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
SUPERVISED BY PHD PROFESSOR’S
Faculty of Mechanical Engineering GIK Institute of Engineering Sciences & Technology
MAY 2011
Abstract In today‟s world, energy is becoming beco ming 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 i nterlocks to augment its safety.
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Table of Contents Abstract Abstract …………………………………………………………………….
I
Dedication …………………………………………………………………
II
Acknowledgments ……………………………… ………………………………………………………… …………………………
III
LIST OF TABLES ………………………………………………………. ………………………………………………………...
IV
FIGURES…………………………………………… ………………………………………….. ………….... LIST OF FIGURES……………
VII
NOMENCLATURE………………………………………………………... ..
IX
CHAPTER 1 INTRODUCTION 1.1 1.2 1.3 1.4 1.5
……………………………………….. 1 Conceptualization of Product ……………………………………… Compressor and its Types ……………………………………..… ……………………………………..…... ... 3 Intercooling………………………………………………………... Intercooling………………………………………………………... 10 Thermodynamics of Air compression……………….………… compression……………….…………… ….. 11 CNG in General………………………………………………… General…………………………………………………..… 14
CHAPTER 2 ANALYTICAL CONSIDERATION CONSIDERAT ION 2.1 2.2 2.3
Compressor Compressor Selection…………………… Selection……………………………………………… ………………………… 22 Variables)……………………………… …………… 30 First Report (Compressor (Compr essor Variables)………………… Second Report (Refurbishing)…………………………………….. (Refurbishing)…………………………………….. 35
2.5 2.6
Factors……………………………... Literature on Compressibility Compressibili ty Factors…………………………… ... 52 compressor…………………… ….…… 56 T-S and P-V P-V diagrams of our compressor…………………
CHAPTER 3 DEVELOPMENTAL WORK 3.1 3.2 3.3 3.4 3.5 3.6
Process Flow Diagram…………………………………………… Diagram …………………………………………….. 61 Development of Trolley Panel………..…………………………… 62 Trolley Fabrication………………..………… Fabrication………………..………………………… ………………..……. ……. 63 Circuit Layout……………..………… Layout……………..……………………………………… …………………………….. 64 Main Circuit Diagram……………………….…… Diagram……………………….…………………… ……………….. .. 65 Safety Interlocks……………… Interlocks…………………………………………… …………………………………… ……… 67
CHAPTER 4 RESULTS 4.1 4.2 4.3 4.4 4.5
Air………………………………….. ……….. 72 Trial Run Data for CNG and Air………………………… Cost Analysis………………………………………………… .…… 74 Alterations…………………………………………………… Alterations…………………………………………………… .….... ….... 75 Power Requirements………………………………….…... Requirements………………………………….…............... ............ 76 Trial Run Comparative Graphs……………..……………………… Graphs ……………..………………………77 77
CHAPTER 5 CONCLUSION AND RECOMMENDATIONS RECOMMENDATIO NS 5.1 5.2
Cost Analysis……………… Analysis………………………………………… ………………………………… ………..…… 79 Alterations….………………..…………………………………….. 80
REFERENCES…………………………………………………………………….. REFERENCES …………………………………………………………………….. 81 Reference Quotation……………………………………………………… 82 Calibration Certificates…………………………………………………… 84-88
List of Tables Table 1.1
Compressed Natural G as Properties………………… Properties……………………………. ………….
17
Table 2.1
Survey T ablature………………………………………………...
32
Table 2.2
Extract Form The Manual……………………………… Manual………………………………………. ……….
43
Table 2.3
Storage Stora ge Systems………………… Systems…………………………………………… ……………………………… ……
48
Table 3.1
Bill of Materials………………………………………………… of Materials…………………………………………………
70
Table 4.1
Trail Run Readings for CNG Filling……………………………. Filling …………………………….
72
Table 4.2
Trail Run Readings for Air Filling…………………………….... Filling……………………………....
