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					Mechanical Engineers’ Handbook: Energy and Power, Volume 4, Third Edition. Edited by Myer Kutz Copyright  2006 by John Wiley & Sons, Inc.

CHAPTER 13 CRYOGENIC SYSTEMS
Leonard A. Wenzel
Lehigh University Bethlehem, Pennsylvania

1 2

CRYOGENICS AND CRYOFLUID PROPERTIES CRYOGENIC REFRIGERATION AND LIQUEFACTION CYCLES 2.1 Cascade Refrigeration 2.2 The Linde or Joule–Thomson Cycle 2.3 The Claude or Expander Cycle 2.4 Low-Temperature Engine Cycles CRYOGENIC HEAT-TRANSFER METHODS 3.1 Coiled-Tube-in-Shell Exchangers 3.2 Plate-Fin Heat Exchangers 3.3 Regenerators INSULATION SYSTEMS 4.1 Vacuum Insulation 4.2 Superinsulation 4.3 Insulating Powders and Fibers MATERIALS FOR CRYOGENIC SERVICE 5.1 Materials of Construction 5.2 Seals and Gaskets

5.3 465 6 471 471 472 473 479 483 484 485 486 8 493 494 495 496 9 497 497 508

Lubricants

508

SPECIAL PROBLEMS IN LOW-TEMPERATURE INSTRUMENTATION 6.1 Temperature Measurement 6.2 Flow Measurement 6.3 Tank Inventory Measurement EXAMPLES OF CRYOGENIC PROCESSING 7.1 Air Separation 7.2 Liquefaction of Natural Gas 7.3 Helium Recovery and Liquefaction SUPERCONDUCTIVITY AND ITS APPLICATIONS 8.1 Superconductivity 8.2 Applications of Superconductivity CRYOBIOLOGY AND CRYOSURGERY REFERENCES

508 509 510 511 511 511 514 520 521 521 524 526 529

7

3

4

5

1

CRYOGENICS AND CRYOFLUID PROPERTIES
The science and technology of deep refrigeration processing occurring at temperatures lower than about 150 K is the field of cryogenics (from the Greek kryos, icy cold). This area has developed as a special discipline because it is characterized by special techniques, requirements imposed by physical limitations, and economic needs, and unique phenomena associated with low-thermal-energy levels. Compounds that are processed within the cryogenic temperature region are sometimes called cryogens. There are only a few of these materials; they are generally small, relatively simple molecules, and they seldom react chemically within the cryogenic region. Table 1 lists the major cryogens along with their major properties, and with a reference giving more complete thermodynamic data. All of the cryogens except hydrogen and helium have conventional thermodynamic and transport properties. If specific data are unavailable, the reduced properties correlation can be used with all the cryogens and their mixtures with at least as much confidence as the correlations generally allow. Qualitatively T–S and P–H diagrams such as those of Figs. 1

465

466

Cryogenic Systems

Table 1 Properties of Principal Cryogens Normal Boiling Point Name Helium Hydrogen Deuterium Neon Nitrogen Air Carbon monoxide Fluorine Argon Oxygen Methane Krypton Nitric oxide Nitrogen trifluoride Refrigerant-14 Ozone Xenon Ethylene T (K) 4.22 20.39 23.56 27.22 77.33 78.78 82.11 85.06 87.28 90.22 111.72 119.83 121.50 144.72 145.11 161.28 164.83 169.39 Liquid Density (kg / m3) 123.9 70.40 170.0 1188.7 800.9 867.7 783.5 1490.6 1390.5 1131.5 421.1 2145.4 1260.2 1525.6 1945.1 1617.8 3035.3 559.4 Latent Heat (J / kg mole) 91,860 902,300 1,253,000 1,737,000 5,579,000 5,929,000 6,024,000 6,530,000 6,504,000 6,801,000 8,163,000 9,009,000 13,809,000 11,561,000 11,969,000 14,321,000 12,609,000 13,514,000 Critical Point T (K) 5.28 33.28 38.28 44.44 126.17 132.9 144.2 151.2 154.8 190.61 209.4 179.2 233.9 227.7 261.1 289.8 282.7 P (kPa) 227 1296 1648 2723 3385 3502 5571 4861 5081 4619 5488 6516 4530 3737 5454 5840 5068 Triple Point T (K) 14.00 18.72 26.28 63.22 68.11 83.78 54.39 90.67 116.00 108.94 89.17 161.39 104.00 P (kPa) 7.20 17.10 43.23 12.55 15.38 Reference 1 2, 3 4 5 6 7, 8 9 10 11, 12, 13 6 14 15

0.14 11.65 73.22

0.12 81.50 0.12

16 17 18

and 2 differ among cryogens only by the location of the critical point and freezing point relative to ambient conditions. Air, ammonia synthesis gas, and some inert atmospheres are considered as single materials although they are actually gas mixtures. The composition of air is shown in Table 12. If a thermodynamic diagram for air has the lines drawn between liquid and vapor boundaries where the pressures are equal for the two phases, these lines will not be at constant temperature, as would be the case for a pure component. Moreover, these liquid and vapor states are not at equilibrium, for the equilibrium states have equal Ts and Ps, but differ in composition. That being so, one or both of these equilibrium mixtures is not air. Except for this difference the properties of air are also conventional. Hydrogen and helium differ in that their molecular mass is small in relation to zeropoint-energy levels. Thus quantum differences are large enough to produce measurable changes in gross thermodynamic properties. Hydrogen and its isotopes behave abnormally because the small molecular weight allows quantum differences stemming from different molecular configurations to affect total thermodynamic properties. The hydrogen molecule consists of two atoms, each containing a single proton and a single electron. The electrons rotate in opposite directions as required by molecular theory. The protons, however, may rotate in opposed or parallel directions. Figure 3 shows a sketch of the two possibilities, the parallel rotating nuclei identifying orthohydrogen and the opposite rotating nuclei identifying the para-hydrogen. The quantum mechanics exhibited by these two molecule forms are different, and produce different thermodynamic properties. Ortho- and para-hydrogen each have conventional thermodynamic properties. However, ortho- and para-hydrogen are interconvertible with the equilibrium fraction of pure H2 existing in para form dependent on temperature, as shown in Table 2. The natural ortho- and para-hydrogen reaction is a relatively slow one and of second order19:

1

Cryogenics and Cryofluid Properties

467

Figure 1 Skeletal T–S diagram.

dx d

0.0114x 2

at

20 K

(1)

where is time in hours and x is the mole fraction of ortho-hydrogen. The reaction rate can be greatly accelerated by a catalyst that interrupts the molecular magnetic field and possesses high surface area. Catalysts such as NiO2 / SiO2 have been able to yield some of the highest heterogeneous reaction rates measured.20 Normally hydrogen exists as a 25 mole % p-H2 , 75 mole % o-H2 mix. Upon liquefaction the hydrogen liquid changes to nearly 100% p-H2 . If this is done as the liquid stands in an insulated flask, the heat of conversion will suffice to evaporate the liquid, even if the insulation is perfect. For this reason the hydrogen is usually converted to para form during refrigeration by the catalyzed reaction, with the energy released added to the refrigeration load. Conversely, liquid para-hydrogen has an enhanced refrigeration capacity if it is converted to the equilibrium state as it is vaporized and warmed to atmospheric condition. In certain applications recovery of this refrigeration is economically justifiable.

468

Cryogenic Systems

Figure 2 Skeletal P–H diagram.

Helium, though twice the molecular weight of hydrogen, also shows the effects of flow molecular weight upon gross properties. The helium molecule is single-atomed and thus free from ortho–para-type complexities. Helium was liquefied conventionally first in 1908 by Onnes of Leiden, and the liquid phase showed conventional behavior at atmospheric pressure. As temperature is lowered, however, a second-order phase change occurs at 2.18 K (0.05 atm) to produce a liquid called HeII. At no point does solidification occur just by evacuating the liquid. This results from the fact that the relationship between molecular volume, thermal energy (especially zero-point energy), and van der Waals attractive forces is such that the atoms cannot be trapped into a close-knit array by temperature reduction alone. Eventually, it was found that helium could be solidified if an adequate pressure is applied, but that the normal liquid helium (HeI)–HeII phase transition occurs at all pressures

Figure 3 Molecular configurations of (a) para- and (b) ortho-hydrogen.

1

Cryogenics and Cryofluid Properties

469

Table 2 Equilibrium Para-Hydrogen Concentration as a Function of T (K) T (K) 20 30 40 60 80 100 150 273 500 Equilibrium Percentage of Para-Hydrogen 99.82 96.98 88.61 65.39 48.39 38.51 28.54 25.13 25.00

up to that of solidification. The phase diagram for helium is shown in Fig. 4. The HeI–HeII phase change has been called the lambda curve from the shape of the heat capacity curve for saturated liquid He, as shown in Fig. 5. The peculiar shape of the heat capacity curve produces a break in the curve for enthalpy of saturated liquid He as shown in Fig. 6. HeII is a unique liquid exhibiting properties that were not well explained until after 1945. As liquid helium is evacuated to increasingly lower pressures, the temperature also drops along the vapor-pressure curve. If this is done in a glass vacuum-insulated flask, heat leaks into the liquid He causing boiling and bubble formation. As the temperature approaches 2.18 K, boiling gets more violent, but then suddenly stops. The liquid He is completely quiescent. This has been found to occur because the thermal conductivity of HeII is extremely large. Thus the temperature is basically constant and all boiling occurs from the surface where the hydrostatic head is least, producing the lowest boiling point. Not only does HeII have very large thermal conductivity, but it also has near zero viscosity. This can be seen by holding liquid He in a glass vessel with a fine porous bottom such that normal He does not flow through. If the temperature is lowered into the HeII

Figure 4 Phase diagram for helium.

470

Cryogenic Systems

Figure 5 Heat capacity of saturated liquid 4He.

Figure 6 Temperature–entropy diagram for saturation region of 4He.

2

Cryogenic Refrigeration and Liquefaction Cycles

471

region, the helium will flow rapidly through the porous bottom. Flow does not seem to be enhanced or hindered by the size of the frit. Conversely, a propeller operated in liquid HeII will produce a secondary movement in a parallel propeller separated from the first by a layer of liquid HeII. Thus HeII has properties of finite and of infinitesimal viscosity. These peculiar flow properties are also shown by the so-called thermal-gravimetric effect. There are two common demonstrations. If a tube with a finely fritted bottom is put into liquid HeII and the helium in the tube is heated, liquid flows from the main vessel into the fritted tube until the liquid level in the tube is much higher than that in the main vessel. A second, related, experiment uses a U-tube, larger on one leg than on the other with the two sections separated by a fine frit. If this tube is immersed, except for the end of the narrow leg, into liquid HeII and a strong light is focused on the liquid He above the frit, liquid He will flow through the frit and out the small tube opening producing a fountain of liquid He several feet high. These and other experiments21 can be explained through the quantum mechanics of HeII. The pertinent relationships, the Bose–Einstein equations, indicate that HeII has a dual nature: it is both a ‘‘superfluid’’ which has zero viscosity and infinite thermal conductivity among other special properties, and a fluid of normal properties. The further the temperature drops below the lambda point the greater the apparent fraction of superfluid in the liquid phase. However, very little superfluid is required. In the flow through the porous frit the superfluid flows, the normal fluid is retained. However, if the temperature does not rise, some of the apparently normal fluid will apparently become superfluid. Although the superfluid flows through the frit, there is no depletion of superfluid in the liquid He left behind. In the thermogravimetric experiments the superfluid flows through the frit but is then changed to normal He. Thus there is no tendency for reverse flow. At this point applications have not developed for HeII. Still, the peculiar phase relationships and energy effects may influence the design of helium processes, and do affect the shape of thermodynamic diagrams for helium.

2

CRYOGENIC REFRIGERATION AND LIQUEFACTION CYCLES
One characteristic aspect of cryogenic processing has been its early and continued emphasis on process efficiency, that is, on energy conservation. This has been forced on the field by the very high cost of deep refrigeration. For any process the minimum work required to produce the process goal is Wmin T0 S H (2) where Wmin is the minimum work required to produce the process goal, S and H are the difference between product and feed entropy and enthalpy, respectively, and T0 is the ambient temperature. Table 3 lists the minimum work required to liquefy 1 kg-mole of several common cryogens. Obviously, the lower the temperature level the greater the cost for unit result. The evident conflict in H2 and He arises from their different molecular weights and properties. However, the temperature differences from ambient to liquid H2 temperature and from ambient to liquid He temperatures are similar. A refrigeration cycle that would approach the minimum work calculated as above would include ideal process steps as, for instance, in a Carnot refrigeration cycle. The cryogenic engineer aims for this goal while satisfying practical processing and capital cost limitations.

