10.3 Refrigerant Piping, Brazing & Soldering Techniques

Refrigerant Piping, Brazing & Soldering Techniques

Refrigerant piping work sits at the technical core of the Fabrication, Installation, and Startup domain: a line set that is undersized, poorly brazed, or improperly pressure-tested can doom an otherwise correctly selected and charged system. This section covers how line sets are sized, how field joints are brazed safely, and the pressure-testing sequence required before a system can be charged.

Line Set Sizing Basics

The liquid line and suction line connecting an outdoor condensing unit to the indoor coil are sized from the equipment manufacturer's line-set sizing tables, which key off system tonnage, total equivalent line length (straight run plus the equivalent length added by every elbow and fitting), and vertical elevation change between the two coils. Sizing isn't a formality:

• An undersized suction line creates excessive pressure drop and can starve the compressor and cause poor oil return at low load.
• An oversized suction line slows refrigerant gas velocity below the point needed to carry compressor oil back with it, risking oil logging in the evaporator and eventual compressor damage.
• Liquid-line sizing errors mostly affect subcooling and flash-gas formation rather than oil return, but still fall outside the manufacturer's approved range if ignored.

Exact nominal tubing sizes vary by manufacturer, refrigerant, and tonnage, so the equipment's own sizing chart — not a generic rule of thumb from a different brand or refrigerant — is the correct reference on the exam and in the field. Once sized, the suction line is insulated along its entire length (commonly with 3/8-inch-wall closed-cell foam insulation) to prevent condensation, often called sweating, on the cold gas line and to protect capacity; the liquid line is typically left uninsulated unless it passes through an unconditioned or unusually hot space where insulation protects against additional heat gain before the refrigerant reaches the metering device.

Brazing With a Nitrogen Purge

Brazing is defined by the American Welding Society as joining metal with a filler metal whose melting (liquidus) temperature is at or above 840°F (450°C); joining below that temperature with filler metal is classified as soldering. Refrigerant-carrying joints are brazed, not soldered, because a brazed joint develops far greater mechanical strength and is rated to withstand the pressures and thermal cycling a refrigeration circuit experiences over its service life. Filler alloys are typically phosphorus-copper (BCuP-series) for copper-to-copper joints, which don't require flux, or silver-bearing alloys when joining copper to brass or steel, or where extra joint strength is needed.

Before applying heat, the technician purges the piping with dry, oxygen-free nitrogen to displace the air and oxygen inside the tubing. Without this purge, heating copper tubing in the presence of oxygen forms a black, flaky internal oxide scale; that scale later breaks loose in operation and can plug the metering device or contaminate the compressor. The typical field sequence:

1. Open the system and begin an initial higher-flow purge (commonly cited around 50 psig or higher, briefly, through a vented cap or plug) to flush out standing air before any heat is applied.
2. Reduce to a low, steady purge flow — roughly 2-5 SCFH (standard cubic feet per hour) at about 1.5-2 psig is a commonly cited target — just enough that a faint hiss is felt at the open end, not a strong blast that would cool the joint and disrupt braze penetration.
3. Keep nitrogen flowing continuously through the entire heating cycle and through cool-down, until the joint is back near ambient temperature, so no oxygen re-enters while the copper is still hot enough to oxidize.

Leak Testing & Pressure Testing Procedures

Once all joints are brazed, the piping system must be pressure-tested before it is evacuated and charged. The test medium must be an inert gas — dry, oxygen-free nitrogen (sometimes with a small trace of helium or hydrogen added as a leak-detection tracer). Oxygen, compressed air, and refrigerant itself are all prohibited as pressurizing test media because they create a fire, explosion, or contamination hazard when combined with refrigerant oil residue inside the piping under pressure.

Target test pressures and hold times vary by the adopted mechanical code and the system's design pressure — for example, some national piping standards require only a short hold at a percentage above design pressure, while other guidance calls for a much longer standing-pressure hold (commonly cited practice includes charging to a few hundred psig and observing the gauge over an extended period, sometimes 24 hours, watching for any drop that indicates a leak). Because these figures differ between code editions and refrigerant systems, always confirm the exact required test pressure and duration against the current adopted mechanical code and the equipment manufacturer's instructions rather than relying on a single memorized number.

Once every joint holds pressure with no drop and no bubbles or response from an electronic leak detector, the system is depressurized and pulled into a deep vacuum to remove residual air and moisture — the widely used industry target is 500 microns or lower, confirmed with a vacuum decay test (isolating the system from the pump and watching that the reading stays low, commonly for around 10 minutes, rather than climbing back up, which would indicate either a leak or residual moisture still boiling off inside the piping). Only after the system holds a deep vacuum does it get its refrigerant charge, covered in Section 10.5.