Viking VGR30, Service Manual

Page: 11 / 40

11
COMBUSTION OF GAS
A.
The Meaning of Combustion - When oxygen
acts with a substance to produce large amounts of
heat rapidly (and usually light), the process is
called combustion or burning.
B. Requirements for Combustion - Three things
needed for combustion to take place are fuel,
oxygen (air) and heat (temperature). All must be
present. Take away any one of the three and
burning will stop. Gas ignition temperature is
approximately 1100 - 1200 degrees Fahrenheit.
C. Basic Chemistry of Combustion - Combustion
of gas is a chemical reaction between fuel gas and
oxygen. The basic elements of common fuel
gases are hydrogen (H) and Carbon (C). When
hydrogen
burns, water vapor (H2O) is
produced. Complete burning of carbon in fuel
gases form carbon dioxide (C02). Complete
combustion produces harmless carbon dioxide
(C02) and water vapor (H20).
Water is produced as a vapor in the burning of
gas.If the flue products remain hot enough, water
is discharged as vapor to the outside through vent
system. If the flue products should become cool,
as in an air conditioned room, this water vapor
will condense out as a liquid on any cooler
surface. The temperature at which water forms
from vapor is known as the dew-point.
D. Controlled Combustion - Controlled combustion
takes place when fuel gas and air are
supplied at proper rates to assure complete
combustion (burning) 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 a gas orifice size and by
gas pressure upstream of the orifice. Burners
which have some air premixed with the gas before
it passes through the BURNER
PORTS
are called “blue flame” burners. This air added to
the gas is called primary air. The rest of the air
required for complete combustion is supplied
to the burner flames in the COMBUSTION
CHAMBER and is called SECONDARY AIR.
Adjustments of gas rate and primary air provide
the key to obtaining stable, blue flames on the
burner using primary air. AIR SHUTTERS or
other devices provide control of primary air.
Proper amounts of primary and secondary air are
needed for quiet and efficient appliance operation
and for complete combustion of the gas. Size of
the inlet openings and flue outlets control
secondary air flow.
E. Explosive Combustion - An explosion is simply
very rapid burning which is not under control.
F. Limits of Flammability - Not all air-gas
mixtures will burn. Mixtures with zero to four
percent natural gas in air are to lean to burn.
Mixtures of four to fourteen percent natural gas in
air can burn with a controlled flame.
Flammability Limits are of interest to those
dealing with problems and everyday operations in
the gas industry. For example consider,
FLASHTUBE ignition of rangetop burners. A
gas-air mixture from the burner head passes
through a charge port into the open end of the
flashtube. This mixture is too rich to burn
without secondary air, but some air also enters
the flashtube at the entrance. The additional air
causes the mixture to become leaner to a
composition that is flammable. When the mixture
travels down the tube to point of ignition, the
flame will carry back from the lighter to the
charge port and light the gas at the burner. If
the mixture in the flashtube has too
much gas (too rich) or has too much air (too lean),
the burner will not light.
Flammability Limits also enter the picture when
primary air adjustments are made on burners. For
example, Infra-red (radiant) burners usually
operate with about 100 percent primary air. In
other word the gas air mixture in the burner head
contains all the air ideally required for complete
combustion. If too much primary air is used, the
mixture may become too lean and fall outside the
lower flammability limit and it will not burn on
the burner surface.
Unburned gas should not be allowed to collect in
combustion chambers or confined spaces because
gas-air mixture within the flammability limits will
explode if ignited.
G. Incomplete Combustion - Causes and Effects
To obtain complete combustion enough air must
be supplied to the process. This air must have a
Reasonably normal oxygen content. Complete
burning of gas produces harmless carbon dioxide
and water vapor. If not enough air is supplied
other products will form. Some of these products
are harmful, especially carbon monoxide.