AIR PREHEATERS

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. (1) Deflecting baffles are installed 1o 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 100° to 200° F below the temperature at which the hot air is discharged.

(2) A drum filled with corrugated sheet-steel plates is rotated about a vertical shaft at4 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. (3) 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 section of the casing between the hot-gas and cold-air ducts, thus minimizing leakage between the two streams of fluid. The depth of the rotor is normally between 3 and 4 ft. (4)

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. (5) Since the stoker limits the heat-recovery possibilities of 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. (6).

Air preheaters are installed to preheat the air required for combustion, the heating medium being the flue gases leaving the economizer. (7)

The final temperature of the air will depend upon the method of firing and classes of coal. For pulverized fuel firing air temperatures of 450° to 650° F are possible, whereas in stoker-fired boilers the maximum permissible temperature would be about 400° though in practice temperatures of 250° to 300° F are more usual for chain-grate stokers. (8)

There are four types of heaters: 1) tubular; 2) plate; 3) rotary or regenerative; 4) tubular-needle or gilled.

In the tubular heater the air is passed across the tubes and the flue gases pass through the tubes or vice versa. (9) The rate of heat transfer is low and the space occupied is generally prohibitive. This type of heater may be used with high temperatures. Trouble is experienced in cleaning long tubes and there is added disadvantage in that considerable space is necessary for withdrawal of the tubes.

The plate type was very popular until the rotary heater was developed. The gilled or needle type of heater is also in use. (10)