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Layered composite thermal insulation system for nonvacuum cryogenic applications
Cryogenics xxx (2015) xxx–xxx
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Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Layered composite thermal insulation system for nonvacuum cryogenic applications J.E. Fesmire NASA Kennedy Space Center, Cryogenics Test Laboratory, UB-R1, KSC, FL 32899, USA
a r t i c l e
i n f o
Article history: Received 4 July 2015 Received in revised form 9 October 2015 Accepted 15 October 2015 Available online xxxx Keywords: Thermal insulation Weathering Compression Piping Valves Tanks Space launch vehicles
a b s t r a c t A problem common to both space launch applications and cryogenic propulsion test facilities is providing suitable thermal insulation for complex cryogenic piping, tanks, and components that cannot be vacuumjacketed or otherwise be broad-area-covered. To meet such requirements and provide a practical solution to the problem, a layered composite insulation system has been developed for nonvacuum applications and extreme environmental exposure conditions. Layered composite insulation system for extreme conditions (or LCX) is particularly suited for complex piping or tank systems that are difficult or practically impossible to insulate by conventional means. Consisting of several functional layers, the aerogel blanket-based system can be tailored to specific thermal and mechanical performance requirements. The operational principle of the system is layer-pairs working in combination. Each layer pair is comprised of a primary insulation layer and a compressible radiant barrier layer. Vacuum-jacketed piping systems, whether part of the ground equipment or the flight vehicle, typically include numerous terminations, disconnects, umbilical connections, or branches that must be insulated by nonvacuum means. Broad-area insulation systems, such as spray foam or rigid foam panels, are often the lightweight materials of choice for vehicle tanks, but the plumbing elements, feedthroughs, appurtenances, and structural supports all create ‘‘hot spot” areas that are not readily insulated by similar means. Finally, the design layouts of valve control skids used for launch pads and test stands can be nearly impossible to insulate because of their complexity and high density of components and instrumentation. Primary requirements for such nonvacuum thermal insulation systems include the combination of harsh conditions, including full weather exposure, vibration, and structural loads. Further requirements include reliability and the right level of system breathability for thermal cycling. The LCX system is suitable for temperatures from approximately 4 K to 400 K and can be designed to insulate liquid hydrogen, liquid nitrogen, liquid oxygen, or liquid methane equipment. Laboratory test data for thermal and mechanical performance are presented. Field demonstration cases and examples in operational cryogenic systems are also given. Published by Elsevier Ltd.
1. Introduction Thermal insulation of working cryogenic systems, that is, the thermal isolation of the working fluid (a cryogen such as liquid oxygen [LO2], liquid hydrogen [LH2], liquefied natural gas [LNG], or liquid methane [LCH4]), is often only an afterthought in system design, yet that isolation is crucial for the control, safety, and reliability of the system and for the energy efficiency and preservation of the cryogen. As shown in Fig. 1, cryogenic systems in most space launch facilities, propulsion test stands, and other aerospace applications are unavoidably complex. And their challenges are dramatically increased by mechanical/vibration loads, weathering/ascent pressure environments, and requirements for accessibility and maintenance. Furthermore, any successful thermal insulation
system must be lightweight and meet a wide range of fire, compatibility, outgassing, and other physical and chemical requirements. High thermal performance (low thermal conductivity) is the overall goal, but it is not always at the top of the list of material requirements. Ambient-air insulation systems for low-temperature (subambient) applications are difficult to achieve because of moisture ingress, environmental degradation, and thermal stress cracking. Most of the accepted methods for externally applied insulation in outdoor environments are fraught with problems centered on moisture ingress and lack of sealing. In response, the Cryogenics Test Laboratory at Kennedy Space Center (KSC) developed LCX – the layered composite insulation system for extreme environments. LCX maintains high thermal performance in ambient air
http://dx.doi.org/10.1016/j.cryogenics.2015.10.008 0011-2275/Published by Elsevier Ltd.
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Fig. 1. Examples of piping complexity in aerospace cryofuel systems: the aft compartment of the Space Shuttle Orbiter Discovery with its main engines removed (left) and the Shuttle External Tank LH2 Vent Umbilical (right).
or a purged environment, needs no sealed outer envelope (vacuum jacket [VJ]), and can be adapted to a wide range of requirements. The common elements of LCX designs are a primary insulation blanket layer, a compressible radiant barrier layer, and an optional overwrap layer, all of which should be hydrophobic or at least waterproof. 2. Thermal insulation system design The LCX system’s primary insulation blanket layer is preferably hydrophobic, such as an aerogel composite blanket, but can be any suitable flexible insulation material, such as polymeric foam. It can consist of one or more layers of blanket or foam and goes onto the cold inner surface of the tank, piping, or other cold-process object. The compressible barrier layer then goes onto the insulation blanket layer and so forth to create a layered stack (Fig. 2). The compressible barrier layer is also an insulating layer, but is primarily selected to offer the mechanical compliance, compressibility, and placement necessary to enable a good fit of the primary insulation layers with optimal closure of seams and gaps. This layer is a polymeric air-sealed material that includes radiant barrier facings (aluminized plastic film or aluminum foil) in a composite. The outermost compressible barrier layer can incorporate an aluminum foil layer to further conform to complex shapes or to close out the blanket stack around a flange, port, or other component. LCX can be field-applied or prefabricated to meet specifications for piping, tanks, or flat panels. 2.1. Heat transfer considerations (full vacuum pressure range) The LCX technology builds on prior layered thermal insulation systems intended for vacuum service (vacuum enclosure or the vacuum of space). Multilayer insulation (MLI) systems are
composed of highly conductive materials, such as aluminum, in combination with excellent insulating materials, such as lightweight polyester netting or fiberglass paper. Designed and installed in the right way, the two-component MLI systems can provide the ultimate in thermal insulation performance, with heat flux values below 1 W/m2 and effective thermal conductivity values below 1 mW/m K for the typical boundary conditions of 300 K and 77 K. However, these systems are strictly for vacuum environments or evacuated metal jackets. Although MLI systems can perform as well as the best closed-cell spray foam systems, the materials will not hold up in the ambient (wet) environment beyond the first cooldown. The KSC Cryogenics Test Laboratory has been developing MLI and other layered, blanket-type insulation systems for the last 15 years [1]. The objective for cryogenic applications is to achieve the best thermal performance according to the specific vacuum environment: high vacuum (HV), soft vacuum (SV), or no vacuum (NV) [2]. Layered composite insulation (LCI) technology has been introduced in the last decade for the ultimate in soft-vacuum performance from 1 to 10 torr [3,4]. For high-vacuum applications, the performance of LCI systems, using three-component architecture within a vacuum envelope or VJ, can be comparable to that of MLI systems. Thus, LCI represents a strategic advantage in that it maintains some effectiveness with degradation in vacuum level. LCI systems designed for soft-vacuum applications do not require expensive, heavy, welded, stainless-steel jackets, but can instead be constructed of inexpensive, lightweight plastics that are glued together for the jackets. For both MLI systems and LCI systems, the approach is to use a combination of materials to address all modes of heat transfer. The total heat flow (Q) consists of solid conduction, gaseous conduction, bulk-fluid convection, and radiation, as indicated by Eq. (1).
Q total ¼ Q solid
conduction
þ Q gaseous
conduction
þ Q conv ection þ Q radiation
ð1Þ
Fig. 2. Basic concept of the LCX system (left) and test specimen construction for laboratory testing (right).
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Cryostat-100 test data for MLI system of almunim foil and mcirofiberglass paper (40 layers at 3.6 layers/mm density) for 293 K / 78 K boundary temperatures and residual gas nitrogen.
100
Heat Flux (W/m2)
MLI High Vacuum Range 10-4 to 101 millitorr
LCX 10
No Vacuum Range 104 to 106 millitorr
1
LCI Soft Vacuum Range 101 to 104 millitorr 0.1 0.001
0.01
0.1
1
10
100
1000
10000
100000
1000000
Cold Vacuum Pressure (millitorr) Fig. 3. Variation of heat flux with cold vacuum pressure (CVP) for an example insulation system. The optimum type of layered system for each sub-range of vacuum level is indicated: MLI for HV, LCI for SV, and LCX for NV.
systems, and other modes may also apply, but these are the rudimentary lines of division. The three very different environments, HV, SV, and NV, call for three very different approaches to achieve the optimum insulation system performance in each range of CVP. 2.2. Multifunctional design considerations (thermal and mechanical)
Fig. 4. Application of an LCX system on vertical cylindrical tank or piping.
