INNOV@ ECFAN DMR -AGU-0912-E • 11 •
4.2 Full vacuum and charge of the unit
Vacuum cycle
In general it is better to apply a “long” rather than a “hard” vacuum: reaching a low pressure too abruptly may in fact
cause that any remaining humidity evaporates instantaneously, thus freezing part of it.
Fig. 7 Vacuum cycle diagram
The Fig. 7 represents a vacuum cycle and an optimal subsequent pressure rise for the refrigeration devices we
manufacture. Generally in bigger refrigeration systems or if there is a suspicion of an extensive quantity of humidity in
the refrigeration circuit, the vacuum needs to be “broken” by using anhydrous nitrogen. Then the steps of evacuation
need to be repeated as described before. This operation facilitates the removal of remaining and/or frozen humidity
during the evacuation process.
4.3 Evacuating a circuit “contaminated” with refrigerant
The first step is to remove the refrigerant from the circuit. To do this a specific machine is necessary with a drying
compressor in order to recover the refrigerant.
Refrigerants all tend to dissolve in oil (compressor sump). The Fig. 8 illustrates a specific property (Charles’ Law) of
gases, which are more soluble in liquids as the pressure increases but less soluble as the temperature increases.
Fig. 8 Charles’ law diagram
If the oil in the sump is held at a constant pressure, an increase in temperature will significantly reduce the amount of
refrigerant dissolved in it, thus ensuring that the lubricating function desired is maintained. The problem of inadequate
lubrication occurs if the crankcase is not duly heated, above all after seasonal interruptions when, due to the suction
effect of the compressor, there is an abrupt drop in pressure inside the sump, which results in considerable evaporation
of the refrigerant previously dissolved in the oil. If heating elements were not installed, this phenomenon would cause
two problems:
1 ) The release of refrigerant from the cooling circuit tends to cool down the oil and thus actually creates the
opposite effect by keeping more refrigerant dissolved in the oil: for this reason, it is advisable to switch on -if
available- the crankcase heater during the evacuation process.
P
200 s
150
T oil
Pressure
% of refrigerant in oil
T0
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