Standard Guide for Thermal Performance Testing of Cryogenic Insulation Systems
|Publication Date:||1 November 2013|
|ICS Code (Cryogenic vessels):||23.020.40|
|ICS Code (Refrigerating technology):||27.200|
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.
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.
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].
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).
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.
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).
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.
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.
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.
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.
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.
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.
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.
2 The boldface numbers in parentheses refer to the list of references at the end of this standard.