heat exchanger and boilers - energy conversion guide

8/6/2019 Heat Exchanger and Boilers - Energy Conversion Guide

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1 4P570 Energy Conversion 2 October 2003

2 October 2003

As a result of the course

4P470 Energy Conversion

Heat exchangers and Boilers

Thijs PaesLiselotte Verhoeven

Gert WitvoetSurjo Adabi

Johan Kunnen

TU/e: click here to visit our website


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Table of Contents

CHAPTER 1 INTRODUCTION 4

1.1 HEAT EXCHANGERS 41.2 DOUBLE PIPE HEAT EXCHANGERS 51.3 SHELL-AND-TUBE HEAT EXCHANGERS 7

CHAPTER 2 HEAT EXCHANGER DESIGN 10

2.1 INTRODUCTION 102.2 THE DOUBLE-PIPE HEAT EXCHANGER 102.3 SHELL-AND-TUBE HEAT EXCHANGER 13

CHAPTER 3 FOULING OF HEAT CHANGERS 18

3.1 INTRODUCTION 183.2 DESCRIPTION OF THE PHENOMENON 183.3 FOULING, AN OVERVIEW 183.4 INFLUENCES ON FOULING 213.5 THE IMPORTANCE OF FOULING 233.6 FOULING SOLUTIONS IN PRACTICE 243.6.1 AUTOMATIC TUBE BRUSHING (ATB) 243.6.2 THE SPIRAL HEAT EXCHANGER 263.6.3 DEPOSIT DETERMINED, FOULING REDUCING MORPHOLOGY (DDEFORM) 263.7 IN CONCLUSION 27

CHAPTER 4 BOILERS 28

4.1 TYPE OF BOILERS 284.1.1 THE CONSTRUCTION 284.1.1.1 The fire tube boiler 284.1.1.2 The Water Tube Boiler 294.1.2 ONCE-THROUGH BOILERS 304.3 ENERGY SOURCE 314.4 MATERIAL 314.4 PRESSURE DROP INSIDE THE TUBE 324.4.1 FRICTION 324.4.2 ACCELERATION 334.4.3 HYDROSTATIC HEAD 334.5 MAINTENANCE 344.5.1 SCALING AND SLUDGE 344.5.2 Mechanism of scale formation 34


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CHAPTER 5 AN INDUSTRIAL BOILER IN PRACTICE 36

5.1 AN POWER PLANT BOILER 365.2 BOILERS AT THE TUE 37

5.3 EMISSIONS 385.3.1 REGULATIONS 385.3.2 NOX-EMISSION 395.3.2.1 NOx-reduction strategies 395.3.2.2 NOx-reduction measures 405.3.3 SO2-EMISSION 415.3.3.1 Wet scrubbers 415.3.3.2 Dry scrubber 42

LITERATURE 43


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Chapter 1 Introduction

By Thijs Paes

This lecture deals with the practical use and applications of heat exchangers and boilers. These twoappliances have much in common, since heat exchangers are vital parts of boilers, and in addition,boilers are the oldest and still one of the most common applications of industrial heat exchangers[1.1].These two devices will be discussed separately, although. After an exposition of all different typesof heat exchangers and examples of their usage, an overview about what has to be dealt with whenone of these types needs to be installed and how to maintain it, will follow. After this, the samewill be done for boilers.

1.1 Heat Exchangers

Heat exchangers can be described by reversing its two terms: they include all devices that aredesigned for exchanging heat. This is a very broad category of devices so first a restriction has tobe made. Only heat exchangers that are meant to exchange heat between two fluids are taken intoaccount. These fluids can be gasses as well as liquids.It is still difficult to have an overview, and a classification needs to be made. It is possible toclassify heat exchangers in a number of ways.

1) A classification of heat exchangers depending on the basic of the fluid paths through the heatexchanger.Difference is made between a parallel flow, counter flow and cross flow. Parallel flow are

those devices in which the warmed and cooled fluids flow past each other in the samedirection, in contrast with the counter flow where these two flow in the opposite direction. Incase of a cross flow, fluids flows pass at right angles to each other. An example of this type isthe heat exchanger in figure 1 [2]. This is a heat exchangers that is found on the top buildingsand is for instance needed for air conditioning inside. Here a fluid is leaded between the platesat the top of the heat exchanger and flows horizontally. Air is blown vertically against theplates to cool the fluid inside.

Fig. 1.1: cross flow air to liquid heat exchanger


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2) The second classification made, is depending on the state of the media in the heat exchanger.Liquid-to-liquid exchangers are those in which two liquids interact. Also gas-to-gas heatexchangers like air preheaters in steam plants and helium-cooled reactor gas turbine plants

have to be mentioned. These devices operate with heat transfer coefficients that are betweenten and one hundred times lower than the coefficients of liquid-to-liquid exchangers. Gas-to-gas exchangers are general much larger and heavier if a same amount of transferred heat isdemanded.A third type is the liquid-to-gas heat exchanger (or vice versa), usually water and air are used,for instance in automotive radiators. Another example was seen in figure 1. Because of thelower heat transfer coefficients on the gas-side there are usually fines placed on the exchangingsurfaces.

3) A third classification method is based the purpose of heat exchanger. In difference with theother classifications, this is not a designers choice but a direct demand to fulfil the need for

lets say an evaporator. So any demand based on this classification is generally a starting point,from which the designer needs to make decisions about the other classifications, like the choicebetween counter flow or cross flow.Some other examples of purpose classification are briefly the cooler, which cools liquids orgases by means of water, the chiller, which cools a fluid with a refrigerant such as freon, tobelow a temperature that would be obtainable if water was used, and condensers, thatcondenses a vapour, often in the presence of a non-condensable gas (only shell tubecondensers; classification on where condensation occurs: horizontal in-shell, vertical in-shell,horizontal in-tube and vertical in-tube)

4) The last classification is actually the most important choice of the designer of a heat

exchanging system. This is the choice what kind of construction he is going to use. Below arediscussed the two most common options: double pipe heat exchangers and shell-and tube heatexchangers.

1.2 Double pipe heat exchangers

The double pipe heat exchangers are quite simple exchangers to analyse. This will be seen inChapter 2. There are two possibilities: the use of a counter flow or parallel flow such as describedin the first classification method. In figure 1.2 [1.1] the development of the temperatures can beseen. From this easily can be concluded that the counter flow is in any case more efficient than theparallel flow since the pipe fluid gets further cooled using this counter flow. While thetemperatures T(of the cooled fluid) and t(of the warmed fluid) in the parallel flow heat exchangercan only approach each other, they can pass each other in the counter flow (Tout< tout) and in thiscase there has to be more heat been transferred.This explains why in practice only counter flow will be seen in case of the double pipe heatexchangers. But there is one other advantage for the counter flow, since the maximum temperaturedifferences between the two flows are much smaller, they suffer less thermal forces.


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Fig. 2.1: counter flow verses parallel flow [1.1]

Double pipe exchangers are mostly built of common water tubing. The use of two single flowareas leads to relatively low flow rates and moderate temperature differences

A straight double pipe heat exchanger as seen in the diagrams will not appear in practice. Mostcommon are U-type or hairpin constructions. Due to the need of a removable bundle constructionand the need for the ability to handle differential thermal expansions the exchanger is implementedin two parts. In figure 1.3 the fluids enter and leave the exchanger by the four nozzles on the rightwhile the exchanger can freely expand to the left which makes the of expansion joints to the othermachinery superfluous and makes demounting much easier.

Fig. 1. 3: U-type or hairpin construction for a double pipe heat exchanger


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Fig. 1.4: practical example: series of U-type constructions

1.3 Shell-and-tube heat exchangers

Because of the many advantages, most liquid-to-liquid exchangers are shell and tube. The highflow rates and heat transfer rates as well as the numerous parameters for the designer to choosemake this type to be more suitable for most applications than a double pipe heat exchanger.The disadvantages are however also that the numerous parameters make it difficult to find anoptimal design and the intricate geometry makes exact calculations impossible and in use it leadsto pressure drops that have to be compensated by the use of a pump.