73
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List of Figures Page Fig 1-1
Compressor and its types………………………………………… types………………………………………… 3
Fig 1-2
Centrifugal Compressor…………………… Compressor…………………………………………. …………………….
4
Fig 1-3
Axial Flow Compressors………………………………………… Compressors …………………………………………
5
Fig 1-4
Reciprocating Compressor………………… Compressor………………………………………. …………………….
6
Fig 1-5
Compressor………………………………………. ……………………. Rotary Screw Sc rew Compressor…………………
7
Fig 1-6
Compressor……………………………… ……… 8 Mechanism of a Scroll Compressor………………………
Fig 1-7
Compressor………………………………………….. ………………….. 9 Diaphragm Diaphrag m Compressor………………………
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 Auto- ignition Temperature…………………… Temperature……………………… ….. 15
Fig 1-12
Comparison of Peak Flame Peak Flame Temperature……………………… Temperature…………………………. …. 16
Fig 2-1
Single Stage Stag e Compression……………………… Compression………………………………………… ………………… 26
Fig 2-2
Multistage Compression………………………………………….. 28
Fig 2-3
Coltri Sub Compressor……………… Compressor………………………………………… ……………………………. …. 34
Fig 2-4
Front View……………………… View…………………………………………………… …………………………………. ……. 36
Fig 2-5
Side View………………..……………………………………….. 37
Fig 2-6
Top View………………………………………………………….. 37
Fig 2-7
Schematic of Safety Controls……………………………………… Controls……………………………………… 44
Fig 2-8
Compressor……………………………………………… ……………………………… … 44 Opted Opted Compressor…………………
Fig 2-9
Cross-Sectional Cross- Sectional View of our Pressure Switch…………………….. Switch…………………….. 45
Fig 2-10
Neo-Dyn Neo-Dyn 232 Pressure Switch…………………………………….. Switch…………………………………….. 45
6
Fig 2-11
Electrical Safety Interlocks Diagram……………………… Diagram……………………………… ……… 46
Fig 2-12
Storage System……………………………………………………. 50
Fig 2-13
Diagram……………………………….. ……….. 53 Compressor Performance Perfo rmance Diagram………………………
Fig 2-14
P-V Diagram of our Compressor………………………………… Compressor ………………………………….. 56
Fig 2-15
T-S diagram of our Compress or…………………………………. . 57
Fig 2-16
Practical P-V P- V diagram…………………………… diagram……………………………………………. ………………. 58
Fig 2-17
Effect of Clearance Cl earance Volume………………………… Volume……………………………………… …………… 59
Fig 3-1
Diagram……………………………… ……… 60 Compressor Process Pro cess Flow Diagram………………………
Fig 3-2
Panel Diagram…………………………………………………… Diagr am…………………………………………………… 61
Fig 3-5
Diagram…………………………………….. ……….. Trolley Fabrication Fabric ation Diagram……………………………
Fig 3-6
Circuit Layout Diagram………………………………………….. 65
Fig 3-7
Diagram……………………………………………. …………………. 65 Main Circuit Circuit Diagram…………………………
Fig 3-10
Safety Interlocks…………………………………………………. Interlocks ………………………………………………….
67
Fig 3-11
Power Supply…………………… Supply………………………………………………… ………………………………. ….
67
Fig 3-12
Thermal O verload Relay……………………… Relay………………………………………… …………………
68
Fig 3-13
Honeywell Temperature Tempe rature Controller………………… Controller……………………………… …………… 68
Fig 3-14
Smoke detector…………………… detector………………………………………………… ……………………………… … 69
Fig 3-15
Three Phase Pha se Contactor……………………………… Contactor…………………………………………… …………… 69
Fig 4-1
CNG Station……………………… Station…………………………………………………… ……………………………… … 75
Fig 4-2
Pressure against Time Graph…………………………………… Graph …………………………………….. .. 77
Fig 4-3
……………………………... Temperature against Pressure Graph …………………………….
