Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
PNEUMATIC CONTROL FUNDAMENTALS
68
Fig. 15. Typical Heatless Desiccant
Dryer Airflow Diagram.
Simultaneously, some of the dried air passes through the
orifice (G) to the right desiccant chamber (E). The air is dry
and the desiccant chamber is open to the atmosphere, which
reduces the chamber pressure to near atmospheric pressure.
Reducing the air pressure lowers the vapor pressure of the air
below that of the desiccant, which allows the moisture to transfer
from the desiccant to the air. The timer controls the cycle, which
lasts approximately 30 minutes.
During the cycle, the desiccant in the left chamber (A) becomes
saturated, and the desiccant in the right chamber (E) becomes
dry. The timer then reverses the flow by switching both of the
solenoid valves (D and H). The desiccant in the right chamber
(E) then becomes the drying agent connected to the compressor
while the desiccant in the left chamber (A) is dried.
The process provides dry air to the control system continually
and requires no heat to drive moisture from the desiccant. A
fine filter should be used after the desiccant dryer to filter out
any desiccant discharged into the air supply.
C1889
DESICCANT
CHAMBERS
CHECK
VALVE
CHECK
VALVE
ORIFICE
ORIFICE
SOLENOID
A
B
C
D
DRY AIR OUT
H
G
F
E
AIR FROM COMPRESSOR
SOLENOID
PRESSURE REDUCING VALVE STATION
The pressure reducing valve station is typically furnished
with an air filter. The filter, high-pressure gage, high pressure
relief valve, pressure reducing valve (PRV), and low-pressure
gage are usually located together at one point in the system
and may be mounted directly on the compressor. The most
important elements are the air filter and the PRV.
AIR FILTER
The air filter (Fig. 16) removes solid particulate matter and
oil aerosols or mist from the control air.
AIR IN
AIR OUT
INNER FOAM
SLEEVE
FILTERING
MEDIUM
OUTER FOAM
SLEEVE
PERFORATED
METAL
CYLINDER
LIQUID
DRAIN
C2601
Fig. 16. Typical Air Filter.
Oil contamination in compressed air appears as a gas or an
aerosol. Gaseous oil usually remains in a vapor state throughout
the system and does not interfere with operation of the
controls. Aerosols, however, can coalesce while flowing
through the system, and turbulence can cause particles to
collect in device filters, orifices, and small passages.
Many filters are available to remove solids from the air.
However, only an oil-coalescing filter can remove oil aerosols
from control air. An oil coalescing filter uses a bonded fibrous
material to combine the small particles of oil mist into larger
droplets. The coalesced liquids and solids gravitate to the bottom
of the outer surface of the filter material, drop off into a sump,
and are automatically discharged or manually drained.