| FILTER
EFFICIENCY TESTING AND AIRFLOW MEASUREMENT IN RELATION
TO NEGATIVE PRESSURE UNITS.
ARTICLE
PROVIDED BY STEVE WILLIAMS, ENVIROVAC SALES AND SERVICE
LTD.
It
is generally accepted that there is a requirement for
negative pressure units and type H vacuum cleaners to
be serviced and tested every six months. As part of
this process the efficiency of the filtration should
be measured and airflow will be determined in order
to confirm the suitability of the equipment for use
with asbestos.
It
is my intention to explain in a little detail the nature
of these measurements and how they are determined.
Filter
efficiency measurements.
For this type of equipment there are
three main methods for the determination of filter efficiency.
1.
Dispersed oil particulate ( DOP test ).
2. Sodium chloride aerosol flame photometer test.
3. Condensation nuclei test.
Of
these three methods DOP testing is the one commonly
used due to the compact nature of the test equipment,
portability and ease of use.
This
method requires the use of an aerosol photometer with
a built in pump which draws a sample of air through
through a flexible tube into the instrument. The sample
then passes through a coarse filter in order to remove
any possible debris. The air is then drawn across an
area of intensive white light in the optical chamber
and any particles of significance present in the area
being sampled deflect the light source on to the sensitive
area of a photomultiplier tube. Each individual burst
of light is converted into a pulse of electrical energy
which is proportional to the amount of light scattered
by the particle.
In
order for the aerosol photometer to measure the efficiency
of a filter system an aerosol generator is required
to provide particles in the form of a dense cloud of
very small oil droplets in a stream of carbon dioxide
or nitrogen. This cloud of particles is then mixed with
air in a duct in order to produce an homogenous sample
for the aerosol photometer to measure.
Put
simply the filter efficiency is determined by measuring
the amount of particles upstream of a filter system,
the concentration being in the region of 80 to 100 micro
grams per litre by comparison to the amount of particles
downstream of the filter system..
Many
instruments by nature of their design and construction
can read filter efficiencies directly in percentages.
These are termed filter penetration. In order for the
complete machine to be suitable for asbestos and related
dangerous dusts the filter penetration should not exceed
0.005 % to DOP test aerosol, this is a PAS 60 requirement.
Since
the efficiency of a filter system is partly dependent
on the volume of air passing through it any testing
carried out should be done at full flow in order to
determine the maximum penetration likely to occur in
an operational situation.
Like
all sensitive instruments it is necessary for correct
maintenance and regular calibration checks to take place.
Calibration certificates traceable to National Standards
should be available for inspection for all equipment
used in testing when requested.
Airflow
measurements
DETERMINING
THE CORRECT MACHINE FOR THE ENCLOSURE SIZE
CALCULATE
THE VOLUME OF THE ENCLOSURE IN CUBIC METRES BY MULTIPLYING
THE LENGTH HEIGHT AND WIDTH IN METRES
FOR
SOME ENCLOSURES YOU MAY HAVE TO MAKE AN APPROXIMATION
DUE TO THE COMPLEXITY OF THE STRUCTURE. THIS CAN BE
DONE BY MEASURING SECTIONS OF THE ENCLOSURE CALCULATING
VOLUMES AND ADDING THEM TOGETHER TO GIVE A TOTAL.
A
SIMPLE EXAMPLE 2.5 METRES HIGH X 5 METRES WIDE X 5 METRES
LONG
VOLUME
OF ENCLOSURE IS 2.5 X 5 X 5 = 62.5 CUBIC METRES
DETERMINE
THE REQUIRED NUMBER OF AIR CHANGES PER HOUR, NORMALLY
A MIMIMUM OF 8
THE
VOLUME FLOW OF THE EXTRACTOR REQUIRED WOULD BE 8 X 62.5
OR 500 M3/HOUR
THIS
IS PURELY A MATHEMATICAL CALCULATION AND AN ALLOWANCE
MUST BE MADE FOR ANY AIRLOCKS FITTED.
IF
THE ENCLOSURE IS NOT CONSTRUCTED CORRECTLY A NEGATIVE
PRESSURE DIFFERENTIAL BETWEEN THE INSIDE AND THE OUTSIDE
OF THE ENCLOSURE MAY NOT BE ACHIEVED
TO
CONVERT M3/HOUR TO CUBIC FEET PER MINUTE DIVIDE BY 1.7
TO
CONVERT CUBIC FEET PER MINUTE TO M3/HOUR MULTIPLY BY
1.7
TO
CONVERT CUBIC METRES TO CUBIC FEET MULTIPLY BY 35.31
TO
CONVERT CUBIC FEET TO CUBIC METRES DIVIDE BY 35.31
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