Classifying Copper Tubes Before Selecting One
Copper tube for refrigeration divides first by material composition. Pure copper tube, typically TP2, has the best thermal conductivity but the highest cost. Brass tube grades such as H62, H65 and H68 have higher strength but lower thermal conductivity. Bronze grades such as QSn6.5-0.1 and cupronickel grades resist corrosion well but are harder to work. The second division is by production process: oxygen free tube, oxygen containing tube and inner-grooved tube. Oxygen free tube is used for capillary and precision components, oxygen containing tube for connection piping where strength, hardness and weldability matter, and inner-grooved tube for evaporator and condenser coils where the grooves raise the heat transfer coefficient. The third division is by hardness: soft O temper tube is ductile and easy to bend, half hard 1/2H temper balances strength and formability, and hard H temper gives the highest stiffness for straight runs.
Material Chemistry and Standards for Refrigeration Tube
TP2 deoxidized copper is the usual refrigeration material. Copper plus silver is 99.90 % minimum and phosphorus is held between 0.015 % and 0.040 %, with tight limits on residual impurities because they reduce conductivity and promote cracking during bending and welding.
| Element | Maximum, % | Element | Maximum, % |
|---|---|---|---|
| Bismuth | 0.001 | Lead | 0.005 |
| Antimony | 0.002 | Sulphur | 0.005 |
| Arsenic | 0.002 | Zinc | 0.005 |
| Iron | 0.005 | Nickel | 0.002 |
| Tin | 0.002 | Phosphorus | 0.015 - 0.040 |
Refrigeration tube is ordered to ASTM B280/B280M and ASME SB280 for air conditioning and refrigeration service, to EN 12735-1 for piping systems and EN 12735-2 for equipment, and to the GB/T 1527 and GB/T 17791 series in the Chinese system, with JIS H3300 covering the Japanese equivalent. The deoxidized chemistry is essential: tube that contains residual oxygen cannot be brazed or annealed without risking hydrogen embrittlement and internal porosity, so the phosphorus addition is what makes refrigeration circuits reliable in service.
Checking Wall Thickness Three Ways
Nominal wall thickness is not chosen by diameter alone. A system is normally checked by three separate methods and the largest result governs the final wall.
Pressure capacity is checked by comparing hoop stress from the design pressure against the allowable stress of the tube material in its delivered temper. The tube must also survive the shop test pressure and any hydrostatic verification applied before dispatch.
Fluid resistance is checked by relating pressure drop to the Reynolds number and the relative roughness of the bore, together with refrigerant density and flow velocity. A wall that is too thin for the intended velocity allows vibration and erosion, while an oversized wall wastes material and reduces heat transfer.
Vibration fatigue is checked from the alternating stress amplitude, which is calculated from the surface quality factor, the fatigue limit of the copper tube material taken as 0.4 to 0.5 of the yield strength, the number of fatigue cycles and the fatigue strength index of 3 to 4, and compared with an allowable alternating stress of 0.6 to 0.7 of the yield strength. Separately, the allowable shear stress used in the resistance checks is taken as one third of the yield strength. Taking the maximum of the three results ensures that the tube is safe under the combined effects of pressure, flow and vibration.
Product Forms and Supply Range
Pancake coil for field installation and service work in air conditioning and general engineering.
Level wound coil for automatic coil, condenser and evaporator production lines.
Straight ACR tube for piping headers, risers and manifold assemblies.
Inner-grooved tube for evaporator and condenser coils with enhanced heat transfer.
Capillary and small diameter oxygen free tube for metering and precision components.
Tube is supplied in soft, half hard and hard tempers with diameters and walls matched to the coil design, cleaned and capped to prevent moisture and contamination entering the bore, and packed in coils or straight bundles for sea, air and courier transport. Selection for a specific project follows cooling capacity, working medium, working temperature and pressure, together with the connection method used on site. Material, temper and dimensions should be fixed together, because changing one of them changes the pressure rating, the pressure drop and the fatigue life of the circuit.
Frequently Asked Questions
Q: Which copper grade is standard for refrigeration tube?
A: TP2 deoxidized copper with 99.90 % minimum copper plus silver and 0.015 - 0.040 % phosphorus, ordered to ASTM B280, ASME SB280, EN 12735 or the GB/T 1527 and GB/T 17791 series.
Q: Why must refrigeration tube be deoxidized?
A: Deoxidation removes oxygen from the metal so that brazing and annealing cannot cause hydrogen embrittlement or internal porosity. Oxygen bearing copper is not suitable for brazed refrigeration circuits.
Q: How is wall thickness decided?
A: It is calculated by three independent checks: pressure capacity, fluid resistance from Reynolds number and relative roughness, and vibration fatigue. The largest of the three calculated values becomes the specified wall.
Q: What does the temper of the tube change?
A: Soft O temper is ductile and easy to bend for coils and field work, half hard 1/2H temper balances strength with formability for general piping, and hard H temper gives the stiffness needed for straight runs that must hold their shape.
Q: When is inner-grooved tube used instead of plain tube?
A: Inner-grooved tube is specified for evaporator and condenser coils because the internal grooves increase the heat transfer coefficient at the same bore size, which allows a more compact coil for the same duty.
Q: What is controlled in the mill test?
A: Chemical composition including the phosphorus and impurity limits, mechanical properties for the ordered temper, grain size on annealed tube, dimensional and wall thickness checks, cleanliness of the bore, and pressure or leak testing where specified.
