Why Copper Is the Default Refrigerant Circuit Material
An air conditioner moves heat rather than creating it, and the refrigerant must absorb heat in the evaporator and reject it in the condenser. The tubing that carries that refrigerant between the indoor and outdoor units is therefore part of the heat transfer surface, not just a pipe. Copper is chosen because it performs four jobs at once: it conducts heat efficiently, it holds the system pressure with a thin wall, it can be bent, flared and brazed on site without special equipment, and it survives decades of wet and dry cycling outdoors. Aluminium can be substituted in some of those roles, but rarely in all of them at the same time.
Thermal and Mechanical Performance
The performance gap between copper and aluminium is best seen side by side. Copper conducts heat at roughly 390 to 400 W per metre kelvin in the tube grades used for refrigeration, while aluminium sits near 205 to 237 W per metre kelvin. Copper is also more than three times denser, which matters for weight but not for efficiency, and it has a lower coefficient of thermal expansion, so a brazed joint is less stressed by temperature cycling.
| Property | Copper tube, C12200 | Aluminium tube |
|---|---|---|
| Thermal conductivity | about 390 to 400 W per metre kelvin | about 205 to 237 W per metre kelvin |
| Density | 8.94 g/cm3 | 2.70 g/cm3 |
| Joining | Brazing and flaring with standard torches | Flux brazing or mechanical joints |
| Field repair | Cut, re-flare or re-braze | Limited, often replaced as a unit |
| Galvanic behaviour with aluminium fins | Noble metal, requires isolation | Compatible with aluminium fins |
Mechanically, annealed copper tube combines high burst strength with enough ductility to be coiled and bent around tight radii, and it work hardens only mildly in service. Aluminium tube of the same outside diameter needs a thicker wall to reach comparable pressure ratings, and it is far less tolerant of repeated bending and vibration. That is why copper remains standard for the connecting lines between the indoor and outdoor unit, where the tube is bent on site and then buried behind a wall or run across a roof.
Grades and Standards for Refrigeration Tube
Air conditioner tubing is made from phosphorus-deoxidised copper, designated C12200 or Cu-DHP, with a copper content of at least 99.9 percent and phosphorus in the range of about 0.015 to 0.04 percent. The phosphorus deoxidises the melt so the tube can be brazed in a reducing atmosphere without the hydrogen embrittlement risk that affects oxygen-bearing copper, while the residual phosphorus costs only a little conductivity, which is unimportant in a tube that is valued for thermal performance rather than electrical conduction. Product standards commonly referenced are ASTM B280 for seamless copper tube for air conditioning and refrigeration, GB/T 17791 for air conditioning and refrigeration equipment, and EN 12735 parts 1 and 2, which cover round seamless copper tubes and the corresponding pressure ratings. Cleanliness is specified as well: the bore is normally supplied dry and capped, with limits on residual oil, moisture and particle content, because contamination left inside the tube migrates to the expansion device and causes blockages or capillary freezing.
Wall Thickness, Temper and Refrigerant Choice
Wall thickness is not a fixed property of a diameter; it follows from the refrigerant and the system pressure. R410A runs at higher pressures than the R22 it replaced, and R32 runs higher again, so the same coil diameter may need a thicker wall or a higher strength temper. Tube selection should therefore be confirmed against the pressure rating tables in the applicable product standard and the system design pressure, not chosen by habit.
| Typical outside diameter | Wall thickness range | Usual temper | Typical duty |
|---|---|---|---|
| 6.35 mm | 0.6 to 0.8 mm | Soft annealed, O60 | Branch and connecting lines |
| 9.52 mm | 0.7 to 0.9 mm | Soft annealed, O60 | Main indoor to outdoor connection |
| 12.7 mm and above | 0.8 to 1.0 mm | Soft or half hard | Larger capacity lines and headers |
The heat transfer surfaces are usually upgraded further. Inner-grooved tube, produced by drawing helical ridges into the bore, raises the internal heat transfer coefficient by promoting nucleate boiling and thinning the liquid film, so a smaller and lighter coil can do the same duty. Grooved tube is the standard for the indoor and outdoor coil, while plain soft tube is used for the longer connecting lines where bending dominates and pressure drop is the controlling factor.
Corrosion and Field Reliability
Copper does not rust, but it is not immune. The most common failure in air conditioning coils is formicary corrosion, in which organic acids, typically formic and acetic acid released from certain insulation foams, adhesives, sealants and cleaning products, attack the copper and produce shallow subsurface tunnels that eventually perforate the wall. The attack is difficult to see until a leak appears, so prevention is a design and installation matter: keep coils clean and dry, avoid insulation materials that release organic acids at elevated temperature, and ensure condensate drains freely. Coastal installations add chloride-driven attack and require closer control of galvanic contact with aluminium fins. Copper is cathodic to aluminium, so any wet bridge between the two creates a galvanic couple that sacrifices the fin and can also accelerate tube attack. Dissimilar metal contact should be broken by coating, by a suitable washer or sleeve, or by drainage arrangements that keep the joint dry.
Copper Versus Aluminium Tubing
Aluminium microchannel condensers have gained ground because they reduce weight and cost, and in some markets they are the standard for the outdoor coil. The trade-off is real. Aluminium coil assemblies are harder to repair, need flux brazing or mechanical joining, and cannot simply be re-flared on site, so a small leak can mean replacing a complete assembly rather than repairing a joint. Aluminium is also more prone to galvanic attack when it touches copper components elsewhere in the circuit and more sensitive to vibration fatigue. The practical split in the market is therefore clear: aluminium is often used inside factory-built coil assemblies where a controlled brazing process is available, while copper remains the material of choice for the field-installed refrigerant lines and for applications where serviceability and long life dominate the specification.
Frequently Asked Questions
Q: Why is copper better than aluminium for air conditioner tubing?
Copper conducts heat roughly twice as effectively, reaches a given burst pressure with a thinner wall, can be bent, flared and brazed on site with ordinary tools, and can be repaired after a leak. Aluminium is lighter and cheaper but harder to join and repair.
Q: What grade of copper is used for refrigeration tube?
Phosphorus-deoxidised copper, C12200 or Cu-DHP, with a minimum copper content of 99.9 percent and about 0.015 to 0.04 percent phosphorus. It is supplied to standards such as ASTM B280, GB/T 17791 or EN 12735, dry and capped for refrigeration service.
Q: Does the refrigerant affect the tube wall thickness?
Yes. Higher pressure refrigerants such as R410A and R32 require a thicker wall or a higher strength temper than R22 at the same diameter. Always confirm the selection against the pressure rating in the applicable tube standard and the design pressure of the system.
Q: What is formicary corrosion?
It is subsurface tunnelling attack caused by organic acids, usually formic or acetic acid released by insulation, adhesives or cleaning agents. It perforates copper coils from the inside out and is prevented mainly by material selection, drainage and keeping the coil dry and clean.
Q: Is inner-grooved tube worth the extra cost?
For coil applications, generally yes. The helical ridges raise the internal heat transfer coefficient, so a smaller and lighter coil achieves the same duty. Plain tube is still preferred for long connecting lines where bending and pressure drop dominate.
Q: How should copper tubing be stored before installation?
Keep the caps in place, store coils indoors and off the ground, and avoid contact with steel and with materials that release organic acids. Moisture, dust and oil left inside the tube are a common cause of early system failure.
