Cold Facts Article on Cryogenic Insulation
The following is an article that appeared in the journal COLD FACTS. It covers forms of insulation used for extreme cold, including vacuum insulation. COLD FACTS is the journal of the Cryogenic Society of America. This society covers all things cryogenic. The article is used with the kind permission of the Cryogenic Society of America. The CSA website is at www.cryogenicsociety.org. CSA has been very supportive of our efforts to bring cryogenic processing into the realm of science. We strongly urge anyone who is interested in cryogenic processing to contact this organization and become a member.
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For further information on vacuum insulation go to http://www.cryofab.com
SURVEY, CRYOGENIC INSULATION: PART 1
by Robert S. Bell, CRYOCO, Inc., firstname.lastname@example.org
As professionals in the field of cryogenics, dealing with the effects of heat transfer is something that we must adapt to every day. No matter what system we work with, heat is always there to challenge our abilities. Every mechanical part, component, instrument, wire and/or structure must address the heat transfer requirements of low temperature systems on which we are working. We must consider all aspects of heat transfer as it relates to the application at hand; conduction (solid and gaseous), convection and radiation. Each must be understood as to its effect on what we wish to accomplish and how best for us to minimize it. There is always a balancing act between the requirements of what we want to accomplish and the constraints we must face: cost, weight, practicality of solution, etc. This has been the concern since the first attempts to liquefy gases.
In the beginning, there was nothing….
The vacuum flask developed by Sir James Dewar should be considered as one of the most important discoveries in the history of cryogenics and was, and still remains, the prime enabling insulating technology in the field of cryogenics today. This simple use of a vacuum is a critical component of nearly all insulation systems and schemes. So important was this development that researchers and scientists even today refer to this vacuum flask as a “dewar.”
Early researchers had produced mists of the common cryogens (oxygen and nitrogen) but the massive heat loads from the surrounding environment never allowed for suitable quantities of liquids to be stored and studied. Without some method of significantly slowing the heat flux from the outside world to the liquid cryogen, the art and science of cryogenics had been stalled. The use of the vacuum has helped to minimize the effects of gas conduction and convection by forming a physical barrier to the outside world and allowing some good common sense engineering to address the effects of the other two heat transport processes, solid conduction and radiation. As the overall techniques of minimizing conduction and its associated thermal mass-that is, the mass of material required to be cooled to near to cryogenic fluid boiling temperature-improved, radiation became the dominant player in a never ending struggle to stifle the flow of heat into the cryogen. So you see, in the beginning of serious research within the field of cryogenics there was the use of vacuum or, if you will, nothing.
So, how are we doing….
Well, in reality not too bad. We will compare the current state of the art in insulation and insulation techniques and then speak of some figures of merit sometimes used to evaluate one insulation scheme in comparison to another. The vacuum is still the primary player in most insulation schemes, though for many storage and transfer aspects of cryogenic technology non-vacuum insulation techniques prove to be more cost effective and simpler to apply. It must be kept in mind, however, that there are several orders of magnitude difference in these two approaches to insulation, and the use of the vacuum is still the best way to go.
Let’s first talk about some relative figures of merit that will allow some general discussion about the relative performance of one insulation technique versus another. The first is Apparent Mean Thermal Conductivity (ka), also known simply as the Apparent Thermal Conductivity. It is a convenient method to compare different insulations over the same temperature regime. Basically, it makes the assumption that the insulation obeys Fourier conduction law and can be thought of as a constant over a set temperature range. The units are usually (energy transferred /unit of time) / ((unit of length) (unit of temperature)). A typical representation would be Watts/(meter – Degree Kelvin). This information is determined experimentally and is listed in any number of cryogenic texts for quite a wide variety of insulations. Using this figure of merit requires care in understanding the temperature range over which the data is given. One should only use this information as long as the system operates within the temperature range given; it is not a substitute for detailed analysis. It is a tool to make comparisons.