Fig. 1.5: cross section of a shell-and-tube heat exchanger

Figure 1.5 shows the design of a standard shell-ad-tube heat exchanger. It consists of a shell on theoutside and tubes placed inside the shell, these are made of standard steel or wrought-iron pipe andtheir thickness depends on the operation pressure. The tubes are attached on front and rear ands intube sheets and by baffles which are also be placed to redirect the shell fluid past the tubes toenhance heat transfer. The so-called channel covers gather on both ends the fluid in the tubes. Thenozzles are the inlet and outlet ports in the shell.


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Fig. 1.6:. Shell-and-tube-heat exchanger with one shell pass and one tube pass; cross- counterflow operation.

Differences in types of shell-and-tube heat exchangers are based on the flows inside the shell andtubes. In figure 1.6 a heat exchanger with one shell flow pass and one tubes flow path can be seen.The baffles lead the shell flow in such a way along the tubes that this type is a cross flow device.So this type could by classified as a Shell-and-tube-heat exchanger with one shell pass and onetube pass; cross- counterflow operation. Thus, only the directions of flows make these devicesvery complicated to make estimations to determine the amount of heat transfer, while still otherparameters like sizes are still not taken into account.

To get a better overview of possibilities for designers, a set of standards has been introduced in the1940s. These define the heat exchanger style, machining and assembly tolerances to be employedin the manufacture of a given exchanger. The association TEMA, which stands for TubularExchanger Manufacturers Association, made up and controls these standards. The New York-based association was formed by a group of heat exchanger manufacturers and their specificationscomprise industry standards that directly relate to recognized quality practices for manufacturing.Vendors who build to TEMA standards can be competitively compared because tolerances andconstruction methods should be very similar for a given design

Also a numbering and type designation has been introduced to TEMA. A certain sequence ofparameters, describes every shell-and-tube heat exchanger, including:

Size of shells and tube bundles Inside diameter of the shell in inches Tube length Type designation by letters describing stationary head, shell and rear head

The designation code can be made with use of a table including three columns, see table 1 [2].Three letters, one from each column, determine the type of certain shell-and-tube heat exchanger.


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Table 1.1: TEMA designation

As an example the heat exchanger in figure 1.7 has a front head type A, because of thedemountable end plate to make cleaning possible. There is a one pass shell fluid path, so this leads

to type E and the rear end is an externally sealed floating tubesheet, type W. So this specimen is tobe described by AEW.

Fig. 1. 7: TEMA designation example: AEW


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Chapter 2 Heat exchanger design

By Liselotte Verhoeven (475117)

2.1 Introduction

First the double-pipe heat exchanger will be discussed. This heat exchanger is very simple andits working principle is easy to understand. Some formulas will be presented used in globaldesigning. These formulas will not be derived and their only purpose is to give an idea of whichparameters play an import role in heat exchanger design.Secondly the shell-and-tube heat exchanger will be discussed. This heat exchanger is frequentlyused in practice in big installations as well as in small installation. The wide range indimensions will be made clear.

2.2 The Double-Pipe Heat exchanger

Heat exchangers can be classified in a number of ways, depending on their construction or onhow the fluids move relative to each other through the device. Now there will be looked at oneparticular heat exchanger to go a little deeper into the working principles and the practicalutilizations.

A double-pipe heat exchanger consists of two concentric pipes or tubes. The outer tube is calledthe annulus. In one of the pipes a warmer fluid flows and in the other a colder one.Due to the temperature difference between the fluids heat is transferred. By the word fluid allsubstances that can flow is meant. So the word fluid means not only liquids but also gases. Inthis part there will be looked at a double-pipe heat exchanger with parallel flow. This means

that the hot fluid and the cold fluid flow in the same directions. There are also counter flow heatexchangers. In this situation the hot fluid and the cold fluid flow in opposite directions.

Schematically a double pipe heat exchanger with parallel flow is drawn in figure 2.1.

I IICold fluid in

Hot fluid in

Cooled fluid out

Warmed fluid out

ThI

TcI

ThII

TcII

Fig. 2.1:A double pipe heat exchanger with parallel flow

To understand what factors influence the dimensions of this heat exchanger when a certain heatrate is expected some simple equations will be examined.


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First a simple heat balance:

( ) ( ) (2.1)cIcIIcchIIhIhh TTcmTTcmq == &&

With:

qh = heat transferred from the hot to the cold fluid (kW)

hm& = mass flow of the hot fluid (kg/s)

ch = specific heat of the hot fluid (kJ/kg/C)ThI = hot fluid at position I (C)ThII = hot fluid at position II (C)

The subscript c stands for cold.

But also the next equation is valid:

(2.2)LMTDAUq =

With:

q = the heat transferred between the hot and the cold fluid (kW)

U = the overall heat transfer coefficient (kW/m2/C)A = the heat transferring surface (m2)LMTD = the log mean temperature difference

For the log mean temperature difference for parallel flow the following can be written down:

( ) ( )(2.4)

(2.3)

=

=

cIhI

cIIhII

cIhIcIIhII

I

II

III

TT

TT

TTTTLMTD

T

T

TTLMTD

ln

ln

The next figure will show how the temperature of the hot and cold fluid changes along thelength of the pipe.

Fig. 2.2:The course of the hot and the cold fluid along the length of the pipe


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( )

epiptheofdiameterouterThe

=

=

p

p

OD

OD

AL

0

When the heat is transferred from the warmer fluid to the colder fluid it encounters resistancesthat will create several losses. There will be losses when the heat in the fluid transfers indirection of the wall, when the heat is transferred through the wall and when the heat istransferred from the wall to the flow in the annulus. In other words there will be a tube-filmresistance, a tube-wall resistance and an annulus-film resistance. These losses are encountered

for in the overall heat transfer coefficient U. When this coefficient is known and it is knownwhat the several temperatures at the beginning and the end are or must be (material boundaries)a heat-transferring surface can be calculated for a desired heat rate.

( )(2.5)

0

00 1ln2

11

hD

D

k

D

AAhU ioii+

+

=

Frequently it is wanted to get an idea of how big a heat exchanger will be for a certainperformance. Size is one of the main factors in costs. The following properties must be knownto perform a global calculation of the dimensions of a double-pipe heat exchanger with parallelflow.

-The required cooling down or heat up of the pipe fluid-The temperatures of both fluids when entering the double-pipe heat exchanger

-The mass flows and heat capacity of both fluids the required heat rate can now be calculateddue to the heat balance equation (2.1)-The LMTD can now be calculated; all the temperatures are known-An estimate of the overall heat coefficient; this is very difficult. U depends, as one can see inequation (2.5), on inner and outer diameters, convection coefficients of both fluids and thetransfer coefficient of the wall of the pipe (depending on wall thickness and material properties).There are tables that can help make a good estimate for this parameter. Nevertheless has thisparameter a great influence on the size of a heat exchanger and estimates must always be donewith great care.

Now with the help of equation (2.2) the heat transferring area can be calculated. Knowing theheat transferring area gives a relation between the diameter of the pipe and the length of the heatexchanger. This gives a rough idea of the dimensions that can be expected.

(2.6)

The next figure (Fig. 2.3) depicts what is called a hairpin exchanger, in which two double-pipeexchangers are connected at one end with U-shaped connectors. The inner tube is very long andis bent into a U-shape. The shell or outer tube is bolted to a connector at the end and completelyencloses the inner tube

Fig. 2.3 Hairpin heat exchanger


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Double-pipe heat exchangers are inexpensive and easily maintained. They are primarily for lowflow rates and are well adapted to high temperature and high pressure due to their relativelysmall diameters. Because of the small amount of heat-transfer surface per section, double-pipeheat exchangers are generally found in small total-surface requirement applications.

