Understanding Cryogenic Heat Pipes

Achieving efficient heat transfer at extremely low temperatures is both a challenge and a necessity for many high-tech applications from space exploration to medical technologies. Cyrogenic heat pipes play a crucial role in maintaining optimal thermal conditions in these challenging environments.

In this blog, we explore the principles, applications, and design considerations of cryogenic heat pipes.

What Are Cryogenic Heat Pipes & how do they work?

Cryogenic heat pipes are specialized passive thermal management devices designed to transfer heat efficiently at low temperatures, typically below -150°C (-238°F). These heat pipes, like room temperature heat pipes, leverage the principles of phase change and capillary action to move heat from one area to another with minimal temperature gradient.

Cryogenic heat pipes rely on a working fluid and a wick structure. The working fluid absorbs heat at the evaporator end, vaporizes, and then travels as vapor to the condenser end where it condenses back into liquid and releases the heat . The wick structure, which can take various configurations such as grooves, mesh, or sintered metal, facilitates the return of the liquid to the evaporator through capillary action. 

Cryogenic Heat Pipes can take on various forms, such as simple heat pipes, Loop Heat Pipes (LHPs), Pulsating Heat Pipes (PHPs), and when gravity field can be used for liquid return, thermosyphons depending on the thermal needs of the solution.

LHPs are highly effective in transferring heat over long distances and are particularly useful in space applications due to their ability to operate efficiently in microgravity and their robustness in handling high heat loads.

Pulsating Heat Pipes operate by the oscillation of the working fluid in a capillary tube. They are simple in design and can handle high heat fluxes. PHPs are advantageous in applications where flexibility and minimal weight are important features. 

Thermosyphons rely on gravity to return the condensed liquid to the evaporator. They are simpler in construction compared to other heat pipes and are highly efficient in situations where gravity can assist in fluid return. Thermosyphons are commonly used in ground-based cryogenic applications.

Working Fluids for Cryogenic Heat Pipes

Cryogenic working fluids include Helium, Hydrogen, Neon, Oxygen, Methane, Argon, Nitrogen, Ethane, and Propylene. Selection of the optimal working fluid depends on the operational temperature range of the heat pipe. 

Figure 1. plots temperature vs the Merit number for the cryogenic heat pipe working fluids.

The Merit number of a working fluid provides a metric for comparing thermal performance of working fluids in specific temperature ranges. The greater the merit number, the higher the heat transfer capacity.  As it can be seen in Figure 1, and in Table 1, working fluids are available for nearly the entire cryogenic temperature range. (Except for 0-2K, 5-14K, and 44-55K). Merit numbers for cryogenic heat pipes are significantly lower than those for higher temperature working fluids such as ammonia (up to 1.3E10 kg/s³) and water (up to 5E11 kg/s³);  Lower Merit numbers for cryogenic working fluids is largely due to their low surface tension and  latent heat of vaporization.

Design Considerations for Cryogenic Heat Pipes

Designing cryogenic heat pipes requires careful consideration of several factors to ensure reliable and efficient operation under extreme conditions.

Material Selection

The envelope material for cryogenic heat pipes must be chemically compatible with the working fluid and maintaining mechanical integrity at both low and high (ground storage and transportation) temperatures. Material incompatibility can result in the formation of non-condensable gas (NCG) that will inhibit heat pipe performance. The noble gases Helium, Neon, and Argon are compatible with all materials due to their inert properties; however, not all materials are suitable for cryogenic temperatures. Table 1. provides a list of common envelope materials for cryogenic heat pipes. These materials are chosen for their compatibility with the working fluid as well as their corrosion resistance, mechanical strength, and favorable thermal properties in cryogenic conditions. 

Contact ACT for a full list of compatible materials.

Wicks and Reservoirs

The wick structure must provide sufficient capillary action to return the condensed fluid to the evaporator. Cryogenic working fluids have low surface tension and thus the wick design requires careful attention to ensure that liquid is consistently returned to the evaporator across the expected operating temperature range of the heat pipe. The required fluid charge, likewise, must be carefully selected to ensure that the wick functions at the desired operational temperature.

Cryogenic heat pipes often require reservoirs to manage pressure at room temperature or even at higher temperatures that would be the standard storage and transportation temperature (60C). At these conditions, cryogenic working fluids in the heat pipes are in supercritical state, resulting in high pressures. The reservoirs would provide the necessary total volume  to limit pressure buildup within the heat pipe. To prevent heat leakage during operation, these reservoirs can be thermally isolated from the heat pipes. Additionally, loop heat pipes need an extra evaporator to prime the primary wick during start-up.

Key Applications of Cryogenic Heat Pipes

Spacecraft Thermal Management

One of the most significant application of cryogenic heat pipes is in spacecraft thermal management. In the vacuum of space, traditional cooling methods are ineffective, and maintaining optimal temperatures for instruments and electronic components is critical. Cryogenic heat pipes are used in satellites and space probes to dissipate heat efficiently, ensuring that sensitive equipment (sensors, detectors, mirrors, electronics) operates within the target temperature ranges. For instance, in satellites, cryogenic heat pipes can be used to cool infrared sensors that need to operate at very low temperatures to detect faint thermal signals from distant celestial objects. They also help maintain the stability of optical components, preventing thermal expansion that could distort images or data.. Cryogenic heat pipes are also vital in scientific missions, such as those involving space telescopes or planetary exploration probes. These missions often require precise low temperature thermal control to achieve their scientific objectives.

Superconducting Technologies

Cryogenic heat pipes find applications in the cooling of superconducting magnets used in medical imaging devices and particle accelerators. Superconducting magnets are essential components in these systems due to their ability to generate powerful magnetic fields with minimal energy loss. However, to maintain their superconducting state, these magnets must be kept at extremely low temperatures, often close to absolute zero. In these applications, cryogenic heat pipes offer several advantages. They provide efficient and reliable heat transfer, crucial for maintaining the low temperatures needed for superconductivity. Additionally, they have no moving parts, which reduces the risk of mechanical failure and increases the longevity of the cooling system.

Cryogenic Storage

In the storage of liquefied gases such as liquid nitrogen, oxygen, and helium, cryogenic heat pipes play a crucial role in maintaining the extremely low temperatures required to keep these substances in their liquid state. These gases have boiling points far below room temperature, and even slight increases in temperature can cause them to vaporize. Cryogenic heat pipes ensure efficient thermal management, preventing heat buildup and maintaining the necessary low temperatures. For liquid nitrogen, commonly used in cryogenics and various industrial applications, cryogenic heat pipes help maintain its temperature at around -196°C (-320°F)*. This is essential for processes such as food preservation, cryosurgery, and materials testing, where the liquid nitrogen must remain in a stable liquid form.

Find Your Solution

Are you involved in a project that requires precise thermal management at cryogenic temperatures? Contact us to learn more about our range of cryogenic thermal solutions and see how ACT can help you achieve your thermal management goals.