KEVOS® MAKEOS · Engineering
Hydrocarbon Combustion Air and Flue Gas Check (Air–Fuel Ratio, Flue Analysis, Water Dew Point)
For complete combustion of this hydrocarbon fuel with this excess air, what air–fuel ratio and air flow are needed, what dry flue oxygen and carbon dioxide will an analyser read, what is the water dew point of the flue gas, and does the flue stay above it?
Complete combustion of a hydrocarbon fuel CₓHᵧ with dry air: the stoichiometric and actual air–fuel ratios, the air and flue-gas flows, the wet flue composition, the dry oxygen and carbon dioxide an analyser reads, the water vapour partial pressure and the water dew point of the flue gas, with the lowest flue or exchanger surface checked against the dew point plus your margin.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Fuel CₓHᵧ | ||
| Carbon atoms in the fuel molecule (x) | Carbon atoms in one molecule of the fuel: 1 for methane, 3 for propane, 4 for butane; an average formula for a gas mixture or liquid fuel from its analysis. | |
| Hydrogen atoms in the fuel molecule (y) | Hydrogen atoms in one molecule of the fuel: 4 for methane, 8 for propane, 10 for butane, 2 for hydrogen. | |
| Fuel mass flow | kg/h | Fuel mass flow at the firing rate, from the gas meter (volume × density) or the burner data. |
| Air and flue | ||
| Excess air | % | Air supplied above the theoretical (stoichiometric) amount, as a percentage of it, from the burner setting or an analyser. |
| Flue gas total pressure (absolute) | kPa | Absolute pressure of the flue gas: atmospheric for most appliances. |
| Lowest flue or heat-exchanger surface temperature | °C | Temperature of the coldest surface the flue gas touches: flue liner, economiser tubes or heat-recovery exchanger. |
| Margin required above the dew point | K | How far above the water dew point the coldest surface must stay, from your specification or the appliance maker. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
Fuel CₓHᵧ
What this means: Carbon atoms in the fuel molecule (x)
- What it is
- The number of carbon atoms in one molecule of the fuel, or the average for a fuel mixture, written as the x in CₓHᵧ.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 30. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More carbon for the same hydrogen raises the flue CO₂ and lowers the share of water vapour and the dew point.
- Get it right
- Natural gas is mostly methane (x = 1); for a mixture, use the mole-weighted average from the gas analysis.
What this means: Hydrogen atoms in the fuel molecule (y)
- What it is
- The number of hydrogen atoms in one molecule of the fuel, or the average for a mixture, written as the y in CₓHᵧ; each pair forms one water molecule.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 to 62. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More hydrogen raises the water vapour in the flue and the dew point.
- Get it right
- Hydrogen-rich fuels such as methane give more water and a higher dew point than oils.
What this means: Fuel mass flow
- What it is
- The mass of fuel burnt each hour at the firing rate being checked, which scales the combustion air and flue-gas flows.
- Units and format
- Enter a number in kg/h: kg/h. The form accepts 0.001 kg/h to 1000000 kg/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 kg/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Air and flue-gas flows rise in proportion; ratios, analyser readings and dew point do not change.
- Get it right
- Convert a gas meter’s volume flow with the gas density at the metering pressure and temperature.
Air and flue
What this means: Excess air
- What it is
- The air supplied beyond what complete combustion theoretically needs, as a percentage of the theoretical air.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 400%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More excess air raises the air–fuel ratio and flue O₂ and lowers the CO₂ and the dew point slightly.
- Get it right
- If an analyser gives dry O₂, adjust this until the tool’s dry O₂ matches the reading.
What this means: Flue gas total pressure (absolute)
- What it is
- The total absolute pressure of the flue gas, which multiplies the water mole fraction into the vapour partial pressure.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 50 kPa to 500 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.325 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher pressure raises the water vapour partial pressure and the dew point.
- Get it right
- Atmospheric for most appliances; only pressurised combustion needs a higher value.
What this means: Lowest flue or heat-exchanger surface temperatu…
- What it is
- The temperature of the coldest surface the flue gas touches, such as the end of a flue, an economiser tube or a heat-recovery coil.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0 °C to 1500 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 70 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A hotter surface increases the margin above the dew point.