2.3 shell-and-tube heat exchanger

Where a high flow rate is involved, the number of double-pipe exchangers required becomesprohibitive, both in ground area required and in funds expended. When high heat-transfers ratesare required, an alternative apparatus, known as a shell-and-tube heat exchanger, can be used.

A shell-and-tube exchanger consists of a large-diameter pipe (on the order of 12 nominal to 24nominal and larger), inside a number of tubes is placed (ranging from about 20 to over 1000tubes!). One fluid is directed through the tubes, and another inside the shell but outside thetubes. Several constructions are possible, but they wont be discussed here. Baffles are used to

direct the shell fluid past the tubes in such a way that heat transfer is enhanced. Turbulence iscaused, and higher heat transfer coefficients, and hence higher rates, result. There are severaltypes of baffles. In the picture below segmental baffles are used.In the figure below the working principle of a shell-and-tube exchanger is graphicallyexplained.

Figure 2.4A shell and-tube heat exchanger


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To show the difference in the size of these heat exchangers a couple of illustrations are givenbelow.

Figure 2.5:A big shell-and-tube Figure 2.6:The tubes of a very small shell- and-tube

In the following table the range of dimensions of several parts of a shell-and-tube heat

exchanger is presented.

Range of dimensionHeat transfer area 0.1 100.000 square meters

Pressure Deep vacuum over 1000 bar

Temperature 0 1400 Kelvin

Diameter of the tubes 6,35 50,8 millimeters

Diameter of the shell 50 millimeter 3,05 meters

Number of tubes used 20 - 1000Table 2.1: Range of dimensions

One can clearly see the many possibilities with the shell-and-tube heat exchanger. The key tosuch flexibility is the wide range of materials of construction, forming and joining methods, anddesign features that can be built into these heat exchangers. Because of this flexibility the shell-and-tube heat exchanger is frequently used. In big installations and in small ones.

Another big factor in heat exchanger design is of course costs. The three main relevant factorsthat have the greatest effect on size and therefore on costs are:

-Pressure drops-Log Mean Temperature Difference-Fouling factors

They will be discussed one by one.

Pressure drops If unrealistically low allowable pressure drops are imposed, the designer isforced to use lower fluid velocities to maintain the pressure drops limitations. Lower velocitiescan result in a large heat exchanger. Higher pressure drops result in a smaller heat exchanger,but a pumping device is needed to maintain this high pressure drop. This pumping device needsenergy and so operating costs must be calculated in the overall cost for the heat exchanger. Onlyby considering the relationship between operating costs and investments can the economicalpressure drop be determined.

Log Mean Temperature Difference The size, or surface, of a heat exchanger is inverselyproportional to the overall heat-transfer coefficient and the corrected LMTD. When looking at a


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shell-and-tube heat exchanger a so-called corrected LMTD must be used instead of the LMTDpresented earlier when the double-pipe heat exchanger was discussed.Assuming that reasonable temperatures have been specified, a designer should try to maximizethe product of the heat-transfer rate and the LMTD.

Fouling factors This wont be discussed here. My colleague Gert has discussed this subjectextensively. A reference is made to his paper.

The following figure (Fig. 2.7) shows the relation between the total tube area and the shelldiameter for several lengths of the heat exchanger. It can be seen that for the same total tubearea the diameter decreases when the length of the heat exchanger is increased. This is a logicalrelation.

Figure 2.7:Diameter of the shell versus the total tube surface

The following figure shows the relation between the surface of a single shell and the costs indollars per square feet. As one can see the costs are going down exponentially with the increaseof the surface. So bigger shell-and-tube heat exchangers are cheaper per square feet than smallones. It must be said that this figure is quite an old figure, namely from 1979. A recent figureinvolving costs and dimensions could not be found. It is very difficult to get hold on suchnumbers. Almost every heat exchanger is custom made and companies dont posses such thingsas price lists for complete heat exchangers and the time was to short to get the information of

the companies. But the figure will give a fairly good impression of how the relations are.


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Figure 2.8: The relation between shell surface and costs [1979]

The final subject that is discussed here is the efficiency of a shell-and-tube heat exchanger. Thisof course is also an important parameter in heat exchanger design. Efficiency stands for the partof the maximum possible heat transfer that is transferred in reality. Efficiency says somethingabout energy losses and so a link to costs can be made. Low efficiency means higher operatingcosts.The effectiveness Number of Transfer Units method of analysis is discussed. The NTU is adimensionless parameter used widely in analysis of heat exchangers. Besides the NTU the ratio ofcapacitances is needed to retrieve the effectiveness e.

(2.7)

(2.8)

To give an idea of how an effectiveness calculation is performed a example is presented. The

following table shows the properties of both the shell and the tube fluid. The raw water is thecooling fluid as one can see.

Distilled water - shell Raw water - tubesMass flow (kg/hour) 77,000 68,000

Capacitance (J/kg/K) 4179 4181

Inlet temperatures (degrees Celsius) 43 18

Table 2.2: Properties of the shell and tube fluid

( )min

:

pcm

AU

UnitsTransferofNumber

( )( )

max

min

pcm

m

pc:escapacitancopRatio


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The overall heat transfer coefficient U and the area required for the heat transfer are as follows.

U = 1987.3 W/m2/KA = 65.4 m2

With the formula for the number of transfer units and the ratio of capacitances the followingnumbers are retrieved.

NTU = 1.64Ratio of capacitance = 0.883

From the following figure the effectiveness can be retrieved. On the horizontal axis the NTUvalues are placed and on the vertical axis the effectiveness can be retrieved. Each line representsa ratio of capacitance.

Figure 2.9Effectiveness vs. number of transferunits for a shell-and-tube exchanger having one

shell pass and any integral multiple of two tube passes.

With the calculated NTU and the ratio of capacitance an effectiveness of 0.58 is retrieved. Thismeans that 58 percent of the maximum heat transfer is transferred in reality. With the real heattransfer the end temperatures of the fluids can be calculated. This can be done by the heatbalance presented in equation (2.1).

( ) MWtTcmq p 71111minmax == &

MWqq 412max ==


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Chapter 3 Fouling of Heat Changers

By Gert Witvoet

In an answer to the proceeding chapters, this chapter will discuss the main difficulty among heatexchangers, fouling. A short description and analysis of this problem will be given, after whichsome examples from the field about prevention and removal will be discussed.

3.1 Introduction

Unfortunately nothing in this world is perfect. Cars arent perfect, women arent perfect, noteven life is perfect. And although heat exchangers may seem to be quite close to being perfect,theyre far from that. Using a heat exchanger brings a lot of problems, which can cost lots ofmoney. During usage they will get dirty, and they need some amount of maintenance. This socalled getting dirty has a name: the fouling of a heat exchanger.

3.2 Description of the phenomenon

What exactly is fouling? According to Garrett-Price (1985) fouling is generally defined as theforming of deposits on heat transfer surfaces, which interferes with heat transfer and/or fluidflow. In other words, by using a heat exchanger small layers of insulating material will beformed on the heat transferring surfaces of that heat exchanger. The influence of this layer istwo-sided:

1) The layer has a high thermal resistance, higher then any other part of the heat exchanger,thereby increasing the total thermal resistance. This will decrease the amount of heattransferred through the surfaces and reduces the efficiency of the heat exchanger.

2) The presence of a layer will decrease cross-sectional flow area of the medium. Toachieve the same throughput through this smaller area, theres a bigger pressure dropneeded. Additional pumping is needed, increasing to total amount of energy added to thesystem, decreasing the efficiency.

So fouling is a absolutely not-wanted phenomenon. The problem is that the heat exchanger thatdoesnt suffer from fouling still has to be invented. Furthermore fouling is extremely difficult todescribe. Thats why recent years theres a lot of emphasis on the analysis of this problem.

