What should be circulated through copper tubing before brazing?

Circulating nitrogen through copper tubing before brazing is essential for several reasons. Firstly, nitrogen acts as a shielding gas that prevents oxidation of the copper surfaces during the brazing process. When copper is heated, it can easily oxidize, and the formation of oxide layers can negatively affect the quality of the joint. By flooding the tubing with nitrogen, any oxygen present is displaced, thereby ensuring a cleaner mating surface for the brazing filler material.

Moreover, using nitrogen helps to control the internal pressure within the tubing during the brazing process. This is important because it can prevent the tubing from collapsing or being contaminated by impurities, which could compromise the integrity of the system once it is in operation.

In contrast, other options such as carbon monoxide, refrigerant, and oil are not suitable for this purpose. Carbon monoxide is toxic and not an appropriate choice for maintaining cleanliness in the joint. Refrigerant inside the tube can also create pressure issues and is unnecessary during the brazing process. Oil, while potentially useful in other contexts, would contaminate the joint and hinder proper brazing by interfering with the bonding of the materials. Thus, nitrogen is the best practice for preparing copper tubing for brazing.