The replaceable element is made of specially
treated convolutions (wrinkles) to increase its
dirt-holding capacity. The element is noncleanable
and should be replaced with a new filter element
during maintenance inspections.
MAGNETIC FILTERS. Some hydraulic
systems have magnetic filters installed at strategic
points. Filters of this type are designed primarily
to trap any ferrous particles that may be in the
Clean, dry gas is required for the efficient
operation of pneumatic systems. Due to the
normal conditions of the atmosphere, free air
seldom satisfies these requirements adequately.
The atmosphere contains both dust and impurities
in various amounts and a substantial amount of
moisture in vapor form.
Solids, such as dust, rust, or pipe scale in
pneumatic systems, may lead to excessive wear
and failure of components and, in some cases,
may prevent the pneumatic devices from operating.
Moisture is also very harmful to the system. It
washes lubrication from moving parts, thereby
aiding corrosion and causing excessive wear of
components. Moisture will also settle in low spots
in the system and freeze during cold weather,
causing a stoppage of the system or ruptured lines.
An ideal filter would remove all dirt and
moisture from a pneumatic system without
causing a pressure drop in the process. Obviously,
such a condition can only be approached; it
cannot be attained.
Removal of Solids
The removal of solids from the gas of
pneumatic systems is generally done by screening
(filtering), centrifugal force, or a combination of
the two. In some cases, the removal of moisture
is done in conjunction with the removal of solids.
Some types of air filters are similar in design
and operation to the hydraulic filters discussed
earlier. Some materials used in the construction
of elements for air filters are woven screen wire,
steel wool, fiber glass, and felt fabrics. Elements
made of these materials are often used in the unit
that filters the air as it enters the compressor.
Porous metal and ceramic elements are
commonly used in filters that are installed in the
compressed air supply lines. These filters also use
a controlled air path to provide some filtration.
Internal design causes the air to flow in a circular
path within the bowl (fig. 9-16). Heavy particles
and water droplets are thrown out of the airstream
and drop to the bottom of the bowl. The air then
flows through the filter element, which filters out
most of the smaller particles. This type of filter
is designed with a drain valve at the bottom of
the bowl. When the valve is opened with air
pressure in the system, the accumulation of solids
and water will be blown out of the bowl.
An air filter that uses moving mechanical
devices as an element is illustrated in figure 9-17.
As compressed air passes through the filter the
force revolves a number of multi-blade rotors at
high speed. Moisture and dirt are caught on the
blades of the rotors. The whirling blades hurl the
impurities by centrifugal force to the outer rims
of the rotors and to the inner walls of the filter
housing. Here, contaminating matter is out of the
airstream and falls to the bottom of the bowl
where it must be drained at periodic intervals.
Removal of Moisture
The removal of moisture from compressed air
is important for a compressed air system. If air
at atmospheric pressure, even at a very low relative
humidity, is compressed to 3000 or 4500 psi, it
becomes saturated with water vapor. Some
moisture is removed by the intercoolers and
aftercoolers (see glossary). Also, air flasks,
receivers, and banks are provided with low point
drains to allow periodic draining of any collected
moisture. However, many uses of air require air
with an even smaller moisture content than can
be obtained through these methods. Moisture in
Figure 9-16.Air filter.