Corresponding values for the heat flux (q) and effective thermal conductivity (ke) are calculated based on the mean area and diameter, respectively [5]. Obviously, some modes of heat transfer are more important than others in a given environment, but all modes must be considered both individually and integrally to achieve the lowest possible heat transmission rate for the intended service conditions. The optimum thermal insulation systems for the three categories of vacuum level are depicted, by principal consideration of the heat transfer properties, in Fig. 3 [6]. The example data given are for an MLI system of 40 layers of aluminum foil and microfiberglass paper tested using liquid nitrogen boiloff calorimetry. The cold vacuum pressure (CVP) is the vacuum level under steady-state thermal operating conditions. The optimum type system for the HV environment, where radiation heat transfer is dominant, is MLI. For the SV environment, where gaseous conduction is dominant, the LCI type system is optimum. In the NV environment, where convection is dominant, the LCX type approach is optimum. Granted, solid conduction is an inherent part of all these insulation
The approach in developing the LCX system was to provide a combination of advantages in thermal performance, structural capability, and operations. The system is particularly suited for the complex piping, tanks, and apparatus subjected to the ambient environment common in the aerospace industry. The low-cost approach also lends the same technology to such industrial applications as building construction and chilled-water piping. By providing compliance and compressibility, the different layers work together not only to make the system easy to work and install but to improve its thermal performance. Without the compressible barrier layer, gaps can form between thermal insulation layers, which can allow additional convection heat transfer and can harbor water or other contaminants. This compression and conformability around the thermal insulation layer ensure
Fig. 5. Cylindrical horizontal tank or piping designs of the LCX system.
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Fig. 6. Example piping application of LCX systems: end view (left) and side view (right).
Fig. 7. Insulation strips for small-diameter piping and tubing insulation with the LCX system.
Fig. 8. Small-diameter piping and tubing insulation wrap: spiral style (left) and cigar style (right).
Fig. 9. Seam closures for tubing insulation wrap: overlap style (left) and flare style (right).
excellent thermal contact, which is essential for the lowest heat leakage rate or the lowest overall thermal conductivity. The system can increase reliability and reduce life cycle costs by mitigating the moisture intrusion and preventing the resulting corrosion that plague subambient-temperature insulation systems operating in the ambient (humidity and rain) environment. Accumulated internal water is allowed to drain and release naturally over to the system’s normal thermal cycles. The thermal insulation
system has a long life expectancy because all layer materials are hydrophobic or otherwise waterproof. LCX systems do not need to be perfectly sealed to handle rain, moisture accumulation, or condensation. Mechanically, the LCX system not only withstands impact, vibration, and the stresses of thermal expansion and contraction, but can help support pipes and other structures, all while maintaining its thermal insulation effectiveness. Conventional
Please cite this article in press as: Fesmire JE. Layered composite thermal insulation system for nonvacuum cryogenic applications. Cryogenics (2015), http://dx.doi.org/10.1016/j.cryogenics.2015.10.008
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Fig. 10. Removable insulation covers showing basic approaches using the LCX system: cryogenic valve (left), piping flange assembly (middle), and water drain seam (right).
Fig. 11. Basic installation sequence of an LCX system on a piping flange assembly (bolted joint).
insulation systems are notoriously difficult to manage around pipe supports because of the cracking and damage that can occur. Used alone or inside another structure or panel, the LCX layering approach can be tailored to provide additional acoustic or vibration damping as a dual function with the thermal insulating benefits. The multilayered composite can be modified, cut, or trimmed without degrading its mechanical performance. Because LCX systems do not require complete sealing from the weather (a practical impossibility for other mechanical insulation systems), it costs less to install. Only minimal amounts of wire, tape, or adhesive are used – materials that ensure an orderly stackup of layers and aid in installation. The materials are generally removable, reusable, and recyclable, a feature not possible with other insulation systems. This feature allows removable insulation covers for valves, flanges, and other components – invaluable benefits for servicing or inspection – to be part of original designs. Thermal performance of the LCX system has been shown to equal or exceed that of the best polyurethane foam systems, which can degrade significantly during the first two years of operation. With its inherent springiness, the system allows for simpler installation and, even more important, better thermal insulation because of its consistency and full contact with the cold surface. Improved contact with the cold surface and better closure of gaps and seams are the keys to superior thermal performance in real systems. Eliminating the requirement for glues, sealants, mastics, expansion joints, and vapor barriers provides dramatic savings in material and labor costs of the installed system. 2.3. System design types and installation methods The LCX system has been developed for different types of geometries and components [5,7]. Fig. 4 shows the basic layup for a vertical cylindrical tank or larger-diameter piping system,
and Fig. 5 shows the same approach for a horizontal configuration. The common feature is that the seams are staggered, both circumferentially and axially. Preferably, the staggering is arranged for each layer, as well as through the stack of layers. As shown in the examples below, each layer pair is secured using stainless steel (SST) lockwire or other fastener as shown. Layers are built up singularly or in pairs. Each pair is an insulation blanket layer, such as aerogel composite blanket, plus a compressible radiant barrier layer. The insulation blanket layer always goes first in order to cover the cold surface as well as possible. The outermost layer is typically a compressible radiant barrier layer that incorporates an aluminum foil external facing. Fig. 6 shows the typical layup details for a piping installation. Depending on the level of permanency desired or ruggedness needed, an outer wrap of weather barrier tape, shrink wrap, or vinyl wrap can also be applied. External seams and joints are normally covered with aluminum tape for a neat and finished appearance. Bottom drain seams that are taped over can be periodically slit with a razor knife to complete the installation. Smaller-size piping (under 50 mm in diameter) can also be handled with insulation strips, as shown in Fig. 7. Each strip is composed of a layer-pair of aerogel blanket combined with a compressible radiant barrier layer. Different methods of installation and closures are shown in Figs. 8 and 9. The insulation system can be one or more wraps installed in opposing directions. In many applications of complex piping and small tubing, complete coverage of the cold surface is impossible for wrapped type installations. Requirements differ widely, but partial coverage will not adversely affect the long-term reliable performance of LCX when properly applied. Applications of LCX systems for valves and flanges have also been developed. These insulation systems can be made semipermanent or fully removable depending on specific requirements.
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Fig. 12. Simplified schematic of the cryogenic insulation thermal performance test apparatus (Cryostat-100).
Table 1 Physical characteristics of Cryostat-100 test specimens.
*
Test series
Description
Thickness (mm)
Circumference (mm)
Mean Area* (m2)
A161 A162 A163 A166
Compressible barriers only: RP (5 layers) Compressible barriers only: RA (5 layers) LCX system: C5/RP/C5/RP/RA LCX system: C10/RP/C10/RP/RA
38.5 35.0 36.0 47.0
767 743 749 821
0.382 0.347 0.357 0.466
Effective heat transfer length of Cryostat-100 = 0.580 m.
Removability can be a key factor for system leak checking, maintenance, or modification, and the system has been used on valves and flanges in both semipermanent and fully removable configurations. Fig. 10 shows the basic approach of removable insulation covers for a cryogenic valve and a bolted flange assembly. A stainless-steel safety wire, hook-and-loop strap, or tape secures the system onto the component and allows for removability as required. Aerogel composite blanket packing is applied around the valve body and between bolts, as desired, for optimum thermal performance, and butt-joint seams at the bottom allow any accumulated water to drain. Fig. 11 shows the basic installation sequence for the bolted joint of a cryogenic piping flange assembly. The combination of compressibility, flexibility, and full elastic recovery makes the LCX system mechanically robust. These robust mechanical properties allow LCX to tolerate thermal contraction/ expansion, impact, vibration, and the stress of providing structural support, without degradation of the overall thermal insulation effectiveness of the installed system.
3. Testing and evaluation of materials Representative LCX systems and their individual materials were tested and evaluated in three areas. For baseline thermal performance, including effective thermal conductivity and heat flux, liquid nitrogen (LN2) boiloff calorimetry was performed, using the cylindrical Cryostat-100 instrument. Mechanical tests of representative LCX specimens were performed to determine the deflection and recovery under compressive loads. Various environmental exposure tests, including weathering, LN2 immersion, and water absorption were also performed.
3.1. Thermal performance testing Using the Cryostat-100 instrument, the KSC Cryogenics Test Laboratory tested five layered composite test specimens. Cryostat-100 is a guarded LN2 boiloff calorimeter with a 1 m-long
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J.E. Fesmire / Cryogenics xxx (2015) xxx–xxx Table 2 Summary of thermal performance results for Cryostat-100 testing.
a b
Test series
Description
A161 Test 2 Test 3
RP (five layers)
A162 Test 1 Test 2
RA (five layers)
A163 Test 2 Test 4
C5/RP/C5/RP/RA
A166 Test 2 Test 3 Test 4
C10/RP/C10/RP/RA
CVPb (millitorr)
Boiloff flow rate (sccm)
Q (W)
kea (mW/m K)
Heat flux (q)a (W/m2)
760,000 760,000
17,400 16,000
72.0 66.2
35.2 32.4
189 173
760,000 760,000
13,900 14,500
57.5 60.0
28.2 29.4
166 173
760,000 760,000
11,900 12,800
49.2 53.0
24.1 25.9
138 148
760,000 760,000 760,000
8900 8800 9500
36.8 36.4 39.3
18.0 17.8 19.2
79.0 78.1 84.4
Boundary temperatures are approximately 293 K and 78 K; ASTM C1774, Annex A1. CVP = cold vacuum pressure (residual gas is nitrogen).