Fig 4-4
Current against Time Graph……………………………………… Graph ……………………………………… 78
64
77
vii
Nomenclature P
Pressure
V
Volume
T
Temperature
R
Universal Gas constant
k
Isentropic Expansion factor
⁰
Fahrenheit
⁰
Celsius
s
Entropy
ASME
American Society of Mechanical Engineers
W
Work
hp
Horsepower
ppm
Parts per million
MPa
Mega Pascal
Bar
10 Pa
Kg
Kilogram
atm
Atmospheric Pressure
hr
hour
gal
Galleon
dB
decibel
psi
Pounds per square inch
PR
Pressure Ratio
PSA
Pressure Switch
F
C
5
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.
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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
4
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.
6
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.
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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 moderat ely high hi gh 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
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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.
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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.
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1.3 Intercooling If you understand that “inter” means between, and that cooler – well – 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 lines,, is also an intercooler of sorts.
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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
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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.
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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 c
p
= γ R /(γ-1) /(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
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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.
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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
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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.
Peak Flame Temperatures
Fig 1-12: Comparison of peak flame temperature
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Table 1.1: Compressed Natural Gas Properties
PROPERTIES
GASOLINE
DIESEL No. 2
LPG (HD5)
CNG
Physical State
LIQUID
LIQUID
GAS
GAS
Boiling Range
80 to 420
320 to 720
-44 to 31
-259
Density (lb/ft )
43 to 49
49 to 55
31
(lb/gal)
5.8 to 6.5
6.5 to 7.3
4.1
1.07
Net Energy Content Btu/lb.
18,700 19,100
18,900
19,800
21,300
Auto ignition o Temperature ( F)
450 - 900
400 - 500
920 - 1,020
1,350
Flashpoint o ( F)
-45
125 (min)
-100 to 150
-300
Octane Number Range (R+M) 2
87 to 93
n/a
104
e
120
Flammability Limits
L = 1.4
L = 0.7
L = 2.4
L = 5.3
(volume % in air)
H = 7.6
H = 5.0
H = 9.6
H = 14
Human Exposure Limit For Fuel (ppm)
500
n/a
n/a
nontoxic
a
o
( F @ 1 atm) 3
b
8
b
c
c
a
e
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Material Safety Data Sheet [Natural Gas] PHYSICAL AND CHEMICAL PROPERTIES
Appearance :
Colourless. 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.
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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
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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.
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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.
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CHAPTER 2 ANALYTICAL ANAL YTICAL 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: 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 compr essors 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 T 1 and T 2 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 interstage 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. 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 P 1 to P 4. The diagram below shows the curve for single stage compression .a-b-c-k-h. The curve for ideal isothermal compression is also shown ab-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 P 1 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 P 3 (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 P 4 (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 3
1m =1000 Liters 200 bar ~ 3000 psi
31 3
Volume Flow rate: 0.009 m /hr 55 liters in 6 hours of filling time 3
~ 9 liters/ hr or 0.009 m /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
3
3
Mass Flow Rate= Volume Flow Rate x Density= (m /hr x kg/m ) = kg/hr
3
Density= Mass Flow Rate/ Volume Flow Rate= 1.33/0.009= 147 kg/m @ 200 bar Tank volume 0.055 m
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.
