First tip I would like to throw out there is cleanliness. Clean and keep clean. Copper, along with most of its adhesive bonding agents (types of solder, brasing, ect.) Generally oxide extremely easily. Especially under heat. Mechanical cleaning is generally sufficient such as sanding or wire brush but occasionally chemicals can be more economical, especially in repair work where severe amounts of tenacious oxide have formed due to environmental circumstances. Next is protection under heat. As mentioned earlier, purge gas of some type can be used. Generally any non reactive gas can be used, but those that displace oxygen and nitrogen are best. They have their own slight hazards, but we're already melting metal here so I hope you can think about ventilation before you start. The careful use of propane is not all that far fetched. I used to recycle large underground gasoline tanks from gas stations when I did pressure vessels and would cut them apart with an oxy-propelyne outfit still with a couple hundred gallons gas inside, I used old shirts soaked is diesel fuel as a wick in the top openings to burn off fumes and controlling oxygen levels within. No oxygen inside means no oxidation and in my case no explosion. Except there was always a spectacular mushroom cloud flame when it finally laid open. Point is, prevent the chemical reaction from happening. Flux can be used if the tubes are flushed with muriatic acid before connecting to the compressor unit. Nitrogen works, but causes embrittlement in copper. So we work harden by bending, heating, the embed nitrogen within the molecular surface. It can mean the difference between 10 years and 25 of service. Getting late for me, 3 am comes fast. More on this later.
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