Heat Pipe

What a heat pipe actually is

A heat pipe looks utterly unremarkable: a thin sealed metal tube, often copper, the diameter of a pencil or thinner. Cut one open and there is no machinery inside — just a porous lining on the wall and a small charge of liquid. Yet that tube moves heat hundreds of times more effectively than the same tube made of solid copper, with no electrical power and nothing that spins or slides. The trick is that it does not conduct heat through a solid at all. It carries heat as latent heat — the energy locked up when a liquid turns to vapor — and ferries it from the hot end to the cold end by a continuous, self-sustaining boiling-and-condensing loop.

Three regions matter. The evaporator is the hot end, bolted to whatever needs cooling — a CPU die, a power transistor, a satellite electronics box. The condenser is the cold end, attached to a heat sink, a fin stack, or a radiator panel. Between them runs the adiabatic section, the insulated transport zone where vapor travels one way and liquid travels the other along the wick. The whole assembly is evacuated of air and back-filled with a precisely metered charge of a working fluid chosen so that it boils and condenses within the intended operating temperature band.

The phase-change cycle, step by step

Follow one molecule of water around the loop:

• Boil. Heat enters the evaporator wall and is conducted a fraction of a millimetre into the saturated wick. The liquid there reaches its boiling point and flashes to vapor, absorbing 2,260 joules per gram. Crucially, this happens at a temperature only slightly above the condenser's, because the pipe is sealed and evacuated — the only gas inside is the working fluid's own vapor, so the boiling point tracks the local pressure.
• Vapor rush. Boiling raises the local vapor pressure a hair above the pressure at the cold end. That tiny pressure difference — sometimes only a few pascals — drives the vapor down the open core of the pipe toward the condenser at speeds that can approach the local speed of sound during startup.
• Condense. At the condenser the vapor touches the cooler wall, gives up its 2,260 J/g of latent heat to the heat sink, and condenses back into liquid that wets the wick.
• Wick return. The porous wick is now full of liquid at the cold end and starved of liquid at the hot end. Surface tension forms tiny menisci in the wick pores, and those menisci pull liquid back toward the evaporator by capillary action — the identical physics that lifts coffee up a sugar cube. No pump is needed; the curvature of the liquid surface does the pumping.

The loop is closed and passive. The hotter you make the evaporator, the harder it boils, the larger the pressure difference, the faster the vapor flows, and the more heat is carried — the device self-regulates over a wide power range. Because the latent heat per gram is so large and the vapor moves so fast, an enormous amount of power crosses the pipe for a temperature drop of just a few degrees end to end.

Why it beats copper by three orders of magnitude

Solid copper transports heat by phonon and electron conduction at a thermal conductivity k ≈ 400 W/m·K. The power a conductor carries follows Fourier's law:

Q = k · A · ΔT / L          (Fourier conduction)

Copper rod:  k = 400 W/m·K,  A = 50 mm²,  L = 0.2 m
To carry  Q = 50 W:
  ΔT = Q·L / (k·A) = 50 × 0.2 / (400 × 50e-6) = 500 °C across the rod

Fifty watts across a 20 cm copper rod of that slender cross-section needs a 500 °C temperature difference — so much that the rod would glow and melt long before it carried the load. That is exactly why solid metal is hopeless over any real distance. A water heat pipe of the same size carries the same 50 W with an end-to-end ΔT of only about 2–3 °C. Plug that measured ΔT back into Fourier's law and you can define an effective conductivity for the pipe:

k_eff = Q·L / (A·ΔT) = 50 × 0.2 / (50e-6 × 2.5) ≈ 80,000 W/m·K

That 80,000 W/m·K is roughly 200× copper — and it is not a material property at all, but a system property of the two-phase loop. The heat never has to conduct down the length of the solid; it only has to conduct the short distance through the wall and wick, then it rides the vapor. The longer the pipe, the bigger the advantage over copper, because solid conduction degrades with length (ΔT ∝ L) while the vapor transport adds almost no extra temperature drop over distance.

The capillary limit — the equation that sizes the wick

The wick is the heart of a terrestrial heat pipe, and its capillary pumping pressure is what most often caps the power. The maximum capillary pressure a wick can develop comes from the Young–Laplace relation for a curved meniscus:

ΔP_cap,max = 2σ / r_eff

where σ is the working fluid's surface tension and r_eff is the effective pore radius of the wick. For the pipe to function, that available capillary pressure must exceed the sum of all the pressure drops the returning liquid and outgoing vapor must overcome:

ΔP_cap,max  ≥  ΔP_liquid + ΔP_vapor + ΔP_gravity

ΔP_gravity = ρ · g · L · sin(φ)        (φ = tilt; positive when evaporator is above condenser)

Worked numbers for a water wick at 60 °C (σ ≈ 0.066 N/m):

