Standard Guide for Thermal Performance Testing of Cryogenic Insulation Systems

Importancia y uso:

5.1 A key aspect in understanding the thermal performance of cryogenic insulation systems is to perform tests under representative and reproducible conditions, simulating the way that the materials are actually put together and used in service. Therefore, a large temperature differential across the insulation and a residual gas environment at some specific pressure are usually required. Added to these requirements are the complexities of thickness measurement at test condition after thermal contraction, verification of surface contact and/or mechanical loading after cooldown, and measurement of high vacuum levels within the material. Accounting for the surface contact resistance can be a particular challenge, especially for rigid materials (32). The imposition of a large differential temperature in generally low density, high surface area materials means that the composition and states of the interstitial species can have drastic changes through the thickness of the system. Even for a single component system such as a sheet of predominately closed-cell foam, the composition of the system will often include air, moisture, and blowing agents at different concentrations and physical states and morphologies throughout the material. The system, as tested under a given set of WBT, CBT, and CVP conditions, includes all of these components (not only the foam material). The CVP can be imposed by design or can vary in response to the change in boundary temperatures as well as the surface effects of the insulation materials. In order for free molecular gas conduction to occur, the mean free path of the gas molecules must be larger than the spacing between the two heat transfer surfaces. The ratio of the mean free path to the distance between surfaces is the Knudsen number (see Guide C740 for further discussion). A Knudsen number greater than 1.0 is termed the molecular flow condition while a Knudsen less than 0.01 is considered a continuum or viscous flow condition. Testing of cryogenic-vacuum insulation systems can cover a number of different intermediate or mixed mode heat transfer conditions.

5.2 Levels of thermal performance can be very high: heat flux values well below 0.5 W/m² are measured. This level of performance could, for example, correspond to a k_e below 0.05 mW/m-K (R-value = 2900 or higher) for the boundary temperatures of 300 K and 77 K and a thickness of 25 mm. At these very low rates of heat transmission, on the order of tens of milliwatts for an average size test apparatus, all details in approach, design, installation, and execution must be carefully considered to obtain a meaningful result. For example, lead wires for temperature sensors can be smaller diameter, longer length, and carefully installed for the lowest possible heat conduction to the cold mass. In the case of boiloff testing, the atmospheric pressure effects, the starting condition of the cryogen, and any vibration forces from surrounding facilities should also be considered. If an absolute test apparatus is to be devised, then the parasitic heat leaks shall be essentially eliminated by the integrated design of the apparatus and test methodology. The higher the level of performance (and usually the higher level of vacuum), the lower the total heat load and thus the parasitic portion shall be near zero. For a comparative apparatus, the parasitic heat leaks must be reduced to a level that is an acceptable fraction of the total heat load to be measured. And most importantly, for the comparative apparatus, the parasitic portion of the heat shall be consistent and repeatable for a given test condition.

5.3 Boiloff Testing—Boiloff testing can be performed with a number of cryogens or refrigerants with normal boiling points below ambient temperature (29). The cold boundary temperature is usually fixed but can be easily adjusted higher by interposing a thermal resistance layer (such as polymer composite or any suitable material) between the cold mass and the specimen. However, the thermal contact resistance shall be fairly well understood and obtaining a specific cold-side temperature can be difficult. Liquid nitrogen (LN₂) is a commonly used cryogen and can be handled and procured with relative ease and economy. Its 77 K boiling point at 1 atmosphere pressure is in a temperature range representative of many applications including liquid oxygen (LO₂), liquid air (LAIR), and liquefied natural gas (LNG). The low level of ullage vapor heating with liquid nitrogen systems means that the vapor correction is minimal or even negligible. Liquid hydrogen (LH₂), with a normal boiling point of 20 K, can be used with the proper additional safety precautions for working with a flammable fluid. Liquid helium (LHE), with a normal boiling point of 4 K, can also be used effectively, but with a significant rise in expense and complexity. The thermal performance, or heat flow rate (W), is a direct relation to the boiloff mass flow rate (g/s) by the heat of vaporization (J/g) of the liquid. Boiloff methods are therefore direct with respect to calculating a k_e or heat flux.

5.4 Electrical Power Testing—In some cases a boiloff method may not be the best option for thermal performance testing. Obtaining a cold boundary temperature below 77 K without additional safety constraints (liquid hydrogen) or unreasonable expense (liquid helium) is often the main reason. The use of electrical power methods provides a wide range of possible approaches without the constraints of a liquid-vapor interface and liquid management. Electrical power apparatus can be designed to use only cryocoolers, cryocoolers in conjunction with cryogens or vapor shields, cryogens to provide the refrigeration to maintain the desired cold boundary temperature, or any combination of these. The key experimental element is the electrical heater system(s), but the key challenge is the temperature sensor calibration at the low temperatures. Temperature sensors are generally silicon diodes or platinum resistance thermometers. These methods are therefore indirect with respect to calculating effective thermal conductivity or heat flux.

5.5 MLI—Multilayer insulation systems are usually evacuated (designed for a vacuum environment). Materials used in MLI systems are highly anisotropic by nature. MLI systems exhibit heat flux values one or two orders of magnitude lower than the best available powder, fiber, or foam insulations under vacuum conditions. The thermal performance of multilayer insulations will vary from specimen to specimen due to differences in the material properties, such as the emittance of the reflective shields, and differences in construction, such as layer density and the way seams or joints are made. MLI systems can vary due to environmental conditioning and the presence of foreign matter such as oxygen or water vapor. MLI systems can vary due to aging, settling, or exposure to excessive mechanical pressures which could wrinkle or otherwise affect the surface texture of the layers. For these reasons, it is imperative that specimen materials be selected carefully to obtain representative specimens. It is recommended that several specimens of any one MLI system be tested with at least three tests performed on each specimen. Further information, including installation methods and typical thermal performance data are given in Guide C740.

5.6 High Performance Insulation Systems—High performance insulation systems, ranging from aerogels at ambient pressure to evacuated powders to MLI under high vacuum conditions, are typical for the more-demanding applications in cryogenic equipment and processes. The requirements of high performance mean low rates of heat energy transfer (in the range of milliwatts) and even more demanding requirements for accurately measuring these small heat leakage rates. Achieving such measurements requires a sound experimental approach and design, specialized vacuum equipment, a well though-out methodology, and careful execution and handling of data.

Note 1: The current lack of Certified Reference Materials (CRMs), or even internal laboratory reference materials, that are characterized under cryogenic-vacuum conditions underscores the need for round robin testing, inter-laboratory studies, and development of robust analytical tools based on these experimental results.

Subcomité:

C16.30

Referida por:

C1130-24

Volúmen:

04.06

Número ICS:

23.020.40 (Cryogenic vessels), 27.200 (Refrigerating technology)

Palabras clave:

boiloff calorimeter; cold boundary temperature; cold vacuum pressure; compressive pressure; cryocooler; cryogenic piping; cryogenic tank; cryogenic temperatures; emittance; evacuated insulations; heat flow; heat flux; heater power measurement; large temperature difference; multilayer insulation; residual gas; steady-state; thermal conductance; thermal conductivity; thermal performance; thermal resistance; thermal transmission; vacuum; warm boundary temperature;