project code: pmo 01/2015 project title:...

UNIVERSITY OF NAIROBI

PROJECT CODE: PMO 01/2015

PROJECT TITLE: DETERMINATION OF THE AMOUNT OF ENERGY

RECOVERABLE FROM A FLUE GAS STREAM IN A COMMERCIAL

INCINERATOR

PROJECT SUPERVISOR: DR. PHILIP MWABE

PROJECT UNDERTAKEN BY:-

KAIMBA BRENCIL GISLOP F18/1427/2010

MAINA TITUS GITHIGIA F18/1429/2010

A FINAL YEAR PROJECT FOR THE PARTIAL FULLFILMENT FOR THE AWARD

OF BACHELORS DEGREE IN MECHANICAL AND MANUFACTURING

ENGINEERING OF THE UNIVERSITY OF NAIROBI

APRIL 2015

DEPARTMENT OF MECHANICAL AND

MANUFACTURING ENGINEERING

i

DECLARATION

We declare that this is our original work and has not been presented for a degree in any other

university.

SIGN DATE

KAIMBA BRENCIL GISLOP ......................................... ………………

REG NO.: F18/1427/2010

MAINA TITUS GITHIGIA ......................................... ………………

REG NO.: F18/1429/2010

This report has been submitted with my approval as University Supervisor:

Dr. Philip Mwabe Date

……………………………………….. …………………………………..

ii

ABSTRACT

The main objective of this project was to determine the amount of energy recoverable from flue

gases produced from a commercial incinerator. The incinerator burns various industrial and

chemical wastes at high temperatures and the flue gases released from the chimney form a viable

source of waste heat. A heat recovery unit has been installed in the chimney line which in design

is a helical tube heat exchanger. Water is passed at an elevated pressure in the helical tube heat

exchanger where it is heated by the hot flue gases.

Inlet and outlet temperatures of the flue gases for both the water and gas stream were measured

and recorded.From this data, the total thermal resistance, heat exchanger correction factor (F),

and effectiveness at the different temperatures of the heat exchanger was calculated. The average

correction factor of the heat exchanger was then calculated.

From the results,it was seen that the higher the temperature difference between the water at entry

and flue gas stream at entry, the higher the amount of heat recovered. While performing the

experiment, the inlet temperature of water was observed to increase due to reduced efficiency of

cooling by the radiator in the pond. The radiator evaporated contaminated water when the hot

water passed through it leaving it exposed to the air leading to a reduction in cooling rate and a

reduction in the amount of heat recovered. The value of thermal resistance also was observed to

increase with increase in temperature.

A theoretical simulation was also doneif the heat exchanger was to be used as an economizer for

low heat recovery. A theoretical simulation was also done for the heat exchanger when the

induced draft fan was running maximum flow rate of 24000 cfm, inlet temperatures of the flue

gases being 700°C and outlet temperature of the flue gases being 200°C. The feed pump was

taken to be running at maximum flow rate of 3m3/hr. From the second analysis, it was seen that

the flue gas stream had a total energy potential of about 2.8 Megawatts.

iii

ACKNOWLEDGEMENT

We would like first to give our sincere gratitude to the Almighty God for guiding us through this

project.

We would like to acknowledge the valuable guidance from our supervisor, Dr. Philip Mwabe and

further thank himfor allowing us to use his state of the art facilities thereby allowing us to

undertake the project smoothly.His monitoring, financial support, advice and mentoring enabled

us tocomplete this project successfully having learned a lot from him.

We also thank our families and friends who gave us physical, financial and emotional assistance

in our project.

We would also like to thank all the staff members of Environmental and Combustion Consultants

Limited who assisted us and helped facilitate our movement from Nairobi to site. Special thanks

go to Titus Magicho and Mulei who were very resourceful on site in answering our questions on

the project.

iv

TABLE OF CONTENTS

DECLARATION............................................................................................................................ i

ABSTRACT ................................................................................................................................... ii

ACKNOWLEDGEMENT ........................................................................................................... iii

ABBREVIATIONS AND SYMBOLS ....................................................................................... vii

LIST OF FIGURES ................................................................................................................... viii

LIST OF TABLES ..................................................................................................................... viii

CHAPTER ONE: INTRODUCTION ......................................................................................... 1

1.1 STATEMENT OF THE PROBLEM ............................................................................... 2

1.2 OBJECTIVES .................................................................................................................. 3

1.3 METHODOLOGY ........................................................................................................... 3

1.4 FIELD STUDY ................................................................................................................ 3

1.5 PROJECT JUSTIFICATION ........................................................................................... 4

1.6 PROJECT LIMITATIONS .............................................................................................. 4

CHAPTER TWO: LITERATURE REVIEW ............................................................................ 5

2.1 FACTORS AFFECTING WASTE HEAT RECOVERY FEASIBILITY ....................... 5

2.2 BENEFITS OF WASTE HEAT RECOVERY ................................................................ 7

2.3 BARRIERS TO WASTE HEAT RECOVERY ............................................................... 8

2.4 HEAT EXCHANGER ANALYSIS ................................................................................. 9

2.4.1 CLASSIFICATION OF HEAT EXCHANGERS .................................................... 9

2.4.2 OVERALL HEAT TRANSFER COEFFICIENT .................................................. 11

2.4.3 USE OF THE LOG MEAN TEMPERATURE DIFFERENCE (LMTD) AND ε-

NTU METHOD IN HEAT EXCHANGER ANALYSIS...................................................... 12

2.5 WASTE HEAT RECOVERY TECHNOLOGIES ........................................................ 15

2.5.1 GAS- AIR HEAT RECOVERY TECHNOLOGIES .............................................. 15

2.5.2 GAS- WATER HEAT RECOVERY TECHNOLOGIES ...................................... 16

2.5.3 POWER GENERATION ........................................................................................ 16

CHAPTER THREE: FEED WATER ANALYSIS .................................................................. 19

3.1 WATER TREATMENT ................................................................................................ 20

3.1.1 EXTERNAL TREATMENT .................................................................................. 20

3.1.2 INTERNAL TREATMENT/INTERNAL CONDITIONING ................................ 21

v

3.2 CHEMICAL ANALYSIS OF SITE WATER ............................................................... 21

3.3 PROPOSED FEED WATER TREATMENT ................................................................ 22

3.3.1 CHEMICAL TREATMENT AGAINST SCALING .............................................. 22

3.3.2 CHEMICAL TREATMENT AGAINST CORROSION BY OXYGEN ............... 22

CHAPTER FOUR: RESULTS AND ANALYSIS ................................................................... 24

4.1 WASTE HEAT RECOVERY SYSTEM DESCRIPTION: ........................................... 24

4.2 RESULTS....................................................................................................................... 25

4.3 DATA ANALYSIS ........................................................................................................ 26

4.3.1 AREA CALCULATION ........................................................................................ 26

4.3.2 SINGLE PHASE ANALYSIS FOR THE HEAT RECOVERY UNIT.................. 26

4.4 ENERGY ANALYSIS OF THE HEAT RECOVERED FROM THE FLUE GASES . 33

4.5 THEORETICAL SIMULATIONS ................................................................................ 34

CHAPTER FIVE: DISCUSSION, CONCLUSION AND RECOMMENDATIONS ........... 37

5.1 DISCUSSION ................................................................................................................ 37

5.2 CONCLUSION .............................................................................................................. 38

5.3 RECOMMENDATIONS ............................................................................................... 39

REFERENCES ............................................................................................................................ 41

APPENDICES ............................................................................................................................. 43

APPENDIX A: FEED WATER TREATMENT ....................................................................... 43

Appendix A1: Recommended feed water characteristics for non fired water boilers. .......... 43

Appendix A2: Flow chart for the selection of the most appropriate water treatment method

............................................................................................................................................... 44

APPENDIX B: GRAPHS ......................................................................................................... 45

Appendix B1: Graph showing variation of heat recovered and available with inlet water

temperature ............................................................................................................................ 45


temperature ............................................................................................................................ 46

Appendix B3: Graph showing the theoretical simulation of variation of heat recovered

against inlet water temperature at Tin = 200°C and Tout = 145°C .......................................... 47

APPENDIX C: MICROSOFT EXCEL WORKBOOK SCREENSHOTS USED IN THE

CALCULATIONS. ................................................................................................................... 48

vi

Appendix C1: Calculation of the convective heat transfer coefficient of the flue gases and the

corresponding thermal resistance. ..................................................................................................... 48

Appendix C2: Calculation of the convective heat transfer coefficient of the water and the

corresponding thermal resistance. ..................................................................................................... 49

Appendix C3: Calculation of effectiveness, fouling factor, total heat available, total heat recovered

and percentage of heat recovered. .................................................................................................... 50

APPENDIX D: CTM 034 ANALYSIS OF FLUE GASES .................................................................................. 51

Appendix D1: CTM 034 analysis showing carbon dioxide concentration in the flue gas stream ....... 51

Appendix D2: CTM 034 analysis showing carbon monoxide concentration in flue gas stream ......... 52

vii

ABBREVIATIONS AND SYMBOLS

V fg − Flue gas volumetric flow rate vw − Velocity of water

Vw − Water volumetric flow rate vfg − Velocity of flue gases

Qfg − Heat availabe (from flue gases) m − mass flow rate

Qw − Heat recoverd (to water) ρ − density

Q − Heat transfered or available di − Inside diameter (helical coil)

P − Pressure do − Outside diameter (helical coil)

Ps − Saturation Pressure Win − Work in

U − Overall heat transfer coefficent Wout − Work out

Uo − External heat transfer heat coefficient

Ui − Internal heat transfer heat coefficient

θM − Log mean temperatur difference

θLM − Corrected log mean temperature difference

RTOT − Total thermal resistance

A − Exchange surface area between two fluids

Ai − Total inside area

Ao − Total outside area

CP − Specific heat capacity at constant pressure

Nu − Nusselt Number

Re − Reynolds Number

Pr− Prandtl Number

Rfi − Fouling factor (inside the heat exchanger)

Rfo − Fouling factor (outside the heat exchanger

μ − Dynamic viscosity

k − Thermal conductivity

DO − Helical diameter (outer coil)

Di − Helical diameter (inner coil)

F − Correction factor

ε − Effectiveness

viii

LIST OF FIGURES

Fig 2.1: Parallel (a) and counter-flow (b) heat exchanger .............................................................. 9

Fig 2.2: Double pipe counter flow heat exchanger ....................................................................... 13

Fig 2.3: The simple ideal Rankine cycle ....................................................................................... 17

Fig 4:1 3D representation of the helical coil heat exchanger........................................................ 25

Fig 4:2 Flow over in line series of tube banks .............................................................................. 29

LIST OF TABLES

Table 2.1: Common low, medium and high temperature waste heat sources and applicable

technologies .................................................................................................................................... 6

Table 2.2: Energy relation for the devices in a Rankine Cycle .................................................... 17

Table 3.1: Table of common water impurities, their effects and removal techniques ................. 19

Table 3.2: Borehole water analysis results ................................................................................... 21

Table 4.1: Components and their corresponding properties ......................................................... 24

Table 4.2: Results as obtained from site ....................................................................................... 25

Table 4.3: Average flue gas composition and their average properties at the mean bulk ............ 31

temperature of 147°C .................................................................................................................... 31

Table 4.4: Table of heat available, correction factor, total thermal resistance and effectiveness for

each set of data obtained from the experiment. ............................................................................ 33

Table 4.5: Table of calculated heat available, heat recovered, ∆Tmax and the percentage of heat

recovered. ...................................................................................................................................... 34

Table 4.6: Table of calculated heat available, heat recovered, percentage of heat recovered and

corresponding Ps values for Tfgin = 200°C .................................................................................... 35

1

CHAPTER ONE: INTRODUCTION

Industrial waste heat refers to energy that is generated in industrial processes without being put

into practical use. Sources of waste heat include hot combustion gases discharged to the

atmosphere, products exiting industrial processes and heat transfer from hot equipment surfaces.

