Lord Fin Tube--Introduce Heat Pipe
Published: 23 November 2015
What is a heat pipe?
A heat pipe is a sealed metal tube that carries heat by boiling a small amount of liquid at one end and condensing it at the other. Nothing inside it moves except vapour and liquid, and no pump or power supply is needed. Before the tube is closed, the air is drawn out and a measured charge of working fluid is put in, so the internal pressure is set by that fluid alone rather than by air.
Heat entering the evaporator section boils the fluid. The vapour crosses the empty core of the tube. At the condenser section the vapour gives up its latent heat and turns back into liquid. A wick lining the inner wall then pulls the liquid back to where it started. As long as the wick can return as much liquid as the evaporator boils away, the loop repeats with no outside help.
| Property | Solid copper rod of the same diameter | Heat pipe |
|---|---|---|
| How heat travels | Conduction through the metal lattice | Latent heat carried by a phase change |
| Equivalent conductivity | About 400 W/(m·K) | Often 5,000 to 100,000 W/(m·K) |
| Temperature drop over 200 mm | Large and proportional to load | A few degrees at rated load |
| Moving parts | None | None |
| Behaviour when tilted | Unaffected | Performance changes with orientation |
How a heat pipe moves heat from one end to the other
Four steps repeat continuously, and each one has to keep up with the others. Break any single step and the whole device stops transferring heat at its rated level.
Which step drives the loop
- Liquid in the evaporator absorbs heat and boils into vapour.
- The vapour pressure pushes vapour towards the colder end of the tube.
- Vapour condenses on the cooler wall and releases its latent heat.
- The wick returns the condensate to the evaporator through capillary pressure.
Why capillary pressure decides the ceiling
The wick generates a small pressure difference between the wet and dry sides of the liquid front. That difference has to overcome three losses: friction as liquid travels through the porous wick, drag as vapour flows the other way through the core, and gravity whenever the condenser sits above the evaporator. When the sum of those losses equals the capillary pressure the wick can produce, the evaporator runs dry and the device reaches its capillary limit.
How gravity changes the number
With the evaporator below the condenser, gravity returns liquid for free and the measured capacity is usually higher than in a horizontal position. With the evaporator on top, gravity works against the wick and the usable load drops, sometimes by half or more depending on wick type and tube length. Any specification that ignores mounting angle is incomplete.
Which parts make up a heat pipe
Envelope
The tube wall has to hold a vacuum, resist the internal pressure at operating temperature, and stay chemically compatible with the fluid. Copper is the usual choice for water and methanol. Aluminium pairs with ammonia. Stainless steel and titanium appear where corrosion or high temperature rules copper out.
Wick
The wick is the engine of the return path. Its pore size sets how much capillary pressure it can build, and its permeability sets how easily liquid flows back. Small pores give strong capillary force but slow flow; large pores do the opposite. That trade-off explains why wick construction is chosen per application rather than by default.
| Wick type | Capillary pressure | Liquid flow | Typical use |
|---|---|---|---|
| Grooved wall | Low | High | Gravity-assisted electronics cooling |
| Woven screen | Medium | Medium | General purpose tubes |
| Sintered powder | High | Medium | Space-rated and anti-gravity work |
| Composite / hybrid | High | High | High-flux devices, long tubes |
Working fluid
The fluid must boil and condense inside the operating band, stay chemically stable, and not attack the wall or the wick. Its figure of merit combines latent heat, liquid thermal conductivity, surface tension, and viscosity, which is why water outperforms most organics in the 30 °C to 250 °C window.
Which working fluid fits which temperature range
Choosing a fluid outside its useful band leads to either a frozen start-up or internal pressure that the envelope cannot hold.
| Working fluid | Useful range | Compatible envelope | Note |
|---|---|---|---|
| Helium | −271 °C to −269 °C | Stainless steel | Very low temperature research |
| Nitrogen | −203 °C to −160 °C | Stainless steel | Cryogenic cooling |
| Ammonia | −60 °C to 100 °C | Aluminium, stainless steel | Strong against copper |
| Acetone | −20 °C to 120 °C | Copper, aluminium | Useful below the water range |
| Methanol | −10 °C to 130 °C | Copper, stainless steel | Common in small electronics tubes |
| Water | 30 °C to 250 °C | Copper, nickel, titanium | Highest figure of merit in its band |
| Mercury | 250 °C to 650 °C | Stainless steel | Handling and safety limits apply |
| Sodium, potassium | 600 °C to 1,200 °C | Stainless steel, superalloys | Liquid metal, industrial heat |
How heat pipe performance is measured
Maximum heat transport, Qmax
This is the wattage at which the evaporator begins to dry out. It is measured by holding the condenser at a fixed temperature and raising the input until the evaporator temperature climbs faster than the load increase. Qmax is not a single fixed number: it falls as the tube gets longer, as the working fluid charge drifts from optimum, and as the mounting angle turns against gravity.
Thermal resistance
Resistance is the temperature difference between the evaporator and the condenser divided by the watts being carried. A well-made water heat pipe can sit between 0.02 °C/W and 0.2 °C/W depending on length and diameter.
