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| Fig. 1: Air Heater of Steam Boiler |
Air heater has an important role to improve the efficiency of the high thermal of the steam boiler by utilizing the thermal energy with low temperature flue gas from the steam boiler before being discharged into atmosphere. Air heater doing heat transfer between flue gas with combustion air. Air heater cooling the flue gas air for every 40o F (22o C)will increase the overall steam boiler efficiency is about 1%. With the utilization of heat from flue gas combustion air will be hotter so it can save fuel requirement needed.
Air heater requires a large amount of heat transfer surface per unit of heat recovered because of the relatively small difference between the flue gas temperature and temperature of combustion air. Air heater is typically located behind steam boiler, where it receives hot flue gas form the economizer and cold combustion air from the force draft fan. The hot air produced by air heater to enhance combustion of all fuel and is needed for drying fuel like coal in stoker steam boiler.
The air heater is usually the last heat trap in the steam boiler. Air heater exit gas temperature should be higher than the corrosion limit of the AH internals and the downstream equipment. Air heaters generally operate in a temperature range of 450–120°C on the gas side and 150–450°C on air side. Design and operation of air heater should consider as follow:
§ Fouling and plugging of air heater
Fouling is the deposition of gas-entrained ash. The deposition at low temperatures occurs due to the adherence of ash to the acid-moistened surfaces at the low end of the air heater. Pressure drop increases as the deposits grow in size. If unchecked, gas passages become progressively choked or plugged. Regular soot blowing can reduce the fouling considerably. Offload water washing is carried out in many installations for deposit removal, as they are mostly water soluble.
§ Erosion in air heater
Employing conservative gas velocities is a design-stage precaution against gas-side erosion, particularly for dusty fuels. Gas velocities have to be high to economize on the surfaces and evolve decent layouts and erosion has to be minimized. Normally erosion is at the entry to the bank where the velocities are the highest, and the maximum velocity limit should be applied at this point. For a tubular air heater it is normal to provide the top row with thicker tubes of, for example erosion shields of typically 3 mm. Heat transfer in tubes covered with erosion shields suffers because there is no direct contact of the tube and gas.
Air heater requires a large amount of heat transfer surface per unit of heat recovered because of the relatively small difference between the flue gas temperature and temperature of combustion air. Air heater is typically located behind steam boiler, where it receives hot flue gas form the economizer and cold combustion air from the force draft fan. The hot air produced by air heater to enhance combustion of all fuel and is needed for drying fuel like coal in stoker steam boiler.
The air heater is usually the last heat trap in the steam boiler. Air heater exit gas temperature should be higher than the corrosion limit of the AH internals and the downstream equipment. Air heaters generally operate in a temperature range of 450–120°C on the gas side and 150–450°C on air side. Design and operation of air heater should consider as follow:
§ Fouling and plugging of air heater
Fouling is the deposition of gas-entrained ash. The deposition at low temperatures occurs due to the adherence of ash to the acid-moistened surfaces at the low end of the air heater. Pressure drop increases as the deposits grow in size. If unchecked, gas passages become progressively choked or plugged. Regular soot blowing can reduce the fouling considerably. Offload water washing is carried out in many installations for deposit removal, as they are mostly water soluble.
§ Erosion in air heater
Employing conservative gas velocities is a design-stage precaution against gas-side erosion, particularly for dusty fuels. Gas velocities have to be high to economize on the surfaces and evolve decent layouts and erosion has to be minimized. Normally erosion is at the entry to the bank where the velocities are the highest, and the maximum velocity limit should be applied at this point. For a tubular air heater it is normal to provide the top row with thicker tubes of, for example erosion shields of typically 3 mm. Heat transfer in tubes covered with erosion shields suffers because there is no direct contact of the tube and gas.
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