According to a report done by the United States Department of energy,1 it is estimated that

between 20 to 50% of industrial energy input is lost as waste heat in the form of hot exhaust

gases, cooling water and heat lost from hot equipment surfaces through conduction convection

and radiation, heated product streams and products.In order to improve energy efficiency, focus

has been laid on reducing energy consumed by equipment or changing processes or techniques to

manufacture products to make them more energy efficient.

To improve the overall efficiency, waste heat can be captured and reused. The recovered

wasteheat is then used for air/ feed water preheating, water heating or power generation. This in

effect leads to increase in efficiency for processes, greenhouse gas free source of energy and

reduction of energy costs for industries.

This project involves a commercial rotary kiln incinerator that is currently being commissioned.

It is owned by Environmental and Combustion Consultants Limited. The incinerator burns

various industrial and chemical wastes at high temperatures. The flue gases released are at a high

temperature and they form a viable source of waste heat. Aheat recovery unit has been installed

along the chimney line, which in design is a double helical coil heat exchanger. Water passing in

the heat exchanger is heated by the hot flue gases. It is from the total heat gained by the water

that the total energy recovered can be determined. From the data obtained and theoretical

analysis done, a proposal for an end use for the water or steam coming out of the heat recovery

unit will be done.

1 BCS, Incorporated (2008) Waste Heat Recovery: Technology and Opportunities in U.S. Industry, U.S. Department

of Energy

2

1.1 STATEMENT OF THE PROBLEM

The commercial incinerator burns various industrial wastes at high temperatures of up to 1200°C

converting various industrial and chemical wastes to ash and flue gases. The flue gases being

released from combustion are at high temperatures.

In many industries, power cost is a key component of the total production and operations costs.

Currently the cost per kWh is KES 82 for a C12 (Commercial 11kV establishment)which is the

classification of the establishment in terms of power ratings. On average 50,000 kWh are used

monthly. This leads to electricity bills incurred to excess of KES 400,000. With the presence of

a hot flue gas stream that is released to the atmosphere from the combustion of the wastes,

viability of the hot flue gases being used to heat water to steam which can be investigated for

potential use in electricity generation.

One disadvantage of releasing the flue gases to the environment at such high temperatures is that

they may have a negative impact on the environment in terms of air pollution. According to

Justin Hill (2011), the impact of energy dissipated as heat into the atmosphere has also been

explored as a possible source of manmade climate change. This released energy has long been

considered as a main cause to the “heat island” in highly industrialized cities.Releasing flue

gases at a lower temperature to the atmosphere leads to a reduction of the “heat island” effect on

the surface of the earth. This has a contribution in reducing global warming.

In an effort to reduce the exit gas temperatures from the chimney and as a long term effort to

reduce energy costs while providing an alternate source of income, the heat recovery unit aids in

cooling the flue gases while at the same time utilizing the hot flue gas stream to heat water

tosteam. From the inlet and exit temperatures of the water passing through the heat recovery unit,

it is possible to determine the total amount of heat recovered. This is the data that will be used in

coming up with theoretical simulations of the heat recovery unit.

2Electricity costs in Kenya accessed from urlhttps://stima.regulusweb.com/ on 6

th April 2015

https://stima.regulusweb.com/

3

1.2 OBJECTIVES

The objective of this project is to determine the amount energy recoverable from a flue gas

stream. Our aim will be to carry out:

Carry out a feed water analysis of the borehole water on site and propose a water

treatment method.

Experimental determination of the temperature of the water at the inlet and outlet of the

heat recovery unit at different flue gas inlet and exit temperatures.

Theoretical calculation of the correction factor (F)of the helical heat exchanger, energy

available from the flue gas stream and the energy recovered by the water.

Determination of the heat exchanger effectiveness (ε) at different flue gas temperatures.

Determination of the most optimum heat that can be recovered from theoretical

simulation.

1.3 METHODOLOGY

Literature review from various text books, designs (for example the steam plant in the

thermodynamics lab), standards and articles.

Experimental determination of the temperature of the water at the outlet of the heat

recovery unit.

Theoretical simulations using Microsoft Excel for the most optimum heat that can be

recovered.

1.4 FIELD STUDY

The field study involved taking temperature readings from thermocouples of the flue gas

stream and water.

A survey of a similar plant was also done in East Africa Portland Cement where a similar

project will be undertaken.

The only form of data acquisition was through reading of values from thermocouples,

pressure gauges and water flow meter.

4

1.5 PROJECT JUSTIFICATION

From the experimental data obtained and from the corresponding theoretical analysis, the project

will seek to find out the most optimum energy that can be recovered. If the energy recoverable is

viable for power generation, a proposal can be made to design a steam power plant on site. This

will cut down on the cost of power that the company pays to run machinery on site. Surplus

power can be sold to the national power supplier, Kenya Power providing an alternative source

of income.

For example, a heat recovery boiler/steam turbine WHP project at a petroleum coke plant in Port

Arthur, Texas, recovers energy from 2,000oF exhaust from three petroleum coke calcining

kilns.The project produces 450,000 lb/hr of steam for process use at an adjacent refinery and 5

MW of power.

1.6 PROJECT LIMITATIONS

The flow rate of the water could not be varied independently thereby making it difficult to obtain

data at different water flow rates.

The actual flue gas flow rate could not be measured. The induced draft fan was run at 80% flow

rate capacity at all times while performing the experiment.

The construction of the plant was finished in early March and was commissioned around the

same time. During commissioning, various problems arose, mainly due to the thermocouples

being faulty and sometimes completely failing. This hindered us from obtaining more results

from the site.

5

CHAPTER TWO: LITERATURE REVIEW

Heatrecovery from waste gas sources provides an opportunity for economical operation and

increased efficiency of thermal systems. Energy present in waste gases cannot be fully

recovered. To determine whether a Heatrecovery project is viable, a few important parameters

must be determined3.

2.1 FACTORS AFFECTING WASTE HEAT RECOVERY FEASIBILITY

a) Heat quantity

This is the amount of energy in a waste heat stream. It is given by:

𝑄 = 𝑚 𝐶𝑝∆𝑇 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 1(𝑎)

Or

𝑄 = 𝑚∆ 𝑕 𝑡 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 1 (𝑏)

Where:

Q- Quantity of waste heat in waste heat stream (J/s)

m- Mass flow rate of waste heat stream (Kg/s)

Cp- Specific heat capacity at constant pressure (Joules/Kg.K)

∆T- Change in temperature of waste heat stream (∆K)

∆h- Change in enthalpy as a function of temperature of waste heat stream

b) Heat quality/ temperature

It is the usefulness of the waste heat stream. The temperature difference between the heat source

and sink determines the waste heat’s utility or quality, heat transfer rate per unit surface area (the

smaller the temperature difference the larger the heat exchange surface area required), maximum

theoretical efficiency material selection in heat exchanger design.

Generally waste heat recovery opportunities are categorized into high (649°C and higher),

medium (232°C to 649°C) and low (232°C) and lower.

3Reiter S: 1983

6

Table 2.1: Common low, medium and high temperature waste heat sources and applicable

technologies

(Courtesy of BCS incorporated:2008)

Temperature

Range

Example Sources Typical Recovery Methods

\Technologies

High 649°C and

higher

Nickel/ Copper/ Steel/

Aluminum/ Glass

melting furnaces

Hydrogen plants

Fume incinerators

Combustion air preheat

Steam generation for heating or power

generation

Furnace load preheating

Transfer to med-low temperature

processes

Medium 232°C to

649°C

Steam boiler exhaust

Gas turbine exhaust

Reciprocating engine

exhaust

Heat treating furnace

Drying and baking

ovens

Cement kiln

Combustion air preheat

Steam generation for heating or power

generation

Furnace load preheating

Transfer to low temperature processes

Low 232°C and

lower

Exhaust gases exiting

recovery devices

Process steam

condensate

Drying, baking and

curing ovens

Hot processed

liquids/solids

Space heating

Domestic water heating

Temperature of the flue gas will affect material selection. Corrosion and oxidation reactionsare

accelerated by temperature increases. Advanced alloys or composite materials are used at higher

temperatures. Carbon steel at temperatures above 425ºC and stainless steel above 650ºC begin to

oxidize. Metallic materials are not used at temperatures above 871ºC. An alternative is to use

ceramic materials that can better withstand the high temperature.

7

c) Waste Stream Composition

The composition of the stream affects the recovery process, material selection, thermal

conductivity and heat capacity. Heat transfer rates in heat exchangers are dependent on the

composition and phase of waste heat streamsand deposition of any fouling substances. Denser

fluids have higher heat transfer coefficients, which enables higher heat transfer rates per unit area

for a given temperature difference.

d) Minimum allowed temperature

This is determined by the flue gas composition. Flue gases contain varying concentrations of

carbon dioxide, water vapor, NOX, SOX, un-oxidized organics and minerals. If the flue gases are

cooled below the dew point temperature, the water vapor in the gas will condense and deposit

corrosive substances on the heat exchanger surface. Heat exchangers are generally designed to

maintain exhaust temperatures above the condensation point.

e) Other factors

Operating schedules: If a waste heat source is only available for a limited time every day, the

heat exchanger may be exposed to both high and low temperatures. One must ensure that the

heat exchange material does not fatigue doe to thermal cycling.

Accessibility: Physical constraints created by equipment arrangements prevent easy access to the

heat source, or prevent the installation of any additional equipment for recovering the heat.

2.2 BENEFITS OF WASTE HEAT RECOVERY

a) Reduced fuel consumption that leads to reduced CO2 emissions.

b) Reduction in equipment sizes of all flue gas handling equipment such as fans, stacks, ducts,

burners etc. due to reduced flue gas produced.

c) It leads to an increase in efficiency. For example, in industrial furnaces, when heat recovery

is done, it leads to an improvement of energy efficiency by 10% to 50%. It also leads to a

reduction in auxiliary energy consumption.

d) It leads can lead to substantial fuel savings. Fuel consumption of a boiler can be reduced by

using waste heat from the plant for preheating both air and boiler feed water.

e) Plants generate their own electricity by using the energy recovered in the form of superheated

steam which runs a turbine.

8

2.3 BARRIERS TO WASTE HEAT RECOVERY

a) Long payback periods due to the high operating, maintenance and materials costs that are

incurred during operation.

b) No direct use in industries for low temperature range flue gas streams.

c) Presence of flue gases with a high chemical activity that will lead to high rates of fouling and

corrosion of the heat recovery unit.

d) It is difficult to access and recover heat from unconventional sources such as hot solid

product streams and hot equipment surfaces and limited space to install waste heat recovery

devices.

A major disadvantage of heat recovery processes is that they reduce the rate of dispersion of

various toxic gases (for example NOX). The heat recovery process in itself does not reduce air

pollution.

A study was conducted on the effect of Heat recovery from flue gases on local humidity and NOx

dispersion in a power plant in Korea4. It was seen that at 115 °C the flue gas containing nitrogen

oxides would be instantaneously diluted at least 60 times as soon as released from the stack. The

direct influencing area was found to be a distance up to 700 m from the emission stack. It

however, stretched over 900m when the flue gas was cooled to 40°C. The lowering of flue gas

temperature therefore, caused an increase in NOx concentration in the region near the power

plant.

4Shi Chang WU, Young Min Jo (2013) Effect of Heat Recovery from flue gas on local humidity and NOx dispersal

in a thermal power station doi: 10.4209/aaqr.2013.01.0028

9

2.4 HEAT EXCHANGER ANALYSIS

Most heat recovery technologies make use of heat exchangers. They facilitate effective heat

transfer between two fluids. Heat transfer is mainly through convection and conduction.

