Fuels, Combustion, and Draft Systems
What this domain tests
Fuels, Combustion, and Draft Systems covers fuel properties, the conditions needed for complete combustion, how draft is produced and measured, and how fuel input and steam output give boiler efficiency. Expect both concept questions and simple arithmetic.
Core rules
- The three T's of combustion are temperature, turbulence, and time, plus sufficient air. Too little air gives carbon monoxide and smoke; too much air wastes heat up the stack.
- Natural gas is mainly methane, about 1000 Btu per cubic foot, and needs roughly 10 cubic feet of air per cubic foot of gas at stoichiometric conditions. Number 2 oil is light and flows easily; number 6 oil must be heated to atomize and burn.
- Draft is measured in inches of water column. Natural draft uses a chimney, forced draft uses an FD fan upstream, induced draft uses an ID fan downstream keeping the furnace negative, and balanced draft uses both.
- Efficiency falls with rising stack temperature and with excessive excess air.
Worked example
A gas-fired boiler burns 10,000 ft3/h of natural gas at 1000 Btu/ft3, so input is 10,000,000 Btu/h. It produces 7,500 lb/h of steam with an enthalpy rise of about 1000 Btu/lb, so output is 7,500,000 Btu/h. Efficiency = output / input x 100 = 75%. The missing 25% leaves as stack loss, radiation, and unburned fuel, so a stack temperature check and an excess air check are the first diagnostic steps.
Traps that cost marks
- Assuming more air always means better combustion; it lowers efficiency.
- Reading draft in psi rather than inches of water column.
- Forcing number 6 oil without preheat and expecting clean atomization.
- Calculating efficiency without converting both figures to the same units per hour.