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Thermal conductivity of evacuated perlite at low temperatures as a function of load and load history
Thermal conductivity of evacuated perlite at low temperatures as a function of load and load history W . P . Dubd, L.L. Sparks and A . J . Slifka Chemical Engineering Science Division, Center for Chemical Technology, National Institute of Standards and Technology (NIST), 3 2 5 Broadway, Boulder, CO 8 0 3 0 3 , USA
Received 10 July 1990 Perlite, a powdered insulation, is commonly used in large cryogenic storage dewars. When these dewars are thermally cycled, the perlite in the evacuated space between the inner and outer vessels of the dewar experiences a changing mechanical load due to thermal expansion and contraction of the inner vessel. Thermal conductivity data were obtained using a boil-off calorimeter. The apparent thermal conductivity of evacuated perlite increases strongly with applied load. Hysteretic behaviour of the conductivity was observed w h e n perlite was subjected to cyclic mechanical loading.
Keywords: thermal conductivity; materials characterization; physical properties
Perlite is subjected to mechanical load in many cryogenic applications, but the apparent thermal conductivity* of evacuated perlite as a function of load has not been studied. Previously, the apparent thermal conductivity of evacuated perlite has been measured as a function of temperature, bulk density, particle size and chemical composition 1'2. When used as insulation between the walls of a storage dewar, contraction and expansion of the inner wall induces mechanical loading on the perlite. Also, some of the newer domestic refrigerators use evacuated plastic bags filled with perlite as insulation 3. Atmospheric pressure loads the evacuated bags of perlite surrounding the cold interior. This study was conducted to determine the effect of load and load history on the insulating ability of evacuated perlite.
for many years and had been thermally cycled an unknown number of times. When viewed under a microscope, perlite looks like glass popcorn or snow. Figure 1 is an enlarged view of new perlite before any conductivity testing or mechanical loading had occurred. The particles were generally large and angular. Figure 2 shows the new perlite after the series of conductivity tests presented here had been performed. There were more small particles, and the large particles appeared more rounded and slightly more opaque than those in the untested material. The used specimen was slightly different in appearance. Figure 3 shows used
Specimens Two specimens of perlite were provided by the Stennis Space Center. One of the specimens was 'new' perlite obtained from the Stennis Space Center stores. This new perlite was of grade IN-91 and had an average density of 160 kg m -3 (10 lb ft -3) according to the manufacturer's specifications. The other specimen was 'used' perlite, which was removed from a large, stationary, liquid hydrogen storage dewar. This dewar had been in service
*Because heat travels through evacuatedperliteby both radiation and solid conduction, the term apparent thermal conductivity is used. If heat transport were by solid conduction alone, the modifier 'apparent' would be unnecessary.
Figure 1 New perlite beforetesting (36 x)
O011 - 2275/91/O 10003 - 04 © 1991 Butterworth - Heinemann Ltd Cryogenics 1991 Vol 31 January
3
Thermal conductivity of evacuated perlite: W.P. Dubd et al. Set Screw (20)
Mylar
/
/
Z - P o w d e r Insulation Specimen
Figure 4
Figure 2
New perlite after testing ( 3 6 x )
Figure 3
Used perlite ( 3 6 x )
perlite. The average particle was much smaller and was much more rounded than in the new material. The bulk density of the used perlite was approximately twice that of the new perlite. To appreciate fully the different characters of new, tested and used perlite, it must be viewed using a binocular microscope. Changes in colour and shape, readily apparent when viewed directly, are not fully captured by black-and-white photographs.
.
