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Volkswagen Golf Service & Repair Manual: Blowing through refrigerant circuit with compressed air and nitrogen

Use compressed air to force moisture and other contaminants out of the refrigerant circuit and use nitrogen to force moisture out of the refrigerant circuit.
The method of blowing through the refrigerant circuit with compressed air and nitrogen often requires a lot more work than purging with refrigerant R134a. When purging with refrigerant R134a, cleaning of the components is considerably better. It is therefore always best to purge the system in the event of complaints (the method of blowing through should only be selected for certain complaints and for individual components).
Only individual components are to be blown through if:
There is no way of purging the refrigerant circuit with refrigerant R134a.
If there is dirt or contamination in individual components of the refrigerant circuit.
If during evacuation of a leak-tight refrigerant circuit the vacuum display is not constant on gauge (moisture in the refrigerant circuit, which generates vapour pressure).
If the refrigerant circuit has been left open for longer than normally required for repairs (e.g. following an accident)
Pressure and temperature measurements in the circuit indicate that there is moisture in the refrigerant circuit.
The vehicle-specific repair manual → Heating, air conditioning deems it necessary after replacement of certain components.
  Note
Blowing through the refrigerant circuit with compressed air and nitrogen will not achieve the same degree of cleanliness as purging with refrigerant R134a. When purging with refrigerant R134a, the refrigerant oil is shifted and cleaning of the components is better → Chapter.
When blowing through, a maximum working pressure of 15 bar must not be exceeded (if necessary, use pressure limiter for compressed air as well).
  WARNING
Nitrogen can escape from the bottle in an uncontrolled way.
Work only with pressure regulator for nitrogen bottles (max. working pressure 15 bar).
The gas mixture escaping from the components must be collected with appropriate extraction equipment.
–  Always blow through components opposite the direction of refrigerant flow.
Compressed air and nitrogen cannot be blown through the restrictor, expansion valve, air conditioner compressor, receiver or reservoir.
–  In cases where condensers have a drying agent cartridge in the integrated receiver, this drying agent cartridge must be removed.
–  First blow out soiling using compressed air and then dry components with nitrogen.
–  Fit adapter for connection of pressure hose to refrigerant circuit → Chapter and → Heating, air conditioning.
To prevent oil and moisture from the compressed air system making its way into the refrigerant circuit, the following points must be observed.
The compressed air must be fed through a compressed air cleaning unit to clean and dry it. Therefore, use compressed air filter and dryer (included in items supplied with tools for painting work) → Workshop equipment and Special tools catalogue.
If refrigerant pipes have threads or union nuts at connections, use adapter -V.A.G 1785/1- to adapter -V.A.G 1785/8- to connect to 5/8" - 18 UNF charging hose. These adapters can be found in the adapter case for VW/Audi passenger vehicle set -VAS 6338/1- and adapter case for commercial vehicle set -VAS 6338/50-.
On refrigerant pipes without thread or union nut on the connection, use adapter from adapter case for VW/Audi passenger vehicle set -VAS 6338/1- or adapter case for commercial vehicle set -VAS 6338/50- or proprietary air gun with rubber nozzle.
  Note
Always extract any escaping air and/or nitrogen from the components using appropriate local exhaust ventilation LEV (e.g. workshop extraction system).
Certain contaminants and old refrigerant oil cannot be shifted from the refrigerant circuit at all with compressed air or if the compressed air is insufficient. These contaminants can only be removed by purging with refrigerant R134a → Chapter.
   

Blowing out refrigerant circuit

  Note
On vehicles where no adapters fit the refrigerant pipes, blow through individual components using an air gun, for example, with rubber nozzle. When doing this, be particularly careful not to damage the connections (do not crush or scratch them).
The evaporator should be blown through from the connection to the low-pressure side (large diameter), with the expansion valve or restrictor removed.
Always blow through components opposite the direction of refrigerant flow.
Examine the expansion valve; renew if dirty or corroded.
If components bear dark, sticky deposits which cannot be removed with compressed air, renew these components.
Thin, light grey deposits on the insides of refrigerant pipes do not impair the function of the components.
After blowing out the system, always renew the receiver or reservoir and the restrictor. If the condenser has a dryer, renew the dryer cartridge or dryer.
   
Clearing refrigerant circuit of contaminants
...

Purging (cleaning) refrigerant circuit with refrigerant R134a
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