PROCESS HEATER AIR INFILTRATION

Process heaters are among the largest energy consumers in industry. Many of them were built years ago and often are not well sealed which leads to excessive air infiltration. Air leaks may be caused by cracks in the wall, by sight ports that are not properly sealed or may even be left open, failure to close air registers for burners that are out of service, improper sealing of penetrations through the heater walls, and by excessive draft levels in the heater. These leaks reduce energy efficiency and indirectly increase pollution emissions as more fuel must be consumed for a given production rate. Leaks may also directly cause NOx emissions to increase due to increased excess oxygen. Excessive air leakage can indirectly cause process burners to operate improperly. The excess oxygen level in a heater is normally controlled to a certain target value, typically about 2-3% by volume for gaseous fuels. Process burners are designed assuming that all of the air for combustion goes through the burner. However, if a significant amount of air is leaking into a heater, the measured excess oxygen level may be on target but not enough of the air is coming through the burner which can adversely affect performance. The flame length may be dramatically increased as the flame searches for air to complete combustion which often causes flame impingement on process tubes. The heat flux distribution may be shifted as the flame length increases. The flames may even become unstable if they are sufficiently starved for air. The draft level in a heater varies with elevation which means that air infiltration depends not only on the size of the leak opening, but also on its location. This paper will include an analysis of how excess air infiltration affects thermal efficiency and how the location of the leak and the heater draft level affect the amount of air infiltration. The impact of air infiltration on burner performance will be discussed. Techniques will be recommended for detecting air leaks and how to correct them.