Gas-expansion turbines for medium-pressure air-distributing devices

Abstract

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.