Design Project Isaac Final

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Design projectTask 3: Individual

design of wasteheat boiler

Department: Chemical & Environmental Engineering

Name: Isaac Mohanadasan

Student ID:

Group 2

Date due: 12/3/2011

Date Submitted: 15/3/2011


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Contents

Summary

Introduction

Design basis and objectives

Design constraints

Existing designs

Operating conditions and distributions

Material selection

Design methodology and steps

Process control

Safety and maintenance

Start up and shutdown Procedures

Economics

Equipment drawing

Equipment specification

Appendix A: References

Appendix B: Design equations

Appendix C: Excel calculations

Appendix D: Mass and energy balances

Appendix E: correlations


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Introduction

Energy conservation and its optimum utilisation are of key importance to all plant design.

The aim of all plant designers is to maximise the use of all available energy to reduce

operating costs and increase potential revenue. Our design group objectives are simply tomaximise profits by minimising incurred costs and manufacturing more sellable products. In

our sulphuric acid manufacturing process, a sulphur burner aids in producing sulphur dioxide,

a raw material required for sulphuric acid manufacture. The conversion of sulphur to sulphur

dioxide is highly exothermic, and the process stream has to be cooled before it meets the

necessary conditions required to enter the sulphur trioxide converter. The thermal energy

released during this cooling process, known as waste heat, is proposed to be used to produce

sellable steam thus generating profit. The production of steam takes place in a waste-heat

boiler and may be used to generate electricity. I have been asked to design a waste-heat boiler

which meets the design group and clients requirements. This report outlines the design

objectives and constraints and provides an idea on the design consideration and methodology

used for the waste heat boiler. The design procedure is broken up into four stages, which

include; materials of construction, thermal design, mechanical design and an

evaluation/optimisation of the design. Other design considerations have been included in the

report such as safety, economic, control and maintenance.

Design objectives and constraints

The overall design objective of the sulphuric acid manufacturing plant is to maximise profits

by producing more sellable products while reducing capital and operating costs. The wasteheat boiler must abide by the overall design objective. To achieve the main objective, there

must be an efficient transfer of thermal energy between the process flue gas and water within

the waste-heat boiler. Effective heat transfer leads to the production of more steam which

increases profits. In addition, the mechanical design of the process equipment must be simple

as possible and easy to maintain. Low maintenance and simplicity reduce both capital and

operating costs. Furthermore, the material of construction, and boiler itself, must be able to

withstand its desired design life. The process should be relatively easy to control, which

reduce operating costs.

The design is constrained by several factors both externally and internally. Externalconstraints are fixed, in the sense that the design must comply with them. Externally, the

design must comply with Government control, economics & resources available.

Governments have imposed emission standards which the plant and therefore the waste heat

boiler must conform to. The economics and resources available heavily influence the design

of any piece of equipment. Mechanical codes and standards are imposed to standardise

equipment sizing, for ease of manufacturing. Physical and chemical laws aid in producing a

reliable design which can withstand its design life. Internal constraints are less rigidand

include the material of construction, choice of process and its conditions. As a designer, we

must meet the requirements of the client whilst abiding by any constraints imposed.


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pressure across the shell drops as the processes gas flows through. There is a steady decrease

in pressure across the tubes as the water flows through. Careful considerations need to be

made for efficient operations. For instance, the operating temperature should not fall below

the dew points of the gases, as this reduces heat transfer area which decreases the production

of steam. Too high temperatures will lead to failures within the structure of the equipment asmechanical properties deteriorate. High temperatures may promote scaling and fouling all of

which reduce thermal efficiencies. Pressures within the boiler affect the stresses applied to

equipment and flow rates of fluids exiting the boiler. Distribution of heat can be considered to

be roughly uniform as baffles ensure the process gas flows evenly across the tubes within the

shell. Hence, temperature and pressure boundaries are defined.

Material Selection

The governing factor in material selection is the ability of said material to resist corrosion.

In addition, mechanical & thermal properties, fabrications, availability and cost all need to beconsidered carefully for the correct selection of construction materials. The most economical

material that satisfies both process and mechanical requirements should be selected. This

will be the material that gives the lowest cost over the working life of the plant, allowing for

maintenance and replacement. Its selection directly affects operating, capital and

maintenance costs and as a result must be chosen with caution.

Steels are the most common materials used for equipment design. This is because of its high

durability, availability, thermal properties and costs. Carbon steel is the cheapest form

available. It has a higher thermal conductivity and strength compared to other forms of steel.

