7
V19
V18
DPS
S
V20
LEFT TOWER
RIGHT TOWER
F4
F3
O3
Figure 4.3
Legend
Item
Description
V18 & V19
Left & Right air sample isolation valves
F3 & F4
Left & Right air sample filters
V20
Air sample switching vlave
DPS
Dew point sensor
O3
Air sample flow orifice
B. Fixed Cycle - Two Stage Regeneration (Optional)
(1) Left Tower Drying - Right Tower Regenerating
At the start of the drying cycle, both inlet valves
(V1,V2) are open. The right inlet valve (V2) is then
closed to isolate the two towers. Wet, compressed
air flows up through the left tower where it is dried.
The dry air exits the dryer through the outlet check
valve (V7).
Next, the right depressurization valve (V6) is opened
and the right tower is slowly depressurized. Air exits
through an exhaust muffler (M2).
In preparation for the first stage regeneration, the
right purge valve (V4) is opened. Next, the blower
(B1) and heater (H1) are energized. The heated air
flows through check valve (V10), down through the
right tower, and exits through the right purge valve
(V4). The blower intake air is filtered (F1) to keep dust
and dirt from entering the dryer. The blower (B1) and
heater (H1) are de-energized when the temperature
at the bottom of the right tower, as sensed by
temperature switch (TS2), reaches the design set
point. This indicates the bed has been fully heated.
To fully seat check valve (V10) and prevent back flow
through the heater (H1) and blower (B1) during the
second stage regeneration, the right tower is partially
pressurized and then depressurized as follows. First,
the right purge valve (V4) is closed and the
repressurization valve (V15) is opened. The pressure
in the right tower begins to rise and check valve (V10)
is seated. Next, the repressurization valve (V15) is
closed and the depressurization valve (V6) is opened,
depressurizing the tower. Finally, the right purge
valve (V4) and repressurization valve (V15) are opened,
starting the second stage regeneration.
A portion of the dry air from the left tower now flows
through repressurization valve (V15). This air is
throttled to near atmospheric pressure by two
orifices (O1,O2). The dry, low pressure air flows down
through the right tower and exits through the purge
valve (V4).
At the end of the second stage regeneration, the
right purge valve (V4) is closed and the tower slowly
repressurizes to full line pressure. The right tower is
now ready to go back on-line.
(2) Right Tower Drying - Left Tower Regenerating
At the start of the drying cycle, both inlet valves
(V1,V2) are open. The left inlet valve (V1) is then
closed to isolate the two towers. The right tower is
now on-line, drying the compressed air. The left
tower will be regenerated, following a similar se-
quence as described in Section 4.3.B.(1) for the right
tower.
C. Demand Cycle (Optional)
Operation of the demand cycle is identical to the
fixed cycle (see Sections 4.3.A and B) except the cycle
is extended until the desiccant bed in the on-line
tower has been fully utilized. The off-line tower is
regenerated and remains in a stand-by mode after
being repressurized.
Figure 4.3 shows the air sampling system for the
demand cycle option. When the left tower is drying
and the right tower is regenerating, an air sample
from the left tower flows through an isolation valve
(V18), a filter (F3), and the sampling valve (V20) to the
dew point sensor (DPS). The sample flow rate is
controlled by a fixed orifice (03). The sample exhausts
to atmosphere.
