Download - Compressor Report Thesis Vohra
DEVELOP A SOLUTION FOR REFUELING
CNG IN A VEHICLE AT HOME BY
INCORPORATING SAFETY INTERLOCKS.
BY
Abdul Wahab Vohra
Faculty of Mechanical Engineering
GIK Institute of Engineering Sciences & Technology
MAY 2011
DEVELOP A SOLUTION FOR REFUELING CNG IN
A VEHICLE AT HOME BY INCORPORATING
SAFETY INTERLOCKS
SENIOR DESIGN PROJECT REPORT
BY
ABDUL WAHAB VOHRA
Approved as to style and content by
_____________________________________________________________
___________________________________________________________
SUPERVISED BY
DR. TARIQ SAEED
SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF BACHELOR OF SCIENCE
Faculty of Mechanical Engineering
GIK Institute of Engineering Sciences & Technology
Abstract
In today‟s world, energy is becoming more and more like a commodity not everyone can
afford. It is gradually slipping out of the grasp of a common man and its consumption is
growing exponentially with each passing year. Pakistan is one of those countries where
the growth of energy sector has received the least bit of attention. And people are growing
poorer and poorer every day. Therefore, more people are inclined to look towards
alternate sources of energy to fulfill their needs. Home Filling CNG Station is one such
step towards achieving the nirvana of financial stability for the common man and within
the comfort of their own residence. It is common knowledge that fuel prices will continue
to hike because most of our vehicles run on non-renewable sources. But when one fills
their vehicle from a commercial fuel station, these prices are tripled or even quadrupled
due to the addition of taxes, rent charges and overhead charges. Our Home Filling CNG
Station avoids that; instead of filling CNG from a commercial station one can now fill
their vehicles with gas at home. It utilizes the natural gas from the domestic gas line and
compresses it slowly until the desired pressure is reached and then automatically shuts it
down. It is safe and easy to use, more like a home appliance. It incorporates a
reciprocating air compressor and is controlled by safety interlocks to augment its safety.
MAY 2011
ii
Dedication
This report is dedicated to our parents, to our mentor and advisor Dr. Tariq Saeed, and to
the people of this country. Their need is our motivation.
iii
Acknowledgements
All thanks is to Allah Almighty, Who gave us strength and perseverance to achieve our goals
and helped us realize our ambition into reality.
We would like to thank our advisor Prof. Dr. Tariq Saeed and Mr. Nasir Mohammed Vohra
for the support that they extended to us during the course of this project. Their dedication and
thoroughness is something that has inspired us, and their example will serve us all our life in
whatever we do.
I would also like to thank the Dean Dr. Javed Chattha for permitting us to work on this high
risk project and trusting us with the safe work ethics.
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Table of Contents
Abstract ……………………………………………………………………. I
Dedication ………………………………………………………………… II
Acknowledgments ………………………………………………………… III
LIST OF TABLES ……………………………………………………….. IV
LIST OF FIGURES………………………………………………………... VII
NOMENCLATURE………………………………………………………... IX
CHAPTER 1 INTRODUCTION
1.1 Conceptualization of Product………………………………………. 1
1.2 Compressor and its Types……………………………………..…... 3
1.3 Intercooling………………………………………………………... 10
1.4 Thermodynamics of Air compression……………….…………….. 11
1.5 CNG in General………………………………………………….… 14
CHAPTER 2 ANALYTICAL CONSIDERATION
2.1 Compressor Selection……………………………………………… 22
2.2 First Report (Compressor Variables)……………………………… 30
2.3 Second Report (Refurbishing)…………………………………….. 35
2.4 Third Report (Storage)…………………………………………….. 47
2.5 Literature on Compressibility Factors……………………………... 52
2.6 T-S and P-V diagrams of our compressor…………………….…… 56
CHAPTER 3 DEVELOPMENTAL WORK
3.1 Process Flow Diagram……………………………………………. 61
3.2 Development of Trolley Panel………..…………………………… 62
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3.3 Trolley Fabrication………………..………………………….……. 63
3.4 Circuit Layout……………..………………………………………. 64
3.5 Main Circuit Diagram……………………….…………………….. 65
3.6 Safety Interlocks…………………………………………………… 67
CHAPTER 4 RESULTS
4.1 Trial Run Data for CNG and Air………………………………….. 72
4.2 Cost Analysis………………………………………………….…… 74
4.3 Alterations…………………………………………………….….... 75
4.4 Power Requirements………………………………….…............... 76
4.5 Trial Run Comparative Graphs……………..……………………… 77
CHAPTER 5 CONCLUSION AND RECOMMENDATIONS
5.1 Cost Analysis………………………………………………….…… 79
5.2 Alterations….………………..…………………………………….. 80
REFERENCES…………………………………………………………………….. 81 Reference Quotation……………………………………………………… 82
Calibration Certificates…………………………………………………… 84-88
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List of Tables
Table 1.1 Compressed Natural Gas Properties……………………………. 17
Table 2.1 Survey Tablature………………………………………………... 32
Table 2.2 Extract Form The Manual………………………………………. 43
Table 2.3 Storage Systems………………………………………………… 48
Table 3.1 Bill of Materials………………………………………………… 70
Table 4.1 Trail Run Readings for CNG Filling……………………………. 72
Table 4.2 Trail Run Readings for Air Filling…………………………….... 73
vii
List of Figures
Page
Fig 1-1 Compressor and its types………………………………………… 3
Fig 1-2 Centrifugal Compressor…………………………………………. 4
Fig 1-3 Axial Flow Compressors………………………………………… 5
Fig 1-4 Reciprocating Compressor………………………………………. 6
Fig 1-5 Rotary Screw Compressor………………………………………. 7
Fig 1-6 Mechanism of a Scroll Compressor……………………………… 8
Fig 1-7 Diaphragm Compressor………………………………………….. 9
Fig 1-8 P-v diagram of Polytropic compression process with Intercooling.. 11
Fig 1-9 T-s diagram of Polytropic compression process with Intercooling.. 12
Fig 1-10 Compression Cycle in a Compressor……………………………… 13
Fig 1-11 Comparison of Auto-ignition Temperature……………………….. 15
Fig 1-12 Comparison of Peak Flame Temperature…………………………. 16
Fig 2-1 Single Stage Compression………………………………………… 26
Fig 2-2 Multistage Compression………………………………………….. 28
Fig 2-3 Coltri Sub Compressor……………………………………………. 34
Fig 2-4 Front View…………………………………………………………. 36
Fig 2-5 Side View………………..……………………………………….. 37
Fig 2-6 Top View………………………………………………………….. 37
Fig 2-7 Schematic of Safety Controls……………………………………… 44
Fig 2-8 Opted Compressor………………………………………………… 44
Fig 2-9 Cross-Sectional View of our Pressure Switch…………………….. 45
Fig 2-10 Neo-Dyn 232 Pressure Switch…………………………………….. 45
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Fig 2-11 Electrical Safety Interlocks Diagram……………………………… 46
Fig 2-12 Storage System……………………………………………………. 50
Fig 2-13 Compressor Performance Diagram……………………………….. 53
Fig 2-14 P-V Diagram of our Compressor…………………………………. 56
Fig 2-15 T-S diagram of our Compressor………………………………….. 57
Fig 2-16 Practical P-V diagram……………………………………………. 58
Fig 2-17 Effect of Clearance Volume……………………………………… 59
Fig 3-1 Compressor Process Flow Diagram……………………………… 60
Fig 3-2 Panel Diagram…………………………………………………… 61
Fig 3-5 Trolley Fabrication Diagram…………………………………….. 64
Fig 3-6 Circuit Layout Diagram………………………………………….. 65
Fig 3-7 Main Circuit Diagram……………………………………………. 65
Fig 3-10 Safety Interlocks…………………………………………………. 67
Fig 3-11 Power Supply……………………………………………………. 67
Fig 3-12 Thermal Overload Relay………………………………………… 68
Fig 3-13 Honeywell Temperature Controller……………………………… 68
Fig 3-14 Smoke detector…………………………………………………… 69
Fig 3-15 Three Phase Contactor…………………………………………… 69
Fig 4-1 CNG Station……………………………………………………… 75
Fig 4-2 Pressure against Time Graph…………………………………….. 77
Fig 4-3 Temperature against Pressure Graph…………………………….. 77
Fig 4-4 Current against Time Graph……………………………………… 78
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Nomenclature
P Pressure
V Volume
T Temperature
R Universal Gas constant
k Isentropic Expansion factor
⁰F Fahrenheit
⁰C Celsius
s Entropy
ASME American Society of Mechanical Engineers
W Work
hp Horsepower
ppm Parts per million
MPa Mega Pascal
Bar 105 Pa
Kg Kilogram
atm Atmospheric Pressure
hr hour
gal Galleon
dB decibel
psi Pounds per square inch
PR Pressure Ratio
PSA Pressure Switch
xii
TISA Temperature Switch
PI Pressure Indicator
V-001 3-way filling valve
H5 Pressure Indication
H6 Temperature Indication
H7 Fire Indication
R1 Pressure Relay
R2 Smoke Relay
R3 Temperature Relay
cfm Cubic Feet per min
1
CHAPTER 1
INTRODUCTION
1.1 Conceptualization of Product
Problem Statement:
Fabrication of a dispensable unit for filling CNG at homes which can achieve the same
pressure of 3000psia as available in commercial stations. The task is to achieve this pressure
by making it cost effective and safe for home usage.
