Heat pipe

Grover's suggestion was taken up by NASA, which played a large role in heat pipe development in the 1960s, particularly regarding applications and reliability in space flight.

NASA has tested heat pipes designed for extreme conditions, with some using liquid sodium metal as the working fluid.

The working fluid mass is chosen so that the heat pipe contains both vapor and liquid over the operating temperature range.

The vast majority of heat pipes for room temperature applications use ammonia (213–373 K), alcohol (methanol (283–403 K) or ethanol (273–403 K)), or water (298–573 K) as the working fluid.

For example, water in an aluminum envelope will develop significant amounts of non-condensable gas within hours or days, hindering normal heat pipe operation.

[16] Since heat pipes were rediscovered by George Grover in 1963, extensive life tests have been conducted to determine compatible envelope/fluid pairs, some going on for decades.

[17][18] The most commonly used envelope (and wick)/fluid pairs include:[19] Other pairs include stainless steel envelopes with nitrogen, oxygen, neon, hydrogen, or helium working fluids at temperatures below 100 K, copper/methanol heat pipes for electronics cooling when the heat pipe must operate below the water range, aluminium/ethane heat pipes for spacecraft thermal control in environments when ammonia can freeze, and refractory metal envelope/lithium working fluid for high temperature (above 1,050 °C (1,320 K; 1,920 °F)) applications.

Most manufacturers cannot make a traditional heat pipe smaller than 3 mm in diameter due to material limitations.

[24] In addition, an internal support structure or a series of posts are generally used in a vapor chamber to accommodate clamping pressures sometimes up to 90 psi (620 kPa).

These thin planar heat pipes are finding their way into "height sensitive" applications, such as notebook computers and surface mount circuit board cores.

Variable conductance heat pipes (VCHPs) are used to passively maintain the temperature of the electronics being cooled as power and sink conditions change.

This non-condensable gas is typically argon for standard Variable conductance heat pipes, and helium for thermosyphons.

A vapor trap diode is fabricated in a similar fashion to a variable conductance heat pipe, with a gas reservoir at the end of the condenser.

During fabrication, the heat pipe is charged with the working fluid and a controlled amount of a non-condensable gas (NCG).

The non-condensable gas is dragged along with the flowing vapor, completely blocking the nominal evaporator, and greatly increasing the thermal resistivity of the heat pipe.

However the maximum adverse elevation (evaporator over condenser) is relatively small, on the order of 25 cm long for a typical water heat pipe.

For example, Storch et al. fabricated a 53 mm I.D., 92 m long propane thermosyphon that carried roughly 6 kW of heat.

Micro loop heat pipes have been developed and successfully employed in a wide sphere of applications both on the ground and in space.

[43][47] Heat pipes employ phase change to transfer thermal energy from one point to another by the vaporization and condensation of a working fluid or coolant.

The latent heat of vaporization absorbed by the working fluid reduces the temperature at the hot end of the pipe.

The second figure shows a typical grooved aluminium/ammonia variable conductance heat pipe (VCHP) for spacecraft thermal control.

Relative efficiencies of the evacuated tube system are reduced however, when compared to flat plate collectors because the latter have a larger aperture size and can absorb more solar energy per unit area.

Evacuated tube collectors reduce the need for anti-freeze additives since the vacuum helps slow heat loss.

The liquid at the bottom of the thermosyphon is vaporized by heat absorbed from the ground, cooling the surrounding permafrost and lowering its temperature.

In the Trans-Alaska Pipeline System initially ammonia was used as the working fluid, however this was replaced with carbon dioxide due to blockages.

The first commercial heat pipe product was the "Thermal Magic Cooking Pin" developed by Energy Conversion Systems, Inc. and first sold in 1966.

The high effective conductivity of the heat pipe reduces the cooking time for large pieces of meat by one-half.

The device consists of a battery of multi-row finned heat pipe tubes located within both the supply and exhaust air streams.

[citation needed] Grover and his colleagues were working on cooling systems for nuclear power cells for space craft, where extreme thermal conditions are encountered.

In case that one heat pipe breaks, only a small amount of liquid is released which is critical for certain industrial processes such as aluminium casting.

A laptop computer heat pipe system
Longitudinal cross-section of a heat pipe. It is closed at both ends. The 'wick' coats the inside surface, while the inner cavity is filled with vapour. The diagram illustrates heat transfer: 1. (left end of the pipe) working fluid evaporates to vapour absorbing thermal energy; 2. vapour migrates along cavity to lower temperature end; 3. vapour condenses back to fluid and is absorbed by the wick, releasing thermal energy; 4. working fluid flows back to the lower temperature end.
Diagram showing components and mechanism for a heat pipe containing a wick
A worker in high visibility clothing and a hard hat examines a long line of pipes about four times his height sticking out of rocky ground.
Heat pipes keep ground frozen and inhibit water transfer into the open pit during mining activities at Ekati Diamond Mine
This 100 mm by 100 mm by 10 mm high thin flat heat pipe (heat spreader) animation was created using high resolution CFD analysis and shows temperature contoured flow trajectories, predicted using a CFD analysis package
This 120 mm diameter vapor chamber (heat spreader) heat sink design thermal animation was created using high-resolution CFD analysis and shows temperature contoured heat sink surface and fluid flow trajectories predicted using a CFD analysis package
Cross section of a heat pipe for cooling the CPU of a laptop computer. Ruler scale is in millimetres
Cut-away view of a 500 μm thick flat heat pipe with a thin planar capillary (aqua coloured)
Thin flat heat pipe (heat spreader) with remote heat sink and fan
Heat pipes on spacecraft typically use a grooved aluminium extrusion as the envelope.
Typical grooved aluminium-ammonia VCHP for spacecraft thermal control, with the evaporator section on the bottom, and the non-condensable gas reservoir just below the valve [ 33 ]
A heat sink (aluminium) with heat pipes (copper)
Typical heat pipe configuration within a consumer laptop. The heat pipes conduct waste heat away from the CPU, GPU and voltage regulators, transferring it to a heatsink coupled with a cooling fan that acts as a fluid-to-fluid heat exchanger.
Alaska pipeline support legs cooled by heat pipe thermosyphons to keep permafrost frozen.