Grooveto Clamp
~ Fiberglass Cloth Bellows
Bellows in Place
Holder for p o w d e r e d specimens
powder while allowing it to be evacuated and mechanically loaded. The top and bottom were of tightly stretched Mylar*, 0.022 nun (0.00085 in) thick. The bellows section was made of two layers of strong, yet flexible, fibre-glass cloth. The top and bottom hoop sets were made of aluminium alloy. These hoop sets clamped the Mylar and fibre-glass cloth together and constrained the specimen to a disk shape. The specimens were ---28 cm (11 in) in diameter and =2.3 cm (0.9 in) thick. After the specimen holder had been filled with perlite and assembled, it was placed in the boil-off calorimeter, which was then sealed and evacuated. Typically, the specimen took 2 - 3 days to outgas. When the vacuum in the specimen chamber reached 0.003 Pa (2 x 10 -5 Torr), the apparatus was cooled with liquid nitrogen (LN2). The system was typically allowed to equilibrate for about 12 h after charging with LN2. If operation with liquid helium (LHe) was needed, the LN2 was blown out, then the cryogen vessels were purged with helium gas and filled with LHe. After the apparatus had reached thermal equilibrium, the specimen was loaded with a force of ~ 1.3 kN (300 lbf). At this load the average mechanical pressure on the specimen surfaces is =18 kPa (2.7 lb in-2). * T h e use of trade names in this paper is solely for identification purposes and does not c o n s t i t u t e a product endorsement. The products mentioned are not necessarily the best or the w o r s t available for the purpose.
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All measurements of apparent thermal conductivity mentioned in this paper were performed with a 30.5 cm (12 in) guarded flat plate boil-off calorimeter 4'5. This apparatus has several unique features which allowed this study to be undertaken. The total mechanical load on the specimen may be varied from 0 to 8.9 kN (1 ton) at any time during the test. The thickness of the specimen is easily measured during the test. The specimen may be surrounded by a vacuum or any non-condensing fill gas. In order to test perlite in our apparatus, a unique specimen holder had to be designed and constructed. This specimen holder, illustrated in Figure 4, contained the
4
Cryogenics 1991 Vol 31 January
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60
90
120
150
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Tmeon (K)
Figure 5 N e w perlite: apparent thermal c o n d u c t i v i t y v e r s u s Kapp = average t e m p e r a t u r e . Load = 6 8 kPa ( 9 . 9 1 b i n - 2 ) . 0.01 37Tmean + 0 . 3 7 3
Thermal conductivity of evacuated perlite: W.P. Dub@ et al. The thickness of the specimen was then measured. When the thermal conductivity had reached the steady state, 100 conductivity data points (measured every 10 s) were stored for subsequent analysis. The load or temperature was then altered and, as soon as steady-state conditions had been attained, another data set was stored. The conductivity data presented here have an imprecision of ± 4 % as measured by one standard deviation. The estimated inaccuracy of these measurements is ± 8 % .
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Figures 5 - 9 show the conductivity data taken during the course of the experiment. Figure 5 plots the apparent thermal conductivity of new perlite versus average temperature at a load of 68 kPa (9.9 lb in-2). Figure 6 plots the conductivity of used perlite versus average temperature at a load of 68 kPa (9.9 lb in-2). These data
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Figure 6 U s e d perlite: a p p a r e n t t h e r m a l c o n d u c t i v i t y v e r s u s a v e r a g e t e m p e r a t u r e . Load = 6 8 kPa. Kaop = 0 . 0 1 3 0 T b a r + 0 . 2 5 0
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were selected from a larger data set containing measurements made at many different loads. Between the data points shown, the load on these specimens was changed, but never exceeded 68 kPa. If the complete data set were plotted, without selection of a specific load, a clear dependence of apparent thermal conductivity on temperature would not be evident. A three-dimensional plot with load, temperature and apparent thermal conductivity as the axes might be more informative but would still be confusing owing to the hysteretic dependence of the apparent thermal conductivity on load. The apparent thermal conductivity of perlite is a strong function of load and load history. Figure 7 shows the conductivity of new perlite versus increasing load at constant average temperature. The points are numbered in the order in which the data were taken. As the specimen was loaded, the conductivity increased dramatically. When the load was reduced, the conductivity also was reduced, but did not return to the original value. Repetitions of the load cycle nearly retraced the unloading curve. Data point 28 in Figure 7 shows the result of increased load after cyclic loading. Data point 28 shows the same trend as points 15, 16 and 17. All of this implies that cyclic loading has little effect on the apparent thermal conductivity at peak load. Before we obtained point number 15 on Figure 7, the specimen had been left unloaded, at room temperature, in a mechanically noisy environment for a number of days. Experience has shown that this treatment tends to allow the specimen to expand and approach its original thermal conductivity. After loading, the specimen never completely returned to its original conductivity. Figure 8 shows the apparent thermal conductivity of used perlite versus load. The used perlite specimen had a slightly lower overall conductivity but the behaviour under load was remarkably similar. Discussion
Initially, one would think that the change in apparent thermal conductivity under load was caused by a correspond-
Cryogenics 1991 Vol 31 January
5
Thermal conductivity of evacuated perlite: W.P. Dubd et al.
