Combustion Calculations Questions and Answers

Answer: A

The first two are correct. The third is wrong, since more excess air means more mass leaving hot and a larger loss, which is why oxygen in the flue gas is trimmed as closely as safety permits. Oxygen trim control exists for exactly this reason.

Answer: B

Twenty kilograms times one point one times the one hundred and seventy-five degree difference gives three thousand eight hundred and fifty kilojoules. Against a calorific value of perhaps forty thousand this is a loss of about ten per cent.

Answer: C

Recovering the latent heat that the net value excludes makes the ratio exceed one, which is an artefact of the definition rather than a violation of anything. Quoted on the gross value the figure stays below one hundred per cent.

Answer: D

Correct answer: consumes oxygen and forms sulphur dioxide

Sulphur burns to sulphur dioxide, so it adds to the theoretical oxygen requirement and appears in the flue gas - both of which the calculation must account for. It also matters beyond the arithmetic: the sulphur dioxide raises the acid dew point of the flue gas, which sets how far the exhaust may be cooled before the heat exchangers begin to corrode.

Answer: A

Both are true and the reason gives the constraint: recovering that last heat requires corrosion resistant materials, which is exactly what a condensing boiler provides. Stainless steel or plastic flues are used in those units.

Answer: B

The first two are correct and the basis must always be stated. The third is wrong, since the ratio depends on the hydrogen to carbon ratio of the fuel and differs between methane and coal. Methane needs roughly seventeen kilograms of air per kilogram.