While Thermosiphons and Heat Pipes often look and behave similarly, there are a few key differences between them.
In our previous blogs, we've gone overWhat Are Thermosiphons?and the variousThermosiphon Configurations and Applications. Here we'll be answering one of the most commonly asked questions regarding Two-Phase Cooling: what is the difference between a Thermosiphon and a Heat Pipe?
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.
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.
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.
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