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Volkswagen Polo Service & Repair Manual: General Information

In order to be able to force moisture and other contaminants out of refrigerant circuit, compressed air and nitrogen (to force moisture out of refrigerant circuit) are to be used.
For the most part, blowing through the refrigerant circuit with compressed air and nitrogen requires significantly more work than flushing with refrigerant R134a. As flushing with refrigerant r134a cleans the components more efficiently, always flush in case of a complaint (blowing through should only be used for certain complaints and individual components).
Only individual components should be flushed if:
there is no possibility of flushing the refrigerant circuit with refrigerant R134a.
dirt or other contaminants are located in individual components of the circuit.
vacuum reading is not maintained on evacuating a leak-free refrigerant circuit (pressure build-up due to moisture in refrigerant circuit).
refrigerant circuit has been left open for longer than normal (e.g. following an accident).
pressure and temperature measurements in the refrigerant circuit indicate the likelihood of moisture.
stipulated by the vehicle-specific repair manual Heating & Air Conditioning following replacement of certain components.
Blowing compressed air and nitrogen through the refrigerant circuit does not achieve the level of cleanliness as flushing with refrigerant R134a. Flushing with refrigerant R134a loosens the refrigerant oil and the components are then cleaner. Refer to → Chapter „Refrigerant Circuit, Flushing (Cleaning) with Refrigerant R134a“.
When blowing, working pressure must not exceed a maximum of 15 bar (218 psi), (also use a pressure reducer for compressed air if necessary).
Nitrogen can leak uncontrolled from the cylinder.
Make exclusive use of pressure reducers for nitrogen cylinders (maximum work pressure 15 bar (218 psi)).
Use appropriate emissions extraction units to draw off gas mixture escaping from components.
–  Always flush components in direction opposite of refrigerant flow direction.
Restrictor, expansion valve, compressor, receiver and reservoir cannot be flushed with compressed air and nitrogen.
–  Remove this dryer cartridge on condensers that have one installed in the integrated receiver/dryer.
–  First blow out dirt with compressed air, then remove moisture from components with nitrogen.
–  Connect adapter to connect the pressure hose to the refrigerant circuit. Refer to → Chapter „Adapter for Assembling Flushing Circuit“ and Heating & Air Conditioning.
Note the following points to prevent oil and moisture from entering the refrigerant circuit from the compressed air system.
The compressed air must be routed through a compressed air purifier for cleaning and drying. Use is therefore to be made of filter and dryer for compressed air (included in scope of delivery as tool for painting work). Refer to the Workshop Equipment And Special Tools Catalog.
For refrigerant lines with threads or union nut at connection, make use of A/C Adapter Set - Adapter 1 -VAG1785/1- through A/C Adapter Set - Adapter 8 -VAG1785/8- for connection of 5/8" -18 UNF charging hoses. This adaptor is located in the Refrigerant Circuits Adapter Set 1 -VAS6338/1- and Refrigerant Circuits Adapter Set 2 -VAS6338/50-.
On refrigerant pipes without thread or union nut on the connection, use adapter from Refrigerant Circuits Adapter Set 1 -VAS6338/1- or Refrigerant Circuits Adapter Set 2 -VAS6338/50- or a commercially available pistol with rubber mouthpiece.
Compressed air/nitrogen emerging from components is to be drawn off by way of an appropriate system (e.g. workshop extraction system).
Certain contaminants cannot be removed or can only be removed insufficiently from the refrigerant circuit with compressed air. These contaminants can only be removed by flushing with refrigerant R134a. Refer to → Chapter „Refrigerant Circuit, Flushing (Cleaning) with Refrigerant R134a“.
Refrigerant Circuit, Flushing with Compressed Air and Nitrogen

Refrigerant Circuit, Flushing
  Note For vehicles in which no adapters fit on to the refrigerant pipes, for example, a blower pistol with rubber mouthpiece is to be used to blow ...

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