2.1

Cascade Refrigeration
The cascade refrigeration cycle was the first means used to liquefy air in the United States.22 It uses conveniently chosen refrigeration cycles, each using the evaporator of the previous

472

Cryogenic Systems
Table 3 Minimum Work Required to Liquefy Some Common Cryogens Normal Boiling Point (K) 4.22 20.39 27.11 77.33 78.8 90.22 111.67 184.50 239.78 Minimum Work of Liquefaction (J / mole) 26,700 23,270 26,190 20,900 20,740 19,700 16,840 9,935 3,961

Gas Helium Hydrogen Neon Nitrogen Air Oxygen Methane Ethane Ammonia

fluid cycle as condenser, which will produce the desired temperature. Figures 7 and 8 show a schematic T–S diagram of such a cycle and the required arrangement of equipment. Obviously, this cycle is mechanically complex. After its early use it was largely replaced by other cryogenic cycles because of its mechanical unreliability, seal leaks, and poor mechanical efficiency. However, the improved reliability and efficiency of modern compressors has fostered a revival in the cascade cycle. Cascade cycles are used today in some base-load natural gas liquefaction (LNG) plants23 and in the some peak-shaving LNG plants. They are also used in a variety of intermediate refrigeration processes. The cascade cycle is potentially the most efficient of cryogenic processes because the major heat-transfer steps are liquefaction–vaporization exchanges with each stream at a constant temperature. Thus, heat-transfer coefficients are high and Ts can be kept very small.

2.2

The Linde or Joule–Thomson Cycle
The Linde cycle was used in the earliest European efforts at gas liquefaction and is conceptually the simplest of cryogenic cycles. A simple flow sheet is shown in Fig. 9. Representation of the cycle as a P–H diagram is shown in Fig. 10. Here the gas to be liquefied or used as refrigerant is compressed through several stages each with its aftercooler. It then enters the main countercurrent heat exchanger where it is cooled by returning low-pressure gas. The gas is then expanded through a valve where it is cooled by the Joule–Thomson effect and partially liquefied. The liquid fraction can then be withdrawn, as shown, or used as a refrigeration source. Making a material and energy balance around a control volume including the main exchanger, JT valve, and liquid receiver for the process shown gives X (H7 H7 H2) QL H5 (3)

where X is the fraction of the compressed gas to be liquefied. Thus process efficiency and even operability depend entirely on the Joule–Thomson effect at the warm end of the main heat exchanger and on the effectiveness of that heat exchanger. Also, if QL becomes large due to inadequate insulation, X quickly goes to zero. Because of its dependence on Joule–Thomson effect at the warm end of the main exchanger, the Joule–Thomson liquefier is not usable for H2 and He refrigeration without

2

Cryogenic Refrigeration and Liquefaction Cycles

473

Figure 7 Cascade refrigeration system on T–S coordinates. Note that T–S diagram for fluids A, B, C, and D are here superimposed. Numbers here refer to Fig. 8 flow points.

precooling. However, if H2 is cooled to liquid N2 temperature before it enters the JT cycle main heat exchanger, or if He is cooled to liquid H2 temperature before entering the JT cycle main heat exchanger, further cooling to liquefaction can be done with this cycle. Even with fluids such as N2 and CH4 it is often advantageous to precool the gas before it enters the JT heat exchanger in order to take advantage of the greater Joule–Thomson effect at the lower temperature.

2.3

The Claude or Expander Cycle
Expander cycles have become workhorses of the cryogenic engineer. A simplified flow sheet is shown in Fig. 11. Here part of the compressed gas is removed from the main exchanger before being fully cooled, and is cooled in an expansion engine in which mechanical work is done. Otherwise, the system is the same as the Joule–Thomson cycle. Figure 12 shows a T–S diagram for this process. The numbers on the diagram refer to those on the process flow sheet. If, as before, energy and material balances are made around a control volume including the main exchanger, expansion valve, liquid receiver, and the expander, one obtains

474

Cryogenic Systems

Figure 8 Cascade liquefaction cycle—simplified flow diagram.

2

Cryogenic Refrigeration and Liquefaction Cycles

475

Figure 9 Simplified Joule–Thomson liquefaction cycle flow diagram.

Figure 10 Representation of the Joule–Thomson liquefaction cycle on a P–H diagram.

476

Cryogenic Systems

Figure 11 Expander cycle simplified flow diagram.

X

(H7

H2)

Y (H9 H10) H7 H5

QL

(4)

where Y is the fraction of the high-pressure stream that is diverted to the expander. Here the liquid yield is not so dependent on the shape of the warm isotherm or the effectiveness of heat exchange since the expander contributes the major part of the refrigeration. Also, the limitations applicable to a JT liquefier do not pertain here. The expander cycle will operate independent of the Joule–Thomson effect of the system gas. The expansion step, line 9–10 on the T–S diagram, is ideally a constant entropy path. However, practical expanders operate at 60–90% efficiency and hence the path is one of modestly increasing entropy. In Fig. 12 the expander discharges a two-phase mixture. The process may be designed to discharge a saturated or a superheated vapor. Most expanders will tolerate a small amount of liquid in the discharge stream. However, this should be checked carefully with the manufacturer, for liquid can rapidly erode some expanders and can markedly reduce the efficiency of others. Any cryogenic process design requires careful consideration of conditions in the main heat exchanger. The cooling curve plotted in Fig. 13 shows the temperature of the process stream being considered, Ti, as a function of the enthalpy difference (Ho Hi), where Ho is the enthalpy for the process stream as it enters or leaves the warm end of the exchanger, and Hi is the enthalpy of that same stream at any point within the main exchanger. The

2

Cryogenic Refrigeration and Liquefaction Cycles

477

Figure 12 Expander cycle shown on a T–S diagram.

Figure 13 Cooling curves showing temperatures throughout the main exchanger for the expander cycle.

478

Cryogenic Systems enthalpy difference is the product of the H obtainable from a thermodynamic diagram and the mass flow rate of the process stream. If the mass flow rate changes, as it does at point 9 in the high-pressure stream, the slope will change. Ho Hi, below such a point would be obtained from Ho Hi (Ho Hi) (1 y) if the calculation is made on the basis of unit mass of high-pressure gas. It is conventional practice to design cryogenic heat exchangers so that the temperature of a given process stream will be the same in each of the multiple passages of the exchanger at a given exchanger cross section. The temperature difference between the high- and lowpressure streams (Th Tc) at that point is the T available for heat transfer. Obviously, the simple Tlm approach to calculation of heat-exchanger area will not be satisfactory here, for that method depends on linear cooling curves. The usual approach here is to divide the exchanger into segments of H such that the cooling curves are linear for the section chosen and to calculate the exchanger area for each section. It is especially important to examine cryogenic heat exchangers in this detail because temperature ranges are likely to be large, thus producing heat-transfer coefficients that vary over the length of the exchanger, and because the curvature of the cooling curves well may produce regions of very small T. In extreme cases the designer may even find that T at some point in the exchanger reaches zero, or becomes negative, thus violating the second law. No exchanger designed in ignorance of this situation would operate as designed. Minimization of cryogenic process power requirements, and hence operating costs, can be done using classical considerations of entropy gain. For any process W Wmin T0 ST (5)

where W is the actual work required by the process, Wmin is the minimum work [see Eq. (1)], and the last term represents the work lost in each process step. In that term T0 is the ambient temperature, and ST is the entropy gain of the universe as a result of each process step. In a heat exchanger T0 ST WL T0 Th Tc dHi ThTc (6)

where Th and Tc represent temperatures of the hot and cold streams and the integration is carried out from end to end of the heat exchanger. A comparison of the Claude cycle (so named because Georges Claude first developed a practical expander cycle for air liquefaction in 1902) with the Joule–Thomson cycle can thus be made by considering the WL in the comparable process steps. In the cooling curve diagram, Fig. 13, the dotted line represents the high-pressure stream cooling curve of a Joule– Thomson cycle operating at the same pressure as does the Claude cycle. In comparison, the Claude cycle produces much smaller Ts at the cold end of the heat exchanger. If this is translated into lost work as done in Fig. 14, there is considerable reduction. The Claude cycle also reduces lost work by passing only a part of the high-pressure gas through a valve, which is a completely irreversible pressure reduction step. The rest of the high-pressure gas is expanded in a machine where most of the pressure lost produces usable work. There are other ways to reduce the T, and hence the WL, in cryogenic heat exchangers. These methods can be used by the engineer as process conditions warrant. Figure 15 shows the effect of (a) intermediate refrigeration, (b) varying the amount of low-pressure gas in the exchanger, and (c) adding a third cold stream to the exchanger.

2

Cryogenic Refrigeration and Liquefaction Cycles

479

Figure 14 Calculation of WL in the main heat exchanger using Eq. (6) and showing the comparison between JT and Claude cycles.

2.4

Low-Temperature Engine Cycles
The possibility that Carnot cycle efficiency could be approached by a refrigeration cycle in which all pressure change occurs by compression and expansion has encouraged the development of several cycles that essentially occur within an engine. In general, these have proven useful for small-scale cryogenic refrigeration under unusual conditions as in space vehicles. However, the Stirling cycle, discussed below, has been used for industrial-scale production situations. The Stirling Cycle In this cycle a noncondensable gas, usually helium, is compressed, cooled both by heat transfer to cooling water and by heat transfer to a cold solid matrix in a regenerator (see Section 3.3), and expanded to produce minimum temperature. The cold gas is warmed by heat transfer to the fluid or space being refrigerated and to the now-warm matrix in the regenerator and returned to the compressor. Figure 16 shows the process path on a T–S diagram. The process efficiency of this idealized cycle is identical to a Carnot cycle efficiency. In application the Stirling cycle is usually operated in an engine where compression and expansion both occur rapidly with compression being nearly adiabatic. Figure 17 shows such a machine. The compressor piston (1) and a displacer piston (16 with cap 17) operate off the same crankshaft. Their motion is displaced by 90 . The result is that the compressor position is near the top or bottom of its cycle as the displacer is in most rapid vertical movement. Thus the cycle can be traced as follows: 1. With the displacer near the top of its stroke the compressor moves up compressing the gas in space 4.

480

Cryogenic Systems

Figure 15 Various cooling curve configurations to reduce WL : (a) Cooling curve for intermediate refrigerator case. (b) Use of reduced warming stream to control Ts. (c) Use of an additional warming stream.

2

Cryogenic Refrigeration and Liquefaction Cycles

481

Figure 16 The idealized Stirling cycle represented on a T–S diagram.

2. The displacer moves down, and the gas moves through the annular water-cooled heat exchanger (13) and the annular regenerator (14) reaching the upper space (5) in a cooled, compressed state. The regenerator packing, fine copper wool, is warmed by this flow. 3. Displacer and compressor pistons move down together. Thus the gas in (5) is expanded and cooled further. This cold gas receives heat from the chamber walls (18) and interior fins (15) thus refrigerating these solid parts and their external finning. 4. The displacer moves up, thus moving the gas from space (5) to space (4). In flowing through the annular passages the gas recools the regenerator packing. The device shown in Fig. 17 is arranged for air liquefaction. Room air enters at (23), passes through the finned structure where water and then CO2 freeze out, and is then liquified as it is further cooled as it flows over the finned surface (18) of the cylinder. The working fluid, usually He, is supplied as needed from the tank (27). Other Engine Cycles The Stirling cycle can be operated as a heat engine instead of as a refrigerator and, in fact, that was the original intent. In 1918 Vuilleumier patented a device that combines these two forms of Stirling cycle to produce a refrigerator that operated on a high-temperature heat source rather than on a source of work. This process has received recent attention24 and is useful in situations where a heat source is readily available but where power is inaccessible or costly. The Gifford–McMahon cycles25 have proven useful for operations requiring a lightweight, compact refrigeration source. Two cycles exist: one with a displacer piston that produces little or no work; the other with a work-producing expander piston. Figure 18 shows the two cycles. In both these cycles the compressor operates continuously maintaining the high-pressure surge volume of P1, T1. The sequence of steps for the system with work-producing piston are

482

Cryogenic Systems

Figure 17 Stirling cycle arranged for air liquefaction reference points have the following meanings: 1, compressor; 2, compression cylinder; 4, working fluid in space between compressor and displacer; 5, working fluid in the cold head region of the machine; 6, two parallel connecting rods with cranks, 7, of the main piston; 8, crankshaft; 9, displacer rod, linked to connecting rod, 10, and crank, 11, of the displacer; 12, ports; 13, cooler; 14, regenerator; 15, freezer; 16, displacer piston, and 17, cap; 18, condenser for the air to be liquefied, with annular channel, 19, tapping pipe (gooseneck) 20, insulating screening cover, 21, and mantel 22; 23, aperture for entry of air; 24, plates of the ice separator, joined by the tubular structure, 25, to the freezer (15); 26, gas-tight shaft seal; 27, gas cylinder supplying refrigerant; 28, supply pipe with one-way valve, 29. (Courtesy U.S. Philips Corp.)

1. Inlet valve opens filling system to P2. 2. Gas enters the cold space below the piston as the piston moves up doing work and thus cooling the gas. The piston continues, reducing the gas pressure to P1. 3. The piston moves down pushing the gas through the heat load area and the regenerator to the storage vessel at P1. The sequence of steps for the system with the displacer is similar except that gas initially enters the warm end of the cylinder, is cooled by the heat exchanger, and then is displaced by the piston so that it moves through the regenerator for further cooling before entering the cold space. Final cooling is done by ‘‘blowing down’’ this gas so that it enters the lowpressure surge volume at P1.

3

Cryogenic Heat-Transfer Methods

483

Figure 18 Gifford–McMahon refrigerator. The dashed line and the cooler are present only when the piston is to be used as a displacer with negligible work production.

