by James Fesmire, senior principal investigator, Cryogenic Test Laboratory, NASA Kennedy Space Center, CSA president, firstname.lastname@example.org
With increasing system control and reliability requirements as well as demands for higher energy efficiencies, thermal insulation in extreme environments is a growing challenge prompting the publication of new technical consensus standards for cryogenic insulation testing and multilayer insulation (MLI) systems . The approach of a layered thermal insulation system provides a means of minimizing the total heat transmission by considering the contribution from each mode of heat transfer:
Qtotal = Qsolid conduction + Qgaseous conduction + Q convection + Qradiation
The operational environment, first and foremost, establishes the relative importance of each of the four heat transfer modes: radiation, gaseous conduction, solid conduction and convection. From the high performance arena of cryogenic equipment, different layered thermal insulation systems such as MLI and Layered Composite Insulation (LCI) have been developed for industrial and commercial applications. In addition to the proven areas in cold-work applications for piping and tanks, a new Layered Composite Extreme (LCX) system has potential for broader industrial use as well as for commercial applications. The LCX technology grew out of solving problems in the insulation of mechanically complex cryogenic systems that must operate in outdoor, humid conditions.
Which thermal insulation system is best?
Thermal insulation provides energy savings, system control and/or safety and reliability. Selection of the “best” insulation system depends on the operational environment, mechanical design and materials. Economic objectives often underscore the technical approach; as such, thermal performance must justify the cost. There are three main questions to answer: 1) vacuum or no vacuum, 2) operational environment (including boundary temperatures and heat flux target) and 3) installation and accessibility (including size, weight and contingencies such as fire).
What are the primary requirements for a thermal insulation system?
The goal is thermal isolation of something located between two different environments. Therefore, a thermal insulation system is first and foremost a system, not a material. Considering the needed thermal isolation as an afterthought or something to be dealt with later in the design process, as is often done, can lead to big problems down the road. The different working fluids for cold work include chilled water, cold air, freon, CO2, LO2, LN2, LNG (or LCH4), LH2, LHe and the cold vent gases for the respective cryogens. Thermal isolation may also be needed for protection of sensitive equipment, goods or living things. Mechanical complexity for below-ambient systems is often the norm and challenges are increased multifold for such things as mechanical/vibration loads, weathering environments and accessibility/maintenance. Thermal insulation systems should also be lightweight and meet a wide range of fire, compatibility, outgassing and other physical and chemical requirements. And while the thermal conductivity of a material is important, it is usually not at the top of the list!
What are the three ranges of thermal performance?
For the full vacuum range, from high vacuum (HV) to soft vacuum (SV) to non-vacuum (NV), there are three categories of layered thermal insulation systems as shown in Figure 1 and summarized as follows according to the respective cold vacuum pressures in Table 1. Designed and installed correctly, MLI systems can provide the ultimate in thermal insulation performance for HV environments. LCI systems can provide the ultimate in thermal performance for SV environments. LCX systems provide excellent, long-life thermal performance for NV environments.
MLI systems are strictly for vacuum environments or evacuated metal jackets. At NV, they are comparable to the best foam insulations in heat leak but will not hold up in ambient (wet) conditions. Long-term vacuum maintenance must be addressed as well as catastrophic loss of vacuum. An MLI system is comprised of alternating layers of reflectors and spacers. Reflector choices include aluminum foil, aluminized Mylar and other metal foils or metalized film products. Spacer choices include microfiberglass paper, polyester net, polyester fabric, silk net and other low thermal conductivity thin sheet or fabric materials. The MLI system will necessarily include attachments, joints, seams, layer density, number of layers, etc., that must all be carefully worked out. Getter pack installation, evacuation and heating processes and many other factors must be understood to render an MLI system that performs its advertised job. The physics-based equation by Glen McIntosh includes three terms respectively addressing radiation, gaseous conduction and solid conduction .
LCI systems are designed for SV. Because the usual HV techniques and materials requirements are not needed, plastic or metal jacketing can be used for more economical systems and simpler engineering approaches . The LCI is distinguished for setting the world record lowest thermal conductivity at ~1 torr air environments (six times better than MLI) and is comparable to MLI systems in HV environments . Unlike MLI systems, the LCI is a three-component system including radiation shield layers, powder layers (aerogel or fumed silica) and carrier layers (non-woven fabric or fiberglass paper). The benchmark thermal performance data for MLI compared to LCI is listed as follows: 0.086 versus 0.091 mW/mK at HV and 10.0 versus 1.6 mW/mK at SV. The aerogel composite blanket (Aspen Aerogel’s Cryogel) performance compared to LCI is given as follows: 4.3 versus 1.6 mW/mK at SV and 11.2 versus 13.4 mW/mK at NV . All data is for the boundary temperatures of approximately 293K and 78K and a typical total thickness of 20 mm.
