ASHRAE REFRIGERATION SI CH 47-2010 CRYOGENICS《低温学》.pdf

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1、47.1CHAPTER 47CRYOGENICSGeneral Applications . 47.1Low-Temperature Properties . 47.1Refrigeration and Liquefaction 47.6Cryocoolers 47.11Separation and Purification of Gases 47.16Equipment 47.20Low-Temperature Insulations. 47.23Storage and Transfer Systems 47.26Instrumentation 47.27Hazards of Cryogen

2、ic Systems. 47.28RYOGENICS is a term normally associated with low tem-Cperatures. However, the location on the temperature scale atwhich refrigeration generally ends and cryogenics begins has neverbeen well defined. Most scientists and engineers working in thisfield restrict cryogenics to a temperat

3、ure below 125 K, because thenormal boiling points of most permanent gases (e.g., helium, hydro-gen, neon, nitrogen, argon, oxygen, and air) occur below this tem-perature. In contrast, most common refrigerants have boiling pointsabove this temperature.Cryogenic engineering therefore is involved with

4、the design anddevelopment of low-temperature systems and components. In suchactivities the designer must be familiar with the properties of fluidsused to achieve these low temperatures as well as the physical prop-erties of components used to produce, maintain, and apply suchtemperatures.GENERAL APP

5、LICATIONSThe application of cryogenic engineering has become extensive.In the United States, for example, nearly 30% of the oxygen pro-duced by cryogenic separation is used by the steel industry to reducethe cost of high-grade steel, and another 20% is used in the chemicalprocess industry to produce

6、 a variety of oxygenated compounds.Liquid hydrogen production has risen from laboratory quantities toover 2.1 kg/s. Similarly, liquid helium demand has required the con-struction of large plants to separate helium from natural gas cryo-genically. Energy demand likewise has accelerated construction o

7、flarge base-load liquefied natural gas (LNG) plants. Applicationsinclude high-field magnets and sophisticated electronic devices thatuse the superconductivity of materials at low temperatures. Spacesimulation requires cryopumping (freezing residual gases in achamber on a cold surface) to provide the

8、 ultrahigh vacuum repre-sentative of conditions in space. This concept has also been used incommercial high-vacuum pumps.The food industry uses large amounts of liquid nitrogen to freezeexpensive foods such as shrimp and to maintain frozen food duringtransport. Liquid-nitrogen-cooled containers are

9、used to preservewhole blood, bone marrow, and animal semen for extended periods.Cryogenic surgery is performed to treat disorders such as Parkin-sons disease. Medical diagnosis uses magnetic resonance imaging(MRI), which requires cryogenically cooled superconducting mag-nets. Superconducting magnets

10、 are now an essential component inhigh-energy accelerators and target chambers. Finally, the chemicalprocessing industry relies on cryogenic temperatures to recovervaluable heavy components or upgrade the heat content of fuel gasfrom natural gas, recover useful components such as argon and neonfrom

11、air, purify various process and waste streams, and produce eth-ylene from a mixture of olefin compounds.LOW-TEMPERATURE PROPERTIESTest data are necessary because properties at low temperaturesare often significantly different from those at ambient temperatures.For example, the onset of ductile-to-br

12、ittle transitions in carbonsteel, the phenomenon of superconductivity, and the vanishing ofspecific heats cannot be inferred from property measurementsobtained at ambient temperatures.Fluid PropertiesSome thermodynamic data for cryogenic fluids are given in Chap-ter 30 of the 2009 ASHRAE HandbookFun

13、damentals. Computer-compiled tabulations include those of MIPROPS prepared by NIST;GASPAK, HEPAK, and PROMIX developed by Cryodata (Arp1998); and EES Klein (continuously updated). Some key propertiesfor selected cryogens are summarized in Table 1, including the nor-mal boiling point (i.e., boiling p

14、oint at atmospheric pressure), criticalpoint, and triple point (nominally equal to the freezing point at atmo-spheric pressure). Table 1 also presents the volumetric enthalpychange associated with evaporation at atmospheric pressure, and thevolumetric enthalpy change associated with heating saturate

15、d vaporat atmospheric pressure to room temperature. These quantities reflectthe value of the cryogen in the conventional situation (where only thelatent heat of evaporation is used) and the less typical situation wherethe sensible heat is also recovered.Several cryogens have unique properties, discu

16、ssed in the fol-lowing sections.Helium. Helium exists in two isotopic forms, the more commonbeing helium 4. The rarer form, helium 3, exhibits a much lowervapor pressure, which has been exploited in the development of thehelium dilution refrigerator to attain temperatures as low as 0.02 to0.05 K. Wh

17、enever helium is referenced without isotopic designa-tion, it can be assumed to be helium 4.As a liquid, helium exhibits two unique phases: liquid helium Iand liquid helium II (Figure 1). Helium I is labeled as the normalfluid and helium II as the superfluid because, under certain condi-tions, the f