20 18
Displacement (mm)
16 14 12 10 8 6 Run 1
4 2 0
Run 2 0
5
10
15
20
25
30
35
40
Compressive Load (kPa) Fig. 13. Displacement as a function of compressive load for a six-layer LCX stack with 39 mm nominal thickness. The test specimen is shown to be settled (run 2) after its initial compression (run 1).
Fig. 14. Compression recovery test fixture showing the installation of a six-layer test specimen.
cold mass for testing cylindrical test specimens and complies with ASTM C1774, Annex A1 [8]. A simplified schematic of the experimental apparatus is given in Fig. 12. The warm and cold boundary temperatures are approximately 293 K and 78 K, respectively, and the cold vacuum pressure (CVP) environment was set at one
atmosphere pressure (760 torr) with gaseous nitrogen. The steady-state boiloff flow rate measurements enable the calculation of effective thermal conductivity (ke) and heat flux (q). Physical characteristics of the test specimens are summarized in Table 1. The following designations apply: RP (compressible radiant barrier layer, standard plastic backed), RA (compressible radiant barrier layer, aluminum backed outer surface), C5 (aerogel blanket, 5 mm thick, without backing), and C10 (aerogel blanket, 10 mm thick, without backing). The RP and RA materials were manufactured by Reflectix, Inc. under the trade name ReflectixÒ Double Reflective Insulation and had a nominal thickness of 8 mm. The aerogel blankets were manufactured by Aspen Aerogels, Inc. under the trade name CryogelÒ and had a nominal thickness of 5 mm or 10 mm. The aerogel blankets can be purchased with backing (CZ5 or CZ10) or without backing (C5 or C10). Additional commercial products include PyrogelÒ and SpaceloftÒ which have been investigated for specialized layered type systems. Other experimental aerogel blankets made by Aspen Aerogels come in thicknesses down to 1 mm and some are optimized for ultralightweight, high-vacuum applications [9]. The results of the thermal performance tests using Cryostat-100 are summarized in Table 2. The data for ke show that the layered composite systems have thermal insulating performance on par with the best foam materials in new condition. For example, the top performing spray-on foam insulation (SOFI) material for the Space Shuttle External Tank application, material BX-265 (new), gave a ke value of 21.2 mW/m K for the same test conditions [10]. The LCX test specimen A166 reported here gave a ke of approximately 18.3 mW/m K. (There is a significant long-term difference in that the spray foams will rapidly degrade in thermal performance due to weathering exposure as well as normal aging.) The cryogenic thermal performance data for the aerogel blanket materials have been previously reported [11]. The lower limit of 12.4 mW/m K as established by similar thermal performance testing of the CryogelÒ aerogel blanket material alone. The ke for the compressible radiant barrier layers alone ranged from approximately 28 mW/m K to 35 mW/m K. The LCX systems can therefore be tailored to the application over a wide range of performance levels from about 15 mW/m K to 25 mW/m K depending on the particular combination of layer materials and their individual thicknesses.
3.2. Load–displacement mechanical testing Mechanical testing of a six-layer LCX test specimen was performed to assess its structural integrity and compliance under
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Compression (mm)
25
20
15
10
5
Fin al
St a 7 4 rt R % u Co n 5 m pr e Re ss la xe d
tR un 4 pr e Re ss la xe d m
Co
St ar
%
62
al In i
St a 2 3 rt R % u Co n 1 m pr e Re ss la xe d St ar 36 t R % u Co n 2 m pr e Re ss la xe d St a 4 9 rt R % u Co n 3 m pr e Re ss la xe d
0
Fig. 15. Compression loading recovery for a six-layer test specimen with 39 mm nominal thickness.
Fig. 16. Extreme (cryo-to-water) exposure test: during LN2 soak (left); immediately after water bath (right).
compressive loading. The 76 mm (3 in.) - diameter test article consisted of the following stackup of materials: C10, RP, C5, RP, C5, and RA (outer layer), for a total thickness of 44 mm. The settled thickness, and nominal test thickness, was 39 mm. A series of load versus displacement tests was performed using a series of known weights for precision. The test data are plotted in Fig. 13. Starting at zero displacement, the run 1 data indicate a settling displacement of approximately 5 mm after release of the compressive load. The subsequent run 2 data, starting and ending at zero displacement, show complete elastic recovery at the conclusion of the test. The six-layer LCX specimen tested is estimated to withstand compressive mechanical loadings of more than 180 kPa (26 psi), corresponding to a compression of 75% of original thickness, with full elastic recovery. The results show that the combination of layers maintains structural integrity under compressive loads and is strong to support system loads. Compression recovery testing was performed using a hydraulic press as shown in Fig. 14. The test data are plotted in Fig. 15. With a nominal starting thickness of 39 mm (settled), five runs were performed with the following amounts of compression at each run: 9 mm, 14 mm, 19 mm, 24 mm, and 29 mm (74% compression). The specimen was observed to return to the nominal thickness of 39 mm. The mechanical tests indicate the unique loadsupporting, mechanically robust capabilities of the combination of aerogel blanket and compressible barrier layers that comprise an LCX system.
3.3. Environmental exposure and cryopumping The following examples of field installations in fully exposed outdoor environments show that LCX is not degraded over time. Proper selection of the outermost layer, or application of a final overwrap (such as RA or VC material), is a key part of optimal protection from UV exposure. Water absorption tests have shown negligible mass increase even after full immersion. Three six-layer test specimens (50 mm diameter by 38 mm thickness) were tested for water absorption by immersion according to ASTM C1763 [12]. Upon immediate removal after the prescribed 24-h immersion period, the average weight increased by 5.7% (presumably because of water droplets resting in the open cut edges of the compressible barrier layers). However, each sample returned to within 0.1% of its original weight after 2 h of drying time. One of these same test specimens was then subjected to an extreme (cryo-to-water) exposure test. The sample was fully immersed in an LN2 bath and allowed to cold-soak for 5 min as shown in Fig. 16. The sample was then taken out of the cryogen and immediately put into a water bath at ambient temperature. The sample quickly thawed and showed no adverse effect or visible change from its original condition. The properly installed system should behave in the same manner by allowing for draining and venting, or freezing and thawing, during all operational cycles. Some level of cryopumping of air and moisture will naturally occur during the cooldown of any cryogenic system operating in
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Fig. 17. Tank dome insulation: schematic of vertical cylindrical tank application (top left), view of completed insulation system installed on top dome of tank (top right), and view of bottom dome of tank during installation with moisture drain/vent features (bottom).
Fig. 18. Vertical cylindrical cryogenic tank (7600-l capacity) for flight tank simulation: completed LCX installation (left); during operation, fully loaded with LN2 (right).
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Table 3 Estimated thermal performance of a 7600-l vertical cylindrical cryogenic tank with different insulation materials/systems (and comparison with Space Shuttle External Tank). System
Description
Heat load – Q (W)
Heat flux – q (W/m2)
Boiloff equivalent LN2 flow (l/min)
LCX (5-layer) LCX (3-layer) Frost Ice Space Shuttle External Tank – LO2 Tank
C10/RP/C5/RP/C5 (36 mm thick) C5/RP/C5 (18 mm thick) Just frost layer (in still air) Just ice layer (in still air) SOFI Insulation (25 mm thick)
2214 5211 13,617 50,180 100,000
90 212 555 1025 200
0.833 1.97 5.26 18.9 37.9
Fig. 19. Completed LCX installation on the Rapid Propellant Loading System at Kennedy Space Center, showing a combination of piping, valves, pipe supports, and flanges.
