5 Steam Turbines Notes

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Subject - THERMAL ENGINEERING - II ME 4003
Branch - Mechanical Engineering
Semester - 4th Semester

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STEAM TURBINES

A steam turbine is a mechanical device that converts the thermal energy of steam into mechanical energy by allowing steam to expand and move through blades mounted on a rotating shaft. Steam turbines are widely used in power generation, marine propulsion, and industrial processes.

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5.1 Classification of Steam Turbines with Examples

Steam turbines can be classified based on different criteria:

(a) Based on Energy Conversion Principle

1. Impulse Turbine – Steam expands completely in nozzles and strikes the blades with high velocity.

   • Example: De Laval Turbine, Curtis Turbine

2. Reaction Turbine – Steam expands partly in nozzles and partly in moving blades, generating reaction force.

   • Example: Parsons Turbine

(b) Based on Steam Flow Direction

1. Axial Flow Turbine – Steam flows parallel to the shaft axis.

2. Radial Flow Turbine – Steam flows radially inward or outward towards the shaft.

(c) Based on Number of Stages

1. Single-Stage Turbine – Steam expands in one set of nozzles and blades.

2. Multi-Stage Turbine – Steam expansion occurs in multiple stages for higher efficiency.

(d) Based on Exhaust Condition

1. Condensing Turbine – Steam exhausts into a condenser, reducing pressure for more work output.

2. Non-Condensing (Back Pressure) Turbine – Steam exits at higher pressure for industrial use.

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5.2 Difference Between Impulse and Reaction Turbines

Feature	Impulse Turbine	Reaction Turbine
Expansion	Complete expansion in nozzles	Partial expansion in nozzles and moving blades
Pressure Drop	Occurs only in nozzles	Occurs in both nozzles and blades
Steam Velocity	High at the nozzle exit	Lower than impulse type
Blade Design	Symmetrical blades	Aerofoil-shaped blades
Energy Transfer	Kinetic energy transfer	Pressure and kinetic energy transfer
Example	De Laval, Curtis Turbine	Parsons Turbine

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5.3 Working of a Simple De Laval Turbine

The De Laval turbine is a single-stage impulse turbine.

Working Principle

1. High-pressure steam expands in a single set of nozzles, increasing its velocity.

2. The high-velocity steam strikes the moving blades, transferring energy to the rotor.

3. The steam pressure remains constant in the moving blades as it only changes direction.

Diagram

• The turbine consists of a nozzle, rotor with moving blades, and exhaust.

• A high-speed rotating shaft is directly coupled to the generator or machinery.

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5.4 Velocity Diagrams

A velocity diagram helps in understanding the velocity components of steam as it moves through the turbine blades.

Components of Velocity

1. Absolute Velocity (V₁, V₂) – Velocity of steam before and after striking the blades.

2. Blade Velocity (U) – Speed of moving blades.

3. Relative Velocity (Vr₁, Vr₂) – Velocity of steam relative to the blade.

4. Inlet and Exit Angles – Determines steam deflection.

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5.5 Expressions for Work Done, Axial & Tangential Thrust, Efficiencies

1. Work Done per Stage:

$$W = m \cdot (V_{w1} + V_{w2}) \cdot U$$

where,

• $m$ = Mass flow rate of steam

• $V_{w1}, V_{w2}$ = Whirl component of velocity

• $U$ = Blade speed

2. Axial Thrust:

$$F_a = m (V_{a1} - V_{a2})$$

where,

• $V_{a1}, V_{a2}$ = Axial velocity components

3. Tangential Thrust:

$$F_t = m (V_{w1} + V_{w2})$$

4. Efficiencies:

• Blade Efficiency:

$$\eta_b = \frac{\text{Work Done on Blades}}{\text{Kinetic Energy Supplied by Steam}}$$

• Stage Efficiency:

$$\eta_s = \frac{\text{Work Done per Stage}}{\text{Energy Supplied per Stage}}$$

• Nozzle Efficiency:

$$\eta_n = \frac{\text{Actual Kinetic Energy at Exit}}{\text{Theoretical Kinetic Energy}}$$

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5.6 Methods of Reducing Rotor Speed

Since turbines operate at very high speeds, they require speed reduction methods:

1. Compounding – Steam expansion is divided into multiple stages.

2. Gear Reduction – A reduction gearbox lowers the output speed.

3. Electrical Speed Control – Generators use frequency control.

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5.7 Compounding of Steam Turbines

Compounding is used to reduce rotor speed while maintaining efficiency.

1. Velocity Compounding (Curtis Turbine) – Uses multiple sets of moving blades with fixed guide blades.

2. Pressure Compounding (Rateau Turbine) – Steam expands in multiple nozzles and blade stages.

3. Pressure-Velocity Compounding – Combination of both types.

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5.8 Working of a Parsons Reaction Turbine

Working Principle:

• Steam expands partially in fixed blades and partially in moving blades.

• The expansion produces reaction force, which rotates the rotor.

• Pressure gradually decreases across the turbine.

Diagram:

• Consists of fixed guide blades and moving blades arranged alternately.

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5.9 Velocity Diagrams for Reaction Turbine

• Reaction turbines use aerofoil-shaped blades, affecting steam flow angles.

• The velocity diagram shows how steam expands and changes direction.

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5.10 Simple Problems on Single-Stage Impulse and Reaction Turbines

Example 1: Work Done in an Impulse Turbine

Given:

• Steam velocity at nozzle exit = 300 m/s

• Blade speed = 100 m/s

• Inlet angle = 20°

Find Work Done per kg of steam:

$$W = V_{w1} \cdot U$$

Using velocity triangle, $V_{w1}$ is calculated and substituted in the equation.

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5.11 Bleeding, Re-heating, and Re-heating Factors

• Bleeding – Extraction of steam for heating purposes.

• Re-heating – Steam is reheated between turbine stages for efficiency.

• Re-heating Factor – Measures how much re-heating improves efficiency.

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5.12 Governing of Steam Turbines

Turbines require governing mechanisms to regulate speed and load.

1. Throttle Governing – Steam flow is controlled using valves at the inlet.

2. Bypass Governing – Part of the steam is diverted, reducing power.

3. Nozzle Control Governing – Selected nozzles are opened/closed based on demand.

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Conclusion

• Impulse turbines use nozzles for expansion, while reaction turbines use moving blades.

• Velocity and pressure compounding help in speed control.

• Turbine efficiency depends on blade design and governing techniques.