4.2 Refrigerant Piping, Copper Tubing Prep, Nitrogen Purge, and Brazing

4.2 Refrigerant Piping, Copper Tubing Prep, Nitrogen Purge, and Brazing

Refrigerant piping serves as the closed vascular network of a vapor-compression system, circulating refrigerant and compressor lubricant between system components. Defective piping preparation, improper sizing, or sloppy joining practices introduce leaks, restrict refrigerant flow, starve compressors of oil, and cause severe internal contamination. Mastery of copper pipe preparation, nitrogen-purged brazing, and oil return mechanics is a core competency evaluated on the NATE certification exam.

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Copper Tubing Classification and Preparation

Copper tubing used in HVAC and refrigeration service is designated as ACR (Air Conditioning and Refrigeration) tubing. ACR copper is washed, degreased, dehydrated, and sealed with protective caps at the factory, nitrogen-charged to maintain internal cleanliness.

Tubing Types and Wall Thickness

Copper pipe wall thickness is categorized by letters, which dictate working pressure ratings:

• Type K (Heavy Wall): Thickest wall construction. Used for underground lines, high-pressure industrial applications, and severe mechanical stress environments.
• Type L (Medium Wall): Standard specification for Residential and Light Commercial HVAC line sets. Available as hard-drawn (stiff straight lengths) or soft-annealed (flexible coils).
• Type M (Thin Wall): Lightweight wall construction used exclusively for domestic water plumbing. Type M copper is NEVER permitted for HVAC refrigerant piping due to its inability to withstand high operating pressures (especially with R-410A or R-454B).

Note on Sizing: Plumbing copper is measured by Nominal Inside Diameter (ID), whereas ACR copper is always measured and specified by its Outside Diameter (OD). For example, 5/8-inch ACR tubing has an outside diameter of exactly 5/8 inch, whereas 1/2-inch nominal plumbing pipe has an outside diameter of 5/8 inch.

Cutting, Reaming, Swaging, and Flaring

1. Cutting: Copper must be cut cleanly using a wheel-type tubing cutter. Using a hacksaw introduces metal filings into the line set that will destroy compressor bearings.
2. Reaming (Deburring): Cutting copper leaves a heavy internal burr or ridge that restricts refrigerant flow and creates turbulent pressure drops. The pipe end must be reamed using a reamer blade while holding the pipe downward so copper shavings fall out of the pipe rather than inside.
3. Swaging: Swaging expands the end of one copper tube using a punch or lever-action swaging tool so that another tube of the same diameter can slip into it, eliminating the need for separate coupling fittings and reducing brazing joints by 50%.
4. Flaring: Flares are used for mechanical connections on mini-split systems. HVAC flares use a 45-degree single flare (never a 37-degree automotive flare or double flare). The pipe must be deburred before flaring, and a drop of refrigerant oil applied to the back of the flare cone prevents copper binding during formation.

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Line Set Sizing and Oil Return Dynamics

Refrigerant lines must be sized accurately to balance pressure drops against liquid velocity and compressor oil return.

Liquid Line Sizing

The liquid line carries liquid refrigerant from the outdoor condenser to the indoor metering device (TXV/piston). If liquid line pressure drops significantly due to undersized piping or excessive vertical lift, the liquid refrigerant will flash into gas before reaching the metering device (producing flash gas). Flash gas severely starves the evaporator coil.

Suction Line Sizing and Vertical Oil Traps

The suction vapor line carries cold refrigerant gas from the evaporator back to the compressor inlet. Because compressor oil mixes with refrigerant, vapor velocity inside the suction line must be high enough to push oil along horizontal runs and carry it upward through vertical risers:

• Minimum Vapor Velocity: Suction gas must maintain a minimum velocity of 1,200 to 1,500 feet per minute (fpm) in horizontal runs and 1,500 to 2,000 fpm in vertical risers to carry oil upward against gravity.
• Vertical Riser Traps: When an evaporator is located below a condensing unit and the vertical suction riser exceeds 25 feet, an oil trap (P-trap) must be installed at the base of the riser. Additional oil traps must be installed for every 20 to 25 feet of vertical rise. As oil drains down the riser wall during off-cycles, it collects in the trap, narrowing the passage; when the compressor starts, high-velocity vapor sweeps the trapped oil upward.

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Dry Nitrogen Purging Protocol During Brazing

Brazing copper tubing involves temperatures exceeding 1,200°F (650°C). When copper is heated in the presence of air (oxygen), the oxygen reacts instantly with copper to form a heavy black scale called cupric oxide (CuO).

The Mechanism of Cupric Oxide Contamination

As the pipe cools, brittle cupric oxide flakes detach from the interior pipe wall and circulate through the refrigeration system. This debris clogs tiny metering orifice ports, plugs TXV inlet screens, restricts filter drier cores, and causes abrasive scoring on compressor scroll sets and bearings.

Nitrogen Purge Procedure

To prevent oxidation, technicians must purge the pipe interior with dry nitrogen during the entire heating and brazing process:

1. Low-Pressure Flow: Connect a dry nitrogen cylinder equipped with a primary regulator and a low-pressure flowmeter to one end of the copper line set.
2. Flow Rate: Set the regulator to deliver a continuous, low-velocity sweep of nitrogen at 2 to 5 Standard Cubic Feet per Hour (SCFH) or 1 to 3 psig.
3. Verification: The nitrogen flow must be low enough that you can barely feel it against your cheek or flame out a lighter placed near the exit port. Excess nitrogen pressure will blow holes through molten brazing alloy.
4. Continuous Sweep: Keep nitrogen flowing continuously through the joint while heating, brazing, and cooling down below 500°F.

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Brazing Alloys and Thermal Protection Practices

Brazing differs from soldering because it utilizes filler metals with liquidus melting temperatures above 840°F (450°C), creating joints with superior tensile strength capable of handling high system operating pressures.

Alloy Selection

• BCuP Series (Copper-to-Copper): Phosphorus-copper filler alloys (e.g., BCuP-5 with 15% silver or BCuP-3 with 5% silver). The phosphorus acts as a self-fluxing agent on copper-to-copper joints. No flux is required.
• BAg Series (Copper-to-Brass or Copper-to-Steel): High-silver content alloys (e.g., 45% or 56% silver). When brazing dissimilar metals like copper to brass service valves or steel compressor stubs, chemical paste flux must be applied to prevent oxidation, and a BAg series rod must be used.

Heat-Sinking Sensitive Components

High brazing temperatures conduct rapidly along copper pipe, endangering internal rubber seals, O-rings, and delicate internal parts of service valves, solenoid valves, and TXV bodies:

1. Wet Rag / Heat-Sink Paste: Wrap a thick, water-soaked cloth tightly around the body of service valves, reversing valves, and TXV assemblies before applying flame.
2. Flame Direction: Angle the oxy-acetylene or air-acetylene torch flame away from the valve body and toward the tubing joint.
3. Cooling: Re-wet the cloth during and immediately after brazing until the joint drops below thermal degradation thresholds.