What Is a Heat Pipe?

A closeup of the Cooler Master Vortex Plus heatsink and fan, showing four heat pipes on the side

Photo from Amazon

A heat pipe is a passive, two-phase heat-transfer device that relocates thermal energy through perpetual cycles of vaporization and condensation. Think of it like the radiator in your car.

A heat pipe incorporates a hollow casing/envelope (e.g. a pipe) made of a thermally-conductive material (e.g. copper, aluminum), a working fluid (i.e. a liquid that can effectively absorb and transmit energy), and a wick structure/lining together in a completely closed/sealed system.

Heat pipes are used for HVAC systems, aerospace applications (e.g. thermal control for spacecraft), and – most commonly – cooling down electronic hot spots. Heat pipes can be made small for individual components (e.g. CPU, GPU) and/or personal devices (e.g. smartphones/tablets, laptops, computers), or large enough to accommodate full-sized enclosures (e.g. data, network, or server racks/enclosures).

How Does a Heat Pipe Work?

The concept behind a heat pipe is similar to that of an automotive radiator or a computer liquid cooling system, but with greater advantages. Heat pipe technology operates by harnessing the mechanics (i.e. physics) of:

  • Thermal conductivity
  • Phase transition
  • Convection
  • Capillary action

The one end of the heat pipe that maintains contact with a high-temperature source (e.g. CPU) is known as the evaporator section. As the evaporator section starts to receive sufficient heat input (thermal conductivity), the local working fluid contained in the wick structure lining the casing is then vaporized from a liquid to a gaseous state (phase transition). The hot gas fills the hollow cavity inside the heat pipe.

As air pressure builds up inside the cavity of the evaporator section, it starts to drive the vapor – carrying latent heat – towards the colder end of the heat pipe (convection). This cold end is known as the condenser section. Vapor in the condenser section cools to the point where it condenses back into a liquid state (phase transition), releasing the latent heat that was absorbed by the vaporization process. The latent heat transfers to the casing (thermal conductivity) where it can be easily removed away from the system (e.g. with a fan and/or heat sink).

The cooled working fluid is soaked up by the wick structure and distributed back towards the evaporator section (capillary action). Once the fluid reaches the evaporator section, it becomes exposed to the heat input, which continues the cycle again.

To visualize the inside of a heat pipe in action, imagine these processes working smoothly in a cycle:

  • Gas flowing through the hollow cavity from hot to cold sections
  • Liquid moving through the wick structure from cold to hot sections

Heat pipes are only able to relocate heat when the temperature gradient falls within the system’s operating range – gases won’t condense when temperatures exceed the element’s condensation point, liquids won’t vaporize when temperatures fall short of the element’s vaporization point. But given the variety of effective materials and working fluids available, manufacturers are able to fine-tune the design of heat pipes and guarantee performance.

Advantages and Benefits of Heat Pipes

Versus conventional methods of electronic cooling, heat pipes offer significant benefits (with few limitations):

  • Passive cooling: Heat pipes don’t need a manual switch or electricity in order to function. All that is required is a temperature difference between the evaporator and condenser sections.
  • No maintenance: Heat pipes are completely closed/sealed systems with zero mechanical/moving parts.
  • Flexible design: Heat pipes can be made with a thickness/diameter as thin as 3 mm, be formed in u-shapes tight enough to coil around the edge of a penny, and work in any direction/orientation (i.e. not dependent on gravity). These flexible design aspects allow heat pipes to meet specific shapes and/or requirements.
  • High conductivity: Heat pipes are made with materials able to handle temperatures up to and over 1000 degrees C. The selection of casing materials, working fluids, and wick structures let designers fine-tune operating temperature ranges.
  • Value: Heat pipes tend to be smaller, lighter, more effective, and more affordable to produce than comparable types of cooling systems.