However, the process gas consists of carbon dioxide, nitrogen, oxygen and sulphur dioxide

entering the boiler at a temperature of 938.96oC. Carbon steels heated for prolonged periods

at temperatures above 455C may be subject to carbon segregation, which is then

transformed into graphite (Ref. E5, p. 23.5). When this occurs the structural strength of the

steel is reduced. In addition, oxygen in the process gas causes the carbon steel surface to

oxidise, which decrease thermal transfer efficiencies. Even though the process gas is non-

corrosive, any leakage within the tube side will create an environment that produces sulphuric

acid. Carbon steel has a low resistance to corrosion, which indicates that it is not ideal for our

design. Studies have found that an increase of chromium content within steel allow for

operating temperatures to be more flexible. According to literature, an alloy of 14% to 18%chromium extends the service to 850C

[5]. Our process gas stream enters the boiler at

938.96oC. As a result, steel containing 26% chromium is selected as the material of

construction, known as stainless steel grade 310. Amongst its flexibility and stability at high

temperatures, there are many other benefits of its use. The high chromium content ensures

resistance to oxidations and surface corrosion. According to literature, the material has a

Good resistance to oxidation in intermittent service in air at temperatures up to 1040C and

1150C in continuous service.it has a Good resistance to thermal fatigue and cyclic heating.

Widely used where sulphur dioxide gas is encountered at elevated temperature[6]s.

In

addition, its high youngs modulus and tensile strength insure the stability of the equipment

design. It has a good resistance to scaling, which greatly improves the thermal efficiencies of


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the boiler. Grade 310, combining excellent high temperature properties with good ductility

and weldability, is designed for high temperature service. It resists oxidation in continuous

service at temperatures up to 1150C[7]. Therefore, mechanically, the material can easily be

welded for ease of fabrications. Steel 310 has a similar thermal expansion coefficient to

carbon steel, and is relatively low which is beneficial. However, a compromise has to bemade as its thermal conductivity is lower than that of carbon steel. Nevertheless, in

comparison to other more suitable materials it has one the highest thermal conductivity. It is

readily available and is relatively cheap. Hence, one can conclude that all the material

requirements have been fulfilled. As a result, stainless steel 310 has been selected as the

material of construction for both the shell and tube.

Design Methodology & Steps

The design selected is a 1-2 shell and tube heat exchanger in which the exit water/steam

composition within the tubes is connected to a steam drum. The heat exchanger selected is an

open floating head heat exchanger to allow for thermal expansions. The number of tube

passes was set as 2 for a greater heat transfer coefficient. A shell and tube heat exchanger was

selected due to its simplicity and versatility. It provides a large surface area within a small

volume. The benefit of a floating head exchanger is that the tubes can be easily removed for

cleaning. In addition, a clamp ring-type internal floating-head exchanger is selected. This

split-flange design reduces the large clearances otherwise needed in other internal floating-

head exchangers.

Before any numbers can be calculated we must first understand the fundamental parameters

that affect the design of the waste heat boiler and thus thermal transfer efficiencies. High flow

rates, fluid velocities and viscosities all ensure the flow regimes within the boiler are

turbulent. Turbulent flow improves thermal transfer efficiencies. However, higher pressure

drop occur. Higher pressure drops incur additional costs of operating as the pumps have to

work harder. In addition, higher velocities increase the rate of erosion of the metal surface. A

benefit is that particles from the burner, such as ash, cannot settle as easily. As a result

phenomenon such as fouling and scaling are reduced. Water has a great chance of causing

fouling the tube surface; hence its velocity is kept high. Common tube velocities are between

the range of 1-3 m/s. The velocities form the basis of the design as most of the critical

parameters are in one way or another related to it. To begin the design process, an estimate of

the overall heat transfer coefficient needs to be made. For the purpose of my design, I have

used an initial value of 100w/m.k. This estimation helps us determine the internal

configuration of the boiler.

The next part of the design involves the selection of the tube diameter and length. In order to

select the optimum tube dimensions, tube lengths were varied at fixed diameters until a fluid

velocity of 1 was found. The obtained values are shown below:


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ID (mm) L(m) V (m/s)

Pressure

drop (Pa)

number

of tubes

10 9 1.03 3888054.55 1897.175

15 15 1.03 4366357.75 878.48

20 21 1.03 4536846.62 496.7388

25 27 1.03 4613501.62 319.73

30 33 1.02 4653191.98 223.12

As you can see from the table above, as the diameter increases, the number of tubes required

decreases. On the other hand, the lengths of the tubes increase with diameter in order to

maintain the same fluid velocity. In addition, pressure drop also increases with diameter.

