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To date only piston-type gas-expansion machines with high pressure differences and relatively low volume flow rates have been used in medium and high pressure air distribution devices. In recent years, however, satisfactory designs of small gas-expansion turbines with a high expansion ratio have been developed. They have a number of advantages over piston machines.Since the flow through a turbomachine is continuous and takes place at a high rate, this machine is much smaller than a comparable piston machine and requires less metal for its construction. For example, the amount of metal needed for the construction of a piston-type gas-expansion machine (in kg per kg of processed gas) is 3-10 while for a turbine-type machine it is only 0.2-0.7. These machines are simpler in design and easier to make and operate. They have no friction surfaces liable to wear (provided that the gas fed into the machine is dry and contains no mechanical impurities and the oil usedis clean). For this reason the service life of a correctly operated machine is practically unlimited whereas the service life of a piston gas-expansion machine is estimated at about 5000 h.The clearances determining the gas leakage in a turbomachine remain practically unchanged so that its efficiency is not dependent on the running time. The efficiency of a piston machine, on the other hand, decreases with time because of the wear of seals. The efficiency of a turbine is also independent of the gas inlet temperature (provided that the optimum shaft speed is retained): the piston machines working with air show a reduction in efficiency with decreasing inlet temperature. The turbine can be mounted directly on the air distribution device casing which considerably reduces the length of the low-temperature piping connecting the expansion machine to the unit. This reduces the absorption of heat from the surrounding medium and obviates the need for the construction of heavy foundations.One of the most important parameters characterizing the operation of a gas-expansion machine is the adiabatic efficiency 77 ad; a comparison of the efficiencies of both machines shows clearly the advantages of the turbine-type machine. However, often the special features of the cold cycle and of the working conditions make the use of a turbine-type machine advantageous even if its efficiency is lower than that of a piston machine. The gains in reliability, capital cost, and operating costs can well outweigh the losses due to lower efficiency. This paper describes the results of the design, construction and operation of a compact expansion turbine whose overall efficiency is equal to that of a piston-type gas-expansion machine. Two turbinetype gas-expansion machines RT-1.3/40 and RT-0.8/50 have been developed for medium pressure airdistribution units AK-1.5 and K-0.4.

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For many years the Geliimash Scientific-Production Association AO has produced small high-speed turbo-expansion engines (TEE) which are used instead of piston expansion engines (PEE) in air-fractionating apparatus operating with mediumpressure (4-7 MPa) and high-pressure (up to 20 MPa) cycles for obtaining both gaseous and liquid products. Geliimash Scientific-Production Association AO also uses TEE instead of PEE in cryogenic helium plant and in hydrogen liquefiers.Compared with the PEE the TEE operates more reliably. The mean time between breakdowns of TEE is, as a rule, four to five times longer than of PEE. Experience with the replacement of PEE by turbo-expansion engines shows that the isentropic efficiency gs of TEE, even of small ones, is higher than of PEE.The TEE require less metal and are considerably simpler to operate, and the field of their application is therefore substantially wider. However, the TEE require greater purity of the gas supplied to them.The changes in the widening fields of application of TEE with radial-axial and radial impellers in various cryogenic installations in dependence on the pressure ahead of the TEE and the volumetric flow rate V 0 (at the temperature T O and the pressure Po at the inlet) are shown in Fig. I.The lower limit of the volumetric flow rate is determined by the resistance to vibration of the rotor at superhigh rotational frequencies. When a PEE is made with plain oil bearings, the lower limit of V o can also be determined by the frictional power absorbed by the plain bearings.The upper limit of the volumetric flow rate is determined solely by the technological possibilities of production. When the volumetric flow rate is very high, it is advisable to use axial TEE.In Russia and the countries of the Commonwealth of Independent States most cryogenic installations operate at present with TEE. There remain only a few small installations operating at pressures of up to 20 MPa which are produced with PEE. The Geliimash Scientific-Production Association AO in particular produces stationary and transportable air-fractionating apparatus SKDS-100 and TKDS-10 with a capacity of 100 liters liquid oxygen or nitrogen per hour and equipped with PEE brand DPV-4.2-200/6-2. The parameters of the PEE are: pressure at the inlet P0 = 18 MPa, at the outlet P2 = 0.65 MPa, temperature at the inlet 280 K, air flow rate 200-250 m3/h, isentropic efficiency r/s = 0.65-0.68. It should be noted that in the course of lengthy operation ~/s decreases.To replace the decompressors DPV-4.2-200/6-2, the turbo-expansion engine DT-0.3/20 was designed; in it the air expands in two macroturbine stages which are connected in series and have the following parameters:Stage I Pressure, MPa:at the inlet Po 18 at the outlet P2 7.5 Temperature at the inlet T o, K 283 Gas flow rate through turbo-expansion engines, V, m3/h 240 Volumetric flow rate V o, m3/h 1