Simulation and Evaluation of Two Different Skin Thermocouples : A Comparison made with Respect to Measured Temperature

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Simulation and Evaluation of Two Different Skin Thermocouples

A Comparison made with Respect to Measured Temperature

Joel Lundh

Applied Thermodynamics and Fluid Mechanics

Degree Project

Department of Management and Engineering

LIU-IEI-TEK-A--07/0076--SE

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Copyright

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Abstract

The demand for more accurate measurements is increasing in today’s industry. One reason for this is to optimize production and thus maximize profits. Another reason is that in some cases government regulations dictate that supervision of certain parameters must be followed. At Preemraff Lysekil there are basically four reasons for measuring skin temperatures inside fired process heaters, namely; because of government regula-tions, in order to estimate the load of the fired process heater, to estimate the lifetime of the tubes inside the fired process heater and finally, to determine the need of decoking. However, only the first three of these reasons are applied to H2301/2/3. The current skin thermocouple design has been in use for many years and now the question of how well it measures surface temperature has risen. Furthermore a new weld-free design is under consideration to replace the old skin thermocouple design. Another question is therefore how well the new design can measure the surface temperature under the same operating conditions as the old one. In order to evaluate this, three–dimensional computer simulations were made of the different designs. As this thesis will show, the differences in calculated skin thermocouple temperature and calculated surface temper-ature is about the same for the two designs. However, the current design will show a lower temperature than the surface temperature, while the new design will show a higher temperature. Regarding the core of the skin thermocouple designs, namely the thermocouple, no hard conclusions can be drawn, although the industry appears to favor type ’N’ over type ’K’.

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Sammanfattning

Efterfrågan på bättre mätningar ökar alltjämt i dagens industri. En anledning till detta är för att optimera produktionen och därigenom maximera vinsten. En annan anledning är att i vissa fall finns det lagkrav som dikterar att övervakning av vissa parametrar måste göras. Vid Preemraff Lysekil finns det i praktiken fyra skäl till att mäta yttemperatur, s.k. skintemperatur, inuti processugnar. Dessa skäl är: det finns myndighetskrav, för att uppskatta ugnens last, för att göra en livslängdsanalys på ugn-stuberna samt för att avgöra när avkoksning skall ske. I H2301/2/3 är det dock bara de första tre anledningarna som är aktuella. I många år så den nuvarande skinelement-designen använts och nu så har frågan om hur pass rätt den mäter dykt upp. Utöver detta så har en svetsfri design fångat Preemraff Lysekils intresse då ett eventuellt byte av design kan vara aktuellt. En annan fråga som har dykt upp är hur den nya desi-gen står sig mot den gamla gällande avvikelse i uppmätt kontra önskad temperatur. För att kunna utvärdera den nya designen mot den gamla utfördes tre–dimensionella datorsimuleringar och som det här examensarbetet visar kommer avvikelsen mellan den beräknade uppmätta temperaturen och den beräknade tubtemperaturen att vara ungefär lika för de två olika skinelementen. Den gamla designen kommer dock att visa en lägre temperatur än tubtemperaturen medan den nya designen kommer att visa en högre. Angående själva kärnan i skinelementet, nämligen termoelementet, kan inga bestämda slutsatser dras. Dock verkar industrin i allmänhet ha en tendens till att favorisera typ ’N’ över typ ’K’.

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Acknowledgments

I would like to give my thanks to the following persons which have, in their own way, contributed to this master thesis.

• Elisabet Blom • Nils Bjørdal • Per Carlsson • Roland Gårdhagen • Stefan Karlsson • Saeid Kharazmi • Nils Larsson

• Faisal Mohamed Ali • Hans Wernergård • Joakim Wren

Furthermore, I would like to give my sincere thanks to Jan-Gunnar Alexandersson for making this master thesis possible and for all guidance.

Last but certainly not least I would like to express my sincere gratitude to professor Dan Loyd. I am truly amazed at his vast knowledge, rich experience and his ability to always find time to sit down and talk problems through. His guidance has truly been invaluable during this master thesis.

Linköping Mars 2007 Joel Lundh

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List of Figures

1.1 This picture shows how burners inside H2301/2/3 heat the vertically placed tubes. [This picture has been published with the permission of Preemraff Lysekil] . . . 2 1.2 This picture shows how a skin thermocouple exits the fired process heater

through the floor. [This picture has been published with the permission of Preemraff Lysekil] . . . 2 3.1 Schematic figure of the current skin thermocouple. The tube outer

di-ameter is 114.3 mm and the thickness 6.02 mm. The didi-ameter of the sheathing is just over 60 mm, the thickness is close to 4 mm and the length is 150 mm. The dimensions for the plate on which the thermo-couple is placed upon is approximately 24 x 24 x 3 mm. . . 10 3.2 Schematic figure of the new skin thermocouple. The height of the thicker

protective part of the skin thermocouple is 13 mm, the length is 38 mm and the maximum breadth is 16 mm. The contact surface towards the tube has the dimension 8 x 38 mm. . . 11 3.3 Picture of the new skin thermocouple when fasten to a tube. [This

picture has been published with the permission of Nils Bjørdal] . . . 11 4.1 Schematic figure of a thermocouple. DMM is an abbreviation for

digital-multi-meter. . . 14 4.2 This figure shows the output signal from the different thermocouples

with respect to the temperature in the measuring point. The reference point is assumed to have a temperature of 0 ◦

