Chapter 10. Cryogenics
Introduction
Materials properties
Refrigeration
Insulation
Cryostat design
Cryogenic systems
dolan swip 2009 1 Introduction
"cryo"= cold "genes" = that which generates
1877-90 liquefaction of N 2, O2 1892-98 Dewar invented vacuum flask
liquified H2 1908-11 H.K. Onnes liquefied He, discovered superconductivity
1934 Kapitza He liquefaction engine 1947 S.C. Collins liquefaction process
dolan swip 2009 2 Applications of Cryogenics Industrial gas production Food preservation Biomedical applications Bearings Elec troni cs Motors and generators Physics research Space technology Fusion research Magnets Neutral beams Vacuum pumps
dolan swip 2009 3 Mechanical Properties
Percent Elongation Mechanical ductility yield stress modulus of elasticity fati gue lif e
Failure of welds at low T. “Ductile-to-brittle Transition temperature”
dolan swip 2009 4 Thermal Properties
Specific heat (J/mole-K): R = 8. 314 J/mole -K
D = Debye Temperature D At T < 0 .08 D, 3 C = 233.8 (T/ D)
Stai nl ess s teel T(K) C (J/kg-K) 293 476 77 159 20 4.6
dolan swip 2009 5 Thermal Properties
T
Enthalpy h = ho + dT C(T) 0 Heat added per kg to raise temperature:
T2 W/M = dT C(T) = h2 –h1 T1 ElExample: A co il con ta ins 100 t onnes of C u st abili zer. If the coil energy of 0.9 GJ is dissipated by a quench, By how much does the Cu temperature rise from 4 K?
h = 0.9x109 J/105 kg = 9000 J/kg = 9 J/g Interpolating in Table, we find T = 93 K. dolan swip 2009 6 Enthalpy vs. Temperature
dolan swip 2009 7 Thermal Conductivity
structural supports coil outside: didesire low k Strength /k relative to SS -304
dolan swip 2009 8 Thermal Expansion
Example: SS-304, 4 K 300 K L/L ≈ 0.00303 Need to compute thermal stresses.
dolan swip 2009 9 Thermal Emissivity Used for radiant heat 293 K transfer in cryostats
dolan swip 2009 10 Electrical Resistivity
DdDepends on purity
B & radiation damage
dolan swip 2009 11 Vapor Pressures of Fluids
From H. Neumann, FZK Summer School, 2008 dolan swip 2009 12 Cryogenic Liquids
Cannot be liquefied above “critical temperature”.
How separate O2 from N2 ? dolan swip 2009 13 Refrigeration and Liquefaction
Joule-Thomson Effect:
If TT T Tinv(H2) = 204 K Tinv(He) = 20 K require precooling dolan swip 2009 14 Expansion Engines Ordinary expansion raises entropy, wastes energy Slow expansion into engine (piston or turbine) cooling with less entropy rise, more efficient. dolan swip 2009 15 Collins Refrigeration System 25% 29K 50% 8K 25% 12% dolan swip 2009 16 Insulation dolan swip 2009 17 Heat Conduction Example dolan swip 2009 18 Convection and Radiation Small cells (styrofoam) or vacuum convection RditiRadiation Stephen-Boltzmann Constant: = 5.67x10-8 W/m2K4 Multilayer radiation barriers: 10 shields with e = 0.20 Prad = 0.003 as large Prad ≈ kapp A(T2-T1)/L dolan swip 2009 19 Apparent Mean Thermal Conductivity 300K 77K Optimum spacing ~ 1 layer/mm Compaction kapp dolan swip 2009 20 Vapor Shielding 293 K 77 K vapor LHe, 4.2 K LN2 dolan swip 2009 21 Cryostat Design 77 K Support JxB and gravity forces, low heat leak Styrofoam 77K 293 K dolan swip 2009 22 Heat Inflows 16 TF coils Bmax = 12 T = dolan swip 2009 23 Superinsulation Installation Poor Better Best From H. Neumann, FZK Summer School, 2008 dolan swip 2009 24 Liquid Nitrogen Storage Dewar From H. Neumann, FZK Summer School, 2008 dolan swip 2009 25 MFTF-B Cryogenic System During quench 10 m3 LHe 7000 m3 gas recovery bags. dolan swip 2009 26 MFTF-B Cryogenic System 7 struts Diameter =27cm Thickness =2.9cm Each ~ 8 W dolan swip 2009 27 MFTF-B Cryogenic Systems Thermal stress limits cooling rate: 80K4K takes ~ 4-5 days. dolan swip 2009 28 Summary Cryogenic systems have many applications (industrial, food, biomedical, mechanical, electrical, physics, space, fusion). Materials properties at low T limit performance (mechanical, thermal, electrical) . Refrigeration technology well developed, but expensive. About 300 W input power per W of heat removed at 4 K. Multilayer aluminized plastic films in vacuum low kapp Structure dominates heat inflow. dolan swip 2009 29 dolan swip 2009 30