Objective:
To develop a cost effective solution to refueling compressed natural gas in a vehicle at home
by incorporating safety interlocks and automated fueling.
Scope
Literature Study
Compressors and Safety Controls.
To purchase an air compressor required to pressurize gas to 200 bar:
High pressure 4 stage reciprocating compressor up to 3000psi (200bar).
Air cooled and Oil lubricated.
Utilizing an air compressor for compressing natural gas with reference to MSDS.
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Perform alterations based on selection criteria of
High Pressure Fixtures
Safety interlocks
Automated fueling
Design and fabrication of dispensing trolley
Trial Runson bothair and natural gas after alterations.
Need Assessment and Motivation:
• Waiting in long queues
• Traveling expenditures and labor
• 24/7 gas supply at home
• Cost effectiveness
• Less hassle, more convenience
Success Criteria
Our goal is to modify an air compressor for compressed natural gas fills on a regular
CNG vehicle and achieving this defines our criteria for project completion.
Once manufactured our home filling prototype unit can be installed in any household
and commercialized, if permitted by OGRA laws in near future.
3
1.2 Compressor and its Types
Gas Compressor:
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its
volume. Compressors are similar to pumps: both increase the pressure on a fluid and both can
transport the fluid through a pipe. As gases are compressible, the compressor also reduces the
volume of a gas.
Fig 1-1: Compressor and its Types
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1) Centrifugal compressors
Fig 1-2: Centrifugal Compressor
Centrifugal compressors use a rotating disk or impeller in a shaped housing to force
the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser
(divergent duct) section converts the velocity energy to pressure energy. They are
primarily used for continuous, stationary service in industries such as oil refineries,
chemical and petrochemical plants and natural gas processing plants. Their
application can be from 100 horsepower (75 kW) to thousands of horsepower. With
multiple staging, they can achieve extremely high output pressures greater than
10,000 psi (69 MPa).
Many large snowmaking operations (like ski resorts) use this type of compressor.
They are also used in internal combustion engines as superchargers and
turbochargers. Centrifugal compressors are used in small gas turbineengines or as the
final compression stage of medium sized gas turbines. Sometimes the capacity of the
compressors is written in NM3/hr. Here 'N' stands for normal temperature pressure
(20°C and 1 atm ) for example 5500 NM3/hr.
2) Diagonal or mixed-flow compressors
Diagonal or mixed-flow compressors are similar to centrifugal compressors, but
have a radial and axial velocity component at the exit from the rotor. The diffuser is
often used to turn diagonal flow to an axial rather than radial direction.
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3) Axial-flow compressors
Fig 1-3: Axial Flow Compressors
Axial-flow compressors are dynamic rotating compressors that use arrays of fan-like
airfoils to progressively compress the working fluid. They are used where there is a
requirement for a high flow rate or a compact design.
The arrays of airfoils are set in rows, usually as pairs: one rotating and one stationary.
The rotating airfoils, also known as blades or rotors, accelerate the fluid. The
stationary airfoils, also known as stators or vanes, decelerate and redirect the flow
direction of the fluid, preparing it for the rotor blades of the next stage.[1]
Axial
compressors are almost always multi-staged, with the cross-sectional area of the gas
passage diminishing along the compressor to maintain an optimum axial Mach
number. Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is
normally used to improve operation.
Axial compressors can have high efficiencies; around 90% Polytropic at their design
conditions. However, they are relatively expensive, requiring a large number of
components, tight tolerances and high quality materials. Axial-flow compressors can
be found in medium to large gas turbine engines, in natural gas pumping stations, and
within certain chemical plants.
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4) Reciprocating compressors
Fig 1-4: A motor-driven six-cylinder reciprocating compressor
Reciprocating compressors use pistons driven by a crankshaft. They can be either
stationary or portable, can be single or multi-staged, and can be driven by electric
motors or internal combustion engines. Small reciprocating compressors from 5 to
30 horsepower (hp) are commonly seen in automotive applications and are typically
for intermittent duty. Larger reciprocating compressors well over 1,000 hp (750 kW)
are commonly found in large industrial and petroleum applications. Discharge
pressures can range from low pressure to very high pressure (>18000 psi or 180
MPa). In certain applications, such as air compression, multi-stage double-acting
compressors are said to be the most efficient compressors available, and are typically
larger, and more costly than comparable rotary units.[6]
Another type of reciprocating
compressor is the swash plate compressor, which uses pistons which are moved by a
swash plate mounted on a shaft - see Axial Piston Pump.
Household, home workshop, and smaller job site compressors are typically
reciprocating compressors 1½ hp or less with an attached receiver tank.
7
5) Rotary screw compressors
Fig 1-5: Diagram of a rotary screw compressor
Rotary screw compressors use two meshed rotating positive-displacement helical
screws to force the gas into a smaller space. These are usually used for continuous
operation in commercial and industrial applications and may be either stationary or
portable. Their application can be from 3 horsepower (2.2 kW) to over
1,200 horsepower (890 kW) and from low pressure to moderately high pressure
(>1,200 psi or 8.3 MPa).
6) Rotary vane compressors
Rotary vane compressors consist of a rotor with a number of blades inserted in
radial slots in the rotor. The rotor is mounted offset in a larger housing which can be
circular or a more complex shape. As the rotor turns, blades slide in and out of the
slots keeping contact with the outer wall of the housing. Thus, a series of decreasing
volumes is created by the rotating blades. Rotary Vane compressors are, with piston
compressors one of the oldest of compressor technologies.
With suitable port connections, the devices may be either a compressor or a vacuum
pump. They can be either stationary or portable, can be single or multi-staged, and
can be driven by electric motors or internal combustion engines. Dry vane machines
are used at relatively low pressures (e.g., 2 bar or 200 kPa; 29 psi) for bulk material
8
movement while oil-injected machines have the necessary volumetric efficiency to
achieve pressures up to about 13 bar (1,300 kPa; 190 psi) in a single stage. A rotary
vane compressor is well suited to electric motor drive and is significantly quieter in
operation than the equivalent piston compressor.
Rotary vane compressors can have mechanical efficiencies of about 90%.
7) Scroll compressors
Fig 1-6: Mechanism of a scroll pump
A scroll compressor, also known as scroll pump and scroll vacuum pump, uses
two interleaved spiral-like vanes to pump or compress fluids such as liquids and
gases. The vane geometry may be involutes, Archimedean spiral, or hybrid curves.
They operate more smoothly, quietly, and reliably than other types of compressors in
the lower volume range
Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating,
thereby trapping and pumping or compressing pockets of fluid or gas between the
scrolls.
This type of compressor was used as the supercharger on Volkswagen G60 and G40
engines in the early 1990s.
9
8) Diaphragm compressors
A diaphragm compressor (also known as a membrane compressor) is a variant of
the conventional reciprocating compressor. The compression of gas occurs by the
movement of a flexible membrane, instead of an intake element. The back and forth
movement of the membrane is driven by a rod and a crankshaft mechanism. Only the
membrane and the compressor box come in contact with the gas being compressed.
Diaphragm compressors are used for hydrogen and compressed natural gas (CNG) as
well as in a number of other applications.
Fig 1-7: A three stage diaphragm compressor
The photograph included in this section depicts a three-stage diaphragm compressor
used to compress hydrogen gas to 6,000 psi (41 MPa) for use in a prototype
compressed hydrogen and compressed natural gas (CNG) fueling station built in
downtown Phoenix, Arizona by the Arizona Public Service company (an electric
utilities company). Reciprocating compressors were used to compress the natural gas.
9) Air bubble compressor
A mixture of air and water generated through turbulence is allowed to fall into a
subterranean chamber where the air separates from the water. The weight of falling
water compresses the air in the top of the chamber. A submerged outlet from the
chamber allows water to flow to the surface at a lower height than the intake. An
outlet in the roof of the chamber supplies the compressed air to the surface.
10
1.3 Intercooling
If you understand that “inter” means between, and that cooler – well that‟s self explanatory
isn‟t it? Particularly if you‟re inclined to a cold beverage drawn from a cooler on a hot
afternoon out on the beach, then you already understand intercoolers.
The process of compressing air elevates the air temperature dramatically. As air is
compressed from single cylinder or from cylinder to cylinder in twin cylinder reciprocating
compressors, or from the compression equipment in rotary screw, or rotary vane compressors
and then into the receiver, the temperature of the compressed air will continue to rise.
In a multi-stage unit compressor, the air is compressed in
succeeding cylinders.
An intercooler will be installed between the cylinders to
help cool the air before it‟s ingested into the next cylinder
for further compression. This aids in the compressor‟s
efficiency.
