While Thermosiphons and Heat Pipes often look and behave similarly, there are a few key differences between them.
The Main Distinction
While both are passive cooling systems that are based on evaporation and condensation of a working fluid, the distinction between a Heat Pipe and a Thermosiphon is the existence of a wicking structure within Heat Pipes that is absent from Thermosiphons. This wicking structure, usually a sintered powder, axially grooved, wire mesh, or screen wick, creates a capillary pressure that allows working fluid to return to the condenser, in any orientation including against gravity. For a Thermosiphon, the working fluid returns via gravity, meaning the heat source and evaporator need to be located below the condenser unit.
What Are Other Benefits and Drawbacks?
The maximum heat transfer capacity (Qmax) for a Thermosiphon is typically going to be greater than that of a Heat Pipe of an equal diameter and length. The wick structure restricts the amount of vapor space and the potential speed of liquid returning to the evaporator through the wick capillaries. In a Thermosiphon, fluid and heat can move more efficiently since it gravity removes the need for a wick.
Distance from Heat Source:
Because Thermosiphons do not rely on a wicking structure to transfer fluid, the length that a Thermosiphon can transfer heat is much longer than that of a Heat Pipe. At Boyd, Thermosiphons have been created upwards of several meters in length. If the gravity is favorable, Thermosiphon length can be virtually unlimited.
Thermosiphons tend to allow for much tighter temperature control over multiple heat sources when compared to Heat Pipes. Since a Thermosiphon doesn't rely on multiple individual tubes, the vapor pressure stays the same throughout the assembly. This means the temperature is going to be similar across the entire Thermosiphon assembly as heat is pulled from the heat sources.
Fewer Tubes and Lower Profile:
When using a Thermosiphon construction with a remote condenser unit, due to the higher heat transfer capacity, the number of tubes needed between the evaporator and condenser is much smaller than the number used for a similar Heat Pipe Assembly. Thermosiphon tubes also have a lower profile than Heat Pipes for similar heat transferability, meaning that they can potentially block less airflow through the system, leading to more efficient cooling.
Design and Complexity:
Thermosiphons are always custom designed for each specific application, based on a variety of factors. This can result in complex and involved design, planning, and development to bring a concept into a manufactured product. Heat Pipe Assemblies are also customized, but individual Heat Pipes have more off-the-shelf constructions readily available for integrating into an assembly.
Boyd has decades of experience creating both Heat Pipe and Thermosiphon assemblies for a wide variety of applications and industries. To learn more about our Two-Phase Cooling capabilities, visit our website or reach out to our experts.
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