the ambient environment. The propensity for cryopumping increases when portions of the cold boundary are bare or inadequately covered by the first insulation layer of aerogel blanket. LCX tolerates gaps and other unavoidable imperfections in installation because the aerogel material has a substantial capacity for adsorption per unit weight. Previous results show that the sorption ratio (nitrogen to aerogel) is about seven to one by mass or 62% by volume [13]. According to previously reported experimental work, the condensed air is safely kept on the surfaces within the nanoporous network of the aerogel in a nonliquid phase, physisorbed state [14]. As the system warms up, the air and other condensables sequestered within the nanopores of the material are gradually liberated and aspirated back into the environment in a manner consistent with the natural thermal state of the overall cryogenic system. 4. Field applications A number of field applications of LCX have been made in recent years, including tanks, piping, valves, umbilical connections, and even liquid–air backpacks. Although each application is a custom design and installation, mechanical robustness and weather tolerance are common requirements of the complex cryogenic systems needed for aerospace functions. Several such applications are described. 4.1. Cryogenic tank A cryogenic test facility being constructed at Kennedy Space Center includes the Rapid Cryogenic Propellant Loading System, a flight simulator tank that holds 7570 l (2000 gallons), is configured vertically, and is constructed of single-wall stainless steel. The system is for the development of advanced cryofuel transfer systems,
including autonomous operations and rapid tank loading [15]. Transient operations with many thermal cycles provide a testbed with harsh operational requirements for any thermal insulation system. The entire system includes a storage tank, pumping skid, valve control skid, and transfer piping, constructed using singlewall (non-VJ) designs that were all subsequently insulated with LCX in different designs. The thermal performance of the tank’s thermal insulation system should be generally representative of a flight tank. This tank is approximately 5.2 m (17 ft) tall and 1.5 m (5 ft) in diameter and is designed for LN2 temperature (77 K) and an operating pressure up to 517 kPa (75 psig). The 4.3 m (14 ft)-tall cylindrical side (or barrel of the tank) is unobstructed, and ports are concentrated on the upper and lower domes, each standing 0.3 m (1 ft) tall. The total surface area of the tank is approximately 25 m2 (270 ft2). A four-layer LCX was designed and installed on this tank. The layup was CZ10 + RP + CZ5 + black vinyl overwap, for a total thickness of approximately 24 mm. The insulation approach for the tank domes is shown in Fig. 17: schematic for vertical cylindrical tank application (top left), view of completed insulation system installed on top dome (top right), and view of bottom dome of tank during installation (bottom). The layers are designed to shed the majority of rain water and condensate. The water that does migrate inward drains through channels (of various materials and installations) that run all the way to the bottom dome. The completed installation is shown in Fig. 18 (left) while the right hand view shows the integrated tank system while fully cold with liquid nitrogen. Temperature sensors were placed through the thickness of the insulation system on the side of the tank. Two Type E thermocouples were affixed on each layer. The flight tank simulator has been thermally cycled numerous times since overall system test operations began. As part of the cooldown and warmup profiles measured by 11 thermocouples installed on the surface of the tank
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11
Fig. 20. Completed LCX installation on a valve skid for a cryofuel servicing system (LCH4).
wall, the insulation system layer temperatures were also monitored. The typical profile shows a starting ambient condition of approximately 296 K and the final temperature of the tank of 78 K in slightly more than 1 h from the start of cooldown. In this case, the layer temperatures after stabilization with a full tank of LN2 were 201 K (layer 1), 239 K (layer 2), and 294 K (layer 3). Of the full temperature difference of 216 K, almost 60% of this difference is produced in the first (aerogel blanket) layer. Through numerous thermal cycles for more than two years of cryogenic flow operations in varying weather conditions, no frost area was observed on the black vinyl outer surface of the tank. The 7600-l vertical cylindrical tank includes a number of appurtenances and is instrumented with temperature sensors and other devices. The design tool TISCALC (incorporating the latest Cryostat100 thermal conductivity data) was used to calculate the thermal performance [16]. The following conditions were applied: 300 K and 77 K boundary temperatures; ambient air with no convection effects; and no compensation for heat leaks through piping nozzles. Table 3 compares the heat leakage for the different systems. Comparison with spray-on foam insulation (SOFI) was based on data previously reported for the polyisocyanurate foams used on the Space Shuttle External Tank [10]. From this information, the boiloff rate was estimated to equal an LN2 flow rate of 1.9 liters per minute (0.5 gpm) for the tank without supports or connecting piping. Analysis from the tanking test measurements indicate a boiloff of 5.3 liters per minute (1.4 gpm) for the total tank system. 4.2. Cryogenic piping The cryogenic piping and components of the Rapid Cryogenic Propellant Loading System were also insulated with various LCX designs according to the component, thermal requirements, and desired level of permanence or removability. The nominal piping sizes insulated ranged from a 530-mm (6-in) pipe to a 25-mm (1-in) tube. Fig. 19 shows a portion of the piping, flanges, and valves on the simulated flight tank interface. The system is operating successfully using LN2 and has seen numerous thermal cycles and full exposure over the last several years. The industry of transport and distribution of LNG, for example, has called for the specification of removable insulation covers for cryogenic valves, but to date, LCX is the only known solution. Four valve skids for cryofuel loading systems (LNG and LO2) were insulated using LCX. The skids connect via VJ bayonets, so top performance for the remaining non-VJ piping was a priority. A seven-layer system was designed and installed (see Fig. 20) for the bare piping. Bulk-fill aerogel granules were also used in the skid-half of the bayonets for the optimum transition to the VJ lines.
The skids are part of a universal propellant servicing system for both ground testing and launch servicing for future methanefueled space launch vehicles. As shown in the field application examples, the LCX system can be used to insulate large, ‘‘clean” surface areas and complex piping systems, for example, those that are full of ports, fittings, flanges, expansion joints, supports, valves, instrumentation components, and so forth. Systems that have been impossible or highly impractical to insulate are now be readily insulated with this approach. 5. Conclusion Overcoming vapor drive toward the cold side and preventing moisture accumulation inside are the major challenges of insulating complex cryogenic equipment in the ambient environment [14]. In response, previous technologies, including cellular glass, rigid foam, and spray foam, are impractical to apply, lack mechanical robustness, and fail to meet life cycle requirements of the systems they support. The use of aerogel composite blankets can provide the lowest heat flux per unit thickness but can also present practical difficulties for certain applications. The LCX system solves these problems with a cost-effective, hybrid approach incorporating the use of aerogel blankets in practical thermal insulation systems with the unique features of breathability and, where desired, removability. Through material development and testing, the LCX system is shown to provide unique mechanical and thermal properties in its integrated and layered approach. Its layered system of materials is physically resilient against damaging mechanical effects, including compression, flexure, impact, vibration, and thermal expansion/contraction. The combination of materials and method of installation achieve low values for effective thermal conductivity by managing all modes of heat transfer (solid conduction, convection, gas conduction, and radiation). The thermal performance of the thermal insulation system is maintained through the life cycle by the hydrophobic properties of the thermal insulation and compressible barrier layers in combination with the built-in moisture draining and venting features of the installed system. Robust mechanical properties are achieved by a tailored combination of materials for both compressibility and flexibility with full elastic recovery. A number of different LCX system designs have been installed successfully for cryogenic tanks, piping, and valve control skids in the field. The LCX technology provides a practical solution for complex systems operating under dynamic, transient conditions in extreme environments. Such conditions are common for aerospace vehicles, launch pad facilities, and propulsion test stands
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and for other industry applications, such as LNG and LH2 cryofuels for transportation and power.
Acknowledgements The author acknowledges and thanks Mr. Stephen Huff of NASA Kennedy Space Center for his assistance and dedication to accomplishing the field installations and Mr. Robert Johnson, also of NASA Kennedy Space Center, for his support through NASA’s Advanced Exploration Systems project.
References [1] Augustynowicz SD, Fesmire JE, Wikstrom JP. Cryogenic insulation systems. In: 20th International congress of refrigeration Sydney, No. 2000-1147. Paris: International Institute of Refrigeration; 2000. [2] Augustynowicz SD, Fesmire JE. Cryogenic insulation system for soft vacuum. Advances in cryogenic engineering, vol. 45. New York: Kluwer Academic/ Plenum Publishers; 2000. p. 1691–8. [3] Fesmire JE, Augustynowicz SD, Scholtens BE. Robust multilayer insulation for cryogenic systems. Advances in cryogenic engineering, vol. 53B. New York: American Institute of Physics; 2008. p. 1359–66. [4] Augustynowicz SD, Fesmire JE. Thermal insulation systems. U.S. Patent 6,967,051 B1; November 22, 2005. [5] ASTM C740. Standard guide for evacuated reflective cryogenic insulation. West Conshohocken (PA, USA): ASTM International; 2013.
