An Analysis of the Key Serviceability and Efficiency Conditions of Loop Heat Pipes

Loop heat pipes (LHPs) are two-phase heat transfer devices with capillary pumping of the working fluid inside a sealed, closed loop, including an evaporator and a condenser connected by smooth-walled pipelines for separate movement of vapor and liquid, the diameter of which usually varies from 2 to 8 mm. This design feature enables easy integration of LHPs into densely packed systems. The LHP evaporator is equipped with a fine-pored capillary structure (wick) (pore radius from 1 to 20 μm), which creates high capillary pressure. Loop heat pipes are used in energy-efficient systems for utilizing low-potential heat, heating or cooling remote objects, and for uniform heat distribution over a large surface area of ​​heat sinks. Currently, the most popular area of ​​application for LHPs is electronic cooling systems operating under a wide variety of conditions. Various liquids can be used as working fluids in LHPs, depending on their operating temperature range, chemical compatibility with structural elements, and the required operating characteristics of the LHPs. This paper presents the theoretical foundations and operational conditions for LHPs. It also discusses the issue of selecting a working fluid to enhance the heat transfer capacity of LHPs. Quality criteria are proposed for assessing working fluids based on their efficiency in these devices. The results of a comparison of six different working fluids using these criteria are presented, showing that ammonia is the most effective working fluid in the operating temperature range relevant for electronics, from 40 to 70°C. Based on the p, t diagram of the working cycle of the working fluid, the thermal resistance of the “heat source–LHP–heat sink” system is visualized, as well as the thermal resistance of the main structural elements.