3
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)
st
2
1 Stage 60 lb/in (Psi) nd
2
rd
2
2 Stage 230 lb/in 3 Stage 1000lb/in th
4 Stage 3200 lb/in
2
3
Mass flow rate:
340 m /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 (25 July 2010)
th
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: 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
USEFUL TECHNICAL DATA:
Compressor
Electric Motor
37
Requirements:
Basic Pressure
: 200 Bar
Volume (avg) : 55 Liter (tank) Flow rate
3
: 2 Nm /hr
Mass Flow rate: 1.33 kg/hr Power
: 1.5-2.0 KW 3
Gas flow Limit: 5 m /hr
Calculated / Derived 1 bar = 14.5 psi 1 atm = 14.7 psi 200 bar ~ 3000 psi
3
Volume Flow rate: 0.009 m /hr 55 liters in 6 hours of filling time ~ 9 liters/ hr or 0.009 m3/hr (@ 200 bar) 3
or 2 Nm /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
38
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
39
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 3
= 4.76 m /hr or 79.3 litre /min
3
Our requirement is over 2 m /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
40
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 system pre-installed
Contains Activated Charcoal & Molecular sieve filter.
41
Table 2.2: Extract from the Manual
42
Schematic for Safety Interlocks
Fig 2-7: Schematic for Safety Controls
Fig 2-8: Our opted compressor
43
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 Fig 2-10: Neo-Dyn Series 232 Pressure Switch
(-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.
44
Safety Interlock
Electrical Safety Interlocks Diagram
Phase
Contactor I
Temperature Temperature Cut off
Smoke Hazard Cut Off
Max Pressure Cut Off
Current Threshold
Compressor C1
Neutral
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.
45
2.5 Literature 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.
46
The thermodynamic definition of a gas k-value as shown the relationship to the specific heat at constant volume, C V V and specific heat at constant pressure, C P. 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
47
Fig 2-13: Compressor Performance with k-value
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: st
57psi
nd
285psi
rd
1000psi
th
3200psi
1 stage 2 stage 3 stage 4 stage
Calculating the pressure ratio:
48
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
2
V1, V2, V3, V4 = (3.925, 0.918, 0.236, 0.059) in respectively. n
n
P1V1 = P2V2 „n‟ for CNG lies from 1.2 to 1.28
Volume of cylinder: 3
V1, V2, V3, V4 = (3.925, 0.918, 0.236, 0.059) in respectively. ‘n’ for CNG lies from 1.2 to 1.28 n
n
P1V1 = P2V2
49 n
57(0.919) = 285(0.2359)
n
n = 1.184 gas. The value of „n‟ obtained is ideal for compressing natural gas.
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
50
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
51
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 (V 2 - V 1 ) but the suction is only from 5 to 2 giving a volume of (V 2 - 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 :
n
Assuming compression curve 2->3 and the expansion curve 4->5 follow the same law PV = c then..
The volumetric ratio of compression (V 2 /V 3 ) = the volumetric ratio of expansion (V 5 /V 4 ) = r v.
52
The volumetric efficiency =
That is
It is clear that the smaller the clearance volume V c 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.
Fi 2-17 2-17:: Act Actua uall com com ress ressio ion n dia dia ram ram
53
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.
CHAPTER 3 DEVELOPMENT DEVELOPMEN TAL 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
54
Fig 3-1: Compressor process flow diagram
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.
55
Fig 3-2: Panel diagram
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.
56
Fig 3-3: Trolley fabrication diagram
57
Fig 3-4: CNG Trolley
58
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
59
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
60
Fig 3-8: Panel Snap
Fig 3-9: CNG Station Powered On
61
3.6 Safety Interlocks & Selection Criteria
Fig 3-10: Safety Interlocks
Power Supply
220v/24VDC/2.1A Selection was based on the safety devices and relays being installed.
Fig 3-11: Power Supply
62
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
Fig 3-12: Thermal Overload Relay
4-6 amps, 3hp, 440 VAC
Digital Temperature ControllerHoneywell
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.
Fig 3-13: Honeywell Temperature Controller
63
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. Fig 3-14: Smoke detector
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
Fig 3-15: Three Phase Contactor
64
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, Transmitter, 0~200 bar, 4~20 ma KH15
Nagano, Japan
4
Three way filling Valve, Valve, 316, 6000 Psi.
Swagelok, USA
5
Filling Nozzle with O rings.
