Tubular Air Preheaters in Large Power Plant Boilers

Tubular Air Preheaters in Large Power Plant Boilers

Tubular air preheaters are composed of numerous thin-walled steel tubes mounted on upper, lower, and intermediate tube sheets to form a tube box. They are most commonly used in power plant boilers.

Core Structure - Tube Box:

Steel Tubes: Typically, thin-walled seamless steel tubes with a diameter of 40-51 mm and a wall thickness of 1.5-2 mm are used. The thin wall helps reduce thermal resistance.

Tube Sheets: Two thick steel plates, upper and lower, secure and support the ends of all the steel tubes, creating a seal. The intermediate tube sheet supports the long steel tubes vertically to prevent bending and vibration.

Flue Gas and Air Flow Paths (Key Design):

Flue Gas Flowing Inside the Tubes (Tube Side): This is the most common design. High-temperature flue gas flows from top to bottom through the steel tubes, transferring heat to the tube walls.

Air Flowing Outside the Tubes (Shell Side): Cool air flows horizontally (or vertically) within the tube box shell, absorbing heat from the tube walls.

Why this design?

Easy to clean: Fly ash contained in the flue gas flows through the straight tubes, making it less likely to accumulate. Even if accumulated, it can be easily removed with steel balls or compressed air.

Reduced wear: The air outside the tubes is relatively clean, causing less wear on the steel. Allowing dust-laden flue gas to flow laterally outside the tubes would cause severe wear on the tube walls.

Structural sealing: The tube box as a single unit makes it easier to ensure air-side sealing, resulting in lower air leakage.

Flow and layout:

To enhance heat transfer, the air flow is often designed as a multi-channel rotary flow. This means that air is directed between multiple tube boxes, circling back and forth multiple times, creating countercurrent or cross-countercurrent heat exchange with the flue gas, thereby maximizing the final air outlet temperature.

The tubular air preheater has become mainstream in power plant boilers primarily due to its:

Simple structure and ease of manufacturing: Compared to complex rotary air preheaters, its structural principles are simple, its manufacturing process is mature, and its cost is relatively low. High reliability and easy maintenance: With no moving parts, the system operates smoothly and has a low failure rate. If individual tubes are damaged, they can be sealed, simplifying maintenance.

Low air leakage: Static sealing is much easier to achieve than dynamic sealing. The air leakage rate of tubular air preheaters can typically be controlled below 5%, while earlier rotary air preheaters could have rates as high as 10%-15%. Low air leakage means higher fan efficiency and lower power consumption.

High pressure resistance: The tube walls can withstand high pressures, making them suitable for boiler systems with high furnace pressures.

No cross-contamination: Flue gas and air are completely separated by the tube walls, eliminating the risk of flue gas leaking into the air side and contaminating the supply air.