Air preheater
[1][2][3][4][5] For instance, as the Ljungström air preheater has been attributed worldwide fuel savings estimated to 4,960,000,000 tons of oil, "few inventions have been as successful in saving fuel as the Ljungström Air Preheater", marked as the 44th International Historic Mechanical Engineering Landmark by the American Society of Mechanical Engineers.
Further, due to dust-laden abrasive flue gases, the tubes outside the ducting wear out faster on the side facing the gas current.
[12] In the tri-sector design, the largest sector (usually spanning about half the cross-section of the casing) is connected to the boiler hot gas outlet.
The third sector is the smallest one and it heats air which is routed into the pulverizers and used to carry the coal-air mixture to coal boiler burners.
The rotor itself is the medium of heat transfer in this system, and is usually composed of some form of steel and/or ceramic structure.
The top end of the rotor has a simple roller bearing to hold the shaft in a vertical position.
The rotor is built up on the vertical shaft with radial supports and cages for holding the baskets in position.
Radial and circumferential seal plates are also provided to avoid leakages of gases or air between the sectors or between the duct and the casing while in rotation.
The boiler flue gas contains many dust particles (due to high ash content) not contributing towards combustion, such as silica, which cause abrasive wear of the baskets, and may also contain corrosive gases depending on the composition of the fuel.
For example, Indian coals generally result in high levels of ash and silica in the flue gas.
In this RAPH, the dust laden, corrosive boiler gases have to pass between the elements of air preheater baskets.
The elements are made up of zig zag corrugated plates pressed into a steel basket giving sufficient annular space in between for the gas to pass through.
Sometimes mild explosions may be detected in the control room by variations in the inlet and outlet temperatures of the combustion air.