3.3 Fouling, an overview

Not two heat exchangers are the same and the same is valid for fouling. To understand thephenomenon better, it is important to understand that there are six types of fouling. These typesare:

1) Precipitation foulingAlso called crystallization fouling. A fluid or gas used in a heat exchanger can containdissolved inorganic salts. Given certain conditions, theres a maximum amount of saltthat can be dissolved in this fluid or gas. When the process conditions inside the heatexchanger differ from the conditions at the entrance, supersaturation may occur. Thismeans that part of the dissolved salt will crystallize on the heat transfer surface. Figure3.1 gives a clear example.


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Figure 3. 1Percipitation fouling

2) Particulate foulingThis is when the gas or fluid inside the heat exchanger contains small particles whichwill attach to the heat transfer surface. Examples are dust or sand. The deposition occursmostly as a result of gravity.

3) Chemical reaction foulingThis type of fouling considers the deposits that are formed as a result of chemicalreactions within the fluid. The heat transfer surface itself is not consumed in thereaction, although it could operate as a catalyst. This type is a common problem in forexample petroleum refining or polymer production.

4) Corrosion foulingThis fouling is also caused by some chemical reaction, but this time the surface is areactant and will be consumed. The surface reacts with the fluid or gas to form corrosionproducts on itself. The rusting of steel parts is a well-known example, as can be seen infigure 3.2.

Figure 3. 2 Corrosion fouling

5) Solidification foulingWhen the heat transfer surface is low enough, a fluid flowing through a heat exchangercan actually freeze at the surfaces. In case of a multicomponent fluid, its the high-


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melting point constituent that will solidify. This is easy to imagine for fluids, like watercooling, but in practice this phenomenon can also occur when the medium is a gas.

6) Biological foulingIts also possible for biological micro- and macro-organisms to stick to the heat transfersurface. In this case not only the attaching of the material is a problem, but also its

growth. In many cases this will result in a slime layer. This can be seen in figure 3.3.

Figure 3. 3Biological fouling

To understand more about the influence of fouling on the performance of a heat exchanger, onemust consider the heat transferred q:

Oq U A LMTD=

Here LMTD stands for the Logarithmic Mean Temperature Difference,AO is the outer surfacearea and Ustands for the overall heat transfer coefficient. The influence of fouling can be seenin the coefficient U. In the past various equations for Uhave been developed to capture foulingfactors, but the most widely used is this:

, ,1 1 1 F I F OW

O O O I I W I O

R RR

U A h A h A A A A

= + + + +

The term inside the first brackets stands for the ordinary heat coefficient, when there is nofouling (or for an unused heat exchanger); h stands for the convective heat transfer coefficient,

RWandAWare thermal resistance resp area of the wall. The second term is the extra termbecause of fouling. HereRFis the fouling resistance. The indices I and O stand for inner andouter surfaces.Its clear to see that for increasing fouling factors, the thermal coefficient Uwill drop, causingthe transferred heat q to drop too. One way to compensate this effect is to overdimensionize theheat exchanger, that is increase the heat transfer area. One disadvantage is of course that thiswill result in a more expensive device.


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3.4 Influences on fouling

Choosing a fouling factor is a rather arbitrary business. In practice, values provided by theTubular Exchanger Manufacturers Association (TEMA) are used. These values are based onexperiences, and are only dependent on the fluid used. An example is given in figure 3.4.

Figure 3. 4 Some values from TEMA standards

Although there are such tables for various temperatures and velocities of the medium, the realvelocity- and temperature-dependency of the fouling factors is unknown. Besides that, foulingfactors are time-dependent and will always increase with time. The way it increases depends onthe specific situation. It could increase linear with time, or increase asymptotically to a certainlimit. The latter case isnt as worse as the linear case. When time increases, fouling resistance

will reach some constant value. This means that one can design a heat exchanger in such a way,that this constant resistance is compensated. In the linear case it is necessary to periodicallyclean the heat exchanger, or else the fouling resistance would reach the sky.In most cases there is also an initial induction period; for a clean heat exchanger with initialfouling factor zero there is a certain time interval for which the fouling resistance is very low.This is illustrated in figure 3.5.


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Figure 3. 5Linear increase with induction time

Of course there are even more parameters that can influence the fouling factor. In general, the

following conditions are known for their influence:1) Velocity of the medium

Increasing medium velocity will in general increase the release rate and thereforedecrease fouling.

2) Temperature of the bulk fluidEspecially precipitation and chemical reaction fouling can depend strongly on bulktemperature, but both in a different way.

3) Temperature of the heat transfer surfaceLowering this temperature may increase solidification or even precipitation fouling.Other fouling mechanisms may increase with increasing temperature.

4) Surface material

The amount of corrosion is strongly dependent on the choice of the surface material.Surface material may also influence biological fouling, e.g. copper is more sensitive tobiological fouling then most other materials.

5) Surface structure and roughnessOf course rough surfaces promote the attachment of any particle. A rough surface alsomeans more area for corrosion or chemical reaction.

6) Heat exchanger configurationFrom experience its known that shell-and-tube heat exchangers are more sensitive tofouling than for example a plate-and-frame or double-piped heat exchangers. This ismostly because velocities and turbulence levels are higher for the latter one.Disadvantage of these heat exchangers is that theyre far more bigger than a shell-and-tube with the same capacity.

Its clear that the precise influence of each of these conditions on the fouling factor depends onthe type of fouling. In some cases this precise influence isnt known yet, and theres still a lot ofresearch done. Still, understanding the impact of these conditions on the fouling resistance isessential to actually control fouling-phenomena, and thereby control the costs. In the design of anew heat exchanger, one must be aware of all the mentioned conditions and other influences offouling factors, and choose those conditions that will result in as less fouling as possible. Onemust also be aware that it must be possible to clean a heat exchanger once in a while, which willhave impact on the chosen construction.


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3.5 The importance of fouling

After the previous description of fouling, one can ask how important the whole issue really is.Does fouling actually have a major impact on a heat exchanger and does it actually occur thatoften?

Since everything in this world is all about money, these questions will also be answered byexpressing the impact of fouling in money. Actual costs e.g. for removing fouling effects for aspecific heat exchanger are unfortunately not available, but there are some data about the totalamount of costs of entire industries.

First of all, fouling costs can be separated according to how they are generated. Roughly taken,there are four types of costs:

1) Additional capital costs or costs for special design considerationsLots of costs in using heat exchangers can be prevented in the R&D departments of acompany. Especially when it comes to fouling. A good design can reduce the effects offouling and thereby the operational costs of the heat exchanger. But of course research

and design costs money.A way to prevent fouling is to choose a bigger heat transferring surface then needed, asdiscussed before. The heat exchanger will become bigger and heavier, and thereby alsomore expensive.

2) Energy costsA heat exchanger that suffers from fouling needs additional energy to keep operating atthe same level. This is because the fouling layer decreases the amount of heat transferredas well as it increases the amount of pressure drop needed to maintain the samethroughput through the smaller cross-section. All this additional energy is pure loss.

3) Maintenance costsA fouled heat changer has to be cleaned once in while, in order to keep the energyneeded for operation low. This cleaning can be online or offline, mechanical orchemical, etc. Sometimes its needed to replace some parts of the heat exchanger, forinstance because of corrosion.

4) Costs of loss production or shutdown costsWhen a heat exchanger is cleaned or maintained offline, there is no production. Noproduction means no income, so this is considered a loss. The effect of this shutdowndepends on the normal plant capacity and the length of the shutdown.

In recent years many research has been done to know more about the magnitude of the costsmentioned above. These amounts were presented in total fouling costs of a complete industry ora country. Two results from history are presented in the following table.

Costcomponent U.S. (1982) U.K. (1978)Capital costs $ 960 - 1280 million 100 million

Energy costs $ 700 - 3500 million 60 million

Maintenance costs $ 2000 million 80 million

Loss production $ 200 million 60 million

Total $ 3860 6980 million 200 million

These are of course enormously high amounts. But it gets even worse when you compare theseresults with the total heat exchanger sales. For example, sales in the U.S. in 1981 totaled $ 1.5billion, or $ 1500 million. This means that fouling costs double the sales by far! And thereby the

importance of fouling analysis is shown.