Intercoolers in multi-stage units may function through air cooling or water cooling.
In air cooling the compressed air will pass through a chamber which, on the outside, offers
substantially increased surface area to the ambient environment. The increased surface area
will allow the heat inside the compressed air line to move more readily to the surface and to
escape.
Water cooling is achieved by passing the compressed air through water-cooled heat
exchanger(s) similar in concept to this one. Cool water will flow around the outside of the air
line, quickly taking heat away, and cooling the compressed air rapidly.
Consider also that the air receiver, that tank that stores your compressed air before use, and
that‟s located between your compressor and your plant air lines, is also an intercooler of
sorts.
11
The longer the air sits in the receiver before use the cooler the air will get and the more
condensation will take place in the receiver. That, and frequent voiding of collected water
through the receiver‟s auto drain will prevent this condensed water from entering the
downstream airline.
Since we know about the arguments to cool any gas as it is compressed. This process reduces
the required work input to the compressor. However, often it is not possible to have sufficient
cooling through the casing of the compressor and now it becomes mandatory to use some
other techniques also to achieve effective cooling. One such technique is multistage
compression with Intercooling.
1.4 Thermodynamics of Air Compression with Intercooling
We know from the previous section that the minimum air compressor work is achieved with
isothermal compression. In practical way, we try to achieve that by involving some cooling
during compression process that leads to Polytropic compression process.
Normally, this can be achieved by dividing air compression into 2 stages. The first stage
builds up the pressure from P1 to Px then the compressed air is cooled by the intercooler and
the second stage compressor builds up the pressure again from Px to the final pressure P2. To
understand how the energy can be saved by using intercooling between each of the following
stages.
Fig 1-8: P-v diagram of Polytropic compression process with Intercooling
12
Fig 1-9: T-s diagram of Polytropic compression process with Intercooling
We can see from Fig. 1 that the amount of compressor work saved is related to the pressure
Px. The size of the colored area (the saved work input) varies with the value of the
intermediate pressure Px, and it is of our interest to determine the conditions under which this
area is maximized. The total work input is the sum of the work inputs for each stage of
compression.
The only variable in this equation is Px. The Px value that minimizes the total work is
determined by differentiating this expression with respect to Px and setting the resulting
expression equal to zero.
Or
That means the pressure ratio of each stage should be identical to get the lowest amount of
work required for air compression.
Although minimum work input is usually achieved with a constant temperature (isothermal)
reversible process, compression in rotary compressors is most often assessed relative to the
reversible adiabatic process ( isentropic-constant „s‟ processes). The p-v diagram below
shows the different processes.
13
Fig 1-10: Compression cycle in a compressor
An ideal compression process with no losses would be adiabatic and real processes are
compared to this by having using the adiabatic- isentropic efficiency which is defined as.
The power for reversible adiabatic compression is calculated from.
c = cycles traced per unit time and m = mass of air pumped per unit time. As cp = γ R /(γ-1)
and cp (T2s- T1 ) = (h2s - h1 ) the above expression can be rewritten
The isentropic efficiency of a uncooled rotary compressor when all the energy is used in
increasing the enthalpy of the fluid can be expressed as
14
1.5 CNG in General
Compressed Natural Gas
Chemical Composition
Natural gas consists of about 90% methane. In its natural form natural gas does not smell.
Therefore, the gas is odorized prior to distribution in order to detect possible leakage. Gas
can therefore be smelled already at a concentration of 0.3%. As CNG requires a
concentration of about 5% to 15% to combust, 0.3% is far below the dangerous combustion
level.
Physical attributes of CNG
Contrariwise the cylinder cools down while driving. When gas expands the density of the
molecules decreases and the temperature drops.
These physical attributes also have an effect on the total storage capacity of the cylinder
when refueling. If the temperature increases, the pressure in the cylinder increases as well.
The dispensers at the filling stations automatically stop dispensing CNG, once a
pressure of 200 bar is reached. If a cylinder can theoretically accommodate 18 kg CNG under
standard conditions (200 bar pressure, 15° Celsius), the cylinder will carry a bit less than 18
kg. Practically this means that the cooler the cylinder and the temperature around the cylinder
is the more kg of CNG can be pumped into the cylinder.
15
Sources of Hazard
Natural Gas, an ideal fuel source for many reasons, includes safety.
Natural Gas is lighter than air. This means that it will not puddle (like
gasoline) or sink to the ground like propane, which is heavier than air.
Instead, Natural Gas will rise and dissipate in the atmosphere.
Natural gas also has a higher ignition temperature. This means that it is much harder to
ignite. Also the storage systems used for compressed natural gas are infinitely stronger that
the gasoline tanks found on cars and trucks today.
Comparison of Auto Ignition Temperature
The auto ignition temperature is the temperature at which a fuel will ignite without the need
for a spark or flame. In respect to auto ignition temperature, CNG is much safer than
gasoline or diesel because the auto ignition temperature is much higher. The following chart
compares the auto ignition temperature of various fuels.
Fig 1-11: Comparison of auto ignition Temperature
16
Comparison of Peak Flame Temperature
The following chart compares the peak flame temperature of various fuels. You can see that
CNG (Compressed Natural Gas) has a peak flame temperature of 1790 C & 3254 F which is
187 C & 337 F or 9.5% cooler than the peak flame temperature of gasoline at 1977 C &
1591 F.
Fig 1-12: Comparison of peak flame temperature
Peak Flame Temperatures
17
Table 1.1: Compressed Natural Gas Properties
PROPERTIES GASOLINE DIESEL No.
2
LPG (HD-
5)
CNG
Physical State LIQUID LIQUID GAS GAS
Boiling Range
(oF @ 1 atm)
80 to 420 320 to 720 -44 to 31 -259a
Density (lb/ft3)
(lb/gal)
43 to 49
5.8 to 6.5
49 to 55
6.5 to 7.3
31b
4.1b
8c
1.07c
Net Energy Content Btu/lb. 18,700 -
19,100
18,900 19,800 21,300a
Auto ignition
Temperature (oF)
450 - 900 400 - 500 920 - 1,020 1,350
Flashpoint
(oF)
-45 125 (min) -100 to -
150
-300
Octane Number Range
(R+M) 2
87 to 93 n/a 104e 120
e
Flammability Limits
(volume % in air)
L = 1.4
H = 7.6
L = 0.7
H = 5.0
L = 2.4
H = 9.6
L = 5.3
H = 14
Human Exposure Limit For Fuel
(ppm)
500 n/a n/a nontoxic
18
Material Safety Data Sheet [Natural Gas]
PHYSICAL AND CHEMICAL PROPERTIES
Appearance : Colourless. Gas.
Odor : Typical gas smell due to addition of odouriser to allow the
detection of product leaks.
Initial Boiling Point and
Boiling Range: -161.5 °C / -258.7 °F
Flash point : -187.8 °C / -306.0 °F
Upper / lower Flammability
or Explosion limits: >= 5 %(V)
<= 15 %(V)
Auto-ignition temperature : 583 °C / 1,081 °F
Density: 420 g/cm3 at -165.5 °C / -265.9 °F Liquid methane at boiling
point.
Water solubility: 0.08 g/l at 25 °C / 77 °F
Vapor density (air=1): Typical 0.58
Emergency Overview
Health Hazards: Vapors may cause drowsiness and dizziness. High gas
concentrations will displace available oxygen from the air;
unconsciousness and death may occur suddenly from lack of
oxygen. Exposure to rapidly expanding gases may cause frost
burns to eyes and/or skin.
19
Safety Hazards : Extremely flammable. May form flammable explosive vapor
air mixture. Electrostatic charges may be generated during
handling. Electrostatic discharge may cause fire.
Environmental Hazards : Not classified as dangerous for the environment.
Explosion limits <= 15 %(V)
Auto ignition temperature : 583 °C / 1,081 °F
Specific Hazards : Forms flammable mixture with air. If released, the resulting
vapours will disperse with the prevailing wind. If a source of
ignition is present where the vapor exists at 5-15%
concentration in air, the vapor will burn along the flame front
toward the source of the fuel.
Suitable Extinguishing
Media: Shut off supply. If not possible and no risk to surroundings, let
the fire burn itself out.
Unsuitable
Extinguishing Media: Do not use water in a jet.
Protective Equipment
For Firefighters: Wear full protective clothing and self-contained breathing
apparatus.
ACCIDENTAL RELEASE MEASURES
Avoid contact with spilled or released material. For guidance
on selection of personal protective
20
Protective measures : Remove all possible sources of ignition in the surrounding
area. Evacuate all personnel. Do not breathe fumes, vapor. Do
not operate electrical equipment. Avoid contact with skin, eyes
and clothing. Ventilate contaminated area thoroughly. Shut off
leaks, if possible without personal risks. Remove all possible
sources of ignition in the surrounding area and evacuate all
personnel. Attempt to disperse the gas or to direct its flow to a
safe location for example by using fog sprays. Take
precautionary measures against static discharge. Ensure
Electrical continuity by bonding and grounding (earthing) all
equipment.
