Precise thermal measurements were done on different types of large surface MLI samples under various boundary conditions. The measurements were focused on the use of MLI for large industrial plants considering quick and simple installation. The results of the measurements aim at optimising MLI parameters, which control the thermal behaviour. Practical recommendations of MLI materials and their installation are given

Benda, V

Bozzini, D

Riddone, G

Vandoni, Giovanna

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsMEASUREMENT ON DIFFERENT MLI SYSTEMS BETWEEN 77 K AND 4 KAND THEIR APPLICATION IN CRYOGENIC ENGINEERINGV. Benda, D. Bozzini, G. Riddone and G. VandoniPrecise thermal measurements were done on different types of large surface MLI samples under variousboundary conditions. The measurements were focused on the use of MLI for large industrial plantsconsidering quick and simple installation. The results of the measurements aim at optimising MLIparameters, which control the thermal behaviour. Practical recommendations of MLI materials and theirinstallation are given.LHC-ACR,  LHC-CRI,  LHC-ECRPresented at the 6th IIR International Conference "Cryogenics 2000"10-13 October 2000, Praha, Czech RepublicGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 457Abstract11 December 20001MEASUREMENT ON DIFFERENT MLI SYSTEMS BETWEEN 77 K AND 4 KAND THEIR APPLICATION IN CRYOGENIC ENGINEERINGV. Benda, D. Bozzini, G. Riddone and G. VandoniDivision LHC, CERN, 1211 Geneva 23ABSTRACTPrecise thermal measurements were done on different types of large surface MLI samplesunder various boundary conditions. The measurements were focused on the use of MLI forlarge industrial plants considering quick and simple installation. The results of themeasurements aim at optimising MLI parameters, which control the thermal behaviour.Practical recommendations of MLI materials and their installation are given.INTRODUCTIONMultilayer insulation (MLI) has been used in cryogenic application for more than 40 years,mainly to operate between 300 K and 77 K or lower. In this region MLI is well investigatedand many results and recommendations are found in literature [1,2]. Less effort has beendone to investigate MLI between 77 K and 4 K or below, so investigations in thistemperature range were launched at CERN in the framework of the LHC project.The heat flow through MLI is shared between radiation, conduction and convection in away depending on the temperature range [3]. Under good vacuum conditions, whilebetween 300 K and 77 K approximately 9/10 of the total heat flux are due to radiation,between 77 K and 4 K radiation accounts for only 1/10 of the heat flux, the main fractionbeing due to solid conduction of the MLI itself. The conductivity depends on the spacermaterial, on its configuration and on the compression of the blanket. The ultimate heat flowwill result from a trade-off between the optimisation of the MLI elements, the installationmethods and the cost of industrial solutions applicable on a large scale.In all the measurements presented here, the MLI blanket contained 10 reflectors. Vacuumconditions vary between 10-5 Pa and 10-1 Pa. Two different laboratory set-ups are used.1. RESULTSThe first measurements we present are done on a cylindrical test cryostat with verticalorientation [4]. The MLI sample, of total surface area ~3 m2, is installed in a vacuumchamber around a vertical cylinder maintained at 4.2 K by saturated liquid helium andsurrounded by a warm cylinder maintained at 78 K by boiling liquid nitrogen. The heat fluxis measured by boil-off. Results are summarised in Table 1 and Figure 1. We measureddifferent reflector (flat or crinkled) and spacer types, bound to reduce the transverseconductivity of the blanket, and studied also the effect of a contamination of the outer layerby concrete dust.2The radiative heat flow between two surfaces depends on the boundary temperatures andemissivities as well as the view factor. To get rid of this parameter, a second test set-up ofhorizontal flat plate geometry has been built [5]. The apparatus consists of awarm-boundary and