Factors Affecting Flue Gas Exit Temperature

In furnaces with bottom-fired heat or preheat zones (firing below the work load), there is often greater resistance to poc gas flow in the bottom zones than in the top zones because the bottom zones usually contain conveying equipment, support 

Flue gas exit temperature is affected by (a) flame length, (b) firing rate (furnace gas velocity), and (c) heat transfer from the furnace gases to the loads, and from furnace gases to the refractory and then to the loads. 

Heat transfer lowers flue gas exit temperatures. Heat transfer rises if 

Increasing flue gas recirculation (FGR) to reduce NOx emissions raises the concentration of inerts in a flame, thereby increasing the flame length. The longer flame raises the flue gas exit temperature and also lowers the reaction (flame) temperature, thereby raising the fuel rate. Using FGR to lower NOx can raise fuel costs considerably. (See sec. 5.12.) 

---