Additional Advice : Notify authorities if any exposure to the general public or the
environment occurs or is likely to occur.
HANDLING AND STORAGE
General Precautions: Take precautionary measures against static discharges.
Handling : Avoid contact with skin, eyes and clothing. Extinguish any
naked flames. Do not smoke. Remove ignition sources. Avoid
sparks. The inherent toxic and olfactory (sense of smell)
fatiguing properties of hydrogen sulphide require that air
monitoring alarms be used if concentrations are expected to
reach harmful levels such as in enclosed spaces, heated
transport vessels and spill or leak situations. If the air
concentration exceeds 50 ppm, the area should be evacuated
unless respiratory protection is in use.
21
Storage : Keep away from sources of ignition - No smoking. Keep
container tightly closed and in a cool, well-ventilated place.
Cleaning, inspection and maintenance of storage tanks is a
specialist operation, which requires the implementation of strict
procedures and precautions.
STABILITY AND REACTIVITY
Stability : Stable under normal use conditions.
Conditions to Avoid : Heat, flames, and sparks. May form explosive mixture on
contact with air.
Materials to Avoid : Strong oxidizing agents.
Hazardous Decomposition
Products: Hazardous decomposition products are not expected to form
during normal storage.
22
CHAPTER 2
ANALYTICAL CONSIDERATIONS
2.1 Compressor Selection
Searching for the right compressor for our task was one monumental job because of the
compressor availability in the market and it having the desired features.
We were mainly comparing between three basic types of air compressors, which are
• reciprocating
• rotary screw
• rotary centrifugal
These types were further disintegrated by each compressor‟s special feature, like:
• the number of compression stages
• cooling method (air, water, oil)
• drive method (motor, engine, steam, other)
• lubrication (oil, Oil-Free where Oil Free means no lubricating oil contacts the
compressed air)
• packaged or custom-built
Reciprocating Air Compressors
Reciprocating air compressors are positive displacement machines, meaning that they
increase the pressure of the air by reducing its volume. This means they are taking in
successive volumes of air which is confined within a closed space and elevating this air to a
higher pressure. The reciprocating air compressor accomplishes this by a piston within a
cylinder as the compressing and displacing element.
23
Single-stage, two-stage and four-stage reciprocating compressors are commercially available.
Single-stage compressors are generally used for pressures in the range of 70 psig to 100 psig.
• Household, home workshop, and smaller job site compressors are typically
reciprocating compressors 1½ hp or less with an attached receiver tank. These
compressors are commonly available.
• Discharge pressures can range from low pressure to very high pressure (>18000 psi or
180 MPa)
Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated
and non-lubricated configurations and provide a wide range of pressure and capacity
selections.
Rotary Screw Compressors
Rotary air compressors are positive displacement compressors. The most common rotary air
compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These
compressors consist of two rotors within a casing where the rotors compress the air
internally. There are no valves. These units are basically oil cooled (with air cooled or water
cooled oil coolers) where the oil seals the internal clearances.
Since the cooling takes place right inside the compressor, the working parts never experience
extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air
cooled or water cooled compressor package.
• Rotary screw air compressors are easy to maintain and operate.
• Advantages of the rotary screw compressor include smooth, pulse-free air output in a
compact size with high output volume over a long life.
The oil free rotary screw air compressor utilizes specially designed air ends to compress air
without oil in the compression chamber yielding true oil free air. Oil free rotary screw air
compressors are available air cooled and water cooled and provide the same flexibility as oil
flooded rotaries when oil free air is required.
24
Centrifugal Compressors
The centrifugal air compressor is a dynamic compressor which depends on transfer of
energy from a rotating impeller to the air.
• Not commonly available and highly expensive for household usage.
Temperature Variation
Compression of a gas naturally increases its temperature, often referred to as the heat of
compression.
Where,
So,
Within taking different values for different compression processes (see below).
Adiabatic - This model assumes that no energy (heat) is transferred to or from the gas
during the compression, and all supplied work is added to the internal energy of the
gas, resulting in increases of temperature and pressure. Theoretical temperature rise is
given by
With T1 and T2 in degrees Rankine or Kelvin, and k = ratio of specific heats (approximately
1.4 for air). Rc is the compression ratio; being the absolute outlet pressure divided by the
absolute inlet pressure. The rise in air and temperature ratio means compression does not
follow a simple pressure to volume ratio. This is less efficient, but quick. Adiabatic
25
compression or expansion more closely model real life when a compressor has good
insulation, a large gas volume, or a short time scale (i.e., a high power level). In practice
there will always be a certain amount of heat flow out of the compressed gas. Thus, making a
perfect adiabatic compressor would require perfect heat insulation of all parts of the machine.
For example, even a bicycle tire pump's metal tube becomes hot as you compress the air to
fill a tire. The relation between temperature and compression ratio described above means
that the value of n for an adiabatic process is k (the ratio of specific heats).
Isothermal - This model assumes that the compressed gas remains at a constant
temperature throughout the compression or expansion process. In this cycle, internal
energy is removed from the system as heat at the same rate that it is added by the
mechanical work of compression. Isothermal compression or expansion more closely
models real life when the compressor has a large heat exchanging surface, a small gas
volume, or a long time scale (i.e., a small power level). Compressors that utilize inter-
stage cooling between compression stages come closest to achieving perfect
isothermal compression. However, with practical devices perfect isothermal
compression is not attainable. For example, unless you have an infinite number of
compression stages with corresponding intercoolers, you will never achieve perfect
isothermal compression.
For an isothermal process, n is 1, so the value of the work integral for an isothermal process
is:
When evaluated, the isothermal work is found to be lower than the adiabatic work.
Polytropic - This model takes into account both a rise in temperature in the gas as
well as some loss of energy (heat) to the compressor's components. This assumes that
heat may enter or leave the system, and that input shaft work can appear as both
increased pressure (usually useful work) and increased temperature above adiabatic
(usually losses due to cycle efficiency). Compression efficiency is then the ratio of
26
temperature rise at theoretical 100 percent (adiabatic) vs. actual (polytropic).
Polytropic compression will use a value of n between 0 (a constant-pressure process)
and infinity (a constant volume process). For the typical case where an effort is made
to cool the gas compressed by an approximately adiabatic process, the value of n will
be between 1 and k.
Staged compression
In the case of small reciprocating compressors, the compressor flywheel may drive a cooling
fan that directs ambient air across the intercooler of a two or more stage compressor.
Limitations of a Single-Stage Air Compressor:
Fig 2-1: Single stage compression
Refer to the enclosed diagram, the single stage air-compressor is compressing from pressure
P1 to Pressure P2, completing the cycle 1234, where 3-4 is the clearance air expansion. Also
V1-V4 is the effective swept volume or the effective volume where the fresh air from
atmosphere is sucked in. The mass of air flowing through the compressor is controlled by this
effective swept volume V1-V4.
If any restriction is placed on the delivery of the air compressor, for example: the discharge
valve throttled, then the delivery pressure of the air compressor increases. From the diagram,
let us say the new delivery pressure is P5. Then the operating cycle will be 1567, where 6-7
27
is the clearance expansion of air and the effective swept volume is V1-V7. Thus it is evident
that the effective swept volume (V1-V4) is more than (V1-V7). Thus when the delivery
pressure of the single-stage air compressor is increased, the effective swept volume is
reduced.
If the delivery pressure is further increased (assuming the compressor is so strong to work),
the delivery pressure reaches P8, and the compression follows the curve 1-8, where there will
be no delivery of compressed air. Thus when the delivery pressure of a single-stage
compressor is increased, the mass flow rate also increases.
Since the delivery pressure increases, the associated temperature also increases. Thus the
temperature of the air after compression is so high as to cause mechanical problems and the
amount of heat is actually the energy loss.
If a single-stage machine is required to deliver a high-pressure compressed air, then it
requires
1. Heavy moving/working components to compress air to such a high pressure.
2. There might be some balancing problems due to heavy moving parts.
3. The power requirement for such heavy parts movement is too high.
4. There will high torque fluctuations.
5. To compensate for the torque fluctuations, a heavy flywheel is required.
6. Better cooling arrangements are required.
7. Lubricating oil which does not get vaporized at such high temperatures.
Conclusion:
Thus it is clear that a single stage compressor cannot contribute to high delivery pressure
demands. In my next article, let us discuss the effects and advantages of multi-staging of an
air compressor.
28
Multi Stage Compressor
When air at high pressure is required, multi-staged compression is more efficient than using
a single stage compressor. Also single stage compressors delivering high pressures result in
high gas temperatures which effect the lubrication and increase the risk of burning.
It is required to compress air from P1 to P4. The diagram below shows the curve for single
stage compression .a-b-c-k-h. The curve for ideal isothermal compression is also shown a-
b-j-h. The area enclosed by the curves indicates the work done per cycle and it is clear that
the work done in the ideal isothermal process is far less than that done in the single stage
compression.
Fig 2-2: Multistage compression
Assume a three stage compressor process is used.
