A vital technology in the refrigerators and liquefiers described in Cold Facts Volume 31 Number 3 is that of turboexpanders. These devices are rotating machines in which the process fluid (e.g., helium) does work against the turboexpander while moving from high pressure to a lower pressure and thus is cooled. Such a process approximates an ideal isentropic expansion. This process can be compared to the isenthalpic or Joule-Thomson expansion described in this column in Cold Facts Winter 2010.
Isentropic expansion has a number of advantages. It will provide cooling at any temperature (unlike isenthalpic expansion, in which the fluid must be below an inversion temperature for cooling rather than heating to take place), and isentropic expansion generally provides a larger temperature drop than isenthalpic expansion for the same pressure change. However, unlike isenthalpic expansion, which occurs through a valve or orifice, isentropic expansion requires that work be done against an external source and therefore requires an expansion engine. Modern refrigerators and liquefiers typically contain anywhere between 2 and 10 expansion engines, depending on the capacity and cycle design of the cryoplant.
In earlier cryoplants, the expansion engines were reciprocating or piston expanders. This style of expansion engine has now mostly been replaced by turboexpanders, which have a number of advantages, including lower maintenance requirements and higher reliability. Turboexpanders are also a key component of small Brayton cycle cryocoolers.The use of turboexpanders in cryogenic plants has been the result of several technological advances. The development of sophisticated computational fluid dynamic models coupled with computer aided design and manufacturing has allowed the design and construction of efficient turboexpanders operating at cryogenic temperatures. The development of reliable rotating bearing systems that function at high speeds and cryogenic temperatures has also been necessary for the successful use of turboexpanders.
Bearing choices for turboexpanders include oil bearings (used only at room temperature), magnetic bearings and gas bearings. Oil bearings for turboexpanders, while still used in air separation, LNG plants and some hydrogen plants, are becoming less common due to the need to maintain a separate oil skid and the due to the risk of oil contamination into the process stream. Helium plants commonly, though not exclusively, are using gas bearings for turboexpanders. Gas bearings can be divided into two types, static and dynamic. In static gas bearings, the bearing gas is provided by an independent helium system and is available regardless of the speed of the turboexpander. In dynamic gas bearings, the bearing gas is taken from the process stream itself. In this approach, supplemental bearings may be required during plant startup.
The work produced by the gas expanding through the turboexpander is available at room temperature. This work may be absorbed as heat into a cooling loop. In some plants, such as the cryoplants at the European Spallation Source, this heat is then recovered for use elsewhere. In some large cryoplants, the turboexpanders may be on the same shaft as compressors elsewhere in the cycle. The work produced by the gas expanding through the turboexpanders in this case then helps drive the compressors.
The ability to design and build efficient, reliable turboexpanders is an important aspect of the capability and intellectual property of cryoplant manufacturers. Figure 1 shows examples of the impellers from the cold end of modern cryogenic turboexpanders.
Further information on turboexpanders and bearings may be found in: “High Performance Cryogenic Turboexpanders,” R. R. Agahi et al. Adv. Cryo. Engr. Vol. 41A (1996), “First Operating Results of a Dynamic Gas Bearing Turbine in an Industrial Hydrogen Liquefier,” S. Bischoff et al., Adv. Cryo. Engr. Vol. 55B (2010), “Development of a Neon Cryogenic Turbo-Expander with Magnetic Bearings,” H. Hirai et al., Adv. Cryo. Engr. Vol. 55B (2010) and “Recent Developments on Air Liquide Advanced Technologies Turbines,” F. Delcayre et al., Adv. Cryo. Engr. Vol. 57A (2012). An example of turboexpander use in a large helium cryoplant is given in “Process Design of Helium Refrigerators Collaborated with the Predesign of Turboexpander,” L. Y. Xiong et al., Adv. Cryo. Engr. Vol. 59A (2014). The use of turboexpanders in small cryocoolers is illustrated in “Demonstration of a High-Capacity Turboalernator for a 20K, 20 W Space-Borne Brayton Cryocooler,” M. Zagarola, Adv. Cryo. Engr. Vol. 59A (2014).