Multistage Turbine

a gas or steam turbine in which the expansion of the steam or gas from its initial pressure to its final pressure and the conversion of its thermal energy into mechanical work takes place in a number of stages arranged in tandem, rather than in one stage. Each stage is basically an elementary turbine and consists of fixed guide vanes and moving rotor blades. The steam or gas expands on the guide vanes, and the kinetic energy of its flow is converted on the rotor blades to the work of turning the turbine’s rotor. Since only a portion of the available pressure and heat differential is used in each stage, the velocity of the steam or gas is moderate. This makes possible good efficiency at a relatively low speed of rotation of the rotor, which is necessary for direct coupling of a turbine to machines such as electric generators and compressors.

The number of stages in a multistage turbine is selected in the design process, taking into account the parameters specified for the working medium and the efficiency and overall dimensions of the turbine. As the number of stages increases, the economy improves because the thermal losses of each stage are used in the subsequent stage, but the size, weight, and cost of the turbine increase. If the number of stages is small (up to ten to 15), they are arranged in a single housing (a cylinder); large numbers of stages (up to 30–40) are placed in two or three housings. Virtually all turbines, except for small auxiliary turbines, are of the multistage type.