C.[20, 19] . . . 17 4.3 This figure shows how the Seebeck coefficient varies with temperature

for the eight IEC standardized thermocouples.[19] . . . 18 4.4 Schematic figure of a MIMS thermocouple. The conventional MIMS

thermocouple has a sheath of Inconel or stainless steel and incorporates Ni-based thermoelements, i.e. type ’K’ or type ’N’.[7] The insulation used between the sheathing and the thermoelements is exclusively MgO.[11] 19 4.5 Schematic figure of a oven. The temperature on the inside is more or less

the same and therefore will the total temperature gradient be placed in the wall. . . 21

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5.1 This figure shows the geometry of the current skin thermocouple on which the simulations were made. The actual tube has an outer diameter of 114 mm, a thickness of 6 mm and a length of 277 mm. The radiation shield has an outer diameter of 60 mm, a thickness of 4 mm and a length of 150 mm. The plate has the dimensions 24 x 24 x 3 mm and is placed on the tube 10 mm from the end of the radiation shield. . . 24 5.2 (a) shows how the new skin thermocouple with the three spray layers

were modeled in COMSOL. The actual tube has an outer diameter of 114 mm, a thickness of 6 mm and a length of 200 mm. The skin thermocouple is 13 mm at its widest and has a contact area of 8 x 38 mm towards the tube. The three spray layers are equally thick with a thickness of 1 mm. (b) is just a variant of (a). The difference is that in (b) the spray area is larger. The total spray area, excluding the contact area of the thermocouple, is about 0.003 m2

. (c) illustrates the reference geometry, namely the tube it self. The outer diameter is 114 mm, it has a thickness of 6 mm and a length of 277 mm. . . 25 5.3 This figure illustrates a simple 2D mesh. . . 28 5.4 This figure shows two different meshes used in the mesh independence

test. As seen, the mesh in (a) is quite more dense than the one in (b) . 29 6.1 Temperature field on the topside of the metal plate on which the

ther-mocouple is placed upon. The z axis is parallel to the length of the tube and the tube height is increased with decreasing z. The measuring point is placed at x = 0 and z = 0.1. . . 34 6.2 Temperature field of the surface between the tube and the skin

thermo-couple. The z axis is parallel to the length of the tube and the tube’s height is increased with decreasing z. The measuring point is placed at x = 0 and z = 0.09. . . 36 6.3 Temperature field of the surface between the tube and the skin

thermo-couple. The z axis is parallel to the length of the tube and the tube’s height is increased with decreasing z. The measuring point is placed at x = 0 and z = 0.09. . . 38 7.1 The absolute deviance between calculated skin thermocouple

temper-ature and calculated skin tempertemper-ature for the various simulations. It should be noted that the current skin thermocouple shows a lower tem-perature than the actual skin temtem-perature. . . 44

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List of Tables

4.1 This table shows the IEC-code, which metal/alloy the positive and nega-tive wire is made of, the field of work and which atmosphere the specific thermocouple is designed for.[20] . . . 16 4.2 This table shows the letter designation of extension wire and

compensa-tion cable according to IEC standard.[20] . . . 20 6.1 Values of the parameters used in the first simulation’s two cases. . . 31 6.2 Values of the parameters used in the second simulation’s three cases. . 32 6.3 Calculated skin temperature at an ambient temperature of 800 and 850 ◦

C. 33 6.4 Calculated skin temperature for three different internal heat transfer

coefficients, at an ambient temperature of 800 ◦

C. . . 33 6.5 Deviation between calculated skin temperature and calculated skin

ther-mocouple temperature for the current installation. . . 35 6.6 Deviation between calculated skin temperature and calculated skin

ther-mocouple temperature for the current installation in an ambient temper-ature of 800 ◦

ther-mocouple temperature for the new skin therther-mocouple. . . 37 6.8 Deviation between calculated skin temperature and calculated skin

ther-mocouple temperature for the new skin therther-mocouple in an ambient tem-perature of 800 ◦

ther-mocouple temperature for the new skin therther-mocouple with a larger spray area. . . 39 6.10 Deviation between calculated skin temperature and calculated skin

ther-mocouple temperature for the new skin therther-mocouple with a larger spray area, in an ambient temperature of 800 ◦

C. . . 39 7.1 Results from the mesh independence check. . . 42 A.1 This table gives an explanation of the abbreviations regarding metals

and alloys that are common in thermocouples.[12] . . . 53 A.2 Parameters used in the boundary heat flux calculation. . . 53 A.3 This table presents an overview of the material parameters used in the

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Chapter 1 Introduction

Today the demand for better and more accurate measurements of various parameters, e.g. temperature, increases. Some of the main reasons for this is that the industry wants to optimize its processes in order to maximize their profits or to more closely check certain parameters. Another reason is that in some cases government regulations dictate that supervision of certain parameters must be done.

1.1 Background

In the oil refinery business unexpected shutdowns of machinery and or processes equal big monetary losses, especially when it comes to critical equipment. This is why all equipment, if possible, must be chosen so that they can be easily repaired, replaced or so that they can last over a long period of time.

Preemraff Lysekil use thermocouples in their fired process heaters to measure the skin temperature1

. Depending on which type of thermocouple used and how these are fasten to the tubes, the error in the measured temperature varies.

In this thesis the skin thermocouples2

in the fired process heater designated H2301/2/3 has been considered. In H2301/2/3 naphtha3

and hydrogen are being heated in tubes placed vertically as figure 1.1 shows. The skin thermocouples are placed both at the ceiling and the floor of the fired process heater. Figure 1.2 gives a more detailed view of the tubes and it also shows how one of these skin thermocouples exits the fired process heater through the floor.

1

Skin temperature is another word for surface temperature.

