Systems that concentrate movement of air also concentrate
the dust and grime contained in it.
Filtration is the only way to remove it.
There are two
criteria used to compare the performance of filters. The first is the pressure
drop which occurs from one side of the filter to the other. This pressure drop
arises due to the resistance that the air encounters passing through the small
pores in the media. The smaller the pores the greater the resistance. The
second criterion is the filters ability to remove dust from the airstream,
measured in terms of the filter efficiency. Various standard test methods exist
(BS6540) (refer to IP1) which involve measuring how much dust there is in the air
upstream and downstream from the filter. The removal efficiency of the filter,
expressed as a percentage can be calculated from these two values.
The efficiency will
depend on the size of the dust particles and the pore size of the filter media.
So for example a coarse filter with a relatively large pore size will have a
high efficiency at collecting large particles but a low efficiency at
collecting smaller particles. Materials with small pore sizes are good at
removing both the large and small suspended matter. They do however, because greater
pressures drop within the system. A large pressure drop will necessitate the
use of a higher capacity fan which will increase the electrical consumption of
the system. A common compromise is to select a medium grade filter even though
this may not necessarily provide the quality of air required. One way of avoiding
this compromise and reducing the pressure loss in higher grade filters, is to
increase the surface area through which the contaminated air can flow. The list
below shows how we increase the working area of filters as the grade increases
to maintain an acceptable working resistance. Face area 600x600mm in all cases
Type Area m2
low grade pad filter
50mm deep 0.36
medium grade 4 bag
filter 400mm deep 1.92
high grade 6 bag
filter 600mm deep 5-5.76
Modern developments
include the 'rigid pack' paper filled filter a 600x600x300mm unit can provide a
working area up to 18m2.
Filter Replacement
If filters are to
carry out their role of dust extraction from the airstream whilst not affecting
the air movement considerably they must be replaced by clean filters at regular
intervals. This can be carried out using routine maintenance or condition based
maintenance.
Routine Maintenance involves making a decision based on previous
experience, knowledge of dust conditions in the building and filter performance
to determine a period after which the filter should be changed. So for example
the filters in an air conditioning system may be routinely changed every six
months. This is a simple method but may mean that the system operates with
dirty filters for a time if the filters have clogged up quicker than expected.
It could also mean that if the dust load is low relatively clean filters are being
removed and replaced.
Condition Based Maintenance
avoids the problems encountered
with routine maintenance. Filter changing is based on the actual
state of the filters rather than an assumption of their condition. The
system works as shown in below figure. Transducers either side of the
filter monitor air pressures. The pressure at point P1 will always be
higher than P2 due to the resistance of the filter. However when the
filter begins to clog up this pressure differential will increase. The
pressure transducers can be observed manually or the signals fed into a
BEMS system which will inform the building operators that the filters
need changing.
Monitoring
filter condition
