Viessmann wall-hung boiler repair answers; the function of the air preheater
(1) Intensify combustion. Increasing the temperature of the boiler's auxiliary combustion air can shorten the drying time of fuel and promote the volatilization of volatile components, thereby allowing the fuel to ignite quickly, accelerating the combustion speed, enhancing combustion stability, and improving combustion efficiency.
(2) Enhance heat transfer. Due to the use of hot air and enhanced combustion, the flue gas temperature in the combustion chamber can be increased, strengthening the radiant heat exchange inside the furnace. If the boiler evaporation rate is fixed, then the evaporative heating surface can be reduced due to the strengthened heat transfer inside the furnace, which means that a low-cost air preheater can be used to save on high-pressure boiler body heating surfaces (such as water-cooled walls, economizers).
(3) Improve the economic operation of the boiler. Modern thermal power plants all adopt feedwater regenerative cycles. Since steam turbine extraction is used to heat the boiler feedwater, the feedwater reaches quite a high temperature (145～65°C) before entering the economizer. At this point, installing only an economizer cannot sufficiently cool the exhaust gases. Adding an air preheater can effectively further reduce the exhaust gas temperature, reduce exhaust gas heat loss, and improve boiler efficiency.
The Beijing Viessmann Wall-Hung Boiler Repair Center has edited and organized the above content.
Modern boilers' air preheaters generally can cool the exhaust gases to a much lower temperature than the feedwater, even reaching below 120°C. A 50°C reduction in exhaust gas temperature can approximately increase the boiler efficiency by 2%. Meanwhile, due to the reduction in exhaust gas temperature, it also improves the operating conditions of the induced draft fan, reducing the electrical consumption of the induced draft fan. Additionally, since hot air intensifies the combustion process of the fuel, it reduces both gaseous and solid incomplete combustion losses of the fuel. Therefore, a lower excess air coefficient can be adopted, further reducing exhaust gas heat loss and the electrical consumption of fans. Besides these three points, hot air can also serve as the drying medium for fuel in the coal powder preparation system, especially when burning fuels with high moisture and ash content, requiring high-temperature air for drying.