Significance and Use
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 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 ke 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 ke 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 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.
Scope
1.1 This guide provides information for the laboratory measurement of the steady-state thermal transmission properties and heat flux of thermal insulation systems under cryogenic conditions. Thermal insulation systems may be composed of one or more materials that may be homogeneous or non-homogeneous; flat, cylindrical, or spherical; at boundary conditions from near absolute zero or 4 K up to 400 K; and in environments from high vacuum to an ambient pressure of air or residual gas. The testing approaches presented as part of this guide are distinct from, and yet complementary to, other ASTM thermal test methods including C177, C518, and C335. A key aspect of this guide is the notion of an insulation system, not an insulation material. Under the practical use environment of most cryogenic applications even a single-material system can still be a complex insulation system (1-3).2 To determine the inherent thermal properties of insulation materials, the standard test methods as cited in this guide should be consulted.
1.2 The function of most cryogenic thermal insulation systems used in these applications is to maintain large temperature differences thereby providing high levels of thermal insulating performance. The combination of warm and cold boundary temperatures can be any two temperatures in the range of near 0 K to 400 K. Cold boundary temperatures typically range from 4 K to 100 K, but can be much higher such as 300 K. Warm boundary temperatures typically range from 250 K to 400 K, but can be much lower such as 40 K. Large temperature differences up to 300 K are typical. Testing for thermal performance at large temperature differences with one boundary at cryogenic temperature is typical and representative of most applications. Thermal performance as a function of temperature can also be evaluated or calculated in accordance with Practices C1058 or C1045 when sufficient information on the temperature profile and physical modeling are available.
1.3 The range of residual gas pressures for this Guide is from 10-7 torr to 10+3 torr (1.33-5 Pa to 133 kPa) with different purge gases as required. Corresponding to the applications in cryogenic systems, three sub-ranges of vacuum are also defined: High Vacuum (HV) from <10-6 torr to 10-3 torr (1.333-4 Pa to 0.133 Pa) [free molecular regime], Soft Vacuum (SV) from 10-2 torr to 10 torr (from 1.33 Pa to 1,333 Pa) [transition regime], No Vacuum (NV) from 100 torr to 1000 torr (13.3 kPa to 133 kPa) [continuum regime].
1.4 Thermal performance can vary by four orders of magnitude over the entire vacuum pressure range. Effective thermal conductivities can range from 0.010 mW/m-K to 100 mW/m-K. The primary governing factor in thermal performance is the pressure of the test environment. High vacuum insulation systems are often in the range from 0.05 mW/m-K to 2 mW/m-K while non-vacuum systems are typically in the range from 10 mW/m-K to 30 mW/m-K. Soft vacuum systems are generally between these two extremes (4). Of particular demand is the very low thermal conductivity (very high thermal resistance) range in sub-ambient temperature environments. For example, careful delineation of test results in the range of 0.01 mW/m-K to 1 mW/m-K (from R-value 14,400 to R-value 144) is required as a matter of normal engineering applications for many cryogenic insulation systems (5-7). The application of effective thermal conductivity values to multilayer insulation (MLI) systems and other combinations of diverse materials, because they are highly anisotropic and specialized, must be done with due caution and full provision of supporting technical information (8). The use of heat flux (W/m2) is, in general, more suitable for reporting the thermal performance of MLI systems (9-11).
1.5 This guide covers different approaches for thermal performance measurement in sub-ambient temperature environments. The test apparatuses (apparatus) are divided into two categories: boiloff calorimetry and electrical power. Both absolute and comparative apparatuses are included.
1.6 This guide sets forth the general design requirements necessary to construct and operate a satisfactory test apparatus. A wide variety of apparatus constructions, test conditions, and operating conditions are covered. Detailed designs are not given but must be developed within the constraints of the general requirements. Examples of different cryogenic test apparatuses are found in the literature (12). These apparatuses include boiloff types (13-17) as well as electrical types (18-21).