Temperature uniformity
Because the internal pressure fixes the saturation temperature, the whole tube tends to sit within a narrow band. This is what makes heat pipes useful for isothermal plates and calibration fixtures, where a few tenths of a degree across a surface matter.
| Test | What it reveals | How it is run |
|---|---|---|
| Helium leak check | Whether air can enter the sealed tube | Mass spectrometer on the finished assembly |
| Vacuum bake | Residual moisture and gas in the wick | Heating under vacuum before charging |
| Qmax test | Dry-out threshold | Stepped load at fixed condenser temperature |
| Resistance test | Temperature drop per watt | ΔT divided by Q at steady state |
| Orientation sweep | Gravity sensitivity | Repeat at several tilt angles |
| Bend and flatten test | Whether forming damaged the wick | Compare before and after shaping |
| Life test | Long-term drift from gas generation | Continuous run with periodic re-measurement |
Where heat pipes are used
The same device shows up in very different industries because the physics does not change with the market.
| Sector | Typical job | Why a heat pipe wins |
|---|---|---|
| Computers and servers | CPU and GPU cooling | Moves watts from a small die to a large fin stack |
| LED lighting | High-power lamp thermal paths | Keeps junction temperature stable without fans |
| Telecom cabinets | Sealed enclosure cooling | Transfers heat out without letting dust in |
| Spacecraft | Payload and battery thermal control | Works without power or maintenance |
| Aircraft | Avionics cooling | Reliable at altitude and under vibration |
| Solar thermal | Evacuated tube collectors | Low loss transfer from absorber to tank |
| Industrial heat recovery | Exhaust-to-supply air exchangers | Recovers waste heat with no cross contamination |
| Medical equipment | Isothermal plates and analysers | Tight temperature uniformity across a surface |
How heat pipes are made and tested
Most quality problems in a finished heat pipe trace back to one of five stages, so each stage is controlled separately.
Cleaning and material preparation
Tubing is cut to length, deburred, and cleaned of oil, oxide, and machining residue. Wick material is handled with gloves because skin oils become a source of non-condensable gas later.
Assembly and sealing
The wick is inserted, end caps are fitted, and the joint between cap and tube is welded. Tungsten inert gas welding is the usual method because it produces a leak-tight joint without filler contamination.
Vacuum baking and charging
The assembly is baked under vacuum to drive off adsorbed water and dissolved gas. The working fluid is then degassed separately and a precise charge is admitted. Charge volume matters: too little fluid causes early dry-out, too much blocks the condenser and raises resistance.
Final sealing and verification
After charging, the fill tube is crimped and welded shut. Every unit passes a helium leak test, and samples from each batch go through Qmax, resistance, orientation, and life testing.
Why a heat pipe stops working
Failure rarely means the tube is broken. It usually means one of the internal limits has been reached.
| Limit | What triggers it | How it shows up |
|---|---|---|
| Capillary | Wick cannot return liquid fast enough | Evaporator temperature climbs sharply |
| Sonic | Vapour reaches the speed of sound at the evaporator | Capacity plateaus regardless of more heat |
| Entrainment | Fast vapour drags liquid droplets towards the condenser | Wick starves even though it is not dry |
| Boiling | Heat flux is too high for the evaporator surface area | Local hot spot, then dry-out |
| Viscous | Vapour pressure is very low at cold start-up | Slow or failed start from frozen state |
| Non-condensable gas | Air or generated gas collects at the condenser | Capacity fades slowly over months |
Why non-condensable gas is the quiet killer
Any gas that is not the working fluid cannot condense. It drifts to the coldest part of the tube and forms a plug that blocks a growing share of the condenser surface. The device still works, but its capacity shrinks a little each month. Correct vacuum baking and clean handling are what prevent this.
Why material pairing matters
Water and aluminium react, producing hydrogen that becomes a non-condensable plug. Ammonia attacks copper in the same way. Choosing the wrong envelope for a fluid can shorten service life from a decade to a few weeks.
Which details a heat pipe supplier needs from you
- Total watts to be moved, and whether the load is steady or pulsed
- Maximum temperature allowed at the heat source
- Temperature available at the cold side, whether ambient air, liquid, or a cold plate
- Acceptable temperature difference between the two ends
- Overall length and the physical path the tube must follow
- Mounting angle, and whether gravity helps or fights the return flow
- Diameter and space envelope, including any flat sections
- Bend locations and minimum bend radius
- Environment: humidity, salt spray, vibration, corrosive atmosphere
- Expected service life and number of start-stop cycles
- Compliance requirements such as RoHS or REACH
Questions that come up about heat pipes
Can a heat pipe work against gravity?
Yes, but the capacity drops. When the evaporator sits above the condenser, the wick must lift liquid instead of letting gravity return it, which consumes part of the available capillary pressure. In a gravity-aided position the same tube often carries noticeably more heat than it does horizontally.
What separates a heat pipe from a vapour chamber?
Both rely on the same phase-change cycle. A heat pipe is long and narrow and moves heat along one axis. A vapour chamber is flat and spreads heat across an area. Vapour chambers are thinner and better at two-dimensional spreading, but they are harder to seal and generally cost more.
How long does a heat pipe last?
With a compatible fluid and envelope, a clean vacuum process, and a leak-tight seal, service life of ten years or more is realistic. Early failures usually come from non-condensable gas or from a fluid that slowly attacks the wall.
Does a heat pipe consume electricity?
No. The loop is driven by the heat input itself, and the liquid return is driven by capillary force. There is no pump, no sensor, and no control circuit inside the tube.
Can a heat pipe be bent or flattened?
Yes, but the shaping has to be planned before the tube is sealed, and the wick must survive it. A bend radius below about three times the tube diameter can crush the wick or narrow the vapour channel. Flattening is usually limited to roughly one third of the original diameter.
What temperature range can one heat pipe cover?
It depends entirely on the working fluid. Ammonia covers roughly −60 °C to 100 °C, water covers about 30 °C to 250 °C, and liquid metals such as sodium reach beyond 600 °C. No single fluid spans the whole range, which is why fluid selection comes before hardware design.
A heat pipe remains one of the few thermal devices that moves large amounts of heat, over distance, with no power input and no moving parts. Matching the fluid, the envelope, and the wick to the actual load and mounting angle is what determines whether it performs for a decade or fails within a season.