Fired heaters flux rate

Fired heaters provide a major part of heat source for reaction and separation. Reliability is the major concern for furnace operation with heat flux for large heaters and tube wall temperature (TWT) for small heaters as the most important reliability parameters. Increasing either heat flux or TWT could increase furnace efficiency. When operating heat flux is much lower than the maximum limit, this is an indication of the furnace being underutilized and thus presents an opportunity for increased feed rate. Increase feed rate is a win-win operation since more feed also results in reduced energy intensity. 

Radiant heat flux is defined as heat intensity on a specific tube surface. Thus, heat flux represents the combustion intensity and is analogous to how hard a fired heater is run. More specifieally, keeping the firing rate within safe limits is equivalent to maintaining the peak heat flux at less than the design limit because high firebox temperatures could cause tubes, tube-sheet support, and refiractory failures. What is the peak flux and why is it so important to keep it within the limit These questions will be answered next. 

The nonuniformity of heat flux around mbe is described by the circumferential flux factor, which is the ratio of peak flux to average flux. Peak flux determines the maximum TWT. The peak flux is typically 1.5-1.8 times the average for a single-fired heater, while it is 1.2 times the average for a double-fired heater. That explains why the double-fired heater has longer run length as it has lower flux rate and hence lower TWT than the single-fired heater. 

A high BWT measurement indicates the heater operates at high radiant flux rates. BWT is the key reliability parameter for fired heaters as high BWT or TWT can cause... 

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