• After the flue gases leave economizer, some further heat can be extracted from them and is used to heat the incoming air for combustion.
• Air preheaters may be of following types:
• Cooling of flue gases by 200increase the efficiency of the plant by 1%.
• The use of air preheaters is more economical with pulverized fuel boilers because the temperature of flue gases going out is sufficiently large and high air temperatures (250 to 3500 C) is always desirable for better combustion.
• Air preheaters should have high thermal efficiency, reliability of operation, less maintance charges, should occupy small space, should be reasonable in initial cost and should be accessible.
• In order to avoid corrosion of the air preheaters, the flue gases should not be cooled below the dew point.