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Figure 9 New perlite: a p p a r e n t t h e r m a l c o n d u c t i v i t y sity. Tmean = 1 0 0 K
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versus
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ing change in bulk density or thickness. As shown in
is increased by selecting powder with more massive but equal-sized particles, then the conductivity of the bulk material decreases. This could be due to decreased radiative transport under conditions of increased reflection and scattering of thermal radiation by the more massive and therefore more opaque particles. If the bulk density is increased by mechanical loading, then the conductivity increases. This could be due to increased thermal contact between adjacent particles along the conduction path. The particles may interlock when the powder is loaded and remain interlocked to some extent when the load is decreased or removed. This could explain the hysteretic dependence of thermal conductivity on applied load. The apparent thermal conductivity of evacuated perlite is a strong function of mechanical load and mechanical load history. When perlite is subjected to mechanical loading in the direction of heat flow, the conductivity increases dramatically. When the load is decreased, but not removed entirely, the conductivity decreases only slightly. When the load is removed entirely in a mechanically noisy environment, the apparent thermal conductivity approaches, but never completely returns to, the original uncompressed value.
Figure 9, however, the bulk density of the new perlite increased just slightly more than 4% during this portion of the measurement while the conductivity nearly doubled. The bulk density was calculated using the thickness, diameter and mass of the specimen. Kropschot and Burgess ~ showed that the thermal conductivity of evacuated perlite decreased slightly with increasing bulk density. They varied the bulk density while maintaining a constant particle size. The bulk density was varied by selecting powders having more or less massive individual particles. They did not vary the bulk density by compacting the powder. Load was not measured in their experiment. Our results confirm this earlier work if data are selected with comparable load andload history. The denser used perlite showed a lower apparent thermal conductivity than the less dense new perlite when both specimens were subjected to comparable load and load history.
Conclusion The apparent thermal conductivity of evacuated perlite is not a function of bulk density alone. If the bulk density
6
Cryogenics 1991 Vol 31 January
Acknowledgement This programme was funded by the Stennis Space Center and NASA. As a contribution from the National Institute of Standards and Technology it is not subject to copyright.
References
°4 1 Kropschot, R.H. and I l u ~ [ I , R.W. Perlite for cryogenic insulation Adv Cryo Eng (1963) 8 425-436 ' 2 Kinser, G.R., Jr Johns-Malwilld~ResearchCenter, Manville, NJ, unpublished data 3 Yarbrough, D.W., Graves, R.S., Weaver, F.J. and McEIrey, D.L. The thermal resistance of perlite-based evacuated insulations for refrigerators ORNL/CON-215, NTIS Report (Sept. 1986) 4 Dul~, W.P., Sparks, L.L. and Slitka, A.J. NBS boil-off calorimeter for measuring thermal conductivity of insulating materials Adv Cryo Eng Mater (1987) 34 67-73 5 ASTM Annual Book of Standards Standard C745, Heat flux through evacuated insulations using a guarded fiat plate boiloff calorimeter, part 18, American Society for Testing and Materials, Philadelphia, PA (1987)
Received 10 July 1990 Perlite, a powdered insulation, is commonly used in large cryogenic storage dewars. When these dewars are thermally cycled, the perlite in the evacuated space between the inner and outer vessels of the dewar experiences a changing mechanical load due to thermal expansion and contraction of the inner vessel. Thermal conductivity data were obtained using a boil-off calorimeter. The apparent thermal conductivity of evacuated perlite increases strongly with applied load. Hysteretic behaviour of the conductivity was observed w h e n perlite was subjected to cyclic mechanical loading.