18、luid exhibits no viscosity. The phase transition betweenthese two liquids is identified as the lambda () line. Intersection ofhelium II with the vapor pressure curve is known as the point.Immediately to the right of the line, the specific heat of helium Iincreases to a large but finite value as the

19、temperature approachesthis line; therefore, although there is no specific volume change orlatent heat associated with the helium I to II transition, a significantenergy change is required. Once below the line, the specific heatof helium II rapidly decreases to zero. Figure 2 illustrates the spe-cifi

20、c heat capacity of helium at low temperatures, both above andbelow the line, and various pressures (data from HEPAK). Noticethe sharp rise in specific heat capacity near 2.17 K (the line) at allpressures (essentially independent of pressure). Also note the spe-cific heat fluctuations at higher tempe

21、ratures, related to the normaltwo-phase behavior of a substance near its critical point.The thermal conductivity of helium I decreases with decreasingtemperature. However, once the transition to helium II has beenmade, the thermal conductivity of the liquid has no real physicalThis preparation of th

22、is chapter is assigned to TC 10.4, Ultralow-TemperatureSystems and Cryogenics.47.2 2010 ASHRAE HandbookRefrigeration (SI)meaning, yet the heat transfer characteristics of helium II are spec-tacular. As the vapor pressure above helium I is reduced, the fluidboils vigorously. As the liquid pressure de

23、creases, its temperaturealso decreases as the liquid boils away. When the temperaturereaches the point and the helium transitions to helium II, allbubbling suddenly stops. The liquid becomes clear and quiet, al-though it is still vaporizing quite rapidly at the surface. The apparentthermal conductiv

24、ity of helium II is so large that vapor bubbles donot have time to form within the body of the fluid before the heatis conducted to the surface of the liquid. Liquid helium I has a ther-mal conductivity of approximately 24 mW/(mK) at 3.3 K, whereasliquid helium II can have an apparent thermal conduc

25、tivity as largeas 85 kW/(mK), approximately six orders of magnitude larger. Thischaracteristic makes He II the ideal coolant for low-temperature ap-plications, including superconducting magnets (Barron 1985).Also, helium 4 has no triple point and requires a pressure of 2.5 MPaor more to exist as a s

26、olid below a temperature of 3 K.Figure 3 illustrates the pressure/volume diagram of helium 4 nearits vapor dome, and clearly shows the critical point and normal boil-ing point. Note that the densities of the liquid and vapor phases ofliquid helium far from the critical point differ only by a factor

27、ofabout 7.5, compared to 1000 for many substances. Also, the latentheat of vaporization for helium is only 21 kJ/kg (or 2.6 kJ/L of liquidhelium), which is very small; thus, the amount of heat that can beabsorbed by a bath of liquid helium is limited, so liquid nitrogenshielding is needed, as well a

28、s stringent thermal isolation. Noticethat the amount of energy that can be absorbed by evaporation of liq-uid helium if the sensible heat capacity of the vapor is included is farlarger: 196 kJ/L (Table 1). Therefore, the flow of the helium vaporas it warms to room temperature is often controlled to

29、ensure thatthis sensible heat is properly used.Hydrogen. A distinctive property of hydrogen is that it can exist intwo molecular forms: orthohydrogen and parahydrogen. These formsdiffer by having parallel (orthohydrogen) or opposed (parahydrogen)Table 1 Key Properties of Selected CryogensCryogenNorm

30、al Boiling Temperature,KCritical Temperature,KTriple-Point Temperature,KDensity ofSaturated Liquidat 101.325 kPa, kg/m3Density ofSaturated Vaporat 101.325 kPa, kg/m3Volumetric Enthalpy of Vaporizationat 101.325 kPa,kJ/L*Volumetric Enthalpy to Warm Vapor to 300 K at101.325 kPa, kJ/L*Helium 4.23 5.20

31、124.7 16.8 2.6 196Hydrogen 20.39 33.19 13.95 70.7 1.3 31.5 279.7Neon 27.10 44.49 24.56 1206 9.5 103 445Oxygen 90.19 154.58 54.36 1141 4.5 243 464Nitrogen 77.35 126.19 63.15 806.1 4.6 161 350Argon 87.30 150.69 83.81 1395.4 5.8 225 382Methane 111.67 190.56 90.69 422.4 1.8 216 387*Per litre of saturate

32、d liquid cryogen at 101.325 kPa.Fig. 1 Phase Diagram for Helium 4Fig. 1 Phase Diagram for Helium 4Fig. 1 Specific Heat Capacity for Helium 4 as Function ofTemperature for Various PressuresFig. 2 Specific Heat for Helium 4 as Function of Temperature for Various PressuresFig. 2 Pressure/Volume Diagram

33、 for Helium 4 near Its VaporDomeFig. 3 Pressure/Volume Diagram for Helium 4 near Its Vapor DomeCryogenics 47.3nuclear spins associated with the two atoms forming the hydrogenmolecule. At ambient temperatures, the equilibrium mixture of 75%orthohydrogen and 25% parahydrogen is designated as normalhyd

