Flared tube fittings are used when a connection is needed in tubing. There are a variety of flared fittings on the market and the ones most often used in HVAC/R are forged. The size of the threads in these fittings will be determined by either National Fine(NF) or National Pipe(NP). They are made from brass usually and are easily cross-threaded so care is recommended.
A flared fitting size corresponds exactly to the size of the tubing being used. 1/2” flare nuts will work with 1/2” sized tubing and so on.
Fitting sizes will also have a code number that identifies it’s size. That coding system follows this numeric categorization…
- 3=3/16” tubing
- 4=4/16” tubing
- 8=8/16” tubing
With some fittings, there will be threads on one end. These threads taper 1/16” for every inch of length. If plastic tubing is being used, plastic fittings may also be used as long as the sizes are matched accurately. Fittings for plastic tubing may also be made of polyethylene, aluminum and brass. These will be compression fittings however, as plastic tubing cannot be flared to match a forged angle of flare. Flared tube fittings may also be used based on the metric system and care must be taken to match sizes correctly.
Soldered and Brazed Fittings
The process of soldering is used for water pipes and drains while brazing is used for refrigerant lines and tubing. The differences in these two joining processes are in the temperatures needed for each action. Essentially, if the temperature required to melt an alloy used to join copper tubing is below 840ºF, it is soldering. If the temperature required is above 840ºf, it is brazing.
Soldering is a process of drawing solder into a joint, connection, or seal. Solders are said to have operating pressures and the choice of solder is based on the temperature of the line being soldered and the work it needs to do.
Tin-antimony solders are most often used for soldering that is being done with moderate pressures and line temperatures. Higher pressures will need greater strengths at the juncture of two lines thus requiring a solder that is 95% tin and 5% antimony.
Antimony is a toxic, semi-metal discovered around 1600BC by the Egyptians who also used it as mascara. It is metallic, hard and brittle. It can be found in semiconductors, infrared detectors diodes, bullets, cable sheathing, and batteries. Antimony is combined with lead and other metals to increase hardness.
Brazing metals are able to produce a stronger bond than soldering metals. The metals used for brazing start melting at 1000ºF. These metals are alloys that are typically 30-60% silver. There are also copper alloys which contain phosphorus.