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Of course research has done a great thing in recent years, decreasing the amounts of costscompared to the amounts of 20 years ago. Still, fouling stays a big problem in industry.

The precise moment to clean a heat exchanger also strongly depends on the costs, of course. Inthe figure below this is illustrated for a simple example for a heat exchanger with a linearly

increasing fouling factor, thus linearly increasing energy costs. The heat exchanger isperiodically cleaned offline. A cleaning operation takes a certain amount of money, so whentime increases this amount per unit time will drop. The right time to clean the exchanger iswhen the sum of the two is at its minimum:

Figure 3. 6Simple example of choosing the cleaning moment

3.6 Fouling solutions in practice

Various manufacturers have dealt with fouling in recent years, and many so called solutionshave been introduced. New cleaning techniques, new cleaning chemicals or even new heatexchanger designs. In this paragraph some of these solutions will be discussed. Of course itsimpossible to discuss all the developments of recent years. The main focus here is just to give asmall idea of

3.6.1 Automatic Tube Brushing (ATB)

When it comes to cleaning of a heat exchanger, there are to ways to do this: offline and online.The advantage of online cleaning is that the heat exchanger will not have to be shut down, andthereby shutdown costs are reduced. This is why online maintenance is often more preferredand therefore all kinds of online cleaning methods were developed in recent years.One method to remove foulants, developed by a firm called Advanced Heat TransferTechnologies, is the Automatic Tube Brushing (ATB). The operating principal will explained inthe light of the following picture.


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Figure 3. 7The principal of the ATB system

The system can be applied to almost any heat exchanger and consists of two main parts. Thefirst is a small nylon bristle brush (white part on the right, within the yellow tube), which isinserted into each tube of the heat exchanger. The size of the brush is chosen in such a way thatthere is an appropriate fit within the tube. The second part is a special plastic cage (blue part onthe left), which is installed at each side of the exchanger tubes. When the exchanger fluid flowsthrough the tubes it takes the brush with it from the cage on the one side to the other one. Byreversing the flow direction, the brush will then be taken to other side again. This way the brushmoves back-and-forth through the tubes, thereby removing all kinds of foulants.The turning of the flow is done by a third component, a special valve, which is activated by anautomatic control panel. This valve normally turns the flow like two or three times a day,depending on the severity of the fouling in the heat exchanger.

Figure 3. 8 The installation of ATB

The big advantage of the ATB system lies in the fact that after installation no more extramaintenance is needed, so the shutdown and maintenance costs almost disappear. The systemkeeps the fouling factor of the heat exchanger at a constant low value. In practice, fouling

factors of 5 till 10 times lower than without ATB are reported. This way also energy costs areminimized. Furthermore no chemicals are needed to clean the heat exchanger, which is anadvantage for the environment.


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The disadvantage of the system lies mostly in the installation, which is rather time consumingfor big heat transfers and also rather expensive because, partially because of the needed controlunit and valves. Capital costs will rise, but a bigger reduction in other costs can be achieved!

3.6.2 The Spiral Heat Exchanger

A spiral heat exchanger isnt so revolutionary as it may sound. It has been used for over 60years in for example asphalt heaters. Later it was discovered that the concept could well be usedin other big industries in order to reduce fouling effects.

A spiral heat exchanger is composed of two long flat plates, which are wrapped around a centertube, to form two concentric spiral channels. This is illustrated in figure 3.9 and 3.10.

Figure 3. 9A spiral heat exchanger Figure 3. 10 Close up

The hot flow enters the heat exchanger in the center, spirals outwards through the long flatchannel to leave the exchanger in tangential direction. The cold fluid enters tangential, flowingthrough the long flat channel to leave in axial direction. This way a counterflow is created,

which maximizes heat transfer.

Why is this concept a good alternative for ordinary shell-and-tube heat exchangers when itcomes to fouling? This is mainly because the curved form of the channels will create turbulenceat any point in the flow, even with low velocities. The same curved form causes high shear ratesat the walls. These two effects can prevent particles from clinging to the wall.The second reason lies in the fact that the spiral heat exchanger is just single-channeled. Inordinary multiple-channel heat exchangers, when some foulant does manage to stick to the wallof a channel, the flow is restricted in that channel and will divert to less fouled channels. Thevelocity in the fouled channel will thereby be reduced, causing even more foulant attachment tothe walls. In spiral heat exchangers, in contrast, there is only one channel, so when some foulantdoes attach, the flow still has to go through. The velocity will locally increase, as will the shearrate, thereby removing the foulant again.

Because of these effects, a spiral heat exchanger will be able to operate three to four timeslonger as a shell-and-tube heat exchanger before cleaning is necessary. Cleaning will still beneeded though, as for every heat exchanger. This cleaning can be done rather easy, as can beseen on figure 3.9: on both sides theres a big cover that can easily be removed to access to heattransferring channels. Spraying some pressure washer will then clean the inside of the device.

3.6.3 Deposit Determined, FOuling Reducing Morphology (DDEFORM)

A final recent development is the DDEFORM, a design by co-operating Universities andlaboratories (Aerodynamics Laboratory of the National Technical University of Athens, the


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Experimental and Computational Laboratory for the Analysis of Turbulence (ECLAT) ofKings College London, GRETh and the PPC of Greece). The design is based on computationalfluid dynamics calculations, experiments from the laboratory and observations from industry.

Figure 3. 11A DDEFORM vs. a common heat exchanger

Special about the design is the shape of the tubes of the heat exchanger, as shown in figure 3.11.This design is based on morphology (study of shape and form) of the first stages of depositformation (that explains the name), which is too complicated to discuss here. What matters, isthat the surface is created in such a way that the attachment of foulants is made very hard,thereby reducing fouling rates, partially because of the reduced frontal area. The elliptic form isfurthermore responsible for a reduction of needed pressure drop. This is the reason that with thisspecial shape, the tubes can be placed closer to each other, increasing the heat transferringsurface. So not only fouling can be reduced, also the efficiency of the heat exchanger can beimproved.The DDEFORM is still a concept and still has to prove itself in industry.

3.7 In conclusion

Fouling is a serious problem in industries dealing with heat exchangers. Its a source of manyexpenses and thats why lots of effort is made to reduce this. The mentioned examples illustrateto possibilities in research. In recent years lots of progress is already made, what gives hope forthe future.As long as research on fouling continues, we may someday be able to fully control fouling. And

make this world a little bit more perfect


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Chapter 4 Boilers

By Surjo Abadi (467017)

4.1 Type of boilers

Boilers can be classified in various ways; depending on firing method used, fuel fired, field ofapplication, type of water circulation employed, and pressure of steam etc

4.1.1 The construction

Depending on their construction boilers can be divided into- Fire tube boilers- Water tube boiler

4.1.1.1 The fire tube boiler

Fire tube boilers have been used in various early forms to produce steam for industrial purposes.Figure 4.1 shows how a simplified of fire tube boilers works.

Fig 4.1

The fire tube boiler is a special form of the shell-tube type boiler. A shell type boiler is a closed, usually cylindrical , vessel, or shell that contain water. Hot gases pass through the tubes duringthe heat transfer process. The shell boiler evolved into modern forms such as the electric boiler, in which heat is supplied by electrodes embedded in the water ( fig 4.2 )

Fig 4.2


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4.1.1.2 The Water Tube Boiler

The difference between fire tubes and water tubes is simply ; water flows through water tubesinstead of fire. (fig 4.3 )

Fig. 4.3

The advantage of the water tube boiler is , it can works in high steam pressure and capacities.With higher steam pressure and capacities, fire tube boilers would need large diameter shell.With such large diameter, the shell would have to operate under such extreme pressure andthermal stresses that their thickness would have been too large.

