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Combustion by the Numbers
Fuel and air, flue gas analysis, available heat, metering, burners, condensing heat recovery and gas safety, worked through one gas-fired furnace
Burn gas well with numbers: stoichiometric air, excess air from flue oxygen, available heat, flame temperature, meter corrections, turndown, flue dew point, CO and cost per useful GJ.
Start readingAbout this book
Most of the heat used in Australian factories for drying, curing, baking, melting and heat treatment still comes from burning gas. A gas burner looks simple: gas and air go in, a flame comes out, and a controller turns it up and down. Behind that simplicity sit a chemical reaction with fixed proportions, a stream of hot flue gas that carries away much of the fuel’s energy, and a set of failure modes, from soot and carbon monoxide to explosions, that make gas plant one of the most regulated equipment types in industry.
This book works through gas combustion with numbers. It starts with the reaction and the air it needs, then reads the flue gas to find out how much air a burner is really getting. It follows the fuel’s energy into the work, the walls and the flue, calculates flame temperature and the benefit of preheating the air, corrects gas meter readings to find a burner’s true firing rate, and explains turndown and air–fuel ratio control. It then recovers heat from the water in the flue gas, deals with carbon monoxide, purging and approvals, and ends by comparing gas and electricity on cost and emissions. Each chapter follows one running case, includes a plausible mistake and how it was found, and ends with an exercise and its worked answer.
It is for process, plant and energy engineers, burner and furnace technicians, operators and technical students. It assumes school algebra and chemistry. Electric process heating has its own book, Industrial Electric Heating by the Numbers, and how long a load takes to heat through is the subject of Heating and Cooling Times by the Numbers; this book links to them rather than repeating them.
The running case
Every chapter follows one fictional furnace. This is an illustration. A heat-treatment job shop runs TF-120, a natural-gas-fired batch tempering furnace. Two nozzle-mix burners of 60 kW each fire into the chamber, for a total input of 120 kW (432 MJ/h) at high fire, with combustion air from a single fan. A typical load is 800 kg of steel parts in 200 kg of baskets, heated to 600 °C and held for 2 hours. The flue gas leaves at about 650 °C, and the burners are set for 3% oxygen in the dry flue gas.
Gas is supplied through a sub-meter at 35 kPa gauge. For the illustration, the network’s heating value is taken as 38.6 MJ/m³ and the gas price as A$30 per GJ including GST. The furnace runs about 500 loads a year.
The job shop, its furnace and its figures are invented. None describes KEVOS® work or a real business.
How to use the book
Read Chapters 1 and 2 first: they set out the reaction, the air and the flue gas analysis that every later chapter uses. Chapters 3 and 4 follow the energy and the flame temperature, Chapter 5 deals with gas metering, and Chapter 6 with burners and control. Chapter 7 covers water in the flue gas and condensing heat recovery, Chapter 8 covers carbon monoxide, purging and approvals, and Chapter 9 compares costs and emissions. Every calculation was checked by running it, using NIST’s enthalpies of combustion and heat-capacity data. Natural gas is treated as methane for the chemistry; Chapter 1 explains why that is accurate enough for air and heat calculations, and where it is not.
Source limits and professional responsibility
The chemistry and thermodynamics are standard, and property data comes from NIST. Burner settings, losses, prices and the furnace itself are illustrations, and real equipment should be judged on its own measurements and its maker’s data. Gas-fired appliances and gas installations are regulated in every Australian state and territory: their design, installation, commissioning, adjustment and servicing are for licensed gasfitters and qualified people working under the applicable standards, the appliance’s approval or acceptance by the technical regulator, the maker’s instructions and work health and safety law. This book explains how combustion works and how to calculate it. It is not a procedure for adjusting, commissioning or approving any appliance.
Who this book is for
Process, plant, maintenance and energy engineers, burner and furnace technicians, heat-treatment and coating operators, and technical students who run, tune, buy or assess gas-fired process heating equipment.
