The majority of HVAC installation instructions necessitate the flow of nitrogen through copper tubing during the process of brazing, This particular step, often referred to as a nitrogen purge, is very important in the production of a HVAC system of quality.
Why Nitrogen Purge?
When oxygen present in the air is combined with copper it produces surface copper-oxide. You can view this on copper tubes in the form of a light onto dark brown discolorations. You may have seen medical gas/ACR copper tubes that are supplied from the tube mills, the nitrogen is capped and charged. This has been designed specifically to stop oxide formation from occurring inside the tubes. When the cap is removed and these tubes are cut for the purpose of installation, this nitrogen-protection will be lost.
At the high-brazing temperatures, heavier cupric oxide (black oxide) forms. When cooled this type of oxide flakes off and forms a “scale.” While mainly cosmetic when it comes to the exterior of the tube, on the interior of these tubes, oxide flakes are transported by the refrigerants inside the system. This type of contaminant has the abilities to restrict flow when it comes to smaller orifices like the pilot-valve capillary tubes and metering devices.
This issue has long since been a problem when it comes to brazing HVAC tubes. This problem is now more than ever important in regards to the change from the HCFC refrigerants such as R-22 which use mineral oils onto the latest HFC refrigerants such as (410a) that uses POE oils. The POE oils have a polar nature their solvent effects have the ability to “scrub” the walls of the copper tubes. This causes the oxide in the tube walls to loosen scale which ends up circulating inside the system.
To avoid oxidation, purging dry nitrogen inside the tubes during the stages of brazing is one of the sought after methods used today. This is because nitrogen is non-reactive (inert) and displaces the oxygen that prevents the scale formation.
The procedure involves introducing nitrogen into the HVAC system through a Schrader valve or through another opening in the system. A tube or hose is connected to a nitrogen cylinder to 1 side of a pipe. These cylinders are equipped with flow-control or regulator valves. There are no universal requirements in regards to the setting for delivery pressure, but the main aim is to use low pressure/volume in order to displace oxygen.
The advisable starting point will be around 2-3 CFH or 1.5 – 2 PSI. Certain technicians set the pressure until a flow is felt at the exit-point. It is a general practice that the technician will initiate this flow before brazing and continues with the flow of this nitrogen until such stage that the tube has cooled. It is suggested to avoid excessive flow rates that can build pressure in the tubes. The higher flow rates can cool down the tube which decreases the efficiency of the brazing heat. In addition, and excessive amount of nitrogen pressure can build in the tubes which decreases the penetration of the braze alloy. Nitrogen purge is an enormously important step to achieve a high quality air conditioning installation.