The Steam pressure

Depending of the steam pressure boilers can be divided into- low pressure more less 1,5 bar- medium pressure , from 1,5 20 bar- high pressure , higher than 20 bar

Water circulation

Boiler can be classified base on water circulation ;- natural- forced

- once-through

Water circulates from the steam drum via downcomer pipes to a bottom header, up the watertubes ( which act as rises ) , where it partially boils , and back to the steam drum.

Fig. 4.4

S = heated riser tubesQ = heating

F = downcomer

V = header


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The density of the saturated water in downcomers is greater than the average density of the twophase-mixture in the risers. Natural circulation is dependent upon the differencethese two densities and the height of the drum above the bottom headers.

But some of this process is required additional help by pumping the single phase flow. Such thatprocess is called forced-circulation.

4.1.2 Once-Through Boilers

The once through boiler is the only type suited to supercritical pressure operation ( above 220bar, for steam ) because the latent heat beyond the critical point is zero ( fig1.5 ) and liquid andvapor are one and the same, so no separation drum is needed

Fig 4.5

black dot lines once-through boiler; black solid subcritical boiler

Fig 4.6: schematic diagram of once-through boiler

Heatevaporator

Water from

condenser

Steam to

turbine


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4.3 Energy Source

Boiler can be also classified by their energy source;1. Coal2. Liquid Fuel

3. Gas4. Biomass5. Geothermal6. Nuclear

4.4 Material

A boiler is classified as a fired pressure vessel. Parts of the boiler that are subjected to highinternal pressure of steam or water are referred to here as pressure parts. Tubes, drum, andheaders are examples of pressure parts. Other component likes burner etc, are not subjected tosuch internal pressure. As such they are classified as nonpressure parts. Selection of materials

for pressure parts of a boiler and their mechanical design and construction are important aspectof a boiler design.

There are several factors that should be taken account for material selection.They are:

- Mechanical propertiesMechanical properties such as ultimate tensile strength , yield strength , creep strength, creeprupture strength, ductility, and toughness are required to be considered for boiler construction

- High temperature propertiesa) creep , it becomes important in boilers at operating of 450 650 C. Creep can take place and

lead to fracture at an extended static load much lower than that will cause yielding orbreakage when the load is of short duration.

b) Creep and Fatigue Interaction, when materials are exposed to cyclic loadings whileoperating at temperatures within their creep range, the creep reduce their fatigue life.

-Manufacturing methodWelding, cold forming, hot forming, and expanding are some of the modern manufacturingmethods used to fabricate a particular material into required shapes. A consideration of thesemethods is important, as the least expensive fabricated product with adequate properties rather

than the cheapest steel should dictate the choice.

- WeldabilityWeldability of material is important manufacturing consideration. Welding problems that haveto be overcome include :a) solidification crackingb) heat affected zone liquefaction crackingc) hydrogen induced crackingd) lamellar crackinge) reheat cracking

-Scaling resistanceNeed for scaling resistance at maximum surface temperature


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For example the tubes and headers from which power station boilers are assembled must bestrong enough to resist internal pressure and system forces, and corrosion both from the waterside and the heat transfer medium, which may be water, steam, flue gas, carbon dioxide, heliumor sodium. Carbon steel have neither the strength nor the corrosion resistance required to

operate at the higher temperature and their resistance is improved by alloying. Table 1.2 showslist of alloys used in boiler construction

Table 4.2:Materials used for boiler tubes

Component Unit type Temperature

range

Heat transfer

fluid

Type of steel

Economizer Conventional 250 350 Flue gasAGR CO2 Carbon/manganseHTR He 2.25% Cr, 1 % Mo

Fast Reactor 250-350 Na

Evaporator Conventional Flue Gas Carbon/manganeseHTR 350 He 9% , 1% MoAGR CO2AGR CO2

Fast Reactor 350 NaPWR 280 Water

-Design Method

Most of the methods include materials selection are standardised. In USA the method usesASME, and TRD in Germany, BS in UK, and IBR in India

4.4 Pressure drop inside the tube

The pressure drop through any section may be written as

PaclPstPfricP ++=

4.4.1 Friction

Pfric = pressure drop is caused by friction. The hydrodynamic resistance in a tube may be dueto friction in a straight length. In case of subcooled water the frictional pressure drop is given

m = mass velocity in the tube

f = the friction coeficient should be taken from the moody diagramd= its surface roughness

= density of water

ld

mfP

subcooled

fric=

2

2


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l = length of the pipe

For steam water mixture

The coefficient K depends on the volume ratio of steam and water , which given as

K = 1 + 0.98 < 0.4

For > 0.4

If the steam flows at very high velocity in the core and water creeps along the wallWe use separated equation

E lies between 0 and 1

4.4.2 Acceleration

The pressure drop due to acceleration is found by integrating over the section over which thesteam fraction changes fromx1 tox2

4.4.3 Hydrostatic Head

The hydrostatic head is given as

( )

+= 111112

22

Kxxld

mfP

ss

phase

fric

( ) ( )[ ] ./1 xx s=

)143.3)(183.1.(6

197.21

166.066.0

66.0

+++

=

K

= es

phase

fricEfE

ld

xmfP

)1(1

1

2

. 222

19.1))1(

1(1

+=

x

xf s

( )( )

2

1

222

1

1x

xs

xxmPaccl

+=

( )[ ] lgPstatic s += .sin1


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4.5 Maintenance

4.5.1 Scaling and sludge

Scaling is the deposit of thermally nonconducting solids inside the tube. Scaling is objectionable

because it interferes with normal heat flow through the boiler metal and may lead overheating.In addition scale may create other probles like

1. waste of fuel2. loss of boiler output3. maintenance problem for removal of scale

In figure 4.7 can be seen the relation betweet cost and thickness of scale

Fig. 4.7

Scaling is caused by the precipitation of calcium and other salts of limited solubility, scale, inaddition to its high insulating value, progressively narrows pipe internal diameters and roughenstube surfaces, thereby impeding proper flow.

4.5.2 Mechanism of scale formation

If the boiler makeup water is not softened, the dissolved bicarbonates break down to carbonates scales

Ca(HCO3)+ Heat = CaCO3 + H2O + CO2 ( calcium carbonate scale )

Mg(HCO3)2 + Heat = MgCO3 + H2O + CO2 ( magnesium bicarbonate scale )

T T

A B

Fig 4.2Temperature distribution in the pipe without scale ( a) and with scale (b)


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Scale is a relatively hard and adherent deposit, while sludge is softer and can be easilydislodged. The buildup of scale ia most severe in most high heat flux areas. Scale buildup isassociated with compound whose solubilities decrease with increasing temperature. Converselysludges are precipitated directly from the boiler water when their solubilities are exceeded.

Scale and sludge increase the resistance to heat transfer and decrease U (Q = U.A.T). Mostimportant problem , sludge and scale raise the tube temperature.

The formation of scale can be prevented by proper treatment of boiler water. It can be removedby chemical cleaning or by mechanical during the maintenance.


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Chapter 5 An industrial boiler in practice

By Johan Kunnen (492061)

In this chapter first a power plant boiler will be discussed. Than a boiler from the Universitywill be considered. Than the NOx and the SO2 emissions will be discussed.

5.1 An power plant boiler

An industrial boiler can be found in a large range of applications. The largest boilers can befound in Power Stations. These large boilers are mostly coal-fired systems. The power of suchlarge power stations is 150+ MW. The boiler in such a large facility is cubic in shape and oftypical dimensions 18m wide, 40m long and 60m high. In the figure below a large boiler is

shown.

