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Integrated Modules
Integrated Modules
Electronics and Gas Theory
2-22
#826498 - Revision B - December, 2016
PRINCIPLES OF GAS COMBUSTION
Combustion
- When oxygen acts with a substance to produce large amounts of heat rapidly.
Requirements for Combustion
- There are three required elements for combustion to occur; Fuel (Gas), Oxygen
(Air) and Heat (Ignition Temperature, which for gas is between 1100°F/593°C and 1200°F/649°C). All must be pres-
ent. Removing any one of the three and combustion will cease.
Chemistry of Combustion
- Combustion of gas is a chemical reaction between fuel gas and oxygen. The basic
elements of common fuel gasses are hydrogen [H] and carbon [C]. When hydrogen burns, water vapor [H
2
O] is pro-
duced. Complete burning of carbon in fuel gases form carbon dioxide [CO
2
] and water vapor [H
2
O].
Controlled Combustion
- Controlled combustion takes place when gas and air are supplied at proper rates to
assure complete combustion of the gas in a steady flame. When a gas appliance is operating properly, burning
starts at the burner ports. Gas flow is controlled by gas orifice size and gas pressure upstream of the orifice. Air is
mixed with the gas before it passes through the burner ports. This added air is called “Primary Air”. The remaining
air required for complete combustion is supplied to the burner at the point of combustion and is called “Secondary
Air”.
Adjustments of the gas-to-air ratio and the secondary air supply is the key to obtaining stable blue flames at a burn-
er. Proper amounts of primary and secondary air are required for quiet and efficient burner operation and for com-
plete combustion of the gas. Air Shutters or other devices provide control of primary air. Inlet opening and flue out-
lets control Secondary Air flow.
Total air
- In an ideal situation, primary and secondary air is all that is needed (for the oxygen required) to burn the
gas, but some additional air is required to assure complete burning of the gas. The total air, “primary”, “secondary”
and “excess” are expressed as percentages of the amount needed. About ten cubic feet of air is required to com-
pletely burn one cubic foot of gas. For this reason an appliance should not be operated in an air tight home.
Limits of Flammability
- Not all air-to-gas mixtures will burn. Mixtures with 0% - 4% natural gas in air are too lean
to burn. Mixtures of 4% - 14% natural gas in air can burn with a controlled flame. Flammability limits come into play
when primary air adjustments are made on burners. If too much primary air is used, the mixture may become too
lean and fall below flammability limits, thus preventing combustion.
Incomplete Combustion
(Causes and Effects)
- To obtain complete combustion, sufficient amounts of air must be
supplied to the process. This air must have a reasonably normal oxygen content. Complete burning of gas pro-
duces harmless carbon dioxide gas and water vapor. If the air supply is insufficient, incomplete combustion occurs
resulting in the formation of toxic by-products, such as carbon monoxide [CO] or aldehydes.
Carbon monoxide is colorless and odorless. Inhaling carbon monoxide in sufficient quantities could cause death by
reducing oxygen levels in the blood.
Aldehydes, which are equally dangerous, have a sharp and penetrating odor which is easily detected by smell at
very low concentrations. The odor caused by aldehydes should not be confused with odorants added to natural gas.
The absence of aldehydes does not assure that carbon monoxide is not present. However, if the odor of aldehydes
is present, then carbon monoxide is virtually always present.
Gas Burner Operation
- A gas burner is a device to burn gas under control in order to produce useful heat.
Primary air is brought into the burner from outside of the appliance at atmospheric pressure. The gas jet streaming
from the orifice draws primary air with it into the burner. The gas/air mixture, combined with a spark at the burner
port(s) and the secondary air creates a controlled burn.
