Air Preheater

Air Preheater

An air preheater is a heat exchanger which is used to preheat the air before entering the furnace by using the waste energy of gases.

The temperature of gases leaving an economizer is an approximate range of 250˚C – 450˚C. This heat energy in exhaust gases is utilized to preheat the surrounding air before being supplied to the combustion chamber.

After the transfer of heat to the air, these gases are finally discharged to the surrounding through the chimney.

Advantages

The advantages of air preheater are listed below.

  • An air preheater is used to improve the rate of combustion of fuel and raises the flame temperature.
  • With the help of higher flame temperatures results in higher heat transfer rates from hot gases to heating surfaces of the boiler.
  • It reduces the amount of excess air needed for the complete combustion of fuel which reduces the heat losses.
  • The use of an air preheater makes it possible to burn low-grade fuels.
  • It improves boiler efficiency and saves fuel costs. (Approximate saving in fuel is 5% per (130˚C-140˚C) air temperature rise)
  • The exit temperature of gases leaving the economizer in the range of 150˚C-200˚C reduces the problem of acid corrosion and helps in removing the gases from the chimney.

Classification of Air Preheater

There are two types of air preheaters;

  • Recuperative air preheater
  • Regenerative air preheater

Recuperative Air Preheater

The heat is directly transferred from the hot gases to the air across the heat transfer surface in a recuperative air preheater.

There are two types of recuperative air preheaters; tubular type and plate type.

In most cases, a tubular type of air preheater is used. In tubular type air preheater, gas flows over the tube. And air passes over the tube or flows over the tubes.

The heat transfer rates are low because of the low heat transfer coefficient. To improve the heat transfer rates either the surface area needs to be increased by using gilled tubes or need to increase the flow of velocity.

In a recuperative air preheater, the baffles are provided to maximize the air contact with the hot tubes.

The diameter of tubes is chosen according to the cost, type of fuel, heat transfer rates, and cleaning requirements.

In most cases, 5cm to 7cm outer diameter tubes are used in modern power plants.

The hollow plates are used in plate-type air preheater. Here, the air is forced to flow while the gas flows in the counter-flow direction.

These types of air preheaters are not used in practice because of choking, corrosion, and maintenance problems.

Regenerative Air Preheater

In regenerative type air preheater, the heat is first transferred from hot flue gases to a storage medium in the form of a matrix and then from matrix to air.

This type of air preheater is also known as storage type air preheater.

The most common type of regenerative or rotary air preheater is the Lungstrom air preheater.

It is a compact heat exchanger with a large heat transfer surface being accommodated in a small volume.

It consists of a rotor driven at low speed by a motor. The speed is reduced through reduction gear at about 2 to 4 rpm (depending upon diameter).

The rotor is divided into 12/24 sectors and each sector is filled with flat or corrugated steel sheets. These sheets are used to form the heat transfer surface.

The stationary sealed sectors on the opposite side of sectors prevent the leakages of gas and air in their respective paths. When the rotor rotates, the gas heats the upper half of the heating surfaces.

When it moves down after half rotation, the air passes over this heated sector and hot air then flows out of the rotor and the cycle repeats itself.

Hence, it can be seen that the heat transfers from the gas to air are periodic.

The disadvantages of regenerative air preheater are;

  • Choking of flow opening by fly ash.
  • Large pressure drops.
  • The problem of the sealing of gas and air paths.

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