a cold-boundary plate connected to a heatmeter, onto which the MLIsample is placed. Both plates are surrounded by a thermal shield and placed in a vacuumchamber.Whereas in the vertical cryostat we could measure installation compression getting rid ofself-compression, in the horizontal set-up we only have self-compression, withoutmounting compression.Table 1: Heat flux measured in the cylindrical, vertical axis cryostat,as a function of helium residual pressure.No Type Installation/ Spacers Heat flux  [W/m2]closing 10-5 Pa 10-2 Pa1 DAMS1 blanket/tape SS 0.040 0.1602 DAMcS1 blanket/tape VS 0.022 0.0823 DAMS2 wrapped DS 0.037 0.1604 DAMS2* wrapped DS 0.049 0.2505 SAMc blanket/tape none 0.032 0.1906 AlS1 blanket/tape SS 0.100 0.2108 DAMVS1 blanket/ Velcro VS 0.063 0.1509 DAMcVS1 blanket/ Velcro VS 0.050 0.11010 DAMVS1 blanket/ Velcro DS 0.055 0.130Figure 1: Heat flux measured in the cylindrical, vertical axis cryostat,as a function of helium residual pressure.In the horizontal cryostat, we measured samples with different configurations of a Velcro1closing, and compared them with the same samples without Velcro. The results at 77 Kwarm boundary are given in Table 2. For better comparison with Table 1, we translated thebare results into the cylindrical geometry of the vertical cryostat, with copper(emissivity 0.07 at 77 K) as the emitter surface. Here, the vacuum is kept below 10-5 Pa.DAM Double aluminised Mylar,SAM Simple aluminised Mylar,c Crinkled reflectorS1 Fibreglass paper spacerS2 Tulle net spacer,V Velcro fastener,* External layer pollutedwith concrete powderSS Single spacerDS Double number of spacersVS Number of spacers variablefrom layer to layer: coldside, 3 spacers between 2reflectors, warm side, 1spacer between reflectors,for a total of 15 spacers.1010010001.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03Residual pressure [m bar]Heat flux [mW/m2]2 DAMcS13 DAMS25 SAMc6  AlS19 DAMcVS132. DISCUSSIONPrevious results showed that at good vacuum, the heat flux is nearly the same if the coldsurface is protected with MLI or with one layer of Al foil [2,4]. The utility of multiplelayers appears at poor vacuum, where the larger the number of layers the better theinsulation [3]. A multi-layer blanket made of 0.05 mm thick Al reflectors with conventionalspacers, yields a higher heat flux than with aluminised MylarTM 2 reflectors ([6] andcurve 6). The reason is the large longitudinal conductivity of the Al foil, which enhancesthe effect of thermal bridges, as well as a larger transverse conductivity resulting from theplasticity of the foil and the consequent number of point contacts.Crinkled MLI always lowers the heat flux. Crinkling reduces the number of point contacts,and hence the transverse conductance. In a cylindrical geometry crinkled reflectorscompensate for the length difference between layers, the spacers taking up the lengthdifference by deformation. The same advantage is obtained with a reflector in one piece,wrapped in 10 layers (Table 1, 3-4), though this is not compatible with all geometries. Athicker spacer material (S1 versus S2) can reduce the heat flux at low residual pressure.Crinkled SAM without spacers is cheap and performs well from a thermal point of view.Crinkled reflectors, alternated with a number of spacers variable from layer to layer, yieldthe lowest heat flux over the whole pressure range (as low as 22 mW/m2 at 10-5 Pa).Table 2: Heat flux in the flat-plate geometry, at 77 K warm boundary, acronyms explained in table 1No Type Spacers Installation Heat flux [W/m2]1 DAMS2 SS 1 circular whole blanket 0.0302 DAMS2 DS 1 circular whole blanket 0.0273 DAMS1 DS 1 circular whole blanket 0.0264 DAMS2 SS 2 semi-circular blankets connected with Velcro,overlapping junction0.0505 DAMS2 SS 2 semi-circular blankets connected with Velcro, edge-to-edge junction0.0426 DAMS2 SS 2 semi-circular blankets connected with Velcro, edge-to-edge, junction covered with aluminized Mylar0.0387 DAMS2 DS 2 semi-circular blankets connected with Velcro,overlapping junction0.063A Velcro closing always increases the heat loads, particularly in good