Process Furnace : efficiency
Definition and quick Estimation
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| Section summary |
|---|
| 1. Process furnace
efficiency definition |
| 2. Process furnace
efficiency estimation |
| 3. Process furnace efficiency shortcut
calculation tool |
1. Process furnace efficiency definition
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
2. Process furnace efficiency estimation
How to calculate the efficiency of a process furnace ?
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
Example :
%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 ?)
3. Process furnace efficiency shortcut Excel calculation tool
Process furnace efficiency Excel calculator
You can access to a free Excel calculation tool to apply the shortcut method mentionned in the this page.
Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.
Process furnace online calculator
Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.
Process Furnace Efficiency Calculator
- Excess air loss = %Oxygen in flue gas / 2
- Temperature loss = Flue gas temperature (°C) / 20
Input Parameters
FAQ: Process Furnace Efficiency
1. What is process furnace efficiency?
Process furnace efficiency is the ratio of heat absorbed by the process fluid to the total heat input from combustion, expressed as a percentage. It reflects how effectively the furnace utilizes energy.
2. How is furnace efficiency calculated?
Efficiency is calculated using: \[ \text{Efficiency (\%)} = \left( \frac{Q_{\text{absorbed}}}{Q_{\text{entered}}} \right) \times 100 \] Alternatively, it can be expressed in terms of losses: \[ \text{Efficiency (\%)} = \left[ \frac{Q_{\text{entered}} - Q_{\text{flue gas}} - Q_{\text{casing}}}{Q_{\text{entered}}} \right] \times 100 \]
3. What are the main sources of heat loss in a furnace?
The main sources of heat loss are flue gas losses (due to excess air and high flue gas temperature) and casing losses (heat escaping through the furnace casing).
4. How are flue gas losses estimated?
Flue gas losses are estimated using: \[ \text{Flue gas loss (\%)} = \frac{\text{Oxygen in flue gas (\%)}}{2} + \frac{\text{Flue gas temperature (°C)}}{20} \]
5. What is a typical value for casing losses?
Casing losses are typically around 2% if the furnace is properly insulated.
6. How can furnace efficiency be improved?
Efficiency can be improved by reducing excess air, lowering flue gas temperature, ensuring proper insulation, and maintaining heat exchangers to prevent fouling.
7. What tools are available to estimate furnace efficiency?
Our website offers a free Excel calculator and an online tool for quick estimation of furnace efficiency using simplified formulas.
8. What precautions should be taken when using the calculator?
The calculator provides approximations for quick estimations. For detailed design, consult a reputable engineer or designer.
9. Why is monitoring furnace efficiency important?
Monitoring efficiency helps optimize energy consumption, reduce costs, and minimize environmental impact by ensuring the furnace operates at peak performance.
10. What should be done if calculated efficiency is low?
If efficiency is low, check for issues such as fouled heat exchangers, inadequate insulation, or improper combustion control, and take corrective actions.