6.5 Refrigerant Line Sizing, Pressure Drop, Brazing, and Piping Practice

6.5 Refrigerant Line Sizing, Pressure Drop, Brazing, and Piping Practice

"Sizing, designing, and installing refrigerant lines," "determine refrigerant line pressure drop and explain the effects of pressure drop on a system," "soldering and brazing using correct techniques," "explaining the proper use and handling of nitrogen," and "flaring" all appear as competencies on the HVAC Excellence task list. Piping is the part of the trade where a technician's decisions are permanently embedded in the building.

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1. Why Line Size Is Not About Fitting the Stub

Every refrigerant line has two competing requirements:

• Low pressure drop, so the compressor does not have to work against friction.
• High enough velocity, so oil entrained in the refrigerant is carried back to the compressor.

Undersize the line and you get excessive pressure drop. Oversize it and velocity falls below the point where the vapor can drag oil up a riser, and oil logs in the evaporator. A line that is too large is as much a failure as one that is too small, which is why "the size of the stub-out" is never the sizing method.

Pressure drop criteria (expressed in saturation temperature)

The industry sizes lines by the equivalent saturation temperature change the pressure drop causes, because that is what actually affects capacity.

Suction line pressure drop is the expensive one. Each 2°F of suction-line saturation loss costs roughly 3–5% of system capacity and raises compressor power draw, because the compressor must pull down to a lower pressure to hold the same evaporator temperature.

Liquid line pressure drop is dangerous rather than merely costly. If total liquid-line pressure drop (friction plus vertical lift) exceeds the available subcooling, the liquid flashes to vapor before reaching the metering device. A TXV fed with flash gas hunts, starves the coil, and produces high superheat with a low-capacity system that looks undercharged but is not. Every foot of vertical lift costs about 0.5 psi on common refrigerants — a 30-foot riser costs roughly 15 psi, which on R-410A is worth several degrees of subcooling.

Velocity criteria for oil return

Double suction risers solve the conflict on systems with capacity control. Two risers are run in parallel with a trap at the bottom of the larger one. At full load both carry gas. At part load, oil collects in the trap and seals the large riser, forcing all flow through the small one, which restores velocity. This is a classic exam topic on unloading compressors and multi-evaporator racks.

Traps and slopes

• Trap the bottom of every suction riser and, on tall risers, add an intermediate trap roughly every 20 feet so the vapor is not asked to lift an unbroken oil column.
• Slope horizontal suction lines toward the compressor, roughly 1/2 inch per 10 feet, so gravity assists oil return.
• An inverted trap at the evaporator outlet, rising above the top of the coil, prevents liquid from draining into the compressor during the off cycle on a system without pump-down.
• Never trap a liquid line; there is no oil-return need and a trap only adds pressure drop.
• Support tubing at code intervals, isolate it from structure with grommets or clamps that allow thermal movement, and insulate the suction line (and, on heat pumps, both lines) with closed-cell insulation of adequate wall thickness.

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2. Brazing and Soldering

The distinction

Refrigerant joints are brazed, not soldered. Soft-soldered refrigerant joints fail under vibration and pressure.

Choosing the alloy

• BCuP-2 (0% silver) and BCuP-5 (15% silver) are self-fluxing on copper-to-copper joints, because the phosphorus reduces copper oxide as it flows. Never use a phosphorus-bearing alloy on ferrous metal, and never on copper-to-nickel or copper-to-steel — brittle phosphides form.
• Copper-to-brass and copper-to-steel joints (service valves, accumulators, driers with steel shells) require a silver alloy (BAg series) with flux.
• Higher silver content lowers the melting range and increases ductility and gap-filling ability — useful on close-tolerance or dissimilar-metal joints.

Technique

1. Cut square with a tubing cutter, then deburr inside and out. Burrs create turbulence and shed copper into the system.
2. Clean the tube end and the fitting socket with an abrasive cloth or nylon pad. Do not use steel wool or emery containing oil.
3. Fit-up matters: capillary action pulls filler into a gap of roughly 0.001–0.005 inch. Too tight and filler will not enter; too loose and it will not bridge.
4. Purge with nitrogen. Flow 2–3 psig of dry nitrogen (a trickle you can barely feel) through the tubing throughout heating and until the joint cools. Without it, oxygen inside the tube reacts with hot copper to form cupric oxide scale, a black flake that breaks loose later and plugs metering devices, screens, and driers. Provide an exit for the nitrogen — never braze a fully closed system under nitrogen pressure.
5. Heat the base metal, not the filler. Heat the fitting and the tube evenly until the copper reaches a dull cherry red, then touch the filler to the joint on the side opposite the flame and let capillary action pull it in.
6. Protect nearby components. Wrap TXV bodies, service valves, and Schrader cores with a wet rag or heat-blocking paste, or remove valve cores entirely — a heated core melts and the valve leaks forever.
7. Wipe or quench the finished joint only after the filler has solidified, and inspect for a continuous fillet all the way around.

Safety recap

Recover refrigerant first (Section 1.5), ventilate, use cadmium-free alloys, keep a fire watch for 30 minutes, secure and cap cylinders, and keep oxygen away from oil.

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3. Mechanical Joints: Flaring and Swaging

Mini-splits, some metering devices, and many service connections use 45° SAE flares.

Making a good flare

1. Cut square and deburr — a burr becomes a leak path.
2. Slide the flare nut on first. Forgetting this is the classic rework.
3. Clamp the tube in the flaring block with the correct amount protruding (roughly the thickness of the tubing wall above the block face for a standard yoke, or per the tool's gauge).
4. Apply a drop of refrigeration oil to the cone; it lubricates the burnishing action and produces a smoother, more uniform flare. (Use the same oil as the system — POE for HFC systems.)
5. Turn the yoke smoothly; back off and re-advance to burnish. Stop before the flare cracks or thins.
6. Inspect: the flare face should be smooth, concentric, and free of cracks or scoring, with the full width bearing on the fitting seat.
7. Torque to specification. Mini-split manufacturers publish flare torque values (for example, roughly 10–12 ft-lb for 1/4", 25–27 ft-lb for 3/8", 35–40 ft-lb for 1/2"). Over-torquing splits the flare; under-torquing leaks. Use a torque wrench, and back up the fitting with a second wrench.

Swaging expands one tube end to accept another of the same size, eliminating a coupling. Anneal work-hardened tube with heat if it resists, expand to a depth of at least the tube diameter, and braze the joint normally.

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4. Pressure Testing and Leak-Test Pressures

The Commercial sheets specifically list "system leak-test pressures and nitrogen regulator installation and adjustment" as a safety competency.

• Always use a two-stage regulator with a relief valve on the nitrogen cylinder. A full nitrogen cylinder exceeds 2,000 psig; connecting it to a system without a regulator will rupture the lowest-rated component, usually the evaporator or a compressor shell, with lethal force.
• Test pressure is set by the equipment's low-side and high-side design pressures marked on the nameplate — never a personal habit. Isolate the compressor and any component whose rating is below the test pressure.
• Never pressure-test with oxygen — it forms an explosive mixture with refrigerant oil — and never with acetylene, compressed air (moisture), or refrigerant plus nitrogen as a "trace gas" for detection, which is prohibited for regulated refrigerants because it constitutes venting when released.
• Standing pressure decay test: pressurize, record pressure and ambient temperature, wait, and re-read both. Pressure changes with temperature even in a perfectly tight system — roughly 0.5 psi per °F on a typical system volume — so a decay test without a temperature reading proves nothing.
• After a successful pressure test, release nitrogen slowly, then evacuate to 500 microns with a micron gauge and confirm with a decay test (Section 10.4).