The air is compressed from P1 to P 2 (a -> c) and the air is transferred into a receiver and
cooled to its original temperature (c -> d) and the air is then transferred from the receiver to
a second cylinder and compressed to P3 (d -> e).
29
The air is then transferred to a second receiver and cooled back to its original temperature
(e -> f) and transferred again to a third cylinder and compressed to P4 (f -> g).
The overall process is represented by curve a-b-c-d-e-f-g-h.
The cooling brings the process closer toward the ideal isothermal (constant temperature)
curve. The saving in work done per cycle is identified by the shaded area.
Electric Motors
There are many kinds of air motors used for powering tools and mechanisms which use
compressed air. These are specially designed units which are very compact and are able to
operate at high speeds with built in torque limitation.
Typical designs of air motors include rotary vane, axial piston, radial piston, gerotor,
turbine, V-type, and diaphragm. Rotary vane, axial- and radial-piston, and gerotor air
motors are most commonly used for industrial applications. Unlike steam air cannot,
conveniently, be used expansively because the resulting cooling effect would result in
freezing of the moisture being carried in the air.
The efficiencies of air motors based on non-expansion cycles is about 20%. With the
efficiencies of compressors being about 60%, then pneumatic drive systems have
efficiencies of less than 12%.
The primary advantages justifying the use of pneumatic drive systems are
Safety - air motors can safely be used in locations with explosive risk resulting from
ignition sources due to electrical devices
Convenience - air motors are generally very compact and include built in overload
protection
Capital Costs - air motors are often very low cost units
Maintenance/Operation - air motors cost little in maintenance and can be easily
operated by semi-skilled operatives
30
2.2 First Report
Objective
To calculate and refer the following variables:
Flow rates, Pressure and Capacity
Electricity and Gas cost
Economic feasibility (Payback)
Volume Flow Rate
Mass Flow Rate
Safety Measures (Pressure switch, Vent, Fail safe etc)
MSDS Natural Gas
Calculated Requirements:
Basic
Pressure: 200 Bar
Volume (avg): 55 Liter
Calculated / Derived
1 bar = 14.5 psi
1 atm = 14.7 psi
1m3=1000 Liters
200 bar ~ 3000 psi
31
Volume Flow rate: 0.009 m3/hr
55 liters in 6 hours of filling time
~ 9 liters/ hr or 0.009 m3/hr
Mass flow rate: 1.33kg/hour
55 liter tank takes in on an average of 8 kg CNG
8 kg in 6 hours of filling time
1.33 kg/hour
Gas Outlet Temperature: 45 deg C
Mass Flow Rate= Volume Flow Rate x Density= (m3/hr x kg/m
3) = kg/hr
Density= Mass Flow Rate/ Volume Flow Rate= 1.33/0.009= 147 kg/m3 @ 200 bar
Tank volume 0.055 m3
Payback calculations shall be preformed
Rough estimation of Electric and Gas units has been carried out.
These shall be validated and practical readings would be substituted.
32
Visit to a local CNG station:
We also did surveys of different commercial CNG filling stations and took readings
of the types of compressors used there, their power ratings, compression ratio, filling
time and even the different types of CNG fuel tanks that are available. Readings in
tabulated form that we took while CNG was being fueled are provided below.
Check/Observation list:
Volume of average gas tank
Filling time needed
Weight of the gas for the tank
Hose temperature of the gas filled
Note Pressure
CNG Plant survey
Gas prices
Table 2.1: Survey Tablature
Volume Of Tank Weight Time to fill Temperature Pressure
Liters Kg Min deg C Bar
1 42 6.00 1.5 45 200
2 45 6.41 2.0 45 200
3 50 7.10 2.2 45 200
4 55 6.25 1.8 45 200
PSO CNG Station, Rashid Minhas
33
4 Stage, Water Cooled Compressor (Cooling Tower)
1st Stage 60 lb/in
2 (Psi)
2nd
Stage 230 lb/in2
3rd
Stage 1000lb/in2
4th
Stage 3200 lb/in2
Mass flow rate: 340 m3/hr
RPM: 600
Supply Pressure (0.3-1.00 bar)
Outlet Pressure (250 bar)
Motor (90 KW)
Gas Temp 50 deg C
Gas Price 55.33 Rs/kg (25th
July 2010)
Compressor Survey
We require a compressor with the following specification or specifications that are close to
this one. We did an enormous survey for this compressor in the Shershah market (Quality,
Tawakal & Fakira Gowdown) and luckily found one that will come close to our requirement
after some modification.
Once we have modified it accordingly, we can put it for use by filling for CNG tank.
Compressor search findings:
Home CNG Compressor 2Nm3/h
Model Number: XF-2/0.017-0.035-200
34
Brand Name: Newtech
For home usage, we can use natural gas to refuel the vehicle, to pressurize the natural gas to
20Mpa. The filling time is 5-6hour. This type features in small size, light weight, excellent
performance, reliable safety, economy and durability.
Discharging volume: 2 Nm3/h
Inlet volume: 0.0017-0.0035 MPa
Discharging pressure: 20 MPa (200bar)
Stage: 4
Stroke: 14mm
Cooling midair cooling
Motors power: 1.1 KW
Voltage: 200-240 V
Nominal current: 6.6 A
*Reference to the supplier‟s quotation is attached at the end of the report.
Preferred option:
Fig 2-3: Used compressor, Coltri Sub make Italy
35
2.3 Second Report
Objective
To research and develop a cost effective solution to refueling compressed natural gas
at home.
Topics of interest: (covered in this report)
i) Compressor refurbishing
a) Servicing
b) Maintenance
ii) Compressor compliance
a) Technical Data
b) Volumetric Data
c) Required Flow rate
d) Required Quality of Gas.
iii) Sources of hazard:
a) Auto Ignition
b) Excess Pressure
d) Safety Interlocks
36
After refurbishing the compressor, it was found to be in good working condition.
Servicing And Maintenance work performed:
High pressure cleaning
Filter cartridges replaced (Activated charcoal and Moisture Filter)
Oil Change
Flange and Valves replaced
Pressure fill pipe installed
Fig 2-4: Front View of our compressor
37
Fig 2-5: Side View
Fig 2-6: Top View
38
USEFUL TECHNICAL DATA:
Compressor
Electric Motor
39
Requirements:
Basic
Pressure : 200 Bar
Volume (avg) : 55 Liter (tank)
Flow rate : 2 Nm3/hr
Mass Flow rate: 1.33 kg/hr
Power : 1.5-2.0 KW
Gas flow Limit: 5 m3/hr
Calculated / Derived
1 bar = 14.5 psi
1 atm = 14.7 psi
200 bar ~ 3000 psi
Volume Flow rate: 0.009 m3/hr
55 liters in 6 hours of filling time
~ 9 liters/ hr or 0.009 m3/hr (@ 200 bar)
or 2 Nm3/hr
Mass flow rate: 1.33kg/hour
55 liter tank takes in on an average of 8 kg CNG
8 kg in 6 hours of filling time
1.33 kg/hour
40
Power Requirements:
Standard Assumptions
Efficiency (Electric Motor) = 0.85
Power Factor (KESC) = 0.90
V = 400volts (Three Phase)
Power (kW) = KVA * Power Factor
Power= Volt * Amp= 400 *6.73= 2.692kW
Efficiency at P.F. =0.90;
Min Current =3.62 *746 / (400 * 1.73 * 0.90 * 0.80)
= 2700.5 / 498.24 = 5.42A (min)
Max Current = (4 *746) / (400*1.73*0.8*0.8)
= 6.73A (max)
Min Horse Power (hp) = (3.0*1000*0.80)/ (746) =3.22 hp (min)
Max Horse Power (hp) = (3.0*1000*0.9)/ (746) =3.62 hp (max)
Range On and Off Load:
5.4 < Current < 6.8 Amp
3.22 <hp< 3.62
41
Volumetric Flow rate of Coltri Compressor:
CFM into m³/h
1 meter = 3.28084 ft
1m³ = 3.28084 x 3.28084 x 3.28084
3.28084³ = 35.31 cubic feet
2.8 cfm = (2.8x 60) / 35.31
= 4.76 m3/hr or 79.3 litre /min
Our requirement is over 2 m3/hr
Volumetric Flow Rate can be reduced by using an electric motor with lower Rpm and
Rated power. This would help us in reducing the energy cost of the system.
All other parameters are acceptable
42
Quality of Gas: (Required for CNG vehicles)
Dehydration of Natural Gas
Natural Gas usually contains significant quantities of water vapor. Changes in
temperature and pressure condense this vapor altering the physical state from gas to
liquid to solid. This water must be removed in order to protect the system from corrosion
and hydrate formation.
All gasses have the capacity to hold water in a vapor state. This water vapor must be
removed from the gas stream in order to prevent the formation of solid ice-like crystals called
hydrates. Hydrates can block pipelines, valves and other process equipment. The dehydration
of natural gas must begin at the source of the gas in order to protect the transmission system.
