It should all be so simple in compressed air filtration: The
comparison of filter performance data is possible with
standard ISO 12500 and registering operating hours is as
easy as reading the differential pressure indicator. However,
the energy conditions of air filtration still receive very little
attention, as Donaldson Filtration Deutschland explains

Compressed air filters use only indirect
energy based on the differential
pressure, which is applied by the
compressor. Dimensioning of a compressed
air system, and therefore the capacity and
number of filters used, is based on the
required compressed air quantity and quality,
location of the compressed air purification
system and type and age of the compressors.

The contamination of the ambient intake air
and the humidity, which varies according to
weather conditions, must also be considered,
as must the impurities that are introduced by
the compression process.

It is generally recommended to call in
filtration experts to measure contamination
that may be caused by the compressors.

Contamination by lubricating oil alone may
amount to 40mg/m3 depending on type of
compressor; this may increase to more than
100mg/m3 in older units. Even in oil-free
compressors, abrasion, moisture and
contamination by water injection and
contamination through the lubricants of the
gears and bearings can add to this.

When the compressed air filters at the
points of use are heavily contaminated, the
differential pressure rises faster. This problem
clarifies the complex interactions of the
components in compressed air systems.

Experienced engineers know that the
condition of a compressed air system can be
seen on the filters.

The Fraunhofer Institute for Systems
Technology and Innovation Research (ISI)
cites the following in a study during the
German Compressed Air Campaign
Druckluft effizient back in 2003: “In the field
of compressed air treatment, it is primarily
important to achieve the compressed air
quality required for the application. It is best
for energy and operating costs to meet the
requirements of the applications. Exceeding
or falling below the air-quality requirements
results in increased operating and energy
costs. The current potential savings per
component part are thereby up to several
thousand Euros. Regularly changing the filter
elements within the recommended intervals
achieves significant savings and thereby
minimises the operating costs.”
Compressed air filter elements must be
replaced to improve the energy balance:
¦ if they are not validated according to ISO
12500 and an objective performance
comparison is impossible or
¦ when the differential pressure of
coalescence filters for the separation of
aerosols and liquid oil/water content
exceeds the limit, or if they have been used
for more than 12 months or 8000
operating hours.

The specification of value ‘350 mbar’ in the
Druckluft effizient campaign is a reference
value, and reflects the differential pressure
curve of compressed air filters. The differential
pressure of a new filter element initially
increases very slowly. With time, the pressure
rises more rapidly and reaches the point at
which filter element replacement is necessary.

The recommendations for differential
pressure limits vary depending on the filter
manufacturer and may well be at 500mbar.

Contamination of the compressed air can
occur at any point on the compressed air
purification system, so it is important that the
filters have reliable differential pressure
gauges. Economizers indicate when the
differential pressure reaches the limit value
and an element replacement is advisable.

Although long-term experience shows that
an element replacement in the coalescence
filters is useful when the differential pressure
limit is reached, it is not the sole criterion. A
sudden steep increase in differential pressure,
for example, is a warning of increased
contamination by particles. Therefore, one
cannot solely rely on the differential pressure.

Even quality filter elements with their great
contamination retention capacity and the high
resistance of their filter
media against the
constant exposure to
corrosive substances
should be replaced at the
latest after 8000
operating hours or 12
months.

This replacement
interval based on
operating hours applies
also to activated carbon
filters that are used to
separate oil vapours and
odours. These are often
used in sensitive areas
such as food and
pharmaceuticals and in the generation of
process air, to achieve a residual oil content of
air pressure of 0.01mg/m3, as referred to in
ISO Standard 8573-1 for compressed air
quality class 1.

Even the use of high quality activated
carbon adsorbents, used as a bed or as an
integrated component of filter media, can
lose effect, despite their high adsorption
capacity, which is not reflected in the
differential pressure. The recommendation
for the filter element replacement is at a
maximum of 2000 operating hours or three
months. Depending on the prevailing oil
vapour content and at inlet temperatures
above 20°C, a replacement is recommended
already at an earlier time.

In compliance with the replacement
intervals and differential pressure limits, the
user can be sure that the productivity and
efficiency of systems which rely on
compressed air, are maintained or increased
and the optimum protection of the workers is
ensured.