Given
Assumptions
In "real world" design problems, an engineer must make
valid assumptions in order to proceed with a design. In an
academic setting, the instructor often guides the initial
design efforts of the students by the choices of assumptions
and constraints. Since this is your first lengthy design
project, some of the basic assumptions you will need to get
started are given below:
1. The ideal gas law applies to natural gas.
2. The CNG
is stored at a maximum pressure of 3000 psi (lb/in2).
3. All
physical properties of natural gas are assumed to be
identical to pure methane.
4. Tanks
are hollow cylindrical shapes (with hemispherical end caps)
made of steel.
Cylinder
Stress
The design
of real "pressure vessels" (such as the CNG tank) is
governed by an elaborate set of codes and standards
developed by the ASME (American Society of Mechanical
Engineers). However, a relatively simple design equation can
be used as a first approximation. The diameters and wall
thickness of the CNG tank are limited by the maximum stress
on the inner surface of the tank. This maximum stress can be
estimated by the formula
The
term on the left side of the equation represents the maximum
tangential stress on the inside surface of an infinitely
long cylinder subjected to an inner pressure of 3000 psi
(pounds per square inch). Engineers generally limit the
maximum allowable stress to a value based on the material
properties with a significant factor of safety built-in. For
this design, a limit of 60,000 psi on the maximum stress
will be satisfactory. Note that this high limit is due to
the use of composite wrapping along the length of the
cylinder and does NOT represent a limit that can be used for
other designs!
Costs for
Conversion to CNG
There are
two costs associated with a conversion of an automobile to
operate on natural gas. There is the fixed cost of the
regulators, hoses, valves, carburetor adaptor, and
installation labor. We will assume that this cost is $1000,
regardless of the vehicle make and model. The other cost is
associated with the CNG tank. For our purposes, we will
assume that the cost of the tank is a function of its
weight, and can be estimated from the formulas fitted to the
data shown below:
Simple
Payback Period
The
Simple Payback Period (SPP) is the time in years required
for the savings to offset the initial, up-front costs.
For a
replacement project: SPP = (Initial Cost) / (Annual Savings)
CNG
Refueling Stations
A number
of service stations across the country provide CNG at a pump
- just like for gasoline.
The table
below indicates the relative costs of CNG and regular
unleaded gasoline at several locations across the U.S.
Station
Location CNG
Regular Unleaded
(equivalent gallon) (gallon)
Amoco Minneapolis, MN
$0.969 $1.269
Exxon Billings, MT
$0.750
$1.299
Unocal Vista, CA
$0.880
$1.319
Total Denver, CO
$0.809
$1.199
Sinclair Salt Lake City, UT
$0.602 $1.159
Mobil Garland, TX
$0.799
$1.179
Shell Houston, TX
$0.899
$1.199
Chevron Houston, TX
$0.799
$1.189
Phillips
66 Oklahoma City, OK
$0.799 $1.069
Amoco Topeka, KS
$0.859 $1.079
Conoco
Mobile, AL
$0.799 $1.149
Shell Palm Beach Gardens, FL
$0.999 $1.339
Amoco Atlanta, GA
$0.799 $1.039
Amoco Tucker, GA
$0.799 $1.029
Amoco Naperville, IL
$0.959
$1.299
Texaco Hartford, CT
$0.999 $1.399
Mobil Brooklyn, NY
$1.139
$1.419
(table
from Natural Gas Fuels, Vol. 4, No. 2, August 1995, page 14)
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