Hence, a compromise needs to be made between the properties above to obtain an optimum

value. Larger tubes will generally result in a lower cost exchanger[9], as it reduces shell

diameter. However, it increases pump requirements. As a result, a tube internal diameter of

25mm (1inch) is selected with a length of 27m based on optimum selection. Once the tubedimensions are determined, we are then able to determine other internal components. Tube

arrangement was selected as triangular as it is the best arrangement for maximum heat

transfer. It does however; increase the pressure drop on the shell side. As a rule of thumb, the

tube pitch is 1.25 time the outer diameter of the tubes. Hence the outer diameters of the tubes

are computed as low as possible to ensure the size of the shell is at its optimum. All

thicknesses in the design include a 2mm corrosion allowance. Baffles play an important role

in the heat transfer process as they direct the process gas flow path. The also provide support

for the tubes within the shell. Hence, the spacing of the baffles along the length is of

importance. According to literature, the optimum spacing is 1/5th

of the shell diameter with a

baffle cut of 25%. These values have been used in the calculations.

The next part of the design is the re-evaluation of the estimated overall thermal coefficient.

Both the kern and bell methods have been compiled and used for the calculations. This is

because the kern method is inaccurate for calculating the shell-side heat transfer coefficient

and shell-side pressure drop[8]. This is because the method does not take into consideration

bypass and leakage factors. These methods are based on experimental work with standard

tolerances. They give reasonable results in most cases and are therefore applied here. A

preliminary estimation was made in order to obtain geometrical data of the equipment to

apply these method. Based on the estimated thermal conductivity, a new calculated thermal

conductivity is derived from the selected methods. These methods consider the tube side and

shell side thermal coefficients independently and equate them together to obtain the overall

thermal coefficient. The estimated value is then re-evaluated until the estimate equals the

value obtained by the kern method. Geometry of the equipment is changed for optimum

sizing and to increase surface area of the tubes, if it is required. Details of the calculations

have been included in the appendix.

The overall thermal coefficient calculated was 196.21w/m.k. The pressure drop on the tube

side was computed as 802441.3 pa. The pressure drop on the shell side was found to be25166081 pa which all seems reasonable. Since the bell method is more accurate for


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determining thermal coefficients, its overall thermal coefficient is used for the design. The

steam drum sits on the top of the heat exchanger, and is a relatively simple design. Details of

the calculations used are shown in the appendix.

Mechanical designThe sizing of the equipment was done in accordance with British codes and standards which

include the following:

BS 3274 (1960): Tubular Heat Exchangers for General Purposes.

BS 3606 (1978): Specification for Shell Tubes in Heat Exchangers.

BS 5500 (1985): Unfired Fusion Welded Pressure Vessels.

Maintenance

If water quality is not good, it may cause the tubes to degrade gradually. This will reduce the design

life of the equipment. As a result, the water must be Deaerated in order to reduce the oxygen content

within the water. Deposition, such as ash, from the gas stream coming from the burner needs to be

handled as it will reduce heat transfer areas thus reducing thermal transfer efficiencies. It is proposed

that a boiler blow down system is used to remove any particulate matter and condensate from the shell

side. The floating head design allows the tubes to be removed for cleaning. Erosion is not considered

as the material of construction is erosion resistant and fluid velocities are within the allowable range..

Start up and shut down procedures

The boiler must initially be preheated so that the material properties within the boiler are

stable. The drum and water feed valves have to be fully open initially so that the tubes are not

empty. The heating of empty tubes may be hazardous. The process gas valve is the last valve

that should be opened. The liquid hold-up volume within the steam drum acts as a time

constraint. To shut down the boiler, the process gas valve must be closed first, to avoid

heating of empty tubes. Next, the feed water, and finally the steam drum valve must be closed

in that order.

Safety considerations

Several factors need to be considered for a successful and safe design. Firstly all parameters must be

controlled with the appropriate monitoring and control mechanisms. If the process gas flow rate was

higher than its design value, then there would be a pressure build-up within the boiler system. More

steam is produced which increase the pressure demands of the steam drum. This could lead to material

failure if not properly controlled. In addition, higher water flow rates will reduce the temperature of

the process gas making its conditions undesirable for the converter. Hence water flow rates need to be

monitored carefully. Any leakages along the tube side could potentially be catastrophic as acid will be

produced from the combination of process gas and water. It is proposed that pigs be used to monitor

the inner surface of the tubes. Pigs are devices that flow through pipes and monitor their integrity.

