2-5. Brazing of pipes
Silver brazing metal
Silver brazing metal is an alloy mainly composed of silver and copper.
It uses iron, copper, or copper alloy, and is relatively expensive though it excels in soldering.
Phosphor bronze brazing metal
Phosphor bronze brazing metal is generally used to join copper or copper alloy.
Low temperature brazing metal
Low temperature brazing metal is generally called solder, and is an alloy of tin and lead. Do not use it for refrigerant piping because its adhesive capacity is low.
NOTE
•Phosphor bronze brazing metal tends to react with sulfur, producing a fragile compound water solution. This may cause gas leakage. Therefore, use other type of brazing metal at a hot spring resort or similar place, and coat the surface with coatings.
•To braze the pipe again while performing service work, use the same type of brazing metal.
Why flux is necessary
•Removing all the oxide film and any foreign matter on the metal surface assists the flow of brazing metal.
•Flux prevents the metal surface from being oxidized in the course of brazing.
•Reducing the brazing metal's surface tension enables the brazing metal to adhere for better metal processing.
Characteristics of flux
•The activation temperature of flux matches the brazing temperature.
•A wide effective temperature range makes flux hard to carbonize.
•It is easy to remove slag after brazing.
•The corrosive action to the treated metal and brazing metal is minimum.
•The good performance of flux gives no harm to a human body.
Since flux works in a complicated manner as described above, select an appropriate type of flux according to metal treatment type, brazing metal and brazing method, or other conditions.
Type of flux
•
It is generally a compound of borax and boric acid. It is effective when brazing temperature is higher than 800 °C.
•Active solvent
Most of this type of flux is generally used for silver brazing.
It features the increase of oxide film while moving the capability to the
Piping materials for brazing and brazing metal / flux
Piping material | Brazing metal to be | Flux to be used |
| used |
|
Copper - Copper | Phosphor copper | Do not use |
|
|
|
Copper - Iron | Silver | Paste flux |
|
|
|
Iron - Iron | Silver | Vapour flux |
|
|
|
NOTE
•Do not enter flux into the refrigerant cycle.
•If chlorine contained in the flux remains within the pipe, the lubricating oil deteriorates. Because of this, use a flux that does not contain chlorine.
•When adding water to the flux, use water that does not contains chlorine. (e.g. distilled water or ion- exchange water)
•Remove the flux after brazing.
Brazing must be performed by a person qualified and experienced with theoretical knowledge since the operation requires sophisticated techniques.
Perform brazing while flowing dry nitrogen gas (N2) to prevent oxide film from forming during brazing application to the inside of the pipe.
NOTE
• Never use gas other than nitrogen gas.
Brazing method to prevent oxidation
1)Attach a reducing valve and a flow meter to the nitrogen cylinder.
2)Use a copper pipe to direct the piping material, and attach the flow meter to the balance.
3)Apply a mark to the clearance between the piping material and the copper pipe filled with nitrogen to prevent the back flow of the nitrogen gas.
4)If the nitrogen gas flows out, be sure to keep open the piping end.
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