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The air heater






The tubular air heater is constructed by expanding vertical tubes into parallel tube sheets which form the top and bottom surfaces, respectively, of the gas inlet and outlet boxes. The tube bank is enclosed in an insulated casing so constructed that the inlet air at room temperature can be admitted to the heating surfaces at the upper end from a fan or blower. The air passes downward around the tubes in a direction opposite to the flow of the hot gases and leaves the air heater at the lower end of the tube bank. Deflecting baffles are installed to guide the air and reduce frictional resistance at the turns. A by-pass damper and baffle permit by-passing the air around the upper half of the tube surface on light load when there is danger of corrosion due to low flue-gas temperatures. Long tubes closely spaced to maintain high air and gas velocities and countercurrent flow of gases and air make it possible in many installations to cool the gases to a temperature 1000 to 200° F below the temperature at which the hot air is discharged.

Let us consider another type of air heater which operates on the regenerative principle. A drum filled with corrugated sheet-steel plates is rotated about a vertical shaft at about 3 rpm by means of a small motor. Hot flue gas passes downward through the right side of the rotor from a duct connected to the economizer or boiler. An induced-draft fan may be connected by a duct to the lower side of the air-heater casing. This fan induces a flow of the gases through the boiler, economizer, and air-heater surfaces, and discharges them to waste up the chimney. The cold air from a forced-draft fan flows upward through the left side of the rotor, where the air is heated, after which it is delivered through suitable duct work to the stoker or burner in the furnace. Any point on the corrugated sheet- metal surface of the rotor is rotated alternately into the hot descending gas stream


and the cold ascending air stream, thus transferring energy from the hot gas to the cold air.

Radial seals with rubbing surfaces on them are mounted on the rotor and make contact with a flat selection of the casing between the hot-gas and cold-air ducts, thus-1rnininiizing leakage between the two streams of fluid. The depth of the rotor is normally between 3 and 4 ft. The unit is also made for operation about a horizontal shaft with horizontal flow of gas and air where building space makes such an arrangement desirable.

The maximum air temperature that can be used in stoker-fired installations without increasing grate maintenance is about 300° F, since the grate surface which supports the hot fuel bed must be cooled by the air to a temperature below which the iron grates-will not be damaged. Air temperatures of 600° F are often used with pulverized coal. Since the stoker limits the heat-recovery possibilities dl the air heater, both economizers and air heaters are usually installed in stoker-fired high-pressure steam-generating units. Where oil, gas, or pulverized coal is burned, an air heater is often installed without an economizer, although in many high- pressure units it may be more economical to reduce the boiler surface and use an economizer. The air heater is necessary in modern pulverized-coal plants since the coal is dried in the pulverizer by hot air to reduce power consumption and increase the capacity of the mill.

 


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