Coltri compressor already has a condensate discharge system pre-installed
Contains Activated Charcoal & Molecular sieve filter.
43
Table 2.2: Extract from the Manual
44
Schematic for Safety Interlocks
Fig 2-7: Schematic for Safety Controls
Fig 2-8: Our opted compressor
45
Excess Pressure Safety
Purchased Neo-Dyn® Series 232P Pressure Switch/ Internal Adjustment
Fig 2-9: Cross-Sectional View of our Pressure Switch
Temperature Range*
Ambient:
-40°F to +180°F
(-40°C to +82°C)
Media: -40°F to +250°F
(-40°C to +121°C)
Adjustment
Internal, slotted adjustment nut
with range scale
This High Pressure Switch offers added protection for excess pressure safety along with
the Safety Valve already installed on the compressor. It also acts as a signal transmitter
and would be used to switch off compressor.
Fig 2-10: Neo-Dyn Series 232
Pressure Switch
46
Safety Interlock
Electrical Safety Interlocks Diagram
Fig. 2-11: Electrical Safety interlocks diagram
Electric panel would further incorporate several safety features. Control screen is likely to be
our focus in making this home refueling station safer and reliable.
Temperature Cut off
Smoke Hazard Cut Off
Max Pressure Cut Off
Current Threshold
Compressor
Contactor I
Phase
C1
Neutral
47
2.4 Third Report
Objective
To research and develop a cost effective solution to refueling compressed natural gas
at home.
Topics of interest: (covered in this report)
Storage System
-Commercial high pressure tanks
-Volume calculation
-Fill Calculation (Pressure Requirements wrt Tank capacity)
-Feasibility and rationale
Volume of a commercially storage tank
BAUER High Pressure Air Storage Systems meet the code requirements of either the
Department of Transportation or the American Society of Mechanical Engineers. Storage
systems are available with 5000 PSI and 6000 PSI DOT cylinders or 5250/7000 PSI dual
rated ASME cylinders. Optional mounting racks are available.
D.O.T. Cylinders ASME Cylinders
Vertical Configuration 5000 PSI or 6000 PSI Vertical Configuration 5000 PSI
Chart for Bauer High Pressure storage cylinders
48
Table 2.3 Storage Systems
VESSEL DOT 5000 PSI DOT 6000 PSI ASME 5000
PSI
ASME 6000
PSI
MATERIAL Lightweight Steel
Alloy
Lightweight Steel
Alloy
Steel ASME SA
372
Class V Type A
AISI 4130
Steel ASME SA
372
Class V Type A
AISI 4130
CAPACITY 486 scf @ 5000
PSI
509 scf @6000
PSI
436 scf @ 5000
PSI
491 scf @ 6000
PSI
WORKING
PRESSURE 5000 PSI 6000 PSI 5000 PSI 6000 PSI
TEST
PRESSURE 7500 PSI 9000 PSI 7875 PSI 10,500 PSI
DIAMETER 9 3/8" 9 9/32" 9 5/8" 9 5/8"
HEIGHT 55" with Valve 55" with Valve 54" without
Valve
54" without
Valve
WEIGHT 175 lbs. 188 lbs. 400 lbs. 400 lbs.
CYLINDER
VALVE CGA DF
Standard Valve
Supplied
Standard Valve
Supplied
FINISH Primer and
Topcoat
Primer and
Topcoat
Primer and
Topcoat
Primer and
Topcoat
49
Volume Calculation:
Conversion
1 US Gallon = 3.785 liters
1 Liters = 0.0353 cubic feet
Considering Avg Tank volume to be 55 liters
55 Liters = 1.9415 scf
Since tanks are mostly available in scf (Standard Cubic Feet)
We shall take a reasonable capacity of tank at 10 scf
10 scf storage tank = 283.2 Liters or 75 Gallons
Fill calculation:
Using Equation
Assumption:
-Gas is assumed to be following ideal gas law.
-Temperature remains same.
50
Figure 2-12 Storage system
The equation suggests that for a storage tank to have a capacity to refill two cars it needs to
have a certain pressure with respect to the volume considered.
Using the equation previously established:
Initial = Final System
(Pressure)(10) = (3200)(1.9415) + (3200)(1.914)
Initial pressure required = 4442.6 Psi
Storage
Tank
Car (a)
Tank
Car (b)
Tank
Initial System
Final System
51
Storage System:
Pros
Cng could be refueled without the expense of time.
Cons
High pressure storage presents safety hazard (~4500 psi) requiring further
safety measures.
Commercial high pressure cylinders are quite expensive. (5000psi)
Most air compressors currently available have a pressure limit of 200 bar
(3000psi).
Added Cost of storage tanks and cylinders increases the payback period.
Compressing natural gas to higher pressure for storage requires additional
power. Thereby increasing the cost of electricity without adding any incentive.
For storing purpose we need to have a commercial CNG compressor.
Feasibility / Conclusion
Considering the compressor ( Coltri Sub) which has been tested to 200 bar Psi.
Incorporates a safety bypass valve which switches on at 200 bar (3000psi). Therefore it
cannot be used for storing purpose.
Direct refueling is more practical and economical. Incorporating a Gas Engine to drive the
compressor would further reduce the requirement and should be our likely focus to reduce
energy demands.
52
2.5 Literature
Compressibility Factors
One of the most important physical properties of a gas is the ratio of specific heats. It is used
in the design and evaluation of many processes. For compressors, it is used in the design of
components and determination of the overall performance of the machine
The ratio of specific heats is a physical property of pure gases and gas mixtures and is known
by many other names including: adiabatic exponent, isentropic exponent, and k-value. It is
used to define basic gas processes including adiabatic and Polytropic compression.
Compressibility Ratio:
,
While Pressure Ratio is defined as the pressure increase:
In calculating the pressure ratio, we assume that an adiabatic compression is carried out (i.e.
that no heat energy is supplied to the gas being compressed, and that any temperature rise is
solely due to the compression). We also assume that air is a perfect gas. With those two
assumptions we can define the relationship between change of volume and change of
pressure as follows:
Where γ (k) is the ratio of specific heats for air (approximately 1.4). The values in the table
above are derived using this formula. Note that in reality the ratio of specific heats changes
with temperature and those significant deviations from adiabatic behavior will occur.
53
The thermodynamic definition of a gas k-value as shown the relationship to the specific heat
at constant volume, CV and specific heat at constant pressure, CP. Both values vary with
temperature and pressure.
Polytropic exponent
K-value Sensitivity Analysis for Compressed Natural Gas:
A natural gas compressor is operating only the k-value is varied from 1.20 to 1.28,
all other given parameters remain constant. Figure illustrates how the “apparent”
performance of a compressor can change by varying the k-value
T2: Gas discharge temp.
PWR: Power to compressor
n: Polytropic exponent
Fig 2-13: Compressor Performance with k-value
deviation
54
It can be seen from Figure that the discharge temperature deviated over 18.8 percent by only
changing the k-value by 6.7 percent. In this case the k-value varied from a value of 1.20 to
1.28; which is the typical range for natural gas.
Similarly, the power changed by 2.5 percent, polytropic exponent by 9.5 percent, and
adiabatic head by 2.5 percent for the same variation of the k-value. The changes in
compressor performance described in Figure can be much larger depending on the gas
composition and the operating temperature and pressure.
Summary
This information has defined the physical property of process gases called the k-value or
ratio of specific heats. It has shown that small changes in the k-value can have a significant
effect on the calculated values of head, power, gas discharge temperature, and polytropic
exponent. Recommendations were also given to improve the accuracy by utilizing different
k-value methods.
Coltri Sub 4 stage Compressor:
1st stage 57psi
2nd
stage 285psi
3rd
stage 1000psi
4th
stage 3200psi
Calculating the pressure ratio:
55
PR1 = 3.89
PR2 = 5.00
PR3 = 3.51
PR4 = 3.20
Compressibility ratio:
Cylinder Dia 1 = 3.1 in
Cylinder Dia 2 = 1.5 in
Cylinder Dia 3 = 0.76 in
Cylinder Dia 4 = 0.38 in
Volume of cylinder
V1, V2, V3, V4 = (3.925, 0.918, 0.236, 0.059) in2
respectively.
P1V1n = P2V2
n
„n‟ for CNG lies from 1.2 to 1.28
Volume of cylinder:
V1, V2, V3, V4 = (3.925, 0.918, 0.236, 0.059) in3 respectively.
‘n’ for CNG lies from 1.2 to 1.28
P1V1n = P2V2
n
57(0.919) n
= 285(0.2359) n
n = 1.184
The value of „n‟ obtained is ideal for compressing natural gas.
56
2.6 T-s and P-v diagram of our Compressor
Below is the P-v diagram of our multistage compressor with air-intercooled coils. As you can
see the compression in 4-stages along with Intercooling is saving a lot of compression work.
The cooling of the gas makes it easier to compress and grants faster compression rates.