2

Skin thermocouples are used for measuring surface temperature, see chapter 3.

3

Naphtha is one of the products from the distillation of crude oil. It usually contains hydrocarbons in the range of C7 – C11.

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Introduction

Figure 1.1: This picture shows how burners inside H2301/2/3 heat the vertically placed tubes. [This picture has been published with the permission of Preemraff Lysekil]

Figure 1.2: This picture shows how a skin thermocouple exits the fired process heater through the floor. [This picture has been published with the permission of Preemraff Lysekil]

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1.2 Objective

The reason why skin thermocouples are used in this fired process heater is threefold. The first reason is due to government regulations. If a fired process heater exceeds a certain output effect than government regulations dictate that supervision of the tem-perature must be done. The second reason is to estimate the load of the H2301/2/3 and the third and final is to estimate the lifetime of the tubes inside the fired process heater. Every fifth year Preemraff Lysekil stops all production for inspection and maintenance during a four week period. During this time the skin thermocouples in H2301/2/3 are being replaced with new ones. The next stop is due in October 2007 and the question of how reliable the current skin thermocouple design really is has risen. Furthermore, an-other type of skin thermocouple, one that requires no welding to the tube, has captured Preemraff Lysekils attention, see section 3.2.

1.2 Objective

Based on the information given in 1.1 this thesis has the following objective: • To give a short repetition of the fundamentals of heat transfer.

• To give an orientation regarding the function and physics behind the thermocouple.

• To give a short survey of the eight IEC4

standardized thermocouples that are used in the industry today and a more detailed one about the type ’K’ and type ’N’ thermocouples.

• To give an analysis, of the present skin thermocouple as well as of the possible re-placement skin thermocouple, regarding deviations in measured skin temperature versus actual skin temperature.

Finally the ultimate goal of this master thesis is to:

• Give a recommendation on which type of thermocouple and which skin thermo-couple that is most suited to use in H2301/2/3.

4

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Chapter 2 Heat Transfer - A Short Repetition

There are basically three fundamental modes that heat can be transferred from one medium to another, namely by conduction, convection or radiation. In all of these modes, heat is always transferred from the higher temperature medium to the lower one.[4]

In real applications however, it is seldom just one mode of heat transfer that is involved, but rather a combination. This chapter will present a brief overview of each of these modes.

2.1 Conduction

Conduction can take place in solids as well as in gases or liquids. In gases and liquids the conduction is due to collisions and diffusion of the molecules during their random motion. In solids, conduction is a combination of the vibrations the molecules in the lattice and the energy transport by the free electrons.[4]

The rate at which heat is transferred by conduction through a homogeneous medium can be described as:

˙

Q = −kAdT

dx (2.1)

where ˙Q is the heat flux, k is the thermal conductivity, A is the area and dT

dx is the

temperature gradient.

This equation, (2.1), is called Fourier’s law of heat conduction. The negative sign on the right-hand-side is a consequence of the second law of thermodynamics which states that heat must flow from a high temperature to a low temperature.[15]

In electrical circuits the flow of electric current can be calculated as the voltage potential divided by the electrical resistance. This method is also applicable for heat flow in a thermal circuit. This states that the heat flux, ˙Q, is equal to the temperature difference, ∆T , divided by the thermal resistance, RT, as equation (2.2) shows.[15]

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Heat Transfer - A Short Repetition

˙

Q = ∆T

RConduction

(2.2) where the thermal resistance for a plate or wall can be written as:

RConduction =

t

Ak (2.3)

or for a tube or pipe it will be as follows: RConduction =

ln(ro

ri)

2πkL (2.4)

2.2 Convection

Convection is a way of energy transfer between a solid and a gas or a liquid. It can be divided into two subcategories, free convection and forced convection. Convection is called forced convection when the fluid is forced over the solid e.g. by a fan or a pump. Convection is called free convection or natural convection when the fluid’s motion is caused by buoyancy forces which are created by density differences due to temperature variations in the fluid.[4]

Convection can be described by Newtons’s law of cooling, equation (2.5): ˙

Q = hA(Ts−T∞) (2.5)

where ˙Q is the heat flux, A is the surface area, Ts is the surface temperature, T∞ is

the temperature of the fluid far from the surface and h is the average convection heat transfer coefficient.

The value of the convection heat transfer coefficient, h, is dependent on many parame-ters such as the geometry of the surface, the physical properties of the fluid, the fluids velocity and in some cases even the temperature difference, Ts−T∞.[15, 4] It is quite

clear that all of these properties are not constant over a surface and that h may vary from point to point. However, in most engineering applications an average value of the convection heat transfer coefficient is satisfactory.

In the same manner as with conduction a thermal resistance can be calculated, as shown in equation (2.6).

RConvection=

1

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2.3 Radiation

and thus the heat flux can be calculated as: ˙

Q = Ts−T∞

RConvection

(2.7)

2.3 Radiation

Radiation is energy emitted from a body by electromagnetic waves. All solids, liquids and gases emit, absorb and/or transmit radiation of varying degrees. However, radia-tion is mostly considered a surface phenomena for opaque solids.[4]

The maximum heat flux a body can emit by radiation is given by the Stefan-Boltzmann law, equation (2.8):

˙

Qemit,max= σAT 4

s (2.8)

where ˙Q is the heat flux, σ is the Stefan-Boltzmann constant, A is the area and T is the temperature in Kelvin.