vacuum, where theheat flux exceeds 50 mW/m2. An edge-to-edge junction is to be preferred with respect tooverlapping the blanket edges, which thermally bridges the warmest layer to the coldestone. The radiation leak through the slit onto the Velcro is effectively screened by onestripe of aluminised Mylar. The measurements also show the effect of local compressiondue to the stitching of the Velcro to the blankets (Table 2, samples 4-7), an effect which isfurther increased by a higher number of spacers with the same tension of the stitches(Table 2, samples 4 and 7). In the absence of Velcro, doubling the number of spacers hasgiven a barely sizeable effect in horizontal geometry.Experience shows that a correct installation of the blankets is as important as the choice ofthe materials, the installation accounting for a spread in the heat flux by a factor 2. Oneadvantage of using Velcro in industrial applications is that the dependence on the abilityof the installator is reduced, allowing for quicker installation and hence reducing the costs.4A contamination by concrete dust (present in installation areas) of the outer layer affects theheat flux, but does not result in a large effect if the spacers are of good quality, since it onlyincreases the emissivity of the outer layer and hence only adds to the less consequentialeffect of radiation with respect to the more determinant effect of solid conduction.3. THERMAL CONTRACTION OF MLI AND VELCRO®Industrial applications need simpler solutions than hand-stitching or adhesive tapefastening. The use of Velcro fastener stitched to the blanket requires some prudence.Results on two measured samples, presented in Table 3, show that the thermal contractioncan vary from one sample to another. The differential thermal contraction between MLI andthe type of Velcro must be minimised to avoid tearing or deforming the blanket.Table 3: Thermal contraction of Velcro and MLI for 2 different samples between 300 K and 77 KSample 1 Sample 2Velcro [mm/m] MLI [mm/m] Velcro [mm/m] MLI [mm/m]2.4 +/-0.5 2.4 +/-0.5 1.3 +/-0.5 2.8 +/-0.5CONCLUSIONSWhen choosing a MLI to work between 77 K and 4 K, the choice should be made on acase-by-case basis, depending on the operational vacuum conditions, the geometry andorientation, the ease of installation and the quantity.Light materials reduce the self-compression in a horizontal set-up. Crinkled reflectors alsoreduce the transverse conductivity, and have dimensional advantages in a cylindricalsystem. Varying the number of spacers from layer to layer permits to put more spacerswhere needed and improves the insulation without making the blanket uselessly heavy.The installation should ensure a low blanket density. Velcro, with respect to adhesive tapeor hand stitching, helps in making the installation more reproducible, and allows fasterinstallation, but should be applied with caution, limiting its length, evaluating thermalcontraction, and closing slits at the junction by a stripe of aluminised MylarTM.REFERENCES[1] T.Nast, Multilayer Insulation Systems, in Handbook of Cryogenic Engineering, ed.J.G.Weisend II, Taylor & Francis, Philadelphia (1998),[2] M. G. Kaganer, Thermal Insulation in Cryogenic Engineering, Jerusalem, Israel Program forScientific Translation, (1969),[3] M.Chorowski et al., Optimisation of Multilayer Insulation - An Engineering Approach, paperpresented at this conference,[4] Ph. Lebrun, et al., Investigation and qualification of thermal insulation systems between 80 Kand 4.2 K, Cryogenics 1992 Vol 32 ICMC Supplement 44-47,[5] D.Camacho et al., Heat Flow Measurements on LHC Components, LHC-Project-Report 329,(1999)[6] V. Benda et al., Qualification of multilayer insulation systems between 80 K and 4.2 K, Proc.3rd. Int. Conf. Kryogenika'94, Usti nad Labem, (1994).                                                          1 Velcro, is the Registered Trade Mark of Velcro Industries B.V2  MylarTM, is a  trademark of DuPont de Nemours

Measurement on Different MLI Systems Between 77 K and 4 K and their Application in Cryogenic Engineering

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