2.4.1 CLASSIFICATION OF HEAT EXCHANGERS

Heat exchangers may be classified according to5:

Nature of heat exchange process

Direct contact heat exchangers- Energy transfer between the hot and cold fluid is brought

about by their complete physical mixing.

Regenerator- A hot fluid is passed through a matrix. Heat is transferred to the solid matrix and

accumulates there. A cold fluid is then passed over the heated matrix and heat is transferred to it

by the matrix.

Recuperator- Fluids flow simultaneously on either side of a separating wall. Heat transfer

occurs between the fluid streams without mixing or physical contact with each other.

Flow arrangement

In the parallel flow heat exchanger, both hot and cold fluids enter the heat exchanger at the

same end and move in the same direction while in the counter flow, the hot and cold fluids enter

the heat exchanger at opposite ends and flow in opposite directions6

Fig2.1: Parallel (a) and counter-flow (b) heat exchanger

(Courtesy of Heat Transfer by Cengel Y.A 2002)

5R.K. Rajput 6Cengel Y.A. 2002

10

Construction

Double Pipe heat exchangers- It is made of two concentric pipes of different diameters where

one fluid flows through the annulus space between the two pipes and the other in the smaller

pipe.

Shell and Tube heat exchangers- They contain a large number of tubes packed in a shell with

their axes parallel to that of the shell. Baffles are placed in the shell to force the shell side fluid to

flow across the shell to enhance heat transfer and to maintain uniform spacing between the tubes.

They are further classified according to the number of shell and tube passes involved.

Plate heat exchangers- They are constructed of thin plates which may be smooth or corrugates.

Hot and cold fluids flow in alternate passages thus each cold fluid stream is surrounded by two

hot fluid steams, resulting in very effective heat transfer.

Heat transfer mechanism

Single phase heat exchanger has no change in phase in both fluids in the heat exchanger while

in the phase change heat exchanger; there is a change in phase in either of the fluids flowing in

the heat exchanger for example, evaporators and condensers.

Compactness

Compact heat exchanger- It has a large heat transfer surface area per unit volume and they are

commonly used in applications with strict limitations on the weight and volume of the heat

exchanger. The ratio of the heat transfer surface area of a heat exchanger to its volume is called

the area density β. Heat exchangers with β > 700m2/m

3 is classified as a compact heat exchanger

e.g. car radiator. Flow in compact heat exchangers may be classified as:

Mixed cross flow- The fluid is free to move in the transverse direction

Unmixed cross flow- Plate fins force the fluid to flow through a particular inter-fin spacing and

prevent it from moving in the transverse direction.

11

In the analysis of a heat exchanger using the log mean temperature difference and ε-NTU

method, the following assumptions are made7

1. The heat exchanger operates under quasi-steady-state conditions (i.e. Constant mass flow

rates)

2. The outside walls are adiabatic. Thereby heat loss or heat gain to the surrounding is

negligible and no thermal energy sources within the heat exchanger.

3. Wall thermal resistance is distributed uniformly in the entire exchanger.

4. Longitudinal heat conduction in the fluids and in the wall is negligible.

5. No phase change occurs in the regenerator.

6. Heat transfer coefficients between the fluids and the matrix wall are constant throughout the

exchanger.

2.4.2 OVERALL HEAT TRANSFER COEFFICIENT

The overall heat transferred between two fluids is given by:

𝑄 = 𝑈𝐴𝜃𝐶𝑂𝑅 = 𝜃𝐶𝑂𝑅𝑅𝑇𝑂𝑇

𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 2

Where:

RTOT- Total thermal resistance

U- Overall heat transfer coefficient

A- Exchange surface area between two fluids.

θCOR- Corrected log mean temperature difference

From equation 4, the relation RTOTand UA is:

𝑅𝑇𝑂𝑇 = 1

𝑈𝐴 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 3

Heat is transferred from a fluid to the wall by convection, through the wall by conduction and

from the wall to another fluid again by convection. Any radiation effects are usually included in

the convection heat transfer coefficients.

Layers of deposits on the heat transfer surfaces cause the rate of heat transfer in a heat exchanger

to decrease resulting to fouling. This introduces a fouling factor Rf (on the inner and outer

surfaces) which is a measure of the thermal resistance introduced by fouling. It is caused by

scaling, particulate fouling, corrosion and solidification fouling.

The total thermal resistance may be given by:

7Shar R., Dusan P. S 2003; Cengel Y.A. 2002

12

RTOT = Convective Resistance (inside) + Fouling resistance (inside) + Thermal resistance (material) +

Fouling resistance (outside) + Convective thermal resistance (outside)

For a tube, the transfer area is not uniform and U may be defined in terms of the internal Ui or

external Uo areas of the pipe.

Therefore we have:

𝑅𝑇𝑂𝑇 = 1

2𝜋𝑟𝑖𝐿𝑕𝑖+

𝑅𝑓𝑖

2𝜋𝑟𝑖𝐿+

1

2𝜋𝐿𝑘𝑙𝑛𝑟𝑜𝑟𝑖

+ 𝑅𝑓𝑜

2𝜋𝑟𝑜𝐿+

1

2𝜋𝑟𝑜𝐿𝑕𝑜=

1

𝐴𝑖𝑈𝑖=

1

𝐴𝑜𝑈𝑜 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 4

Where

hi and ho -Convection heat transfer coefficients for the inside and outside surfaces of the heat

exchanger tubes.

Rfi and Rfo- Fouling resistances due to deposits and other modifications on the surface

k- Thermal conductivity

Dividing the above equation by the internal and external areas where Ai = 2πriL and Ao = 2πroL

we obtain the expression for Uias8

𝑈𝑖 = 1

1

𝑕𝑖+ 𝑅𝑓𝑖 +

𝑟𝑖

𝑘ln

𝑟𝑜

𝑟𝑖+

𝑟𝑖

𝑟𝑜𝑅𝑓𝑜 +

𝑟𝑖

𝑟𝑜𝑕𝑜


The expression for Uo is then obtained as:

𝑈𝑜 = 1

𝑟𝑜

𝑟𝑖𝑕𝑖+

𝑟𝑜

𝑟𝑖𝑅𝑓𝑖 +

𝑟𝑜

𝑘ln

𝑟𝑜

𝑟𝑖+ 𝑅𝑓𝑜 +

1

𝑕𝑜


2.4.3 USE OF THE LOG MEAN TEMPERATURE DIFFERENCE (LMTD) AND

ε-NTU METHOD IN HEAT EXCHANGER ANALYSIS

2.4.3.1 LOG MEAN TEMPERATURE DIFFERENCE (LMTD)

Consider a double pipe counter flow heat exchanger as shown below. The hot fluid passes

through the smaller pipe and the cold fluid passes through the annulus of the larger pipe.

8Luti F.M 2012

13

Fig 2.2: Double pipe counter flow heat exchanger


From the log mean temperature difference (LMTD) method of analysis, the total heat transferred

may be given by:

𝑄 = 𝑈𝐴𝜃𝑀 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 7

Where θM is the log mean temperature difference. This is given by:

𝜃𝑀 = ∆𝑇1 − ∆𝑇2

𝑙𝑛 ∆𝑇1

∆𝑇2


∆T1 and ∆T2 are the temperature differences at the inlet and outlets of the heat exchangers.

The above expression is limited to parallel flow and counter flow heat exchangers only.

However, various expressions have been made for cross flow and multi-pass shell and tube heat

exchangers. The resulting expressions are too complicated because of complex flow conditions.

It is therefore convenient to relate equivalent temperature difference to LMTD relation for

counter flow case as:

𝜃𝐶𝑂𝑅𝑅 = 𝐹𝜃𝑀 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 9

Where F is the correction factor

This is a measure of deviation of the θM for corresponding values of the counter flow case. F is a

function of the geometry plus two parameters P and R. Therefore,

𝐹 = 𝑓(𝑅,𝑃,𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑦) 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 10

Where:

𝑅 = (𝑇𝑖𝑛 − 𝑇𝑜𝑢𝑡 )𝑠𝑕𝑒𝑙𝑙(𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛 )𝑡𝑢𝑏𝑒

= (𝑚 𝐶𝑝)𝑡𝑢𝑏𝑒

(𝑚 𝐶𝑝)𝑠𝑕𝑒𝑙𝑙 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 11 (𝑎)

And:

14

𝑃 = (𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛 )𝑠𝑕𝑒𝑙𝑙

(𝑇𝑖𝑛 𝑠𝑕𝑒𝑙𝑙 − 𝑇𝑖𝑛 𝑡𝑢𝑏𝑒 ) 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 11 (𝑏)

2.4.3.2 ε-NTU METHOD

Effectiveness (ε) for a heat exchanger is the ration of the actual heat transfer rate to themaximum

heat transfer possible9.

𝜀 = 𝐴𝑐𝑡𝑢𝑎𝑙 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑟𝑎𝑡𝑒

𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑕𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑟𝑎𝑡𝑒 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒=

𝑄

𝑄𝑀𝐴𝑋 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 12

QMAX is given by:

𝑄𝑀𝐴𝑋 = 𝐶𝑚𝑖𝑛 𝑇𝑕𝑜𝑡 𝑖𝑛 − 𝑇𝑐𝑜𝑙𝑑 𝑖𝑛 = 𝐶𝑚𝑖𝑛 ∆𝑇𝑚𝑎𝑥 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 13

Where; 𝐶𝑚𝑖𝑛 = 𝑚 ∗ 𝐶𝑝(min )

Equation 12 may be written as:

𝜀 = 𝐶𝑕(𝑇𝑕𝑖 − 𝑇𝑕𝑜)

𝐶𝑚𝑖𝑛 (𝑇𝑕𝑖 − 𝑇𝑐𝑖)=

𝐶𝑐(𝑇𝑐𝑜 − 𝑇𝑐𝑖)

𝐶𝑚𝑖𝑛 (𝑇𝑕𝑖 − 𝑇𝑐𝑖)=

𝑈𝐴𝐹𝜃𝑀𝐶𝑚𝑖𝑛 ∆𝑇𝑚𝑎𝑥


Effectiveness of a heat exchanger enables us to determine the heat transfer rate without knowing

the outlet temperature of the fluids. The effectiveness of a heat exchanger depends on the

geometry of the heat exchanger as well as the flow arrangement.

Effectiveness relations of the heat exchangers typically involve the dimensionless group

UAs/Cmin. This quantity is called the number of transfer units (NTU). Normally for a higher

value of NTU the larger the heat exchanger.

Effectiveness of a heat exchanger is a function of the number of transfer units (NTU), flow

arrangement and the capacity ratio c where c = Cmin/Cmax.

𝜀 = 𝑓 𝑈𝐴𝑠𝐶𝑚𝑖𝑛

, 𝑐 ,𝐹𝑙𝑜𝑤 𝑎𝑟𝑟𝑎𝑛𝑔𝑒𝑚𝑒𝑛𝑡 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 15

Effectiveness relations have been developed for a large number of heat exchangers.

9Luti F.M 2012

15

2.5 WASTE HEAT RECOVERY TECHNOLOGIES

As earlier stated, most heat recovery technologies make use of heat exchangers. The following

are the common types waste heat recovery technologies10

.

2.5.1 GAS- AIR HEAT RECOVERY TECHNOLOGIES

There are used most commonly used to heat combustion air by heat exchange with flue gas. It is

also used in direct fired air or gas heaters in process work. Some of the technologies used

include:

a) Recuperators

They recover exhaust gas waste heat in medium to high temperatures that is used to preheat air

for combustion. They are constructed of either metallic or ceramic materials. They can generally

be categorized into three:

Metallic radiation recuperator- It consists of two concentric lengths of ductwork. Hot waste gases

pass through the inner duct and heat is radiated to the wall and the cold incoming air in the outer

shell.

Convective or tube type recuperator- Hot gases are carried through a number of parallel small

diameter tubes, while the incoming air to be heated enters a shell surrounding the tubes.

