IPF
Page A1
Appendix A
Instrument Air and
Filtration
The selection and use of a good air filtration system
is essential in ensuring optimum performance of
pneumatic instrumentation and devices. Most users
find that it is much less expensive and troublesome
to design a system that includes a good air filtration
than to deal with downtime and repairs later. The
cleaner the air, the longer the time before servicing
will be needed.
Obtaining good instrument-quality air involves
removing solids, oil, and water after compression.
Oversizing elements helps to avoid performance
aberrations, and should reduce the need for periodic
maintenance. Redundancy should be used where
possible to avoid shutdown during maintenance.
The IPF is available with an optional coalescing filter/
regulator module, the FR1 Option, that combines a
0.01 micron air filter and a miniature supply-line
regulator. The unit has a 1/4-inch NPT female port
and a 4 scfm maximum flow at 90 psig inlet pressure.
It is furnished with an integrally mounted pressure
gauge scaled in both psi (0-60) and bars (0-4).
The Problems
Oil:
Oil is the most common problem in compressed
air instrument systems. A coalescing filter removes
sub-micron liquid oil droplets from the air, and is
usually supplied with an automatic drain.
The filter works by trapping oil and water droplets in
a bed of microfibers. Droplets run together at fiber
cross-over points, form large liquid drops, and are
forced by air flow to a drain. A filter system consist-
ing of a general purpose first-stage filter (about 5
micron) and a high-efficiency coalescing final filter is
recommended to obtain contaminant-free air.
The exact location of the first-stage filter in the loop
is not important; it can be located just ahead of each
final filter, or a single first-stage filter can be located
on a main line to protect a number of final filters on
branch lines. Each final filter (coalescing) should be
located just ahead of each pressure regulator.
Water:
The amount of water in an air system
depends on temperature, pressure, and the relative
humidity of the air. The amount of contamination,
therefore, tends to vary widely with geographic
location and weather.
To obtain intrument-quality air, sufficient water must
be removed to lower the dew point of the air to a
temperature below ambient. The dew point (at line
pressure) is expressed as the temperature at which
any moisture in the system begins to condense.
Water may be removed using a number of tech-
niques, including coalescing filters, refrigeration
dryers and desiccant-type dryers. Care must be
taken in the selection and location of the filter,
because cooling, downstream of the filter, can cause
more condensation of water.
Typically, a coalescing filter should be installed
immediately upstream of the pressure regulator. In
this way the filter removes most of the water before
the air enters the regulator. Air leaving the regulator
then continues to dry due to expansion.
Solids:
Random solid dirt, such as pipe scale and
rust, is occaisionally a problem in compressed air
instrument systems. A good filter will remove these
solids. However, if there is a desiccant dryer in the
line, a high-efficiency sub-micron filter is recom-
mended for removing the highly abrasive sub-micron
particles produced by the dryer. This sort of filter is
desirable in any system, and is often a feature of
coalescing filters.
For systems subjected to freezing temperatures, the
portion of the system that runs outdoors should have
a dryer installed. The dryer reduces the dew point
below the lowest expected outdoor temperature.
A desiccant dryer is used upstream from a coalesc-
ing filter to keep the dryer from being damaged by oil
or from being overloaded with excessive condensed
water. Another high-efficiency coalescing filter is
recommended downstream of the dryer, to remove
desiccant particles.
Figures A1 and A2 illustrate typical non-redundant
systems with multiple branch lines. They both work
in any environment above freezing and differ only in
the placement of the first-stage, general purpose
filter.
