| The performance of an air filter is generally
evaluated based on four parameters. These include:
Contaminant removal efficiency
Contaminant holding capacity
Resistance to airflow
Safety with regard to support or spread of fire and smoke
Contaminant removal efficiency is tested by challenging
the filter with contaminant on the upstream side and measuring
the residual contaminant on the downstream side of the filter
after the air has passed through the media. Various methods
and instruments have been developed for moving the air through
the filter, holding the filter, generating and dispursing
the challenge, and measuring the challenge on the upstream
and downstream side of the filter.
Contaminant holding capacity is determined by measuring
the mass of contaminant removed before the filter reaches
its maximum differential pressure in the case of particle
filters or before the contaminant breaks through the filter
in the case of a gaseous filter.
Resistance to airflow is determined by measuring the pressure
of the air upstream of the filter and downstream of the
filter and comparing those values. The value of resistance
to airflow must be accompanied by the value of airflow velocity
in order to truly understand the performance of the filter.
Safety in a situation where a filter is exposed to flame
is measured by its support of fire when no other fuel source
is present (the filter is therefore a fuel source), the
amount of smoke generated by the filter, and the release
of sparks by the filter when exposed to the heat of a flame.
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70 % of the cost is energy!
Calculations reveal that energy normally accounts for 70
% of the total LCC of the system. The energy consumption
is directly proportional to the average pressure drop over
the life of the filter.
Choosing the right filter saves energy
Many people assume that the filter efficiency is the largest
contributor to initial pressure drop, however, it is the
actual design of the filter that has the main impact!
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Air Filter Testing
Nothing is ever black and white. A perfect air filter would
operate at 100% efficiency on the target contaminants, require
zero energy input, and last forever. However, no filter of
this type has been invented.
Filter efficiency, dust holding capacity and differential
pressure can be measured in many ways, and the performance
of an air filter changes over time. The challenge imposed
on air filters changes as the environment inside and outside
of a building changes. Many air filter testing methods have
been developed by various organizations for predicting the
in-use performance of filters and for comparing the performance
of air filters of different designs.
It is important to understand the complexity of differentiating
air filters. Many variables impact the results of a comparison
study, some of which are obvious and some of which aren't.
Most air filters will be in a system for months or even
years. However, testing of these filters often occurs in
a few minutes or hours. During its life, an air filter will
see dozens or hundreds of environmental changes such as
temperature, humidity, airflow velocity, and particle load.
However, testing of filters often occurs in a controlled
environment. Add to this the imperfect design of testing
methods and the various motivations of the people developing
test methods and you can conclude that you must fully understand
how to interpret the results of any air filter test you
are presented with prior to using these results to make
important decisions.
Organizations involved in setting filter standards and
testing methods include:
American Society of Heating, Refrigeration, and Air-Conditioning
Engineers (ASHRAE)
Institute of Environmental Sciences and Technology (IEST)
Underwriters Laboratories (UL)
International Organization for Standardization (ISO).
Each of these organizations has an area of focus but their
standards and testing methods may overlap in some cases.
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