A major problem for all Central Heating-and-Power Plants is to achieve as much as possible fuel savings. The improvement the efficiency of such plants includes, also, the maintaining the temperature of combustion air at maximum level. In the case of dirty gaseous fuels, such as the coal, the most used air-preheater system is rotary Ljungstrom type preheater. Due to the many sources of boiler inefficiency, the temperature of flue gas changes and the number of rotation of heat wheel must change also. This paper presents a calculation method to determine the number of rotation of the heat wheel on the basis the maintaining time necessary for the metallic matrix of the heat wheel to absorb maximum energy from the flue-gas.
