains the genesis of creating a teachable method
of measure for those without practical experience using a
laser particle counter in the field. As we look at the
potential for this kind of field measuring of performance
verification and recognize that this was done in 1996 the
future will be brighter with the emergence of particle
counters that are more user friendly and easier to transport
with the likes of the ARTI and Lighthouse versions.
Each of these devices
allow lightweight multiple channel-sizing particle counts with
very economical purchase prices. One of the features that make
them valuable is that they can be set up to data-log and make
several counts in the same area without physical attention.
There is still no
shortcut or substitute for experience in diagnosis of what the
data means once collected. This is profound when in the
Eurovent 4/10 examples where, at the 3.00 to 4.50 micron size,
the filter in test is negatively efficient while calculated at
83% at 0.75 to1.00 micron. This is not the possibility of
unreliable information if it is true through all 12 counts!
(page 22)This phenomenon has been witnessed in several lower
efficiency filters and reported as unloading or sloughing,
should it be any different in high efficiency filters, or In
Situ testing?
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