Combined radiation/convection recuperator- The system has a radiation section followed by

convection section.

b) Regenerators

These are heat exchangers through which hot and cold air flow alternatively. Flue gases flow

through one chamber, from where the surfaces absorb heat and increase in temperature. The flow

of air is them adjusted so that so that incoming combustion air passes over the surfaces which

transfers heat to the combustion air entering the furnace.

c) Heat wheel (Rotary Regenerative Heat Exchanger)They make use of a rotating porous

disc of a high heat capacity material placed across two parallel ducts, one containing hot

waste gas the other containing cold gas. The disc rotates between two ducts and transfers heat

from the hot gas duct to the cold gas duct.

10Boyen J.L, 1975; Reiter S. 1983; BCS, Incorporated 2008

16

d) Passive Air Pre-heaters

They are generally of two types:

Plate type exchanger

Heat pipe heat exchanger- It consists of several pipes with sealed ends. Each pipe contains a

capillary wick structure that facilitates movement of the working fluid between the hot and cold

ends of the pipe. Hot gases pass over one end of the heat pipe, causing the working fluid inside

the pipe to evaporate. Pressure gradients along the pipe cause the hot vapor to move to the other

end of the pipe, where the vapor condenses and transfers heat to the cold gas. The condensate

then cycles back to the hot side of the pipe via capillary action.

2.5.2 GAS- WATER HEAT RECOVERY TECHNOLOGIES

a) Boiler feed water preheating (Economizers)

They are found in process boilers where waste heat in the exhaust gas is passed along finned

tubes that carry the boiler feed water. The feed water receives heat from hot gases flowing across

the tubes. The feed water is then fed into the boiler. This in effect increases the efficiency of the

power plant.

2.5.3 POWER GENERATION

This involves using waste heat to heat water forming steam which is expanded in a turbine

coupled to an electric generator. The tubes carry the feed water which is heated and converted to

steam. In cases where the waste heat is not sufficient for producing desired levels of steam,

auxiliary burners or an afterburner can be added to attain a higher steam output.

Generating electrical power via mechanical work makes use of the Steam Rankine Cycle,

Organic Rankine Cycle and the Kalina Cycle.

a) Steam Rankine Cycle

Water/steamis used as the working fluid in the closed loop. Steam is the most common working

fluid used in vapor power cycles because of its many desirable characteristics such as low cost,

availability and high enthalpy of vaporization.

Wateris first pumped to elevated pressure before entering a heat recovery boiler. The pressurized

water is vaporized by the hot exhaust and then expanded to lower temperature and pressure in a

turbine, generating mechanical power that can drive an electric generator. The low-pressure

steam is then exhausted to a condenser at vacuum conditions, where heat is removed by

17

condensing the vapor back into a liquid. The condensate from the condenser is then returned to

the pump and the cycle continues.

This is illustrated in the figure below:

Fig 2.3: The simple ideal Rankine cycle

(Courtesy of Thermodynamics by Cengel Y.A 2002)

Energy analysis of the Ideal Rankine Cycle

The kinetic energy changes are usually small relative to the work and heat transfer terms and are

therefore usually neglected. The steady flow energy equation per unit mass of steam reduced to.

𝑄𝑖𝑛 − 𝑄𝑜𝑢𝑡 + 𝑊𝑖𝑛 −𝑊𝑜𝑢𝑡 = 𝑕𝑒 − 𝑕𝑖 (𝑘𝐽 𝑘𝑔) 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 16

Boiler and condenser don’t involve any work, and the pump and the turbine are assumed to be

isentropic. Then the conservation of energy relation for each device can be expressed as follows.

Table 2.2: Energy relation for the devices in a Rankine Cycle

Device Energy Relation

Pump h2-h1 or v(P2-P1) h1 where h1 = hf at P1 and v = vfat P1

Boiler Qin = h3-h2

Turbine Wturbine out = h3-h4

Condenser Qout = h4-h1

18

The thermal efficiency of the Rankine cycle is determined from

𝜂 𝑡𝑕 = 𝑊𝑛𝑒𝑡

𝑄𝑖𝑛= 1 −

𝑊𝑖𝑛

𝑄𝑜𝑢𝑡

b) Organic Rankine cycle

Organic working fluids (e.g. ammonia, toluene, Freon, haloalkanes, and iso-pentane)can be used

below a temperature of 400°C.The fluids have better efficiencies at lower heat source

temperatures and have lower boiling point, higher vapor pressure, higher molecular mass, do not

have to be superheated, and higher mass flow compared to water If the degree of superheating is

reduced, more steam can be generated and hence more energy can be recovered from the heat

source. In this cycle, the working fluid superheats as the pressure is reduced unlike steam which

becomes wet during expansion11

.

c) Kalina cycle

The working fluid is an ammonia water mixture. Its main advantage over the conventional

Rankine cycle is that the ammonia water mixture has a varying boiling and condensing

temperature, which enables the fluid to extract more energy from the hot stream by matching the

hot source better than a system with a constant boiling and condensing temperature. This results

in significant energy recovery from hot gas streams, particularly those at low temperatures. It is

15-25% more efficient than the Organic Rankine Cycles working at the same temperature level.

11V. Ganapathy 2003

19

CHAPTER THREE: FEED WATER ANALYSIS

Water treatment and conditioning is the process of making water from any natural source such as

river, dam or lake suitable for feeding water into a boiler and mixing appropriate chemicals to

treated water to prevent damage to internal surfaces of a boiler.

The table below shows the common water impurities their effects on heating surfaces and

removal techniques.

Table 3.1: Table of common water impurities, their effects and removal techniques

(Table courtesy of Kumar Rayaprolu 2009)

Item Description Effects Removal

Hardness Ca and Mg salts as

CaCO3

Formation of scales S, DM, internal

treatment,

surface agents

Alkalinity HCO3, CO3, OH Foaming, carryover,

embrittlement, CO2

in steam causing corrosion

in condensate lines

S, DM, HX softening,

dealkalization by AX

Free Acids HCl, H2SO4,

Carbonic acid

Corrosion Neutralization with

alkalies

CO2 Corrosion in steam and

condensate lines

A, Da, neutralization with

alkalies

SO42-

Formation of CaSO4

scales

DM

Cl-

Adds to corrosive nature

of water

DM

Na+

Corrosion by combining

with OH

DM

SiO2 Scale in band insoluble

deposits in turbine

Adsorption in high basic

AX in DM

Iron and Mn Deposits in water lines

O2 Corrosion in water lines

and heating tubes

by pitting

Da, Na2SO3, corrosion

inhibitors

Suspended

solids

Deposits in boiler and

heating tubes

Filtration

Oil Foaming and hence

carryover

Dual media or activated

carbon filtration

Abbreviations: S, softener; DM, demineralizer; A, aeration; Da, daeration; F, filtration; AX anion exchanger; CX

cation exchanger; SS, suspended solids; HX, heat exchanger

20

3.1 WATER TREATMENT

Water treatment is grouped into two main areas external and internal treatment12

.

3.1.1 EXTERNAL TREATMENT

Its the removal of suspended solids (through sedimentation and filtration) hardness, other soluble

impurities and degasification (Deaeration).

Oxygen scavengers are also used to further remove dissolved oxygen. They can be divided into

non-volatile (e.g. Sodium Sulfite, Iso ascorbic acid and Tannins) and volatile chemicals

(Hydrazine, Diethyl Hydroxylamine (DEHA) and Carbohydrazide).

3.1.1.1 Removal of Hardness

Thiscan be done using the following methods:

a) Lime Softening- Slaked lime (Calcium hydroxide) is added to hard water and it reacts with

some of the calcium, magnesium and silica forming a solid precipitate. The reaction that

takes place as shown below:

Ca OH 2 s + Ca(HCO3)2(aq ) → 2CaCO3(s) + 2H2O(l)

b) Base Exchange Softening- Hard water enters an ion exchange unit that is partially filled

with an ion exchange resin. The calcium and magnesium ions are exchanged for sodium ions.

The resin is regenerated by passing a concentrated solution of sodium chloride (brine)

through it.

c) Dealkalization-Water is passed through a weak acid cation exchanger where calcium and

magnesium ions are exchanged for hydrogen ions. The hydrogen carbonates are converted to

carbonic acid which is removed in a de-gassing tower.

d) Ion exchange demineralization- Water is passed through a one or more cation exchange

beds (calcium and magnesium are exchanged for hydrogen ions) then through anion

exchange beds (sulphate, chloride, carbonate and silica are ex-changed for hydroxide ions).

e) Reverse osmosis- The process makes use of a applying a pressure greater than the osmotic

pressure to a more concentrated solution that is separated from a less concentrated solution

by a semi-permeable membrane. Water from the more concentrated solution is forced back

through the semi permeable membrane to the more dilute solution on the other side. Water

12CIBO Handbook, 1997; Kumar Rayaprolu 2009; BS 2486-1997

21

containing high levels of hardness should be pre-treated prior to being fed to the reverse

osmosis membranes to prevent them becoming fouled with calcium salts or other foulants.

The process is very effective as it removes 95%-99% 13

of all dissolved salts in the water.

3.1.2 INTERNAL TREATMENT/INTERNAL CONDITIONING

This is the dosing of appropriate chemicals at specific places to the treated feed water to prevent

damage to the internal surfaces of the heating surfaces. They assist with managing corrosion,

scaling, removing traces of dissolved O2 and maintain correct chemical balance in feed water.

The four main types are:

a) Conventional Phosphate treatment. A pH of 10.5-11.2 is maintained with excess OH and

converting the hardness constituents as flocculent precipitate. Orthophosphate residuals are

maintained between 20 and 60 ppm as PO4 and hydrate alkalinity.

b) Coordinated Phosphate treatmentChemicals used are a combination of tri- and disodium

phosphates. pH correction is done by altering the ratio of tri- and disodium phosphates.

c) All-volatile treatmentmethod makes use of ammonia and hydrazine.

3.2 CHEMICAL ANALYSIS OF SITE WATER

Table 3.2: Borehole water analysis results

(Courtesy of Aquachem Chemicals)

Parameters Unit Result Parameters Unit Result

Ph pH Scale 8.56 Total Alkalinity mgCaCO3/l 430

Color mgPt/1 <5 Chloride mg/l 600

Turbidity N.T.U. 4.8 Fluoride mg/l 8.99

Conductivity (25°C) µS/cm 2980 Nitrate mg/l 3.36

Iron mg/l 0.2 Nitrite mg/l <0.01

Manganese mg/l <0.01 Sulphate mg/l 166

Calcium mg/l 8.0 Free Carbon Dioxide mg/l NIL

Magnesium mg/l 9.72 Total Dissolved solids mg/l 1847.6

Sodium mg/l 654.6 Arsenic µg/l -

Potassium mg/l 5.1 Others -

Total Hardness mgCaCO3/l 60

13

Basics of reverse osmosis- Accessed from urlhttp://puretecwater.com/resources/basics-of-reverse-osmosis.pdf on

1st October 2014

http://puretecwater.com/resources/basics-of-reverse-osmosis.pdf

22

3.3 PROPOSED FEED WATER TREATMENT

In order to perform the experiment, the main feed water problems that must be handled are those

that will cause scaling in the tubes and corrosion. From table 3.2, it’s seen that the amounts of

calcium and magnesium in the site water and the amount of hardness were extremely high.

From BS 2486-199714

, the heat recovery steam unit can be classified as a non-fired water tube

boiler common in heat recovery applications. It is difficult to generalize on the water chemistry

since they cover a wide range of operating pressures designs and applications. Recommended

water standards for a non-fired water boiler is as shown in Appendix A1.

3.3.1 CHEMICAL TREATMENT AGAINST SCALING

The chemical chosen for use was Sodium triphosphate (Na3PO4). It prevents scale formation

by converting impurities into a slug that settles at the bottom of the boiler removed by

blowdown15

.