- Get it right
- Check it at low fire and on cold starts, when surfaces run coolest.
What this means: Margin required above the dew point
- What it is
- The least temperature difference you accept between the coldest surface and the water dew point of the flue gas.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0 K to 100 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the line between above the dew point and a margin-too-small warning.
- Get it right
- Allow more where the fuel may contain sulfur, because acid condenses above the water dew point.
Method and formulas
- Theoretical oxygen per kmol of fuel a_th = x + y ÷ 4; air supplies (1 + e) a_th kmol O₂ with 3.76 kmol N₂ per kmol O₂.
- Products per kmol of fuel: x CO₂, y ÷ 2 H₂O, e a_th O₂ and 3.76 (1 + e) a_th N₂; the dry basis leaves out the water.
- Air–fuel ratio by mass = 4.76 (1 + e) a_th × 28.97 ÷ (12.011 x + 1.008 y), with molar masses of air, carbon and hydrogen.
- Water vapour partial pressure p_v = (y ÷ 2) ÷ N_total × P; dew point where the Hyland–Wexler saturation pressure equals p_v.
- Example: methane (CH₄) with 15 % excess air needs 19.77 kg of air per kg of fuel (17.191 at stoichiometric); the dry flue reads 3.02 % O₂ and 10.05 % CO₂ and the water dew point is 56.54 °C, 13.46 K below a 70 °C surface.
Limitations
- Complete combustion with dry air: carbon monoxide, unburnt fuel, sulfur, nitrogen oxides, dissociation and the moisture carried in by humid air or a wet fuel are ignored. Humid air raises the dew point a little.
- Fuels containing sulfur form acid that condenses well above the water dew point; this tool does not predict the acid dew point.
- A screening calculation; burner setting, flue design and appliance approval follow AS/NZS 5601 and AS 3814 and the work of licensed gas fitters.
- Example values are illustrations. Replace them with values for your actual fuel and burner.
Typical uses
- Work out the combustion air a burner needs at a chosen excess air
- Predict the dry O₂ and CO₂ readings to expect from a flue-gas analyser
- Check whether a flue, economiser or heat-recovery exchanger will run wet
Who it is for
- Combustion and boiler engineers
- Gas fitters and burner technicians
- Energy auditors
What you get
- Stoichiometric and actual air–fuel ratio by mass
- Combustion air and flue-gas mass flows
- Wet flue composition and the dry O₂ and CO₂ analyser readings
- Water vapour partial pressure and the flue-gas water dew point
- Margin of the coldest surface above the dew point
Interpreting the result
- Each extra 10 % of excess air lowers the dew point only a little but carries more heat up the flue; analysers read O₂ on a dry basis, as reported here.
- Condensing boilers and heat-recovery units are designed to run below the dew point to recover latent heat; other flues must stay above it.
Before relying on the result
- Compare the predicted dry O₂ and CO₂ with a calibrated flue-gas analyser reading.
- Measure the flue and exchanger surface temperatures at low fire, when they are coldest.
References
- Çengel Y. A. and Boles M. A., Thermodynamics: An Engineering Approach, 5th edition (McGraw-Hill, 2006), chapter 15 (sections 15-1 and 15-2: fuels and combustion, theoretical and actual combustion, air–fuel ratio, dew point of products) and Table A-1 molar masses. Method reference; no tables, figures or examples are reproduced.
- ASHRAE Handbook—Fundamentals (SI), chapter 1 Psychrometrics: Hyland–Wexler saturation pressure.
- AS/NZS 5601 Gas installations and AS 3814 Industrial and commercial gas-fired appliances (named by number and scope only).
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Explains the concept · KEVOS® original book
Fuel, air and the reaction
What it covers: The combustion reaction for natural gas and propane, the air each needs, the flue gas it makes, higher and lower heating values, and why a burner's rating basis matters.
Also in Excess air and the flue gas analyser, Water in the flue gas and condensing heat recovery
Useful alongside · KEVOS® original book
Real engines: fuel, heat rate and the energy balance
In On-Site Power and Heat by the Numbers
What it covers: Gross and net heating values, heat rate at full and part load, ratings and ambient conditions, the full energy balance of a gas engine, and exhaust flow and dew point from the air–fuel ratio.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.