Keywords: thermal conductivity; materials characterization; physical properties
Perlite is subjected to mechanical load in many cryogenic applications, but the apparent thermal conductivity* of evacuated perlite as a function of load has not been studied. Previously, the apparent thermal conductivity of evacuated perlite has been measured as a function of temperature, bulk density, particle size and chemical composition 1'2. When used as insulation between the walls of a storage dewar, contraction and expansion of the inner wall induces mechanical loading on the perlite. Also, some of the newer domestic refrigerators use evacuated plastic bags filled with perlite as insulation 3. Atmospheric pressure loads the evacuated bags of perlite surrounding the cold interior. This study was conducted to determine the effect of load and load history on the insulating ability of evacuated perlite.
for many years and had been thermally cycled an unknown number of times. When viewed under a microscope, perlite looks like glass popcorn or snow. Figure 1 is an enlarged view of new perlite before any conductivity testing or mechanical loading had occurred. The particles were generally large and angular. Figure 2 shows the new perlite after the series of conductivity tests presented here had been performed. There were more small particles, and the large particles appeared more rounded and slightly more opaque than those in the untested material. The used specimen was slightly different in appearance. Figure 3 shows used
Specimens Two specimens of perlite were provided by the Stennis Space Center. One of the specimens was 'new' perlite obtained from the Stennis Space Center stores. This new perlite was of grade IN-91 and had an average density of 160 kg m -3 (10 lb ft -3) according to the manufacturer's specifications. The other specimen was 'used' perlite, which was removed from a large, stationary, liquid hydrogen storage dewar. This dewar had been in service
*Because heat travels through evacuatedperliteby both radiation and solid conduction, the term apparent thermal conductivity is used. If heat transport were by solid conduction alone, the modifier 'apparent' would be unnecessary.
Figure 1 New perlite beforetesting (36 x)
O011 - 2275/91/O 10003 - 04 © 1991 Butterworth - Heinemann Ltd Cryogenics 1991 Vol 31 January
3
Thermal conductivity of evacuated perlite: W.P. Dubd et al. Set Screw (20)
Mylar
/
/
Z - P o w d e r Insulation Specimen
Figure 4
Figure 2
New perlite after testing ( 3 6 x )
Figure 3
Used perlite ( 3 6 x )
perlite. The average particle was much smaller and was much more rounded than in the new material. The bulk density of the used perlite was approximately twice that of the new perlite. To appreciate fully the different characters of new, tested and used perlite, it must be viewed using a binocular microscope. Changes in colour and shape, readily apparent when viewed directly, are not fully captured by black-and-white photographs.
.
Grooveto Clamp
~ Fiberglass Cloth Bellows
Bellows in Place
Holder for p o w d e r e d specimens
powder while allowing it to be evacuated and mechanically loaded. The top and bottom were of tightly stretched Mylar*, 0.022 nun (0.00085 in) thick. The bellows section was made of two layers of strong, yet flexible, fibre-glass cloth. The top and bottom hoop sets were made of aluminium alloy. These hoop sets clamped the Mylar and fibre-glass cloth together and constrained the specimen to a disk shape. The specimens were ---28 cm (11 in) in diameter and =2.3 cm (0.9 in) thick. After the specimen holder had been filled with perlite and assembled, it was placed in the boil-off calorimeter, which was then sealed and evacuated. Typically, the specimen took 2 - 3 days to outgas. When the vacuum in the specimen chamber reached 0.003 Pa (2 x 10 -5 Torr), the apparatus was cooled with liquid nitrogen (LN2). The system was typically allowed to equilibrate for about 12 h after charging with LN2. If operation with liquid helium (LHe) was needed, the LN2 was blown out, then the cryogen vessels were purged with helium gas and filled with LHe. After the apparatus had reached thermal equilibrium, the specimen was loaded with a force of ~ 1.3 kN (300 lbf). At this load the average mechanical pressure on the specimen surfaces is =18 kPa (2.7 lb in-2). * T h e use of trade names in this paper is solely for identification purposes and does not c o n s t i t u t e a product endorsement. The products mentioned are not necessarily the best or the w o r s t available for the purpose.