Fig. 5.1: Coal fired steam boiler at the Alcao Power plant in Anglesea, Australia

The steam leaves the boiler for the steam turbines. The flames in the boiler reach temperaturesclose to 2000C. The boiler pressure may typically be 100-150 bars and be at a temperature of500-550C. The boiler shown in the figure above produces steam at 104 bar and 538C [5.1].A 660 MW system converts over 2 million liters of water into steam an hour. This is done inmore than 450 km boiler tubes [5.2]. In the figure below the inside of a boiler is shown.


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Fig. 5.2:Inside the boiler

The fuel consumption of such a boiler is enormous. It consumes 250 tons of coal an hour. Thisis around 70 kg/second. The total efficiency of a power station, this means from the energy inthe coal to electrical energy is in the order of 35-40 %. The purchasing costs of a large coal firedsteam boiler are in the order of 30-40 euro/kW [5.3].

5.2 Boilers at the TUE

Boilers are not only used in large industrial plants to create steam. Boilers are also used forother applications. In the ketelhuis (boiler home) at the university there are 5 boilers. Theseboilers do not create steam; they provide the water, which is used to heat the buildings. Thewater they provide is heated from temperatures of 120 C up to 180 C, which means the waterwill have pressures up to 10 bars [5.4].The total capacity of these 5 boilers together is 56 MW [5.5]. One of these boilers is build by

Bronswerk N.V. (www.bronswerk.nl) in Rotterdam. This gas-fired boiler has a capacity of 14MW. Although it is capacity is considerably less than the previous discussed coal fired system,it is still more than 6 meters high, 4 meters wide and 6 meters long. The amount of gasconsumed depends on the load of the boiler. With full load the boiler consumes around 1300m3/hour of gas [5.6]. The empty weight of the boiler is 40.000 kg and it can contain 9.000 litersof water. The water used in the boiler is treated before it enters the system. It is dehardened anda substance is being added which binds oxygen. This substance prevents corrosion in thesystem. The boiler can heat up to 255 m3 of water an hour. In the boiler the water flows throughpipes with a diameter of 63.5 mm. The pipes are 2.9 mm thick. The heat exchange surface is440 m2. The total length of pipes in the boiler is thus 2.2 km.

There are a few instructions for the maintenance of the boiler. The maintenance of the boiler canbe divided in maintenance on the burner and the rest of boiler [5.7]. Every year a burningrapport has to be made. For this rapport the performances of the burner are tested each year.Typical measurements involve consumptions, emissions and temperatures. The burner is thepart of the boiler, which causes the most problems. Because of this the boiler is inspected andcleaned two times a year. The rest of the boiler, the pipes, is cleaned only once a year. This issufficient for the boiler to perform properly.Every two years the boiler also has to be inspected and certificated by the Stoomwezen-LloydsRegister. During this inspection a number tests take place. One of these test is as followed. Themachine is disconnected from the water circuit. Also al the safety measures are decoupled. Thenthe boiler is put under a pressure of 25 bars. In this way leakages are being tracked down.

During this testing also the thickness of the boiler wall is being measured with the help of x-raydevice.


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Every month there also a number of other test has to be done by the personal of the HousingService (Dienst Huisvesting). The low-water security system has to be tested. It is very harmfulfor the boiler if it contains too little water. Than the temperature in the boiler could rise todangerous values. In that case the pipes of the boiler could melt down or the boiler couldexplode. Other safety-measures that have to be checked monthly are the flame-safety, the

pressure over the boiler, the temperature-safety and the flow-safety. The cleaning costs of theburner are 4000-5000 euros a year. The costs for cleaning the pipes and the walls are around3500 euros a year. The total cleaning cost for the rest of the burner is 1000-2000 euros a year.

5.3 Emissions5.3.1 Regulations

There are different environment-problems to which an industrial boiler contributes. Two ofthese problems are: smog and the acidification of the environment. These problems are amongothers caused by emissions of SO2, NOx and Fly ash in the flue gases. In the table below the air

emission-rates of these products are showed for a typical coal fired steam generator [5.8]. Thesevalues are for a moderate size boiler.

Typical Air EmissionsFor a Coal-fired Steam Generator of

75 MW

Constituent

SO2NOx as NO2 ( Includes Low NOx burners)Fly ash

Uncontrolled Emissions rate

0.63 ton/hr0.05 ton/hr1.50 ton/hr

Table 5.1

In 1990 the governments of the different countries in the European Community have madeprograms for reducing the amount of NOx and SO2 emissioned. These programs contain themaximum amount emission that may be thrust out for each country each year. They also containa time schedule. The maximum amount of emission has to become less every year. In the tablebelow the schedules for SO2 in The Netherlands and the whole European Community is given[5.9].

SO2 Emissions Max emissions (1000 tons/year)

Year 1980 1993 1998 2003

NL 299 180 120 90EU 14430 11065 8402 6140Table 5.2

In order to reach these requirements a number of measures were taken by the government.Among others, laws were made involving the maximum amount of emission for a boiler andother polluting machinery. In order to control the amount produced the emissions have to bemonitored. By law it is required for machinery bigger as 300 MW to monitor the emissionscontinuous. For equipment smaller than 300 MW measures have to be taken on a regally basis.In the table below the maximum allowable emission values are given [5.9].


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Emission Guideline 2001 [mg/m3]

SO2 (Solid Fuels) 850 ( < 100 MW)200 ( > 100 MW)

SO2 (Liquid Fuels) 850 ( < 100 MW)200 ( > 300 MW)

SO2 (Refineries) 450NOx (Solid and Liquid Fuels) 400 ( < 100 MW)

300 ( 100-300 MW)200 ( > 300 MW)

NOx (Natural Gas) 150 ( < 300 MW)100 ( > 300 MW)

Dust (Solid and Liquid Fuels) 50 ( < 100 MW)30 ( > 100 MW)

Table 5.3

Smaller systems, which are running on gases or liquids, are allowed to have larger emissions

than larger systems. As a result for smaller systems do not required to treat their flue gasesafterwards. Because of the evermore-stricter laws the amount of exhausted emissions by thelarge industrial systems have decreased very much. As can be seen in the table below [5.9].

1980 1997

SO2 299 54

NOx 120 53Table 5.3

5.3.2 NOx-emission

The emission of NOx is one of the major sources for acidification and smog. NOx formation ina boiler can occur in two ways. NOx is mainly formed in the boiler during the combustionprocess. During the combustion process oxidation of nitrogen that is available in the suppliedair is taking place. The NOx formed in this way is called thermal NOx. The rate of the oxidationdepends on the temperature and off course the amount of Oxygen available [5.10], [5.11].It is also possible for NOx to be formed in a different way. The NOx formed in this way iscalled Fuel NOx. In this case NOx is formed because of the nitrogen that is organically bondedin the fuel. Coal contains relatively large amounts of nitrogen. As a result around 80 % of theNOx formed in a coal boiler will be Fuel NOx. The amount of fuel NOx formed does notdepend on the temperature, but on the amount of oxygen available.

5.3.2.1 NOx-reduction strategies

There are different strategies to reduce the amount of NOx. Lowering the temperature of thecombustion can reduce the forming of thermal NOx. The lower temperature makes nitrogen andoxygen less reactive with each other. Especially if peek temperatures occur during combustionNOx will be formed. Peek temperatures may occur when fuel and air are not properly mixed.By controlling the mixing process in the right way the combustion can be made more uniform.Thus preventing the appearance of peek temperatures and thus limiting the production of NOx.The forming of NOx will also be decreased if the oxygen concentration is limited.Thermal NOx is formed because of the amount of nitrogen and other pollution in coal. These

amounts depend on the place where the coal is found. The figure below shows the amount ofNOx formed by a burner with coal from different mining sites in the USA [5.11].


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Illinois # 6

Coal

Mahoning # 7A

Coal

Ohio

Horizontal

Coal

0

50

100

150

200

250

300

350

NOxconcen

tration[ppm]

Fig. 5.3

Although it is not a real solution, emissions of NOx can be reduced if a coal from a differentmine is used. The amount of fuel NOx can also be reduced if the amount of oxygen availableduring the combustion process is limited.