The second figure of merit to discuss is the thermal conductivity over strength ratio. This ratio compares the ka of a material to the structural material properties of that same material. One can use any mechanical property desired, but the one most popular is tensile strength. The units would be ((energy transferred /unit of time) X (unit of length)) / ((mass) X (unit of temperature)). Again, typically, units of (Watts X meter)/ (Kilogram) Degree-Kelvin) can be found in any number of references. This figure of merit is important when dealing with structural members at low temperature and trying the match requirements of low mass or high strength within the cryogenic system. With these two figures of merit, many cryogenic system trades can be made with regards to insulation selection and material selection.
General categories of insulations, which would you prefer….
First, and the one we have spoken about to this point, is vacuum insulation. It is really a closed space formed out of either glass, metals and in some cases composite materials that can be evacuated with vacuum pumps. The details of vacuum technology are not within the scope of this article, however it is important to say that the degree of the vacuum within the space has a direct impact on the overall efficiency of the insulation characteristics. Typically, a vacuum on the order of I X 10-4 torr is an excellent place to start. Just for reference, a torr is defined as 1/760 times standard atmospheric pressure. At this vacuum level, the gas molecules act not as a continuous medium but more in a free-molecular flow regime, thus significantly reducing the flow of heat into the space. In all cases, as the inner walls of the vacuum space become very cold, gaseous molecules with a freezing temperature greater than the cryogenic wall temperature will stick to those walls when they strike, and thus continue to lower or at least maintain the low pressure within that space. Many vacuum systems use low emissivity coating, e.g. silver, on the walls of the vacuum vessel to attempt to reduce the radiative heat input into the vessel.
It should be understood that a vacuum is not always required in a cryogenic system, particularly in short-term storage or transfer applications. There are a number of applications where the cost, weight and complexity of a vacuum system would not be required. Foams, fiberglass batting, composite coatings, and in some cases, woods (balsa, pine, maple) work exceptionally well, depending upon the needs and requirements of the system. In fact, many valve and other components in a cryogenic system are insulated this way. It is a matter of need driven by the desired end state of the fluid and/or the length of the storage time requirement.
Within a vacuum, however, a number of solid materials have been introduced to reduce the radiation heat transfer within the system. These include, but are not limited to, expanded perlite, silica aerogel, diatomaceous earth, lampblack, glass and phenolic microspheres, and many others. Between the room temperature and liquid nitrogen temperature in a vacuum, radiation is the dominant heat transfer mechanism. The introduction of these materials and the subsequent removal of the gases in the vacuum space allow the material to shield a significant portion of the radiation between the inner and outer walls of the vacuum space. This approach is relatively inexpensive and has demonstrated excellent results. It is currently the state of the art with commercially available cryogenic storage tanks and cryogenic transfer stations.
There has developed the need, however, to improve the performance of the cryogenic insulation system above the state-of-the art commercial units. These needs were driven by low temperature physics research projects, civil and military aerospace requirements and medical research. A number of techniques and approaches have been developed over the years. These include multi-layer insulation blankets, also known as super insulation, and vapor cooled shields, which use the ever-present boil-off vapor of a cryogenic fluid to establish a series of temperature zones within a vacuum space. This fundamentally makes use of the fact that the primary driving parameter within the radiation transfer process is the difference of the outer temperature to the fourth power and lower temperature to the fourth power. The closer these two temperatures are to one another, the smaller the difference, therefore the smaller the amount of radiant heat transfer. A long-term version of this scheme is the use of a guard fluid, basically as a sacrificial fluid used to protect a more sensitive fluid or maintain a low temperature system in a long-term stable condition. Finally, mechanical refrigerators have been used for a number of years to produce and insulate low temperature systems. These do require constant power and maintenance. If they do stop working, care must be made to ensure that they are thermally disconnected from the cryogenic system or they will become a thermal short into the system severely degrading the system. By combining some of these systems together we get what is sometimes referred to as hybrid insulation systems.
All of these descriptions are, of course, terribly oversimplified but should give you an idea of the most general categories of cryogenic insulations.
