Lennox INNOV DM R Series Application Manual page 47

The figure Fig. 9 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. All refrigerants all tend to be dissolve the in oil in the compressor
sump). The "Oil" Fig. 10 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. 10 Charles' law diagram
Pressure
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 the crankcase heater during the evacuation process.
2 ) If a high % of refrigerant gets in contact with the Pirani gauge (vacuum sensor), it may "mislead" this
sensitive sensor and misinterpret the value for a certain period of time. For this reason (if no machine for
recovering refrigerant is available) it is nonetheless advisable to switch on the crankcase heater and to
avoid full vacuum before the circuit has been adequately purged of refrigerant. The refrigerant may in
fact dissolve in the oil of the vacuum pump, reducing its performance for a long time (hours).
4.4 Charging positions (single point)
The best position to charge the unit is the section between the thermostatic valve and the evaporator. take care to
avoid the fixing of the thermostat bulb until the operation is completed. It is important to ensure that the valve
orifice remains open in order to allow the passage of refrigerant also towards the condenser / liquid receiver.
If possible, avoid the charge of refrigerant into the suction line of the compressor as this may cause excessive
dilution of the lubricant.
In case of air cooled units, in the attached "Piping Design Criteria" is described how to calculate the estimate
refrigerant charge.
INNOV@ ECFAN DHR -AGU-0912-E
T oil
T2
% of refrigerant in oil
T1
• 15 •