Sintered-copper wick, effective pore radius r_eff = 30 µm:
  ΔP_cap,max = 2 × 0.066 / 30e-6 ≈ 4,400 Pa

Adverse gravity, evaporator 0.1 m above condenser (φ = 90°), water ρ ≈ 983 kg/m³:
  ΔP_gravity = 983 × 9.81 × 0.1 = 964 Pa

Budget left for liquid + vapor friction:  4,400 − 964 ≈ 3,440 Pa

Finer pores raise the capillary pressure (good) but also throttle the liquid flow and raise ΔP_liquid (bad). Wick design is the engineering of that trade-off: a 30 µm sintered-copper powder wick pumps hard; a coarse 100 µm screen mesh pumps weakly but flows freely. When the heat load demands more liquid than the wick can return against gravity and friction, the evaporator dries out, its wall temperature spikes, and the pipe fails — this is the dreaded capillary limit, also called dryout.

The other four operating limits

The capillary limit dominates at normal operating temperatures, but four other ceilings can bite, and the lowest one at any given temperature is the real maximum power. Plotted against temperature they form the classic heat-pipe performance envelope:

• Viscous limit. At very low temperatures the vapor pressure is so small that viscous drag in the vapor swamps the driving pressure difference. The vapor can barely flow. Dominant near a fluid's freezing point.
• Sonic limit. Vapor velocity cannot exceed Mach 1 at the evaporator exit. During startup from a cold condition, the vapor density is tiny, so even a modest power demands choking-flow velocity. This caps the power until the pipe warms up and vapor density rises.
• Entrainment limit. Fast-moving vapor shears droplets off the liquid surface in the wick and carries them back to the condenser — the wrong way. The liquid return is interrupted and the evaporator starves. Set by the Weber number of the vapor–liquid interface.
• Boiling limit. If the radial heat flux into the wick is too high, vapor bubbles nucleate inside the wick rather than at its surface. These bubbles block the pores and cut off liquid return locally — a hot-spot burnout. This is a flux limit (W/cm²), not a total-power limit, and it is what caps the heat density of vapor chambers under bare CPU dies.

Choosing the working fluid

A fluid only works between its triple point and critical point, and practically only over the band where its vapor pressure and transport properties are favorable. The figure of merit for capillary heat pipes weighs latent heat, surface tension, density, and viscosity together; in practice engineers pick from a short menu by temperature range:

Working fluid	Useful range	Latent heat	Typical use
Helium	2–4 K	21 kJ/kg	Superconducting magnets, cryogenics
Nitrogen	70–110 K	199 kJ/kg	Cryogenic sensors, IR detectors
Ammonia	−60 to 100 °C	1,370 kJ/kg	Spacecraft thermal control
Acetone	0 to 120 °C	518 kJ/kg	Electronics, aerospace avionics
Water	30 to 200 °C	2,260 kJ/kg	CPU/GPU coolers, LED, telecom
Sodium	500 to 1,100 °C	4,300 kJ/kg	Solar receivers, high-temp process

Water is the workhorse of electronics cooling precisely because of that 2,260 kJ/kg latent heat and its high surface tension, which gives strong capillary pumping. Its one weakness is the freezing point: a water heat pipe must never be allowed to freeze charged, because the expanding ice can rupture the wick or burst the envelope. For sub-zero operation — spacecraft, outdoor telecom — ammonia takes over. And note the fluid must be chemically compatible with the wall: water with copper is fine, but water with aluminium generates hydrogen gas that accumulates at the condenser as a non-condensable slug and slowly kills the pipe. That is why aluminium-walled spacecraft pipes use ammonia, not water.

Variants — vapor chambers, loop heat pipes, thermosiphons

• Thermosiphon (wickless heat pipe). Remove the wick and let gravity return the condensate. Dead simple and capable of very high power, but the condenser must sit above the evaporator. The trans-Alaska pipeline's 124,000 vertical thermosiphons exploit this to pull heat out of the soil in winter and keep the permafrost frozen around the pipe supports.
• Vapor chamber. A flat, two-dimensional heat pipe — a thin sealed plate with wick on both faces. It spreads heat from a concentrated die across a large area, smoothing out hot spots. Standard under high-power GPUs and gaming-console SoCs, and increasingly inside flagship smartphones.
• Loop heat pipe (LHP). Separates the vapor and liquid into distinct smooth-walled lines connected to a finely structured capillary evaporator (the wick is concentrated only at the evaporator). This lets the working fluid travel metres, around bends, and against gravity. LHPs are the backbone of modern spacecraft thermal control because they can move kilowatts from an electronics deck to a remote radiator with no pump.
• Variable-conductance heat pipe (VCHP). A reservoir of non-condensable gas blocks part of the condenser; as the load rises the vapor pressure pushes the gas back, exposing more condenser area. The result is near-constant evaporator temperature across a wide load range — used where electronics demand tight temperature control in space.
• Pulsating (oscillating) heat pipe. A long capillary tube bent into many turns with no wick; slugs of liquid and vapor oscillate back and forth, transferring heat by sensible and latent exchange. Cheap, flexible, and effective at high flux, increasingly used in compact electronics.

