James E. Fesmire
Cryogenics Test Laboratory
NASA Kennedy Space Center
james.e.fesmire@nasa.gov
Introduction
In today’s world, the use of cryogenics and low-temperature refrigeration is taking a more and more significant role. From the food industry, transportation, energy, and medical applications to the Space Shuttle, cryogenic liquids must be stored, handled, and transferred from one point to another. To minimize heat leaks into storage tanks and transfer lines, high-performance materials are needed to provide high levels of thermal isolation. Complete knowledge of thermal insulation is a key part of enabling the development of efficient, low-maintenance cryogenic systems. The need for insulation is never a direct one. What is important is to save money on the energy bill or to be able to effectively control a system. Thermal insulation systems therefore provide energy conservation and allow system control for process systems.
Cryogenics is an energy intensive field and insulation is needed for economic effectiveness. The term “superinsulation” has a number of different meanings to people in different technical areas. To the cryogenic engineer superinsulation typically means many layers of alternating reflective films and low-conductivity spacers, or multilayer insulation (MLI). Vacuum insulation panels for appliances are sometimes referred to as superinsulation. House construction using straw bales is also called superinsulation. A fitting analogy between house superinsulation and cryogenic superinsulation is given below:
Superinsulation is more than simply piling up large amounts of insulation in the ceiling. It is an integrated system of building practices and components that achieves very low energy use by careful design, selection and installation of all of the elements that go into a home: walls, ceilings, vapor barriers, floors, windows, ventilation, and heating and cooling systems. Close attention to details and careful workmanship are required during construction to ensure that the installed insulation performs up to its full potential. [The Kentucky Energy Saving Home, Kentucky Division of Energy, 11/23/2003]
The point here is to emphasis the importance of a well-designed and properly executed system of thermal insulation. The extreme environmental conditions imposed by cryogenic systems lead to even greater and more complex technological problems that must be solved.
Materials
The overall efficiency of a cryogenic thermal insulation system can be summarized by the following four factors: 1) thermal conductivity, 2) vacuum level, 3) density or weight, and 4) cost of labor and materials. Materials typically come in three basic forms: bulk fill, foam, or multilayer. The vacuum level, or cold vacuum pressure (CVP), is the major cost driver for the design, fabrication, and maintenance of most systems. After the actual operating conditions are considered, an analysis of the total heat leak of the mechanical system is needed to determine the insulation requirements. Often only a common-sense thermal review of the system is needed to ascertain which level of insulation material should be selected. The performance level will dictate the insulation materials and mechanical support structures or joining devices to be used.
An insulation material’s performance under a large temperature difference is given in terms of milliwatt per meter-kelvin (mW/m-K) and is referred to as the apparent thermal conductivity or k-value. To compare k-values for different materials one must understand the warm and cold boundary temperatures, the vacuum level, the residual gas composition, and the installed thickness. The designer has a very wide range of k-values with which to work: as low as 0.03 mW/m-K for perforated MLI blankets up to approximately 40 mW/m-K for cellular glass. As in all good designs, the performance must justify the cost. The performance of the total thermal insulation system as it is actually put to use is defined as the overall k-value for actual field installation or koafi.
Testing
Several test methods are usually needed to adequately test and evaluate the overall performance of an insulation system. Standardized material test methods can be employed for basic thermal, mechanical, and compatibility properties. Cryostat test methods provide the apparent thermal conductivity values for the insulation systems. Prototype testing is then needed to determine the actual performance for a specific mechanical system. The use of MLI systems illustrates the need for this three step testing process. The k-value for an MLI system under ideal laboratory conditions may be around 0.05 mW/m-K while the koafi can easily be 10 times worse.
Applications
Applications of cryogenic insulation systems can be divided into three main categories according to the vacuum environments in which they operate. These three categories of cold vacuum pressure (CVP) are listed as follows: below 0.0001 torr or high vacuum (HV), from about 1 to 10 torr or soft vacuum (SV), and about 760 torr or no vacuum (NV). Materials used in high vacuum systems include, for example, MLI, micro-fiberglass, fine perlite, LCI, vacuum panels, and aerogels. Materials used in the newer soft vacuum systems include aerogels, LCI, and vacuum panels. Materials for no vacuum applications include foams, cellular glass, perlite, aerogels, and many others.
A number of new materials are now commercially available for cryogenic thermal insulation application. These new materials include aerogel blankets by Aspen Aerogels (Pyrogel® and Spaceloft®), aerogel beads by Cabot (Nanogel®), and polyimide foams by Sordal (SOLREX®) and Inspec Foams (SOLIMIDE®). Other materials and composites under development are nearing the commercialization phase.
Technologies and markets forecast for rapid expansion into the 21st century, such as superconducting power distribution and hydrogen-based transportation, will require more efficient approaches to energy management for a wide variety of low-temperature applications. Cryogenic insulation technology is expected to be a foundational support to these developments.