What is the point….
The key to being a good cryogenic engineer or researcher is being a good system engineer and architect, to understand what a system is supposed to do and how the insulation is to perform within that system.
This means that you must know:
1) What cryogenic fluid is to be used and what are its physical properties?
2) How is that cryogenic system to be used?
3. What physical state is required while the cryogen is used?
So what is new….
There follow a number of submissions by fellow members of the CSA about their recent work in the field of cryogenic insulation. Some are working with microspheres, others with multilayer insulation blankets, and other insulating materials. Within the CSA itself no fewer than 10 organizations claim to deal with insulation and there are any number that deal with fabrication, transfer systems, cryocoolers, heat exchangers, etc. Some ideas have been published, some are more guarded; one thing is for sure, as long as the field of cryogenics is required, the industry will continue to work at controlling the heat that we deal with every day.
News on insulation from other corporate sustaining companies
CAD Cut manufactures Multi-layer Insulation Blankets to customer specification. They stock many of the materials used in blanket construction, including metallized mylar in a number of thicknesses and a variety of spacer materials. Utilizing their CAD capability, they can assist in developing blanket shapes by producing flat patterns from measurements, drawings, templates or physical objects. They can efficiently manufacture blankets up to 96″ wide and 520″ long. Class 100,000 clean room processing is available. CAD Cut has been providing specialty fabric cutting and manufacturing services for over 10 years.
From individual member Steve Bates, Thoughtventions Unlimited LLC, email@example.com: “Radiant heat transfer dominates heat transfer at low temperatures in vacuum without solid contact. At cryogenic temperatures a 10K blackbody radiates 5.67 x 10-4 W/M2, peaking at 10 nun, with most of the power at longer wavelengths. Cryogenic devices rely on low emissivity radiation shields to minimize radiant heating of the cryogenically cooled structure.
“The primary recent advance in radiation shielding has been the ability to provide high quality low emissivity, (e) coatings and surfaces. Gold is used most widely because of its inertness and a nominal e of 2% (1 mm) and 1% (8 mm). Cheaper aluminum is often used, where multiple parallel surfaces at 3%/surface give low total emissivity. Radiation from holes (windows) can dominate the overall heat transfer.
,,At low temperatures and long wavelengths adequate surface finishes are relatively easy to obtain. Coatings are relatively thin (inexpensive), but require polished surfaces. Practical emissivity are very sensitive to surface condition, including polish, surface strain, and, most of all, contamination. Anomalous skin effects are important to cryogenics, where the electron mean free path becomes large compared with the depth of ‘ penetration of the electromagnetic wave into the metal. Reflectivity quoted are for normal reflection, whereas reflectivity can greatly decrease at high angles. Metal purity is also important in determining surface emissivity. Low level impurities may have affected measurements over the past 50 years when impurity detection and elimination were far less developed. Potential radiation shield advances include higher purity, better surface control, high accuracy measurements, and the use of even lower emissivity materials, such as silver.”
Technology Applications, Inc., a CSA corporate sustaining member, is currently working on a two-year NASA SBIR (Small Business Innovation Research) Phase 11 program to develop and demonstrate microsphere insulation systems to meet NASA and commercial requirements. Insulation applications include cryogen transfer and storage, space vehicles, aircraft, scientific freezers and ovens, and donor organ medical transport containers. While microspheres have been recognized as a legitimate insulation material for decades, actual implementation has not been pursued.
Microsphere insulation, which consists of hollow glass bubbles, combines in a single material the desirable properties that other insulations only have individually. The bulk microsphere material has a crush strength of 250 psi, a density of only 4 lb/ft3, is noncombustible, inexpensive, and available in large quantities. Initial testing during 2001 indicates that the thermal performance is competitive with other bulk fill materials. Comprehensive thermal conductivity measurements of the microsphere bulk material will be performed this year at the Cryogenics Test Laboratory located at NASA!s Kennedy Space Center, a CSA corpora.te sustaining member.
