Understanding Basic Combustion

There’s more to a Tune-up than a quick “Once-Over”…

Understanding Basic Combustion

Useful Information on Combustion Efficient Fuel Use

A Definition:
Combustion is the rapid oxidation of a fuel resulting in the release of usable heat and the production of a visible flame.

The Combustion Equation:

Natural Gas With Air:
CH₄ + 2O₂ + 8N₂ ———–>

CO₂ + 2H₂O + 8N₂ + 1000 BTU Heat

Examples of Combustion

Perfect Combustion Provides:
The hottest flame
Minimum exhaust volume

Perfect Combustion

Characteristics:

• All fuel combusted
• Blue near burner tile
• Forms a yellow conical flame shape
• Produces the highest flame temperature
• Provides the minimum exhaust volume

Lean Combustion

Characteristics:

• Flue products oxidizing (Free O₂)
• Pale blue color
• Forms a more conical flame
• All fuel combusted
• Flame temperature drops (Heating excess air)

Incomplete Combustion

Characteristics:

• Air starved or fuel rich
• Carbon Monoxide and H₂ formed
• Reducing atmosphere
• Flame color is predominantly yellow color
• The flame is less defined
• Flame temperature drops

Optimum Energy Utilization Occurs When Perfect Combustion Ratios Are Maintained

When A Furnace Tune-Up Is Done Right The Technician will have taken time to:

• Verify proper mix and directed air/fuel supply
• Confirm that you have stable combustion
• Check to make sure that the system is venting all the by-products of the exhaust flue gases

A Definition of Combustion Terms

Excess Air Definition:

Air remaining after the fuel has been combusted
-or-
Air supplied in addition to the quantity needed for stoichiometric combustion

Primary & Secondary Air:

Primary Air —

• Air mixed with the fuel prior to ignition

Secondary Air —

• Air supplied to the flame after it is ignited

Secondary Air Effects —

• Imprecise air/fuel ratio control
• Exhaust volume increases
• Fuel rate rises
• Less available heat