[6] Fesmire JE. A thermal insulation system for non-vacuum applications including a multilayered composite. International patent application, publication number WO2014164591A1; October 9, 2014. [7] Fesmire JE. Thermal insulation system for non-vacuum applications including a multilayer composite. U.S. patent application, publication number US20140255628A1; September 11, 2014. [8] ASTM C1774. Standard guide for thermal performance testing of cryogenic insulation systems. West Conshohocken (PA, USA): ASTM International; 2013. [9] White S, Begag R, Fesmire J, Mihalak D, Kerce J, Mills G, et al. Multi-layer aerogel insulation for cryogenic applications. In: Cryogenic engineering conference, Tucson, Arizona; July 2015. [10] Fesmire JE, Coffman BE, Meneghelli BJ, Heckle KW. Spray-on foam insulations for launch vehicle cryogenic tanks. Cryogenics. http://dx.doi.org/10.1016/j. cryogenics.2012.01.018. [11] Coffman BE, Fesmire JE, Augustynowicz SD, Gould G, White S. Aerogel blanket insulation materials for cryogenic applications. AIP Conf Proc Adv Cryogenic Eng 2010;1218:913–20. [12] ASTM C1763. Standard test method for water absorption by immersion of thermal insulation materials. West Conshohocken (PA, USA): ASTM International; 2014. [13] Fesmire JE, Sass JP. Aerogel insulation applications for liquid hydrogen launch vehicle tanks. Cryogenics 2008. http://dx.doi.org/10.1016/j.cryogenics. 2008.03.014. [14] Fesmire JE. Aerogel insulation systems for space launch applications. Cryogenics 2006;46(2–3):111–7. [15] Toro Medina J, Sass JP, Schmitz WD. Simulated propellant loading system: testbed for cryogenic component and control systems research & development. In: Cryogenic engineering conference, Tucson, AZ; July 2015. [16] Demko JA, Fesmire JE, Augustynowicz SD. Design tool for cryogenic thermal insulation systems. Advances in cryogenic engineering, vol. 53A. New York: American Institute of Physics; 2008. p. 145–51.
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Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Layered composite thermal insulation system for nonvacuum cryogenic applications J.E. Fesmire NASA Kennedy Space Center, Cryogenics Test Laboratory, UB-R1, KSC, FL 32899, USA
a r t i c l e
i n f o
Article history: Received 4 July 2015 Received in revised form 9 October 2015 Accepted 15 October 2015 Available online xxxx Keywords: Thermal insulation Weathering Compression Piping Valves Tanks Space launch vehicles
a b s t r a c t A problem common to both space launch applications and cryogenic propulsion test facilities is providing suitable thermal insulation for complex cryogenic piping, tanks, and components that cannot be vacuumjacketed or otherwise be broad-area-covered. To meet such requirements and provide a practical solution to the problem, a layered composite insulation system has been developed for nonvacuum applications and extreme environmental exposure conditions. Layered composite insulation system for extreme conditions (or LCX) is particularly suited for complex piping or tank systems that are difficult or practically impossible to insulate by conventional means. Consisting of several functional layers, the aerogel blanket-based system can be tailored to specific thermal and mechanical performance requirements. The operational principle of the system is layer-pairs working in combination. Each layer pair is comprised of a primary insulation layer and a compressible radiant barrier layer. Vacuum-jacketed piping systems, whether part of the ground equipment or the flight vehicle, typically include numerous terminations, disconnects, umbilical connections, or branches that must be insulated by nonvacuum means. Broad-area insulation systems, such as spray foam or rigid foam panels, are often the lightweight materials of choice for vehicle tanks, but the plumbing elements, feedthroughs, appurtenances, and structural supports all create ‘‘hot spot” areas that are not readily insulated by similar means. Finally, the design layouts of valve control skids used for launch pads and test stands can be nearly impossible to insulate because of their complexity and high density of components and instrumentation. Primary requirements for such nonvacuum thermal insulation systems include the combination of harsh conditions, including full weather exposure, vibration, and structural loads. Further requirements include reliability and the right level of system breathability for thermal cycling. The LCX system is suitable for temperatures from approximately 4 K to 400 K and can be designed to insulate liquid hydrogen, liquid nitrogen, liquid oxygen, or liquid methane equipment. Laboratory test data for thermal and mechanical performance are presented. Field demonstration cases and examples in operational cryogenic systems are also given. Published by Elsevier Ltd.
1. Introduction Thermal insulation of working cryogenic systems, that is, the thermal isolation of the working fluid (a cryogen such as liquid oxygen [LO2], liquid hydrogen [LH2], liquefied natural gas [LNG], or liquid methane [LCH4]), is often only an afterthought in system design, yet that isolation is crucial for the control, safety, and reliability of the system and for the energy efficiency and preservation of the cryogen. As shown in Fig. 1, cryogenic systems in most space launch facilities, propulsion test stands, and other aerospace applications are unavoidably complex. And their challenges are dramatically increased by mechanical/vibration loads, weathering/ascent pressure environments, and requirements for accessibility and maintenance. Furthermore, any successful thermal insulation
system must be lightweight and meet a wide range of fire, compatibility, outgassing, and other physical and chemical requirements. High thermal performance (low thermal conductivity) is the overall goal, but it is not always at the top of the list of material requirements. Ambient-air insulation systems for low-temperature (subambient) applications are difficult to achieve because of moisture ingress, environmental degradation, and thermal stress cracking. Most of the accepted methods for externally applied insulation in outdoor environments are fraught with problems centered on moisture ingress and lack of sealing. In response, the Cryogenics Test Laboratory at Kennedy Space Center (KSC) developed LCX – the layered composite insulation system for extreme environments. LCX maintains high thermal performance in ambient air
http://dx.doi.org/10.1016/j.cryogenics.2015.10.008 0011-2275/Published by Elsevier Ltd.
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Fig. 1. Examples of piping complexity in aerospace cryofuel systems: the aft compartment of the Space Shuttle Orbiter Discovery with its main engines removed (left) and the Shuttle External Tank LH2 Vent Umbilical (right).
or a purged environment, needs no sealed outer envelope (vacuum jacket [VJ]), and can be adapted to a wide range of requirements. The common elements of LCX designs are a primary insulation blanket layer, a compressible radiant barrier layer, and an optional overwrap layer, all of which should be hydrophobic or at least waterproof. 2. Thermal insulation system design The LCX system’s primary insulation blanket layer is preferably hydrophobic, such as an aerogel composite blanket, but can be any suitable flexible insulation material, such as polymeric foam. It can consist of one or more layers of blanket or foam and goes onto the cold inner surface of the tank, piping, or other cold-process object. The compressible barrier layer then goes onto the insulation blanket layer and so forth to create a layered stack (Fig. 2). The compressible barrier layer is also an insulating layer, but is primarily selected to offer the mechanical compliance, compressibility, and placement necessary to enable a good fit of the primary insulation layers with optimal closure of seams and gaps. This layer is a polymeric air-sealed material that includes radiant barrier facings (aluminized plastic film or aluminum foil) in a composite. The outermost compressible barrier layer can incorporate an aluminum foil layer to further conform to complex shapes or to close out the blanket stack around a flange, port, or other component. LCX can be field-applied or prefabricated to meet specifications for piping, tanks, or flat panels. 2.1. Heat transfer considerations (full vacuum pressure range) The LCX technology builds on prior layered thermal insulation systems intended for vacuum service (vacuum enclosure or the vacuum of space). Multilayer insulation (MLI) systems are
composed of highly conductive materials, such as aluminum, in combination with excellent insulating materials, such as lightweight polyester netting or fiberglass paper. Designed and installed in the right way, the two-component MLI systems can provide the ultimate in thermal insulation performance, with heat flux values below 1 W/m2 and effective thermal conductivity values below 1 mW/m K for the typical boundary conditions of 300 K and 77 K. However, these systems are strictly for vacuum environments or evacuated metal jackets. Although MLI systems can perform as well as the best closed-cell spray foam systems, the materials will not hold up in the ambient (wet) environment beyond the first cooldown. The KSC Cryogenics Test Laboratory has been developing MLI and other layered, blanket-type insulation systems for the last 15 years [1]. The objective for cryogenic applications is to achieve the best thermal performance according to the specific vacuum environment: high vacuum (HV), soft vacuum (SV), or no vacuum (NV) [2]. Layered composite insulation (LCI) technology has been introduced in the last decade for the ultimate in soft-vacuum performance from 1 to 10 torr [3,4]. For high-vacuum applications, the performance of LCI systems, using three-component architecture within a vacuum envelope or VJ, can be comparable to that of MLI systems. Thus, LCI represents a strategic advantage in that it maintains some effectiveness with degradation in vacuum level. LCI systems designed for soft-vacuum applications do not require expensive, heavy, welded, stainless-steel jackets, but can instead be constructed of inexpensive, lightweight plastics that are glued together for the jackets. For both MLI systems and LCI systems, the approach is to use a combination of materials to address all modes of heat transfer. The total heat flow (Q) consists of solid conduction, gaseous conduction, bulk-fluid convection, and radiation, as indicated by Eq. (1).
Q total ¼ Q solid
conduction
þ Q gaseous
conduction
þ Q conv ection þ Q radiation
ð1Þ
Fig. 2. Basic concept of the LCX system (left) and test specimen construction for laboratory testing (right).
