Manual for Refrigeration Servicing Technicians - UNEP - Division of ...

4CHAPITRE 4
PAGE 06
EVACUATION
Servicing Practices
Procedures to perform evacuation
To evacuate and dehydrate a system, before filling with refrigerant, take the
following steps:
1 - First, the system should be tightness tested (i.e., “leak tested”). This
can be done by pressurising the system with oxygen free, dry nitrogen
(OFDN). Shut off the supply of nitrogen and check the pressure over a
period of time (a minimum of 15 minutes, but it depends upon the size
of the system; a larger system requires more time). Keep checking the
pressure gauge to see if the pressure reduces.
2 - If the pressure does fall, it is likely that the system has a leak, so
leak searching and repair procedures must be carried out.
OPEN
OPEN
Vacuum pump connexion
OPEN
Exhaust
3 - When the system is confirmed to be leak-tight, release the OFDN and
immediately connect a proper vacuum pump to both suction side
and discharge the side of the compressor (see diagrams), and make
sure the vacuum gauge is connected. Open all the valves, including
solenoid valves, so there is no part of the circuit that is “locked” in.
4 - Switch on the vacuum pump and wait.
5 - When satisfactory vacuum has been reached (below 100 Pa abs.)
stop the pump and leave it for an appropriate length of time (around
half an hour for a small hermetic system, to several hours for a
large site-installed system) to see if the vacuum gauge indicates an
increase in internal pressure. If the pressure rises there could be two
reasons for it: either there is a leak or moisture still in the system.
In this case, the evacuation
procedure should continue,
but if a constant vacuum
pressure is never achieved,
then it is likely that a leak is
present and the tightness
test should be repeated.
Vacuum Pump
6 - If the vacuum pressure
remains constant over a
period of time, the circuit is
correctly evacuated; dry and
free of leakage.
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