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ABSTRACT
Ensuring a cryogenic environment means first limiting heat loads from any source including the ambient environment. Second, it means ensuring an efficient heat exchange, which is necessary to remove heat. This article presents those characteristics of the low temperature domain that are needed for the understanding and estimation of heat transfer in cryogenics. Conduction, convection and radiation are examined in the light of the two complementary objectives of reducing and promoting heat exchanges. Practical examples are given to illustrate technical data.
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Read the articleAUTHORS
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Bertrand BAUDOUY: Doctor, Research Engineer at the French Atomic Energy and Alternative Energies Commission (CEA), Saclay, France, in the Accelerator, Cryogenics and Magnetism Department.
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Gérard DEFRESNE: Associate Professor, lecturer at the Orsay University Institute of Technology (Université de Paris Sud), member of the Computer Science Laboratory for Mechanics and Engineering Sciences (LIMSI), France
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Patxi DUTHIL: Ph.D., Research Engineer, Centre National de la Recherche Scientifique (CNRS), Institut de Physique Nucléaire, Orsay, France
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Jean-Pierre THERMEAU: CNRS research engineer at the Institut de Physique Nucléaire, Orsay, France
INTRODUCTION
At low temperatures, all three types of heat transfer are present (conduction, convection, radiation). However, their intensity is very different from that observed at room temperature. Several parameters explain this difference. The physical characteristics of materials and fluids vary by several orders of magnitude between room temperature and low temperatures. What's more, most variations in these characteristics do not evolve proportionally with temperature. This non-linear evolution of fluid and material characteristics at low temperatures complicates the evaluation of the various heat flows exchanged. It is therefore essential to know the behavior of materials and fluids in order to draw up a heat balance for a cryosystem.
Numerous industrial and scientific facilities operate at low temperatures, and have confirmed the validity of the relationships used for the thermal dimensioning of equipment in this field. The emergence of new materials for construction (composites with low thermal conductivity) and insulation (superinsulation foils) has improved the thermal and thermodynamic performance of cryosystems. Improvements in the energy performance of cryosystems and superconducting magnets have gone hand in hand with major developments in cryogenic equipment, such as that used for medical imaging.
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KEYWORDS
heat intercepts | thermal shields | convection | heat transfers | thermal insulation
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Low-temperature heat transfer
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CryoComp © Eckels Engineering Inc. 1993-2012. Database of thermal and electrical properties of materials. Software distributed in France by Cryoforum http://www.cryoforum.com
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