How to Flow Nitrogen While Brazing
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First, you will want to remove the Schrader cores from the suction and liquid line service ports. Nitrogen will flow into the system through one port, and it may flow out through the other service port or out of another opening in the copper. The nitrogen just needs to flow across the joint being brazed.
Then, you purge all the air from the lines by sending nitrogen through at a high velocity; purging flushes out the oxygen and will prevent oxidation from forming inside the copper tubing and creating restrictions later. We can control the velocity of nitrogen with the help of a regulator; some nitrogen regulators have specific preset settings for testing, purging, and brazing. Other regulators have SCFH gauges or a floating ball to indicate the nitrogen velocity. During the actual brazing phase, we can flow nitrogen at a very low speed to prevent air from getting into the lines.
When brazing, we also need to protect heat-sensitive components and surfaces with a wet towel or a heat-blocking putty (like Refrigeration Technologies WetRag). The copper should also be clean, and we need to be thinking about where the nitrogen will be entering and exiting the system.