It should be noted that this equation, equation (2.8), is only valid for an idealized surface, called a blackbody. In real life no body can emit that much radiation.[4] Because of this, the property emissivity is introduced and equation (2.8) can now be written as:

˙

Qemit = σǫATs4 (2.9)

It should also be noted that the emissivity is in the interval of 0 <ǫ <1 and is dependent on the form and texture of the body.

However, a body will not only emit but also absorb radiation. In other words it is the net rate of radiation heat transfer that is interesting. Between two surfaces this can be described as: ˙ Q = σǫA(T4 s −T 4 ∞) (2.10)

In this case, equation (2.10), the emissivity factor ǫ is based not only on the body, for which the calculations are made, but also on the surrounding surfaces.

As for conduction and convection a thermal resistance can also be described for radia-tion, as equation (2.11) shows.

RRadiation = Ts−T∞ Aǫσ(T4 s −T 4 ∞) (2.11) The heat flux can then be calculated as:

˙

Q = Ts−T∞

RRadiation

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Chapter 3 Skin Thermocouples

Thermocouples can be used to measure temperature at many various locations, e.g. the temperature of the air in a duct, the temperature inside a wall or the surface temper-ature of a tube. Another expression for surface tempertemper-ature is skin tempertemper-ature hence the term skin thermocouple.

It is however not uncommon to mount the thermocouple to the surface in quite different ways, according to the situation. In some cases it might be more practical, from an installation point of view, to have the thermocouple welded to a plate which later on is placed on the objects surface. In this case the whole design with thermocouple and plate is referred to as the skin thermocouple.

In this thesis two types of skin thermocouples were studied. First the current installation which has been in use for many years and secondly a new solution which has not been tried out at Preemraff Lysekil yet.

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Skin Thermocouples

3.1 The Current Skin Thermocouple

Figure 3.1 illustrates the current installation. Here the whole design including metal plate, thermocouple, insulation and radiation shield is referred to as the skin thermo-couple.

(a) Cross-sectional view from above (b) Cross-sectional view from side

Figure 3.1: Schematic figure of the current skin thermocouple. The tube outer diameter is 114.3 mm and the thickness 6.02 mm. The diameter of the sheathing is just over 60 mm, the thickness is close to 4 mm and the length is 150 mm. The dimensions for the plate on which the thermocouple is placed upon is approximately 24 x 24 x 3 mm.

In this current installation, see figure 3.1, the metal plate is welded to the tube on three sides allowing for expansion in one direction. This is also the case with the thermocouple which is welded on three sides to the metal plate. The protective shield placed over the thermocouple is welded to the tube and the space between the shield and the tube is filled with verilight, which is a kind of cement. The thermocouple used in this design is of type ’K’. More information of this specific type of thermocouple can be found in section 4.4.

3.2 The New Skin Thermocouple

This type of skin thermocouple is, compared to the current installation, very simple. In fact it is just a thermocouple which has been fasten to the tube by two layers of metal and finally an additional layer of aluminiumoxide. In order to protect the measuring section of the thermocouple its’ sheathing is a bit thicker in that area, otherwise it is a normal thermocouple. In order to fasten the thermocouple to the tube, it is first smoothen by blasting. This is done in order to get a good contact surface between the thermocouple and the tube. The thermocouple is then fasten to the tube with metal ties and a metal spray of hastalloy fixates it. In order to make the installation mechanically robust a second layer of Ni-200 is added upon the thermocouple and finally to enclose and protect the thermocouple from oxidation and reduction a layer for aluminiumoxide is added. A big advantage with this design is that no welding has to take place. As stated in the beginning of this chapter this type of skin thermocouple has not been

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3.2 The New Skin Thermocouple

tested at Preemraff Lysekil yet. Figure 3.2 gives a schematic picture of the new skin thermocouple and figure 3.3 shows how it will look when fasten to a tube.

(a) Cross-sectional view from above (b) Cross-sectional view from side

Figure 3.2: Schematic figure of the new skin thermocouple. The height of the thicker protective part of the skin thermocouple is 13 mm, the length is 38 mm and the max-imum breadth is 16 mm. The contact surface towards the tube has the dimension 8 x 38 mm.

Figure 3.3: Picture of the new skin thermocouple when fasten to a tube. [This picture has been published with the permission of Nils Bjørdal]

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Chapter 4 Thermocouples

The first and foremost thing to know about thermocouples is that they do not measure an absolute temperature, but rather a temperature difference.[20] This chapter will give a brief description of the principle and the physics behind the thermocouple. This chapter also includes a short presentation of different thermocouples and a concluding section in which a more detailed study on type ’K’ and type ’N’ thermocouples is presented.

4.1 The Principle Behind the Thermocouple

In 1821 Thomas Johann Seebeck discovered that when a conductor, e.g. a metal bar, is exposed to a temperature gradient, it will generate a voltage. When two different metals, which are subjected to a temperature gradient, are put together to form a closed circuit, a continuous current will flow in the conductors. This is due to the voltage, the thermoelectric emf1

, between the two metals. This effect is called the Seebeck effect or the thermoelectric effect.[21]

Consider the the circuit shown i figure (4.1).

If T2 = T1 + ∆T and ∆V is the voltage observed at b-c, then the thermopower, i.e.

Seebeck coefficient, is defined by:

SAB = lim ∆T →0

∆V

∆T (4.1)

As stated before, the thermoelectric effect occurs only when two dissimilar conductors are used. This means that the thermoelectric effects are determined by the properties of the individual conductors and thus:

SAB = SB−SA (4.2)

Where SA and SB are the Seebeck coefficients of the metals A and B. Both SA and SB

depend on the respective conductors’ material, molecular structure and usually also the 1

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Thermocouples

temperature.[10] Furthermore it is important to point out that the resulting Seebeck coefficient SAB is a non-linear parameter.

