General Gas Knowledge
The General Gas Knowledge domain tests the physical and combustion fundamentals that every later calculation depends on: what natural gas and propane are, how they behave at different pressures and temperatures, how they burn, and how appliance input is measured. Expect definition-style questions, simple gas-law calculations, and questions that ask you to predict flame behaviour from an air/gas ratio.
Fuel properties
- Natural gas is mostly methane (typically 90–95%), with a specific gravity of about 0.60–0.65 (air = 1.00) and a heating value near 1,000–1,050 Btu per cubic foot.
- Propane (C3H8) has a specific gravity of about 1.52, so it pools in low areas; its vapour heating value is roughly 2,500 Btu/ft³ and liquid about 91,500 Btu/gal.
- Both fuels are odorized; natural gas is lighter than air, propane is heavier.
- Stoichiometric combustion needs about 9.5 volumes of air per volume of natural gas and about 24 volumes per volume of propane.
Pressure and gas laws
Gas pressure is usually stated in inches of water column (in. w.c.). 1 psi = 27.7 in. w.c.; 1 in. w.c. ≈ 0.036 psi ≈ 249 Pa. Typical natural gas supply pressure is 7 in. w.c. and propane 11 in. w.c. Boyle's law relates pressure and volume at constant temperature; Charles's law relates volume and absolute temperature at constant pressure. Always use absolute pressure and absolute temperature (Rankine or Kelvin) in these calculations.
Combustion and flame
A correctly adjusted natural gas flame is blue with a well-defined inner cone; propane shows a similar but shorter flame. Insufficient primary air produces yellow tipping and carbon; excess primary air can lift the flame. Yellow flame alone does not prove a shortage of air - a blocked flue or high CO can also be present.
Worked example
A 1 ft³ test dial on a natural gas meter makes one revolution in 36 seconds. Gas rate = 3,600 ÷ 36 = 100 ft³/h. At 1,000 Btu/ft³, input = 100 × 1,000 = 100,000 Btu/h. If the appliance rating plate says 80,000 Btu/h, the measured input is 25% high, so check orifice size, manifold pressure, and gas density before condemning the appliance.
Traps that cost marks
- Treating propane as lighter than air.
- Mixing gauge and absolute pressure in gas-law questions.
- Using 3,600 s/h but forgetting the dial volume.
- Assuming a yellow flame always means too little primary air.