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

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Servicing Practices
Evacuation
A refrigerating system must contain only the refrigerant in liquid or
vapour state along with dry oil. All other vapours, gases, and fluids
must be removed. Connecting the system to a vacuum pump and
allowing the pump to run continuously for some time while a deep
vacuum is drawn on the system can best remove these substances.
It is sometimes necessary to warm the parts to around +50°C
while under a high vacuum; in order to accelerate the removal of all
unwanted moisture, heat the parts using warm air, heat lamps, or
water. Never use a brazing torch. If any part of the system is below
0°C, the moisture may freeze and it will take a considerably longer
time for the ice to sublimate to vapour during the evacuation process.
The equipment necessary to carry out the evacuation is:
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PROCEDURES TO PERFORMS
• vacuum pump
• manifold gauges
two servicing valves (in the case system is not equipped with
servicing valves)
• vacuum gauge.
It is essential to know that conventional manifold gauges have
low sensitivity, particularly at lower pressures. As such, they are
ineffective at determining whether or not a sufficient vacuum has
been achieved. Therefore it is essential to ensure that a proper
vacuum gauge (such as a Pirani gauge) is used.
To understand why system evacuation is very important for moisture
elimination, it is useful to remember the concept of vacuum and the
relationship between boiling temperature and pressure. For a pure
substance, like water, the boiling temperature for a fixed pressure
is called saturation temperature at this pressure, and the pressure
at which the water evaporates at a fixed temperature is called
saturation pressure at this temperature.
The relationship between these two thermodynamic properties (a natural
law) is presented in the figure for water:
PRESSURE [kPa]
160
140
120
100
80
60
40
20
0
-75 -35 5 45 85 125
TEMPERATURE [ ° C]
Saturation pressure
and temperature curve
for water
It can be seen in this figure that as the pressure reduces, the
boiling temperature will be lower. If one wants to remove moisture
in DONE
vapour phase from a refrigeration system, it is very important to
lower the system pressure because this will facilitate the change of
the moisture from liquid to vapour phase (through heat transfer from
the surrounding environment) making its removal easier.
Always evacuate a system when:
• replacing a circuit component (compressor, condenser, filterdrier,
evaporator, etc.)
• whilst the system has no refrigerant
• if the refrigerant is contaminated
• after the lubricant is charged.
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