ASHRAE REFRIGERATION IP 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 235F (225R),because the normal boiling points of most permanent gases (e.g.,helium, hydrogen, neon, nitrogen, argon, oxygen, and air) occurbelow this temperature. In contrast, most common refrigerants haveboiling points above this temperature.Cryogenic engineering therefore is involved wit

4、h 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 A

5、PPLICATIONSThe 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 produ

6、ce a variety of oxygenated compounds.Liquid hydrogen production has risen from laboratory quantities toover 200 tons/day. Similarly, liquid helium demand has required theconstruction of large plants to separate helium from natural gascryogenically. Energy demand likewise has accelerated construc-tio

7、n of large base-load liquefied natural gas (LNG) plants. Appli-cations include high-field magnets and sophisticated electronicdevices that use the superconductivity of materials at low tempera-tures. Space simulation requires cryopumping (freezing residualgases in a chamber on a cold surface) to pro

8、vide the ultrahigh vac-uum representative of conditions in space. This concept has alsobeen used in commercial 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 contain

9、ers are 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

10、 magnets 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 n

11、eonfrom 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 ducti

12、le-to-brittle 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 Han

13、dbookFundamentals. 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., b

14、oiling point 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

15、saturated 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 propertie

16、s, discussed 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 0.03 to0.

17、09R. Whenever 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

18、, the fluid 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

19、as the 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 s

20、pe-cific 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 455.76F (3.9R) (the line) at all pressures (essentially independent of pressure). Alsonote the specific heat fluctuations a

21、t higher temperatures, related tothe normal two-phase behavior of a substance near its criticalpoint.The thermal conductivity of helium I decreases with decreasingtemperature. However, once the transition to helium II has beenThis preparation of this chapter is assigned to TC 10.4, Ultralow-Temperat

22、ureSystems and Cryogenics.47.2 2010 ASHRAE HandbookRefrigerationmade, the thermal conductivity of the liquid has no real physicalmeaning, 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 pre

23、ssure decreases, 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 c

24、onductivity 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 0.0139 Btu/hftR, whereasliquid helium II can have an apparent thermal co

25、nductivity as largeas 49,112 Btu/hftR, approximately six orders of magnitudelarger. This characteristic makes He II the ideal coolant for low-temperature applications, including superconducting magnets (Bar-ron 1985). Also, helium 4 has no triple point and requires a pressureof 360 psia or more to e

26、xist as a solid below a temperature of 455F(5R).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 o

27、nly by a factor ofabout 7.5, compared to 1000 for many substances. Also, the latentheat of vaporization for helium is only 9 Btu/lb (or 70 Btu/ft3ofliquid helium), which is very small; thus, the amount of heat that canbe absorbed by a bath of liquid helium is limited, so liquid nitrogenshielding is

28、needed, as well as 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: 5260 Btu/lb3(Table 1). Therefore, the flow of the heliumvapor as it warms to room temperature is

29、often controlled to ensurethat this 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

30、 CryogensCryogenNormal Boiling Temperature,RCritical Temperature,RTriple-Point Temperature,RDensity of SaturatedLiquid at1atm, lbm/ft3Density of SaturatedVapor at1atm, lbm/ft3Volumetric Enthalpy of Vaporization at 1atm, Btu/ft3*Volumetric Enthalpy to Warm Vapor to 537R at 1atm, Btu/ft3*Helium 7.61 9

31、.35 7.787 1.046 69.4 5212Hydrogen 36.70 59.74 25.12 4.420 0.0836 845.5 4005Neon 48.79 80.09 44.21 75.35 0.5979 2774 7476Oxygen 162.34 278.25 97.85 71.24 0.2789 6535 8707Nitrogen 139.24 227.15 113.67 50.32 0.2879 4303 6370Argon 157.14 271.24 150.85 87.11 0.3604 6035 7629Methane 201.00 343.02 163.25 2

32、6.37 0.1134 5790 7125*Per cubic foot of saturated liquid cryogen at 1 atm.Fig. 1 Phase Diagram for Helium 4Fig. 1 Phase Diagram for Helium 4Fig. 2 Specific Heat Capacity for Helium 4 as Function ofTemperature for Various PressuresFig. 2 Specific Heat for Helium 4 as Function of Temperature for Vario

33、us PressuresFig. 3 Pressure/Volume Diagram 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 2

34、5% parahydrogen is designated as normalhydrogen. With decreasing temperatures, the thermodynamics shiftto 99.79% parahydrogen at 423F (36.7R), the normal boilingpoint of hydrogen. Conversion from normal hydrogen to parahydro-gen is exothermic and evolves sufficient energy to vaporize 1% ofthe stored

35、 liquid per hour, assuming negligible heat leak into the stor-age container. 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 th

36、en(2)Here mois the mass of normal hydrogen at = 0 and 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 summari

37、zes the calculations.To minimize such losses in commercial 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 a

38、s catalysts. The latter combination isabout 90 times 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

39、 boiling point. This characteristic has prompted the 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 seri

40、oussafety problem, systems using liquid oxygen must 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 clean

41、liness.Nitrogen. Liquid nitrogen (LIN) is of considerable 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 illustr

42、ates the pressure/volume diagram fornitrogen near its 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 g

43、as reduces itsspecific volume by a factor of approximately 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 ar

44、eof major interest at low temperatures.Specific Heat. 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, diffic

45、ulties areencountered when applying the Debye theory to alloys and com-pounds.mmo-ln1.571.33 0.0114+- 1 . 1 8=Fig. 4 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 Conversi

46、on as Function of Storage TimeFig. 5 Pressure/Volume Diagram for Helium 4 near Its VaporDomeFig. 5 Pressure/Volume Diagram for Hydrogen near Its Vapor DomeFig. 6 Pressure/Volume Diagram for Nitrogen near Its VaporDomeFig. 6 Pressure/Volume Diagram for Nitrogen near Its Vapor Dome47.4 2010 ASHRAE Han

47、dbookRefrigerationSeveral computer programs provide 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 decreas

48、es in amanner similar to that observed for crystalline 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 illustr

49、ates the specific heat capacity of several com-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(80.3F) 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 va