T2

T1 DMM T1

A B

a

b c

Figure 4.1: Schematic figure of a thermocouple. DMM is an abbreviation for digital-multi-meter.

If there is a finite difference between the temperature T1 and T2 then the voltage

gen-erated in the circuit shown in figure (4.1) is:

Vc −Vb = Z T2 T1 SB(T ) − SA(T ) dT (4.3)

or rewritten with equation 4.2:

∆V = Z T2

T1

SAB(T )dT (4.4)

In some cases equation 4.4 can be simplified into:

∆V = SAB(T2−T1) (4.5)

The simplification from equation (4.4) to equation (4.5) is only valid when the Seebeck coefficient is constant enough. The variation of the Seebeck coefficient, for the eight IEC standardized thermocouples, with respect to the temperature can be found in figure 4.3 in section 4.3.

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4.2 The Physics Behind the Thermocouple

As stated in section 4.1 it is the Seebeck effect which allows a conversion of temperature, or rather temperature difference, into electricity. This is due to two effects, namely charge carrier diffusion and phonon drag.[10]

4.2.1 Charge Carrier Diffusion

If there is a thermal gradient in a conductor, the charge carriers2

will diffuse from one end to the other. This means that the hot carriers will diffuse to the cold end of the conductor due to the lower density of hot carriers there. By the same token cold charge carriers will diffuse to the hot end of the conductor. The conductors however are not perfect. There are imperfections and impurities which scatter the diffusing charges and if the scattering is energy dependent, e.g. if the hot electrons scatter more then the cold ones, the hot and cold carriers will diffuse at a different rate. This will create a density difference between the two ends of the conductor and thus create a potential difference, i.e. a voltage.[10]

4.2.2 Phonon Drag

In the simplest model of charge carrier diffusion, it’s assumed that the phonons3

always are in thermal equilibrium. This of course is not completely true and phonons will move along the temperature gradient.[10]

The phonons will interact with crystal imperfections and with electrons and thus lose some of their momentum. If the greater part of the interaction is between phonons and electrons then the phonons will lose their momentum to the electrons and push them to one side of the conductor. This will contribute to the thermoelectric field.The phonon drag contribution will be at its’ most when the phonon-electron interaction is predomi-nant. This occurs for temperatures T ≈ 15ΘD where ΘD is the Debye temperature

4

.[10]

2

In metals the charge carriers are electrons and in semiconductors they are electrons and holes.[10]

3

”The quantum of acoustic or vibrational energy, considered a discrete particle and used especially in mathematical models to calculate thermal and vibrational properties of solids.”.[2, 3]

4

”Debye temperature: In the Debye model of the heat capacity of a crystalline solid, ΘD = hv

D

k ,

where h is Planck’s constant, k is the Boltzmann constant, and vD is the maximum vibrational

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Thermocouples

4.3 The Eight IEC Standardized Thermocouples

The IEC has standardized eight types of thermocouples out of the very many that exist in the world today.[20] Out of these eight there are five that are made of non-precious metal alloys and three that are. These thermocouples have the designations E, J, K, N, T and S, R, B respectively.[20] Table 4.1 gives a quick overview over the eight IEC standardized thermocouples. In table A.1 in Appendix a more detailed explanation of the metal and alloy abbreviations can be found.

Type IEC-code +Wire/-Wire Field of Work Atmosphere

E Violet Chromel/Constantan -200 - 900◦

C Good in oxidizing atmosphere

J Black Fe/Constantan -200 - 760◦

C

Not for use in oxidizing atmosphere or in acids K Green Chromel/Alumel -200 - 1200 ◦ C Good in oxidizing atmosphere N Pink Nicorsil/Nisil 0 - 1300 ◦ C Good in oxidizing atmosphere T Brown Cu/Constantan -200 - 370◦

C Not for use in

oxidizing atmosphere S Orange Pt-10%Rh/Pt 0 - 1480 ◦ C Ceramic protective pipe R Orange Pt-13%Rh/Pt 0 - 1480 ◦ C Ceramic protective pipe B Gray Pt-30%Rh/Pt-6%Rh 0 - 1700 ◦ C Ceramic protective pipe

Table 4.1: This table shows the IEC-code, which metal/alloy the positive and negative wire is made of, the field of work and which atmosphere the specific thermocouple is designed for.[20]

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4.3 The Eight IEC Standardized Thermocouples

As figure 4.2 shows, the difference in output voltage between the thermocouples of type ’R’ and type ’S’ is extremely small and as table 4.1 indicates it is but a small difference in composition in the positive thermoelement that separates them. The reason that they are both included in the standard is that Europeans and Americans could not agree upon which of the thermocouples, type ’S’ from Germany or type ’R’ from the US, to use.[20] 0 200 400 600 800 1000 1200 1400 1600 1800 2000 −10 0 10 20 30 40 50 60 70 80 Temperature [°C] emf [mV] E J T K N R S B

Figure 4.2: This figure shows the output signal from the different thermocouples with respect to the temperature in the measuring point. The reference point is assumed to have a temperature of 0◦

C.[20, 19]

As can be seen in figure 4.2 the eight different thermocouples differs from each other regarding the output in voltage with change in temperature. Since all real systems have a certain amount of background noise, it is favorable to have a large difference in output voltage per degree Celsius since changes in output signal will be easier to detect.

