Figure 9-17.Air filter using rotating blades as element.
air lines can create problems which are potentially
hazardous, such as the freezing of valves and
controls. This can occur, for example, if very high
pressure air is throttled to a very low pressure at
a high flow rate. The venturi effect of the throttled
air produces very low temperatures which will
cause any moisture in the air to freeze into ice.
This makes the valve (especially an automatic
valve) either very difficult or impossible to
operate. Also, droplets of water can cause serious
water hammer in an air system which has high
pressure and a high flow rate and can cause
corrosion, rust, and dilution of lubricants within
For these reasons, air driers
(dehydrator, air purifier, and desiccator are all
terms used by different manufacturers to identify
these components) are used to dry the compressed
air. Some water removal devices are similar in
design and operation to the filters shown in figures
9-16 and 9-17. Two basic types of air dehydrators
are the refrigerated-type and the desiccant-type.
In refrigerated-type dehydrators, compressed air
is passed over a set of refrigerated cooling coils. Oil
and moisture vapors condense from the air and can
be collected and removed via a low point drain.
DESICCANT-TYPE DEHYDRATORS. A
desiccant is a chemical substance with a high
capacity to absorb water or moisture. It also has
the capacity to give off that moisture so that the
desiccant can be reused.
Some compressed air system dehydrators use
a pair of desiccant towers (flasks full of desiccant).
One is kept in service dehydrating the compressed
air, while the other one is being reactivated. A
desiccant tower is normally reactivated by passing
dry, heated air through it in the direction opposite
the normal dehydration airflow.
Another type of chemical drier is shown in
figure 9-18. This unit consists of the housing, a
cartridge containing a chemical agent, a filter
(sintered bronze), and a spring. Various types of
absorbent chemicals are used by the different
manufacturers in the construction of the
cartridges. To ensure proper filtering, the air must
pass through the drier in the proper direction. The
correct direction of flow is indicated by an arrow
and the word FLOW printed on the side of the
Figure 9-18.Chemical drier.