𝐹𝑒𝑒𝑑 𝑟𝑒𝑞𝑢𝑖𝑟𝑚𝑒𝑛𝑡 𝑁𝑎3𝑃𝑂4 = 𝐹𝑊 𝐶𝑎 ∗ 1.15 + 1.7 ∗ 𝑑𝑒𝑠𝑖𝑟𝑒𝑑 𝑃𝑂4−3 𝑟𝑒𝑠𝑖𝑑𝑢𝑎𝑙/𝐶𝑦𝑐𝑙𝑒𝑠

Hardness concentration = 60mgCaCO3/l

Desired residual = 12 mg/L (chosen arbitrarily by the designer to be between 5-30 mg/L)

𝐹𝑒𝑒𝑑 𝑟𝑒𝑞𝑢𝑖𝑟𝑚𝑒𝑛𝑡 𝑁𝑎3𝑃𝑂4 = 60 ∗ 1.15 + 1.7 ∗ 12

1.0336= 88.74 𝑚𝑔/𝑙

To treat 1000 liters of water we use:

88.74 ∗ 10−3 ∗ 1000 = 88.74 𝑔

The chemical dosing of sodium tri-phosphatewas to be done in the upper feed tank.

3.3.2 CHEMICAL TREATMENT AGAINST CORROSION BY OXYGEN

After removal of hardness, the water was channeled to a second tank where oxygen removal was

to be done using Sodium Sulphite.It is very reactive and reduces oxygen to very low levels to

less than 5ppb if used correctly.

The feed requirement for Na2SO3 mg/L per mg/L of dissolved Oxygen is 7.88 mg/L.

Sulfite residual maintained in the boiler is about 5 to 20 mg/L divided by boiler

cycles.(Association of Water Technologies, Technical Manual)

Have the equation below:

14

BS 2486-1997: Recommendations for treatment of water for steam boilers and water heaters 15

Association of Water Technologies, Technical Manual

23

𝐹𝑒𝑒𝑑 𝑟𝑒𝑞𝑢𝑖𝑟𝑚𝑒𝑛𝑡 (𝑁𝑎2𝑆𝑂3) = 7.88 ∗ 𝑚𝑔 𝐿 𝑂2 + 5 − 20𝑚𝑔 𝐿

𝐵𝑜𝑖𝑙𝑒𝑟 𝐶𝑦𝑐𝑙𝑒𝑠

Taking the feed water to be at a temperature of 20°C, oxygen content is 10 mg/l.

Residual SO3 taken as 6 mg/L (chosen arbitrarily by the designer to be between 5-20 mg/L)

From the water analysis done:

The total dissolved solids 1847.6mg/l

The total hardness 60mg/l

Theoretical Boiler cycles;

𝐵𝑜𝑖𝑙𝑒𝑟 𝐶𝑦𝑐𝑙𝑒𝑠 = 𝐵𝐷 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛

𝐹𝑊 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛=

1847.6

1787.6= 1.0336

Feeding the values in the feed requirement equation we have:

𝐹𝑒𝑒𝑑 𝑟𝑒𝑞𝑢𝑖𝑟𝑚𝑒𝑛𝑡 (𝑁𝑎2𝑆𝑂3) = 7.88 ∗ 10 + 6

1.0336

𝐹𝑒𝑒𝑑 𝑟𝑒𝑞𝑢𝑖𝑟𝑚𝑒𝑛𝑡 𝑁𝑎2𝑆𝑂3 = 78.8 + 5.7477 = 84.6052 𝑚𝑔/𝑙

To treat 1000 liters of water we use:

84.6052 ∗ 10−3 ∗ 1000 = 84.6052 𝑔

For the experimental setup, the main aim here was to treat the feed water to prevent scaling and

corrosion. From the water analysis results done, it can be seen that the chloride concentration is

very high (600 mg/l). This may have little effect in terms of corrosion and scaling boiler feed

water problems. However, when a turbine is installed, the chloride ions may cause priming when

they settle on the turbine blades leading to a decline in turbine efficiency.

24

CHAPTER FOUR: RESULTS AND ANALYSIS

4.1 WASTE HEAT RECOVERY SYSTEM DESCRIPTION:

Table 4.1: Components and their corresponding properties

Component Description Parameter Value

Inner Coil (IC) SS 304 Stainless

steel pipe

Inner Diameter (di)

Outer Diameter (do)

Length

Helical Diameter (DI)

Number of Turns

0.02664m

0.0334m

26.49m

0.527

17

Outer Coil (OC) SS 304 Stainless

steel pipe

Inner Diameter (di)

Outer Diameter (do)

Length

Helical Diameter (DO)

Number of Turns

0.02664m

0.0334m

55.048m

1.095m

16.5

Pipe (Feed Water) 1”Schedule mild

steel pipe

Inner diameter

Outer Diameter

0.02664m

0.0334m

Pump Vertical

Centrifugal

Grundfos

CR-3-IOA-FGJ-

A-E-HOOE

Maximum head

Flow rate (Ṽw)

46.4m

3m3/s

Reservoir Tank Mild Steel casing

insulated with

Fiber Glass

Volume Capacity 2500Litres

Make up Water

Tank

Mild steel Volume Capacity

1000Litres

Radiator Mild Steel The radiator is made

of a series of long

elongated 1 and 2 inch

steel pipes with fins

that increase the

surface area for heat

exchange.

It is submerged

in a pool

containing

contaminated

water.

25

Fig 4:13D representation of the helical coil heat exchanger

4.2 RESULTS

Table 4.2: Results as obtained from site

Water Flue Gas

Tin Tout Tin Tout

56 0C 116

0C 162

0C 133

0C

580C 117

0C 163

0C 134

0C

58 0C 120

0C 163

0C 134

0C

58 0C 120

0C 166

0C 136

0C

60 0C 124

0C 172

0C 141

0C

620C 138

0C 182

0C 150

0C

64 0C 138

0C 190

0C 154

0C

64 0C 136

0C 195

0C 157

0C

660C 130

0C 200

0C 159

0C

Induced draft fan flow rate = Ṽfg = 24,000Cfm

The IDFan was run at 80 % capacity, hence the flow rate was taken as Ṽfg = 0.8*24,000= 19,200

cfm and at a pressure of about 0.99 Bar.

Water flow rate = Ṽw= 1.783m3/hr

26

Pump Pressure (Gauge) = 2 Bar

4.3 DATA ANALYSIS

4.3.1 AREA CALCULATION

Total inner area (Ai):

2.217 + 4.6071 = 6.8241 𝑚2

Total outer area (Ao):

2.7796 + 5.7761 = 8.5557 𝑚2

4.3.2 SINGLE PHASE ANALYSIS FOR THE HEAT RECOVERY UNIT

The analysis below was done for the heat exchanger in single phase. (i.e. The water did not

change phase)

4.3.2.1 CALCULATION OF TOTAL THERMAL RESISTANCE

From equation 7, the different contributors to the total resistance (RTOT) are as calculated below:

a) Fouling Resistance (boiler feed water fouling resistance)

Taking fouling coefficient16

Rfi for boiler feed water as 0.0002; the fouling resistance is given by:

𝐼𝑛𝑠𝑖𝑑𝑒 𝑓𝑜𝑢𝑙𝑖𝑛𝑔 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = 𝑅𝑓𝑖

𝐴𝑖=

0.0002

6.8241= 2.9308 ∗ 10−5 °𝐶 𝑊

b) Fouling Resistance (flue gases fouling resistance)

Taking fouling coefficient Rfo for flue gases as 0.0002; the fouling resistance is given by:

𝑂𝑢𝑡𝑠𝑖𝑑𝑒 𝑓𝑜𝑢𝑙𝑖𝑛𝑔 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = 𝑅𝑓𝑜

𝐴𝑜=

0.0002

8.5557= 2.3376 ∗ 10−5 °𝐶 𝑊

c) Conductivity Resistance

For steel the thermal conductivity (k) is 16 W/m.K. The conductivity resistance is given by:

𝑅𝑐𝑜𝑛𝑑 = 1

2𝜋𝐿𝑘ln𝑟𝑜𝑟𝑖

= 1

2 ∗ 𝜋 ∗ 81.538 ∗ 16.2ln

16.7 ∗ 10−3

13.32 ∗ 10−3 = 2.7247 ∗ 10−5 °𝐶 𝑊

16

Cengel YA Heat Transfer: A Practical Approach

Inner Helical Coil Outer Helical Coil

Outer Area 2.7796 m2 5.7761 m

2

Inner Area 2.217 m2 4.6071 m

2

27

d) Convective Heat Transfer Resistance (Inside the helical heat exchanger)

Assumptions made in the flow are that the flow rate remains constant.

Velocity of flow of water:

𝑣𝑤 = 𝑉 𝑤

𝑇𝑢𝑏𝑒 𝑐𝑟𝑜𝑠𝑠𝑒𝑐𝑡𝑖𝑜𝑛 𝑎𝑟𝑒𝑎=

1.78299/3600

𝜋 ∗ 0.013322= 0.88856 𝑚/𝑠

Calculating the convective heat transfer coefficient

The convective heat transfer coefficient is a function of Reynolds number and Prandtl number.

The two terms vary with temperature of the fluid flowing in the tube.

Mori and Nakayama17

(1967b) proposed a formula relating the Nusselt number of flow in a

helical heat exchanger with Reynolds number, Prandtl the ratio (D/d). It is given by:

𝑁𝑢𝑐 = 0.023 1 + 0.061

𝑅𝑒 𝑑 𝐷 2.5 1 6 𝑑 𝐷

1

12𝑅𝑒0.833𝑃𝑟0.4

The above equation is valid for:

𝑃𝑟 > 1, 𝑅𝑒(𝑑 𝐷 ) > 0.4

Sample calculation

Taking the temperatures at Tin = 56°C and Tout = 116°C for water:

Mean bulk temperature of the water flowing inside the helical heat exchanger coil:

𝑇𝑀𝐵 = 56 + 116

2= 86℃

Cp at mean bulk temperature is 4204 J/kgK and density at the mean bulk temperature is 968.244

kg/m3.

In order to use the Nusselt number relation developed by Mori and Nakayama (1967b), the

helical heat exchanger can be analyzed as two separate heat exchangers that are arranged in

series.

i. Inner helical coil analysis

An assumed mean bulk temperature of the inner helical heat exchanger coil is given by:

𝑇𝑀𝐵𝐼 = 56 + 86

2= 71℃

17Mori and Nakayama Coronel P., Sandeep K.P.: Heat transfer coefficient in Helical Heat Exchangers under

Turbulent Flow conditions

28

Properties of the water at 71°C: Density (ρ) = 976.9455 Kg/m3, Specific heat capacity at constant

pressure (Cp) = 4191 J/Kg.K, Dynamic viscosity (µ) = 0.0003948 kg/ms, Prandtl number (Pr) =

2.4968, thermal conductivity (k) = 0.6628.

Reynolds number:

𝑅𝑒𝐼 = 𝜌𝑑𝑣𝑤𝜇

= 976.9455 ∗ 0.02664 ∗ 0.88856

0.0003948= 58575.76

The ratio (di/DI)

𝑑𝑖𝐷𝐼

= 0.02664

0.527= 0.05055

Substituting the above values in the relation developed by Mori and Nakayama:

𝑁𝑢𝑐 = 0.023 1 + 0.061

58575.76 0.05055 2.5 1 6 0.05055

1

12 ∗ 58575.760.833 ∗ 2.49680.4

= 250.4163

Convective heat transfer coefficient can be obtained by the relation below:

𝑁𝑢𝑐 =𝑕𝐼𝑑𝑖𝑘

= 250.4163 ∴ 𝑕𝑐 = 6230.329 𝑊/𝑚2℃

ii. Outer helical coil analysis

Mean bulk temperature of the outer helical heat exchanger coil:

𝑇𝑀𝐵𝑂 = 86 + 116

2= 101℃

Properties of the water at 101°C: Density (ρ) = 957.1232 Kg/m3, Specific heat capacity at

constant pressure (Cp) = 4220.4 J/Kg.K, Dynamic viscosity (µ) = 0.0002762 kg/ms, Prandtl

number (Pr) = 1.7107, thermal conductivity (k) = 0.6814.

