Finned tubes in boiler economizers utilize waste heat from flue gas to preheat boiler feedwater, thus saving fuel; while in air preheaters, finned tubes heat the combustion air, improving furnace efficiency. Both recover waste heat, but their functions differ. Economizers typically use durable materials to withstand water-side conditions, while air preheaters prioritize increasing the gas-side surface area to improve air heating efficiency, although specific anti-fouling/corrosion designs vary depending on the application.
Boiler Economizer
Function: Preheats feedwater before it enters the boiler drum, reducing the fuel required to convert water into steam.
Fluids: Water (inside the tubes) and high-temperature flue gas (outside the tubes).
Tube Type: Typically uses H-type or spiral finned tubes to increase the flue gas heat transfer area, but requires durable materials (e.g., carbon steel or stainless steel, especially in condensing economizers) to withstand water pressure and potential corrosion/scaling.
Advantages: Significant fuel savings (5-12%) and reduced emissions.
Air Preheater
Function: Heats combustion air using waste heat from flue gas before it enters the furnace, improving combustion efficiency and boiler performance.
Fluids: Air (outside the tubes) and flue gas (inside/around the tubes).
Tube Type: Common types include G-type fins (grooved) or E-type fins (wound), to achieve a large surface area, typically using aluminum or steel fins for efficient air-side heat transfer.
Advantages: Maximizes heat recovery from flue gas to aid combustion, which is particularly important for solid fuel boilers.
Key Differences
Purpose: Economizers preheat boiler feedwater; air preheaters preheat combustion air for the furnace.
Fluids: Economizers handle water/flue gas; air preheaters handle air/flue gas.
Design Focus: Economizers require a balance between heat transfer efficiency and water-side operating limitations; air preheaters focus on maximizing the gas-side surface area to improve air heating efficiency. Material Selection: Economizers require high-strength materials (such as stainless steel in condensing economizers) to handle water; air preheaters prioritize heat transfer efficiency and typically use aluminum fins to improve air-side performance.
The core difference in the application of finned tubes in boiler economizers and air preheaters lies in the different operating temperatures of the media, leading to different fin structure selections and heat exchange priorities—economizers focus on heating feedwater with high-temperature flue gas, using high-frequency welded spiral finned tubes; air preheaters focus on preheating cold air, often using low-finned tubes, and the two differ in fin spacing, materials, and wear-resistant design.
Economizer: Recovers waste heat from flue gas to heat boiler feedwater, thereby reducing fuel consumption and improving boiler efficiency. Its design focuses on efficiently absorbing heat from high-temperature flue gas while addressing dust abrasion and corrosion risks.
Core Function: Recovers waste heat from the boiler’s exhaust gas, heats boiler feedwater, and reduces exhaust gas temperature and heat energy waste.
Structure: Typically consists of steel finned tubes (welded or rolled), with fin types including H-type and spiral type, enhancing heat transfer by expanding the heat transfer surface.
Air Preheater: Utilizes waste heat from flue gas to heat the air required for combustion, improving combustion efficiency and boiler thermal efficiency. Its design focuses on enhancing heat transfer on the air side, as the heat transfer coefficient between gases is generally low.
As a core component of boiler waste heat recovery, the selection of finned tubes directly affects thermal efficiency, and the application of high-efficiency and energy-saving finned tubes is an industry trend in industrial boiler retrofitting.
I. Differences in Operating Conditions: Temperature and Medium Dictate Selection Direction
Economizers utilize high-temperature flue gas to heat feedwater, resulting in large temperature differences and stress. Finned tubes need to withstand high temperatures and corrosion, and heat-resistant steel is often chosen. Air preheaters use medium-to-low temperature flue gas to preheat cold air, resulting in smaller temperature differences and stress. The focus is on improving air-side heat transfer efficiency, and a combination of carbon steel and heat-resistant steel can be used for better cost-effectiveness.
II. Fin Structure Design: Detailed Differences to Meet Heat Exchange Requirements
Due to the high flue gas temperature and high dust content in the economizer, high-frequency welded spiral finned tubes are used to prevent dust accumulation and blockage. The fin spacing is 8-12mm, balancing heat exchange area and wear resistance. The air preheater has a low heat transfer coefficient on the air side, so high-fin, small-pitch finned tubes are selected, with high-frequency welding technology ensuring strong bonding. Integral rolled finned tubes are used in some operating conditions to optimize anti-fouling performance.
III. Heat Exchange Objectives Determine Design Focus
The boiler economizer prioritizes efficient recovery of high-temperature waste heat and increasing feedwater temperature. The finned tube design balances heat exchange efficiency and resistance to high-temperature deformation, with some parts adding wear-resistant covers to reduce dust erosion. The core of the air preheater is to increase the temperature of the cold air to optimize combustion, focusing on enhancing air-side heat transfer and reducing flow resistance. Low-temperature conditions require anti-corrosion treatment to prevent dew point corrosion.
IV. High-Efficiency and Energy-Saving Finned Tubes
In industrial boiler energy-saving retrofits, the application of high-efficiency and energy-saving finned tubes has become a hot topic. Economizers are developing towards high-efficiency, low-resistance, high-frequency welded spiral finned tubes, while air preheaters focus on large heat exchange areas and low-energy-consumption finned tubes. Both aim to improve waste heat recovery efficiency and extend service life.