If the working fluid is assumed to be an ideal gas, all process steps are ideal, and compression is isothermal, the COPs for the two cycles are: COP (work producing) COP (displacer) 1 RT1 1n P2 / P1 CP 0T3[1 (P4 / P3)(k
1) / k

]

1

P3(T1 / T3

P3 P1 P1 / P3) 1n P2 / P1

In these equations states 1 and 2 are those immediately before and after the compressor. State 3 is after the cooling step but before expansion, and state 4 is after the expansion at the lowest temperature.

3

CRYOGENIC HEAT-TRANSFER METHODS
In dealing with heat-transfer requirements the cryogenic engineer must effect large quantities of heat transfer over small Ts through wide temperature ranges. Commonly heat capacities and / or mass flows change along the length of the heat-transfer path, and often condensation or evaporation takes place. To minimize heat leak these complexities must be handled using exchangers with as large a heat-transfer surface area per exchanger volume as possible.

484

Cryogenic Systems Compact heat-exchanger designs of many sorts have been used, but only the most common types will be discussed here.

3.1

Coiled-Tube-in-Shell Exchangers
The traditional heat exchanger for cryogenic service is the Hampson or coiled-tube-in-shell exchanger as shown in Fig. 19. The exchanger is built by turning a mandrel on a specially built lathe, and wrapping it with successive layers of tubing and spacer wires. Since longitudinal heat transfer down the mandrel is not desired, the mandrel is usually made of a poorly conducting material such as stainless steel, and its interior is packed with an insulating material to prevent gas circulation. Copper or aluminum tubing is generally used. To prevent uneven flow distribution from tube to tube, tube winding is planned so that the total length of each tube is constant independent of the layer on which the tube is wound. This results in a constant winding angle, as shown in Fig. 20. For example, the tube layer next to the mandrel might have five parallel tubes, whereas the layer next to the shell might have 20 parallel tubes. Spacer wires may be laid longitudinally on each layer of tubes, or they may be wound counter to the tube winding direction, or omitted. Their presence and size depends on the flow requirements for fluid in the exchanger shell. Successive tube layers may be wound in opposite or in the same direction. After the tubes are wound on the mandrel they are fed into manifolds at each end of the tube bundle. The mandrel itself may be used for this purpose, or hook-shaped manifolds of large diameter tubing can be looped around the mandrel and connected to each tube in the bundle. Finally, the exchanger is closed by wrapping a shell, usually thin-walled stainless steel, over the bundle and welding on the required heads and nozzles. In application the low-pressure fluid flows through the exchanger shell, and highpressure fluids flow through the tubes. This exchanger is easily adapted for use by three or more fluids by putting in a pair of manifolds for each tube-side fluid to be carried. However, tube arrangement must be carefully engineered so that the temperatures of all the cooling streams (or all the warming streams) will be identical at any given exchanger cross section. The exchanger is typically mounted vertically so that condensation and gravity effects will not result in uneven flow distribution. Most often the cold end is located at the top so that any liquids not carried by the process stream will move toward warmer temperatures and be evaporated.

Figure 19 Section of a coiled-tube-in-shell heat exchanger.

3

Cryogenic Heat-Transfer Methods

485

Figure 20 Winding relationships for a coiled-tube-in-shell exchanger.

Heat-transfer coefficients in these exchangers will usually vary from end to end of the exchangers because of the wide temperature range experienced. For this reason, and because of the nonlinear T variations, the exchanger area must be determined by sections, the section lengths chosen so that linear Ts can be used and so that temperature ranges are not excessive. For inside tube heat-transfer coefficients with single-phase flow the Dittus–Boelter equation is used altered to account for the spiral flow: hD k 0.023N 0.8N 0.32 1 Re Pn 3.5 d D (7)

where D is the diameter of the helix and d the inside diameter. For outside heat-transfer coefficients the standard design methods for heat transfer for flow across tube banks with in-line tubes are used. Usually the metal wall resistance is negligible. In some cases adjacent tubes are brazed or soldered together to promote heat transfer from one to the other. Even here wall resistance is usually a very small part of the total heat-transfer resistance. Pressure drop calculations are made using equivalent design tools. Usually the lowpressure-side P is critical in designing a usable exchanger. The coiled-tube-in-shell exchanger is expensive, requiring a large amount of hand labor. Its advantages are that it can be operated at any pressure the tube can withstand, and that it can be built over very wide size ranges and with great flexibility of design. Currently these exchangers are little used in standard industrial cryogenic applications. However, in very large sizes (14 ft diameter 120 ft length) they are used in base-load natural gas liquefaction plants, and in very small size (finger sized) they are used in cooling sensors for space and military applications.

3.2

Plate-Fin Heat Exchangers
The plate-fin exchanger has become the most common type used for cryogenic service. This results from its relatively low cost and high concentration of surface area per cubic foot of exchanger volume. It is made by clamping together a stack of alternate flat plates and

486

Cryogenic Systems corrugated sheets of aluminum coated with brazing flux. This assembly is then immersed in molten salt where the aluminum brazes together at points of contact. After removal from the bath the salt and flux are washed from the exchanger paths, and the assembly is enclosed in end plates and nozzles designed to give the desired flow arrangement. Usually the exchanger is roughly cubic, and is limited in size by the size of the available salt bath and the ability to make good braze seals in the center of the core. The core can be arranged for countercurrent flow or for cross flow. Figure 21 shows the construction of a typical plate-fin exchanger. Procedures for calculating heat-transfer and pressure loss characteristics for plate-fin exchangers have been developed and published by the exchanger manufacturers. Table 4 and Fig. 22 present one set of these.

3.3

Regenerators
A regenerator is essentially a storage vessel filled with particulate solids through which hot and cold fluid flow alternately in opposite directions. The solids absorb energy as the hot fluid flows through, and then transfer this energy to the cold fluid. Thus this solid acts as a short-term energy-storage medium. It should have high heat capacity and a large surface area, but should be designed as to avoid excessive flow pressure drop. In cryogenic service regenerators have been used in two very different applications. In engine liquefiers very small regenerators packed with, for example, fine copper wire have been used. In these situations the alternating flow direction has been produced by the intake and exhaust strokes of the engine. In air separation plants very large regenerators in the form of tanks filled with pebbles have been used. In this application the regenerators have been used in pairs with one regenerator receiving hot fluid as cold fluid enters the other. Switch valves and check valves are used to alternate flow to the regenerator bodies, as shown in Fig. 23. The regenerator operates in cyclical, unsteady-state conditions. Partial differential equations can be written to express temperatures of gas and of solid phase as a function of time and bed position under given conditions of flow rates, properties of gaseous and solid phases, and switch time. Usually these equations are solved assuming constant heat capacities, thermal conductivities, heat-transfer coefficients, and flow rates. It is generally assumed that flow is uniform throughout the bed cross section, that the bed has infinite conductivity in the radial direction but zero in the longitudinal direction, and that there is no condensation or vaporization occurring. Thermal gradients through the solid particles are usually ignored. These equations can then be solved by computer approximation. The results are often expressed graphically.26 An alternative approach compares the regenerator with a steady-state heat exchanger and uses exchanger design methods for calculating regenerator size.27 Figure 24 shows the temperature–time relationship at several points in a regenerator body. In the central part of the regenerator Ts are nearly constant throughout the cycle. Folding the figure at the switch points superimposes the temperature data for this central section as shown in Fig. 25. It is clear that the solid plays only a time-delaying function as energy flows from the hot stream to the cold one. Temperature levels are set by the thermodynamics of the cooling curve such UA T equation can be used for small sections of the as Fig. 15 presents. Thus, the q regenerator if a proper U can be determined. During any half cycle the resistance to heat transfer from the gas to the solid packing will be just the gas-phase film coefficient. It can be calculated from empirical correlations for the packing material in use. For pebbles, the correlations for heat transfer to spheres in

3

Cryogenic Heat-Transfer Methods

487

Figure 21 Construction features of a plate-fin heat exchanger. (a) Detail of plate and fin. (b) Flow arrangements. (c) Total assembly arrangement.

488

Cryogenic Systems
Table 4 Computation of Fin Surface Geometricsa Fin Height (in.) 0.200 0.200 0.250 0.375 0.375 0.250 0.250 0.375 0.455
a

Type of Surface Plain or perforated Plain or perforated Plain or perforated Plain or perforated Plain or perforated 1 ⁄8 lanced 1 ⁄8 lanced 1 ⁄8 lanced Ruffled

Fin Spacing (FPI) 14 14 10 8 15 15 14 15 16

Fin Thickness (in.) 0.008 0.012 0.025 0.025 0.008 0.012 0.020 0.008 0.005

Ac 0.001185 0.001086 0.001172 0.001944 0.00224 0.001355 0.001150 0.00224 0.002811

Aht 0.596 0.577 0.500 0.600 1.064 0.732 0.655 1.064 1.437

B 437 415 288 230 409 420 378 409 465

rh 0.001986 0.001884 0.00234 0.003240 0.00211 0.001855 0.001751 0.002108 0.001956

Ar / Aht 0.751 0.760 0.750 0.778 0.862 0.813 0.817 0.862 0.893

Definition and use of terms: FPI fins per inch Ac free stream area factor, ft2 / passage / in. of effective passage width Aht heat-transfer area factor, ft2 / passage / in. / ft of effective length B heat-transfer area per unit volume between plates, ft2 / ft3 rh hydraulic radius cross section area / wetted perimeter, ft Ar effective heat-transfer area Aht 0 Aht total heat-transfer area weighted surface effectiveness factor 0 1 (Ar / Aht)(1 f) Af fin heat-transfer area fin efficiency factor [tanh (mL)] / ml f ml fin geometry and material factor (b / s) 2h / k b fin height, ft h film coefficient for heat transfer, Btu / hr ft2 F k thermal conductivity of the fin material, Btu / hr ft F s fin thickness, ft U overal heat transfer coefficient 1 / (A / ha Aa A / hb Ab) a,b subscripts indicating the two fluids between which heat is being transferred Courtesy Stewart-Warner Corp.

a packed bed28 is normally used to obtain the film coefficient for heat transfer from gas to solid: hgs where hgs G d 1.31(G / d)0,93 (8)

heat transfer from gas to regenerator packing or reverse, J / hr m2 K mass flow of gas, kg / hr m2 particle diameter, m

The heat that flows to the packing surface diffuses into the packing by a conductive mode. Usually this transfer is fast relative to the transfer from the gas phase, but it may be necessary to calculate solid surface temperatures as a function of heat-transfer rate and adjust the overall T accordingly. The heat-transfer mechanisms are typically symmetrical and hence the design equation becomes

3

Cryogenic Heat-Transfer Methods

489

Figure 22 Heat-transfer and flow friction factors in plate and fin heat exchangers. Curves 1: plain fin (0.200 in. height, 14 fins / in.—0.008 in. thick). Curves 2: ruffled fin (0.445 in. height, 16 fins / in.—0.005 in. thick). Curves 3: perforated fin (0.375 in. height, 8 fins / in.—0.025 in. thick). (Courtesy StewartWarner Corp.)

A

q U T

q h/2 T/2

˙

4q hgs T

This calculation can be done for each section of the cooling curve until the entire regenerator area is calculated. However, at the ends of the regenerator temperatures are not symmetrical nor is the T constant throughout the cycle. Figure 26 gives a correction factor that must be used to adjust the calculated area for these end effects. Usually a 10–20% increase in area results. The cyclical nature of regenerator operation allows their use as trapping media for contaminants simultaneously with their heat-transfer function. If the contaminant is condensable, it will condense and solidify on the solid surfaces as the cooling phase flows through the regenerator. During the warming phase flow, this deposited condensed phase will evaporate flowing out with the return media. Consider an air-separation process in which crude air at a moderate pressure is cooled by flow through a regenerator pair. The warmed regenerator is then used to warm up the returning nitrogen at low pressure. The water and CO2 in the air deposit on the regenerator

490

Cryogenic Systems

Figure 23 Regenerator pair configuration.

surfaces and then reevaporate into the nitrogen. If deposition occurs at thermodynamic equilibrium, and assuming Raoult’s law, yH2O or yCO2 where y P P P H2O or CO2 P (9)

mole fraction of H2O or CO2 in the gas phase saturation vapor pressure of H2 O or CO2 total pressure of flowing stream

This equation can be applied to both the depositing, incoming situation and the reevaporating, outgoing situation. If the contaminant is completely removed in the regenerator, and the return gas is pure as it enters the regenerator, the moles of incoming gas times the mole fraction of contaminant must equal that same product for the outgoing stream if the contaminant does not accumulate in the regenerator. Since the vapor pressure is a function of temperature, and the returning stream pressure is lower than the incoming stream pressure, these relations can be combined to give the maximum stream-to-stream T that may exist at any location in the regenerator. Figure 27 shows the results for one regenerator design condition. Also plotted on Fig. 27 is a cooling curve for these same design conditions. At the conditions given H2O will be removed down to very low concentrations, but CO2 solids may accumulate in the bottom of the regenerator. To prevent this it would be necessary to remove some of the air stream in the middle of the regenerator for further purification and cooling elsewhere.

3

Cryogenic Heat-Transfer Methods

491

Figure 24 Time–temperature histories in a regenerator.

Figure 25 Time–temperature history for a central slice through a regenerator.

