Differences Between Fired Heaters and Boilers
The differences are:
1. On a fired heater an economizer can generate steam as well as preheat water. On a boiler, it only preheats boiler feedwater. In economizers on fired heaters, the steam pressure levels can be optimized with the available flue gas temperatures and steam balance. Where more steam is being generated than the plant can use, air preheaters may be preferred if steam cannot be used elsewhere.
2. On a fired heater the economizer will heat the process as a first priority if the flue gas and process stream temperatures permit. After all economic process heat has been recovered, economizer sections can be added to make steam. They also can be added to heat water or intermediate heat transfer fluids.
3. On a fired heater, the temperature available for waste heat recovery is not a constant amount above some boiling temperature. On fired heaters, it is determined by the temperature of the hydrocarbon in the fired heater tubes. For example, in an Atmospheric Crude Unit fired heater, this stack temperature would be (without heat recovery) about 750°F to 850°F; for the Vacuum fired heater, 900°F to 1000°F; and in a Hydrogen Plant fired heater, about 1500°F. Without waste heat recovery, these would be the stack outlet temperatures.