In industrial boiler systems, economizer recirculation is a key component that contributes to energy efficiency, operational safety, and environmental sustainability. Its role extends beyond simple heat management, making it an essential part of modern industrial boiler design. This article details the core functions and working principles of economizer recirculation, providing clarity for industry professionals and technical enthusiasts.
1. Core Functions of Economizer Recirculation
Economizer recirculation serves three primary purposes, all of which align with industrial goals of cost reduction, efficiency improvement, and equipment protection.
1.1 Enhances Energy Efficiency and Reduces Fuel Consumption
A major advantage of economizer recirculation is its ability to recover waste heat from flue gas. In boiler operations, flue gas is typically discharged at high temperatures, carrying significant unused thermal energy. The recirculation system captures this residual heat and transfers it to the feedwater entering the boiler. By preheating the feedwater, the boiler requires less fuel to reach the desired steam temperature, directly lowering fuel consumption. This not only cuts operational costs but also aligns with global efforts to reduce energy waste.
1.2 Lowers Flue Gas Emission Temperature and Reduces Environmental Impact
High-temperature flue gas emissions can contribute to thermal pollution and reduce overall system efficiency. Economizer recirculation addresses this by absorbing heat from the flue gas before it is released. As the flue gas passes through the economizer, its temperature decreases significantly—often by 50–150°C, depending on system design. This reduction in emission temperature minimizes thermal impact on the surrounding environment and helps industrial facilities meet environmental standards for emissions.
1.3 Protects Economizer Tubes During Startup and Shutdown
The most critical safety function of economizer recirculation is preventing overheating of economizer tubes during boiler startup or shutdown. During these phases, the boiler may stop feeding water temporarily (e.g., when water supply is interrupted or during initial ignition). Without water flow, the stagnant water in economizer tubes cannot dissipate heat, leading to overheating, tube deformation, or even burnout.
To avoid this, the recirculation system includes a dedicated recirculation pipe and valve. When water supply stops, the valve opens, creating a loop: the water in the economizer, heated by flue gas, becomes less dense and rises into the boiler drum. Cooler water from the drum then flows back into the economizer, maintaining continuous circulation. This process keeps the tubes cool and intact, extending the economizer’s lifespan and reducing maintenance costs.
2. Working Principles of Economizer Recirculation
The operation of economizer recirculation is based on heat exchange and natural circulation, creating a closed-loop system that maximizes heat recovery. Below is a step-by-step breakdown of its working process:
2.1 Initial Heat Exchange Between Flue Gas and Feedwater
When the boiler operates normally, flue gas (generated by fuel combustion) flows through the economizer first, before being discharged. At the same time, cold feedwater (from the boiler’s water supply system) enters the economizer. Inside the economizer, the flue gas and feedwater pass through separate channels (usually metal tubes and fins). Heat transfers from the high-temperature flue gas to the low-temperature feedwater through the tube walls, raising the feedwater temperature by 30–80°C (varies by system).
2.2 Recirculation for Further Heat Utilization
Even after the initial heat exchange, the feedwater may not reach the optimal temperature for boiler efficiency. To address this, a recirculation pipe is installed at the economizer’s outlet. A portion of the preheated feedwater is redirected back into the economizer inlet via this pipe, rather than being sent directly to the boiler drum. This redirected water mixes with the incoming cold feedwater, creating a higher initial temperature for the next round of heat exchange with flue gas.
2.3 Continuous Circulation Loop
The recirculation process repeats continuously during boiler operation:
Flue gas releases heat to the mixed feedwater (cold + recirculated preheated water) in the economizer.
The now-warmer feedwater splits: most flows to the boiler drum to generate steam, while a small portion is recirculated.
The recirculated water blends with new cold feedwater, starting the heat exchange cycle again.
This loop ensures that flue gas heat is fully utilized, minimizing heat loss and maximizing the boiler’s thermal efficiency.
3. Factors Influencing Economizer Recirculation Performance
In practical applications, the effectiveness of economizer recirculation depends on several variables. Adjusting these factors can optimize system performance:
Flue Gas Parameters: Flue gas temperature, flow rate, and composition (e.g., dust content) affect heat transfer efficiency. Higher flue gas temperatures or flow rates may require adjustments to recirculation flow.
Feedwater Conditions: Feedwater temperature, pressure, and flow rate impact how much heat can be absorbed. For example, lower incoming feedwater temperatures may require increasing the recirculation ratio to maintain preheating efficiency.
Conclusion
Economizer recirculation is a cost-effective, eco-friendly technology for industrial boilers. By recovering flue gas waste heat, reducing fuel consumption, lowering emissions, and protecting equipment, it supports both operational efficiency and sustainability goals. Understanding its functions, principles, and influencing factors allows industrial facilities to optimize their boiler systems, cut costs, and align with global energy-saving and environmental standards. As industrial practices continue to prioritize sustainability, economizer recirculation will remain a key technology in boiler design and operation.
