Cryogenic Systems

Introduction

Cryogenic engineering is the technical field that is concerned with developing and utilizing low-temperature techniques, processes, and phenomena. The point on the temperature scale at which conventional refrigeration ends and cryogenics begins is somewhat arbitrary. In the 1950s, engineers and scientists at the National Bureau of Standards (now called NIST) in Boulder, CO, suggested that the field of cryogenics be defined as that temperature region below –150°C (123 K or –240°F) (Scott, 1959). This point was selected because the refrigerants used in air conditioning systems and domestic refrigeratorsboil at temperatures above –150°C; whereas, the gases utilized for cryogenic applications, such asoxygen, nitrogen, hydrogen, and helium, boil at temperatures below –150°C.Heat transfer in cryogenic systems is an important factor in the design of all low-temperature systems.The cost of removing energy from a low-temperature region is significant. A power input of approximately14 kW (19 hp) would be required to drive a refrigerator (cryocooler) removing 1 kW (3412 Btu/hr)from a space at 90 K (–298°F) and rejecting the energy to the ambient surroundings. The average cost

of the cryocooler is about $38,000 (in 1974 dollars) or about $415,000 in 1999 dollars (Strobridge,1974). Because of the high cost of removing energy from a cryogenic system, heat transfer is examinedclosely in the design of these systems.

Joule-Thomson Cryocooler

Any liquefaction system that uses the expansion valve or J-T valve to produce low temperatures maybe classified as a J-T cryocooler. A schematic of the basic J-T cryocooler is shown in Figure 4.12.4. Therefrigeration effect, Qa/m, for the J-T cryocooler is give by:(4.12.6)The quantities are the same as those used in Equation 4.12.1.The liquid in the evaporator boils at a constant temperature, so the temperature level achieved by thecryocooler is dependent on the liquid used as the working fluid. Liquid nitrogen may be used as thecoolant for the temperature range from about 65 to 115 K. The low-temperature limit is set by the triplepoint for the working fluid. If the evaporator pressure is lowered below the triple point pressure, nitrogen“snow” forms in the evaporator. Unless special heat transfer surfaces are provided, the heat transfer

between solid nitrogen and a surface, such as the evaporator tube wall, is much less effective than boilingheat transfer that would occur for the liquid phase. The high-temperature limit is set by the criticaltemperature for the working fluid. If the evaporator is operated at pressures above the critical pressure,there is no liquid to evaporate.Because the J-T cryocooler relies on the positive Joule-Thomson effect for production of low temperatures,it cannot be used with hydrogen, helium, or neon gases unless the system is precooled belowthe maximum inversion temperature for the respective gas.Microminiature J-T cryocoolers have been developed for cooling infrared sensors and for thermal imagingsystems (Little, 1990). These systems use nitrogen-hydrocarbon gas mixtures as the working fluid to producehigh refrigeration capacities and more rapid cool-down. The refrigeration capacity of the J-T cryocoolerusing the gas mixture has been found to be as much as five times that for the system using nitrogen alone.

Air LiquefactionCommercial systems for the production of liquid nitrogen, liquid oxygen, liquid argon, and liquid neonuse atmospheric air as the raw stock. The first commercial air liquefaction system was developed inGermany by Karl von Linde (1897) for the production of oxygen-enriched air for use in the steel-makingindustries (Ruhemann, 1949). Hampson (1895) developed a similar system with a more efficient heatexchanger. A schematic of the Linde-Hampson system is shown in Figure 4.12.1.In the Linde-Hampson liquefaction system, air is compressed to 200 atm (20.3 MPa or 2940 psia),and the high-pressure stream is cooled in a counterflow heat exchanger. The cold gas is finally expandedthrough an expansion valve or Joule-Thomson valve to a pressure on the order of atmospheric pressure.At the exit of the expansion valve, the stream is in the two-phase (liquid-vapor) condition. The liquidis collected in the liquid receiver, and the cold vapor is returned through the heat exchanger to providecooling for the incoming warm gas stream.The fraction of the gas from the compressor that is liquefied (the liquid yield) for the Linde-Hampsonsystem is given by:(4.12.1)

where h1 = enthalpy of the low-pressure stream at pressure p1 and temperature T1 = T2; h2 = enthalpy ofthe high-pressure stream at the inlet at the warm end of the exchanger; hg is the enthalpy of the vaporreturning from the liquid receiver; andε is the heat exchanger effectiveness. The power requirement perunit mass flow rate through the compressor is given by:(4.12.2)

Helium Liquefaction

Helium is one of the most difficult (i.e., expensive) gases to liquefy because the maximum inversiontemperature for helium is only 45 K (-379°F). Prior to 1946, low-temperature research laboratoriesgenerally designed and constructed their own liquefiers (Croft, 1961), some of which used liquidhydrogen as a precoolant. Samuel Collins (1947) developed a helium liquefier, shown in Figure 4.12.3,that was a modification of the basic Claude liquefier. The commercial introduction of the Collins liquefierhad a significant positive impact on the ease and economy of helium liquefaction.

In the Collins liquefier, helium gas is compressed to a pressure of 1275 kPa (185 psia), and passedthrough the first heat exchanger. After leaving the first heat exchanger, a portion (about 16%) of thestream is bypassed through the first expander. The gas temperature at the expander inlet is between 40and 45 K (–388 to –379°F). The remainder of the helium gas flows through the second heat exchanger,and leaves the exchanger at about 15 K (–433°F). A fraction (about 56%) of this stream is bypassedthrough a second expander after the helium has passed through a third exchanger. The remaining flowpasses through two more exchangers and expands through the J-T valve, in which a portion of the streamis liquefied. The vapor formed during the expansion process is returned through the heat exchangers toprovide cooling for the incoming gas stream. Although it is not necessary, liquid nitrogen precooling isusually used to improve the liquid yield and offset some of the heat exchanger’s inefficiencies.Collins liquefiers have been constructed with as many as five expanders, depending on the designinlet pressure of the helium gas.