Microsphere Insulation Panels (MIP) are configurable to virtually any shape and are ideal for field retrofit installations; i.e., replacement of deteriorated foam insulation. Since the microspheres are load bearing, a panel can be created that is filled with microspheres and enveloped with a lightweight rigid or flexible vacuum-barrier material. MIP can be used to insulate areas that have complicated geometry, require intermittent access such as around instrumentation and piping components, or for applications that require formability and minimal weight. Microsphere insulation is also a superior replacement for perlite since it does not experience compaction and excessive water absorption.
The MIP development program involves several key activities to demonstrate high-efficiency thermal insulation systems. A full-scale 60-foot long MIP-insulated cryogenic transfer line will be installed at the Cryogenics Test Laboratory pipeline facility for thermal performance testing. Prototype configurations will be used in actual operating conditions to prove that MIP is a viable insulation technology with substantial improvements over conventional insulations in the areas
of weight, performance, safety, and durability. TAI has a patent pending for microsphere insulation systems. Contact Mark Allen, Technology Applications, Inc., 303/442-2262 ext. 120.
Thermal Conductivity of High Performance Polyimide Foams
by Martha Williams and James Fesmire,
NASA Kennedy Space Center, Erik Weiser,
NASA Langley Research Center, and Stan
Augustynowicz, Dynacs Inc. at Kennedy Space Center
A new class of material is now being developed for cryogenic thermal insulation systems for the next generation reusable launch vehicles, commercial and defense marine ships, commercial aircraft, and numerous industrial applications. Recent advancements in high temperature polymeric materials at the NASA Langley Research Center have led to the development of new polyimide foam systems with attractive properties for applications in thermal and acoustic insulations To understand the performance parameters of these new foam systems at low temperatures and pressures, experimental studies are being conducted at the NASA Kennedy Space Center Cryogenics Test Laboratory. Liquid nitrogen cryostats developed by the Cryogenics Test Laboratory are being used to perform detailed thermal characterizations of these novel materials under full range cryogenic-vacuum conditions. Although common polymeric foams such as polyurethane have outstanding properties, they have been limited in their applications by temperature and poor fire resistance, and also their susceptibility to thermal cycling and UV light exposure.2 These high performance new foam systems of cyclic imide polymers have excellent structural integrity, fire resistance3,4, thermal aging resistance, and thermal cycling properties.
In general, thermal conductivity of foamed systems is the lowest of any solid materials, and is determined by the gaseous conduction within the pores, the conduction via the solid structure of the foam, convection through the cells and by radiative heat transfer.5,6 This research allows for all of these parameters to be studied, including density, surface area, and open or closed cell content effects. In closed cell foams the heat transfer coefficient in the cell will change as the blowing agent is replaced by air with time, and in open cell foams the overall thermal conductivity of the system will increase because of the open transfer of air into the cells through convection. The thermal performance of the material depends strongly on the vacuum pressure level of the material’s environment. Optimum material properties for one vacuum level are different from those of another vacuum level, and so on, for all eight decades of vacuum pressure from no vacuum to high vacuum. This vacuum level dependence coupled with the low temperature effects makes essential the need for experimental testing under actual use conditions. I
Over 100 cryogenic tests of a number of different materials have been performed to establish a baseline of research information. This pioneering work involves never before investigated parameters for these foam systems and has led to important technology development for the NASA Space Launch Initiative / 2nd Generation Launch Vehicles Program. The research and development is now being extended for the future utilization of novel polyimide composite materials in both aerospace launch systems and a number of industrial cryogenic equipment applications.
1. Weiser, E.S.; Johnson, T.F.; St. Clair, T.L.; Echigo, Y.; Kaneshiro, H.; Grimsley, B. High Temperature Polyimide Foams for Aerospace Vehicles, Journal of High Performance Polymers, March 2000, Vol. 12,1,1-12.
