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Electronics and Gas Theory
Integrated Modules
Integrated Modules
2-21
#826498 - Revision B - December, 2016
GAS THEORY - CG152TF/S and MM15TF/S
OPERATION INFORMATION
A service technician should understand how a gas appliance operates before attempting to service the appliance.
This section provides descriptions of the different types of fuel gases and explains gas heating values. A definition
of specific gravity of gas is given along with its characteristics and effects. Gas combustion principles are explained
and gas burner components are described and illustrated.
Types of Fuel Gas
Gases used to supply heat energy are called fuel gases. Common fuel gases are not simply one kind of hydrocar-
bon, they are mixtures of hydrocarbon gases. They contain other gases as well, such as free hydrogen, carbon
dioxide and nitrogen. As an example, natural gas might contain 85% methane, 12% ethane and 3% of other gases.
The presence of each of these gases in the fuel gas has some effect on the nature of the gas.
Some common fuel gasses are methane [CH
4
], ethane [C
2
H
6
], Propane [C
3
H
8
] and butane [C
4
H
10
]. Propane and
butane are nearly odorless. Natural gas that is processed to remove condensables and moisture, has little or no
odor and no color. Odorants are added to natural gas before distribution to aid in leak detection. A common odorant
used is a colorless liquid containing sulfur compounds.
Heating Value of Gas
Heat energy produced when burning a fuel gas is commonly expressed in British Thermal Units (BTU). One BTU of
heat will raise the temperature of one pound of water one degree Fahrenheit.
The more carbon and hydrogen atoms in each molecule of a fuel gas, the higher its heating value. Natural gas
which is high in methane has a heating value of about 950 to 1150 BTU per cubic foot. The variance is due to vari-
ous other substances found in natural gases. The more ethane, propane or butane in the gas raises the heating
value. Propane, or LP gas, has a heating value of about 2500-2800 BTU per cubic foot, and butane about 3200
BTU per cubic foot.
Specific Gravity of Gas
The specific gravity of a gas is the weight of one cubic foot of the gas compared to one cubic foot of dry air. When
stating the specific gravity of a gas, a pressure and temperature must be clearly stated. In the gas industry, the
standard conditions of pressure and temperature are 30.0 inches of mercury and 60° F. A pressure of 30.0 inches of
mercury will sustain a column of mercury 30 inches high in a tube with a vacuum on top of the column. Since air is
used as the reference, its specific gravity is always 1.0. This value of 1.0 has no direct physical meaning with
regard to air, such as its density. It is only a relative number or ratio used to express specific gravity of other gases.
The specific gravity of a gas will determine if the gas will rise or fall when released into the air. Natural gas will rise
since its specific gravity is less than 1.0 at 0.4 to 0.8. Propane has a specific gravity of 1.5 and butane 2.0. These
gases will fall when released into the air. Gases with a specific gravity greater than 1.0 can sometimes pool in low
spots, which become a hazard if a spark or open flame is present.
In addition, specific gravity has two other characteristics. It has an important effect on the flow of gases through ori-
fices, and hence the rating of the burners. Gas flow through an orifice is dependent upon the orifice size and the
gas pressure upstream of the orifice. More of a lighter gas will flow through a given orifice size than a heavier gas
at the same gas pressure. This effect is taken into account in tables and calculators used to select orifice sizes for
burners.
Specific gravity also affects gas flow in pipes. A given driving pressure at a pipe inlet will move more lighter gas
than heavier gas through that pipe.