1.7 These testing approaches are applicable to the measurement of a wide variety of specimens, ranging from opaque solids to porous or transparent materials, and a wide range of environmental conditions including measurements conducted at extremes of temperature and with various gases and over a range of pressures. Of particular importance is the ability to test highly anisotropic materials and systems such as multilayer insulation (MLI) systems (22-25). Other test methods are limited in this regard and do not cover the testing of MLI and other layered systems under the extreme cryogenic and vacuum conditions that are typical for these systems.
1.8 In order to ensure the level of precision and accuracy expected, users applying this standard must possess a working knowledge of the requirements of thermal measurements and testing practice and of the practical application of heat transfer theory relating to thermal insulation materials and systems. Detailed operating procedures, including design schematics and electrical drawings, should be available for each apparatus to ensure that tests are in accordance with this Guide. In addition, automated data collecting and handling systems connected to the apparatus must be verified as to their accuracy. Verification can be done by calibration and comparing data sets, which have known results associated with them, using computer models.
1.9 It is impractical to establish all details of design and construction of thermal insulation test equipment and to provide procedures covering all contingencies associated with the measurement of heat flow, extremely delicate thermal balances, high vacuum, temperature measurements, and general testing practices. The user may also find it necessary, when repairing or modifying the apparatus, to become a designer or builder, or both, on whom the demands for fundamental understanding and careful experimental technique are even greater. The test methodologies given here are for practical use and adaptation as well as to enable future development of improved equipment or procedures.
1.10 This guide does not specify all details necessary for the operation of the apparatus. Decisions on sampling, specimen selection, preconditioning, specimen mounting and positioning, the choice of test conditions, and the evaluation of test data shall follow applicable ASTM Test Methods, Guides, Practices or Product Specifications or governmental regulations. If no applicable standard exists, sound engineering judgment that reflects accepted heat transfer principles must be used and documented.
1.11 This guide allows a wide range of apparatus design and design accuracy to be used in order to satisfy the requirements of specific measurement problems. Compliance with a further specified test method should include a report with a discussion of the significant error factors involved as well the uncertainty of each reported variable.
1.12 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only. Either SI or Imperial units may be used in the report, unless otherwise specified.
1.13 Safety precautions including normal handling and usage practices for the cryogen of use. Prior to operation of the apparatus with any potentially hazardous cryogen or fluid, a complete review of the design, construction, and installation of all systems shall be conducted. Safety practices and procedures regarding handling of hazardous fluids have been extensively developed and proven through many years of use. For systems containing hydrogen, particular attention shall be given to ensure the following precautions are addressed: (1) adequate ventilation in the test area, (2) prevention of leaks, (3) elimination of ignition sources, (4) fail safe design, and (5) redundancy provisions for fluid fill and vent lines. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
1.14 Major sections within this standard are arranged as follows:
|
| Section | |
| Scope | 1 | |
| Referenced Documents | 2 | |
| Terminology | 3 | |
| Summary of Test Methods | 4 | |
| Significance and Use | 5 | |
| Apparatus | 6 | |
| Test Specimens and Preparation | 7 | |
| Procedure | 8 | |
| Calculation of Results | 9 | |
| Report | 10 | |
| Keywords | 11 | |
| Annexes |
| |
| Cylindrical Boiloff Calorimeter (Absolute) | Annex A1 | |
| Cylindrical Boiloff Calorimeter (Comparative) | Annex A2 | |
| Flat Plate Boiloff Calorimeter (Absolute) | Annex A3 | |
| Flat Plate Boiloff Calorimeter (Comparative) | Annex A4 | |
| Electrical Power Cryostat Apparatus (Cryogen) | Annex A5 | |
| Electrical Power Cryostat Apparatus (Cryocooler) | Annex A6 | |
| Appendix |
| |
| Rationale | Appendix X1 | |
| References |
| |