The Seebeck coefficient for a thermocouple is not linear and depends on the physical parameters of the included thermoelements as well as the temperature, i.e. the Seebeck coefficient is a material parameter dependent on the temperature. Figure 4.3 shows how the Seebeck coefficient varies with increased temperature for the eight IEC standardized thermocouples.

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Thermocouples 0 200 400 600 800 1000 1200 1400 1600 1800 2000 −10 0 10 20 30 40 50 60 70 80 90 Temperature [°C] Seebeck coefficient [ µ V/°C] E J T K N R S B

Figure 4.3: This figure shows how the Seebeck coefficient varies with temperature for the eight IEC standardized thermocouples.[19]

4.4 The Type ’K’ and Type ’N’ Thermocouples

The type ’N’ thermocouple was developed to overcome the instabilities of the conven-tional type ’K’ thermocouple.[6] The main difference between the two is the fact that type ’N’ does not experience the aging process of type ’K’. The aging process will re-sult in a drift in output signal. If this drift is large enough the output signal will be unreliable. However, type ’N’ will not give these unreliable output signals, to the same extent, since it will most likely fail before that occurs.

In the industry, when measuring higher temperatures, it is necessarily to protect the thermoelements that makes up the thermocouple because ordinary insulation like PVC5

will melt. The thermocouples used are so called MIMS thermocouples, where MIMS stands for Mineral-insulated metal-sheathed. A schematic picture of the MIMS ther-mocouple can be found in figure 4.4.

5

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4.4 The Type ’K’ and Type ’N’ Thermocouples

Figure 4.4: Schematic figure of a MIMS thermocouple. The conventional MIMS ther-mocouple has a sheath of Inconel or stainless steel and incorporates Ni-based thermoele-ments, i.e. type ’K’ or type ’N’.[7] The insulation used between the sheathing and the thermoelements is exclusively MgO.[11]

4.4.1 Corrosion

The most known and most severe material change that can occur in the type ’K’ ther-mocouple is the corrosion of chromium. In unfavorable conditions, i.e. at 850 – 1000◦

C and with a low content of oxygen, some risk of corrosion of the Chromel wire exists even in MIMS thermocouples.[11, 20] What happens is that the chromium oxidizes at the surface of the wire. This means that less metallic chromium is at the surface of the wire and thus there will be a continuous flow of chromium towards the surface until it’s depleted. The change in material composition will give a lower Seebeck coefficient which means that the measured output emf will differ from the expected one.[11] The change in output emf due to this corrosion may give deviations in temperature of around 10 ◦

C/1000h in operation around 1000 ◦

C.[13]

In type ’N’ thermocouples the amount of chromium is increased and the large amount of silicon gives a stronger oxide layer. There has been no reports of corrosion problems in type ’N’ thermocouples.[11]

4.4.2 Hysteresis and In Situ Drift

Hysteresis6

in the type ’K’ thermocouple predominates at lower temperatures, i.e. around 400 ◦

C. For the type ’N’ alloys it is in the vicinity of 700 ◦

C that hystere-sis is greatest. For both types of thermocouples these peaks cause changes in the net Seebeck coefficient of about 1-1.5 %. This means that hyseteresis contributes with about 2◦

C to long time drift.[7, 8] 6

Phenomenon in which the response of a physical system to an external influence depends not only on the present magnitude of that influence but also on the previous history of the system.[1]

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Thermocouples

In situ drifts7

in the emf, due to changes in the Seebeck coefficient, are smaller for wires in Inconel8

sheathing than for those in stainless steel. This change is even less if a thermoelement of type ’N’ is used rather than one of type ’K’. The change in emf is linked to the presence of the element manganese in stainless steel, Alumel and to some extent in Inconel.[9]

If a type ’N’ thermocouple is sheathed with stainless steel and is used at high tem-peratures, i.e. temperatures in the vicinity of 1100 ◦

C, then an in situ drift in emf corresponding to 25 – 30 ◦

C is to be expected when used for 1000 h. If however a sheathing of Inconel is used instead, the variation in emf would be equivalent to 3 – 5

◦

C.[9]

4.5 Extension Wire and Compensation Cable

The difference between the extension wire and the compensation cable is that the com-pensation cable is made out of a different material than the thermocouple but has the same thermoelectric properties, at least in a narrow temperature range. The extension wire on the other hand is made of the same material as the thermocouple although the tolerance demands have a limited temperature range compared to the thermocouple.[20] Regarding thermocouples of type ’K’ and type ’N’ there is virtually no reason for using anything other than thermocouple cable. However, regarding thermocouples of type ’S’, ’R’ and ’B’, i.e. noble thermocouples, there is much money to be saved using a compensation cable.

Extension wire and compensation cable for different thermocouples have been stan-dardized. Table 4.2 shows the different designations according to the IEC.

Designation Explanation

K Type of thermocouple, according to standard.

KX Extension wire of the same material as the thermocouple.

KCA Compensation cable of a different material than the thermocouple. The last letter, i.e. the A, stands for the type of alloy used.

Table 4.2: This table shows the letter designation of extension wire and compensation cable according to IEC standard.[20]

7

In situ _{drift refers to the changes in emf while the emf is monitored at a fixed T, i.e. with a} constant temperature profile along the MIMS thermocouple.[9]

8

Inconel is a registered trademark referring to a family of austenitic nickel-based high-performance alloys.

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4.6 Considerations at a Thermocouple Installation

As described in section 4.1 a thermocouple will generate a voltage, i.e. an emf, when subjected to a temperature gradient. This means that every part of the thermocouple, that is in a temperature gradient, will contribute to the output emf.

