The Role of the Tube in a Heat Pipe
A heat pipe moves heat by evaporating a working fluid at the hot end and condensing it at the cold end, returning the liquid through a wick or by gravity. The envelope that contains this cycle is usually seamless copper tube, and ASTM B111/B111M is the specification that defines the chemical, mechanical and testing requirements for it. UNS C12200, the phosphorus-deoxidised DHP grade, is the material of choice for most water and methanol heat pipes.
The specification covers seamless round, rectangular and square copper tube in a range of UNS numbers including C12200. Tube is manufactured by hot and cold working, cold drawn to the finished size and wall thickness, and supplied in the cold-drawn or annealed temper. Chemical composition for copper, phosphorus and arsenic is verified by analysis, and mechanical requirements including tensile strength, yield strength, hardness and straightening are checked before dispatch.
Why Deoxidised Copper Is Specified
| Requirement | Why it matters in a sealed tube |
|---|---|
| Copper 99.90 % minimum, phosphorus 0.015-0.040 % | Deoxidation removes oxygen so the metal cannot embrittle during welding or high-temperature bake-out |
| Residual oxygen at trace level | Prevents internal steam voids and non-condensable gas generation inside a sealed envelope |
| Controlled grain size | Fine uniform grains allow tight bending and flattening without cracking |
| Clean, scale-free bore | Supports uniform wick wetting and reproducible thermal performance |
| Dimensional tolerance and straightness | Controls wall thickness for pressure containment and allows wick insertion along the full length |
Phosphorus also raises the softening temperature of work-hardened copper and promotes a fine grain size, so a bent or flattened heat pipe keeps more of its strength than an equivalent high-purity copper part. Conductivity is lower than in oxygen-free copper - roughly 70-90 percent IACS instead of 101 percent - but the heat pipe relies on phase change rather than conduction along the wall, so this reduction has little effect on overall transport capacity.
Working Fluids and Compatibility
Water is the usual working fluid for electronics and industrial cooling between about 30 °C and 200 °C and is fully compatible with copper.
Methanol and ethanol are used for low-temperature duty, and both are compatible with copper envelopes.
Ammonia and ammonia-bearing fluids are not compatible with copper and should not be used in this envelope.
Refrigerants used in HVAC and refrigeration circuits are handled in copper tube as a matter of course, provided the circuit is clean and dry.
Fabrication of the Envelope
Tube is cut to length, degreased and pickled to remove drawing lubricant and oxide, then fitted with the wick or internal groove structure. The ends are closed by crimping and welding, or by welding end caps, after which the assembly is evacuated, charged with the working fluid and sealed. Annealing between 250 °C and 650 °C can be applied before or after forming; stress relief at 200 °C to 250 °C stabilises dimensions in bent sections, and hot working is carried out between about 750 °C and 950 °C.
Cleanliness is the single most important fabrication variable. Trapped flux, oil or moisture becomes non-condensable gas or a corrosion site inside the sealed envelope, and performance degrades within weeks. Welding should be done with dry, oxygen-free shielding gas and with the bore protected from oxidation, and every part should be handled with gloves after cleaning.
Testing and Performance Checks
Eddy-current testing of the tube detects longitudinal defects before fabrication begins.
Hydrostatic or pneumatic pressure testing confirms the wall can contain the internal pressure at the maximum working temperature.
Expansion and flattening tests verify that the tube will survive bending and flattening into flat heat pipes or vapour chambers.
Helium leak testing and a thermal performance run confirm that the sealed unit holds vacuum and meets its rated thermal resistance.
Grain size and hydrogen embrittlement checks, as required by ASTM B111, guard against batch-to-batch variation in forming behaviour.
FAQ
Q: Why is C12200 used instead of oxygen-free copper for heat pipes?
The deoxidised chemistry gives the same freedom from hydrogen embrittlement while offering better bending and flattening behaviour, higher softening resistance and lower cost. Oxygen-free grades are chosen when maximum thermal conductivity of the wall is the priority.
Q: Which working fluids suit a copper heat pipe?
Water is the most common, with methanol and ethanol for lower temperatures. Ammonia must not be used, because it attacks copper.
Q: What temper should be ordered?
Soft annealed tube is supplied where the envelope will be bent, flattened or grooved, while drawn tempers are used for straight tubes that must contain higher internal pressure.
Q: How clean does the tube have to be?
Very clean. A degreased, pickled and dried bore with no residual drawing lubricant is essential, because contamination inside a sealed envelope becomes non-condensable gas and reduces performance.
Q: What tests are recommended before assembly?
Eddy-current examination, dimensional and wall-thickness checks, and where the design demands it a pressure test. The finished heat pipe is then helium leak tested and performance tested after charging.
Q: Can rectangular or square tube be supplied?
Yes. ASTM B111 covers seamless round, rectangular and square copper tube, which allows flat heat pipes and vapour chamber profiles to be produced directly from specified stock.
