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.

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.