3.11 EVACUATION OF SYSTEM WITH
COMPRESSOR AND VACUUM PUMP FOR
FREON PLANTS:
Before charging the system with refrigerant, air and
water must be cleared from the compressor and plant.
A Freon plant must be thoroughly dried before charging
it with gas. otherwise reaction between the water and
Freon will produce acids which will attack the system
and cause corrosion scale to collect internally and
impair the performance and reliability of the plant. In
addition water may cause blockage through freezing.
particularly in a low temperature plants.
Water at a temperature of 20°C boils at a vacuum of 743
mm mercury (equal to an absolute of 17 mm mercury or
0.0228 atm). The water vapour contained in the air in a
system decreases proportionally with an increase in
vacuum. As the compressor cannot create a high
vacuum on its own, a high vacuum pump should be used
to evacuate the plant. This is particularly important for
low temperature plants. It is recommended that the
evaporators, condensers, etc. should be warmed during
evacuation. The time required for this cannot be laid
down strictly and depends on the capacity of the vacuum
pump and the volume of the plant. The reading of most
vacuum gauges is not sufficiently accurate to determine
whether the plant has been evacuated adequately. As this
is very important for Freon plants, the pump outlet
should be provided with a tube or hose discharging into
a vessel filled with oil. The evacuation should be
stopped only when there are no longer any air bubbles
visible in the oil. Evacuating the plant after first filling it
with Nitrogen will ensure that it is thoroughly dried.
Any moisture still present in the system is absorbed by
the Nitrogen and exhausted with it. This operation
should be carried out as follows:
a. Shut suction and delivery stop valves and
disconnect delivery pressure gauge line from
delivery manifold. Shut all drain valves and also oil
return stop valve on the oil separator.
b. Open all remaining system stop valves, thus
ensuring that the system is open from delivery stop
valve to suction stop valve.
c. Start compressor and open suction stop valve
slowly, as far as is necessary. Air is discharged from
the plant via the open delivery pressure gauge
connection.
d. Stop compressor as soon as oil jet behind oil level
glass disappears.
e. Shut suction stop valve.
f. Connect high vacuum pumps to the system (some
plants have a special connection for this purpose)
g. Start vacuum pump and open stop valve between
vacuum pump and system.
h. When most of the air has been evacuated from the
system, attach a hose. discharging into an oil filled
vessel. to the outlet from the vacuum pump.
i. Shut stop valve between vacuum pump and
system when gas bubbles are no longer seen to rise
in the oil, and then stop pump.
j. Fill system with Nitrogen.
k. Release Nitrogen via a hose leading into the open air
until an atmospheric pressure remains in the
system.
l. Shut stop valve between release line and system.
m. Start compressor and open suction stop valve
slowly.
n. Repeat steps 'd', 'e', 'g' and 'k'.
o. Shut delivery and suction stop valves and hold a
vacuum in the system for 24 hours.
p. Repeat again steps 'I' up to 'p' inclusive.
q. Reconnect delivery pressure gauge line to delivery
manifold.
The plant is now ready for charging it with Freon.
3.12 EVACUATION OF THE SYSTEM WITH
COMPRESSOR ALONE FOR AMMONIA
PLANT:
Before charging the system with refrigerant, air and
water must be cleared from the compressor and the
plant. Ammonia plants are generally evacuated with the
refrigeration compressor only.
a. Shut all stop valves connecting to the atmosphere,
and stop valve in the oil return line from the oil
separator I and also suction and delivery stop valves
(on two stage compressors HP delivery and LP
suction stop valve only).
b. Open all other stop valves.
c. Disconnect LP pressostat (if fitted).
d. Disconnect delivery pressure gauge line from
delivery manifold.
e. Start compressor with suction valve shut. After
starting up the compressor, open suction stop valve
slowly, but only as needed, to prevent delivery
pressure from rising too much and the compressor
from running hot.
g. Reconnect delivery pressure gauge line to
compressor. The plant is ready for charging it with
refrigerant.
3.13 REFRIGERANT CHARGING:
Only after the system leak test, evacuation and drying
out in the case of Freon Plants has been carried out the
system should be charged with refrigerant. It Is
recommended to ventilate the plant room well while
charging the system refrigerant. The following
procedure should be adopted.
a. Make sure that a vacuum exists in the system and
that all stop valves in the circuit are open.
b. Weigh refrigerant bottle and check that it contains
the proper refrigerant.
c. Connect the charging line to the refrigerant bottle.
d. Connect charging line to refrigerant charging stop
valve in the system, but do not tighten the union nut.
e. Open bottle stop valve and tighten the union nut as
soon as refrigerant gas escapes through it. The
charging line is now purged. Open bottle stop valve
completely.
f. Open refrigerant charging stop valve slowly.
Note : On small plants it will now be necessary to
process directly with step 'k'. as the bottle cannot be
completely emptied.
g. Shut bottle stop and then refrigerant charging stop
valve once the bottle is empty. (Frosted charging
line thaws). Disconnect charging line from bottle
and weigh empty bottle.
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