How to Know When to Change Air
Conditioning Filters
By
Thomas
Yoon
Although we don't
usually realize it, we can actually reduce the running cost of a
building by removing the dust from the atmosphere. The furniture,
walls and fittings are preserved longer. It also improves the health
of the occupants.
Removing dust is
usually done by installing filters in the incoming air stream or the
circulating air stream. In air-conditioning systems, these filters
are installed at the air handling units or AHU's.
How do we know when
to replace the filters?
As more and more
dust accumulates at the filters, the differential pressures between
the inlet and the outlet will increase. By measuring the
differential pressures, we will have a very good indication of how
much the filter has been choked by dusts. Persons maintaining the
air-conditioning plant should know at what differential a filter
should be replaced. Some of the dust particles are so minute, that
they cannot be seen by the naked eye. In all appearances, the filter
looks as good as new.
These are some
factors to consider when determining when to change the filter:
Mechanical
strength of the filter
The final pressure
of a filter is usually stated in the product. That is the strength
of the filtration medium. However, other factors need to be
considered - the strength of the filter walls, frames and holders if
not adequately designed can result in the sagging of the filter
housing that will result in leaks.
Changes in
efficiency of the filter
This is
particularly important in low efficiency filters. Their efficiencies
can be radically impaired as a result of high final pressure losses.
Particles that have been collected by the filter can become detached
and accompany the air stream. Fibrous particles are less likely to
become detached compared to granular particles. Because of this,
higher final pressure drops are permissible for the former, although
this should not exceed about 200 Pa in a pre-filter.
With granular
particles, the final pressure loss should not be allowed to exceed
100 to 150 Pa.
Fine filters and
HEPA filters are less sensitive to changes in quality. In principle,
they can be allowed to have a final pressure loss up to the limit of
their mechanical strength.
Permissible Air
flow variations in the system
In this case, the
flow pattern is more important than the particles carried along the
air stream. The permissible final pressure loss here will depend on
the total pressure and characteristics of the fan and any other
control facilities.
The filter will
affect the air flow as a result of dust accumulation. The filter has
to be changed when the velocity of the air in a laminar flow
installation cannot be maintained or has fallen below the required
specifications.
Bacteriological
reasons
Sometimes, the
filters are replaced at regular intervals instead of pre-defined
pressure loss indications. This is to prevent accumulation of
bacteria and other microorganisms.
Suitable time
window periods
Filters are often
changed to coincide with pre-planned plant shutdown periods. In
these cases, production disturbances are minimized. For example they
can be arranged to be changed during holidays.
Economic
considerations
In terms of
operating costs, there is an optimum final pressure point to
balance. By having longer intervals between filter replacements, we
do achieve lower filter replacement costs. On the other hand, this
will increase the operating energy costs due to inefficiency.
Many years of
working experience in Marine, Facilities, Construction has given the
author material for writing e-books and articles related to
engineering, and management.
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