Brazing Today's Air Conditioners
As CFC refrigerants are phased out, there has been much discussion about braze joint requirements for the new replacement blends. There are two primary reasons why contractors should pay closer attention to braze quality:
- The non-CFC replacements, such as R-410a, operate at pressures considerably higher than current refrigerants such as R-22. For example, at 70°F the R-22 operating pressure is 120 PSIG, while R-410a is 200 PSIG. Currently, R-410a refrigerant is significantly more expensive than R-22 so if you have a leak, the lost refrigerant will cost more to replace.
- A related development is the transition to higher SEER air conditioning ratings. To meet the government mandated 13 SEER, many manufacturers will likely increase coil size. This usually means more brazed joints. While not affecting the contractor, HVAC manufacturers also have an added incentive to improve braze quality.
In either case, one thing hasn't changed. The key to producing leak-proof brazed connections is proper technique. Sound brazed joints are the result of following several key procedures.
This is an easy step to overlook, especially on new installations. Contaminants such as dirt, oil, or heavy surface oxide will inhibit "wetting"; the ability of the molten braze alloy to spread out and adhere to the surface. Wipe parts with a rag, and then use a stainless steel wire brush, or Scotchbrite® to remove the oxide layer.
This is another critical aspect. Flame settings depend on the type of heat source. For oxygen-acetylene a neutral flame is recommended. A slightly excess acetylene, (carburizing), flame is also suitable. Avoid using a flame with excessive oxygen, (oxidizing flame.) This flame chemistry increases surface oxides and inhibits filler metal wetting.
Air-acetylene torches using swirl type tips have become very popular. These have a single flame adjustment. Regardless of the type of torch, as tube size increases or decreases change to a larger or smaller tip, rather than simply increasing pressure settings.
Incorrect heating is the primary reason for poorly made joints. Adequate joint strength and ductility depend on pulling the molten braze filler metal into the connection. The first joint appears sound from the outside. While this connection may be adequate, under the right circumstances, high vibration or stress might cause a crack to develop.
To prevent this we need to use a heating method that gets both parts, including down inside the coupling, to brazing temperature. We recommend this multi-step approach to heating:
1. Begin by heating the tube. Heat around the tube to conduct heat inside the coupling.
2. After this preheat, move the torch to the coupling. Angle the flame towards the tube.
3. Sweep the torch between the tube and coupling to bring both parts to brazing temperature.
4. Move the flame towards the coupling base and apply filler metal. The molten braze alloy will follow the heat to the base of the cup.
5. Steps 3 & 4 should be done on both sides of the tube/connection. Also, as tube diameter increases, you need to repeat these steps at several points around the circumference. This will promote even heating and ensure there is adequate filler metal, and no "starved" areas.
It's important to wait for both pieces to absorb sufficient heat before applying the filler metal. The rod or wire should easily melt and flow when it is applied to the base metal. Avoid melting the rod with only the torch flame.
Brazing Filler Metal
Available brazing filler metals are quite suitable for producing leak-free connections. The Harris Filler Metal Chart lists recommended brazing filler metals for various base metal combinations.
When brazing copper or brass a flux is required to reduce surface oxides, protect the base metal during heating, and reduce filler metal surface tension. On copper to copper applications the phosphorus in braze rods, (Stay Silv® 5, 15, Blockade®, Dynaflow®), provide this function. When brazing other metals a separate chemical flux is required.
Flux should be sparingly used and applied with a brush. A suggested method is to leave ¼" of un-fluxed area at the tube end. During heating capillary action will pull the melted flux through the joint. Be sure to remove flux residue with a wet rag after brazing.
A somewhat controversial area has been the use of soldering as a replacement for brazed connections. Most commercial solders, notably the tin/lead and tin/antimony grades, are not recommended for high vibration cooling applications. The tin/silver alloys, including our Stay Brite®, do possess the required strength and ductility for residential HVAC applications. The exception is applications where service temperatures exceed 200°F.
It's worth noting the International Mechanical Code provides that copper tube soldered joints on "Group A1" refrigerants are acceptable. R-410A is included in this refrigerant group.
The benefits of tin/silver joints are well documented. You eliminate the necessary nitrogen purge during heating, and external cooling of heat sensitive valves is unnecessary.
Remember, solder fluxes are corrosive. Take care to avoid excess flux on the tube and fitting that may end up inside the closed system. To help eliminate this make sure you apply flux with a brush.
Keep in mind that some air conditioning manufacturer's installation instructions and service bulletins stipulate brazed connections. Contractors should follow the manufacturer's instructions, and any local or national code requirements.