5.3.2.2 NOx-reduction measures

An example of an application that uses the techniques of lowering combustion temperature andlowering oxygen levels is the Overfire Air System (OFA) for a coal-fired steam boiler. OverfireAir is also referred to as Two Stage combustion. The estimated costs of a UFA-system are250,000 euro [5.12]. If an OFA-system is used part of the combustion air (typically 10-25 %) isdiverted and introduced downstream [13]. The total process is as followed: Primary air (75-90%) is mixed with the fuel. Thus a fuel-rich, oxygen deficient zone is created.During its combustion a relatively low temperature occurs and therefore moderate amounts offuel NOx are formed. During the secondary stage of the combustion the diverted air is injected

above the combustion zone through a special wind-box with air introducing ports and/ornozzles. Combustion is completed at this increased flame volume. Again the relatively low-temperature secondary-stage combustion limits the production of thermal NOx. Also theefficiency of the boiler is increased because the design of the system provides an almostcomplete burnout of the fly and bottom ash carbon. The efficiency of a boiler containing theOFA-system is projected to be increased 5%. The increase of efficiency could lead to savings ofaround 1500 tons of coal/year for one moderate boiler [5.12]. Which means savings of around60,000 euro for each boiler each year. The emission reduction should be at a level proportionalto the efficiency increase. This would also mean each boiler would thrust out in the order of1,000 tons of SO2 and 1,000 tons of NOx a year.

Another way to reduce NOx production is by Flue Gas Re-circulation [5.14]. Using thistechnology means that 20-30 % of the flue gases with a temperature of 350-400C is re-circulated and mixed with the combustion air. The resulting dilution in the flame decreases thetemperature and the availability of oxygen. As a result the formation of thermal NOx is reduced.For a coal boiler the amount of NOx reduction is limited if only flue gas re-circulation is used (< 20 %). This is due to the low ratio of thermal NOx in the total NOx emissions. If the amountof re-circulation gases is increased to much the flame can become instable. Also the increasedflow of gases through the boiler may affect the performances .


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5.3.3 SO2-emission

SO2 is formed during the combustion process. When fuel that is containing sulfur is beingburned the sulfur is being burned as well to form SO2. Especially when coal is being used as afuel, SO2 will be formed. Coal contains large amounts of sulfur. Typical sulfur amounts are 2.5-

3 % [5.8]. In order to reduce the amount of emissions, desulfurization of the flue gases has tooccur. The desulfurization of the flue gases is usually done in a scrubber. There are two types ofscrubbers, wet scrubbers and dry scrubbers.

5.3.3.1 Wet scrubbers

Before the flue gases enter the scrubber, the gases are filtered. The gases are filtered off the flyash. After the filtering the flue gases enters a large vessel, this is the spray tower or absorber.

Fig. 5.4:A wet scrubber

There the gases are sprayed with water slurry, which is containing approximately 10% lime or

limestone. The calcium in the slurry reacts with the SO2 to form calcium sulfite or calciumsulfate. A portion of the slurry from the reaction tank is pumped into a thickener, where thesolids settle before going to a filter for final dewatering to about 50 percent solids. The calciumsulfite waste product is usually mixed with fly ash and disposed of in landfills. Alternatively,gypsum can be produced from the waste. Gypsum is a useful by-product. The efficiency of awet scrubber can be up to 90-95 % and higher. There have been reports of efficiencies of99,99% of removal. The total costs of a wet scrubber are around 60-130 euro/kW [5.15].


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5.3.3.2 Dry scrubber

In dry scrubbers, calcium hydroxide slurry is introduced into a spray dryer tower. The slurry isatomized and injected (close to saturation) into the flue gases. There, the droplets react with SO2as they evaporate in the vessel. The resulting dry by-product is collected in the bottom of the

spray dryer and in a filter.

Fig. 5.5

These byproducts are the same as when wet scrubbing is used. The efficiency of a dry scrubberis less than for a wet scrubber. Typical values are 70-90 %. The capital costs of a dry scrubbersystem are 100-150 euro/kW [5.16]. A dry scrubber system is simpler and easier to operate andmaintain.


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Literature

The following literature was used for Chapter 1:

[1.1] William S. Janna, Engineering heat transfer, 2nd ed., London : Van NostrandReinhold, 2000

[1.2] Arthur P. Fraas, Heat exchanger design, 2nd ed., Chichester : Wiley-Interscience,1989

The following literature was used for Chapter 2

[2.1] Process heat Exchange, edited by Vincent Cavaseno and the staff of ChemicalEngineering, 1979, New York.

[2.2] The CRC Handbook of Thermal Engineering, editor-in-chief Frank Kreith, 2000,U.S.A.

[2.3] Engineering Heat Transfer, by William S.Janna, 2000, U.S.A.[2.4] The Internet site ofAPI Heat Transferworld leaders in heat transfer technology


[3.1] Fouling of Heat ExchangersB.A. Garrett-Price et al. 1985 (New Jersey)

[3.2] Heat Exchangers; Selection, design and constructionE.A.D. Saunders 1988 (New York)

[3.3] A New Heat Exchanger Designwww.eee.kcl.ac.uk/mecheng/sb/fouling/DDEFORM_HX.pdf

[3.4] No More Fouling: The Spiral Heat Exchangerhttp://www.process-heating.com/CDA/ArticleInformation

/coverstory/BNPCoverStoryItem/0,3154,18383,00.html[3.5] Automatic Tube Brushing (ATB) Systems

http://www.fbhx-usa.com/ATB1.html


[4.1] Basu Prabir, Kefa Cen , Jestin Louis , Boilers and Burners , Springer Verlag New York2000

[4.2] Wakil-El M M , Power Plant Technology,McGraw-Hill, London, 1984


[5.1] Alcoa in Australiahttp://www.alcoa.com/australia/en/info_page/boiler.asp

[5.2] Vales Point Power Stationhttp://www.de.com.au/Online/Default.asp?DeptID=192

[5.3] Babcock & Wilcox Newshttp://www.babcock.com/pgg/pr/bwbcpuch.html

[5.4] Archive Housing Service TU/e:Drawing 21B67016:

Heetwaterketel 20 samenstellingtekening


44/44


[5.5] Archive Housing Service TU/e:Bedrijfs- en bedieningsvoorschriften betreffende heetwaterinstallatie in gebouw Ceres.

[5.6] Archive Housing Service TU/e:Stookrapport ketel 8, 5-10-2000

[5.7] R.J.M. Van Herk, Maintenance operator of the Housing Service at the TU/e

[5.8] Air pollution control for industrial boiler systems.http://www.babcock.com/pgg/tt/pdf/BR-1624.pdf

[5.9] Overzicht van EU-beleidhttp://eu-milieubeleid.nl/ch06.html

[5.10] B&Ws Experience reducing NOx-emissions in Tangentially Fired Boilershttp://www.babcock.com/pgg/tt/pdf/BR-1726.pdf

[5.11] B&Ws Low NOx Burner operating Experiencehttp://www.babcock.com/pgg/tt/pdf/BR-1684.pdf

[5.12] Advanced Overfire Air System for Stoker Boilers and furnaceshttp://www.oit.doe.gov/inventions/factsheets/berkau.pdf

[5.13] Air staging for NOx control (Overfire Air (OFA) or two-stage combustion)

http://www.iea-coal.org.uk/CCTdatabase/airstag.htm[5.14] Flue gas re-circulation for NOx control - Clean Coal Technologies

http://www.iea-coal.co.uk/site/database/cct%20databases/fgr.htm[5.15] Wet Flue Gas Desulfurization

http://www.worldbank.org/html/fpd/em/power/EA/mitigatn/aqsowet.stm[5.16] Dry Flue Gas Desulfurization

http://www.worldbank.org/html/fpd/em/power/EA/mitigatn/aqsodry.stm

heat exchanger and boilers - energy conversion guide

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