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Cryostat-100 test data for MLI system of almunim foil and mcirofiberglass paper (40 layers at 3.6 layers/mm density) for 293 K / 78 K boundary temperatures and residual gas nitrogen.
100
Heat Flux (W/m2)
MLI High Vacuum Range 10-4 to 101 millitorr
LCX 10
No Vacuum Range 104 to 106 millitorr
1
LCI Soft Vacuum Range 101 to 104 millitorr 0.1 0.001
0.01
0.1
1
10
100
1000
10000
100000
1000000
Cold Vacuum Pressure (millitorr) Fig. 3. Variation of heat flux with cold vacuum pressure (CVP) for an example insulation system. The optimum type of layered system for each sub-range of vacuum level is indicated: MLI for HV, LCI for SV, and LCX for NV.
systems, and other modes may also apply, but these are the rudimentary lines of division. The three very different environments, HV, SV, and NV, call for three very different approaches to achieve the optimum insulation system performance in each range of CVP. 2.2. Multifunctional design considerations (thermal and mechanical)
Fig. 4. Application of an LCX system on vertical cylindrical tank or piping.
Corresponding values for the heat flux (q) and effective thermal conductivity (ke) are calculated based on the mean area and diameter, respectively [5]. Obviously, some modes of heat transfer are more important than others in a given environment, but all modes must be considered both individually and integrally to achieve the lowest possible heat transmission rate for the intended service conditions. The optimum thermal insulation systems for the three categories of vacuum level are depicted, by principal consideration of the heat transfer properties, in Fig. 3 [6]. The example data given are for an MLI system of 40 layers of aluminum foil and microfiberglass paper tested using liquid nitrogen boiloff calorimetry. The cold vacuum pressure (CVP) is the vacuum level under steady-state thermal operating conditions. The optimum type system for the HV environment, where radiation heat transfer is dominant, is MLI. For the SV environment, where gaseous conduction is dominant, the LCI type system is optimum. In the NV environment, where convection is dominant, the LCX type approach is optimum. Granted, solid conduction is an inherent part of all these insulation
The approach in developing the LCX system was to provide a combination of advantages in thermal performance, structural capability, and operations. The system is particularly suited for the complex piping, tanks, and apparatus subjected to the ambient environment common in the aerospace industry. The low-cost approach also lends the same technology to such industrial applications as building construction and chilled-water piping. By providing compliance and compressibility, the different layers work together not only to make the system easy to work and install but to improve its thermal performance. Without the compressible barrier layer, gaps can form between thermal insulation layers, which can allow additional convection heat transfer and can harbor water or other contaminants. This compression and conformability around the thermal insulation layer ensure
Fig. 5. Cylindrical horizontal tank or piping designs of the LCX system.
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Fig. 6. Example piping application of LCX systems: end view (left) and side view (right).
Fig. 7. Insulation strips for small-diameter piping and tubing insulation with the LCX system.
Fig. 8. Small-diameter piping and tubing insulation wrap: spiral style (left) and cigar style (right).
Fig. 9. Seam closures for tubing insulation wrap: overlap style (left) and flare style (right).
excellent thermal contact, which is essential for the lowest heat leakage rate or the lowest overall thermal conductivity. The system can increase reliability and reduce life cycle costs by mitigating the moisture intrusion and preventing the resulting corrosion that plague subambient-temperature insulation systems operating in the ambient (humidity and rain) environment. Accumulated internal water is allowed to drain and release naturally over to the system’s normal thermal cycles. The thermal insulation
system has a long life expectancy because all layer materials are hydrophobic or otherwise waterproof. LCX systems do not need to be perfectly sealed to handle rain, moisture accumulation, or condensation. Mechanically, the LCX system not only withstands impact, vibration, and the stresses of thermal expansion and contraction, but can help support pipes and other structures, all while maintaining its thermal insulation effectiveness. Conventional
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Fig. 10. Removable insulation covers showing basic approaches using the LCX system: cryogenic valve (left), piping flange assembly (middle), and water drain seam (right).
Fig. 11. Basic installation sequence of an LCX system on a piping flange assembly (bolted joint).
insulation systems are notoriously difficult to manage around pipe supports because of the cracking and damage that can occur. Used alone or inside another structure or panel, the LCX layering approach can be tailored to provide additional acoustic or vibration damping as a dual function with the thermal insulating benefits. The multilayered composite can be modified, cut, or trimmed without degrading its mechanical performance. Because LCX systems do not require complete sealing from the weather (a practical impossibility for other mechanical insulation systems), it costs less to install. Only minimal amounts of wire, tape, or adhesive are used – materials that ensure an orderly stackup of layers and aid in installation. The materials are generally removable, reusable, and recyclable, a feature not possible with other insulation systems. This feature allows removable insulation covers for valves, flanges, and other components – invaluable benefits for servicing or inspection – to be part of original designs. Thermal performance of the LCX system has been shown to equal or exceed that of the best polyurethane foam systems, which can degrade significantly during the first two years of operation. With its inherent springiness, the system allows for simpler installation and, even more important, better thermal insulation because of its consistency and full contact with the cold surface. Improved contact with the cold surface and better closure of gaps and seams are the keys to superior thermal performance in real systems. Eliminating the requirement for glues, sealants, mastics, expansion joints, and vapor barriers provides dramatic savings in material and labor costs of the installed system. 2.3. System design types and installation methods The LCX system has been developed for different types of geometries and components [5,7]. Fig. 4 shows the basic layup for a vertical cylindrical tank or larger-diameter piping system,
and Fig. 5 shows the same approach for a horizontal configuration. The common feature is that the seams are staggered, both circumferentially and axially. Preferably, the staggering is arranged for each layer, as well as through the stack of layers. As shown in the examples below, each layer pair is secured using stainless steel (SST) lockwire or other fastener as shown. Layers are built up singularly or in pairs. Each pair is an insulation blanket layer, such as aerogel composite blanket, plus a compressible radiant barrier layer. The insulation blanket layer always goes first in order to cover the cold surface as well as possible. The outermost layer is typically a compressible radiant barrier layer that incorporates an aluminum foil external facing. Fig. 6 shows the typical layup details for a piping installation. Depending on the level of permanency desired or ruggedness needed, an outer wrap of weather barrier tape, shrink wrap, or vinyl wrap can also be applied. External seams and joints are normally covered with aluminum tape for a neat and finished appearance. Bottom drain seams that are taped over can be periodically slit with a razor knife to complete the installation. Smaller-size piping (under 50 mm in diameter) can also be handled with insulation strips, as shown in Fig. 7. Each strip is composed of a layer-pair of aerogel blanket combined with a compressible radiant barrier layer. Different methods of installation and closures are shown in Figs. 8 and 9. The insulation system can be one or more wraps installed in opposing directions. In many applications of complex piping and small tubing, complete coverage of the cold surface is impossible for wrapped type installations. Requirements differ widely, but partial coverage will not adversely affect the long-term reliable performance of LCX when properly applied. Applications of LCX systems for valves and flanges have also been developed. These insulation systems can be made semipermanent or fully removable depending on specific requirements.
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Fig. 12. Simplified schematic of the cryogenic insulation thermal performance test apparatus (Cryostat-100).
Table 1 Physical characteristics of Cryostat-100 test specimens.
*
Test series
Description
Thickness (mm)
Circumference (mm)
Mean Area* (m2)
A161 A162 A163 A166
Compressible barriers only: RP (5 layers) Compressible barriers only: RA (5 layers) LCX system: C5/RP/C5/RP/RA LCX system: C10/RP/C10/RP/RA
38.5 35.0 36.0 47.0
767 743 749 821
0.382 0.347 0.357 0.466
Effective heat transfer length of Cryostat-100 = 0.580 m.
Removability can be a key factor for system leak checking, maintenance, or modification, and the system has been used on valves and flanges in both semipermanent and fully removable configurations. Fig. 10 shows the basic approach of removable insulation covers for a cryogenic valve and a bolted flange assembly. A stainless-steel safety wire, hook-and-loop strap, or tape secures the system onto the component and allows for removability as required. Aerogel composite blanket packing is applied around the valve body and between bolts, as desired, for optimum thermal performance, and butt-joint seams at the bottom allow any accumulated water to drain. Fig. 11 shows the basic installation sequence for the bolted joint of a cryogenic piping flange assembly. The combination of compressibility, flexibility, and full elastic recovery makes the LCX system mechanically robust. These robust mechanical properties allow LCX to tolerate thermal contraction/ expansion, impact, vibration, and the stress of providing structural support, without degradation of the overall thermal insulation effectiveness of the installed system.
