Air preheater is a device for recovery of waste heat from flue gas and is placed in the path of the waste flue gas going to chimney. Waste heat if the flue gas is transferred to the air before entering into the furnace to support the combustion of fuel. It is placed in chimney and above economiser. If fuels used in furnace are oil, gases or pulverized coal, the hot air supply is possible. But, in case of stoker firing, maximum temperature of air is limited due to overheating of stoker parts. The constructional details of an air preheater are shown in Figure 4.25.
Figure 4.25 Air Preheater
- Due to high furnace temperature, water evaporation rate increases.
- Boiler efficiency increases by 2–10%.
- Low grade fuel can be used.
Disadvantages
- Capital cost increases due to use of preheater and two fans (induced fan and force draught fan) to create artificial draught.
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