Garland G Series, Техническое справочное руководство

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Part # GCTRM Rev 3 (12/10/09) Page 26
Properties And Characteristics Of Fuel Gases
The petroleum gases most commonly used in the gas
industry are natural gas and it two processed derivatives,
propane and butane. The two most commonly used fuels in
commercial cooking equipment are natural gas and propane.
The major ingredient in fuel gas in methane.
Natural Gas
Natural gas exists because of a naturally occurring mixture
of Hydrogen (H) and Carbon (C) and small amounts of other
gases. Since natural gas is made up mostly of methane, it is
usually represented by the chemical formula CH4.
Propane
LP gases (Liquid petroleum gases) are hydrocarbons that are
refined, stored and transported as liquid under pressure. Your
BBQ propane tank is an example of how this gas is stored as a
liquid. It then vaporized into a fuel gas when it is released for
use in a gas-burning appliance. LP gas is extracted from both
oil and natural gas. Under normal atmospheric temperature
and pressure, propane is a gas. At cooler temperatures and
at higher pressures, it is easily changed to a liquid form.
Propane is represented by the chemical formula C3H8.
General Properties
Natural gas and propane are nontoxic, tasteless, colorless,
invisible and odorless. Since they are undetectable, for safety,
an odorant known as mercaptan must be added to the
natural and propane gas. Mercaptan lets the user know that
gas has escaped. Approximately one pound of mercaptan is
added to every million cubic feet of gas.
There are other properties of natural and propane gas that you
should be aware of. They are specific gravity, expansion factors,
boiling point and calorific value of natural and propane gas.
Specific Gravity
Also known as relative density, specific gravity is the
comparison between the weight of the volume of vapor or
liquid and the weight of an equal volume of air or water.
Specific Gravity of Vapor Gases
To be able to compare the specific gravity of air and gas, both
must be at the same pressure and temperature. Air has a
specific gravity of 1.0. Natural gas had a specific gravity of 0.6.
Natural gas is lighter than air and will rise to the ceiling when
there is a leak. Propane gas has a specific gravity of 1.52
compared to and will usually settle in low areas.
Specific Gravity of LP
When compared to water LP has a specific gravity of 0.51.
Propane is lighter than water and if it were to be spilled in a
liquid form on water, it would float on the surface.
Expansion Factors
When the temperature of a liquid increases, it causes the
liquid to expand, causing the volume of the liquid to increase
as well.
Your propane tank at home is usually only filled to 80% of
its capacity to allow for expansion. The space between the
liquid and top of the tank is replaced by vapor, which you
use as propane gas. The pressure caused by the molecular
movement of LP gas vapors is called vapor pressure. When
you remove this vapor from the container, it lowers the
vapor pressure, it causes the remaining liquid gas to boil and
replace the vapor. This stabilizes the reduced pressure in the
tank.
When a fuel changes from a liquid to a gas, expansion takes
place. Each gas has a different ratio of expansion. Natural gas
expands at a ratio of 600:1 and propane at a ratio of 270:1 at
atmospheric pressure.
Boiling Point
The pressure of a tank is determined by the temperature of
the liquid inside it. At atmospheric pressure, natural gas has
a boiling point of -258°F. Propane has a boiling temperature
of -44°F.
Natural gas is almost always provided by a utility company in
the form of gas but the utility company may store it liquefied.
If the temperature of a fuel gas is lowed below its boiling
point, it will not vaporize and no vapor pressure will develop.
The boiling in a tank only stops when the vapor pressure
is high enough to suppress the boiling of the liquid at
that particular temperature in the tank. This pressure only
varies with the temperature not with the liquid content of
the cylinder tank. The pressure in a cylinder is at the same
pressure whether the tank is 20% or 70% full.
Calorific Value (Heat Content)
The calorific value of a fuel is also known as the heat content.
It is described as the energy released when a given volume of
fuel is burned. The unit of measure commonly used is British
Thermal Units per cubic foot (BTU/ft
3
). A BTU is the amount
of heat required to raise the temperature of 1 pound of
water 1° F.
The calorific value of natural gas varies because of different
components of the natural gas that my be present. The
general value used in industry is
1000 BTU/ft
3
for natural gas and 2520 BTU/ft
3
vaporized.
When calculating the input rate of an appliance, the calorific
value of the gas is taken into consideration.