What Is The Basic Principle of An Air Preheater?

What is the basic principle of an air preheater?

The basic principle of an air preheater is to increase the air temperature by transferring heat from the flue gas to the air entering the burner.

An air preheater is a typical air-to-air heat exchanger, and its operation is as follows:

Hot-side fluid: High-temperature flue gas (typically 200-400°C or even higher) from a boiler or industrial furnace.

Cold-side fluid: Ambient-temperature air entering the boiler burner.

Heat exchange process: The high-temperature flue gas flows through the heat exchange elements, transferring the heat it carries to the heat transfer surfaces. Subsequently, the cold air flows over these heated heat transfer surfaces, absorbing the heat and raising its temperature.

Main Purposes and Benefits

The benefits of increasing air temperature are significant and directly impact the efficiency and economy of the boiler:

Improving boiler efficiency (primary purpose):

After the hot air enters the furnace, it accelerates the drying, preheating, and ignition of the fuel, resulting in faster, more complete, and more stable combustion.

The increased theoretical combustion temperature enhances radiant heat transfer within the furnace. The end result: less fuel is consumed for the same steam or heat output. Generally, every 20°C increase in inlet air temperature increases boiler efficiency by approximately 1%.

Enhanced combustion and reduced incomplete combustion losses:

The high-temperature air provides physical heat to the fuel, reducing the amount of heat required to ignite it and enabling stable combustion of even difficult-to-burn fuels (such as low-quality coal or high-moisture fuel).

It reduces unburned carbon particles and combustible gases in the flue gas, saving fuel and reducing pollutant emissions.

Improved heat exchange within the furnace:

The increased combustion temperature enhances radiative heat transfer within the furnace, allowing more heat to be absorbed within the furnace and reducing the burden on subsequent heating surfaces.

Auxiliary function: Used for drying (e.g., in power plant pulverized coal boilers):

In large pulverized coal boilers, this hot air also serves as "primary air," used to transport and dry the pulverized coal, making it an integral part of the pulverizing system.