492

Cryogenic Systems

Figure 26 End correction for regenerator heat-transfer calculation using symmetrical cycle theory27 (Courtesy Plenum Press): 4 H0 S(Tc Tw ) Cc Tc Cw Tw 12 H0(Tc Tw ) c sd U0 where Tw , Tc S U0 U Cw , Cc c d
s

reduced length

reduced period

1 1 4 h

0.1d k

switching times of warm and cold streams, respectively, hr regenerator surface area, m2 overall heat transfer coefficient uncorrected for hysteresis, kcal / m2 hr C overall heat transfer coefficient heat capacity of warm and cold stream, respectively, kcal / hr C specific heat of packing, kcal / kg C particle diameter, m density of solid, kg / m3

Cryogenic heat exchangers often are called on to condense or evaporate and two-phase heat-transfer commonly occurs, sometimes on both sides of a given heat exchanger. Heattransfer coefficients and flow pressure losses are calculated using correlations taken from high-temperature data.29 The distribution of multiphase processing streams into parallel channels is, however, a common and severe problem in cryogenic processing. In heat exchangers thousands of parallel paths may exist. Thus the designer must ensure that all possible paths

4

Insulation Systems

493

Figure 27

T limitation for contaminant cleanup in a regenerator.

offer the same flow resistance and that the two phases are well distributed in the flow stream approaching the distribution point. Streams that cool during passage through an exchanger are likely to be modestly self-compensating in that the viscosity of a cold gas is lower than that of a warmer gas. Thus, a stream that is relatively high in temperature (as would be the case if that passage received more than its share of fluid) will have a greater flow resistance than a cooler system, so flow will be reduced. The opposite effect occurs for streams being warmed, so that these streams must be carefully balanced at the exchanger entrance.

4

INSULATION SYSTEMS
Successful cryogenic processing requires high-efficiency insulation. Sometimes this is a processing necessity, as in the Joule–Thomson liquefier, and sometimes it is primarily an economic requirement, as in the storage and transportation of cryogens. For large-scale cryogenic processes, especially those operating at liquid nitrogen temperatures and above, thick blankets of fiber or powder insulation, air or N2 filled, have generally been used. For lower temperatures and for smaller units, vacuum insulation has been enhanced by adding one or many radiation shields, sometimes in the form of fibers or pellets, but often as reflective metal barriers. The use of many radiation barriers in the form of metal-coated plastic sheets

494

Cryogenic Systems wrapped around the processing vessel within the vacuum space has been used for most applications at temperatures approaching absolute zero.

4.1

Vacuum Insulation
Heat transfer occurs by convection, conduction, and radiation mechanisms. A vacuum space ideally eliminates convective and conductive heat transfer but does not interrupt radiative transfer. Thus heat transfer through a vacuum space can be calculated from the classic equation: q where q F12 T1,T2
4 AF12(T 1 4 T 2)

(10)

rate of heat transfer, J / sec Stefan-Boltzmann constant, 5.73 10 8 J / sec m2 K combined emissivity and geometry factor temperature (K) of radiating and receiving body, respectively

In this formulation of the Stefan–Boltzmann equation it is assumed that both radiator and receiver are gray bodies, that is, emissivity and absorptivity are equal and independent of temperature. It is also assumed that the radiating body loses energy to a totally uniform surroundings and receives energy from this same environment. The form of the Stefan–Boltzmann equation shows that the rate of radiant energy transfer is controlled by the temperature of the hot surface. If the vacuum space is interrupted by a shielding surface, the temperature of that surface will become Ts, so that q/A
4 F1s (T 1

T 4) s

Fs2 (T 4 s

4 T 2)

(11)

Since q / A will be the same through each region of this vacuum space, and assuming F1s Fs2 F12 Ts
4 4 T1 4 T2

2

(12)

For two infinite parallel plates or concentric cylinders or spheres with diffuse radiation transfer from one to the other, F12 1 1
1

A1 A2

1
2

1

(13)

If A1 is a small body in a large enclosure, F12 1. If radiator or receiver has an emissivity that varies with temperature, or if radiation is spectral, F12 must be found from a detailed statistical analysis of the various possible radiant beams.30 Table 5 lists emissivities for several surfaces of low emissivity that are useful in vacuum insulation.31 It is often desirable to control the temperature of the shield. This may be done by arranging for heat transfer between escaping vapors and the shield, or by using a doublewalled shield in which is contained a boiling cryogen. It is possible to use more than one radiation shield in an evacuated space. The temperature of intermediate streams can be determined as noted above, although the algebra becomes clumsy. However, mechanical complexities usually outweigh the insulating advantages.

4

Insulation Systems

495

Table 5 Emissivities of Materials Used for Cryogenic Radiation Shields Emissivity at Material Aluminum plate Aluminum foil (bright finish) Copper (commercial polish) Monel 304 stainless steel Silver Titanium 300 K 0.08 0.03 0.03 0.17 0.15 0.022 0.1 77.8 K 0.03 0.018 0.019 0.11 0.061 4.33 K 0.011 0.015

4.2

Superinsulation
The advantages of radiation shields in an evacuated space have been extended to their logical conclusion in superinsulation, where a very large number of radiation shields are used. A thin, low emissivity material is wrapped around the cold surface so that the radiation train is interrupted often. The material is usually aluminum foil or aluminum-coated Mylar. Since the conductivity path must also be blocked, the individual layers must be separated. This may be done with glass fibers, perlite bits, or even with wrinkles in the insulating material; 25 surfaces / in. of thickness is quite common. Usually the wrapping does not fill in the insulating space. Table 6 gives properties of some available superinsulations. Superinsulation has enormous advantages over other available insulation systems as can be seen from Table 6. In this table insulation performance is given in terms of effective thermal conductivity ke where ke L T q/A T/L (14)

effective, or apparent, thermal conductivity thickness of the insulation T1 T2

Table 6 Properties of Various Multilayer Insulations (Warm Wall at 300 K) Sample Thickness (cm) 3.7 3.7 2.5 1.5 4.5 2.2 3.2 1.3
a

Shields per Centimeter 26 26 24 76 6 6 24 47

Density (g / cm3) 0.12 0.12 0.09 0.76 0.03 0.03 0.045 0.09

Cold Wall T (K) 76 20 76 76 76 76 76 76

Conductivity ( W / cm K) 0.7 0.5 2.3 5.2 3.9 3.0 0.85 1.8

Materiala 1 1 2 3 4 5 5 5

1, Al foil with glass fiber mat separator; 2, Al foil with nylon net spacer; 3, Al foil with glass fabric spacer; 4, Al foil with glass fiber, unbonded spacer; 5, aluminized Mylar, no spacer.

496

Cryogenic Systems This insulating advantage translates into thin insulation space for a given rate of heat transfer, and into low weight. Hence designers have favored the use of superinsulation for most cryogen containers built for transport, especially where liquid H2 or liquid He is involved, and for extraterrestrial space applications. On the other hand, superinsulation must usually be installed in the field, and hence uniformity is difficult to achieve. Connections, tees in lines, and bends are especially difficult to wrap effectively. Present practice requires that layers of insulation be overlapped at a joint to ensure continuous coverage. Some configurations are shown in Fig. 28. Also, it has been found that the effectiveness of superinsulation drops rapidly as the pressure increases. Pressures must be kept below 10 3 torr; evacuation is slow; a getter is required in the evacuated space; and all joints must be absolutely vacuum tight. Thus the total system cost is high.

4.3

Insulating Powders and Fibers
Fibers and powders have been used as insulating materials since the earliest of insulation needs. They retain the enormous advantage of ease of installation, especially when used in air, and low cost. Table 7 lists common insulating powders and fibers along with values of effective thermal conductivity.32 Since the actual thermal conductivity is a function of temperature, these values may only be used for the temperature ranges shown. For cryogenic processes of modest size and at temperatures down to liquid nitrogen temperature, it is usual practice to immerse the process equipment to be insulated in a cold

Figure 28 Superinsulation coverage at joints and nozzles: (a) Lapped joint at corner. Also usable for nozzle or for pipe bend. (b) Rolled joint used at surface discontinuity, diameter change, or for jointure of insulation sections. (c) Multilayer insulation at a nozzle.

5

Materials for Cryogenic Service

497

Table 7 Effective Thermal Conductivity of Various Common Cryogenic Insulating Materials (300 to 76 K) Material Silica aerogel (250A) Perlite ( 30 mesh) Polystyrene foam Polyurethane foam Foamglas Gas Pressure (mm Hg) 10 4 N2 at 628 10 5 N2 at 628 Air, 1 atm Air, 1 atm Air, 1 atm P (g / cm2) 0.096 0.096 0.096 0.096 0.046 0.128 0.144 K (W / cm K) 20.8 195.5 18.2 334 259 328 346 10 10 10 10 10 10 10
6 6 6 6 6 6 6

box, a box filled with powder or fiber insulation. Insulation thickness must be large, and the coldest units must have the thickest insulation layer. This determines the placing of the process units within the cold box. Such a cold box may be assembled in the plant and shipped as a unit, or it can be constructed in the field. It is important to prevent moisture from migrating into the insulation and forming ice layers. Hence the box is usually operated at a positive gauge pressure using a dry gas, such as dry nitrogen. If rock wool or another such fiber is used, repairs can be made by tunneling through the insulation to the process unit. If an equivalent insulating powder, perlite, is used, the insulation will flow from the box through an opening into a retaining bag. After repairs are made, the insulation may be poured back into the box. Polymer foams have also been used as cryogenic insulators. Foam-in-place insulations have proven difficult to use because as the foaming takes place cavities are likely to develop behind process units. However, where the shape is simple and assembly can be done in the shop, good insulating characteristics can be obtained. In some applications powders or fibers have been used in evacuated spaces. The absence of gas in the insulation pores reduces heat transfer by convection and conduction. Figure 29 shows the effect on a powder insulation of reducing pressure in the insulating space. Note that the pressures may be somewhat greater than that needed in a superinsulation system.

5

MATERIALS FOR CRYOGENIC SERVICE
Materials to be used in cryogenic service must operate satisfactorily in both ambient and cryogenic temperatures. The repeated temperature cycling that comes from starting up, operating, and shutting down this equipment is particularly destructive because of expansion and contraction that occur at every boundary and jointure.

5.1

Materials of Construction
Metals Many of the normal metals used in equipment construction become brittle at low temperatures and fail with none of the prewarning of strain and deformation usually expected. Sometimes failure occurs at very low stress levels. The mechanism of brittle failure is still a topic for research. However, those metals that exhibit face-centered-cubic crystal lattice structure do not usually become brittle. The austenitic stainless steels, aluminum, copper, and nickel alloys are materials of this type. On the other hand, materials with body-centered-cubic

498

Cryogenic Systems

Figure 29 Effect of residual gas pressure on the effective thermal conductivity of a powder insulation— perlite, 30–80 mesh, 300 to 78 K.

crystal lattice forms or close-packed-hexagonal lattices are usually subject to a brittle transformation as the temperature is lowered. Such materials include the low-carbon steels and certain titanium and magensium alloys. Figure 30 shows these crystal forms and gives examples of notch toughness at room temperature and at liquid N2 temperature for several example metals. In general carbon acts to raise the brittle transition temperature, and nickel lowers it. Additional lowering can be obtained by fully killing steels by deoxidation with silicon and aluminum and by effecting a fine grain structure through normalizing by addition of selected elements. In selecting a material for cryogenic service, several significant properties should be considered. The toughness or ductibility is of prime importance. Actually, these are distinctively different properties. A material that is ductile, as measured by elongation, may have poor toughness as measured by a notch impact test, particularly at cryogenic temperatures. Thus both these properties should be examined. Figures 31 and 32 show the effect of nickel content and heat treatment on Charpy impact values for steels. Figure 33 shows the tensile elongation before rupture of several materials used in cryogenic service. Tensile and yield strength generally increase as temperature decreases. However, this is not always true, and the behavior of the particular material of interest should be examined. Obviously if the material becomes brittle, it is unusable regardless of tensile strength. Figure 34 shows the tensile and yield strength for several stainless steels. Fatigue strength is especially important where temperature cycles from ambient to cryogenic are frequent, especially if stresses also vary. In cryogenic vessels maximum stress

5

Materials for Cryogenic Service

499

Figure 30 Effect of crystal structure on brittle impact strengths of some metals. (Courtesy American Society for Metals.)

Figure 31 Effect of nickel content in steels on Charpy impact values. (Courtesy American Iron and Steel Institute.)

500

Cryogenic Systems

Figure 32 Effect of heat treatment on Charpy impact values of steel. (Courtesy American Iron and Steel Institute.)

