Gas fundamentals
Properties and gas laws
- Natural gas is mostly methane with a relative density of about 0.60 (air = 1), so it rises and collects at high points. Propane is about 1.52 and collects at low points.
- Gross heating values are near 37 MJ/m³ for natural gas and 93 MJ/m³ for propane vapour at 15°C and 101.325 kPa. Compare appliance inputs on the same basis.
- Ideal gas law: P1V1/T1 = P2V2/T2. Use absolute pressure (gauge + 101.325 kPa) and absolute temperature (°C + 273.15).
- Wobbe index = heating value ÷ √relative density. It predicts heat input through a fixed orifice at the same pressure; a gas with a different Wobbe index needs a different orifice or pressure.
- Flammable limits are roughly 5–15% gas in air for natural gas and 2.1–9.5% for propane. Ignition temperatures are high enough that a spark or hot surface is needed.
Combustion
- Methane: CH4 + 2O2 → CO2 + 2H2O. Stoichiometric air is about 9.5 m³ air per m³ of natural gas; propane needs about 23.8 m³ air per m³ of vapour.
- Complete combustion produces CO2 and water vapour. CO appears when air is insufficient, the flame impinges, or the burner is cold or dirty.
- Excess air dilutes flue gases and lowers CO2; too little air raises CO. Condensing appliances recover latent heat and need a drain and a suitable vent material.
Pressure and flow
- 1 in w.c. = 0.249 kPa; 1 psi = 27.7 in w.c. = 6.895 kPa. Manifold pressures are typically 3.5 in w.c. for natural gas and 10 in w.c. for propane.
- Orifice flow rises with the square root of pressure and falls with the square root of gas density.
Worked example: 2.00 m³ of gas at 101.325 kPa and 15°C is heated to 35°C at constant pressure. V2 = 2.00 × 308.15/288.15 = 2.14 m³. The 6.9% volume increase follows Charles's law; using 35/15 directly would give a wrong 4.67 m³.
Traps: using gauge pressure in gas-law calculations; treating relative density as heating value; assuming propane rises; using °C directly; assuming more excess air always improves safety; ignoring altitude effects on orifice sizing.