3 Steam Generators Notes

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

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STEAM GENERATORS

Steam generators, commonly known as steam boilers, are devices that convert water into steam using heat energy. These are essential in power plants, industries, and locomotives where steam is used for power generation, heating, and mechanical work.

This section covers types of steam boilers, their classification, comparison between water-tube and fire-tube boilers, high-pressure boilers, boiler mountings, and boiler accessories.

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3.1 Types and Classification of Steam Boilers

A steam boiler is a closed vessel in which water is heated to produce steam for various applications.

Classification of Steam Boilers

1. Based on the Flow of Water and Hot Gases

   • Fire-Tube Boiler: Hot gases pass through tubes, and water surrounds them.

     • Example: Cochran Boiler, Lancashire Boiler

   • Water-Tube Boiler: Water flows inside the tubes, and hot gases surround them.

     • Example: Babcock & Wilcox Boiler

2. Based on Pressure

   • Low-Pressure Boilers: Operate below 15 bar.

   • High-Pressure Boilers: Operate above 15 bar (e.g., Benson Boiler, Lamont Boiler).

3. Based on Circulation Method

   • Natural Circulation Boilers: Water circulates naturally due to density differences.

   • Forced Circulation Boilers: A pump is used to circulate water.

4. Based on Fuel Used

   • Coal-Fired Boilers

   • Oil-Fired Boilers

   • Gas-Fired Boilers

   • Electric Boilers

5. Based on Application

   • Stationary Boilers: Used in power plants and industries.

   • Mobile Boilers: Used in locomotives, ships, and vehicles.

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3.2 Comparison of Water-Tube and Fire-Tube Boilers

Feature	Fire-Tube Boiler	Water-Tube Boiler
Water and Gas Flow	Hot gases inside tubes, water outside	Water inside tubes, hot gases outside
Steam Generation Rate	Low	High
Operating Pressure	Low to moderate	High
Size	Large and bulky	Compact
Efficiency	Lower	Higher
Response to Load Changes	Slow	Fast
Application	Small industries, heating	Power plants, marine applications

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3.3 High-Pressure Boilers

High-pressure boilers operate at pressures above 15 bar and are used for efficient steam generation in power plants. They have improved heat transfer, forced circulation, and high steam generation rates.

3.3.1 Lamont Boiler

• Type: Water-Tube, Forced Circulation, High-Pressure Boiler.

• Pressure Range: 30-50 bar.

• Steam Generation Rate: 50,000 kg/hr.

• Working Principle:

  • A pump forces water through water tubes where it gets heated by hot gases.

  • The steam-water mixture enters the separator drum, where steam is separated and sent to the turbine.

  • The remaining water is recirculated.

• Applications: Used in thermal power plants and industries.

3.3.2 Benson Boiler

• Type: Water-Tube, Supercritical, High-Pressure Boiler.

• Pressure Range: Above 225 bar (Supercritical).

• Special Feature: No steam drum (unlike other boilers).

• Working Principle:

  • Water is directly converted into steam at supercritical pressure (above 225 bar).

  • This eliminates the need for a steam separator drum.

  • It provides higher efficiency and faster steam generation.

• Applications: Used in modern power plants.

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3.4 Boiler Mountings

Boiler mountings are safety devices and control instruments mounted on a boiler to ensure safe and efficient operation.

Important Boiler Mountings:

1. Safety Valve – Prevents excessive pressure buildup.

2. Water Level Indicator – Shows water level inside the boiler.

3. Pressure Gauge – Measures steam pressure inside the boiler.

4. Blow-Off Valve – Removes sediments from the boiler.

5. Feed Check Valve – Controls water supply into the boiler.

6. Stop Valve – Controls the steam flow from the boiler.

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3.5 Boiler Accessories

Boiler accessories are devices that improve boiler performance and efficiency.

Important Boiler Accessories:

1. Economizer – Heats the feedwater using exhaust gases to improve efficiency.

2. Superheater – Heats the steam beyond saturation temperature to improve energy efficiency.

3. Air Preheater – Warms the air before it enters the furnace, increasing combustion efficiency.

4. Feed Pump – Supplies water to the boiler.

5. Steam Separator – Removes water droplets from steam to produce dry steam.

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Conclusion

Steam generators (boilers) are essential for power generation and industrial applications. Different types of boilers are used based on pressure, efficiency, and application. Water-tube boilers are preferred in power plants due to their higher efficiency and fast response. High-pressure boilers like Lamont and Benson boilers are used in modern thermal power plants for improved efficiency.