Thermosiphon

Lore & Background

Thermosiphoning occurs naturally across temperature gradients, as seen in wood-fire chimneys or solar chimneys. The circulation can be open-loop or vertical closed-loop, simplifying liquid or gas transfer while avoiding the cost and complexity of a conventional pump. In computing, thermosiphons are used for watercooling internal components, most commonly the processor, where water is the easiest liquid to use. The heated water (which may become vapor) rises to a heat exchanger—typically a radiator with fans—where it cools and condenses, then falls back to repeat the cycle.

In some situations, if the loop is not entirely full of liquid, the system no longer convects and becomes a heat pipe thermosyphon, relying on evaporation and condensation. A thermosiphon is sometimes incorrectly called a 'gravity return heat pipe,' but heat pipes use a wick for capillary action, while thermosiphons rely on gravity. Thermosiphons can only transfer heat upward, making orientation critical, and they can fail if a bubble forms in the loop.

Historically, some early cars and engines used thermosiphon circulation for cooling, but as engine power increased, pumps were added. In permafrost regions, thermosiphons are used to chill the ground beneath buildings and pipelines to prevent thawing. In solar heating, water is heated passively in a collector and stored in a tank above it.

Reader's Guide

The thermosiphon's significance lies in its ability to transfer heat without moving parts, reducing cost and complexity. In computing, it offers a quieter and potentially more compact alternative to traditional watercooling, maintaining processor temperatures 10–20 °C cooler than a heat sink and fan. However, it requires careful mounting to ensure vapor rises and liquid falls, with no bends that trap liquid. The system must be completely airtight; otherwise, water will evaporate over time and circulation will stop. The fan cooling the gas needs cool air to operate. While thermosiphons can cover multiple heat sources, they are sensitive to low coolant levels and can fail if a bubble forms. Their legacy includes use in early automotive cooling, permafrost stabilization, and solar water heating, demonstrating a simple, gravity-driven approach to thermal management that remains relevant in specialized applications.

Did You Know?

• A thermosiphon does not require a mechanical pump; it relies on natural convection.
• In computing, a thermosiphon can maintain processor temperatures 10–20 °C cooler than a traditional heat sink and fan.
• Thermosiphons are used to chill permafrost beneath buildings and pipelines to prevent thawing.