steam and Gas turbines

1. Steam and Gas Turbines

2. What is a turbine
• A Turbine is a device which converts the heat energy of steam into the kinetic
energy & then to rotational energy.
• The Power in a steam turbine is obtained by the rate of change in momentum of
a high velocity jet of steam impinging on a curved blade which is free to rotate.
• The basic cycle for the steam turbine power plant is the Rankine cycle.
• The modern Power plant uses the rankine cycle modified to include superheating,
regenerative feed water heating & reheating.

3. INTRODUCTION OF STEAM TURBINE
• STEAM TURBINE is a prime mover in which pressure energy of steam is converted
into mechanical energy & further electrical energy.
• CLASSIFICATION OF STEAM TURBINE
• Simple impulse turbine
• Reaction turbine

4. Construction of steam turbines

5. Principles of operation
• The power in a steam turbine is obtained by the rate of change in momentum of
a high velocity jet of steam impinging on a curved blade which is free to rotate.
• The steam from the boiler is expanded in a nozzle, resulting in the emission of a
high velocity jet. This jet of steam impinges on the moving vanes or blades,
mounted on a shaft. Here it undergoes a change of direction of motion which
gives rise to a change in momentum and therefore a force.
• Steam turbines are mostly 'axial flow' types; the steam flows over the blades in a
direction Parallel to the axis of the wheel. 'Radial flow' types are rarely used.

6. Simple Impulse turbine
• In this type, the drop in pressure takes place in fixed nozzles as well as moving
blades. The pressure drops suffered by steam while passing through the moving
blades causes a further generation of kinetic energy within these blades, giving
rise to reaction and add to the propelling force, which is applied through the
rotor to the turbine shaft. The blade passage cross-sectional area is varied
(converging type).

7. Reaction turbine
• In the reaction turbine high pressure steam from the boiler, passes through the
nozzle. When the steam comes out through these nozzles the velocity of steam
increases & then strike on fixed blade. In this type of turbine there is a gradually
pressure drop takes place over fixed & moving blade.

8. Difference b/w Impulse and Reaction Turbine
The main difference between impulse and
reaction turbine lies in the way in which
steam is expanded while it moves throw
them In the former type steam expands in
the nozzle and its pressure doesn’t change as
it moves over the blades while in the later
types the steam expands continuously as it
passes over the blades and thus there is a
gradual fall in pressure during expansion

9. Advantages of Steam turbines:
• Thermal Efficiency of a Steam Turbine is much higher than that of a steam engine
• The Steam Turbine develops power at a uniform rate and hence does not
required Flywheel
• If the Steam Turbine is properly designed and constructed then it is the most
durable Prime Mover
• In a steam turbine there is no loss due to initial condensation of steam
• In Steam Turbine no friction losses are there

10. Disadvantages of steam turbines
• High efficiency is ordinarily obtained only at high speed
• Gas turbine locomotives had similar problems, together with a range of
other difficulties
• These devices are heavy and cumbersome
• Turbines can rotate in only one direction

11. Steam turbines are constructed step by step in a logical
manner

12. Steam turbines have air foiled shape
When high energy fluid passes over it this air
foiled shape creates pressure difference
This will create a lift force
Lift Force

13. • The lift force rotates the turbine
• The energy of the fluid is transferred
to mechanical energy of rotor
Flow Energy Mechanical Energy
Fluid has three forms of Energy
1. Speed (Kinetic Energy)
2. Pressure
3. Temperature

14. • To increase velocity the fluid is
entered through stator section
• Stator is stationary and is attached to
turbine case
• Flow area decreases along stator
• The speed thus increases
• The stator acts like a nozel
• As the speed of jet increses in stator,
K.E increases
• As there is not net energy transfer in
fluid in stator section, the pressure
and temperature of the jet should
decrease to keep total energy
constant

15. • Next row of rotor is added the stator also makes sure that flow coming out is at
optimum angle of attack to the next rotor set.
• After that another nozel set is added
• Many such sets are used to th esteam turbine
An important Term
Degree of reaction

16. • The degree of reaction decides what type of steam turbine it is
• Impulse or Reaction
• Pressure of steam drastically decreases its volume increses
proportionally
Ideal Gas Law
𝑉 ∝
1
𝑃

17. • To accommodate such an expended steam we have to increase the flow area
otherwise the flow speed will become too high
• This is why steam turbine blades are too long towards the outlet
Last Stage blade
First Stage blade
Twisted blade
High Velocity
Tips of such blades have very
high velocity compared to
the root
Twist is given to all blade
cross-section should remain
at an optimum angle of attack

18. This type of turbine uses to such
symmetrical units
Steam is equally divided between
these units

19. • High capacity power plants uses different stages of steam turbines

20. • All units are attached to single rotating shaft
Shaft connected
to the generator

21. Reason of different stages
Greater steam temperature results in greater plant efficiency
Second Law of Thermodynamics
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦𝑚𝑎𝑥 = 1 −
𝑇𝐶
𝑇𝐻
Maximum allowed temperature is 600 deree celcius above this
temperature the blades of turbine will melt

22. Temperature decreases as steam flows along blades
Add more heat after first stage

23. After first stage pipe is passed to boiler and more heat is added
This will increase steam temperature again leading to higher plant
efficiency and output

24. One challenging problem is to keep turbine speed constant
This is important because the frequency of electricity produced is
proportional to generator speed
Depending on the load demand
the steam turbine will vary

25. To keep steam turbine constant a steam flow governing mechanism is used
If steam turbine rotates at higher speed the control valve will automatically reduce
the speed of flow rate to turbine until the speed becomes normal
Balance of power demand and power supply will perfectly synchronized