A flue gas air preheating apparatus for combustion air in a boiler of a power plant. An air pipe is placed in a flue gas duct. An air guide sleeve is provided at an initial end of the air pipe at least partly inside the air pipe. The air guide sleeve is made of a poorly heat conductive material and is designed to diminish turbulence in air flow and has an inflow end and a second end placed inside the air pipe. The shaping of the second end is designed to diminish turbulence in the air flow. The air guide sleeve includes splits or notches that become wider towards the second end. A method and a corrosion shield.
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