STEAM TURBINES Steam turbine is a device which is used to convert kinetic energy of steam into mechanical energy. In this, enthalpy of steam is first converted into kinetic energy in nozzle or blade passages The high velocity steam impinges on the curved blades and its direction of flow is changed. This causes a change of momentum and thus force developed drives the turbine shaft The steam turbine has been used as a prime mover in all steam power plants.

STEAM TURBINES Now – a days, single steam turbine of 1000MW capacity is built in many countries. In large sizes, it is used for driving electric generator. In small sizes, it is used to drive pumps, fans, compressors

Steam turbine -Parts Nozzle: The heat energy of high pressure steam is converted into kinetic energy, so that the steam issues from the nozzle with a very high velocity Blades: The blades which change the direction of steam issuing from the nozzle, so that a force acts on the blades due to change of momentum and propel them

Advantages of ST over Reciprocating steam engine Steam turbine can be rotated with very high speed The efficiency of S.T is high The steam consumptions is less The S.T is compact& requires less space Frictional loss is less due to lesser sliding parts Less operating & maintenance cost

Classification of steam turbines According to mode of steam action 1.Impulse turbine 2.Reaction turbine According to direction of steam flow 1.Axial flow turbine 2.Radial flow turbine According to exhaust condition of steam 1.Condensing type turbine 2.Non condensing type turbine According to no of stages 1.Single stage turbine 2.Multi stage turbine According to pressure of steam 1.High pressure turbine 2.Low pressure turbine 3.Medium pressure turbine

Impulse Turbine An impulse turbine ,as the name indicates, is a turbine which runs by the impulse of steam jet .In this turbine, the steam is first made to flow through a nozzle. Then the steam jet impinges on the turbine blades(which are curved like buckets) and are mounted on the circumference of the wheel. The steam jet after impinging glides over the concave surface of the blades and finally leave the turbine.

Impulse Turbine

Components Nozzle: Runner and blades: Casing:

Reaction Turbine

In reaction turbine there is no sudden pressure drop In reaction turbine there is no sudden pressure drop. There is a gradual pressure drop and takes place continuously over the fixed and moving blades. A number of moving blades are fitted to the rotating shaft. Fixed blades are provided in between such pairs of moving blades The function of fixed blades is that they guide the steam as well as allow it to expand in a larger velocity. It is similar to that of nozzles as in impulse turbine The moving blade serves the following functions It converts the kinetic energy into useful mechanical energy The steam expands in moving blades and gives reaction to the moving blades

Impulse Turbine Reaction Turbine It consists of nozzles and moving blades It consists of fixed and moving blades Pressure drop occurs in nozzles and not in moving blades Pressure drop occurs in nozzles and moving blades Steam strikes the blades with kinetic energy Steam passes over the blades with pressure and kinetic energy Blades are of symmetrical type Blades are of varying cross section Power developed is less Power developed is more It occupies less space for the same power It occupies more space for the same power output Velocity of turbine is high Velocity of turbine is more Efficiency is less Efficiency is more Blade manufacturing is simple as they are of symmetrical cross section No of stages required are less for the same power developed Blade manufacturing is difficult as it is of varying cross section No of stages required are more for the same power developed

Compounding It is the process of increasing the number of nozzle rows and moving blade rows in order to increase the power output and efficiency of the turbine. Purpose of compounding: 1.To increase the power output of turbine by utilising most of the energy available in the steam 2.To increase the efficiency of the turbine 3.To reduce the exit steam loss

Types of compounding Velocity compounding Pressure compounding Pressure velocity compounding