What you will learn
- Write the combustion reaction for natural gas and propane, and calculate the air needed, the products formed and the heating values per cubic metre.
- Turn a flue gas oxygen reading into excess air and CO₂, and spot an analyser reading that cannot be right.
- Calculate available heat from flue temperature and excess air, and use it to plan a furnace load's fuel, time and cost.
- Correct a gas meter reading to standard conditions and check a burner's firing rate against its rating.
- Explain turndown, air–fuel ratio control, flue gas condensation, carbon monoxide and purging, and compare gas and electric heat on cost and emissions.
Inside this book
Chapters
- 01Fuel, air and the reactionThe 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.
- 02Excess air and the flue gas analyserWhat a flue gas analyser measures and calculates, how to turn an oxygen reading into excess air and CO₂, air-free carbon monoxide, where to sample, and a probe that was reading room air.
- 03Where the heat goes: available heat and the furnace loadFollow a gas furnace's fuel into the load, the walls and the flue, calculate available heat from flue temperature and excess air, and plan a load's fuel, time and cost.
- 04Flame temperature and preheated airAdiabatic flame temperature and why excess air lowers it, dissociation, how preheating combustion air raises available heat, and whether a recuperator pays on a small batch furnace.
- 05Metering gas and checking the firing rateWhat a gas meter measures, correcting metered volumes to standard conditions, turning cubic metres into megajoules, clocking a meter to check a burner's input, and a correction left out.
- 06Burners, turndown and air–fuel ratio controlBurner types, turndown, and four ways of controlling a burner's input; why fixed air wastes fuel at low fire, sizing the combustion air fan, and a blocked impulse line found only at low fire.
- 07Water in the flue gas and condensing heat recoveryHow much water burning gas makes, the flue gas dew point and how excess air moves it, recovering latent heat with a condensing exchanger, and a flue that was never meant to be wet.
- 08Carbon monoxide, purging and approvalsThe hazards of gas-fired plant in numbers: flammable limits, how fast a chamber can fill, purging, the safety chain, carbon monoxide in a room, and why changes need the regulator's acceptance.
- 09Cost, emissions and choosing gas or electricCompare gas and electric process heat per useful gigajoule, calculate carbon dioxide from the chemistry, find the break-even price and grid factor, and rank the improvements for TF-120.
Apply This in Practice
Use these KEVOS® tools to try the method from this reading with your own numbers.
Explained in this book
Hydrocarbon Combustion Air and Flue Gas Check (Air–Fuel Ratio, Flue Analysis, Water Dew Point)
Balances the combustion of any CₓHᵧ fuel with air and gives the stoichiometric and actual air–fuel ratio, the reaction this chapter sets out for methane.
Used in Fuel, air and the reaction, Excess air and the flue gas analyser, Water in the flue gas and condensing heat recovery
Applies this method
Equipment Investment Appraisal (NPV, IRR, Payback, B/C, EUAC)
Appraises an equipment purchase with yearly savings: NPV, IRR, payback and annual cost; used to test whether a recuperator pays on a small batch furnace.
Applies this method
Fan Duty, Fan Laws and Belt Drive
Gives fan air and shaft power from flow, pressure and efficiency, and applies the fan laws; used to size a burner's combustion air fan.
Applies this method
Furnace Wall Design (Layers, Convection and Radiation from the Surface)
Finds a furnace wall's outside surface temperature and heat loss with convection and radiation from the surface; used for the gas furnace's standing loss.
Used in Where the heat goes: available heat and the furnace load
Applies this method
Gas Volume at Pressure and Temperature
Converts a gas volume or flow between pressures and temperatures, which corrects a meter reading at line pressure to standard conditions.
Applies this method
Heat Exchanger LMTD and Area
Calculates the log mean temperature difference and the area for a duty and overall coefficient; used to size a flue gas to water heat exchanger.