However, a pig catcher needs to be installed, adding to capital costs. In the event of a tube leakage,the process gas must be by-passed.


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Process control

According to literature, the most effective type of control is a PID controller. Hence, this control has

been used in the design to regulate parameters. What it does is basically restores any deviated variable

back to its set point. It has been employed to control the flow rates of the water and process gas. Flow

rates directly affect other parameters such as temperature and pressures. The exit process gas must be

at a temperature suitable for the converter. Hence temperature control has been used in the design by

manipulating flow rates. The following diagram shows the control mechanisms of the design:

Where the exit steam enters a steam drum.

Costing

Costing was determined using plant design equations as follows.

[ ]

*+2]

Cost of the equipment was found to be


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References

1. Coulson-Richardson-s-Chemical-Engineering-Vol-6 design2. Equitment description of componets & design considerations3. Types control and maitainance evaluation components page 24. Design calculation and samples p1985. Design calculation and samples p1986. http://www.azom.com/article.aspx?ArticleID=9667. http://www.azom.com/article.aspx?ArticleID=9668.http://www.engr.iupui.edu/me/courses/shellandtube 9. Coulson-Richardson-s-Chemical-Engineering-Vol-6 design
http://www.azom.com/article.aspx?ArticleID=966http://www.azom.com/article.aspx?ArticleID=966http://www.engr.iupui.edu/me/courses/shellandtubehttp://www.engr.iupui.edu/me/courses/shellandtubehttp://www.engr.iupui.edu/me/courses/shellandtubehttp://www.engr.iupui.edu/me/courses/shellandtubehttp://www.azom.com/article.aspx?ArticleID=966http://www.azom.com/article.aspx?ArticleID=966


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Appendix: Calculations

For the purpose of these calculations, I have assumed the flowing

Water is vaporises within the tubes The process gas has the same properties as that of air

The energy evolved from the process gas:

Where 1 represents properties of the process gas and 2 is that of the water

From the above equation, the mass of the water can be calculated

Calculation of mean temperature difference

For counter current flow, log mean temperature difference is

The log mean temperature difference is corrected by the correlations given in appendix B

Heat transfer area

Where the overall heat transfers coefficient is estimated a 100w/m.k for determination of equitment

dimensions.

Based on trial and error method for determining the most effective dimensions of the tubes, a tube

with an internal diameter of an inch and a length of 27 was selected

Number of tubes:

Tube cross sectional area

=

Volumetric flow rates:


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Tube side velocity

Tube side velocity ( The tube bundle Diameter:

Shell clearance:

Area of shell:

tube pitch =1.25

baffle spacing =

From the area of the shell we can then calculate the the velocity as follows;

Tube side heat transfer coefficient

(kf/di)

Shell side heat transfer coefficient (kern method)

( )


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Shell side heat transfer coefficient (Bell method)

hs = hoc Fn Fw Fb FL

where hs heat transfer coefficient calculated for cross-flow over an ideal tube bank,

no leakage or bypassing.

Fn = correction factor to allow for the effect of the number of vertical tube rows,

Fw = window effect correction factor

Fb = bypass stream correction factor,FL = leakage correction factor.

Overall heat transfer coefficient:

Tube side pressure drop:

[ ] Shell side pressure drop: (Kerns method),

Shell side pressure drop: (bell method)

Steam drum design

Settling velocity:


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Volumetric flow rates

= Lv/Dv ratio was at 4 based on the outlet steam pressure

Vapour velocity:

Vapour residence time:

=

Assuming real time is the same as actual time, we can calculate the diameter of the vessel and hence

the length.