2. Cellular materials Encyclopedia of Polymer Science and Engineering, New York: Wiley l985, Vol. 3,1-59.
3.Williams, Martha K., et al., High Performance Polyimide Foams, G. L. Nelson and C. A. Wilkie, eds., Fire and Polymers: Materials and Solutions for Hazard Prevention, ACS Symp6sium Series 797, American Chemical Society/Oxford Press, 2001, 49-62.
4. Williams, M. K., Nelson, G.L. Brenner, J.R., Weiser, E.S., and St.Clair, T.L., Cell Surface Area and Foam Flammability, Proceedings of Recent Advances in Flame Retardancy of Polymeric Materials, May, 2001.
5. Gibson, L. J. and Ashby M. F.; Cellular Solids – Structures and Properties, 2nd Edition.; Cambridge University Press, New York, NY, 1997.
6. Barron, R.F.; Cryogenic Heat Transfer, Series in Chemical and Mechanical Engineering, Taylor and Frances, Philadelphia, PA, 1999, 24-28.
Lydall Corporation’s cryogenic insulations, trade named Cryotherm™, are used as a separating media between layers of foil or aluminized polyester film to form a multi-layered “superinsulation” for cryogenic tanks and vacuum insulated pipe (VIP). The tanks, which can range in size from the large DOT over the road pressurized vessels to portable oxygen tanks, serve as storage vessels for super-cold liquefied gases. These can be nitrogen, hydrogen, helium, argon, carbon dioxide, and the previously mentioned oxygen. The SI holds these liquids at low temperatures for prolonged periods.
The key feature Lydall products offer is that based on their formulations, which are 100’/,, microfiberglass, there is no danger of binder outgassing which equates to improved long-term performance of the thermal system. These products are also certified to meet the DOT oxygen compatibility requirements. A proven performer, CryoTherm has been in use for 20 years in tanker applications without report of slumping or sagging.
Lydall also offers a product called CRS Wrap, which is the CryoTherm collated with foil or double aluminized Mylar in roll form. This is available in a single or multiple layer roll and is manufactured to custom widths. This is a value added product and designed to increase wrap speeds and efficiencies.
These Multi-Layer designs are typically used in a hard vacuum atmosphere and provide a far superior performance to the perlite insulation systems in a much thinner and lighter weight design which is very important in over-the-road tanker designs because they can carry far more payload, longer distances, with less loss due to evaporation.
Technical data is available on Lydall’s three cryogenic grades which are manufactured by the wet laid fiber forming process. The products can be offered in a variety of widths ranging from 2″ – 100″. Standard diameters offered are 15″ and 18″.
Lydall has expanded their product line to include Cryo-Liteo, a high efficiency thermal insulation blanket manufactured by Johns Manville. Lydall has been given the exclusive global distributorship for this revolutionary product. Cryo-Lite is achieving worldwide acceptance in many tank applications formerly addressed by perlite insulation. This material would typically be used in soft or no vacuum applications. It is preferred over the perlite insulation because it does not slump, settle or compact which would leave voids in the system causing severe thermal leaks and loss of performance as well as loss of payload due to increased evaporation.
For technical literature and details, contact Bill Luerman, Market Manager, Cryogenic Insulation, firstname.lastname@example.org, www.lydall.com.
Aerogel Beads as Cryogenic Thermal Insulation System
by J. Fesmire, NASA Kennedy Space Center, S. Augustynowicz, Dynacs Inc. at Kennedy Space Center, and S. Rouanet, Cabot Corporation
Studies of thermal insulation systems are a key technology focus area of the Cryogenics Test Laboratory at the NASA Kennedy Space Center. The development of cost effective, robust cryogenic insulation systems that operate at soft vacuum level is a primary target from the energy and economics point of view.1 This applied research and development work includes the test, evaluation, characterization, and application of silica aerogel beads produced by Cabot Corporation. The basic aerogel bead material has numerous possibilities for thermal insulation uses in cryogenic and other higher performance insulation needs in industry. Evaluation activities include novel composite constructions and larger-scale applications such as cold boxes. The material has been proposed for insulating cryogenic umbilical connections for new commercial launch platforms, retrofit perlite-insulated storage dewars, and insulation of a miles-long cryogen transfer line. Investigation of the use of aerogel beads as thermal insulation for cryogenic applications is being conducted. Over 100 liquid nitrogen boil-off tests of the aerogel products using research cryostats have been performed. The thermal performance data obtained are being used in the preliminary development of future space travel and space launch applications. Other characterization information such as evacuation, outgassing, and ease-of-use is also being obtained.
