A drum type boiler controls the final steam temperature primarily via the use of a spray water attemporator. It is normally installed midway thru the superheaters, and will operate at a near steady state when the boiler is at a fixed load, fixed excess air, fixed burner row in service, and fixed operating pressure.The final steam temperature can also be changed by modifying the above listed fixed items or temporarily by use of wall blowers or sootblowers.
refer to standard texts such as B+W "Steam" for the general design philosophy for these items.