Reynolds number:

𝑅𝑒𝐼 = 𝜌𝑑𝑣𝑤𝜇

= 957.1232 ∗ 0.02664 ∗ 0.88856

0.0002432= 82029.28

The ratio (di/DO)

𝑑𝑖𝐷𝑂

= 0.02664

1.095= 0.024329

Substituting the above values in the relation developed by Mori and Nakayama18

:

18

Mori and Nakayama adapted from Coronel P., Sandeep K.P.: Heat transfer coefficient in Helical Heat Exchangers

under Turbulent Flow conditions

29

𝑁𝑢𝑐 = 0.023 1 + 0.061

82029.28 0.024329 2.5 1 6 0.024329

1

12 82029.280.833 1.71070.4

= 270.6131


𝑁𝑢𝑐 =𝑕𝑂𝑑𝑖𝑘

= 270.6131 ∴ 𝑕𝑐 = 6921.763 𝑊/𝑚2℃

Adding the inside and outside helical convective heat transfer coefficient inside the tubes we

obtain:

𝑕𝑇𝑂𝑇 = 6230.329 + 6921.763 = 13152.09148 𝑊/𝑚2℃

Convective heat resistance (inside the tubes) is given by:

1

𝐴𝑖𝑕𝑖=

1

6.8241 ∗ 20224.2567= 1.11419 ∗ 10−5

e) Convective Heat Transfer Resistance (Flow over the helical coil heat exchanger)

Assumptions made in the flow over the helical coil heat exchanger are the flow rate of the flue

gases is constant.

Flue gas velocity:

𝑣𝑓𝑔 = 𝑉 𝑓𝑔

𝑇𝑢𝑏𝑒 𝑐𝑟𝑜𝑠𝑠𝑒𝑐𝑡𝑖𝑜𝑛 𝑎𝑟𝑒𝑎=

19200 ∗ 1.699/3600

𝜋 ∗ 0.62= 8.01198 𝑚/𝑠

Consider the diagram below:

Fig 4:2 Flow over in line series of tube banks


30

The maximum velocity for an inline flow as in the case above can be approximated at the point

where the separation distance between the inner and outer helical coil is minimum. Have

maximum velocity calculated as:

𝑣𝑚𝑎𝑥 = 𝑆𝑇

𝑆𝑇 − 𝐷 𝑣𝑓𝑔 =

0.284

0.284 − 0.0334 ∗ 8.01198 = 9.0798 𝑚/𝑠

Calculating the convective heat transfer coefficient

The convective heat transfer coefficient is a function of Reynolds number and Prandtl number.

The two terms vary with temperature of the fluid flowing in the tube.

There is no empirical formula that has been developed for this particular type of helical heat

exchanger. For flow over a helical heat exchanger, it was suggested by Avina19

that the Nusselt

number can be approximated as flow over tube banks and Zukauskas (1987) co-relations can be

used.Zukauskas (1987)20

co-relation for flow over tube banks.

𝑁𝑢 = 𝐹𝑐𝑜𝑟 ∗ 0.033𝑅𝑒0.8𝑃𝑟0.4 𝑃𝑟 𝑃𝑟𝑠

0.25

Fcor for four in line tubes is = 0.9 The correction factor was chosen as four because when the

helical coil heat exchanger is split laterally, it looks like it is the flow over a series of tube banks

made up of four columns.

Sample calculation

Taking the temperatures at Tin = 162°C and Tout = 133°C for the flue gas:

Mean bulk temperature of flue gases flowing over the helical heat exchanger coil:

𝑇𝑀𝐵 = 162 + 133

2= 147.5℃

Flue gas composition varies depending on the chemical and industrial wastes being incinerated.

The average flue gas composition21

is as shown in the table below. The properties at the mean

bulk temperature (147°C) of the flue gas components are as shown below:

19

Kharat Rahul, Nitin Bhardwaj.: Development of heat transfer coefficient co-relation for concentric helical coil heat

exchanger. International Journal of Thermal Sciences 20

Zukauskas (1987) adapted from Cengel Y.A. 2002

21 Source: CTM-034 Analysis of flue gases at a similar incinerator plant owned by ECCL (See Appendix D)

31

Table 4.3: Average flue gas composition and their average properties at the mean bulk

temperature of 147°C

The above flue gas analysis is based on the Conditional Test Methods (CTM-034): Draft method

for the determination of O2, CO, CO2, NO and NO2 for periodic monitoring.

The mean values at the mean bulk temperatures were calculated as shown

below22𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔𝑕𝑡 = (𝑦𝑖𝑀𝑊𝑖) = 28.57

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝐶𝑝 = 𝐶𝑝𝑖𝑀𝑊𝑖𝑦𝑖 𝑀𝑊𝑖𝑦𝑖

= 1019.87 𝐽/𝑘𝑔.𝐾

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑘 = 𝑘𝑖𝑦𝑖 𝑀𝑊

3

𝑖

𝑦𝑖 𝑀𝑊𝑖3

= 0.03387

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝜇 = 𝜇𝑖𝑦𝑖 𝑀𝑊

2

𝑖

𝜇𝑖 𝑀𝑊𝑖2

= 2.6455𝐸 − 05𝑘𝑔/𝑚𝑠

𝑅𝑓𝑙𝑢𝑒 𝑔𝑎𝑠 = 𝑅𝑜

𝑀𝑊𝑓𝑙𝑢𝑒 𝑔𝑎𝑠=

8314.5

29.9609= 291.0024 𝐽/𝑘𝑔𝐾

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 (𝜌)𝑓𝑙𝑢𝑒 𝑔𝑎𝑠 = 𝑃

𝑅𝑓𝑙𝑢𝑒 𝑔𝑎𝑠𝑇=

99000

277.5119 ∗ 455.5= 0.8090 𝑘𝑔/𝑚3

𝑅𝑒 = 𝜌𝐷𝑣

𝜇=

0.7832 ∗ 1.2 ∗ 9.0798

2.36899 𝐸 − 5= 333210.1788

22V. Ganapathy 2003

GAS

%

Volume

Molecular

Weight Cp k µ

Nitrogen 0.8044 28.013 1049.5 0.0355 2.3768E-05

Carbon Dioxide 0.006 44.01 977.10 0.03 2.08000E-05

Oxygen 0.1804 32 954.6 0.04 2.79E-05

Nitric Oxide 0.000047 30.01 1028.06 0.03 1.29E-05

Nitrogen Dioxide 0.000002 46 497.83 0.02 2.25E-05

Argon 0.009 39.948 520.30 2.4133E-02 3.0956E-04

Carbon

Monoxide 0.000064 28.011 1054.40 0.03 2.3510E-05

Trace gases 0.000087

Total 1

Mean values 28.57 1019.87 0.03387 2.6455E-05

32

𝑃𝑟 = 𝜇𝐶𝑝

𝑘=

2.36899 𝐸 − 5 ∗ 1024.4391

0.034623= 0.7964

To obtain the Prandtl number at the tube wall temperature of 162°C (Prs), the same procedure

described above wasas repeated for the flue gas analysis. The Prandtl number was obtained as:

𝑃𝑟𝑠 = 𝜇𝐶𝑝

𝑘=

2.55404E − 05 ∗ 1022.2498

0.033064123= 0.7942

Substituting the above values in the relation developed by Zukauskas:

𝑁𝑢 = 0.9 ∗ 0.033 ∗ 333210.17880.8 ∗ 0.79640.4 0.79640.7942

0.25= 710.7289


𝑁𝑢 =𝑕𝐷

𝑘= 70.4372 ∴ 𝑕𝑐 = 720.879 𝑊/𝑚2℃

Convective heat resistance (flow over the tubes) is given by:

1

𝐴𝑂𝑇𝑕𝑖=

1

8.5557 ∗ 743.6814= 0.0001621 °𝐶 𝑊

Total thermal resistance

From equation 6, the total resistance RTOT is then given by:

2.9308 ∗ 10−5 + 2.3376 ∗ 10−5 + 2.7247 ∗ 10−5 + 1.11419 ∗ 10−5 + 0.0001621

= 2.5321 ∗ 10−4 °𝐶 𝑊

4.3.2.2 CALCULATION OF HEAT EXCHANGER CORRECTION FACTOR

Sample calculation

Using the same temperature range used earlier in the sample calculations: i.e. Flue gases Tin =

162°C, Tout = 133°C; Water Tin = 56°C, Tout = 116°C.

From equation 1 the heat available is given by:

𝑄𝑓𝑔 = 9.06133 ∗ 0.8483 ∗ 1018.9623 ∗ 162 − 133 = 216825.0601

𝜃𝑀 = ∆𝑇1 − ∆𝑇2

𝑙𝑛 ∆𝑇1

∆𝑇2

= 162 − 116 − (133 − 56)

𝑙𝑛 162−116

(133−56)

= 60.17501

From equation 11, the correction factor calculated is:

𝑄𝑓𝑔 =1

𝑅𝑇𝑂𝑇𝐹𝜃𝑀 =

1

2.5321 ∗ 10−4∗ 𝐹 ∗ 60.17501 ∴ 𝐹 = 0.91

Using equation 15, the effectiveness was found to be:

𝜀 = 𝑄𝑤

𝑚 𝑓𝑔 ∗ 𝐶𝑓𝑔 ∗ ∆𝑇𝑀𝐴𝑋=

122048.689

9.06133 ∗ 0.8483 ∗ 1018.9623 ∗ (162 − 56)= 0.14699

33

The table below gives a summary of the heat available, correction factor (F), total thermal

resistance (RTOT) and effectiveness of the heat exchanger at different temperatures.

The average correction factor (F) of the heat exchanger was calculated and found to be = 0.95.

Table 4.4: Table of heat available, correction factor, total thermal resistance and effectiveness

for each set of data obtained from the experiment.

Water Flue Gas Q(fg) Correction

factor(F)

RTOT°C/W ε

Effectiveness

Tin Tout Tin Tout

56 116 162 133 216825.0601 0.91 2.5321 * 10-4

0.14699

58 117 163 134 226673.6867 0.9430 2.4857 * 10-4

0.1461

58 120 163 134 226673.6867 0.97212 2.4849 *10-4

0.1536

58 120 166 136 233237.1419 0.9584 2.4902 * 10-4

0.1501

60 124 172 141 238216.3594 0.9443 2.5001 * 10-4

0.151

62 138 182 150 241086.3342 0.9557 2.5165 * 10-4

0.1706

64 138 190 154 267920.0476 0.9779 2.5284 * 10-4

0.1607

64 136 195 157 280532.3593 0.9526 2.5317 * 10-4

0.151

66 130 200 159 300569.9876 0.9449 2.5451 * 10-4

0.1309

4.4 ENERGY ANALYSIS OF THE HEAT RECOVERED FROM THE

FLUE GASES

Sample calculation (using the same results used above in the sample calculations)

From equation 1 the heat available is given by:

𝑄𝑓𝑔 = 9.06133 ∗ 0.8483 ∗ 1018.9623 ∗ 162 − 133 = 216825 𝑊

Also using equation 1, the heat recovered by the water is:

𝑄𝑤 = 0.000495278 ∗ 968.244 ∗ 4191.6 ∗ 116 − 56 = 122048.689 𝑊

Percentage of heat recovered is:

𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑 = 𝑄𝑤𝑄𝑓𝑔

∗ 100 = 122048.689

227162.1506= 53.7275%

34

Table 4.5: Table of calculated heat available, heat recovered, ∆Tmax and the percentage of heat

recovered.

