Loop Heat Pipe - Limitations of Heat Pipes

Limitations of Heat Pipes

Heat pipes are excellent heat transfer devices but their sphere of application is mainly confined to transferring relatively small heat loads over relatively short distances when the evaporator and condenser are at same horizontal level. This limitation on the part of the heat pipes is mainly related to the major pressure losses associated with the liquid flow through the porous structure, present along the entire length of the heat pipe and viscous interaction between the vapor and liquid phases, also called entrainment losses. For the applications involving transfer of large heat loads over long distances, the thermal performance of the heat pipes is badly affected by increase in these losses. For the same reason conventional heat pipes are very sensitive to the change in orientation in gravitational field. For the unfavorable slopes in evaporator-above-condenser configuration, the pressure losses due to the mass forces in gravity field adds to the total pressure losses and further affect the efficiency of the heat transfer process.

As a result of these limitations, different solutions involving structural modifications to the conventional heat pipe have been proposed. Some of these modifications incorporate arterial tubes with considerably low hydraulic resistance for liquid return to the heat source (arterial heat pipes), while others provide spatial separation of the vapor and liquid phases of the working fluid at the transportation section (separated line heat pipes).

Though these new forms of heat pipes are able to transfer significant heat flows and can increase heat transport length, they remain very sensitive to spatial orientation relative to gravity. To extend functional possibilities of two-phase systems towards applications involving otherwise inoperable slopes in gravity, the advantages provided by the spatial separation of the transportation line and the usage of non-capillary arteries are combined in the loop scheme. This scheme allows heat pipes to be created with higher heat transfer characteristics while maintaining normal operation in any directional orientation. The loop scheme forms the basis of the physical concept of Two-Phase Loops (TPLs).