6
Filling Hose Length 300 mm, 6000 Psi.
7
Main Filling Hose Length 2500 mm, 6000 Psi. 518C518C-8, Parker
8
On/Off Push button
ABN 111
9
Indication lights 220 VAC.
AD16AD16-22DS/31
Wintop, China
10
Indication lights 24 VDC.
AD16AD16-24DS/31
Japan
AEY 1
Swagelok, USA
145923
Weather USA.
Parflex, USA
Idec Izumi, Japan
65
S/No.
Equipment/ Component
Model
Make
11
Buzzer 24 VDC.
Japan
12
Three Phase Circuit Breaker 10 A.
Xs50NS
13
Three phase Contactor, 10 hp.
CL02A310T
GE, Poland
14
Thermal Overload Relay, 4 ~ 6 Amps.
GKTGKT-22
GE, Korea
15
Power Supply 220v/24VDC/2.1A
SW--50SW 50-24
16
Smoke Detector
Remote type
Korea
17
Bottle fuse connector type.
2.5mm
Japan
18
Control Relay with base
11 pin 24 VDC
19
Ventilation Fan
220 V AC
Korea
20
Digital Temperature Controller
220 VAC
Honeywell Japan
21
Analouge Pressure Indicator
46 x 96
Japan
Emergency Stop Push Button
Klockner& Moller
Japan
22
Japan
Tarasaki, Japan
SwitchWell, Japan
Finder 60.13. Japan
66
Chapter 4: Results
4.1 Trail Run data for CNG and Air
Time Hrs.
ELECTRIC MOTOR C
NO ISE
PRESSURE
dB
Psi
Bar
o
s p m A
. p m e T
o
TEMPERATURE C. 1 E G A T S
2 E G A T S
3 E G A T S
4 E G A T S
Comp. Outlet
At Car Nozzle
SUI GAS METER READING
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.
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Table 4.2: Trial Run Readings Air Filling Time Hrs.
NOI SE
o
PRESSURE dB
Psi
TEMPERATURE C. Bar
s p m A
1 E G A T S
2 E G A T S
3 E G A T S
Remarks
4 E G A T S
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.
S N I A M E R . E C ° . R g U e T d A 3 R 4 E P O T M E 0 T 4 R N O E T E O W T M E C I B R T C E L E
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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
o
Total Kilowatts Hrs = 3.75 * 3.1 = 11.625 kWh 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 – =5400 – 3087 3087
= Rs. 2313 •
•
Breakeven Time = 60,000/2313 = 25.94 months ~ 25 months & 28 days or 2yrs & 2 months.
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4.2 Alterations 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 o
Servicing And Maintenance work performed:
o
Oil Change.
o
Filter cartridges replaced (Moisture Filter).
o
Pressure fill pipe connected
Fig 4-1: CNG Station
70
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 3
2.8 cfm = (2.8x 60) / 35.31 = 4.76 m /hr 3
Our requirement is over 2 m /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
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4.4 Trial Run Comparative Graphs for CNG and Air Fillings
200 175 150
) r a 125 B ( e r 100 u s s e 75 r P
CNG Pressure Pressure Air
50 25
0 0
15
30
45
60
75
90
105
120
135
150
180
210
225
Time (min)
Fig 4-2: Pressure against time diagram
45
40 ) C ° ( 35 e r u t a r e p 30 m e T
CNG outlet temp. Air outlet temp.
25
20 0
15
30
45
60
75
90
105
Pressure (Bar)
Fig 4-3: Temperature against pressure diagram
120
135
150
180
210
225
72
6
5.5
s 5 e r e p m A4.5
Amperes CNG trial Amperes Air trial
4
3.5 0
15
30
45
60
75
90
105
Time (min)
Fig 4-4: Current against time graph
120
135
150
180
210
225
73
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 g auges‟ 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.
74
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: 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 heatexchanger 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.
75
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.
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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.
77
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 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.
