195
6.2 Working Principles of Steam Turbines
typical application is drive of turbo-compressors. Drive of turbo-pumps exists also,
but is less common. Opting for a steam turbine as a motor is attractive if the industrial application also needs process heat, which then can be supplied by the outlet
steam from a backpressure turbine or by the extraction steam from a condenserturbine. Cogeneration of electrical power and process heat is similar. Steam is then
produced at high pressure and temperature and fed to a steam turbine driving an
electric generator. The process heat is supplied by extraction steam or backpressure
steam from the steam turbine. The generated electric power is consumed locally or
fed into the electricity grid. Industrial steam turbines function with lower values of
inlet pressure and inlet temperature than big power station steam turbines, up to a
maximum of about 100 bar, 500 °C. This is due to the cost and to the need for quick
adaptability of the machine load (no big thermal inertia). Dependent on the application, industrial steam turbines may differ highly in power, with the biggest around
250 MW. The biggest machines also serve in combined-cycle plants. In current
cogeneration applications, steam turbines experience strong competition from gas
turbines. Gas turbines burn natural gas and process steam is produced in a recovery
steam generator by means of turbine outlet gas. Investment costs for gas turbines
are considerably lower than for steam turbines. Waste gas from the process may also
be burnt in a recovery steam generator (co-firing).
Steam turbines have had an important role in ship propulsion, but this application is nowadays extremely exceptional due to the power allowed by modern diesel engines, up till 80 MW. Large slow running diesel engines also have a higher
efficiency (45 %). Pressure and temperature conditions of ship propulsion steam
turbines equal those of industrial steam turbines. The efficiency amounts to about
38 %. At present, ships are mainly propelled by diesel engines, but gas turbines are
applied for high power when available space is strongly limited: high speed battle
ships, coast guard patrol boats, fast ferries and fast container ships. Some ships with
gas turbines feature both gas turbines and diesel engines for propulsion. Use of the
diesel engine at low speed and the gas turbine or both the gas turbine and the diesel
engine for high speed is typical.
6.2 Working Principles of Steam Turbines
Present steam turbines are almost exclusively built in the axial form. The functioning is analogous with that of the axial hydraulic turbine discussed as an example
in Chap. 1. A flow is generated in stator components by converting static enthalpy
into kinetic energy. Mechanical work is produced by change of flow direction in the
downstream rotor, i.e. by using the kinetic energy. Static enthalpy can be converted
into kinetic energy during work in the rotor. A degree of reaction is then present.
Even though the principle of a steam turbine is the same as for a hydraulic one, it
has totally different appearance due to the nature of the fluid. The available enthalpy
drop with steam is very high: of the 1000 kJ/kg order. Even under extreme circumstances, a hydraulic turbine ranks two orders lower: a 1000 m height difference
6.2 Working Principles of Steam Turbines
typical application is drive of turbo-compressors. Drive of turbo-pumps exists also,
but is less common. Opting for a steam turbine as a motor is attractive if the industrial application also needs process heat, which then can be supplied by the outlet
steam from a backpressure turbine or by the extraction steam from a condenserturbine. Cogeneration of electrical power and process heat is similar. Steam is then
produced at high pressure and temperature and fed to a steam turbine driving an
electric generator. The process heat is supplied by extraction steam or backpressure
steam from the steam turbine. The generated electric power is consumed locally or
fed into the electricity grid. Industrial steam turbines function with lower values of
inlet pressure and inlet temperature than big power station steam turbines, up to a
maximum of about 100 bar, 500 °C. This is due to the cost and to the need for quick
adaptability of the machine load (no big thermal inertia). Dependent on the application, industrial steam turbines may differ highly in power, with the biggest around
250 MW. The biggest machines also serve in combined-cycle plants. In current
cogeneration applications, steam turbines experience strong competition from gas
turbines. Gas turbines burn natural gas and process steam is produced in a recovery
steam generator by means of turbine outlet gas. Investment costs for gas turbines
are considerably lower than for steam turbines. Waste gas from the process may also
be burnt in a recovery steam generator (co-firing).
Steam turbines have had an important role in ship propulsion, but this application is nowadays extremely exceptional due to the power allowed by modern diesel engines, up till 80 MW. Large slow running diesel engines also have a higher
efficiency (45 %). Pressure and temperature conditions of ship propulsion steam
turbines equal those of industrial steam turbines. The efficiency amounts to about
38 %. At present, ships are mainly propelled by diesel engines, but gas turbines are
applied for high power when available space is strongly limited: high speed battle
ships, coast guard patrol boats, fast ferries and fast container ships. Some ships with
gas turbines feature both gas turbines and diesel engines for propulsion. Use of the
diesel engine at low speed and the gas turbine or both the gas turbine and the diesel
engine for high speed is typical.
6.2 Working Principles of Steam Turbines
Present steam turbines are almost exclusively built in the axial form. The functioning is analogous with that of the axial hydraulic turbine discussed as an example
in Chap. 1. A flow is generated in stator components by converting static enthalpy
into kinetic energy. Mechanical work is produced by change of flow direction in the
downstream rotor, i.e. by using the kinetic energy. Static enthalpy can be converted
into kinetic energy during work in the rotor. A degree of reaction is then present.
Even though the principle of a steam turbine is the same as for a hydraulic one, it
has totally different appearance due to the nature of the fluid. The available enthalpy
drop with steam is very high: of the 1000 kJ/kg order. Even under extreme circumstances, a hydraulic turbine ranks two orders lower: a 1000 m height difference