Figure 33 Percent elongation before rupture of some materials used in cryogenic service.33

5

Materials for Cryogenic Service

501

Figure 34 Yield and tensile strength of several AISI 300 series stainless steels.33 (Courtesy American Iron and Steel Institute.)

cycles for design are about 10,000–20,000 rather than the millions of cycles used for highertemperature machinery design. Because fatigue strength data for low-temperature applications are scarce, steels used in cryogenic rotating equipment are commonly designed using standard room-temperature fatigue values. This allows a factor of safety because fatigue strength usually increases as temperature decreases. Coefficient of expansion information is critical because of the stress that can be set up as temperatures are reduced to cryogenic or raised to ambient. This is particularly important where dissimilar materials are joined. For example, a 36-ft-long piece of 18-8 stainless will contract more than an inch in cooling from ambient to the boiling point of liquid H2. And stainless steel has a coefficient of linear expansion much lower than that of copper or aluminum. This is seen in Fig. 35. Thermal conductivity is an important property because of the economic impact of heat leaks into a cryogenic space. Figure 36 shows the thermal conductivity of some metals in the cryogenic temperature range. Note that pure copper shows a maximum at very low temperatures, but most alloys show only modest effect of temperature on thermal conduc-

502

Cryogenic Systems

Figure 35 Coefficient of linear thermal expansion of several metals as a function of temperature. (Courtesy American Institute of Chemical Engineers.)

tivity. One measure of the suitability of a material for cryogenic service is the ratio of tensile strength to thermal conductivity. On this basis stainless steel looks very attractive and copper much less so. The most common materials used in cryogenic service have been the austenitic stainless steels, aluminum alloys, copper alloys, and aluminum-alloyed steels. Fine grained carbonmanganese steel and aluminum-killed steel and the 2.5% Ni steels can be used to temperatures as low as 50 C. A 3.5% Ni steel may be used roughly to 100 C; 5% Ni steels have been developed especially for applications in liquified natural gas processing, that is, for temperatures down to about 170 C. Austenitic stainless steels with about 9% Ni such as the common 304 and 316 types are usable well into the liquid H2 range ( 252 C). Aluminum and copper alloys have been used throughout the cryogenic temperature range. However, in selecting a particular alloy for a given application the engineer should consider carefully all of the properties of the material as they apply to that application. Stainless steel may be joined by welding. However, the welding rod chosen and the joint design must both be selected for the material being welded and the expected service. For example, 9% nickel steel can be welded using nickel-based electrodes and a 60–80 single V joint design. Inert gas welding using Inconel-type electrodes is also acceptable. Where stress levels will not be high types 309 and 310 austenitic-stainless-steel electrodes can be used despite large differences in thermal expansion between the weld and the base metal. Dissimilar metals can be joined for cryogenic service by soft soldering, silver brazing, or welding. For copper-to-copper joints a 50% tin / 50% lead solder can be used. However, these joints have little ductility and so cannot stand high stress levels. Soft solder should not be used with aluminum, silicon-bronze, or stainless steel. Silver soldering is preferred for aluminum and silicon bronze and may also be used with copper and stainless steel. Polymers Polymers are frequently used as structural materials in research apparatus, as windows into cryogenic spaces, and for gaskets, O-rings, and other seals. Their suitability for the intended

5

Materials for Cryogenic Service

503

Figure 36 Thermal conductivity of materials useful in low-temperature service. (1) 2024TA aluminum; (2) beryllium copper; (3) K-Monel; (4) titanium; (5) 304 stainless steel; (6) C1020 carbon steel; (7) pure copper; (8) Teflon.35

service should be as carefully considered as metals. At this point there is little accumulated, correlated data on polymer properties because of the wide variation in these materials from source to source. Hence properties should be obtained from the manufacturer and suitability for cryogenic service determined case by case. Tables 8 and 9 list properties of some common polymeric materials. These are not all the available suitable polymers, but have been chosen especially for their compatibility with liquid O2. For this service chemical inertness and resistance to flammability are particularly important. In addition to these, nylon is often used in cryogenic service because of its machinability and relative strength. Teflon and similar materials have the peculiar property of losing some of their dimensional stability at low temperatures; thus they should be used in confined spaces or at low stress levels. Glass Glasses, especially Pyrex and quartz, have proven satisfactory for cryogenic service because of their amorphous structure and very small coefficient of thermal expansion. They are

504
Silicone Rubber Fluorosilicone Silastic LS-53a 50–60 1.41–1.46 0.051 1000 — 350 — — 27 10
5 a b

Table 8 Properties of Polymers Used in Cryogenic Service

Elastomer Type Silastic Silicone Rubber e,f Viton Fluorec 55–90 1.4–1.85 0.051–0.067 2000 45–60 1.17–1.46 0.045 Under 400 600–1500

Vinylidene Fluoride Hexafluoropropylene

Polytrifluorochloroethylene Kel-Fc,d 55–90 1.4–1.85 0.051–0.067 350–600

Trade Name

— 200 — 0.13 45 10 Good
5

— 500–800 — — — Excellent

Under 200 200–800 0.13 45 10 5 Excellent Very good Very good Good to excellent Excellent

Physical and Mechanical Properties Durometer range (shore A) Specific gravity (base elastomer) Density, lb / in.3 (base elastomer) Tensile strength; psi: Pure gum Reinforced Elongation, percent: Pure gum Reinforced Thermal conductivity, g, Btu / hr / ft2 / ( F / ft) Coefficient of thermal expansion, cubical, in.3 / in.3 / F Electrical insulation Rebound Cold Hot Compression set Good Excellent Good to excellent

Very good Very good Good

— — Good to excellent

— — — — 90 — — Excellent

Resistance Properties Temperature: Tensile strength at 250 F, psi Tensile strength at 400 F, psi Elongation at 250 F, percent Elongation at 400 F, percent Low temperature brittle point, F Low temperature range of rapid stiffening, F Drift, room temperature Drift, elevated temperature (158 to 212 F)

850 400 350 200 90 to 200 60 to 120 Poor to excellent Excellent

300–800 150–300 100–350 50–160 10 to 60 20 to 30 Good Good to excellent

300–800 150–300 100–350 50–160 10 to 60 20 to 30 Good Good to excellent

Excellent 480 178 50 90 60

Excellent 450

Excellent 500

Excellent 400

Poor Poor Poor Poor Poor Excellent Excellent Excellent Excellent Very good Fair to excellent Poor to good Excellent — — — — Excellent Fair Excellent Excellent Excellent Very good Good Fair to excellent Good Poor to fair Poor Good Excellent Good Poor to fair Excellent Good to excellent Excellent Good Good Poor Good Excellent Poor Good Good Good Excellent Excellent Excellent

Poor to good Good Poor to good

—

Poor to good Good Poor to good Excellent

↓

Heat aging (212 F) Maximum recommended continuous service temperature, F Minimum recommended service temperature, F Mechanical: Tear resistance Abrasion resistance Impact resistance (fatigue) Chemical: Sunlight aging Weather resistance Oxidation Acids: Dilute Concentrated Alkali Alcohol Petroleum products, resistance Coal tar derivatives, resistance Chlorinated solvents, resistance Hydraulic oils: Silicates Phosphates Water swell resistance Permeability to gases Poor to fair Excellent Good to excellent Excellent Good Good Poor Good Excellent

a

b

Dow Corning Corp. E. I. duPont de Nemours. c Minnesota Mining and Manufacturing Co. d CTFE compounded with vinylidine fluoride. e General Electric. f Union Carbon and Carbide.

505

506
Fluorinated Ethylene Propylene Polychlorotrifluoroethylene Polyvinylidene Fluoride Polytetrafluoroethylene
d,e

Table 9 Properties of Polymers Used in Cryogenic Service Polyimide

Common Name

Trade Name

Teflon FEP a

Kel-Fb

Kynar c

Fluorosint Teflon TFEa Halon TFEf

Kapton Ha Kapton Fa Vespela Polymer SP-1a

2.14–2.17 2700–3100 250–330 0.5 105 2200 — No break 105 R25 1.75 R110–R115 0.9

2.1–2.2 4500–6000 30–250 1.5 105 3 32,000–80,000 7400–9300 0.8–5.0

1.76–1.77 7000 100–300 1.2 105 10,000 — 3.5 D80 (Shore) 0.9

2.13–2.22 2000–4500 200–400 0.58 105 1700 — 3.0 D50–D65 (Shore) 1.75

1.42 25,000g; 10,500 70g; 6–8 4.3 105 24,400 14,000 0.9 H85–H95 2.2

Physical and Mechanical Properties Specific gravity Tensile strength, psi Elongation, percent Tensile modulus, psi Compressive strength, psi Flexural strength, psi Impact strength, ft-lb / in. of notch Rockwell hardness Thermal conductivity, Btu / hr / ft2 / ( F / in.) Specific heat, Btu / lbm / F Coefficient of linear expansion, in. / in. / F 10 5 Volume resistivity, ohm-cm Clarity 0.28 4.7 10 5 to 5.8 10 5 2 1018 Transparent to translucent Excellent 625–760 0.03–0.06 Excellent 0.22 5 10 5 to 15 10 5 1.2 1018 Transparent to translucent Excellent 440–600 0.005–0.010 Excellent

0.33 6.7 10

5

0.25 5.5 10 2 1014 Transparent to translucent Excellent 450–550 0.030 Excellent 1018 Opaque

5

0.27 28 10 5 to 35 10 5 1018 Opaque

Processing Properties Molding qualities Injection molding temperature, F Mold shrinkage, in. / in. Machining qualities

— — — Excellent

— — — —

—

0.01

17.6

—

—

None 420 400 — None 258 (66 psi) 300 (66 psi), 195 (264 psi) Slight bleaching on long exposure 350–390 300 550 250 (66 psi) None

None 400

Self-extinguishing 80

None 420 500

— —

Resistance Properties Mechanical abrasion and wear Tabor CS 17 wheel mg, loss / 1000 cycles Temperature: Flammability Low temperature brittle point, F Resistance to heat, F (continuous) Deflection temperature under load, F Chemical: Effect of sunlight None

— Degrades after prolonged exposure None

Effect of weak acids Effect of strong acids
↓ ↓

Effect of weak alkalies Effect of strong alkalies Effect of organic solvents Halogenated compounds cause slight swelling

None Attacked by fuming sulfuric None None Resists most solvents

↓

— Attacked Resistant to most organic solvents

a

b

E. I. duPont de Nemours. Minnesota Mining and Manufacturing Co. c Pennsalt Chemicals Corp. d Polymer Corp. of Pennsylvania. e Polypenco, Inc. f Allied Chemical Corp. g Film.

507

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Cryogenic Systems commonly used in laboratory equipment, even down to the lowest cryogenic temperatures. They have also successfully been used as windows into devices such as hydrogen bubble chambers that are built primarily of metal.

5.2

Seals and Gaskets
In addition to careful selection of materials, seals must be specially designed for cryogenic service. Gaskets and O-rings are particularly subject to failure during thermal cycling. Thus they are best if confined and / or constructed of a metal–polymer combination. Such seals would be in the form of metal rings with C or wedge cross sections coated with a sealant such as Kel-F, Teflon, or soft metal. Various designs are available with complex cross sections for varying degrees of deflection. The surfaces against which these seal should be ground to specified finish. Elastomers such as neoprene and Viton-A have proven to be excellent sealants if captured in a space where they are subjected to 80% linear compression. This is true despite the fact that they are both extremely brittle at cryogenic temperatures without this stress. Adhesive use at low temperatures is strictly done on an empirical basis. Still, adhesives have been used successfully to join insulating and vapor barrier blankets to metal surfaces. In every case the criteria are that the adhesive must not become crystalline at the operating temperature, must be resistant to aging, and must have a coefficient of contraction close to that of the base surface. Polyurethane, silicone, and various epoxy compounds have been used successfully in various cryogenic applications.

5.3

Lubricants
The lubrication of cryogenic machinery such as valves, pumps, and expanders is a problem that has generally been solved by avoidance. Valves usually have a long extension between the seat and the packing gland. This extension is gas filled so that the packing gland temperature stays close to ambient. For low-speed bearings babbitting is usually acceptable, as is graphite and molybdenum sulfide. For high-speed bearings, such as those in turboexpanders, gas bearings are generally used. In these devices some of the gas is leaked into the rotating bearing and forms a cushion for rotation. If out-leakage of the contained gas is undesirable, N2 can be fed to the bearing and controlled so that leakage of N2 goes to the room and not into the cryogenic system. Bearings of this sort have been operated at speeds up to 100,000 rpm.

6

SPECIAL PROBLEMS IN LOW-TEMPERATURE INSTRUMENTATION
Cryogenic systems usually are relatively clean and free flowing, and they often exist at a phase boundary where the degrees of freedom are reduced by one. Although these factors ease measurement problems, the fact that the system is immersed in insulation and therefore not easily accessible, the desire to limit thermal leaks to the system, and the likelihood that vaporization or condensation will occur in instrument lines all add difficulties. Despite these differences all of the standard measurement techniques are used with lowtemperature systems, often with ingenious changes to adapt the device to low-temperature use.

