turbine thermal power installation designed for simultaneous utilization of two working mediums in the thermal cycle: steam and gaseous fuel combustion products. Both separate and contact thermal schemes are possible. The former uses steam and gas in loops with separate steam and gas turbines. In the latter, the gas and steam are combined to flow together into the turbine.

Steam-gas turbine installations first came into practical use in 1932 with the Velox high-pressure steam generators of the firm Brown Boveri & Cie A.G. of Switzerland. In this steam-gas turbine installation, a gas turbine was driven by exhaust gases from a steam generator and actuated a turboblower, which supercharged the combustion chamber to a pressure of 200–300 kilonewtons (kN)/m², or 2–3 kilograms-force (kgf)/cm², with a resulting substantial increase in heat transfer. Steam generators of the Velox type came into widespread use in the USSR but were built with a relatively low power output.

High-pressure high-capacity steam generators have been constructed in the USSR for large power generating stations. The steam from such a generator is piped to a steam turbine, and the combustion products are piped to a gas turbine, which drives an air compressor and electric generator (Figure 1).

Figure 1. Schematic of a steam-gas turbine installation with a high-pressure steam generator: (1) gas turbine, (2) high-pressure steam generator, (3) compressor, (4) steam turbine, (5) electric generator

A unit with an output capacity of 200 megawatts (MW) was installed at the Nevinnomyssk State Regional Power Plant in 1972. It was the first to make use of a multiple arrangement consisting of a high-pressure VPG-450–140 steam generator, working at a combustion chamber pressure of 650 kN/m²(6.5 kgf/cm²), a gas turbine unit with a capacity of 43 MW, and a steam turbine unit with a capacity of 160 MW. The joint use of steam and gas cycles lowers the heat duty by 4–7 percent, as compared to a steam turbine unit of a similar capacity and having the same parameters; at the same time, capital expenditures are decreased by 10–12 percent.

In steam-gas turbine installations abroad—for example, in the USA and West Germany—there have come into widespread use thermal schemes wherein the hot exhaust gases from the gas-turbine unit are fed directly into the combustion chamber of the steam boiler, the temperature of which is thereby increased. Alternatively, the hot exhaust gases may be piped into gas heat exchangers of the economizer type for preheating the feedwater.