Consider figure 4.5. The temperature is, more or less, the same inside the oven. This means that there will be no output emf from the part of the thermocouple that is inside the oven. However, the part of the thermocouple that is in the wall is subjected to a large temperature gradient. It is here that all of the output emf is generated. This behavior is quite an important thing to have in mind when installing a thermocouple.

6

-T

x

Figure 4.5: Schematic figure of a oven. The temperature on the inside is more or less the same and therefore will the total temperature gradient be placed in the wall.

Consider figure 4.5 again. If the extension is made inside the oven all of the emf will be generated in the extension cable. This might be very undesirable if the extension cable and the thermocouple cable differs in thermoelectric properties. If this in fact is the case, then the output signal from the thermocouple will correspond to a temperature that differs from the real temperature.

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Chapter 5 Method

The method used to evaluate the skin thermocouples is based on two cornerstones, namely simulation and the experience of competent people in and around the temper-ature measuring business. Regarding the simulations made, it is important to realize that these are based on a model which is just an approximation of the real world and not an exact replica.

5.1 Three–Dimension Simulation

In the three–dimension simulation process it was the program COMSOL, formerly FEMLAB, that was used. In fact all of the simulation process, the implementation of the geometry, the meshing, the simulation and the after study were done in COM-SOL.

The simulations were divided into two sets with two and three cases respectively. All geometries, see 5.1.1, were used in each case. The fundamental difference between the simulation sets is the boundary condition on the inside of the tube, see 5.1.2. In the first simulation set a constant heat flux is used and in the second simulation set a convective boundary condition is utilized.

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Method

5.1.1 Geometries

In all, four different geometries has been considered. The first geometry was that of the current skin thermocouple. As figure 5.1 shows some simplifications were made. For one, the actual thermocouple has been left out. Another is that, due to symmetry, the tube has been cut in half. The reason for these simplifications is non other than to reduce the number of cells in the mesh and thus minimize the number of calculations.

Figure 5.1: This figure shows the geometry of the current skin thermocouple on which the simulations were made. The actual tube has an outer diameter of 114 mm, a thickness of 6 mm and a length of 277 mm. The radiation shield has an outer diameter of 60 mm, a thickness of 4 mm and a length of 150 mm. The plate has the dimensions 24 x 24 x 3 mm and is placed on the tube 10 mm from the end of the radiation shield.

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5.1 Three–Dimension Simulation

The second geometry studied was the new skin thermocouple, as shown in figure 5.2 (a). Even here some simplifications to the geometry has been made. Just as for the current skin thermocouple, the actual thermocouple were left out and the tube were cut in two. Furthermore the spray were idealized to cover just the thermocouple and only a small area of the tube. The third geometry considered, figure 5.2 (b), were in fact a variant of the second geometry. In this case however the spray were applied on a larger area of the tube, as can bee seen in 5.2. The fourth and last geometry considered is used as a reference in the oncoming simulations, figure 5.2 (c).

(a) Geometry 2: New Skin Thermocouple (b) Geometry 3: New Skin Thermocouple – Larger Spray Area

(c) Geometry 4: Reference geometry

Figure 5.2: (a) shows how the new skin thermocouple with the three spray layers were modeled in COMSOL. The actual tube has an outer diameter of 114 mm, a thickness of 6 mm and a length of 200 mm. The skin thermocouple is 13 mm at its widest and has a contact area of 8 x 38 mm towards the tube. The three spray layers are equally thick with a thickness of 1 mm. (b) is just a variant of (a). The difference is that in (b) the spray area is larger. The total spray area, excluding the contact area of the thermocouple, is about 0.003 m2

. (c) illustrates the reference geometry, namely the tube it self. The outer diameter is 114 mm, it has a thickness of 6 mm and a length of 277 mm.

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Method

5.1.2 Assumptions and Boundary Conditions

The validity of a model is, as in all models, based on the boundary conditions and assumptions made for the model. In this master thesis two simulation sets were utilized and the assumptions and boundary conditions for these are as follows.

Simulation Set 1

For the two simulation cases in the first simulation set the following boundary conditions were set:

• BC 1: The inside of the tube has a heat flux boundary condition.

• BC 2: The outer side of the tube and the skin thermocouple has a radiative and convective boundary condition.

• BC 3: The cut surfaces of the tube has symmetric boundary conditions.

The boundary condition on the inside of the tube can be justified, at least from an engineers point of view, as follows. The temperature of the naphtha is known in both ends of the pipe and thus can a mean value of the heat transfered be calculated. The values of the parameters used in the calculation of the heat flux can be found in table A.2.

Simulation Set 2

For the three simulation cases in the second simulation set the following boundary conditions were set:

• BC 1: A convective boundary condition is used on the inside of the tube.

• BC 2: The outer side of the tube and the skin thermocouple has a radiative and convective boundary condition.

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The assumptions made in the two simulation sets were as follows: • A steady state situation.

• A constant ambient temperature.

• The heat flux from the inside of the tube to the naphtha is uniformed over the whole pipe. (First simulation set)

• The heat transfer coefficient is the same along the inside of the tube. (Second simulation set)

• All sides of the skin thermocouple is subjected to radiation.

• The measured temperature is the same as the temperature in the point where the measuring point of the thermocouple is placed.

• No heat flux due to convection on the outer side of the skin thermocouple. Regarding the last assumption, it has been made due to two facts. The first one is that in this high temperature environment the heat flux due to radiation is much greater than the heat flux due to convection. Secondly, it is very hard to calculate/approximate the convective heat transfer coefficient from the flue gas to the skin thermocouple.