Fig 2-14: P-v diagram of our compressor
57
And this is the T-S model of our compressor. The pressure lines indicate the compression
pressure of different stages of our compressor. The line pressure is also indicated in this
diagram which is approximately the same as atmospheric pressure. If we observe clearly, our
compression process is not ideal because the Intercooling is unable to bring the temperatures
down to initial temperature. But it still significantly helps in lowering the temperatures.
Fig 2-15: T-s diagram of our compressor
58
Clearance Volume effect
A practical single stage compressor cylinder will have a small clearance at the end of the
stroke. This clearance will have a significant effect on the work done per cycle.
In operation the air in the clearance volume expands to 5 before any fresh air is drawn into
the cylinder. The stroke is from 1 to 2 with a swept volume of (V2 - V1 ) but the suction is
only from 5 to 2 giving a volume of (V2 - V5 ) taken into the cylinder on each stroke.
Figure 2.16 Effect of Clearance Volume
The volumetric efficiency obtained from the hypothetical indicator diagram is :
Assuming compression curve 2->3 and the expansion curve 4->5 follow the same law PVn =
c then..
The volumetric ratio of compression (V2 /V 3 ) = the volumetric ratio of expansion (V5 /V 4 )
= r v.
The volumetric efficiency =
59
That is
It is clear that the smaller the clearance volume Vc the larger the volumetric efficiency will
be.
In practice is is possible to get the clearance volume down to 3 to 5% of the stroke.
When clearance is taken into account the work done per cycle =
The hypothetical power of a single stage compressor (kW working on c cycles /s)
The actual compressor diagrams differ from hypothetical diagrams because of valve opening
and closing delays and component inertia. A typical actual indicator diagram is shown
below.
A good approximation of the volumetric efficiency is indicated by the ratio of x to y
measured at the atmospheric pressure line. The actual performance of a reciprocating
compressor used as pump is measured by the ratio.
Fig 2-17: Actual compression diagram
60
CHAPTER 3
DEVELOPMENTAL WORK
3.1 Process Flow Diagram
During the development phase, before the start of manufacturing, work was done to develop
a compression process flow diagram. The idea was to understand how our unit will work
along with the installed safety switches. As is visible from the diagram all the safety switches
are installed in series so that failure of one switch would disrupt the process flow and turn off
the system. The four stages of compression and the charcoal filter is also shown.
The following codes are used:
PSA = Pressure Switch
TISA= Temperature Switch
PI = Pressure Indicator
V-001 = 3-way filling valve
Fig 3-1: Compressor process flow diagram
61
3.2 Development of Trolley Panel
This is the trolley panel which was designed for the unit. All the indication meters and
controls are installed on this panel. H5, H6 and H7 are the indicating lights which will switch
on during a pressure, temperature and fire hazard. HOT-1 is the hooter, which will also sound
simultaneously along with any of the aforementioned hazards. A digital pressure and
temperature (Honeywell) display is also installed on the panel. An emergency push button is
given to switch off the system during an emergency.
Fig 3-2: Panel diagram
62
3.3 Trolley Fabrication
Below is the trolley fabrication design which was employed. A-36 or ASTM-36 steel alloy
was used for trolley plates. The bottom compartment is where the compressor is placed
whereas the top compartment is where most of the circuitry is attached to the display and
control switches. For proper ventilation grills have been carved into the trolley and an
additional draft fan has been installed.
Fig 3-3: Trolley fabrication diagram
63
Fig 3-4: CNG Trolley
64
3.4 Circuit layout
A circuit component layout is shown below. Where R1, R2 and R3 are pressure, smoke and
temperature relays respectively. L1, L2 and L3 are the three live wires from the three phases
main. H7 is the hooter and C1 is the main Circuit Contactor.
Fig 3-5: Circuit layout diagram
Fig 3-6: Circuit Snap
65
3.5 Main Circuit Diagram
Below is the summarized circuit diagram of our system. The +DC1 line is 25volt line which
is used to run some safety controls including the pressure sensor, smoke detector,
temperature indicator and smoke detector.
Fig 3-7: Main circuit diagram
66
Fig 3-8: Panel Snap
Fig 3-9: CNG Station Powered On
67
3.6 Safety Interlocks & Selection Criteria
Fig 3-10: Safety Interlocks
Fig 3-11: Power Supply
Power Supply
220v/24VDC/2.1A
Selection was based on the safety
devices and relays being installed.
68
Fig 3-12: Thermal Overload Relay
Fig 3-13: Honeywell Temperature Controller
Thermal Overload Relay, 4~6 Amps.
From our trial runs we ascertained the
current drawn by the electric motor to be
4-5 amps.
Hence, opted for a thermal overload relay
which should trip the motor if there is an
overload or a short circuit.
Phase-Loss protection
Overload Protection
Rated current adjustment
4-6 amps, 3hp, 440 VAC
Digital Temperature Controller-
Honeywell
Programmable digital controller which
uses thermocouple to sense temperature.
It has the option where we can define the
safe limits on the bases of auto ignition
temperature of natural gas.
69
Fig 3-14: Smoke detector
Fig 3-15: Three Phase Contactor
Smoke Detector
Having an early warning system is
crucial for the safety.
This detector not only indicates but has
an inbuilt alarm and shuts of the three
phase contactor, turning the unit off.
Three Phase Contactor
6 amps: 3 hp motor: 440 VAC
When the power is “on”,
Relay switches the supply to the
contactor coils energizing the magnetic
coils and making the plunger move.
This switches the contact from normally
70
Table 3.1: Bill of Material (BOM)
This is a bill of materials and equipment‟s which were employed in our project.
S/No. Equipment/ Component Model Make
1 Compressor Coltri Portable MCH6/EM Italy
2
High Pressure Adjustable Switch upto
5000 psi. 232P NEMA 4 USA
3 Pressure Transmitter, 0~200 bar, 4~20 ma KH15 Nagano, Japan
4 Three way filling Valve, 316, 6000 Psi. AEY 1 Swagelok, USA
5 Filling Nozzle with O rings. Swagelok, USA
6 Filling Hose Length 300 mm, 6000 Psi. 145923 Weather USA.
7
Main Filling Hose Length 2500 mm, 6000
Psi. 518C-8, Parker Parflex, USA
8 On/Off Push button ABN 111 Idec Izumi, Japan
9 Indication lights 220 VAC. AD16-22DS/31 Wintop, China
10 Indication lights 24 VDC. AD16-24DS/31 Japan
71
S/No. Equipment/ Component Model Make
11 Buzzer 24 VDC. Japan Japan
12 Three Phase Circuit Breaker 10 A. Xs50NS Tarasaki, Japan
13 Three phase Contactor, 10 hp. CL02A310T GE, Poland
14 Thermal Overload Relay, 4 ~ 6 Amps. GKT-22 GE, Korea
15 Power Supply 220v/24VDC/2.1A SW-50-24 SwitchWell, Japan
16 Smoke Detector Remote type Korea
17 Bottle fuse connector type. 2.5mm Japan
18 Control Relay with base 11 pin 24 VDC Finder 60.13. Japan
19 Ventilation Fan 220 V AC Korea
20 Digital Temperature Controller 220 VAC Honeywell Japan
21 Analouge Pressure Indicator 46 x 96 Japan
22 Emergency Stop Push Button
Klockner&
Moller Japan
72
Chapter 4: Results
4.1 Trail Run data for CNG and Air
Time Hrs.
ELECTRIC MOTOR
NOISE
PRESSURE TEMPERATURE o C. SUI GAS METER READING
Am
ps
Tem
p.
o C
dB
Psi
Bar
STA
GE
1
STA
GE
2
STA
GE
3
STA
GE
4
Comp. Outlet
At Car Nozzle
1015 4.3 25 88 0 0 38 30 28 24 26 22 13015
1045 4.4 34 89 435 30 40 42 35 34 29 20 13016
1115 4.6 35 88 870 60 40 46 37 38 30 24 13017
1145 4.9 35 88 1160 80 41 47 39 43 31 24 13019
1215 5.0 36 89 1668 115 41 48 40 44 33 24 13021
1245 5.0 37 88 2030 140 41 49 40 46 33 25 13023
1315 5.0 38 89 2248 155 42 49 40 46 33 25 13024
1345 5.1 38 89 2610 180 43 50 42 48 34 26 13025
1400 5.2 39 89 2900 200 44 50 43 48 35 26 13026
Table 4.1: Trial Run Readings CNG Filling
As performed, using Natural gas to fill a 55 liter CNG cylinder.
73
Table 4.2: Trial Run Readings Air Filling
Time Hrs.
NOISE
PRESSURE TEMPERATURE o C. Remarks
Am
ps
dB
Psi
Bar
STA
GE
1
STA
GE
2
STA
GE
3
STA
GE
4
Comp. Outlet
At Car Nozzle
1100 5.2 88 0 0 36 44 44 34 26 33
1115 4.9 89 290 20 40 50 50 35 29 33
1130 5.1 88 580 40 40 53 50 36 30 32
1145 5.1 87 870 60 41 47 49 37 31 33
1200 5.6 88 1160 80 44 51 50 39 33 38
1215 5.2 87 1450 100 43 53 55 42 33 39
1230 5.5 88 1740 120 40 51 66 57 33 41
1245 5.5 89 2030 140 46 54 66 48 34 40
1300 5.5 90 2320 160 48 57 67 51 35 41
1320 5.5 81 2610 180 47 56 68 54 42 2610
1335 5.5 88 2900 200 48 57 67 51 43 2900
As performed, using Air to fill a 55 liter CNG cylinder.
