For the FD, calculate your boiler´s total air requirements. The total air flow is the sum of theoretical air plus excess air. Once you have the air in mass units ( kg/s , lb/h ) convert it to volume units ( multiply by the specific volume of air at a standard temperature, for example 20 deg C and correct for the ambient temperature and altitude of your locality ).
For the fan discharge pressure, you will need to add all of the pressure drops of your boiler system ( air pressure drop in air preheater and, in the air ductwork, in the burner wind box and register. Flue gas pressure drops in the radiant superheater panels, in the reheater, in the convective superheater panels, in the boiler bank, in the economizer banks, in the gas side of your air preheater and, in the scrubbers, cyclone dust collectors and bag or electrostatic collectors)
If your boiler system is of the balanced draught type, the FD handles air pressure drops and the ID fan flue gas pressure drops and the flue gas flow converted to units of volume flow. Remember to correct the flue gas pressure drops for altitude.
If you have a package boiler or a field erected oil or gas fired unit, the FD fan handles ALL of the pressure drops.
Pressure drops have to be corrected for altitude and the FD fan vendor can select the fan for a design air temperature greater than the ambient temperature provided for the location of your boiler.
Margins are added on the total volumetric air, flue gas volumetric flows and on pressure drops, since wear and time reduce fan capacity. For industrial oil and gas fired boilers 15% on flow and 30% on pressure drops are common.