34、rogen. With decreasing temperatures, the thermodynamics shiftto 99.79% parahydrogen at 20.4 K, the normal boiling point ofhydrogen. Conversion from normal hydrogen to parahydrogen isexothermic and evolves sufficient energy to vaporize 1% of thestored liquid per hour, assuming negligible heat leak in

35、to the storagecontainer. The fractional rate of conversion is given bydx/d = kx2(1)where x is the orthohydrogen fraction at time in hours and k is thereaction rate constant, 0.0114/h. The fraction of liquid remaining ina storage dewar at time is then(2)Here mois the mass of normal hydrogen at = 0 an

36、d m is the massof remaining liquid at time . If the original composition of the liq-uid is not normal hydrogen at = 0, a new constant of integrationbased on the initial orthohydrogen concentration can be evaluatedfrom Equation (1). Figure 4 summarizes the calculations.To minimize such losses in comm

37、ercial production of liquidhydrogen, a catalyst is used to hasten the conversion from normalhydrogen to the thermodynamic equilibrium concentration duringliquefaction. Hydrous iron oxide, Cr2O3on an Al2O3gel carrier, orNiO on an Al2O3gel are used as catalysts. The latter combination isabout 90 times

38、 as rapid as the others and is therefore the preferredchoice.Figure 5 shows a pressure/volume diagram for hydrogen.Oxygen. Unlike other cryogenic fluids, liquid oxygen (LOX) isslightly magnetic. Its paramagnetic susceptibility is 1.003 at itsnormal boiling point. This characteristic has prompted the

39、 use of amagnetic field in a liquid oxygen dewar to separate the liquid andgaseous phases under zero-gravity conditions.Both gaseous and liquid oxygen are chemically reactive, particu-larly with hydrocarbon materials. Because oxygen presents a serioussafety problem, systems using liquid oxygen must

40、be maintainedscrupulously clean of any foreign matter. Liquid oxygen cleanlinessin the space industry has come to be associated with a set of elaboratecleaning and inspection specifications representing a near ultimate inlarge-scale equipment cleanliness.Nitrogen. Liquid nitrogen (LIN) is of conside

41、rable importanceas a cryogen because it is a safe refrigerant. Because it is rather inac-tive chemically and is neither explosive nor toxic, liquid nitrogen iscommonly used in hydrogen and helium liquefaction cycles as aprecoolant. Figure 6 illustrates the pressure/volume diagram fornitrogen near it

42、s vapor dome.Liquefied Natural Gas (LNG). Liquefied natural gas is theliquid form of natural gas, consisting primarily of methane, a mix-ture of heavier hydrocarbons, and other impurities such as nitro-gen and hydrogen sulfide. Liquefying natural gas reduces itsspecific volume by a factor of approxi

43、mately 600 to 1, whichmakes handling and storage economically possible despite theadded cost of liquefaction and the need for insulated transport andstorage equipment.Thermal PropertiesSpecific heat, thermal conductivity, and thermal expansivity areof major interest at low temperatures.Specific Heat

44、. Specific heat can be predicted fairly accurately bymathematical models through statistical mechanics and quantum the-ory. For solids, the Debye model gives a satisfactory representation ofspecific heat with changes in temperature. However, difficulties areencountered when applying the Debye theory

45、 to alloys and com-pounds.mmo-ln1.571.33 0.0114+- 1 . 1 8=Fig. 3 Fraction of Liquid Hydrogen Evaporated due to Ortho-Parahydrogen Conversion as Function of Storage TimeFig. 4 Fraction of Liquid Hydrogen Evaporated due to Ortho-Parahydrogen Conversion as Function of Storage TimeFig. 3 Pressure/Volume

46、 Diagram for Helium 4 near Its VaporDomeFig. 5 Pressure/Volume Diagram for Hydrogen near Its Vapor DomeFig. 3 Pressure/Volume Diagram for Nitrogen near Its VaporDomeFig. 6 Pressure/Volume Diagram for Nitrogen near Its Vapor Dome47.4 2010 ASHRAE HandbookRefrigeration (SI)Several computer programs pro

47、vide thermal data for many met-als used in low-temperature equipment. METALPAK (Arp 1997),for example, is a reference program for the thermal properties of 13metals used in low-temperature systems.The specific heat of cryogenic liquids generally decreases in amanner similar to that observed for crys

48、talline solids as the temper-ature is lowered. At low pressures, specific heat decreases with adecrease in temperature. However, at high pressures near the criti-cal point, humps appear in the specific heat curve for all cryogenicfluids.Figure 7 illustrates the specific heat capacity of several com-

49、monly used solid materials as a function of temperature. In general,specific heat decreases with decreasing temperature.Often, the specific heat must be known to determine the amountof energy to remove from a material to cool it from room temperature(300 K) to cryogenic temperatures. The integrated average specificheat is useful for this type of calculation:(3)Figure 8 illustrates the integrated average (from room tempera-ture) specific heat for various solid materials.Table 2 summarizes the integrated average spec