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Combustion practice questions

30 questions from the gas fitting (natural gas and lp) practice test.

Combustion practice test

10 questions, graded as you go, with the reason for every answer. No sign-up.

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Sample questions

About how many cubic feet of air are needed to completely burn one cubic foot of natural gas?

  1. 2 cubic feet
  2. 5 cubic feet
  3. 10 cubic feet
  4. 24 cubic feet
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C. 10 cubic feet

Natural gas needs about 10 cubic feet of air per cubic foot of gas for complete combustion. About 24 cubic feet is the figure for propane, which has more carbon and hydrogen in each cubic foot.

Reference: Combustion fundamentals; NFPA 54 Annex

About how many cubic feet of air are needed to completely burn one cubic foot of propane vapor?

  1. 5 cubic feet
  2. 10 cubic feet
  3. 15 cubic feet
  4. 24 cubic feet
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D. 24 cubic feet

Propane needs roughly 24 cubic feet of air per cubic foot of vapor. The figure of 10 applies to natural gas; the air needed per Btu is nearly the same for both fuels, but propane packs 2.5 times the heat in each cubic foot.

Reference: Combustion fundamentals; NFPA 58 Annex B

What is primary air?

  1. Air that mixes with the gas before it reaches the burner ports
  2. Air that reaches the flame from the space around the burner
  3. Air that enters the draft hood and mixes with the flue gas
  4. Air that is blown across the outside of the heat exchanger
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A. Air that mixes with the gas before it reaches the burner ports

Primary air is drawn in at the orifice and mixed with the gas inside the burner before ignition. Air that joins the flame from around the burner is secondary air, and air entering the draft hood is dilution air.

Reference: Burner fundamentals; NFPA 54 definitions

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