Q & A > Question Details
In our refinery we have 2 atmospheric crude distillation units:
Unit 1
Fired heater stack reads 550C

Unit 2
Fired heater stack reads 440C
Can I know what is the accepted range ?
24/02/2020 A: S Sivasankaran SABANAYAGAM, CPCL, cpclsiva@yahoo.com
Simply we can conclude that both the heaters are not designed / or operated properly wrt energy efficiency point of view. Normally heaters are designed to recover maximum heat in the form of steam generating coils and APH in the convection path.
Even in the absence of heat recovery sections (Steam . Air preheaters) other than hydrocarbon coils in convection section, the temperature range indicated in your query seems to be on higher side and scope of heat recovery is more.
Mostly flue gas temperature will be around 180 Deg C - 200 Deg C. Heaters can be designed for further reduction in flue gas temperature if fuel composition used in heater is of low sulfur.
You may check the design condition and verify whether encon equipment installed is working properly.
22/02/2020 A: Apoorv Gupta, IOCL Haldia, apoorv.work19@gmail.com
Both limits can be acceptable. Please check with the heater design parameters. Difference in convection section design may yield different stack temperatures.

For acceptable limits, stack temp temperature (typically 160-165°C) more than 15°C from present dew point temperature is acceptable. Heat recovery options may be explored.
21/02/2020 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The higher the temperature the less efficient the furnace. There is not really a maximum stack temperature. The higher it is the greater your incentive is to install a heat recovery system like air preheat or waste steam generation. If you get too low on stack temperature you can have an acid dew point that leads to corrosion. That temperature depends on the quality of your fuel.
21/02/2020 A: RUPAM MUKHERJEE, ENGINEERS INDIA LIMITED, rupammukherjee86@gmail.com
You are wasting lots of energy to the atmosphere. Your fuel efficiency is of the order 75 percent and even only 70 percent for the first unit. Modern day furnaces can easily achieve 92 percent efficiency where the stack temperature is around 140-150 degC.
You may think of an APH retrofit project which will help you in savings millions on fuel. Exact economics or NPV may need to be worked out.