# Research Developments of Aerostatic Thrust Bearings: A Review
## 5 The Applications of ATBs
### High-Speed Applications  
A typical machine of high-speed applications using ATBs is the turboexpander. In the development of high-speed turboexpanders, the Air Liquid Company, Xi'an Jiaotong University, the Technology Institute of Physics and Chemistry, the Chinese Academy of Science, and other institutions have accumulated profound technical expertise. A lot of research has been conducted on air turboexpanders, hydrogen turboexpanders, and helium turboexpanders using ATBs.  
Helium turboexpanders produced by Air Liquid Company have been used in many large scientific devices, such as the tokamak device KSTAR of the Korean National Fusion Research Center (NFRC), and the International Thermonuclear Experimental Reactor (ITER). The 9 kW @ 4.5 K helium refrigerator of KSTAR was equipped with six TC4-500 and TC5-500 helium turboexpanders supported by ATBs [111]. Each of ITER's 65 kW @ 4.5 K helium refrigerators was equipped with three TC-4 and one TC-5 turboexpander supported by ATBs [112]. The power of three TC-4 turboexpanders is 15, 13, and 11 kW, respectively. The power of the TC-5 turboexpander is 11 kW. The efficiency of all the turboexpanders is greater than 74%. The Creare company in the United States has been working on research for a two-phase helium turbine expander [113]. A helium expander has been designed with a length of 365 mm, a diameter of 150 mm, and a weight of approximately 17 kg. The rotor is supported by two aerostatic journal bearings and one ATB. This turboexpander has been successfully applied in a small helium liquefaction device. The expander works safely in the two-phase region for more than 7500 h. The rotation speed is 354 krm, with a flow rate of 43 g/s. The isentropic efficiency is about 63%, and the liquefaction is 84 L/h. The Technology Institute of Physics and Chemistry of The Chinese Academy of Science has successfully developed a helium turbine expander. The wheel diameter of the turboexpander is 35 mm, and the brake wheel diameter is 60 mm. Aerostatic thrust bearings have been applied to support the shaft. During testing, the outlet temperature of the helium turboexpander has reached 15.4 K. The maximum cooling capacity is 2.2 kW, and the adiabatic efficiency exceeds 72% [114].  
Xi'an Jiaotong University has also conducted a lot of research on turboexpanders. A helium turboexpander supported by ATBs was developed for the space environment simulator KM6 in the late 1990s [115]. The working wheel diameter was 35 mm, and the brake fan wheel diameter was 60 mm. In the low-temperature air environment test, the highest speed reached 127 krpm, and adiabatic efficiency was 68%. While the outlet temperature was 12.8 K, the efficiency of the turboexpander could reach 75%. The maximum cooling capacity was close to 2 kW. In addition, a series of hydrogen and helium turboexpanders have been developed recently [116]. To meet the requirements of axial load, ATBs have been applied as thrust bearings in these hydrogen and helium turboexpanders, as shown in Figure 16. The power of the hydrogen turboexpanders is 20-40 kW. The helium turboexpander is 39.7 kW with a rotation speed of 74.5 krpm. In the air environment test, the rotation speed of the helium turboexpander can reach 60 krpm, and the amplitude is less than 0.023 \(\upmu\)m.