2.5 -
/ / / / J /
20 / I/ /
8
/ / /
Experimental
E
/ / /
©
All measurements of apparent thermal conductivity mentioned in this paper were performed with a 30.5 cm (12 in) guarded flat plate boil-off calorimeter 4'5. This apparatus has several unique features which allowed this study to be undertaken. The total mechanical load on the specimen may be varied from 0 to 8.9 kN (1 ton) at any time during the test. The thickness of the specimen is easily measured during the test. The specimen may be surrounded by a vacuum or any non-condensing fill gas. In order to test perlite in our apparatus, a unique specimen holder had to be designed and constructed. This specimen holder, illustrated in Figure 4, contained the
4
Cryogenics 1991 Vol 31 January
/ /
~10 / ©
/ / / 05
O0
,30
60
90
120
150
180
Tmeon (K)
Figure 5 N e w perlite: apparent thermal c o n d u c t i v i t y v e r s u s Kapp = average t e m p e r a t u r e . Load = 6 8 kPa ( 9 . 9 1 b i n - 2 ) . 0.01 37Tmean + 0 . 3 7 3
Thermal conductivity of evacuated perlite: W.P. Dub@ et al. The thickness of the specimen was then measured. When the thermal conductivity had reached the steady state, 100 conductivity data points (measured every 10 s) were stored for subsequent analysis. The load or temperature was then altered and, as soon as steady-state conditions had been attained, another data set was stored. The conductivity data presented here have an imprecision of ± 4 % as measured by one standard deviation. The estimated inaccuracy of these measurements is ± 8 % .
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Results
Figures 5 - 9 show the conductivity data taken during the course of the experiment. Figure 5 plots the apparent thermal conductivity of new perlite versus average temperature at a load of 68 kPa (9.9 lb in-2). Figure 6 plots the conductivity of used perlite versus average temperature at a load of 68 kPa (9.9 lb in-2). These data
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I ..... 120
I .... 150
180
Figure 6 U s e d perlite: a p p a r e n t t h e r m a l c o n d u c t i v i t y v e r s u s a v e r a g e t e m p e r a t u r e . Load = 6 8 kPa. Kaop = 0 . 0 1 3 0 T b a r + 0 . 2 5 0
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Figure 8 U s e d perlite: a p p a r e n t thermal c o n d u c t i v i t y versus load• Tmean = 9 9 . 5 K
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load.
were selected from a larger data set containing measurements made at many different loads. Between the data points shown, the load on these specimens was changed, but never exceeded 68 kPa. If the complete data set were plotted, without selection of a specific load, a clear dependence of apparent thermal conductivity on temperature would not be evident. A three-dimensional plot with load, temperature and apparent thermal conductivity as the axes might be more informative but would still be confusing owing to the hysteretic dependence of the apparent thermal conductivity on load. The apparent thermal conductivity of perlite is a strong function of load and load history. Figure 7 shows the conductivity of new perlite versus increasing load at constant average temperature. The points are numbered in the order in which the data were taken. As the specimen was loaded, the conductivity increased dramatically. When the load was reduced, the conductivity also was reduced, but did not return to the original value. Repetitions of the load cycle nearly retraced the unloading curve. Data point 28 in Figure 7 shows the result of increased load after cyclic loading. Data point 28 shows the same trend as points 15, 16 and 17. All of this implies that cyclic loading has little effect on the apparent thermal conductivity at peak load. Before we obtained point number 15 on Figure 7, the specimen had been left unloaded, at room temperature, in a mechanically noisy environment for a number of days. Experience has shown that this treatment tends to allow the specimen to expand and approach its original thermal conductivity. After loading, the specimen never completely returned to its original conductivity. Figure 8 shows the apparent thermal conductivity of used perlite versus load. The used perlite specimen had a slightly lower overall conductivity but the behaviour under load was remarkably similar. Discussion
Initially, one would think that the change in apparent thermal conductivity under load was caused by a correspond-
Cryogenics 1991 Vol 31 January
5
Thermal conductivity of evacuated perlite: W.P. Dubd et al.
