Process furnaces in refineries or petrochemical factories are consuming a lot of energy. At a time when energy prices go up but also when environmental concerns are rising as well as manifestation of climate change are becoming more visible, the monitoring and optimization of furnaces efficiency is becoming more and more critical.
The efficiency is actually related to the heat losses that happen within the furnace. Indeed, not all the energy released by the combustion can be used, and some of this energy is lost, for example simply through the casing of the furnace. The higher the losses, the lower the furnace efficiency ; the lower the losses, the higher the furnace efficiency.
Being able to calculate the furnace efficiency is thus mandatory to be able to optimize the economics of the combustion process.
The graph below illustrate that the energy introduced in the furnace, through combustion, is split in between the heat absorbed by the process fluid, the heat that escapes with the flue gas (it is a loss), and the heat that escapes through the furnace casing (it is a loss as well).
Qentered = Qabsorbed + Qfluegas + Qcasing
Figure 1 : Process furnace
efficiency - losses and heat absorbed
It thus means that only the heat absorbed is interesting. The heat that escapes must be minimized. The higher the heat absorbed, the higher the efficiency, which can thus be expressed the following way :
Efficiency (%) = (Qabsorbed / Qentered) *100
Actually, the determination of Qabsorbed is not easy ; which means that it can be useful to actually express it as a function of the losses :
Efficiency (%) = [(Qentered - Qfluegas - Qcasing)/ Qentered] *100
It is possible to estimate a process furnace efficiency thanks to the following method :
Efficiency (%) = 100 - %flue gas losses - %casing losses
The casing losses are generally 2% if the furnace has been properly insulated.
The losses through the flue gas are due to the excess air, and to the heat (= the temperature) that is carried away. A method to estimate those losses is the following :
% loss excess air = %Oxygen in flue gas / 2
% loss temperature = Flue gas temperature (c) / 20
An analyser and a temperature probe positioned in the stack, after the last convection zone of the furnace will help to measure those values and will allow an estimation of the process furnace that can be very useful for a process operator to make sure the furnace performance is not drifting.
Efficiency (%) = 100 - (%Oxygen in flue gas / 2) - (Flue gas temperature (c) / 20) - %casing losses
%O2 in exhaust fumes = 3.8 %
Temperature of exhaust fumes = 372 c
%casing losses = 2%
Efficiency = 100 - 3.8/2 - 372/20 - 2 = 100 - 1.9 - 18.6 - 2 = 77.5 %
This efficiency is quite low, the factory operator should
especially check the heat exchangers in the furnace (tubes fouled ?)
You can access to a free Excel calculation tool to apply the shortcut method mentionned in the this page.