Where heat pipes actually show up

• Laptop and desktop cooling. A 3–6 mm flattened copper-water pipe carries 15–45 W from a laptop CPU/GPU to an edge-mounted fin stack. Desktop tower coolers stack four to eight 6–8 mm pipes pressed into a copper baseplate; a good one moves 250 W at under 15 °C of spread.
• GPUs and game consoles. Vapor chambers spread 300+ W from a bare die across the cooler base. The PlayStation 5 and high-end graphics cards use them where a round pipe cannot cover the die area.
• Spacecraft. Nearly every satellite uses constant-conductance and loop heat pipes filled with ammonia to carry waste heat from electronics to external radiator panels. There is no convection in vacuum and no coolant pump to fail, so passive two-phase transport is ideal.
• The trans-Alaska pipeline. 124,000 ammonia thermosiphons in the vertical support members keep the surrounding permafrost frozen, preventing the warm oil pipe from thawing the ground and sinking. The Qinghai–Tibet railway uses the same trick to stabilize its embankments.
• LED lighting and telecom. Sealed outdoor enclosures cannot use fans reliably, so heat pipes and vapor chambers move heat from LED arrays and RF power amplifiers to finned housings for fanless, maintenance-free cooling.
• Solar and process heat. Evacuated-tube solar collectors use water/methanol heat pipes to carry absorbed sunlight to a manifold; sodium heat pipes appear in concentrated-solar receivers and high-temperature industrial furnaces.

Heat pipe vs solid copper vs pumped liquid loop

Property	Heat pipe	Solid copper	Pumped liquid loop
Heat transport	Latent heat (two-phase)	Conduction	Sensible heat (single-phase)
Effective conductivity	10,000–100,000 W/m·K	400 W/m·K	Set by flow rate, not k
Moving parts	None — passive	None	Pump (can fail)
Electrical power	Zero	Zero	Pump power required
Distance over which it stays efficient	Metres (LHP); ΔT ≈ const	Centimetres; ΔT ∝ L	Metres
Orientation	Any (wicked); gravity-aided improves	Any	Any
Failure modes	Dryout, NCG, freeze rupture	None practically	Pump failure, leaks, air lock
Typical use	Electronics, satellites, permafrost	Short heat spreaders, bus bars	Data-center racks, EV batteries

Failure modes — how heat pipes actually die

• Dryout (capillary limit exceeded). The load demands more liquid return than the wick can pump; the evaporator runs dry, its temperature runs away, and the cooled device throttles or overheats. Cured by finer wicks, gravity-favorable mounting, or a loop heat pipe.
• Non-condensable gas (NCG) accumulation. Any gas that does not condense — from chemical incompatibility (water + aluminium → hydrogen) or a slow leak — collects at the cold end of the condenser, progressively blocking condensing area and shrinking the active length. Diagnosed as a creeping rise in source temperature over months or years. Cured by correct material/fluid pairing and rigorous outgassing during manufacture.
• Freeze rupture. A water-charged pipe stored or operated below 0 °C can have its wick or envelope split by expanding ice. Spacecraft and outdoor units use ammonia or methanol to dodge this.
• Boiling-limit burnout. Local heat flux too high — bubbles form inside the wick, block liquid return, and create a local hot spot even though the total power is modest. The reason vapor chambers, not round pipes, are used under bare high-flux dies.
• Wall corrosion / wick fouling. Over years, trace impurities or incompatible materials corrode the wall or clog wick pores, raising flow resistance and lowering capillary performance. Controlled by ultra-clean charging and compatible materials.

Common pitfalls when designing with heat pipes

• Ignoring orientation in the spec. A wicked pipe's capacity drops sharply when the evaporator is above the condenser. Quote the power against gravity, not the favorable best case, or the laptop will overheat only when held at a certain angle.
• Pairing the wrong fluid with the wall. Water with copper: fine. Water with aluminium: generates hydrogen and slowly fails. Always check compatibility tables before choosing materials.
• Bending too tightly. Flattening and bending a pipe to fit a chassis collapses the vapor core and crushes the wick, slashing capacity. Manufacturers publish minimum bend radius and maximum flattening for each diameter — respect them.
• Treating the pipe as the whole solution. The heat pipe only moves heat; it still has to be rejected. A pipe with a starved fin stack or a dead fan saturates and the device cooks. Size the condenser-side heat exchanger to the load.
• Forgetting startup limits. A pipe that meets the capillary limit at steady state can still hit the sonic or viscous limit on a cold start. Validate the full temperature envelope, not just the nominal operating point.