Air preheater is a key component in thermal equipment such as boilers, and its working principle is based on the basic law of heat transfer. It preheats air by recovering waste heat from flue gas. The following introduces the working principles of different types of air preheaters:
Tubular air preheater
Structural foundation
The tubular air preheater is composed of many thin-walled steel pipes arranged neatly and fixed on the upper, lower, and middle tube plates to form a tube box. The pipe box is usually installed in the flue at the tail of the boiler, where the flue gas flows longitudinally inside the pipe and the air flows laterally outside the pipe to flush the pipe.
heat transfer process
Heat release from flue gas: The high-temperature flue gas discharged from the tail of the boiler enters the tubes of the tubular air preheater, where the flue gas temperature is relatively high and the tube wall temperature is relatively low. According to the principle of heat conduction, heat will be transferred from the high-temperature flue gas to the lower temperature pipe wall.
Air absorbs heat: At the same time, cooler air flows laterally outside the tube, scouring the heated tube wall. Due to the air temperature being lower than the pipe wall temperature, heat will be transferred to the air through the pipe wall, causing the air temperature to rise.
Cycle process: The flue gas continuously transfers heat to the pipe wall, and then transfers it to the air through the pipe wall. Its own temperature gradually decreases, and finally it is discharged from the preheater; And the air continuously absorbs heat, the temperature gradually increases, and then enters the boiler to participate in the combustion process.
Plate air preheater
Structural characteristics
The plate type air preheater is composed of multiple thin steel plates welded into long square plate bundles, which allows the flue gas and air to form specific flow channels between the plate bundles.
Heat transfer mechanism
Smoke process: Smoke flows from top to bottom through the outer channel of the plate bundle, and the heat in the smoke is transferred to the other side of the plate bundle through the thin steel plate.
Air flow: The air passes horizontally through the plate bundle and enters the internal channel, absorbing the heat transferred from the steel plate during the flow process to achieve an increase in temperature. At the bottom of the plate air preheater, the air will turn back upwards and undergo a second reverse heat exchange with the flue gas, further improving the heat exchange efficiency.
Rotary air preheater
Overall structure
The rotary air preheater is mainly composed of a casing, rotor, transmission device, and sealing device. The outer shell consists of a polygonal cylinder, upper and lower end plates, and a fan-shaped plate. The end plate is equipped with openings for smoke and air ducts to connect with the flue and air ducts; The rotor is the core component of a rotary air preheater, which contains a large number of heat transfer elements such as corrugated plates inside.
operational principle
Heating stage of flue gas: When the rotor rotates to the flue gas side, high-temperature flue gas enters the flue gas side of the air preheater from the flue and flows through the heat transfer elements in the rotor. The flue gas transfers heat to the heat transfer element, causing the temperature of the heat transfer element to increase while the temperature of the flue gas itself decreases.
Air preheating stage: As the rotor continues to rotate, the heated heat transfer element moves to the air side. At this point, the cooler air enters the air side of the air preheater through the duct and flows through the heat transfer element. The heat transfer element transfers the previously absorbed heat to the air, causing the air temperature to rise while the temperature of the heat transfer element decreases.
Cyclic rotation: The rotor continuously rotates, and the flue gas and air flow alternately through the heat transfer element, achieving continuous heat exchange. In this way, the rotary air preheater can transfer heat from the flue gas to the air.