Air Removal - Trane PRGD series Installation, Operation And Maintenance Manual

collected in the purge tank limits the available
condensing surface in the tank, the condensing-unit
compressor suction temperature begins to fall.
The purge controller initiates a pump-out cycle when
the suction temperature reaches the pump-out initiate
value that is calculated within the purge control. During
the pump-out cycle, the small pump-out compressor
pulls any non-condensables from the purge tank and
discharges them through the carbon tank. As the non-
Figure 7. Purge operating limits

Air Removal

If no air is in the purge tank, the refrigerant returning to
the purge condensing unit compressor suction has a
high superheat (heat added past the point of
evaporation), because of the heat removed from the
condensing chiller refrigerant vapor in the purge tank.
As air accumulates in the purge tank, it displaces the
chiller refrigerant vapor and decreases the amount of
coil surface that is exposed to the vapor. Less heat is
removed from the vapor, and the available superheat
at the purge condensing unit compressor suction
consequently falls. When the purge refrigerant
PRGD-SVX01F-EN
S S e e q q u u e e n n c c e e o o f f O O p p e e r r a a t t i i o o n n s s
condensables are removed from the purge tank, the
condensing-unit compressor suction temperature
increases. The purge controller monitors the
compressor suction temperature and cycles or stops
the pump-out, depending on the temperature that is
present.
The 1/4 hp air-cooled condensing unit of the
refrigeration system operates effectively when it is in
the operating range shown in the following figure.
compressor suction temperature falls far enough to
reach the pump-out initiate value, the purge control
activates the solenoids and the pump-out compressor
to remove the accumulated air. The following figure
illustrates this process.
As air is removed from the purge tank, the inside coil is
once again exposed to chiller refrigerant vapor. As
more chiller refrigerant vapor condenses on the coil,
more heat is removed from the vapor, and the purge
refrigerant compressor suction temperature rises. The
purge control cycles or stops the pump-out process in
response to the compressor suction temperature.
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