Fan basically pulls out hot flue gas from the furnace of boiler via dust collector. It's located between dust precipitators(ESPs) and Chimney and also handles hot air/dust. The ID Fans serves two main functions:

Creates a negative pressure in the furnace as compared with the outer pressure. If the furnaces have leaks, a positive pressure in the furnaces will cause gases to leak out.

Once the furnace flue gases get cool, it increases the flow velocity of the furnace.

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