Steam Turbine Working Principle• In the figures shown in the next slides, Rankine cycle represent the following

processes:– ab, compression of liquid water by condensate extraction pump and feed

pump– bc, raising feedwater temperature in the feed heaters and steam generator– cd, evaporation of water in the steam generator– de, superheating of vapour in the steam generator– ef, expansion of steam in the turbine– fa, condensation of exhaust steam in the condenser

2

Steam Turbine Introduction• Steam turbine is a rotating machine, which converts heat energy of steam to

mechanical energy• The steam turbine process can be explained by the Rankine cycle• Each of the three types of diagrams (as shown in the subsequent slides) indicate

the same Rankine cycle, but display the changes taking place in the process from different considerations/point of view

3

Steam Turbine Working Principle (contd…)• The thermal efficiency of the Rankine cycle is:

4

η =Net work done

=W

=He – Hf - ( Hb - Ha) =

He – Hf

Heat added Q He – Ha - (Hb –Ha) He – Ha

Steam Turbine

5

Steam Turbine

6

Steam Turbine Working Principles (contd…)• Steam expanding through a narrow orifice (nozzle), acquires kinetic energy at the

expense of its heat energy• This kinetic energy of steam jet is used to rotate a rotor of turbine• There are two basic types of turbines:

– Impulse turbine– Reaction turbine

7

Steam Turbine Impulse Turbine• In an ideal impulse turbine,

the steam expansion occurs through the stationary nozzle only

• The moving blades change steam velocity and direction

• The motive force is due to the change in the resultant momentum of the steam

8

Impulse Turbine

Steam Turbine Reaction Turbine• In ideal reaction turbine, steam

expansion occur in the moving blades

• The turbine rotor is forced to turn by the active force of the steam jet leaving the nozzle

• In commercial turbines, fixed nozzles (blades) admit steam in moving nozzles (blades), leading pressure reduction in both, i.e., moving and fixed blades

9

Reaction Turbine

Steam Turbine

10Expansion of Steam in Impulse and Reaction

Turbine

Steam Turbine

11

Rotor (set of moving blades) Diaphragm (set of fixed blades)The above figure shows construction of a typical lower half diaphragm, which is fixed in the lower half of the turbine casing

Steam Turbine

12

Steam TurbineConstruction features

• The figure shown in the previous slide is a simplified view of a typical turbine

• It has a single rotating shaft which has a number of wheels attached to it

• The steam passing the turbine is contained within a casing

• The casing is usually split into upper and lower half, which are bolted together. This enables upper half to be raised for maintenance.

• Attached to the casing are diaphragm, which support the fixed blades nozzles

• The casing gets progressively larger from high pressure end to low pressure end. This is to accommodate the expansion of the steam as pressure is reduced.

• Steam temperature for modern nuclear turbine is typically around 250°C and 4000 kPa

13

Steam Turbine (Photo of Steam Turbine Rotor)

14Turbine Rotor

Steam Turbine (Photo of Steam Turbine Rotor)

15Turbine Rotor