FD, SA, and ID Fans in a Steam Power Plant

SA - secondary air (provides secondary combustion air)FD - forced draft (provides air under pressure to burner/boiler)ID - induced draft (draws air out of boiler to maintain draft/negative pressure in the boiler)
A boiler may have no fan, FD fan only, ID fan only, or both FD and ID fans.
here's a link from several years ago discussing primary and secondary air:http://cr4.globalspec.com/thread/51131

Generally, FD fans push air into and through a process; ID fans pull air through a process and discharge it to atmosphere. SA = supply air, which is usually pushed. (If pulled, it is normally called exhaust air.)

These terms are not necessarily universal.

In any boiler we have to maintain balanced draft(with negative and positive drafts) is the basic...PA Fan will provide air for combustion of coal which is positive draft to compensate it ID Fan will provide negative draft. SA FAn will provide EXCESS AIR, which will be required for the COMPLETE combustion of coal without any unburnt. IN CFBC Boilers, Secondary air will also be used in fluidization process as Tertiary air.