Air Preheater in Thermal Power Plant
- As the entire heat of the flue gases cannot be extracted through the economizers, air preheaters are used to recover some of the heat from these gases.
- On an average increase of 20°C in the air temperature results in an increase in the boiler efficiency by 1%.
- The use of air preheater is more economical in the case of boilers using pulverized fuel because temperature of gases going out is sufficiently large and high air temperature (250°C to 350°C) is always desirable for better combustion of fuel.
- Air preheaters raise the temperature of the furnace gases, improves combustion rates and efficiency and lowers the stack temperature, thus the overall efficiency of the boiler is increased.
- Air preheaters are of two types:
(i) Recuperative type
(ii) Regenerative type.
- The recuperative type uses a bank of straight light gauge steel tubes 25 mm to 75 mm in diameter, usually with the flue gases inside the tubes and air outside.
- The regenerative type transfers the heat by using slowly revolving drum, revolving at 1 or 2 rpm. of corrugated metal which moves alternately through the hot gas and cold air streams.