Failing to purge a 410a system with nitrogen before/during brazing wont necessarily cause a problem but increases the chance of a problem..the filter/drier in the liquid line will catch the "flakes" that form on the inside of the pipe and circulate through the system but even if you locate the drier near the indoor coil,you still have the solder joint at the outlet of the drier who's flakes wont be captured by the drier screen because they are downsteam of the body of the drier...those flakes could clog the strainer inside the txv,or otherwise affect the txv...
I dont know why 410a systems are more prone to problems from
failing to purge while brazing but i've been told that either the
refrigerant itself or the "poe"oil in the compressor has a greater
attraction to moisture than r-22 systems and their type oil..therfore
doing everything by the book is more important on a 410a system,such as
installing a large liquid drier [thats rated for puron],,purging with
nitrogen while brazing,and evacuating system thoroughly before
charging.......the flakes produced while brazing without nitrogen are
caused by the presence of oxygen,not moisture..
Nitrogen is basically air without the oxygen and moisture...it doesnt require very much nitrogen to purge the system,,just need to have a LITTLE pressure going in while leaving the other port open so it can flow without building any pressure that could cause solder joint leak from nitrogen exiting systen through the solder joint while brazing.....
Also be sure to take pressure and temperature readings while charging, and charge per mfr recommendations....usually 10-12 deg "subcooling" is required... dont just add puron until the line gets cold........
Nitrogen Purge and Brazing
Most HVAC installation instructions require flowing nitrogen through the copper tube during brazing. This is an important step in producing a quality HVAC system.
Why nitrogen purge?
Oxygen in the air combines with copper to form surface copper oxide. We see this on copper tube as a light to dark brown discoloration. You've probably seen ACR/medical gas copper tube supplied from the tube mill nitrogen charged and capped. This is designed to prevent this oxide formation inside the tube. Once the caps are removed and the tube is cut for installation, the nitrogen protection is lost.
At high brazing temperature a heavier black oxide forms (cupric oxide). On cooling this oxide flakes off to form "scale".
While mostly cosmetic on the tube exterior, inside the tube the oxide flakes are carried by the refrigerant through the system. This contaminant can restrict flow through small orifices such as metering devices or or the pilot valve capillary tube in a reversing valve.
This problem has long been an issue in brazing HVAC tube. It has become more important with the change from HCFC refrigerants like R-22 that use mineral oil to the new HFC refrigerants (410a) using POE oils. Due to their polar nature, POE oils have a solvent effect and can "scrub" the copper tube walls. Oxide from tube walls and loose scale can circulate through the system.
What's the procedure?
To prevent oxidation, flow dry nitrogen through the tube during brazing. Nitrogen is inert, (non- reactive), and will displace the oxygen to prevent scale formation.
Nitrogen is typically introduced into the system throught the Schrader valve (after removing the core), or other system opening.
Connect a hose or tube from the nitrogen cylinder to one end of the pipe. The cylinder will be equipped with a regulator or flow control valve. There is no universal requirement for the delivery pressure setting, but the goal is to use low volume/pressure to displace the oxygen. A suggested starting point is 2 -3 CFH or 1.5 - 2 PSI. Some users will set pressure until they feel a slight flow at the exit point on the back of their hand. It's good practice to initiate flow before heating and continue to flow nitrogen until the part has cooled.
Avoid an excessive flow rate that builds pressure inside the tube. A high flow rate will tend to cool the tube reducing brazing heat efficiency. Excess nitrogen pressure can build up inside the tube and reduce braze alloy penetration. A small hole in a cap at the end of the line will allow the nitrogen to escape.
It's a good idea to experiment with flow rates by test brazing parts on the bench. Section the finished assemblies and inspect for a clean inner tube wall.
Take this step during installation and eliminate problems down the road.