Hi Philip
First thing you should do is obtain (if you have not already got
them) the certification and performance data for the filters installed.
Filters are designed to give a standard (for that filter) pressure
drop (PD) at a certain flow. They then have working range (of
differential pressure) as they slowly 'blind' and choke off the
flow. This is typically (but not definitively) 200Pa.
Your design engineers should have calculated the total air that was
required to maintain the room DP while supplying the air change rate
that the room classification specified. They should then have sized
the number of filters and the physical size of the filters required
to accommodate that flow, allowing a 20% margin for blinding. I.E.
the working range of the filters.
So when you come to commissioning you expect to see, a flow through
the each filter that is around the design 'nominal' flow for that
specific filter.
This nominal flow should result in a DP that is also around the
nominal DP for a clean filter at its rated flow. The filter will
then slowly choke off the flow as it 'blinds' through its working
life. It will be changed at the PD that the makers specify as the
'dirty filter condition'. Briefly the filters have:
An initial rated working air flow rate.
A nominal PD at this air flow rate.
A working range of PD.
The nominal air flow rate you find invariably differs from the filter
design specified nominal flow. Therefore the initial PD will also
differ from the specified DP. You only know when to consider the
filter 'as dirty', when the working range of the filter is
exhausted. I.E. Dirty filter change PD is calculated as initial
actual clean filter PD plus filter working range PD.
So if you start with a standard flow rate that is less than the
filter specified one; you will have a lower initial filer PD. Then
add the working range PD onto this figure then you will have a lower
PD that the filter has to be changed at.
The regulators are very aware of this and will ask for a written
justification on how you calculated your actual clean filter DP and
so derived your dirty filter change PD.
Further details on this are available at:
http://quality.validation-online.net/HVAC.html
There are half a dozen or more filter manufacturers contact them and
give them the flow you require and the physical dimensions and see if
they have filters that can give the performance you require.
Alex
validation-online.net
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