4.5 THEORETICAL SIMULATIONS

The theoretical simulation done in this section makes an assumption of steady state operation for

an entry temperature of flue gases of 145 °C and 200 °C.

From the results obtained, and using a correction factor of 0.95 theoretical simulations were done

for the heat recovery unit at inlet flue gas temperatures of Tin = 200°C and Tout = 145°C.

Sample calculation

Water Flue Gas Qfg

Qw

%

Recovered

∆Tmax

Tin Tout Tin Tout

56 116 162 133 216825 122049 56.29 106

58 117 163 134 226674 119934 52.91 105

58 120 163 134 226674 126022 55.6 105

58 120 166 136 233237 126022 54.03 108

60 124 172 141 238216 129990 54.56 112

62 138 182 150 241086 154242 63.98 120

64 138 190 154 267920 150735 56.21 126

64 136 195 157 280532 146039 52.06 131

66 130 200 159 300570 150735 51.4547 134

35

Take the inlet flue gas temperature as Tin = 200 °C and Tout = 145 °C (as stated above) and Tin of

water = 20 °C,

Using a similar flue gas analysis as done in table 4.3 and 4.4 and using a mean bulk temperature

of around 70°C,

From equation 1, the heat available from the flue gas stream is given by:

𝑄𝑓𝑔 = 9.06133 ∗ 0.8007 ∗ 1021.6525 ∗ 200 − 145 = 407721.7097 𝐽/𝑠

From equation 4, the Total thermal resistance is:

𝑅𝑇𝑂𝑇 = 2.4713 ∗ 10−4 °𝐶 𝑊

From equation 2, the overall heat transfer between the two fluids is given by:

𝑄 = 𝑈𝐴𝜃𝐶𝑂𝑅 = 𝜃𝐶𝑂𝑅𝑅𝑇𝑂𝑇

Where θCORR is given as shown in equation 9:

𝜃𝐶𝑂𝑅𝑅 = 𝐹𝜃𝑀

Substituting for Q, F = 0.95, and RTOT = 2.4713 * 10-4°C/W. Using a combination of equation 4

and equation 11,

407721.7097 ∗ 2.4713 ∗ 10−4

0.95 = 𝜃𝑀 = 105.42

Using alteration, from equation 10:

𝜃𝑀 = ∆𝑇1 − ∆𝑇2

𝑙𝑛 ∆𝑇1

∆𝑇2

= 106.06 = 200 − 𝑥 − (145 − 20)

𝑙𝑛 200−𝑥

(145−20)

The value x therefore is obtained as 112°C. From the steam table, the corresponding saturation

pressure is 1.53Bar.

Using equation 1, the Energy gained by the water is given by:

𝑄𝑤 = 0.000495378 ∗ 992.7227 ∗ 4161 ∗ 112 − 20 = 188257 𝐽/𝑠

The percentage of heat recovered may be given as:

% 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑 = 𝑄𝑤𝑄𝑓𝑔

∗ 100 = 188257

407722∗ 100 = 46.17%

Table 4.6: Table of calculated heat available, heat recovered, percentage of heat recovered and

corresponding Ps values for Tfgin = 200°C

36

Tin

(Water)

T

out(Water)

Qfg

Qw

%

Recovered

20 112 407722 188257 46.17

25 107 407722 167602 41.61

30 104 407722 151131 37.07

35 99 407722 130589 32.03

40 94 407722 110093 27

45 90 407722 91660 22.5

50 84 407722 69202 17

55 78 407722 46777 11.47

60 70 407722 20326 4.99

The corresponding saturation pressures at feed water inlet temperatures of 20 °C and expected

outlet temperature of 112°C(as obtained from alteration) was found to be 1.53 Bar and 1.3 Bar

for a feed water inlet temperature of 25 °C and expected outlet temperature of 107 °C.

In steady state working conditions, it is expected that the flue gas at entry to the heat exchanger

unit is 700 °C with an outlet temperature of 200°C. The flow rate of the flue gases at full

capacity is taken as 24000 cfm (full capacity) and water flow rate taken to be 3m3/hr. Using

equation 1, the total heat available is therefore:

Taking mfg= 5.3297kg/s Cpg= 1.062kJ/Kg.K Tgin=7000C Tgout = 200

0C

𝑄 = 𝑚 𝐶𝑝∆𝑇 = 5.3297 ∗ 1062 ∗ 500 = 2,831,384𝑊

For the water side;

Taking mw=0.8333Kg/s at 250C h1=104.8*10

3 At 130

0 Ch2 =2720*10

3

𝑄 = 𝑚∆ 𝑕 𝑡 = 0.8333 ∗ 2720 − 104.8 1000 = 2179333.25𝑊

This is 75% of the total energy available.

37

CHAPTER FIVE: DISCUSSION, CONCLUSION AND

RECOMMENDATIONS

5.1 DISCUSSION

A water analysis was done and the total dissolved solids content in the water was found to be

1847.6 mg/l. The proposed method of hardness removal was use of sodium triphosphate.The

quantity to be used wascalculated and found to be 88.74g for 1000 liters of water tobetreated.

Use of hydrazine was considered as an oxygen remover but because of it was highly poisonous

nature, Sodium Sulphite was proposed.Sodium Sulphite is very reactive and reduces oxygen

levels to less than 5ppb. The quantity to be used was calculated and found to be 84.6052g for

every 1000liters of water.

The inlet and outlet water temperatures were obtained and recorded in tables. These values

ranged from 560 to66

0 and 116

0 to 138

0 for the inlet and outlet respectively.

From the results obtainedit is observed that the higher the temperature difference between the

flue gas stream at entry and water at entry, the higher the amount of heat recovered.

For optimum heat recovery, the water temperature at entry should be as low as possible. This is

as evidenced by the graph plotted (Appendix B1).After the water temperature at entry reached

62°C, the heat recovered began to decline even with increase in the heat available for recovery

from the flue gas stream. It is also evidenced by the theoretical simulation graphs that were

plotted (Appendix B3) that the heat recovered is high when the inlet water temperature is low.

The inlet temperature increased due reduced efficiency of cooling by the radiator in the pond.

The radiator evaporated contaminated water when the hot water passed through it leaving it

exposed to the air. This reduced the cooling rate and in effect it hadas discussed above in

reducing the heat recovered and also reducing the outlet temperature of the water (see table 4.2).

The highest temperature of water at the outlet attained was 138°C. It then reduced to 136°C and

finally to 130°C.

From the results, it is also seen that the value of thermal resistance increases with increase in

temperature. With the thermal resistance being larger, the reciprocal, decreases (the product of

overall heat transfer coefficient U and surface area A). This leads therefore to a larger amount of

38

heat recovered. Therefore, for more heat to be recovered, the flue gas stream should be at high

temperatures.

From the results obtained, the water did not change phase to steam. However, with proper

lagging of the chimney, the flue gas has an energy potential of about 2.8 Megawatts. From the

analysis above, the flue gas stream can be used to heat water up to saturated steam conditions.

Further provision may be made for a super heater.

5.2 CONCLUSION

The borehole water was analyzed to determine the salts in solution. From the analysis, awater

treatment method was recommended to enable the experimental analysis to be done. This was to

prevent damage of the helical coil heat exchanger by scaling and corrosion (by dissolved

oxygen). The water treatment method chosen involved dosing the water with 88.74 g of Sodium

Triphosphate for every 1000 liters of water and 84.6 g of Sodium Sulphite for every 1000 liters.

The average correction factor (F) of the heat exchanger was calculated and found to be 0.95.

Thermal resistance and effectiveness (ε) at different flue gas and water temperatures were

calculated using the data obtained experimentally. This is as tabulated in table 4.6. The thermal

resistance was seen to increase with increase in temperature.

The most optimum heat that can be recovered was found to be at a very low water inlet

temperature and at a high flue gas temperature. From the theoretical simulation done(without

change in phase), the most optimum heat that can be recovered with an inlet flue gas temperature

of 200 °C, outlet flue gas temperature of 145°C and water inlet temperature of 20 °C was found

to be 188257 J/s.

For an inlet flue gas temperature of 700°C and outlet flue gas temperature of 200 °C , the flue gas

stream has an energy potential of about 2.8 Megawatts that can be harnessed to heat water to

saturated steam conditions at 2.8 Bar.

A Microsoft Excel workbook has been built to aid in further analysis of this project. Screen shots

of the excel workbook are attached in appendix C.

39

5.3 RECOMMENDATIONS

Good working instruments should be installed for proper pressure, temperature and flow rate

readings to facilitate further experimental analysis.

In the flue gas stream, a pitot static pressure gauge should be installed so as to determine the

pressure drop. A CTM-034 flue gas analyzer should also be installed to determine the flue

gas composition.

The chimney should have a thicker lining of insulation installed. A lot of heat is lost in the

upper regions of the chimney stack by radiation and convection.

Further experimental analysis of the heat recovery unit should be done to determine the

optimal mass flow rates of water, flue gases and corresponding temperature values when the

incinerator plant is in steady state operating conditions (i.e. temperatures for both the water

inlet and flue gas inlet are constant). The results obtained were when the incinerator was

working in transient state (the temperatures did not reach a constant value).

For maximum heat recovery and efficiency, the water inlet temperature should be maintained

at low temperatures of about 25°C-30°C. From the analysis done, the lower the inlet

temperature of the water, the higher the heat recovered Qw. It is proposed that a vacuum

surface condenser be designed and installed. It will make use of a water circuit that will be

connected to the radiator in the cooling pond. It is proposed that the radiator in the cooling

pond have its surface area increased to allow a larger area of evaporation and the

contaminated water pool be completely filled with cold water for faster heat loss.

The borehole contains water with a high content of total dissolved solids (1847.6 mg/l). After

further experimental analysis has been done and it has been established that the project is

fully viable for power generation, it is recommended that Reverse Osmosis to be used. This

is the recommended method of feed water treatment for total dissolved solid contents of

greater than 1000 mg/l (see Appendix A2). This treatment is capable of removing up to 99%

of dissolved solids. This will remove the need for further chemical dosing (except oxygen

scavenging) and filming amines in the condenser.

As described earlier, fouling is a general term that includes any kind of deposition of

extraneous material that appears upon the heat transfer surface during the lifetime of the heat

exchanger. Whatever the cause or exact nature of the deposit, an additional resistance to heat

transfer is introduces and the operational capability of the heat exchanger is correspondingly

40

reduced. If fouling cannot be prevented (as it is for our case) it’s necessary to make some

provisions for its periodic removal. By installing a blower that will occasionally blow out the

soot in the heat exchanger, the amount of fouling will be reduced.

Further academic research, further laboratory experiments, analysis and simulations should

be done to determine more Nusselt number relations for flow over helical tubes in different

configurations. Approximations of Nusselt number are done using relations for flow over

tube banks deloped by Zukauskas if the relation is not available for the particular heat

exchanger (like in this case).

41

REFERENCES

TEXT BOOKS

1. Boyen J.L.: Practical Heat Recovery, John Wiley and Sons Inc, Toronto, 1975.

2. BS 2486-1997: Recommendations for treatment of water for steam boilers and water heaters.

3. Cengel Y. A., BolesM. A: Thermodynamics: An Engineering Approach, (5th

Edition),

McGraw-Hill, 2003

4. Luti F.M., University of Nairobi: Notes on Heat Transfer: An Introduction, Lecture notes

2012.

5. Incropera, Bergman D., Lavine: Fundamentals of Heat and Mass Transfer.

6. RayaproluKumar.: Boilers for power and processes, CRC Press, Boca Raton, FL, 2009.

7. Reiter S.: Industrial and Commercial Heat Recovery Systems, Van Nostrand Reinhold

Company New York, 1983.

8. Shar R., Dusan P. S.: Fundamentals of Heat Exchanger Design, John Wiley and Sons Inc,

New Jersey, 2003.

