Repeat the process of prefilter installation until all housings are filled
with latched prefilters as shown below.
3.8 Pressure Gauge: It is recommended that a pressure gauge be
installed across the filter bank to indicate when the prefilter needs to
be replaced. The pressure gauge will register the pressure differential
across the bank. As the prefilter loads with dirt the resistance to the
airflow, and consequently, the pressure across the filter will increase.
The pressure across the gas phase chemical filter will remain
constant since this filter collects gas molecules and not particulate. If
a pressure gauge has not been included in the customer s purchase
order, discuss the pressure gauge options with your AAF representative.
If a pressure gauge has been included in the order, refer to the
specific installation instructions supplied with the gauge.
3.9 After-filters: Some gas phase chemical filter media may
discharge a small plume of dust on fan start-up when the filter system
is first operated. This may result in dust being deposited on the
downstream side of the filter bank. There are generally two ways to
deal with this:
— In industrial applications that are not particularly sensitive to small
amounts of dust, this initial plume is allowed to blow off and the
deposited dust is swept up or vacuumed.
— In applications that are sensitive to dust, it is recommended that
an after-filter similar to the prefilter discussed above be supplied on
the downstream side of the filter bank. This will require an
additional frame bank to be mounted in the duct on the downstream
side of the gas phase chemical filters. In addition to the MERV 7
after-filter a higher rated air filter may also be installed to provide a
significantly improved level of air cleanliness in the space to be
ventilated. Many options are available up to HEPA and ULPA filter
systems. If an after-filter or higher efficiency final filter with
appropriate support frames have not been included in the
customer s purchase order, discuss the additional filter options with
your AAF representative. If any of these items have been included
in the order, refer to the specific installation instructions supplied
with those items.
4.0 Start-up Instructions
When the fan system is started up, ensure that the airflow volume is
controlled so that the actual airflow velocity across the filter bank does
not exceed the rated face velocity shown on the AAF submittal
drawing. If there is any doubt about the required face velocity the
default maximum velocities shall be as follows:
— MD and CG cassettes: 500 feet per minute
— HD cassettes: 250 feet per minute
Immediately on start-up examine the filter system for any apparent air
leaks or other anomalies. Air leaks may be detected by noise or by
use of a synthetic smoke puffing device at the internal and perimeter
joints and seams of the filter bank installation. Correct or repair any
discrepancies, as necessary. Repeat this examination after 24 hours
of operation and again after one week of operation.
Monitor the pressure drop across the filter bank to ensure that the
filters are operating within the pressure range expected and to ensure
that the pressure drop is not too high. Consult the AAF drawing for the
expected clean filter pressure differentials.
5.0 Maintenance
5.1 Prefilter: If a prefilter has been included as part of the system,
record the pressure drop weekly to obtain the status of the particulate
filter. A normal particulate prefilter can be expected to last for between
2 and 3 months before reaching its final recommended pressure drop
(see the AAF drawings for details). However, under heavier or lighter
dust loading conditions this may vary. At an airflow velocity of 500 feet
per minute the 2 deep AAF PerfectPleat filter can be expected to
have an initial pressure drop in the range of 0.35 water gauge. The
recommended final pressure drop is 1.0 water gauge. This means
that when a pressure drop increase of 0.65 water gauge has been
recorded it is time to change the prefilter.
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