Short Answer:

An air preheater is a heat recovery device used in boilers and furnaces to heat the air before it enters the combustion chamber. It uses the waste heat of flue gases to raise the temperature of the incoming air, which improves fuel combustion and increases the efficiency of the boiler.

In simple words, an air preheater transfers heat from the exhaust gases leaving the boiler to the fresh air entering the furnace. By preheating the air, it ensures better fuel burning, reduces fuel consumption, and enhances the overall efficiency and performance of the boiler system.

Detailed Explanation :

Air Preheater

An air preheater is an important heat exchange device in a boiler system that improves the thermal efficiency of a power plant or industrial furnace. Its primary function is to recover waste heat from the flue gases coming out of the boiler and use this heat to preheat the air before it enters the combustion chamber.

When cold air is supplied to the furnace, a large portion of heat energy from fuel combustion is used just to raise the temperature of the air, reducing overall efficiency. By using an air preheater, this loss is minimized as the air enters the furnace already heated, which helps in better combustion, fuel savings, and reduction in exhaust losses.

1. Purpose of Air Preheater

The main purpose of an air preheater is to increase the efficiency of the boiler by recovering heat from flue gases. The key purposes include:

1. To Improve Combustion Efficiency:
   • Hot air mixes with fuel more easily, ensuring complete combustion.
2. To Save Fuel:
   • Since air is already preheated, less fuel energy is required to reach the desired furnace temperature.
3. To Increase Boiler Efficiency:
   • Reduces the temperature of exhaust gases, lowering heat loss.
4. To Reduce Moisture in Fuel:
   • Hot air helps in drying moist fuel, improving burning quality.
5. To Enhance Steam Generation Rate:
   • With improved combustion, the furnace can achieve higher temperatures and steam production rates.

2. Construction of Air Preheater

An air preheater is typically installed between the economizer and the chimney in a boiler system. The hot flue gases from the economizer pass through the air preheater before leaving the chimney.

The main components are:

1. Casing:
   • The outer shell that holds all the internal parts and prevents heat leakage.
2. Tubes or Plates:
   • Heat exchange surfaces where air and flue gases come into indirect contact for heat transfer.
3. Fans or Ducts:
   • Supply cold air and remove heated air for use in combustion.
4. Rotating or Fixed Heat Transfer Elements:
   • In some designs, rotating elements are used to continuously exchange heat between air and gases.
5. Bypass Dampers:
   • Allow control of air and gas flow depending on load and operating conditions.

The air preheater must be made of heat-resistant and corrosion-resistant materials, as it operates under high temperatures and harsh conditions.

3. Working of Air Preheater

The working principle of an air preheater is heat exchange between hot flue gases and cold air.

1. The flue gases from the boiler furnace, still containing significant heat, enter the air preheater.
2. The cold air required for combustion is simultaneously passed through the preheater in the opposite direction.
3. As the flue gases and air pass through, heat is transferred from the hot gases to the cold air through metal surfaces (tubes or plates).
4. The flue gases become cooler before exiting through the chimney, and the air gets heated before entering the furnace.
5. The heated air now supports more efficient and complete fuel combustion in the furnace.

This counter-flow arrangement ensures maximum heat recovery and efficiency.

4. Types of Air Preheaters

There are mainly three types of air preheaters used in boilers and furnaces:

1. Recuperative Air Preheater

• It consists of metal tubes or plates through which hot flue gases and air pass separately.
• Heat transfer occurs through the wall separating them.
• Commonly used in industrial boilers.
• Advantages: Simple design and low maintenance.

1. Regenerative Air Preheater

• In this type, the same surface is alternately heated by flue gases and cooled by air.
• A rotating heat storage element (rotor) absorbs heat from flue gases and transfers it to the incoming air.
• Commonly used in large power plants.
• Advantages: Very high efficiency and compact design.

1. Plate Type Air Preheater

• Uses a series of corrugated plates for heat exchange between air and flue gases.
• Suitable for small industrial applications.

5. Advantages of Air Preheater

1. Increases Boiler Efficiency:
   • Reduces fuel consumption by recovering heat from flue gases.
2. Improves Combustion:
   • Hot air ensures better fuel-air mixing and more complete combustion.
3. Reduces Flue Gas Temperature:
   • Minimizes heat loss through the chimney.
4. Reduces Fuel Moisture:
   • Especially useful for fuels like coal, where preheated air dries the fuel before burning.
5. Enhances Power Output:
   • Higher furnace temperature leads to greater steam generation and improved turbine performance.
6. Environmental Benefits:
   • Better combustion means lower smoke and carbon emissions.

6. Disadvantages of Air Preheater

1. Initial Cost:
   • Adds to the installation cost of the boiler.
2. Maintenance Needs:
   • Soot and dust deposits can reduce heat transfer efficiency.
3. Corrosion Risk:
   • Flue gases may contain acidic moisture leading to corrosion of tubes.
4. Pressure Drop:
   • Air and gas flow may face resistance through narrow passages.
5. Temperature Control Issues:
   • Excessive heating of air can cause overheating in the furnace.

7. Applications of Air Preheater

Air preheaters are widely used in:

• Thermal power plants for efficient combustion of coal or oil.
• Industrial furnaces in steel and cement industries.
• Marine boilers to save fuel on ships.
• Refineries and chemical plants for heat recovery.

In modern power plants, they form an essential part of the boiler heat recovery system, working alongside economizers and superheaters.

8. Efficiency and Energy Saving

The efficiency gain due to an air preheater can be estimated as:

Generally, the addition of an air preheater can improve boiler efficiency by 5% to 10%, and the flue gas temperature is reduced by up to 100°C, leading to substantial energy savings.

Conclusion

An air preheater is an important component in a boiler system designed to recover heat from flue gases and preheat the air used for combustion. It improves combustion efficiency, saves fuel, reduces heat loss, and increases overall plant efficiency. Though it involves additional cost and maintenance, the long-term benefits in fuel economy and energy conservation make it an essential device in modern thermal power plants and industrial boilers. In combination with economizers and superheaters, air preheaters help achieve maximum thermal efficiency in boiler operations.