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 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.

ii

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

2.5 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

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

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

v

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

6

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

vii

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.

2

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

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.

5

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.

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: 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 : 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

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 <hp< 3.62

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

= 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

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 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

(-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

44

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

45

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.

46

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

47

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:

Fig 2-13: Compressor Performance with k-value

deviation

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

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

49

57(0.919) n

= 285(0.2359) n

n = 1.184

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 (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.

52

The volumetric efficiency =

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.

Fig 2-17: Actual compression diagram

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

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

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

Fig 3-11: Power Supply

Power Supply

220v/24VDC/2.1A

Selection was based on the safety

devices and relays being installed.

62

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.

63

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

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, 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

65

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

66

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.

67

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.

68

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

• Breakeven Time = 60,000/2313 = 25.94 months ~ 25 months & 28 days or 2yrs & 2

months.

•

69

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

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

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

71

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.

72

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

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 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.

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:

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.

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.

76

Reference to the supplier’s quotation:

We received an email from the supplier.

回复： [vohra.wahab@gmail.com]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 newtech.f@hotmail.com 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: adamhuang08@hotmail.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.