STEAM TURBINES, Mechanical Engg 3rd Semester Notes, ME3001

 

3. STEAM TURBINES

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam and converts it into mechanical energy (rotational motion). Steam turbines are widely used in power plants for electricity generation.

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3.1 Impulse and Reaction Turbines

Steam turbines are classified into two main types based on their working principles: Impulse Turbines and Reaction Turbines.

Impulse Turbine:

• Working Principle: In an impulse turbine, the steam jet is directed onto the blades of the turbine rotor, causing the rotor to spin. The steam expands in a nozzle before hitting the blades. The pressure of the steam decreases as it passes through the nozzle, converting pressure energy into kinetic energy (speed of steam). This high-speed steam strikes the blades at an angle, causing them to rotate. The pressure of the steam does not change significantly as it passes over the blades. The energy is transferred purely by the velocity (impulse).

  • Diagram:

        High-Pressure Steam
          ↓
   +---------------------+
   |       Nozzle        | → Steam expands to high velocity
   +---------------------+
          ↓
     +------------------+      ← Blades (move due to high velocity)
     |   Rotor Blade    |
     +------------------+
          ↓
    +------------------+
    |   Generator      |
    +------------------+
          ↓
       Low-Pressure
         Steam
  

• Characteristics:

  • The steam pressure remains nearly constant during the expansion process in the blades.
  • The energy is transferred to the turbine via the high-speed steam (impulse).
  • The impulse turbine typically operates at higher steam pressures.

Reaction Turbine:

• Working Principle: In a reaction turbine, steam flows over the blades, and the pressure of the steam decreases as it passes over the blades. As the steam expands, it creates a reaction force that causes the blades to rotate. Unlike the impulse turbine, the blades in a reaction turbine are designed to harness both pressure and velocity changes in the steam. The reaction force is the result of both the change in pressure and the momentum of the steam.

  • Diagram:

        High-Pressure Steam
          ↓
   +---------------------+
   |       Nozzle        | → Steam expands and increases velocity
   +---------------------+
          ↓
    +-------------------+
    | Rotor Blades      | ← Steam passes over blades, causing them to rotate
    +-------------------+
          ↓
    +------------------+
    |   Generator      |
    +------------------+
          ↓
       Low-Pressure
         Steam
  

• Characteristics:

  • The steam pressure decreases as it passes over the rotor blades.
  • Energy is transferred to the rotor both through the kinetic energy of the steam and its pressure.
  • Reaction turbines have a more complex design compared to impulse turbines but offer better efficiency at lower pressures.

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3.2 Condensers: Jet & Surface Condensers (Only Working Principle)

A condenser is a heat exchanger used in steam power plants to condense the exhaust steam from the turbine into water, which is then pumped back into the boiler. There are two main types of condensers: Jet Condensers and Surface Condensers.

Jet Condenser:

• Working Principle: In a jet condenser, exhaust steam from the turbine is directly mixed with cool water (usually from a cooling tower or nearby water source) to condense the steam. The mixture of steam and water (known as a “jet” or "water-steam mixture") is then passed through a separator to remove the condensed water. The cooling water absorbs the heat from the steam, causing the steam to condense.

  • Diagram:

    +-----------------+
    |   Exhaust Steam |  ← From turbine
    +-----------------+      ↓
             ↓       +---------------------+
    +----------------+|   Cooling Water     |  ← From cooling tower
    |   Condenser    | +---------------------+
    +----------------+        ↓
             ↓           +---------------+
    +--------------------+| Water-Steam  |  ← Jet mixture
    | Condensed Water    | +---------------+ 
    +--------------------+
             ↓
       Discharge Water
  

• Characteristics:

  • Simpler and less expensive than surface condensers.
  • Requires a large amount of cooling water.
  • Condensing efficiency is lower compared to surface condensers.

Surface Condenser:

• Working Principle: In a surface condenser, the exhaust steam does not mix with the cooling water. Instead, the exhaust steam flows over a series of tubes, and cool water flows inside these tubes. The heat from the steam is transferred to the cooling water through the tube walls, causing the steam to condense on the outer surface of the tubes. The condensed steam (water) is then collected and pumped back to the boiler.

  • Diagram:

    +-----------------+ 
    |   Exhaust Steam |  ← From turbine
    +-----------------+      ↓
             ↓       +---------------------+
    +----------------+|   Surface Tubes     |  ← Water flows inside
    |   Condenser    | +---------------------+
    +----------------+        ↓
             ↓           +---------------+
    +--------------------+|  Condensed Water| 
    |  Condensed Water   | +---------------+ 
    +--------------------+
             ↓
       Discharge Water
  

• Characteristics:

  • More efficient than jet condensers due to better heat transfer.
  • Requires a large surface area for heat exchange, leading to higher costs.
  • The cooling water and steam do not mix, preventing contamination.

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3.3 Cooling Towers (Only Working Principle and Types)

Cooling Towers are large structures used to dissipate waste heat from a thermal power plant into the atmosphere. They use water to remove heat from the plant’s condenser system and discharge it as vapor.

Working Principle:

The cooling tower operates on the principle of evaporative cooling. Warm water from the condenser is pumped to the top of the tower. As the water descends through the tower, air is blown across it, and some of the water evaporates, carrying heat away. The remaining cooled water is collected at the bottom and recirculated back to the condenser.

• Diagram:

          +----------------------+
          |   Warm Water Inlet   |
          |      (From Condenser) |
          +----------------------+
                  ↓
            +-----------------+
            | Cooling Water   |  ← Air flow through tower (natural or forced)
            | (Evaporative)   |
            +-----------------+
                  ↓
        +-----------------------+
        | Cooled Water Outlet   |  ← Cooled water is recirculated
        +-----------------------+
  

Types of Cooling Towers:

1. Natural Draft Cooling Towers:

   • Working Principle: These towers rely on natural convection to move air through the tower. The warm water rises, and as it cools, the air enters the tower through the bottom. The shape of the tower (usually hyperbolic) helps create a natural draft that pulls air up through the tower.

   • Diagram:

              ^
              |
      +---------------------+
      |    Hot Water Inlet  |
      +---------------------+   
              |
              ↓
     +-------------------------+
     |  Large Hyperbolic Tower |  
     | (Natural Air Flow)      |
     +-------------------------+
              ↓
     +---------------------+
     | Cooled Water Outlet |
     +---------------------+
     

2. Mechanical Draft Cooling Towers:

   • Working Principle: These towers use fans (either forced or induced) to circulate air through the tower. In forced draft towers, fans blow air into the tower, while in induced draft towers, fans suck air through the tower. Mechanical draft cooling towers can be smaller and more efficient than natural draft towers.

   • Diagram:

          +---------------------+
          |  Hot Water Inlet    |
          +---------------------+
                  ↓
          +---------------------+    ← Fans induce air circulation
          |  Cooling Tower      |
          | (Mechanical Draft)   |
          +---------------------+
                  ↓
          +---------------------+
          | Cooled Water Outlet |
          +---------------------+
     

Comparison:

Feature	Natural Draft Cooling Towers	Mechanical Draft Cooling Towers
Air Flow	Natural convection	Forced/induced draft with fans
Size	Large (tall towers)	Smaller, compact
Efficiency	Lower (due to less air control)	Higher (better air control)
Cost	Higher initial cost	Lower initial cost