The LCX technology builds on prior work in the areas of layered thermal insulation systems including LCI, MLI and aerogel blanket development. The focus is on NV systems in the below-ambient environment. Because the effects of vapor drive toward the cold side, preventing moisture accumulation inside is the major challenge of insulating below-ambient temperature equipment in ambient environments . The top three problems can be summarized as follows:
- Moisture (dramatically degrades thermal performance)
- Moisture (leads to corrosion under insulation)
- Moisture (ice bridging and cracking)
Added to these problems are environmental degradation and mechanical damage from personnel/equipment. The LCX system is designed to provide a favorable combination of thermal, mechanical and weathering properties with an integrated/layered approach. Low effective thermal conductivity is achieved by managing all modes of heat transfer by combination of materials and method of installation. Physical resilience against damaging mechanical effects, including compression, flexure, impact, vibration and thermal expansion/contraction, is a key part. Long life is ensured by the hydrophobic properties and compressible barrier layers in combination with moisture draining and venting features of the installed system.
Design basics of LCX
The LCX system works using two main components: a primary insulation blanket layer and a compressible barrier blanket layer (both hydrophobic). The insulation blanket layer is always the first layer (cold inner surface) and may be comprised of an aerogel composite blanket or flexible foam material. The compressible barrier layer is always the second layer. This layer is also an insulating layer, but primarily offers mechanical compliance, compressibility and placement to enable overall good fit-up with optimal closure of seams and gaps. Layer pairs are applied to comprise a stack (per the heat leak design requirements). An overwrap layer is employed as needed for the total system requirements. Appearance and level of permanence are key features of the overwrap layer. In some cases, the overwrap may incorporate an aluminum foil layer for conforming to complex shapes or for close-out around a component. Installation of the LCX system can be field-fit or pre-fabricated (or a combination) per specifications for piping, tanks or flat panels .
The LCX system, like all layered systems, is designed to address the total heat transmission (i.e., all modes of heat transfer). Its structural capability is enhanced by the compliance and compressibility of the two different material layers working together for an easy to work and install system. Without the compressible barrier layer, gaps between thermal insulation layers will occur, which allows additional convection heat transfer to occur and localized areas to harbor water or other contaminants. The compressible barrier layer also provides thermal radiation shielding. Examples of LCX system designs are shown in Figure 2.
System design types and installation methods
The LCX system can be applied to tanks or piping in either vertical or horizontal orientations. The key differences center on the handling of joints, seams and overlaps. In all cases, the system is designed to reduce moisture intrusion and at the same time allow proper venting and draining during cycles in weather or operation. Another LCX design, a narrow strip product, covers small-diameter piping and tubing insulation wrap by spiral-style or cigar-style installation. Removable LCX systems are available for pipe flanges, valves or other components. These flange or valve covers can be quickly and easily removed and replaced to facilitate system maintenance, inspections or modifications. Finally, the LCX system can also be produced in standard-sized sheet product for building construction, for fabrication tiles or for shipping boxes for refrigerated transport of perishable or temperature-sensitive products such as pharmaceuticals, food or electronics.
Thermal, mechanical and environmental properties
Using the Cryostat-100, a guarded LN2 boiloff calorimeter with a one-meter-long cold mass for testing cylindrical test specimens , thermal properties of a variety of different LCX systems were determined. For the ambient pressure test condition, the effective thermal conductivity (ke) for a typical five-layer system was found to be approximately 18.3 mW/mK for the boundary temperatures of 293K and 78K. The corresponding heat flux for the same 37-mm thick system was found to be approximately 81 W/m2 . As the LCX system is tailored to the given design requirements, thermal performance will vary according to the materials selected, their thicknesses and the final arrangement of layers. The usual range of ke is from 12 to 24 mW/mK. Load-displacement mechanical testing of a six-layer LCX test specimen showed that the system can be compressed to more than 50 percent of its thickness, and up to approximately 75 percent, with full elastic recovery when the load is removed. Water absorption tests indicate negligible mass increase even after full immersion in water (the test specimen returned to within 0.1 percent of its initial weight after two hours ambient air drying). Extreme exposure testing (LN2 cold soak followed by water bath) showed no adverse effect and no visible change. Previous cryopumping and cryo-adsorption testing show that any condensed air is safely kept within the nanoporous aerogel in a (non-liquid phase) physisorbed state .
Different LCX systems have been successfully executed for field installations of cryogenic tanks, piping and valve control skid applications . A 2,000 gallon LN2 tank has now been in service for more than two years with numerous thermal cycles. Four valve skids for LNG and LO2 service are scheduled to go into service next year.
Layered thermal insulation systems for high performance cryogenic applications include three distinct categories described by their respective operational pressure environment (or cold vacuum pressure). These categories include MLI for HV, LCI for SV and LCX for NV (ambient pressure). Overcoming the effects of vapor drive toward the cold side (and preventing moisture accumulation inside) is the major challenge of insulating below-ambient temperature equipment. LCX technology for NV applications provides a practical solution for complex systems operating in extreme environments. The LCX system provides a unique and highly favorable combination of mechanical, thermal and environmental properties with its integrated/layered approach. Such conditions are common for aerospace vehicles, launch pad facilities, and propulsion test stands and cryofuels (LNG and LH2) for transportation and power. Additional industry applications include HVAC systems, hot water piping, building construction, refrigerated trucks, cold chain shipping containers and various consumer products.
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