Aerogel beads produced by Cabot Corporation have a bulk density of approximately 80 kilograms per cubic meter (kg/m3) and a mean particle diameter of I millimeter (mm). Production of the gel beads: 25.4mm, 94 kg/M3 (carbon blackR300);perlite powder: 25.4mm, 115 kg/M3 (50 x 50 mesh); and multilayer insulation (MLI): 21.3 mm, 92 kg/M3 (60 layers).
The values for density and thickness correspond to the installed condition. The MLI specimens were composed of aluminum foil and fiberglass paper, as are typically used for highly evacuated cryogenic insulation systems.
Results showed the performance of the aerogel beads was significantly better than the conventional materials in both soft-vacuum (1 to 10 torr) and no-vacuum (760 torr ranges) while the opacified aerogel beads performed better than perlite powder under high-vacuum conditions.
Tests of the Cabot aerogel beads show this new product offers advantages compared to the more conventional thermal insulation products currently available for cryogenic applications.
Some key advantages of the aerogel beads are free flowing, easily filling small cavities; minimal dusting; nonsettling, does not compact; no preconditioning needed; easily molded or formed using binders.
Many of the above advantages are due to Cabot’s unique process of this scale of aerogel bead production. In summary, a silica hydrogel is sylilated in water where the surface hydroxyl groups of the hydrogel are replaced by trimethyl-silil groups rendering the surface hydrophobic. The water is displaced by an organic solvent immiscible with water, the solvent exchange being very fast as it is not based on washing but on immiscible displacement. The gel is then dried in an oven and the solvent is recovered.
Properties of the aerogel beads can be summarized as follows: Nominal diameter, lmm – with a very narrow bead size distribution due to a patented spraying system; bead density, 140 kg/M3; bulk density, 81 kg/M3; surface area, 650 M2/g; pore volume, 3.17 CM3/g; outgassing, less than 1% total mass loss; minimum ignition temperature, 400’C.
Although the beads are treated to remain hydrophobic, a hydrophilic (untreated) product is available for oxygen service. Cabot’s unique manufacturing process, utilizing ambient pressure drying, makes highly efficient aerogel beads a cost effective solution for many insulation problems. Cabot’s new plant, coming online by the summer of 2002, will contribute significantly to the maturation of the insulation industry.
1. Fesmire, J.E., Augustynowicz, S.D. and Rouanet, S., “Aerogel Beads as Cryogenic Thermal Insulation System,” Cryogenic Engineering Conference, Madison, WI, July 2001
2. Fesmire, J.E., Augustynowicz, S.D., “Insulation Testing Using Cryostat Apparatus With Sleeve,” Advances in Cryogenic Engineering, Vol. 45, Kluwer Academic/Plenum Publishers, New York, 2000, pp. 1683-1690.
3. Adams’ L. “Thermal Conductivity of Evacuated Perlite,” in Cryogenic Technology Journal of the Cryogenic Society of America, Vol. 1, No. 6, 1965, pp. 249-251
4. Kaganer, M.G., “Thermal Insulation in Cryogenic Engineering,” in Israel Program for Scientific Translations, Inc., IPST Press, Jerusalem, 1969, pp. 114116.
Editor’s note: For a complete version of the papers presented here, with figures and illustrations, visit our website, www.cryogenicsociety.org.
To go back to our cryoprocessor page click here.
For further information on vacuum insulation go to http://www.cryofab.com