Why Copper Tube Dominates Refrigeration Equipment
In a refrigeration circuit copper tube serves two distinct functions. Inside the heat exchangers it is the heat transfer surface of the condenser and the evaporator, and in the connecting pipework it carries refrigerant between the compressor, the expansion device and the coils. The material is chosen because it offers thermal conductivity of roughly 390 W/(m·K), sufficient strength to contain the refrigerant, good plasticity for bending and flaring, and an oxide film that resists both the refrigerant and the surrounding atmosphere.
Practical selection is therefore a compromise between three requirements: the tube must transfer heat efficiently, it must contain the design pressure with an adequate margin, and it must be clean enough inside that lubricant, oxide and moisture do not contaminate the circuit.
Classification of Refrigeration Copper Tube
Refrigeration tube is classified in three ways, and all three descriptions usually appear on the same purchase order.
By material composition. Pure copper tube offers the best thermal conductivity. Brass tube has higher strength but lower conductivity. Bronze and copper-nickel tube resist aggressive cooling water and marine atmospheres but are harder to form and to braze, so they are used mainly in seawater-cooled condensers rather than in a factory-built refrigeration unit.
By production route. Oxygen-free tube is used for capillary lines and precision components. Deoxidised and oxygen-bearing copper tube is the standard for connecting pipework because it combines suitable strength and hardness with reliable brazing behaviour. Internally grooved tube carries a helical ridge on the bore that increases the internal surface area and promotes turbulence, improving the heat transfer coefficient of an evaporator or condenser without increasing its size.
By temper. Soft annealed tube has the highest ductility and is supplied in coils for field bending. Hard tube has the highest strength and is used in straight runs and in some heat exchanger structures. Half-hard tube balances strength and formability and is frequently preferred for refrigeration pipework built in a workshop.
Performance Characteristics That Govern Selection
| Property | Significance in a refrigeration circuit |
|---|---|
| Thermal conductivity | Determines the heat exchanger surface area needed for a given duty |
| Tensile and yield strength | Sets the pressure capability of the tube wall at the design temperature |
| Elongation and ductility | Allows bending, flaring, swaging and coil forming without cracking |
| Corrosion resistance | Protects against condensate, cleaning agents, salt air and aggressive cooling water |
| Low-temperature behaviour | Keeps the material tough at evaporating temperatures well below zero |
| Internal cleanliness | Prevents blockages, wax formation and compressor wear caused by residual oil, oxide or moisture |
Grades and standards used for refrigeration and air conditioning tube include ASTM B280 for seamless copper tube in field service, GB/T 17791 for copper tube for air conditioners and refrigerators, and EN 12735 parts 1 and 2 for European piping and heat exchanger applications. The phosphorus-deoxidised grade corresponding to C12200 is the ordinary choice, because it can be brazed repeatedly without any risk of hydrogen embrittlement.
Wall Thickness and Pressure Design
Wall thickness is not a matter of habit; it must be matched to the design pressure, the tube diameter and the maximum operating temperature of the circuit. The saturated pressure of modern high-pressure refrigerants is substantially higher than that of the older chlorofluorocarbon fluids, and the design pressure of a system must therefore be taken from the refrigerant data at the maximum expected ambient or condensing temperature, with an additional margin required by the applicable pressure equipment or piping code.
Three further factors reduce the working capability of a tube and must be allowed for:
Wall thickness tolerance, which means the minimum wall of a delivered lot is below the nominal value.
Wall thinning on the outside of a bend, which can be substantial when the bending radius is small.
Reduction in allowable stress if the circuit is operated at elevated condensing temperature, since the strength of annealed copper falls as temperature rises.
Where the calculation cannot be carried out in full, the correct approach is to order to the wall thickness series published in the applicable product standard for the duty concerned, and to increase the wall for a small bending radius rather than to rely on the nominal dimension.
Piping Design, Installation and Cleanliness
Use a bending radius of not less than about three to five times the outside diameter for annealed tube, and larger where the bend is subsequently pulled or vibrated.
Support horizontal and vertical runs at intervals that prevent sagging and vibration contact with the structure, and avoid trapping oil in low points of a suction line.
Size suction lines to keep refrigerant velocity high enough to return oil to the compressor while keeping pressure drop within an acceptable fraction of the saturation pressure.
Purge the bore with dry nitrogen while brazing so that no internal oxide scale forms; the scale would otherwise travel through the circuit and block the expansion device.
Keep the tube capped and dry during storage, and never leave an open end on a site where moisture can enter.
Separate copper from steel or aluminium at any connection where an electrolyte could bridge the joint, to avoid galvanic corrosion.
It is also worth stating that any wall thickness chosen from a rule of thumb is only provisional. Final selection should consider the bend radius, the support spacing, the connection details and the vibration environment, and should be confirmed against the latest edition of the relevant refrigeration piping standard before the design is frozen.
FAQ
Q: Which copper tube grade is standard for refrigeration and air conditioning?
Phosphorus-deoxidised copper corresponding to C12200, supplied in the annealed temper for coils and in a drawn temper for straight runs. The phosphorus content makes the tube safe to braze in air without risk of hydrogen embrittlement.
Q: What is ACR tube?
ACR stands for air conditioning and refrigeration. Such tube is produced to ASTM B280 or GB/T 17791 with a clean, dry bore, controlled dimensions and capped ends so that it can be installed without an internal cleaning operation.
Q: What does an internally grooved tube do?
The helical grooves on the bore increase the internal surface area and disturb the boundary layer of the refrigerant, which raises the heat transfer coefficient of the evaporator or condenser for the same external dimensions.
Q: Can hard temper tube be used for refrigeration piping?
Yes, for straight runs that are cut and brazed in a workshop. Bends should be made from annealed tube or the bend area should be annealed before forming, because hard tube will crack if bent cold to a small radius.
Q: Why must nitrogen be used during brazing?
A dry nitrogen purge displaces air from the bore and prevents copper oxide scale from forming on the hot inner surface. Scale carried around the circuit would contaminate the refrigerant and can block capillary tubes and expansion valves.
Q: How is the correct wall thickness determined?
From the design pressure and maximum operating temperature of the refrigerant, the tube diameter, the wall thickness tolerance and any wall thinning at bends, using the allowable stress permitted by the applicable pressure or piping standard for the copper grade and temper concerned.