9. GanapathyV.: Industial Boilers and Heat Recovery Steam Generators, Mercel Dekker Inc,

New York 2003.

10. BarasaW.A., S.K. Ngugi: Designing a Boiler Chimney Heat Recovery System against

Fouling, Nairobi, University of Nairobi 2009.

11. CengelY.A.: Heat Transfer: A practical Approach, (2nd

Edition), McGraw-Hill, 2002

12. . Rajput R. K: Heat and Mass Transfer

INTERNET SOURCES

1. Association of Water Technologies, Technical Manual. Accessed from url:

www.steamforum.com/pictures/water%20treat%20boilers(1).pdf on 1st October 2014.

2. Basics of reverse osmosis- Accessed from urlhttp://puretecwater.com/resources/basics-of-

reverse-osmosis.pdf on 1st October 2014.

3. BCS, Incorporated: Waste Heat Recovery: Technology and Opportunities in U.S. Industry,

U.S. Department of Energy, 2008. Accessed from

urlhttp://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery

.pdf on 1st October 2014.

4. Business and Government Energy Management Divison, Department of Energy Mines and

Resources: Waste Heat Recovery, Ottawa. Accessed from url: http://www.em-

http://www.steamforum.com/pictures/water%20treat%20boilers(1).pdf



http://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf



http://www.em-ea.org/Guide%20Books/book-2/2.8%20Waste%20Heat%20Recovery.pdf

42

ea.org/Guide%20Books/book-2/2.8%20Waste%20Heat%20Recovery.pdf on 1st October

2014.

5. Coronel P., Sandeep K.P.: Heat transfer coefficient in Helical Heat Exchangers under

Turbulent Flow conditions. International Journal of Food Engineering Volume 4 Issue 1

Article 4 2008. Doi: 10.2202/1556-3758.1209, January 2008

6. Council of Industrial Boiler Owners (CIBO): Energy Efficiency Handbook, 1997 accessed

from url:

https://www.eere.energy.gov/manufacturing/technical_assistance/pdfs/steamhandbook.pdf on

6th April 2015.

7. Electricity costs in Kenya accessed from urlhttps://stima.regulusweb.com/ on 6th

April 2015

8. Conditional Test Methods (CTM)-034 accessed from

urlwww.epa.gov/ttemc01/ctm.html/ctm-034.pdf on 6th

April 2015.

9. J.M. Hill (2011) Study of Low-Grade Waste Heat Recovery and Energy Transportation

Systems In Industrial Applications (A thesis). Tuscaloosa, University of Alabama. Accessed

from url:

http://acumen.lib.ua.edu/content/u0015/0000001/0000628/u0015_0000001_0000628.pdf on

1st October 2014.

10. Kharat Rahul, Nitin Bhardwaj.: Development of heat transfer coefficient co-relation for

concentric helical coil heat exchanger. International Journal of Thermal Sciences, 48, 2009,

2300-2308. Accessed from url: http://aaqr.org/VOL14_No3_April2014/23_AAQR-13-01-

OA-0028_840-849.pdf on 2nd March 2015.

11. Shi Chang WU, Young Min Jo: Effect of Heat Recovery from flue gas on local humidity and

NOx dispersal in a thermal power station, Taiwan Association for Aerosol Research,doi:

10.4209/aaqr.2013.01.0028, 2013.

https://www.eere.energy.gov/manufacturing/technical_assistance/pdfs/steamhandbook.pdf

https://stima.regulusweb.com/

http://www.epa.gov/ttemc01/ctm.html/ctm-034.pdf

http://acumen.lib.ua.edu/content/u0015/0000001/0000628/u0015_0000001_0000628.pdf

http://aaqr.org/VOL14_No3_April2014/23_AAQR-13-01-OA-0028_840-849.pdf%20on%202nd%20March%202015

http://aaqr.org/VOL14_No3_April2014/23_AAQR-13-01-OA-0028_840-849.pdf%20on%202nd%20March%202015

43

APPENDICES

APPENDIX A: FEED WATER TREATMENT

Appendix A1: Recommended feed water characteristics for non fired water boilers.

Parameter Unit Recommended

Standard Value

Feed water at economizer inlet

Total hardness mg/kg CaCO3 Not Detectable

pH value at 25°C pH scale 9.3 to 9.8

Oxygen O2 max. mg/l 0.02

Iron and copper and nickel max. mg/l 0.05

Boiler water (Chemicals Added)

Phosphate P04 mg/l 20 to 40

Caustic alkalinity mg/l CaCO3 25 to 50

Silica SiO2 mg/l 20

Oxygen scavenger

Sodium sulfite mg/kg Na2SO3

or

Hydrazine mg/kgN2H4

or

DEHA mg/kg

or

Tannin or

Iso ascorbic acid

mg/l

mg/l

mg/l

15 to 20

0.1 to 0.5

0.1 to 0.25

in feed water

120 to 160

15 to 30

Dissolved solids max mg/kg 1000

Conductivity µS/cm 2000

Table adapted from BS: 2486-1997

44

Appendix A2:Flow chart for the selection of the most appropriate water treatment method

Flow chart adapted from BS: 2486-1997

45

APPENDIX B: GRAPHS


temperature

0

50000

100000

150000

200000

250000

300000

350000

54 56 58 60 62 64 66 68

He

at (

J/s)

Inlet Temperature °C

Graph of the heat available and heat recovered against inlet water temperature

Heat available

Heat recovered

46


temperature

0

50000

100000

150000

200000

250000

300000

350000

0 5 10 15 20 25 30 35 40 45

He

at r

eco

vere

d (

j/s)

Flue gas inlet and outlet temperature difference

Graph of variation of heat recovered against temperature difference of flue gases

47

Appendix B3: Graph showing the theoretical simulation of variation of heat recovered

against inlet water temperature at Tin = 200°C and Tout = 145°C

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

0 10 20 30 40 50 60 70

He

at R

eco

vere

d (

J/s)

Inlet Temperature °C

Graph of heat recovered against inlet water temperature at flue gas T in = 200 ° C

48

APPENDIX C: MICROSOFT EXCEL WORKBOOK SCREENSHOTS

USED IN THE CALCULATIONS.

Appendix C1: Calculation of the convective heat transfer coefficient of the flue gases and

the corresponding thermal resistance.

49

Appendix C2: Calculation of the convective heat transfer coefficient of the water and the

corresponding thermal resistance.

50

Appendix C3: Calculation of effectiveness, fouling factor, total heat available, total heat

recovered and percentage of heat recovered.

51

APPENDIX D: CTM 034 ANALYSIS OF FLUE GASES

The CTM 034 analysis of the flue gas stream shown was obtained from another plant run by the

same company. An assumption was made that the flue gas stream produced by this plant is

similar to the plant where the project was carried out

Appendix D1: CTM 034 analysis showing carbon dioxide concentration in the flue gas

stream

PERIODIC MONITORING REPORT

Facility Name & Address Phone

ECCL LTD –INCINERATOR Emission Point: AFTER I.D.

ECCL

72828-00200 , NAIROBI

020-551878

Analyzer make & model: TESTO 350 Serial # 02510852(9)

Calibration Gas Verification Information Calibration Gas Info.(manufacturer,expiration,etc.)

Gas Type

O2 % CO2 NO ppm NO2 ppm

Concent. 21 12 47 2

MEASUREMENT CYCLE(circle measurement task below). Pre-test Verification / Repeatability / Source Test / Post-Test Verification

(zero,span,interference) (once per five days) (_______________) (zero,span,interference)

Three Phases

23/9/2014 Analyzer Response Stack Temp

Flow Rate

Start time: 01:16:10 pm O2 %

CO2 % NO ppm

NO2 ppm °C l/min

RAMP-UP

Phase

t1 17.95 0.67 47 2 167.3 0.57

t2 17.95 0.61 47 2 167.4 0.57

t3 17.97 0.66 47 2 168.7 0.57

t4 17.99 0.61 47 2 169.5 0.57

t5 18.03 0.62 46 2 169.7 0.57

TEST DATA Phase

t5:15 18.04 0.62 47 2 170 0.57

t5:30 18.05 0.63 46 2 170 0.57

t5:45 18.05 0.63 47 2 170.9 0.57

t6:00 18.06 0.63 47 2 171.1 0.57

t6:15 18.04 0.64 47 2 172.3 0.57

t6:30 18.03 0.65 47 2 172.7 0.57

t6:45 18.02 0.65 47 2 173.2 0.57

t7:00 18.04 0.66 47 2 173.8 0.57

Mean Average Concentration(sum of t5:15 through t7:00 ÷8) 18.04 0.6 46.9 2 171.8 0.57

Mean Deviation(no single reading exceeds± 2% of mean average)

52

Acceptable "Test Data Phase" Yes or No

Yes or No

Yes or No

Yes or No

REFRESH Phase

t7 18.06 0.65 46 2 173.9 0.57

t8 18.07 0.66 45 2 174.1 0.57

t9 18.07 0.65 45 2 174.1 0.57

t10 18.06 0.66 45 2 174.6 0.57

t11 18.05 0.66 45 2 175 0.57

t12 18.03 0.65 45 2 175.4 0.57

t13 18.03 0.65 46 2 175.5 0.57

t14 17.98 0.64 46 2 175.5 0.57

t15 17.95 0.64 48 2 176.1 0.57

Stop Time : 1:16:39 pm

Appendix D2: CTM 034 analysis showing carbon monoxide concentration in flue gas

stream

PERIODIC MONITORING REPORT

Facility Name & Address Phone

ECCL LTD -INCINERATOR Emission Point: AFTER I.D.

ECCL

72828-00200 , NAIROBI

020-551878

Analyzer make & model: TESTO 350 Serial # 02510852(9)

Calibration Gas Verification Information Calibration Gas Info.(manufacturer,expiration,etc.)

Gas Type O2 % CO ppm NO ppm NO2 ppm

Concent.

MEASUREMENT CYCLE(circle measurement task below). Pre-test Verification / Repeatability / Source Test / Post-Test Verification

(zero,span,interference) (once per five days) (_______________) (zero,span,interference)

Three Phases

23/9/2014 Analyzer Response Stack Temp

Flow Rate

Start time: 01:16:10 pm O2 % CO ppm NO ppm

NO2 ppm °C l/min

RAMP-UP

Phase

t1 17.95 60 47 2 167.3 0.57

t2 17.95 60 47 2 167.4 0.57

t3 17.97 61 47 2 168.7 0.57

t4 17.99 61 47 2 169.5 0.57

t5 18.03 62 46 2 169.7 0.57

TEST DATA Phase

t5:15 18.04 62 47 2 170 0.57

t5:30 18.05 62 46 2 170 0.57

t5:45 18.05 63 47 2 170.9 0.57

53

t6:00 18.06 63 47 2 171.1 0.57

t6:15 18.04 64 47 2 172.3 0.57

t6:30 18.03 65 47 2 172.7 0.57

t6:45 18.02 65 47 2 173.2 0.57

t7:00 18.04 66 47 2 173.8 0.57

Mean Average Concentration(sum of t5:15 through t7:00 ÷8) 18.04 63.8 46.9 2 171.8 0.57

Mean Deviation(no single reading exceeds± 2% of mean average)

Acceptable "Test Data Phase" Yes or No

Yes or No

Yes or No

Yes or No

REFRESH Phase

t7 18.06 65 46 2 173.9 0.57

t8 18.07 66 45 2 174.1 0.57

t9 18.07 65 45 2 174.1 0.57

t10 18.06 66 45 2 174.6 0.57

t11 18.05 66 45 2 175 0.57

t12 18.03 65 45 2 175.4 0.57

t13 18.03 65 46 2 175.5 0.57

t14 17.98 64 46 2 175.5 0.57

t15 17.95 64 48 2 176.1 0.57

Stop Time : 1:16:39 pm

project code: pmo 01/2015 project title:...

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