We do torch brazing of copper and steel tubing/piping for the HVAC industry, and we try to do a nitrogen purge of the ID of the tubing during our brazing process to prevent oxidation of the inside of the assemblies we’re brazing. We do this by inserting a rubber hose into the opening of the header that we are brazing and flow nitrogen gas through the rubber hose into the header. We have been told this is ineffective. Why would that be true, and what should we be doing instead?
Yes, the way you are using the nitrogen hose is probably quite ineffective and is actually wasting a lot of pure nitrogen gas (and your money along with it). Here’s why.
By merely inserting a nitrogen hose loosely into the end of the header, as shown in Figure 1A, any nitrogen gas flowing from the hose into the header is actually drawing in a lot of surrounding air (by a venture-effect) along with the pure nitrogen from the hose.
Thus, instead of flowing only pure nitrogen through the inside of the header, you are actually causing a lot of surrounding air (oxygen) to be drawn in around the hose tube. It is even possible, therefore, that you are causing more oxygen to get inside the header than if you did not use the nitrogen hose at all.
To be effective, the nitrogen hose must completely seal off the entrance of the header where it is being introduced so that only the pure nitrogen is entering the header and no surrounding air is drawn in along with the nitrogen. This can be accomplished by running the nitrogen hose through a tightly fitting rubber stopper, as shown in Fig. 1B.
The flow rate of the nitrogen should only be such that it will purge out all the air inside the header but not flow so rapidly that the gas has a “cooling effect” on the header. If this is the case, the brazing process might be adversely effected, taking much longer to complete than normally required.
Similarly, the brazing can only be realistically started once the header has been completely purged of air or else any remaining oxygen inside the header can cause oxidation of the inside surfaces during heating, the very thing that is trying to be prevented. To properly purge out the header prior to brazing you need to know the volume inside the header that is being purged and the flow rate of the nitrogen gas so that you can then calculate how many seconds (or minutes) it will take to completely purge the system prior to the start of the torch braze.