/<
2.2
End #28/~
2.0
1.8
d
c(b ©
1.6
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0088
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0.090 0091 0092 Density (g/crT?)
0.093
Figure 9 New perlite: a p p a r e n t t h e r m a l c o n d u c t i v i t y sity. Tmean = 1 0 0 K
0094
versus
den-
ing change in bulk density or thickness. As shown in
is increased by selecting powder with more massive but equal-sized particles, then the conductivity of the bulk material decreases. This could be due to decreased radiative transport under conditions of increased reflection and scattering of thermal radiation by the more massive and therefore more opaque particles. If the bulk density is increased by mechanical loading, then the conductivity increases. This could be due to increased thermal contact between adjacent particles along the conduction path. The particles may interlock when the powder is loaded and remain interlocked to some extent when the load is decreased or removed. This could explain the hysteretic dependence of thermal conductivity on applied load. The apparent thermal conductivity of evacuated perlite is a strong function of mechanical load and mechanical load history. When perlite is subjected to mechanical loading in the direction of heat flow, the conductivity increases dramatically. When the load is decreased, but not removed entirely, the conductivity decreases only slightly. When the load is removed entirely in a mechanically noisy environment, the apparent thermal conductivity approaches, but never completely returns to, the original uncompressed value.
Figure 9, however, the bulk density of the new perlite increased just slightly more than 4% during this portion of the measurement while the conductivity nearly doubled. The bulk density was calculated using the thickness, diameter and mass of the specimen. Kropschot and Burgess ~ showed that the thermal conductivity of evacuated perlite decreased slightly with increasing bulk density. They varied the bulk density while maintaining a constant particle size. The bulk density was varied by selecting powders having more or less massive individual particles. They did not vary the bulk density by compacting the powder. Load was not measured in their experiment. Our results confirm this earlier work if data are selected with comparable load andload history. The denser used perlite showed a lower apparent thermal conductivity than the less dense new perlite when both specimens were subjected to comparable load and load history.
Conclusion The apparent thermal conductivity of evacuated perlite is not a function of bulk density alone. If the bulk density
6
Cryogenics 1991 Vol 31 January
Acknowledgement This programme was funded by the Stennis Space Center and NASA. As a contribution from the National Institute of Standards and Technology it is not subject to copyright.
References
°4 1 Kropschot, R.H. and I l u ~ [ I , R.W. Perlite for cryogenic insulation Adv Cryo Eng (1963) 8 425-436 ' 2 Kinser, G.R., Jr Johns-Malwilld~ResearchCenter, Manville, NJ, unpublished data 3 Yarbrough, D.W., Graves, R.S., Weaver, F.J. and McEIrey, D.L. The thermal resistance of perlite-based evacuated insulations for refrigerators ORNL/CON-215, NTIS Report (Sept. 1986) 4 Dul~, W.P., Sparks, L.L. and Slitka, A.J. NBS boil-off calorimeter for measuring thermal conductivity of insulating materials Adv Cryo Eng Mater (1987) 34 67-73 5 ASTM Annual Book of Standards Standard C745, Heat flux through evacuated insulations using a guarded fiat plate boiloff calorimeter, part 18, American Society for Testing and Materials, Philadelphia, PA (1987)