Gravity-Driven Two-Phase Cooling for the 21st Century

LTS Integration

Figure 4. Example of a liquid cold plate evaporator and tubefin condenser LTS.

Loop thermosyphons have actually been in use for many decades in industries like automotive engine cooling (circa 1935), chemical processing plants, and even nuclear reactors. Evaporator and condenser geometry combinations are near infinite, but the most typical configuration utilizes a liquid cold plate evaporator and a tube-fin condenser like the one shown in Figure 4. Some other potential implementations could involve a two-circuit liquid heat exchanger for cooling a liquid loop or a liquid-to-air heat exchanger for cooling air streams. Similarly, the condenser of an LTS could be any type of heat exchanger that allows heat to be removed from the system. This flexibility in evaporator and condenser design is one of the major benefits of utilizing LTS technology.

For more demanding applications, surface area enhancement features, like fins, are possible on the inside of the evaporator to increase the maximum heat flux capability. Traditional passive cooling techniques, like heat pipes, are limited to heat fluxes of less than 50 W/cm2. With an internal fin structure in an LTS evaporator and sufficient height for fluid flow, heat fluxes greater than 100 W/cm2 have been demonstrated. That makes LTS technology one of the highest heat flux capable passive cooling solutions currently available.

Another benefit of utilizing LTS technology takes advantage of the two-phase nature of the working fluid inside of the loop. The phase change process from liquid to vapor occurs along a line of constant temperature. Therefore, an LTS is capable of maintaining numerous heat sources mounted on the same evaporator at around the same temperature. This phenomenon is only possible in a two-phase system, whether it is active or passive. A pumped single-phase liquid loop would require a substantial amount of fluid flow in order to achieve the same effect which results in higher energy consumption, higher pump noise, and increased reliability concerns.


Loop thermosyphons offer a wide range of benefits to system designers including passive operation, high heat flux capability, isothermality, and low cost. As long as a vertical operating orientation can be achieved, an LTS is an optimal cooling solution for a wide range of applications. Increasing component powers and shrinking system sizes will continue to demand the highest of performance from cooling solutions, and system designers will not relent in their pursuit of the lowest cost and most reliable solution that meets their system needs. Thermal management no longer has to be a leash on our technological advancements. The opportunity to expand our system capabilities is out there, and loop thermosyphons provide a viable solution.

This article was written by Devin Pellicone, Lead Engineer Custom Products, Advanced Cooling Technologies, Inc. (Lancaster, PA). For more information, Click Here.