Daikin R410A Technical Manual page 29

Special Precautions When Charging Unit With Scroll Compressor
These precautions are intended for use with Scroll compressors only with R22, and R410A refrigerants
but are not applied to reciprocating compressors or competitive Scroll compressors. Scroll compressors
have a very high volumetric efficiency and quickly pump a deep vacuum if there is insufficient refrigerant
in the system or if refrigerant is added too slowly. Operation with low suction pressure will quickly lead
to very high discharge temperatures. While this process is happening, the scrolls are not being well
lubricated – scrolls depend on the oil mist in the refrigerant for lubrication. A lack of lubrication leads to
high friction between the scroll flanks and tips and generates additional heat. The combination of heat of
compression and heat from increased friction is concentrated in a small localized discharge area where
temperatures can quickly rise to more than 300°C. These extreme temperatures damage the Scroll
spirals and the orbiting Scroll bearing. This damage can occur in less than one minute especially on
larger compressors. Failure may occur in the first few hours or the damage done during field charging
may show up some time later. Other typical field charging problems include undercharging,
overcharging, moisture or air in the system etc. In time each one of these problems can cause
compressor failure. Minimal equipment is required for field charging. The minimum equipment required
to do a satisfactory job is:-
The proper refrigerant charge should follow the volume as recommended by manufacturer and
recommendation should be followed by the installer.
1. Charging procedures – Single phase compressors
Evacuate the system to -760mmHg. To reduce evacuation time, use short, large diameter hoses and
connect to unrestricted service ports on the system. Quality of vacuum cannot be determined by time
– a reliable vacuum gauge must be used. (etc. electronic vacuum gauge) Ensure that the refrigerant
cylinder is in the correct orientation (see Figure F), purge the charging hose and charge liquid
through the liquid line charging port until refrigerant no longer flows or until the correct charge has
been weighed in. If additional charge is required start the system and slowly bleed liquid into the
suction side until the system is full.
It recommends charging liquid in a CONTROLLED manner into the suction side until the system
is full. This recommendat ion does not hold true for reciprocating compressors where liquid charging
into the suction side could cause severe damage.
Carefully monitor the suction and discharge pressures – ensure that the suction pressure does not fall
below 25 psig (1.7 bar) at any time during the charging process.
A
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Set of service gauges
Hoses
Vacuum pump
Figure F
Dip-pipe
Vacuum gauge
Scales
Thermometer
Invert cylinder
without dip-pipe
Liquid withdrawal
There are many ways of charging liquid in a "controlled
manner" into the suction side:-
1. Use valve A on the manifold gauge set
2. Use the valve on the refrigerant cylinder
3. Charge through a Shredder valve
4. Use a hose with a Shredder valve depressor
5. Charge into the suction side at some distance from
the compressor
6. All of the above