3. Testing and evaluation of materials Representative LCX systems and their individual materials were tested and evaluated in three areas. For baseline thermal performance, including effective thermal conductivity and heat flux, liquid nitrogen (LN2) boiloff calorimetry was performed, using the cylindrical Cryostat-100 instrument. Mechanical tests of representative LCX specimens were performed to determine the deflection and recovery under compressive loads. Various environmental exposure tests, including weathering, LN2 immersion, and water absorption were also performed.
3.1. Thermal performance testing Using the Cryostat-100 instrument, the KSC Cryogenics Test Laboratory tested five layered composite test specimens. Cryostat-100 is a guarded LN2 boiloff calorimeter with a 1 m-long
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J.E. Fesmire / Cryogenics xxx (2015) xxx–xxx Table 2 Summary of thermal performance results for Cryostat-100 testing.
a b
Test series
Description
A161 Test 2 Test 3
RP (five layers)
A162 Test 1 Test 2
RA (five layers)
A163 Test 2 Test 4
C5/RP/C5/RP/RA
A166 Test 2 Test 3 Test 4
C10/RP/C10/RP/RA
CVPb (millitorr)
Boiloff flow rate (sccm)
Q (W)
kea (mW/m K)
Heat flux (q)a (W/m2)
760,000 760,000
17,400 16,000
72.0 66.2
35.2 32.4
189 173
760,000 760,000
13,900 14,500
57.5 60.0
28.2 29.4
166 173
760,000 760,000
11,900 12,800
49.2 53.0
24.1 25.9
138 148
760,000 760,000 760,000
8900 8800 9500
36.8 36.4 39.3
18.0 17.8 19.2
79.0 78.1 84.4
Boundary temperatures are approximately 293 K and 78 K; ASTM C1774, Annex A1. CVP = cold vacuum pressure (residual gas is nitrogen).
20 18
Displacement (mm)
16 14 12 10 8 6 Run 1
4 2 0
Run 2 0
5
10
15
20
25
30
35
40
Compressive Load (kPa) Fig. 13. Displacement as a function of compressive load for a six-layer LCX stack with 39 mm nominal thickness. The test specimen is shown to be settled (run 2) after its initial compression (run 1).
Fig. 14. Compression recovery test fixture showing the installation of a six-layer test specimen.
cold mass for testing cylindrical test specimens and complies with ASTM C1774, Annex A1 [8]. A simplified schematic of the experimental apparatus is given in Fig. 12. The warm and cold boundary temperatures are approximately 293 K and 78 K, respectively, and the cold vacuum pressure (CVP) environment was set at one
atmosphere pressure (760 torr) with gaseous nitrogen. The steady-state boiloff flow rate measurements enable the calculation of effective thermal conductivity (ke) and heat flux (q). Physical characteristics of the test specimens are summarized in Table 1. The following designations apply: RP (compressible radiant barrier layer, standard plastic backed), RA (compressible radiant barrier layer, aluminum backed outer surface), C5 (aerogel blanket, 5 mm thick, without backing), and C10 (aerogel blanket, 10 mm thick, without backing). The RP and RA materials were manufactured by Reflectix, Inc. under the trade name ReflectixÒ Double Reflective Insulation and had a nominal thickness of 8 mm. The aerogel blankets were manufactured by Aspen Aerogels, Inc. under the trade name CryogelÒ and had a nominal thickness of 5 mm or 10 mm. The aerogel blankets can be purchased with backing (CZ5 or CZ10) or without backing (C5 or C10). Additional commercial products include PyrogelÒ and SpaceloftÒ which have been investigated for specialized layered type systems. Other experimental aerogel blankets made by Aspen Aerogels come in thicknesses down to 1 mm and some are optimized for ultralightweight, high-vacuum applications [9]. The results of the thermal performance tests using Cryostat-100 are summarized in Table 2. The data for ke show that the layered composite systems have thermal insulating performance on par with the best foam materials in new condition. For example, the top performing spray-on foam insulation (SOFI) material for the Space Shuttle External Tank application, material BX-265 (new), gave a ke value of 21.2 mW/m K for the same test conditions [10]. The LCX test specimen A166 reported here gave a ke of approximately 18.3 mW/m K. (There is a significant long-term difference in that the spray foams will rapidly degrade in thermal performance due to weathering exposure as well as normal aging.) The cryogenic thermal performance data for the aerogel blanket materials have been previously reported [11]. The lower limit of 12.4 mW/m K as established by similar thermal performance testing of the CryogelÒ aerogel blanket material alone. The ke for the compressible radiant barrier layers alone ranged from approximately 28 mW/m K to 35 mW/m K. The LCX systems can therefore be tailored to the application over a wide range of performance levels from about 15 mW/m K to 25 mW/m K depending on the particular combination of layer materials and their individual thicknesses.
3.2. Load–displacement mechanical testing Mechanical testing of a six-layer LCX test specimen was performed to assess its structural integrity and compliance under
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Compression (mm)
25
20
15
10
5
Fin al
St a 7 4 rt R % u Co n 5 m pr e Re ss la xe d
tR un 4 pr e Re ss la xe d m
Co
St ar
%
62
al In i
St a 2 3 rt R % u Co n 1 m pr e Re ss la xe d St ar 36 t R % u Co n 2 m pr e Re ss la xe d St a 4 9 rt R % u Co n 3 m pr e Re ss la xe d
0
Fig. 15. Compression loading recovery for a six-layer test specimen with 39 mm nominal thickness.
Fig. 16. Extreme (cryo-to-water) exposure test: during LN2 soak (left); immediately after water bath (right).
compressive loading. The 76 mm (3 in.) - diameter test article consisted of the following stackup of materials: C10, RP, C5, RP, C5, and RA (outer layer), for a total thickness of 44 mm. The settled thickness, and nominal test thickness, was 39 mm. A series of load versus displacement tests was performed using a series of known weights for precision. The test data are plotted in Fig. 13. Starting at zero displacement, the run 1 data indicate a settling displacement of approximately 5 mm after release of the compressive load. The subsequent run 2 data, starting and ending at zero displacement, show complete elastic recovery at the conclusion of the test. The six-layer LCX specimen tested is estimated to withstand compressive mechanical loadings of more than 180 kPa (26 psi), corresponding to a compression of 75% of original thickness, with full elastic recovery. The results show that the combination of layers maintains structural integrity under compressive loads and is strong to support system loads. Compression recovery testing was performed using a hydraulic press as shown in Fig. 14. The test data are plotted in Fig. 15. With a nominal starting thickness of 39 mm (settled), five runs were performed with the following amounts of compression at each run: 9 mm, 14 mm, 19 mm, 24 mm, and 29 mm (74% compression). The specimen was observed to return to the nominal thickness of 39 mm. The mechanical tests indicate the unique loadsupporting, mechanically robust capabilities of the combination of aerogel blanket and compressible barrier layers that comprise an LCX system.
3.3. Environmental exposure and cryopumping The following examples of field installations in fully exposed outdoor environments show that LCX is not degraded over time. Proper selection of the outermost layer, or application of a final overwrap (such as RA or VC material), is a key part of optimal protection from UV exposure. Water absorption tests have shown negligible mass increase even after full immersion. Three six-layer test specimens (50 mm diameter by 38 mm thickness) were tested for water absorption by immersion according to ASTM C1763 [12]. Upon immediate removal after the prescribed 24-h immersion period, the average weight increased by 5.7% (presumably because of water droplets resting in the open cut edges of the compressible barrier layers). However, each sample returned to within 0.1% of its original weight after 2 h of drying time. One of these same test specimens was then subjected to an extreme (cryo-to-water) exposure test. The sample was fully immersed in an LN2 bath and allowed to cold-soak for 5 min as shown in Fig. 16. The sample was then taken out of the cryogen and immediately put into a water bath at ambient temperature. The sample quickly thawed and showed no adverse effect or visible change from its original condition. The properly installed system should behave in the same manner by allowing for draining and venting, or freezing and thawing, during all operational cycles. Some level of cryopumping of air and moisture will naturally occur during the cooldown of any cryogenic system operating in
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Fig. 17. Tank dome insulation: schematic of vertical cylindrical tank application (top left), view of completed insulation system installed on top dome of tank (top right), and view of bottom dome of tank during installation with moisture drain/vent features (bottom).
Fig. 18. Vertical cylindrical cryogenic tank (7600-l capacity) for flight tank simulation: completed LCX installation (left); during operation, fully loaded with LN2 (right).
