GC: n

CT: Combustion Air is the air drawn through the firebox by the draughting system which allows combustion to take place. Only the oxygen content of the air (approx 18%) is used in the combustion process, the remainder (mostly nitrogen) being inert and serving no function other than wasting energy and cooling the fire.
Combustion Air comes in two forms:
Primary Combustion Air which is drawn upwards through the ashpan, grate and firebed, and
Secondary Combustion Air which is drawn in over the top of the fire – e.g. through the firehole door.
Essentially, the primary air releases heat from the fuel and the secondary combustion air releases heat from volatile gases released from the hot coal in the firebed. The chemical reactions that release heat from the fuel are sometimes complex, but the end-product is a combination of carbon dioxide gas and water vapour mixed with small quantities of carbon monoxide and oxides of other impurities (plus large volumes of nitrogen). These reaction products are termed Combustion Gases.

S: http://5at.co.uk/index.php/definitions/terrms-and-definitions/combustion-air.html (last access: 30 December 2014)

N: 1. primary (adj): early 15c., “of the first order,” from Latin primarius “of the first rank, chief, principal, excellent,” from primus “first” (see prime (adj.)).
air (n): c.1300, “invisible gases that make up the atmosphere,” from Old French air “atmosphere, breeze, weather” (12c.), from Latin aerem (nominative aer) “air, lower atmosphere, sky,” from Greek aer (genitive aeros) “air” (related to aenai “to blow, breathe”), of unknown origin, possibly from a base awer- and thus related to aeirein “to raise” and arteria “windpipe, artery” (see aorta) on notion of “lifting, that which rises.” In Homer mostly “thick air, mist;” later “air” as one of the four elements.
2. Combustion and Fuels (Fireplaces) and Burners and Steamfitting: The air introduced into a burner which mixes with the fuel before it reaches the combustion chamber. Synonym: primary combustion air.
3. Air Conditioning and Heating: Induction. Process by which the primary air sets into motion an air volume, called secondary air, in the room.
4. Environmental engineering: Treated air introduced to a diffuser where it mixes with room recirculated air by induction.
5. Air for combustion is divided into four types depending upon its role and the design of the particular burner. Air will be referenced in this manual and seminar as primary, secondary, excess and dilution air.

• Primary air provides a percentage of the combustion air, but more importantly, controls the amount of fuel that can be burned.
• Secondary air improves combustion efficiency by promoting the fuel to burn completely. Power burners generally do not require secondary air. However, air leaking in through access/clean out doors, burner mounting flanges, boiler sections, etc., dilutes the flame and flue gas temperatures, reducing operating efficiencies as well as our ability to accurately monitor combustion conditions.

Excess air is supplied to the combustion process to ensure each fuel molecule is completely surrounded by sufficient combustion air. As a burner tune-up improves the rate at which mixing occurs, the amount of excess air required can be reduced.
Dilution air does not participate directly in the combustion process and is primarily required to attempt to control stack draft and reduce the likelihood that moisture in the flue gases will condense in the vent system – which directly influences combustion air intake, safety and efficiency.

S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=primary&searchmode=none; http://www.etymonline.com/index.php?allowed_in_frame=0&search=air&searchmode=none (last access: 30 December 2014). 2 & 3. TERMIUM PLUS (last access: 30 December 2014). 4. GDT (last access: 30 December 2014). 5. http://www.combustiontechnology.co.za/training/combustion.htm (last access: 30 December 2014).

SYN:
S:

CR: fuel