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Special Problems in Low-Temperature Instrumentation

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6.1

Temperature Measurement
Temperature may be measured using liquid-in-glass thermometers down to about 40 C, using thermocouples down to about liquid H2 temperature, and using resistance thermometers and thermistors down to about 1 K. Although these are the usual devices of engineering measurement laboratory measurements have been done at all temperatures using gas thermometers and vapor pressure thermometers. Table 10 lists the defining fixed points of the International Practical Temperature Scale of 1968. This scale does not define fixed points below the triple point of equilibrium He.36 Below that range the NBS has defined a temperature scale to 1 K using gas thermometry.37 At still lower temperatures measurement must be based on the fundamental theories of solids such as paramagnetic and superconducting phenomena.38 The usefulness of vapor pressure thermometry is limited by the properties of available fluids. This is evident from Table 11. For example, in the temperature range from 20.4 to 24.5 K there is no usable material. Despite this, vapor pressure thermometers are accurate and convenient. The major problem in their use is that the hydraulic head represented by the vapor line between point of measurement and the readout point must be taken into account. Also, the measurement point must be the coldest point experienced by the device. If not, pockets of liquid will form in the line between the point of measurement and the readout point greatly affecting the reading accuracy. Standard thermocouples may be used through most of the cryogenic range, but, as shown in Fig. 37 for copper–constantan, the sensitivity with which they measure temperature drops as the temperature decreases. At low temperatures heat transfer down the thermocouple wire may markedly affect the junction temperature. This is especially dangerous with copper wires, as can be seen from Fig. 36. Also, some thermocouple materials, for example, iron, become brittle as temperature decreases. To overcome these difficulties special thermocouple pairs have been used. These usually involve alloys of the noble metals. Figure 37 shows the thermoelectric power, and hence sensitivity of three of these thermocouple pairs. Resistance thermometers are also very commonly used for cryogenic temperature measurement. Metal resistors, especially platinum, can be used from ambient to liquid He temperatures. They are extremely stable and can be read to high accuracy. However, expensive instrumentation is required because resistance differences are small requiring precise bridge circuitry. Resistance as a function of temperature for platinum is well known.36

Table 10 Defining Fixed Points of the International Practical Temperature Scale, 1968 Equilibrium Point Triple point of equilibrium H2 Boiling point of equilibrium H2 (P Boiling point of equilibrium H2 (P Boiling point of neon (P 1 atm) Triple point of O2 Boiling point of O2 (P 1 atm) Triple point of H2O (P 1 atm) Freezing point of Zn (P 1 atm) Freezing point of Ag (P 1 atm) Freezing point of Au (P 1 atm) 33330.6 N / m2) 1 atm) T (K) 13.81 17.042 20.28 27.102 54.361 90.188 273.16 692.73 1235.08 1337.58

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Cryogenic Systems
Table 11 Properties of Cryogens Useful in Vapor Pressure Thermometers Triple Point (K) — — 13.80 24.54 63.15 83.81 54.35 Boiling Point (K) 3.19 4.215 20.27 27.09 77.36 87.30 90.18 Critical Point (K) 3.32 5.20 32.98 44.40 126.26 150.70 154.80 Hydraulic Heat at Boiling Point (K / cm2) 0.000054 0.00013 0.00023 0.0039 0.0067 0.013 0.011

Substance He He p-H2 (20.4 K equilibrium) Ne N2 Ar O2
4 3

dP / dT (mm / K) 790 715 224 230 89 80 79

At temperatures below 60 K, carbon resistors have been found to be convenient and sensitive temperature sensors. Since the change in resistance per given temperature difference is large (580 ohms / K would be typical at 4 K) the instrument range is small, and the resistor must be selected and calibrated for use in the narrow temperature range required. Germanium resistors that are single crystals of germanium doped with minute quantities of impurities are also used throughout the cryogenic range. Their resistance varies approximately logarithmically with temperature, but the shape of this relation depends on the amount and type of dopant. Again, the germanium semiconductor must be selected and calibrated for the desired service. Thermistors, that is, mixed, multicrystal semiconductors, like carbon and germanium resistors, give exponential resistance calibrations. They may be selected for order-ofmagnitude resistance changes over very short temperature ranges or for service over wide temperature ranges. Calibration is necessary and may change with successive temperature cycling. For this reason they should be temperature-cycled several times before use. These sensors are cheap, extremely sensitive, easily read, and available in many forms. Thus they are excellent indicators of modest accuracy but of high sensitivity, such as sensors for control action. They do not, however, have the stability required for high accuracy.

6.2

Flow Measurement
Measurement of flow in cryogenic systems is often made difficult because of the need to deal with a liquid at its boiling point. Thus any significant pressure drop causes vaporization,

Figure 37 Thermoelectric power of some thermocouples useful for cryogenic temperature measurement (courtesy Plenum Press): 1, copper versus constantan; 2, Au 2 at % Co versus silver normal (Ag 0.37 at % Au); 3, Au 0.03 at 0.03 at % Fe versus % Fe versus silver normal; 4, Au Chromel.

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Examples of Cryogenic Processing

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which disrupts the measurement. This may be avoided by subcooling the liquid before measurement. Where this is possible, most measurement problems disappear, for cryogenic fluids are clean, low-viscosity liquids. Where subcooling is not possible, flow is most often measured using turbine flow meters or momentum meters. A turbine meter has a rotor mounted axially in the flow stream and moved by the passing fluid. The rate of rotation, which is directly proportional to the volumetric flow rate, is sensed by an electronic counter that senses the passage of each rotor blade. There are two problems in the use of turbine meters in cryogenic fluids. First, these fluids are nonlubricating. Hence the meter rotor must be self-lubricated. Second, during cool-down or warm-up slugs of vapor are likely to flow past the rotor. These can flow rapidly enough to overspeed and damage the rotor. This can be avoided by locating a bypass around the turbine meter shutting off the meter during unsteady operation. Momentum meters have a bob located in the flow stream to the support of which a strain gage is attached. The strain gage measures the force on the bob, which can be related through drag calculations or correlation to the rate of fluid flow past the bob. These meters are flexible and can be wide of range. They are sensitive to cavitation problems and to overstrain during upsets. Generally, each instrument must be calibrated.

6.3

Tank Inventory Measurement
The measurement of liquid level in a tank is made difficult by the cryogenic insulation requirements. This is true of stationary tanks, but even more so when the tank is in motion, as on a truck or spaceship, and the liquid is sloshing. The simplest inventory measurement is by weight, either with conventional scales or by a strain gage applied to a support structure. The sensing of level itself can be done using a succession of sensors that read differently when in liquid than they do in vapor. For instance, thermistors can be heated by a small electric current. Such devices cool quickly in liquid, and a resistance meter can ‘‘count’’ the number of thermistors in its circuit that are submerged. A similar device that gives a continuous reading of liquid depth would be a vertical resistance wire, gently heated, while the total wire resistance is measured. The cold, submerged, fraction of the wire can be easily determined. Other continuous reading devices include pressure gages, either with or without a vapor bleed, that read hydrostatic head, capacitance probes that indicate the fraction of their length that is submerged, ultrasonic systems that sense the time required for a wave to return from its reflectance off the liquid level, and light-reflecting devices.

7

EXAMPLES OF CRYOGENIC PROCESSING
Here three common, but greatly different, cryogenic technologies are described so that the interaction of the cryogenic techniques discussed above can be shown.

7.1

Air Separation
Among the products from air separation, nitrogen, oxygen, and argon are primary and are each major items of commerce. In 1994 nitrogen was second to sulfuric acid in production volume of industrial inorganic chemicals, with 932 billion standard cubic feet produced. Oxygen was third at 600 billion standard cubic feet produced. These materials are so widely used that their demand reflects the general trend in national industrial activity. Demand

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Cryogenic Systems generally increases by 3 to 5% / year. Nitrogen is widely used for inert atmosphere generation in the metals, electronics and semiconductor, and chemical industries, and as a source of deep refrigeration, especially for food freezing and transporation. Oxygen is used in the steel industry for blast furnace air enrichment, for welding and scarfing, and for alloying operation. It is also used in the chemical industry in oxidation steps, for wastewater treatment, for welding and cutting, and for breathing. Argon, mainly used in welding, in stainless steel making, and in the production of specialized inert atmospheres, has a demand of only about 2% of that of oxygen. However, this represents about 25% of the value of oxygen shipments, and the argon demand is growing faster than that of oxygen or nitrogen. Since all of the industrial gases are expensive to ship long distances, the industry was developed by locating a large number of plants close to markets and sized to meet nearby market demand. Maximum oxygen plant size has now grown into the 3000 ton / day range, but these plants are also located close to the consumer with the product delivered by pipe line. Use contracts are often long-term take-or-pay rental arrangements. Air is a mixture of about the composition shown in Table 12. In an air separation plant O2 is typically removed and distilled from liquified air. N2 may also be recovered. In large plants argon may be recovered in a supplemental distillation operation. In such a plant the minor constituents (H2–Xe) would have to be removed in bleed streams, but they are rarely collected. When this is done the Ne, Kr, Xe are usually adsorbed onto activated carbon at low temperature and separated by laboratory distillation. Figure 38 is a simplified flow sheet of a typical small merchant oxygen plant meeting a variety of O2 needs. Argon is not separated, and no use is made of the effluent N2. Inlet air is filtered and compressed in the first of four compression stages. It is then sent to an air purifier where the CO2 is removed by reaction with a recycling NaOH solution in a countercurrent packed tower. Usually the caustic solution inventory is changed daily. The CO2free gas is returned to the compressor for the final three stages after each of which the gas is cooled and water is separated from it. The compressed gas then goes to an adsorbent drier where the remaining water is removed onto silica gel or alumina. Driers are usually switched each shift and regenerated by using a slip stream of dry, hot N2 and cooled to operating temperature with unheated N2 flow. The compressed, purified air is then cooled in the main exchanger (here a coiled tube type, but more usually of the plate-fin type) by transferring heat to both the returning N2 and O2. The process is basically a variation of that invented by Georges Claude where part of the high-pressure stream is withdrawn to the expansion engine (or turbine). The remainder of the air is further cooled in the main exchanger and expanded through a valve.

Table 12 Approximate Composition of Dry Air Component N2 O2 Ar CO2 H2 Ne He Kr Xe Composition (mole %) 78.03 20.99 0.93 0.03 0.01 0.0015 0.0005 0.00011 0.000008

Figure 38 Flow sheet of a merchant oxygen plant. (Courtesy Air Product and Chemicals, Inc.)

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Cryogenic Systems The combined air stream, nearly saturated or partly liquefied, enters the bottom of the high-pressure column. This distillation column condenses nearly pure N2 at its top using boiling O2 in the low-pressure column as heat sink. If the low-pressure column operates at about 140 kN / m2 (20 psia), the high-pressure column must operate at about 690 kN / m2 (100 psia). The bottom product, called crude O2, is about 65 mole % N2. The top product from the high-pressure column, nearly pure N2, is used as N2 reflux in the low-pressure column. The crude O2 is fed to an activated carbon bed where hydrocarbons are removed, is expanded to low-pressure column pressure, goes through a subcooler in which it supplies refrigeration to the liquid O2 product, and is fed to the low-pressure column. The hydrocarbons removed in the adsorber may come in as impurities in the feed or may be generated by decomposition of the compressor oil. If they are not fully removed, they are likely to precipitate in the liquid O2 at the bottom of the low-pressure column. They accumulate there and can form an explosive mixture with oxygen whenever the plant is warmed up. Acetylene is especially dangerous in this regard because it is so little soluble in liquid oxygen. The separation of O2 and N2 is completed in the low-pressure column. In the column, argon accumulates below the crude O2 feed and may be withdrawn at about 10 mole % for further distillation. If it is not so removed, it leaves as impurity in the N2 product. Light contaminants (H2 and He) must be removed periodically from the top of the condenser / reboiler. Heavy contaminants are likely to leave as part of the O2 product. This plant produces O2 in three forms: liquid, high-pressure O2 for cylinder filling, and lower-pressure O2 gas for pipe line distribution. The liquid O2 goes directly from the lowpressure column to the storage tank. The rest of the liquid O2 product is pumped to high pressure in a plunger pump after it is subcooled so as to avoid cavitation. This high-pressure liquid is vaporized and heated to ambient in the main heat exchanger. An alternate approach would be to warm the O2 to ambient at high-pressure column pressure and then compress it as a gas. Cylinder pressure is usually too great for a plate-and-fin exchanger, so if the option shown in this flow sheet is used, the main exchanger must be of the coiled tube sort. The nitrogen product, after supplying some refrigeration to the N2 reflux, is warmed to ambient in the shell of the main exchanger. Here the N2 product is shown as being vented to atmospheric. However, some of it would be required to regenerate the adsorbers and to pressurize the cold box in which the distillation columns, condenser / reboiler, main exchanger, hydrocarbon adsorber, subcoolers, throttling valves, and the liquid end of the liquid oxygen pump are probably contained. This process is self-cooling. At startup refrigeration needed to cool the unit to operating temperatures is supplied by the expansion engine and the three throttling valves. During that time the unit is probably run at maximum pressure. During routine operation that pressure may be reduced. The lower the liquid O2 demand, the less refrigeration is required and the lower the operating pressure may be.