EL
EC
TR
IC M
OT
OR
TE
MP
ER
AT
UR
E R
EM
AIN
S
BE
TW
EE
N 4
0 T
O 4
3 d
eg.
°C.
74
4.2 Cost Analysis
• Total Gas Cost:
o Gas Consumption= 11 unit
o Gas domestic rate= Rs. 5.50/ unit
o Gas Cost= 11 * 5.50 = Rs. 60.50
• Total Electricity Cost:
o Motor Rating = 5Amps
o Power Consumption = 3.1kW
o Total Running Hrs. = 3hrs & 45 min
o Total Kilowatts Hrs = 3.75 * 3.1 = 11.625 kWh
o Electricity domestic rate = 12.5 Rs/kWh
Electricity Cost = 12.5 * 11.625 = Rs. 145.3
Breakeven Analysis
• Total Cost = 60.50 + 145.3 = Rs. 205.8
• Approximately 2days/refill for an average car
• Thus, an average filling cost of a car from a CNG Home station per month
= 205.8 * 15 = Rs. 3087
• Thus, an average filling cost of a car from a CNG Fuel station per month
= 360 * 15 = Rs. 5400
• Amount saved per month =5400 – 3087
= Rs. 2313
• Initial Investment = Rs. 60,000
• Breakeven Time = 60,000/2313 = 25.94 months ~ 25 months & 28 days or 2yrs & 2
months.
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4.2 Alterations
• Natural Gas inlet/outlet fixture
o Addition of moisture filter/assembly.
o Pressurized pipe, certified for zero static discharge.
o Feed valves & bypass valve.
o Electric panel would further incorporate several safety features. User interface
and Control screen is likely to be our focus in making this home refueling
station safer and reliable.
Refurbishing before Alterations
• After refurbishing the compressor, it was found to be in good working condition.
o Servicing And Maintenance work performed:
o Oil Change.
o Paint restored.
o Motor belt replaced.
o Filter cartridges replaced (Moisture Filter).
o Pressure fill pipe connected
Fig 4-1: CNG Station
76
4.3 Power Requirements
Calculations Involving Our Compressor
Standard Assumptions
Efficiency (Electric Motor) = 0.85
Power Factor (KESC) = 0.90
V = 400volts (Three Phase)
Power (kW) = Line amps x Line volts x Power Factor x 1.73
Volumetric Flow rate of Coltri Compressor:
CFM into m³/h
1 meter = 3.28084 ft
1m³ = 3.28084 x 3.28084 x 3.28084
3.28084³ = 35.31 cubic feet
2.8 cfm = (2.8x 60) / 35.31 = 4.76 m3/hr
Our requirement is over 2 m3/hr
Initial Deduction:
Volumetric Flow Rate can be reduced by using an electric motor with lower Rpm and
Rated power. This would help us in reducing the energy cost of the system.
Mass flow rate: 3.20kg/hour
◦ 55 liter tank takes in on an average of 8 kg CNG
◦ 8 kg in 2.5 hours of filling time
◦ 3.2 kg/hour
77
4.4 Trial Run Comparative Graphs for CNG and Air Fillings
Fig 4-2: Pressure against time diagram
Fig 4-3: Temperature against pressure diagram
0
25
50
75
100
125
150
175
200
0 15 30 45 60 75 90 105 120 135 150 180 210 225
Pre
ssu
re
(Bar)
CNG Pressure
Pressure Air
Time (min)
20
25
30
35
40
45
0 15 30 45 60 75 90 105 120 135 150 180 210 225
Tem
pe
ratu
re (
°C
)
Pressure (Bar)
CNG outlettemp.
Air outlettemp.
78
Fig 4-4: Current against time graph
3.5
4
4.5
5
5.5
6
0 15 30 45 60 75 90 105 120 135 150 180 210 225
Am
pe
res
Time (min)
AmperesCNG trial
AmperesAir trial
79
CHAPTER 5
Conclusion and Recommendations
5.1 Conclusion
As is seen from the trial runs, the cost analysis and the data gathered from results, this
prototype home filling CNG station is fully functional for home usage. Every care has been
taken to make it safe with the installation of safety interlocks. All the gauges have been
masterfully calibrated, for references these calibration certificates are attached to this
report[7]. The percentage error in the in the gauges‟ calibration is also present, but none of
these errors exceed beyond 0.5%.
Initial trial runs were first performed on Air. The first trial run was performed without any
additional safety controls. Testing of all the built in safety feature was carried out which
allowed the compressor to switch off when the pressure reached 3000psi, which is also the
working pressure of this compressor. Even though the highest achievable pressure from our
compressor is 5000psi, the built in safety feature allowed it to switch off at 3000psi. This was
a necessity because the compressor was bought from a scrap market and it was impossible to
use it for the project before refurbishing it. Later the Air trial runs were performed with
safety features intact and once the functionality of our control parameters were confirmed,
the experimentation was moved to CNG.
The CNG trial runs were different, in that, the first major issue was the filling time which is
3hr 45min compared to the 2hr 40min time taken by air. Even though the filling time is high
but such small compression units take long to achieve high pressures. And automatic
switching after filling of the tank ensures that there is no need to worry about switching the
machine off at completion. Furthermore, running this unit for this long still cost less as
compared to filling the vehicle from a commercial station.
80
Secondly there were a few discrepancies which were noticed in the temperature/pressure
graphs of both CNG and air. But as you can see that the compression of air is quicker due to
greater density than CNG, therefore there is more temperature rise and greater irreversibility
in compression. Whereas, the compression of CNG is much slower and it follows a more
quasi-equilibrium form of compression, therefore there are less irreversibilities and the
temperature rise is less.
5.2 Recommendations:
Since compression is very slow, some users might find it revolting due to their impatience.
However, this compression time can be brought down by introducing a water cooled heat-
exchanger between the Intercooling lines. The theory behind it is that it is easier to compress
a cooler gas and therefore there is lesser power consumption, the process time is thus
automatically decreased. The only catch faced was the cost effectiveness of a heat exchanger
for our unit; this would hugely influence our unit cost and might be a turn off for potential
customer. But if some firm decides to commercialize our unit and plans to do its mass
production, the cost/unit will decrease and then it will be ideal to install a heat exchange as
well to make the process more efficient and cost effective.
81
REFERENCES
1. Compressor literature study
http://www.coltrisubmaldives.com/catalogue/portable/mch6-em/
2. Compressed Natural Gas Study
http://www.brighthub.com/engineering/mechanical/articles/63720.aspx
3. Heat Exchanger „tube in tube‟ compliance
http://www.roymech.co.uk/Related/Thermos/Thermos_Air_com_mot.html
4. Book Fundamentals Of Engineering Thermodynamics by Michael J. Moran
Compression Cycles and Formulation.
5. Principles Of Material Science And Engineering by William F. Smith
6. Reference to the supplier‟s quotation attached.
7. Calibration certificates attached.
82
Reference to the supplier’s quotation:
We received an email from the supplier.
回复: [[email protected]]I'm interested in your home cng compressor, xf-
2/0017-0.035-200
Dear Mr Abdul Wahab Vohra,
Thank you for your inquiry! First, I'd like to introduce that we are the professional company
dealing with CNG refueling unit and the relevant units. Our products have been exported to
many countries and enjoyed a high popularity all over the world for our first-class quality
and best service.
The sample price of our 2Nm3/h home CNG refueling unit is FOBChina USD2500/set. If
you want to buy two units, we'd like to give you the favourable price, FOBChina
USD2100/set. So the total price is USD4200. And the method of payment is T/T before
shipment.
My MSN is [email protected] and SKYPE is newtech.f. For any further questions,
please feel free to contact me and I will do my best to cooperate with you.
Your quick reply will always be highly appreciated!
Warm regards.
Sally
Adam Huang/President
Shenyang Newtech International Co.,Ltd.
83
ADD: Rm.C-8-2 No.168 ShifuRoad,HepingDistrict,Shenyang,China.
TEL: 0086 24 22532086
FAX: 0086 24 22532084
MOBILE: 0086 13804042544
Email: [email protected]
Website:www.synewtech.com www.chinagascompressor.cn
Shenyang Newtech International Co., Ltd. locates in Shenyang, China. Our company
professionally deals with CNG compressor, CNG cylinders for automobiles, automobile care
equipment and spare parts.
Our home CNG filling unit and small CNG filling station are our star products which are
popular among the worldwide.
By now, our products have been exported all around the world, including America, Canada,
Czech Republic, United Kindom, Australia, Iran, Bangladesh, Singapore, Malaysia, etc.