Applies this method
Heat-Treatment Furnace Load Plan (Time, Energy, Cost)
Plans a furnace load's heat, heat-up time, energy and cost; for a gas furnace, enter the available heat less standing loss as the share reaching the load.
Used in Where the heat goes: available heat and the furnace load
Applies this method
Heating Energy and Time
Gives the energy to heat a mass through a temperature range; used for the daily heat a parts washer's make-up water needs.
Worked example
Radiant Heat Loss
Gives net radiation from a surface to large surroundings; with an emissivity of 1 it shows the heat a hot furnace loses through its open door.
Used in Where the heat goes: available heat and the furnace load
Related Tools and Examples
Useful alongside
Energy and Work Converter
Converts energy between kWh, MJ, GJ and other units, the first step in comparing a gas bill with an electricity bill.
Related to Cost, emissions and choosing gas or electric
Useful alongside
Fuel Oil Heating Value from Baumé Gravity (Specific Gravity, Heat per kg and per Litre, Fuel Use)
Estimates the higher heating value of a fuel oil from its gravity, the gross basis this chapter explains, and the oil a burner uses for a heat output.
Related to Fuel, air and the reaction
References and source notes
The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 29 October 2026.
- C. Kittel and H. Kroemer. Thermal Physics, second edition (1980). Supplied source, © W. H. Freeman and Company, in copyright. Read as a subject map only: heat and work, enthalpy, the Gibbs free energy and chemical reactions, the law of mass action, the ideal gas and kinetic theory. No text, derivations as set out, figures, tables or problems are reproduced; the treatment, case, numbers and exercises in this book are its own.
- National Institute of Standards and Technology. NIST Chemistry WebBook, SRD 69: methane, propane, nitrogen, oxygen, carbon dioxide and water (2026). Source of the enthalpies of combustion (methane −890.7 kJ/mol, propane −2,219.2 kJ/mol, both Pittam and Pilcher 1972) and of the Shomate heat-capacity coefficients (Chase 1998) used for every flue gas enthalpy, available heat and flame temperature in the book.
- US Department of Energy, Industrial Technologies Program. Check burner air to fuel ratios (Process Heating Tip Sheet #2) (2007). Source of the guidance that most high-temperature direct-fired furnaces, radiant tubes and boilers run with about 10 to 20% excess air at high fire, of its worked example pairing 10% excess air with 2.1% oxygen in the flue gas, which the book's own calculation reproduces, and of the method of estimating savings from available heat at present and target oxygen levels.
- Australian Gas Networks. Understanding your gas bill (2026). Describes how a network converts a metered volume to energy with a heating value and a pressure correction factor, the method followed in Chapter 5. The heating value used in the running case is an illustration.
- NOAA CAMEO Chemicals. Methane: chemical datasheet (2026). Source of methane's lower and upper explosive limits in air, 5% and 15% by volume, used in Chapter 8.
- SafeWork NSW. Carbon monoxide technical fact sheet (2026). Source of the workplace exposure standard for carbon monoxide of 30 ppm averaged over 8 hours, used in Chapter 8. Readers should check Safe Work Australia's current list for the value in force.
- Energy Safe Victoria. GIS 54: Schedule 9, additional information required for Type B appliance acceptance (2024). An example of a technical regulator's requirements for accepting a Type B gas appliance, cited in Chapter 8. Requirements differ between states and territories.
- Standards Australia. AS 3814 Industrial and commercial gas-fired appliances; AS/NZS 5601.1 Gas installations, general installations (editions as called up). Named by number and scope as the standards for the design of Type B appliances and for gas installations, including ventilation and flueing. No requirements or values from them are quoted.
- Australian Government, Department of Climate Change, Energy, the Environment and Water. National Greenhouse Accounts Factors (published annually). Named as the source of official emission factors for fuels and state electricity grids. The book's own carbon dioxide figures are calculated from the combustion chemistry of methane; no factor from the publication is quoted.