7.2

Liquefaction of Natural Gas
Natural gas liquefaction has been commercially done in two very different situations. Companies that distribute and market natural gas have to meet a demand curve with a sharp maximum in midwinter. It has been found to be much more economic to maintain a local supply of natural gas liquid that can be vaporized and distributed at peak demand time than to build the gas pipe line big enough to meet this demand and to contract with the supplier for this quantity of gas. Thus, the gas company liquefies part of its supply all year. The liquid is stored locally until demand rises high enough to require augmenting the incoming

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Examples of Cryogenic Processing

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gas. Then the stored liquid is vaporized and added to the network. These ‘‘peak-shaving’’ plants consist of a small liquefier, an immense storage capacity, and a large capacity vaporizer. They can be found in most large metropolitan areas where winters are cold, especially in the northern United States, Canada, and Europe. The second situation is that of the oil / gas field itself. These fields are likely to be at long distances from the market. Oil can be readily transported, since it is in a relatively concentrated form. Gas is not. This concentration is done by liquefaction prior to shipment, thus reducing the volume about 600-fold. Subsequently, revaporization occurs at the port near the market. These ‘‘base-load’’ LNG systems consist of a large liquefaction plant, relatively modest storage facilities near the source field, a train of ships moving the liquid from the field to the port near the market, another storage facility near the market, and a large capacity vaporizer. Such a system is a very large project. Because of the large required investment, world political and economic instability, and safety and environmental concerns in some developed nations, especially the United States, only a few such systems are now in operation or actively in progress. See Table 13 for data on world LNG trade. Peak-Shaving Plants The liquefaction process in a peak-shaving installation is relatively small capacity, since it will be operating over the bulk of the year to produce the gas required in excess of normal capacity for two to six weeks of the year. It usually operates in a region of high energy cost but also of readily available mechanical service and spare parts, and it liquefies relatively pure methane. Finally, operating reliability is not usually critical because the plant has capacity to liquefy the required gas in less time than in the maximum available. For these reasons efficiency is more important than system reliability and simplicity. Cascade and various expander cycles are generally used, although a wide variety of processes have been used including the Stirling cycle. Figure 39 shows a process in which an N2 expander cycle is used for low-temperature refrigeration, whereas the methane itself is expanded to supply intermediate refrigeration. This is done because of the higher efficiency of N2 expanders at low temperature and the reduced need for methane purification. The feed natural gas is purified and filtered and then

Table 13 Data on World LNG Trade World’s LNG Plants, 1994 Capacity, Million Metric Parallel Tons / yr Liq. Trains 2.9 6.2 16.4 1.3 3.2 4.3 9.0 13.2 5.3 7.5 6.0 2 8 12 1 4 2 5 7 5 3 3 World’s LNG Imports, 1994 World’s LNG Trade

Location Kenai, Alaska Skikda, Algeria Arzew, Algeria Camel, Algeria Mersa, Libya Das Is., Abu Dhabi Arun, Indonesia Bontang, Indonesia Lamut, Brunei Bintulu, Malaysia Barrup, Australia

Country Japan S. Korea Taiwan France Other Europe U.S.A.

Quantity, Amount, Million Metric Million Metric Year Tons / yr Tons / yr 38.9 4.4 1.7 6.6 7.8 1.7 1980 22 1990 65 2000 90–95 (est) 2010 130–160 (est)

516
Figure 39 Flow sheet of an LNG process using N2 refrigeration.

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Examples of Cryogenic Processing

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split into two streams. The larger is cooled in part of the main exchanger, expanded in a turboexpander, and rewarmed to supply much of the warm end refrigeration, after which it is sent to the distribution system. The smaller fraction is cooled both by methane and by N2 refrigeration until it is largely liquid, whereupon it goes to storage. Heavier liquids are removed by phase separation along the cooling path. Low-temperature refrigeration is supplied by a two-stage Claude cycle using N2 as working fluid. The LNG is stored in very large, insulated storage tanks. Typically such a tank might be 300 ft in diameter and 300 ft high. The height is made possible by the low density of LNG compared to other hydrocarbon liquids. LNG tanks have been built in ground as well as aboveground and of concrete as well as steel. However, the vast majority are aboveground steel tanks. In designing and building LNG tanks the structural and thermal requirements added to the large size lead to many special design features. A strong foundation is necessary, and so the tank is often set on a concrete pad placed on piles. At the same time the earth underneath must be kept from freezing and later thawing and heaving. Thus electric cables or steam pipes are buried in the concrete to keep the soil above freezing. Over this pad a structurally sound layer of insulation, such as foam glass, is put to reduce heat leak to the LNG. The vertical tank walls are erected onto the concrete pad. The inner one is of stainless steel, the outer one is usually of carbon steel, and the interwall distance would be about 4 ft. The walls are field erected with welders carried in a tram attached to the top of the wall and lifted as the wall proceeds. The wall thickness is, of course, greater at the bottom than it is higher up. The floor of the tank is steel laid over the foam glass and attached to the inner wall with a flexible joint. This is necessary because the tank walls will shrink upon cooling and expand when reheated. The dish roof is usually built within the walls over the floor. When the walls are completed, a flexible insulating blanket is put on the inside wall and the rest of the interwall space is filled with perlite. The blanket is necessary to counter the wall movement and prevent settling and crushing the perlite. At the end of construction the roof is lifted into position with slight air pressure. Usually this roof has hanging from it an insulated subroof that also rises and protects the LNG from heat leak to the roof. When this structure is in place, it is welded in and cover plates are put over the insulated wall spaces. For safety considerations these tanks are usually surrounded by a berm designed to confine any LNG that escapes. LNG fire studies have shown such a fire to be less dangerous than a fire in an equivalent volume of gasoline. Still, the mass of LNG is so large that opportunities for disaster are seen as equally large. The fire danger will be reduced if the spill is more closely confined, and hence these berms tend to be high rather than large in diameter. In fact, a concrete tank berm built by the Philadelphia Gas Works is integral with the outside tank wall. That berm is of prestressed concrete thick enough to withstand the impact of a major commercial airliner crash. Revaporization of LNG is done in large heat exchangers using air or water as heat sink. Shell and tube exchangers, radiators with fan-driven air for warming, and cascading liquid exchangers have all been used successfully, although the air-blown radiators tend to be noisy and subject to icing. Base-Load LNG Plant Table 13 lists the base-load LNG plants in operation in 1994. Products from these plants produce much of the natural gas used in Europe and in Japan, but United States use has been low, primarily because of the availability of large domestic gas fields. In contrast to peak-shaving plants, liquefiers for these projects are large, primarily limited by the size of compressors and heat exchangers available in international trade. Also,

518

Cryogenic Systems these plants are located in remote areas where energy is cheap but repair facilities expensive or nonexistent. Thus, only two types of processes have been used: the classic cascade cycle and the mixed refrigerant cascade. Of these the mixed refrigerant cascade has gradually become dominant because of its mechanical simplicity and reliability. Figure 40 shows a simplified process flow sheet of a mixed refrigerant cascade liquefier for natural gas. Here the natural gas passes through a succession of heat exchangers, or of bundles in a single heat exchanger, until liquified. The necessary refrigeration is supplied by a multicomponent refrigeration loop, which is essentially a Joule-Thomson cycle with successive phase separators to remove liquids as they are formed. These liquid streams are subcooled, expanded to low pressure, and used to supply the refrigeration required both by the natural gas and by the refrigerant mixture.

Figure 40 Mixed refrigerant LNG process flow sheet. (Courtesy Plenum Press.)

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Examples of Cryogenic Processing

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The success of this process depends on a selection of refrigerant composition that gives a cooling curve with shape closely matching the shape of the natural gas cooling curve. Thus all heat transfer will be across small Ts. This is shown in Fig. 41, a cooling curve for a mixed refrigerant cycle. The need to deal with a mixed refrigerant and to control the composition of the refrigerant mixture are the major difficulties with these processes. They complicate design, control, and general operation. For instance, a second process plant, nearly as large as the LNG plant, must be at hand to separate refrigerant components and supply makeup as needed by the liquefier. Also not shown in this flow sheet is the initial cleanup of the feed natural gas. This stream must be filtered, dried, purified of CO2 before it enters the process shown here. As noted above, both compressors and heat exchangers will be at the commercial maximum. The heat exchanger is of the coiled-tube-in-shell sort. Typically it would have 3⁄4-in. aluminum tubes wrapped on a 2–3 ft diameter mandrel to a maximum 14-ft diameter. The exchanger is probably in two sections totaling about 120 ft in length. Shipping these exchanger bundles across the world challenges rail and ship capacities. Ships used to transport LNG from the terminal by the plant to the receiving site are essentially supertankers with insulated storage tanks. These tanks are usually built to fit the ship hull. There may be four or five of them along the ship’s length. Usually they are constructed at the shipyard, but in one design they are built in a separate facility, shipped by barge to the shipyard, and hoisted into position. Boiloff from these tanks is used as fuel for the ship. On long ocean hauls 6–10% of the LNG will be so consumed. In port the

Figure 41 Mixed refrigerant process cooling curve. (Courtesy Plenum Press.)

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Cryogenic Systems evaporated LNG must be reliquefied, for which purpose a small liquefier circuit is available onboard.

7.3

Helium Recovery and Liquefaction
Helium exists in minute concentrations in air (see Table 12). However, this concentration is well below the 0.3 vol % that is considered to be the minimum for economic recovery. It exists at higher concentrations in a few natural gas deposits in the United States, as shown in Table 14, and in like concentrations in some deposits in Russia, Poland, and Venezuela. This fossil material is apparently the total world supply. The vital role that helium plays in welding, superconductivity applications, space program operations, medicine, in certain heat transfer and inert atmosphere needs, and in a wide variety of research requirements lead to the demand that helium be conserved. This was undertaken by the Bureau of Mines after World War II. A series of helium-separation plants was built in the Southwest. Generally these produced an 80% helium stream from high Hecontent steams of natural gas that would otherwise have gone directly to the municipal markets. The processes used a modified Joule-Thompson cooling system that depended on the methane accompanying the He. This crude He was stored in the Cliffside Field, a depleted gas reservoir, from which it could be withdrawn and purified. Most of these plants shut down during the 1970s because of shifting government policies and budgetary limitations. In 1995 the last of these plants was closed down, as was the Bureau of Mines itself. The fate of the stored crude helium is being debated now (1996). There are now about 30 billion standard cubic feet of crude He stored in the Cliffside reservoir, more than enough to supply the U.S. government needs estimated, at 10 Bcf

Table 14 Helium in Natural Gases in the United States A. Composition of Some He-Rich Natural Gases in the United States Typical Composition (vol %) Location Colorado (Las Animas Co.) Kansas (Waubaunsee, Elk, McPherson Cos.) Michigan (Isabella Co.) Montana (Musselshell) Utah (Grand) CH4 0 30 57.9 17 30 25.5 C2H6 N2 77.6 66.4 14.3 54 1.0 CO2 14.7 0.2 0 30 3.5 O2 0.3 0 0.3 He 7.4 3.4 2.0 16 7.1

B. Estimated Helium Reserves (1994) Location Rocky Mountain area (Arizona, Colorado, Montana, New Mexico, Utah, Wyoming) Midcontinent area (Kansas, Oklahoma, Texas) He stored in the cliffside structure Total

Estimated Reserve (SCF) 25 109

169 30 224

109 109 109

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Superconductivity and Its Applications

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through 2015. Total demand for U.S. helium is nearly constant at about 3 Bcf / yr (in 1994). Private industry supplies about 89% of this market, the rest coming from the stored government supply. The estimated He resources in helium-rich natural gas in the United States is about 240 Bcf as of 1994. With the stored He, this makes a total supply of about 270 Bcf, probably enough to supply the demand until the middle of the 21st century. Eventually technology will be needed to economically recover He from more dilute sources. The liquefaction of He, or the production of refrigeration at temperatures in the liquid He range, requires special techniques. He, and also H2, have negative Joule–Thomson coefficients at room temperature. Thus cooling must first be done with a modified Claude process to a temperature level of 30 K or less. Often expanders are used in series to obtain temperatures close to the final temperature desired. An expansion valve may then be used to effect the actual liquefaction. Such a process is shown in Fig. 42. The goal of this process is the maintenance of a temperature low enough to sustain superconductivity (see below) using a conventional low-temperature superconductor. Since such processes are usually small, and since entropy gains at very low temperature are especially damaging to process efficiency, these processes must use very small T ’s for heat transfer, require high-efficiency expanders, and must be insulated nearly perfectly. Note that in heat exchanger X4 the T at the cold end is 0.55 K.

8

SUPERCONDUCTIVITY AND ITS APPLICATIONS
For normal electrical conductors the resistance decreases sharply as temperature decreases, as shown in Fig. 43. For pure materials this decrease tends to level off at very low temperatures. This results from the fact that the resistance to electron flow results from two factors: the collision of electrons with crystal lattice imperfections and electron collisions with the lattice atoms themselves. The former effect is not temperature dependent, but the latter is. This relationship has, itself, proven of interest to engineers, and much thought and development has gone toward the building of power transmission lines operating at cryogenic temperatures and taking advantage of the reduced resistance.

8.1

Superconductivity
In 1911 Dr. Onnes of Leiden was investigating the electrical properties of metals at very low temperatures, helium having just been discovered and liquefied. He was measuring the resistance of frozen mercury as the temperature was reduced into the liquid He range. Suddenly the sample showed zero resistance. At first a short circuit was suspected. However, very careful experiments showed that the electrical conductivity of the sample had dropped discontinuously to a very low value. The phenomenon of superconductivity has since been found to occur in a wide range of metals and alloys. The resistance of a superconductor has been found to be smaller than can be measured by the best instrumentation available. Possibly it is zero. Early on this was demonstrated by initiating a current in a superconducting ring which could then be maintained, undiminished, for months. The phenomenon of superconductivity has been studied ever since in attempts to learn the extent of the phenomena, to develop a theory that will explain the basic mechanism and predict superconductive properties, and to use superconductivity in practical ways. On an empirical basis it has been found that superconductors are diamagnetic, that is, they exclude a magnetic field, and that they exist within a region bounded by temperature and magnetic field strength. This is shown in Fig. 44. In becoming superconductive a material also changes in specific heat and in thermal conductivity.

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Cryogenic Systems

Figure 42 Helium liquefier flow sheet.

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Superconductivity and Its Applications

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Figure 43 Variation of resistance of metals with temperature.

Figure 44 Limits to superconductive behavior of some elements.