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Table 3 Estimated thermal performance of a 7600-l vertical cylindrical cryogenic tank with different insulation materials/systems (and comparison with Space Shuttle External Tank). System
Description
Heat load – Q (W)
Heat flux – q (W/m2)
Boiloff equivalent LN2 flow (l/min)
LCX (5-layer) LCX (3-layer) Frost Ice Space Shuttle External Tank – LO2 Tank
C10/RP/C5/RP/C5 (36 mm thick) C5/RP/C5 (18 mm thick) Just frost layer (in still air) Just ice layer (in still air) SOFI Insulation (25 mm thick)
2214 5211 13,617 50,180 100,000
90 212 555 1025 200
0.833 1.97 5.26 18.9 37.9
Fig. 19. Completed LCX installation on the Rapid Propellant Loading System at Kennedy Space Center, showing a combination of piping, valves, pipe supports, and flanges.
the ambient environment. The propensity for cryopumping increases when portions of the cold boundary are bare or inadequately covered by the first insulation layer of aerogel blanket. LCX tolerates gaps and other unavoidable imperfections in installation because the aerogel material has a substantial capacity for adsorption per unit weight. Previous results show that the sorption ratio (nitrogen to aerogel) is about seven to one by mass or 62% by volume [13]. According to previously reported experimental work, the condensed air is safely kept on the surfaces within the nanoporous network of the aerogel in a nonliquid phase, physisorbed state [14]. As the system warms up, the air and other condensables sequestered within the nanopores of the material are gradually liberated and aspirated back into the environment in a manner consistent with the natural thermal state of the overall cryogenic system. 4. Field applications A number of field applications of LCX have been made in recent years, including tanks, piping, valves, umbilical connections, and even liquid–air backpacks. Although each application is a custom design and installation, mechanical robustness and weather tolerance are common requirements of the complex cryogenic systems needed for aerospace functions. Several such applications are described. 4.1. Cryogenic tank A cryogenic test facility being constructed at Kennedy Space Center includes the Rapid Cryogenic Propellant Loading System, a flight simulator tank that holds 7570 l (2000 gallons), is configured vertically, and is constructed of single-wall stainless steel. The system is for the development of advanced cryofuel transfer systems,
including autonomous operations and rapid tank loading [15]. Transient operations with many thermal cycles provide a testbed with harsh operational requirements for any thermal insulation system. The entire system includes a storage tank, pumping skid, valve control skid, and transfer piping, constructed using singlewall (non-VJ) designs that were all subsequently insulated with LCX in different designs. The thermal performance of the tank’s thermal insulation system should be generally representative of a flight tank. This tank is approximately 5.2 m (17 ft) tall and 1.5 m (5 ft) in diameter and is designed for LN2 temperature (77 K) and an operating pressure up to 517 kPa (75 psig). The 4.3 m (14 ft)-tall cylindrical side (or barrel of the tank) is unobstructed, and ports are concentrated on the upper and lower domes, each standing 0.3 m (1 ft) tall. The total surface area of the tank is approximately 25 m2 (270 ft2). A four-layer LCX was designed and installed on this tank. The layup was CZ10 + RP + CZ5 + black vinyl overwap, for a total thickness of approximately 24 mm. The insulation approach for the tank domes is shown in Fig. 17: schematic for vertical cylindrical tank application (top left), view of completed insulation system installed on top dome (top right), and view of bottom dome of tank during installation (bottom). The layers are designed to shed the majority of rain water and condensate. The water that does migrate inward drains through channels (of various materials and installations) that run all the way to the bottom dome. The completed installation is shown in Fig. 18 (left) while the right hand view shows the integrated tank system while fully cold with liquid nitrogen. Temperature sensors were placed through the thickness of the insulation system on the side of the tank. Two Type E thermocouples were affixed on each layer. The flight tank simulator has been thermally cycled numerous times since overall system test operations began. As part of the cooldown and warmup profiles measured by 11 thermocouples installed on the surface of the tank
Please cite this article in press as: Fesmire JE. Layered composite thermal insulation system for nonvacuum cryogenic applications. Cryogenics (2015), http://dx.doi.org/10.1016/j.cryogenics.2015.10.008
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Fig. 20. Completed LCX installation on a valve skid for a cryofuel servicing system (LCH4).
wall, the insulation system layer temperatures were also monitored. The typical profile shows a starting ambient condition of approximately 296 K and the final temperature of the tank of 78 K in slightly more than 1 h from the start of cooldown. In this case, the layer temperatures after stabilization with a full tank of LN2 were 201 K (layer 1), 239 K (layer 2), and 294 K (layer 3). Of the full temperature difference of 216 K, almost 60% of this difference is produced in the first (aerogel blanket) layer. Through numerous thermal cycles for more than two years of cryogenic flow operations in varying weather conditions, no frost area was observed on the black vinyl outer surface of the tank. The 7600-l vertical cylindrical tank includes a number of appurtenances and is instrumented with temperature sensors and other devices. The design tool TISCALC (incorporating the latest Cryostat100 thermal conductivity data) was used to calculate the thermal performance [16]. The following conditions were applied: 300 K and 77 K boundary temperatures; ambient air with no convection effects; and no compensation for heat leaks through piping nozzles. Table 3 compares the heat leakage for the different systems. Comparison with spray-on foam insulation (SOFI) was based on data previously reported for the polyisocyanurate foams used on the Space Shuttle External Tank [10]. From this information, the boiloff rate was estimated to equal an LN2 flow rate of 1.9 liters per minute (0.5 gpm) for the tank without supports or connecting piping. Analysis from the tanking test measurements indicate a boiloff of 5.3 liters per minute (1.4 gpm) for the total tank system. 4.2. Cryogenic piping The cryogenic piping and components of the Rapid Cryogenic Propellant Loading System were also insulated with various LCX designs according to the component, thermal requirements, and desired level of permanence or removability. The nominal piping sizes insulated ranged from a 530-mm (6-in) pipe to a 25-mm (1-in) tube. Fig. 19 shows a portion of the piping, flanges, and valves on the simulated flight tank interface. The system is operating successfully using LN2 and has seen numerous thermal cycles and full exposure over the last several years. The industry of transport and distribution of LNG, for example, has called for the specification of removable insulation covers for cryogenic valves, but to date, LCX is the only known solution. Four valve skids for cryofuel loading systems (LNG and LO2) were insulated using LCX. The skids connect via VJ bayonets, so top performance for the remaining non-VJ piping was a priority. A seven-layer system was designed and installed (see Fig. 20) for the bare piping. Bulk-fill aerogel granules were also used in the skid-half of the bayonets for the optimum transition to the VJ lines.
The skids are part of a universal propellant servicing system for both ground testing and launch servicing for future methanefueled space launch vehicles. As shown in the field application examples, the LCX system can be used to insulate large, ‘‘clean” surface areas and complex piping systems, for example, those that are full of ports, fittings, flanges, expansion joints, supports, valves, instrumentation components, and so forth. Systems that have been impossible or highly impractical to insulate are now be readily insulated with this approach. 5. Conclusion Overcoming vapor drive toward the cold side and preventing moisture accumulation inside are the major challenges of insulating complex cryogenic equipment in the ambient environment [14]. In response, previous technologies, including cellular glass, rigid foam, and spray foam, are impractical to apply, lack mechanical robustness, and fail to meet life cycle requirements of the systems they support. The use of aerogel composite blankets can provide the lowest heat flux per unit thickness but can also present practical difficulties for certain applications. The LCX system solves these problems with a cost-effective, hybrid approach incorporating the use of aerogel blankets in practical thermal insulation systems with the unique features of breathability and, where desired, removability. Through material development and testing, the LCX system is shown to provide unique mechanical and thermal properties in its integrated and layered approach. Its layered system of materials is physically resilient against damaging mechanical effects, including compression, flexure, impact, vibration, and thermal expansion/contraction. The combination of materials and method of installation achieve low values for effective thermal conductivity by managing all modes of heat transfer (solid conduction, convection, gas conduction, and radiation). The thermal performance of the thermal insulation system is maintained through the life cycle by the hydrophobic properties of the thermal insulation and compressible barrier layers in combination with the built-in moisture draining and venting features of the installed system. Robust mechanical properties are achieved by a tailored combination of materials for both compressibility and flexibility with full elastic recovery. A number of different LCX system designs have been installed successfully for cryogenic tanks, piping, and valve control skids in the field. The LCX technology provides a practical solution for complex systems operating under dynamic, transient conditions in extreme environments. Such conditions are common for aerospace vehicles, launch pad facilities, and propulsion test stands
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and for other industry applications, such as LNG and LH2 cryofuels for transportation and power.
Acknowledgements The author acknowledges and thanks Mr. Stephen Huff of NASA Kennedy Space Center for his assistance and dedication to accomplishing the field installations and Mr. Robert Johnson, also of NASA Kennedy Space Center, for his support through NASA’s Advanced Exploration Systems project.
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Please cite this article in press as: Fesmire JE. Layered composite thermal insulation system for nonvacuum cryogenic applications. Cryogenics (2015), http://dx.doi.org/10.1016/j.cryogenics.2015.10.008