Air preheaters can be divided into two main categories based on their heat transfer methods: conductive type and regenerative type. In conductive-type air preheaters, the most commonly used is the tubular air preheater. As boiler parameters and capacities increase, the heating surface of tubular air preheaters also increases, posing challenges to the arrangement of the rear heating surface. Therefore, in large-capacity units, compact and lightweight rotary air preheaters are often used. Air preheaters are generally classified into plate-type, rotary-type, and tubular-type.

 

 

This type of air preheater is mostly made of thin steel plates ranging from 1.5 to 4mm. Steel plates are welded to form rectangular boxes, and several boxes are assembled into a group, with the entire air preheater consisting of 2-4 boxes. The flue gas passes from top to bottom, passing through the outer side of the boxes, while the air passes horizontally through the inside of the boxes, turning upward at the bottom. This interaction occurs twice, transferring energy between the flue gas and air in a counter-flow manner, achieving efficient heat transfer.

 

Due to the high steel consumption, lack of compactness in structure, and susceptibility to weld leakage, plate-type air preheaters are now rarely used.

 

 

The main heat transfer components of a tubular air preheater are thin-walled steel tubes. Tubular air preheaters are often cubic, with steel tubes arranged in a vertical and staggered manner, welded at both ends to upper and lower tube sheets. Inside the tube box, there is a middle tube sheet. The flue gas passes through the preheater vertically along the steel tubes, while the air passes horizontally, completing heat conduction.

 

The advantages of tubular air preheaters include good sealing, high heat transfer efficiency, ease of manufacturing and processing, making them suitable for power station boilers and industrial boilers. However, they have drawbacks such as large volume, susceptibility to ash blockage in the tubes, difficulty in cleaning, and wear at the flue gas inlet.

 

 

Rotary air preheaters are the most common form of regenerative air preheaters, utilizing alternating passages of flue gas and air through metal heating surfaces to heat the air. Rotary air preheaters can be divided into two types based on their movement: rotating heating surface and rotating air duct. In this furnace, two air preheaters are of the three-cell rotating heating surface type.

 

The rotary air preheater consists of a cylindrical rotor, a fixed cylindrical shell, and a driving device.

 

Rotary air preheaters can be further classified into two types: one with a rotating heating surface rotor and the other with a rotating air duct.

 

The rotor rotary air preheater consists of a rotating circular rotor and a fixed shell. The rotor has many compartments, each containing a heat storage plate that absorbs heat from the flue gas and accumulates it. When the rotor rotates to the air side, the accumulated heat is released to the air, lowering its temperature. As the heating surface keeps rotating, heat is continuously transferred from the flue gas to the air, heating the air and cooling the flue gas. This is the working principle of the rotary air preheater.

 

The main advantages of the rotary air preheater are its small size, light weight, allowance for significant wear on heat transfer elements, making it particularly suitable for large boilers. However, its disadvantages include complex structure, power consumption, and relatively large air leakage.