524

Cryogenic Systems The theory of supercoductivity developed after the discovery in 1933 by Meissner of the magnetic field exclusion. This led to a qualitative ‘‘two-fluid’’ model analogous to the theory underlying HeII. Since then this theory has been recast in quantum mechanical theory terms, most completely and successfully by J. Bardeen, L. N. Cooper, and R. Schrieffer of the University of Illinois in 1957 (BCS theory). The BCS theory accounts for the Meissner effect and for other physical behavior phenomena of superconductors. It does not yet allow the prediction of superconductive transition points for new materials. The theory predicts an energy gap between normal and superconductive states existing simultaneously and visualizes the flow of paired electrons through the crystal lattice and the quantization of the magnetic flux. The quantized flux lines are fundamental to the explanation of the difference between type I superconductors that exhibit perfect Meissner effects, and which have relatively low transition temperatures and field tolerance, and type II superconductors that have imperfect Meissner effects, higher transition temperatures, and greater tolerance of magnetic fields. For example, Nb3Sn, which is a type II superconductor, can be used for generation of magnetic fields of 100,000 gauss. These materials allow the penetration of magnetic field above a lower critical field strength, Hc1, but remain superconductive to a much greater field strength, Hc2. At fields above Hc1, flux enters the material in the form of quantized bundles that are pinned by dislocations so that the flux does not move easily and lead to normalization of the material. Thus, both HeII and superconductors may be considered examples of superfluids. Each exhibits a nondissipative process for mass transfer. In HeII the mass transferred is the fluid itself in inviscid flow; with a superconductor it is the electrons flowing without encountering resistance. In both cases a flow velocity greater than a critical value restores normality. In superconductors circulating currents are set up in the penetration layer at the surface to cancel the applied magnetic field. When this field is increased to a critical value, the critical current is reached and the material reverts to the normal state.

8.2

Applications of Superconductivity
Development of applications of superconductivity has proceeded very rapidly over the past decade. However, by far the widest use of superconductivity has been in magnet applications. Superconductive magnets were constructed and tested soon after such practical superconductors as Nb3Sn and rhodium–zirconium were discovered about 1960. Field strengths as high as 7 T were predicted then. Since then magnets in a wide range of sizes and shapes have been made, tested, and used with field strengths approaching 20 T. The first three Nb alloys listed in Table 15 are ductile to the point that they are readily fabricated by conventional wire-drawing techniques. These form the cheapest finished product, but cannot be used at high field strengths. The Nb3Sn, which is the most widely used superconductive material, and V3Ga are formed into tape by chemical vapor deposition. The tape is clad with copper for stability and stainless steel for strength. Materials for multifilament conductor formation are produced by the bronze process. In this process filaments of Nb or V are drawn down in a matrix of Sn–Cu or Sn–Ga alloy, the bronze. Heat treatment then produces the Nb3Sn or V3Ga. The residual matrix is too resistive for satisfactory stabilization. Hence copper filaments are incorporated in the final conductors. Multifilament conductors are then made by assembling superconductive filaments in a stabilizing matrix. For example, in one such conductor groups of 241 Ni–Ti filaments are sheathed in copper and cupronickel and packed in a copper matrix to make a 13,255-filament conductor. Such a conductor can be wound into an electromagnet or other large-scale electrical device.

8

Superconductivity and Its Applications

525

Table 15 Some Commercially Available Superconductive Materialsa Hc 2 (T) at 4.2 K 7.0 8.0 12.0 22.0 23.0 Jc (105 A / cm2 at 4.2 K) 2.5 T 1.1 0.9 2.5 17.0 5.0 5T 0.8 0.8 1.5 10.0 2.5 10 T 0 0 0.3 4.0 1.4 15 T 0 0 0 0.5 0.9 Fabrication Fairly ductile Fairly ductile Ductile CVD diffusion bronze Diffusion bronze

Material Nb–25 wt% Zr Nb–33 wt% Zr Nb–48 wt% Ti Nb3Sn V3Ga
a

Tc (K) 11 11.5 9.5 18.0 15.0

Courtesy Plenum Press.

Superconductive magnets have been used, or are planned to be used for particle acceleration in linear accelerators, for producing the magnetic fields in the plasma step of magnetohydrodynamics, for hydrogen bubble chambers, for producing magnetic ‘‘bottles’’ for nuclear fusion reactors such as the Tokomak, for both levitation and propulsion of ultra high speed trains, for research in solid-state physics, for field windings in motors, and for a host of small uses usually centered on research studies. In fact superconductive magnetics with field strength approaching 10 T are an item of commerce. They are usable where liquid helium temperatures are available and produce magnetic fields more conveniently and cheaply than can be done with a conventional electromagnet. Table 16 lists the superconductive magnets in use for various energy related applications. Perhaps the most interesting of these applications is in high-speed railroads. Studies in Japan, Germany, Canada, and the United States are aimed at developing passenger trains that will operate at 300 mph and above. The trains would be levitated over the track by superconductive magnets, sinking to track level only at start and stop. Propulsion systems vary but are generally motors often with superconductive field windings. Such railroads are proposed for travel from Osaka to Tokyo and from San Diego to Los Angeles. Design criteria for the Japanese train are given in Table 17. Superconductive electrical power transmission has been seriously considered for areas of high density use. Superconductors make it possible to bring the capacity of a single line up to 10,000–30,000 MW at a current density two orders of magnitude greater than conventional practice. The resulting small size and reduced energy losses reduce operating costs of transmission substantially. The economic attraction of a superconductive transmission line depends on the cost of construction and the demand for power, but also on the cost of refrigeration. Thus a shield is built in and kept at liquid N2 temperature to conserve on helium. Also, superinsulation is used around the liquid N2 shield. Other applications of superconductivity have been found in the microelectronics field. Superconductive switches have been proposed as high-speed, high-density memory devices and switches for computers and other electronic circuits. The ability of the superconductor to revert to normal and again to superconductive in the presence or absence of a magnetic field makes an electric gate or a record of the presence of an electric current. However, these devices have been at least temporarily overshadowed by the rapid development of the electronic chip. Ultimately, of course, these chips will be immersed in a cryogen to reduce resistance and dissipate resistive heat.

Table 16 General Characteristics of Superconductive Magnets for Energy Conversion and Storage Systems40,a Operated dc Yes No Yes, from the MHD fluid No Yes, in case of unbalanced load Pulsed Transients

Application 500–5000

Magnet Type

Typical Stored Energy in the Winding (MJ)

Largest Prototype So Far 60-MJ magnet

MHD generators

Homopolar machines Synchronous machines

Dipole magnet with warm aperature, possibly tapered Solenoid Rotating dipole or quadrupole winding 10–100 Power plant machines, 50–100; airborne systems, 0.5–1 105 Yes No 103 Yes, in case of fast voltage control Yes Yes No No

3-MW generator 5-MVA generator

Fusion magnets Tokamak or similar lowconfinement Poloidal field coils

Toroidal field coils

—

Mirror confinement

Baseball coils

105

Yes

No

Yes, pulsed field harmonic components of poloidal field Yes, dc field components from the toroidal field No

Baseball coils with 9 MJ

Energy storage; operation of pulsed fusion magnets Theta pinch 100 per unit No optimal shape defined yet No optimal shape defined yet No optimal shape defined yet 104

No

— 300 kJ No —

Tokamak

Load leveling in the grid

108

No

Yes, transfer time about 30 msec Yes, transfer time about seconds Yes, transfer time about hours

—

—

a

Courtesy Plenum Press.

526

9

Cryobiology and Cryosurgery

527

Table 17 Design Criteria for Japanese High-Speed Trains41,a Maximum number of coaches / train Maximum operation speed Maximum acceleration and deceleration: Acceleration Deceleration, normal brake Deceleration, emergency brake Starting speed of levitation Effective levitation height (between coil centers) Accuracy of the track Hours of operation Period of operation without maintenance service Number of superconduction magnets Levitation Guiding and drive Carriage weight dimensions Propulsion
a

16 550 km / hr 3 km / hr / sec 5 km / hr / sec 10 km / hr / sec 100 km / hr 250 mm 10 mm / 10 m From 6 AM to 12 18 hr

PM

at 15-min intervals

4 2 rows / coach 4 2 rows / coach 30 tons 25 m 3.4 m 3.4 m Linear synchronous motor

Courtesy Plenum Press.

9

CRYOBIOLOGY AND CRYOSURGERY
Cryogenics has found applications in medicine, food storage and transportation, and agriculture. In these areas the low temperature can be used to produce rapid tissue freezing and to maintain biological materials free of decay over long periods. The freezing of food with liquid N2 has become commonplace. Typically the loose, prepared food material is fed through an insulated chamber on a conveyor belt. Liquid N2 is sprayed onto the food, and the evaporated N2 flows countercurrent to the food movement to escape the chamber at the end in which the food enters. The required time of exposure depends on the size of individual food pieces and the characteristics of the food itself. For example, hamburger patties freeze relatively quickly because there is little resistance to nitrogen penetration. Conversely, whole fish may freeze rapidly on the surface, but the enclosing membranes prevent nitrogen penetration, so internal freezing occurs by conductive transfer of heat through the flesh. Usually a refrigerated holding period is required after the liquid N2 spray chambers to complete the freezing process. The advantages of liquid N2 food freezing relative to more conventional refrigeration lie in the speed of freezing that produces less tissue damage and less chance for spoilage, and the inert nature of nitrogen, which causes no health hazard for the freezer plant worker or the consumer. Liquid N2 freezing and storage has also been used with parts of living beings such as red blood cells, bull semen, bones, and various other cells. Here the concern is for the survival of the cells upon thawing, for in the freezing process ice crystals form which may rupture cell walls upon freezing and thawing. The rate of survival has been found to depend on the rate of cooling and heating, with each class of material showing individual optima. Figure 45 shows the survival fractions of several cell types as a function of cooling velocity. Better than half the red blood cells survive at cooling rates of about 3000 K / min. Such a cooling rate would kill all of the yeast cells. The mechanism of cell death is not clearly understood, and may result from any of several effects. The cell-wall rupture by crystals is the most obvious possibility. Another is

528

Cryogenic Systems

Figure 45 Survival rate for various cells frozen to liquid N2 temperature.42 (Courtesy Plenum Press.)

the dehydration of the cell by water migration during the freezing process. In any case the use of additives such as glycerol, dimethyl sulfoxide, pyridine n-oxide, and methyl and dimethyl acetamide has greatly reduced cell mortality in various specific cases. The amount and type of additive that is most effective depends upon the specific cell being treated. Controlled freezing has proven useful in several surgical procedures. In each of these the destruction of carefully selected cells and / or their removal has been the goal of the operation. In treating Parkinson disease destruction of some cells in the thalmus can lead to sharp reduction in tremors and muscular rigidity. The operation is done under local anesthetic using a very fine probe consisting of three concentric tubes. Liquid N2 flows in through the center tube, returning as vapor through the central annulus. The outer annulus is evacuated and insulates all but the probe tip. The surgeon inserts the probe using X-ray pictures for guidance. He or she gently cools that probe tip using temperatures just below freezing. If the patient’s tremors subside without other side effects, the right location has been found. Freezing of a quarter inch sphere around the probe tip can proceed. In ophthalmic surgery cryogenic probes are used to lift cataracts from the lens of the eye. Here the cataract is frozen to the cryo-probe tip and carefully separated from the eye. Liquid N2 is not needed and Freons or Joule–Thomson cooling is sufficient. Malignant or surface tumors can also be removed cryogenically. The freezing of such a cell mass helps to prevent the escape of some of the cells into the blood stream or the body cavity.

REFERENCES
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34. R. J. Coruccini, Chem. Eng. Prog. 342 (July 1957). 35. R. B. Stewart and V. J. Johnson (eds.), ‘‘A Compedium of Materials at Low Temperatures, Phases I and II,’’ WADD Tech. Rept. 60-56, NBS, Boulder, CO, 1961. 36. C. R. Barber et al., ‘‘The International Practical Temperature Scale,’’ Metrologia 5, 35 (1969). 37. F. G. Brickwedde, H. van Diyk, M. Durieux, J. M. Clement, and J. K. Logan, J. Res. Natl. Bur. Stds. 64A, 1 (1960). 38. R. P. Reis and D. E. Mapother, Temperature, Its Measurement in Science and Industry, H. H. Plumb (ed.), 4, 885–895 (1972). 39. L. L. Sperikr, R. L. Powell, and W. J. Hall, ‘‘Progress in Cryogenic Thermocouples,’’ Adv. Cryo. Eng. 14, 316 (1968). 40. P. Komacek, ‘‘Applications of Superconductive Magnets to Energy with Particular Emphasis on Fusion Power,’’ Adv. Cryo. Eng. 21, 115 (1975). 41. K. Oshima and Y. Kyotani, ‘‘High Speed Transportation Levitated by Superconducting Magnet,’’ Adv. Cryo. Eng. 19, 154 (1974). 42. E. G. Cravalho, ‘‘The Application of Cryogenics to the Reversible Storage of Biomaterials,’’ Adv. Cryo. Eng. 21, 399 (1975). 43. K. D. Timmerhaus, ‘‘Cryogenics and Its Applications: Recent Developments and Outlook,’’ Bulletin of the Int. Inst. of Refrigeration 66(5), 3 (1994).


				
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