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Excel calculations

Mechanical design of waste heat boiler

Shell side

Variables

Flue gas inlet pressure

Flue gas inlet temperature

Shell thickness

Pressure 0.011 N/mm2

Dsi 616.0714269 mmStress factor 515 na

Joint factor 0.8 na

Thickness 0.008224364 mm

with

corrosion 2.008224 mm

Tube side


Ds 25 mm

Stress factor 515 na

Joint factor 0.8 na

tube thickness 0.01167537

with


Steam drum


Ds 1934.280457 mm

Stress factor 0.515 na

Joint factor 0.8 na

Shell thickness 615.9619323

with


Physical and chemical data

Process gas

Propertise inlet mean outlet units


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temperature 938 677.5 417 c

cp 1.1427 1.10885 1.075 kj/kgmol.k

k 0.068 0.06018 0.05236 w/m.c

density 0.34 0.425 0.51 kg/m3

viscosity 0.000042 0.00003766 0.00003332 pa.s

water propertise

Propertise inlet mean outlet units

temperature 25 133.5 242 c

cp 4.181322 3.999261 3.8172 kj/kgmol.k

k 0.607 0.328175 0.04935 w/m.c

density 997.13 507.315 17.5 kg/m3viscosity 0.00089 0.0004536 0.0000172 pa.s

energy calculations

Mass flow rate of gas 61.34451514 Kg/s

heat evolved 35439.39198 kj/s

Mass of water 40.83633712 kg/s

Overall heat transfer coefficient estimate

U 196 w/m2C

Mean temperature difference

log Mean temperature 450.2489347 C

Heat transfer area

A 401.585 m2

Number of tubes And tube sizes

tube OD 29.02335 mm

Tube ID 25 mm i

Length 27 m

Number of tubes 163.128228

Tube cross area 0.000490859 m2

area per pass 0.04003651 m2


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Tube side velocity

Volumetric flow 0.080495032 m3/s

tube side velocity 2.010540677 m/s

Bundle and shell diameter

Nt 163.1282

db 548.0714 mm Diameter of tube bundle

K 0.249

Shell ID 616.0714 mm

Area of shell

tube pitch 36.27918843 mm

baffle spacing 123.2142854 mm

Ds 616.0714269 mmdo 29.02335074 mm

Area of shell 0.01518176 m2

Mass flow rate gas 220840.2545 Kg/hr input

Mass flowrate/s 61.34451514

Gs 4040.672139

Vs 9507.463856 m/s

Tube side heat coifficient

Kern

Re 65262.71

Pr 0.005528

di 25

L/di 1080

Jh 0.0035 Input

hi 539.4351 w/m2c

Shell side heat transfer coificient

Re 2211107.101

equialent diameter 20.60803019 mmPr 0.000693906

Jh 0.007 input

h0 4.099707088 w/m2c

Overall heat rransfer coificient

thermal conductivity of steel 310 18.7 w/mC

1/u 0.246189459

fi 0.0002 Input

fo 0.0001

U 4.061912342 -0.97928


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pressure drop tube side

Number of passes 2

Jf 0.045 input

pressure drop 802441.3121 pa

Pressure drop shell side

Jf 0.025 input

pressure drop 25166080.57 pa

steam drum

settling velocity 0.523733138 m/s

without pad 0.078559971 m/s

vapor vol flow rate 3.505400865 m3/s

selected Lv/Dv ratio 4

Lv Lv = 0.5Dv

DV

cross sectional area 0.393 Dv^2

vapour velocity 8.919595077 Dv^-2

vapour res time 1.677852349 Dv

actual res time 0.448450851 Dv^3

Dv 1.934280457 m

Liquid hold up 0.000648196 m3/s

liq cross sec 1.469217069 m2

length 7.73712183 2

hold up vol 11.36751145 m3/s

hold up time 17537.15617 s

Bells method calculations

crossflow area 2.634740615 m2

shell side mass

velocity 23.28294284 Kg/sm2

equivalent diameter 20.60803019 mm

Mean temp 677.5 C

density 0.425 kg/m3

Kf 0.06018 w/mk

Re 618240.649

Pr 0.000693906

Baffle

cut 0.25

Jh 0.002 input

ho 327.5160302


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CORRECTION FACTOR Tube row

Vertical tube pitch 31.56289393 mm

Baffle height 154.0178567 mm

height between baf 308.0357135

number of res cross 9.75942555

Fn 1 find

CORRECTION FACTOR window

height of baffle cord -42.75446418

ra 0.14

Tubes in one window 22.83795192

Tubes in cross flow 117.4523241

Rw 0.28

Fw 1.08

CORRECTION FACTOR bypass

Ab 0.008378571

ratio 0.003180037

Fb 1

1

Hs 353.7173126

Overall heat transfer coieffiecent

1/U 0.00509671

U 196.2050128 0.001045983

Pipe sizing

Designed so fluid velocity is between 1-2 m/s

ID (mm) L(m) V Pressuredrop number oftubes

10 9 1.03 3888054.55 1897.175

15 15 1.03 4366357.75 878.48

20 21 1.03 4536846.62 496.7388

25 27 1.03 4613501.62 319.73

30 33 1.02 4653191.98 223.12

tubes selected are of 1 inch diamter and a length of 21


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Mass balance


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Energy balance


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Design Project Isaac Final