Since some parameters of certain materials could not be found, they were substituted with those of materials with similar characteristics. Table A.3 shows the values of all material parameters used in the calculations.

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Method

5.1.3 Mesh

As stated early in chapter 5.1, COMSOL were used to generate a mesh. The mesh generation in COMSOL is not complex per se. However if a mesh is made to fine, a great deal of computational power is required since the equations are to be solved in all nodes. To illustrate this problem consider figure 5.3. This figure illustrates a simple two dimensional mesh where ∆x and ∆y are the distance between the nodes. Given a fix area the number of nodes will increase with a decreasing distance in x and y direction and hence a increased number of calculations for a finer mesh.

z }| { ∆x ∆y   

Figure 5.3: This figure illustrates a simple 2D mesh.

In order to validate that the mesh did not have any fundamental impact on the solution, several simulations were made with the same assumptions and boundary conditions but with different meshes. Figure 5.4 illustrates two of the different meshes used in the three dimensional simulations.

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(a) Mesh with about 112 000 elements

(b) Mesh with about 8 100 elements

Figure 5.4: This figure shows two different meshes used in the mesh independence test. As seen, the mesh in (a) is quite more dense than the one in (b)

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Chapter 6 Results

The results presented in section 6.1 were calculated with the values given in table 6.1 and in table 6.2. As pointed out in section 5.1.2 all geometries were used in all of the simulation cases. As stated in section 5.1 the main difference between the two simula-tion sets is the inside boundary condisimula-tion, namely constant heat flux and a convective boundary condition.

Simulation set 1

Case 1

Parameter Value Description

Tamb, ambient temperature 800 ◦C Based on measurements of

the flue gas temperature

ǫ, thermal emissivity 0.9

Estimated value on the outside of the tube and the radiation sheathing ˙q, heat flux 43500 W/m2 Calculated from process data

given by Preemraff Lysekil ho, heat transfer coefficient 0 W/m2 ◦C No convective heat flux added

on the outside of the tube

Case 2

Tamb, ambient temperature 850 ◦C Based on measurements of_{the flue gas temperature}

Estimated value on the outside of the tube and the radiation sheathing ˙q, heat flux 43500 W/m2 Calculated from process data

on the outside of the tube Table 6.1: Values of the parameters used in the first simulation’s two cases.

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Results

Simulation set 2

Case 1

Estimated value on the outside of the tube and the radiation sheathing hi, heat transfer coefficient 50 W/m2 ◦C _{given by Preemraff Lysekil}Based on process data

ho, heat transfer coefficient 0 W/m2 ◦C No convective heat flux added

Case 2

Tamb, ambient temperature 800 ◦C Based on measurements of_{the flue gas temperature}

Estimated value on the outside of the tube and the radiation sheathing hi, heat transfer coefficient 100 W/m2 ◦C Based on process data

Case 3

ǫ, thermal Emissivity 0.9

Estimated value on the outside of the tube and the radiation sheathing hi, heat transfer coefficient 800 W/m2 ◦C Based on process data

given by Preemraff Lysekil ho, heat transfer coefficient 0 W/m2 ◦C No convective heat flux added_{on the outside of the tube}

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6.1 Three–Dimension Simulations

Just as in the beginning of chapter 5 it is important to stress that these results are based upon a model which is a simplification of the real world. With this in mind one should realize that there will be deviations between the model and the real world.

6.1.1 Reference Simulation – The Tube

Simulation 1

Ambient Calculated

Temperature Skin Temperature 800 ◦ C 593 ◦ C 850 ◦ C 681 ◦ C

Table 6.3: Calculated skin temperature at an ambient temperature of 800 and 850 ◦

C. Simulation 2

Heat Transfer Calculated

Coefficient Skin Temperature 50 W/m2 ◦ C 748 ◦ C 100 W/m2 ◦ C 708 ◦ C 800 W/m2 ◦ C 548 ◦ C

Table 6.4: Calculated skin temperature for three different internal heat transfer coeffi-cients, at an ambient temperature of 800 ◦

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Results

6.1.2 Current Skin Thermocouple

Simulation 1

(a) Ambient temperature of 800◦C

(b) Ambient temperature of 850◦C

Figure 6.1: Temperature field on the topside of the metal plate on which the thermo-couple is placed upon. The z axis is parallel to the length of the tube and the tube height is increased with decreasing z. The measuring point is placed at x = 0 and z = 0.1.

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Ambient Calculated Calculated

∆T

Temperature Skin Temperature Skin Thermocouple

Temperature 800 ◦ C 593 ◦ C 574 ◦ C - 19 ◦ C 850 ◦ C 681 ◦ C 646 ◦ C - 35 ◦ C

Table 6.5: Deviation between calculated skin temperature and calculated skin thermo-couple temperature for the current installation.

Simulation 2

Heat Transfer Calculated Calculated

∆T

Coefficient Skin Temperature Skin Thermocouple

Temperature 50 W/m2 ◦ C 748 ◦ C 739 ◦ C - 9 ◦ C 100 W/m2 ◦ C 708 ◦ C 695 ◦ C - 13 ◦ C 800 W/m2 ◦ C 548 ◦ C 540 ◦ C - 8 ◦ C

Table 6.6: Deviation between calculated skin temperature and calculated skin ther-mocouple temperature for the current installation in an ambient temperature of 800

◦

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Results

6.1.3 New Skin Thermocouple

Simulation 1

Figure 6.2: Temperature field of the surface between the tube and the skin thermocou-ple. The z axis is parallel to the length of the tube and the tube’s height is increased with decreasing z. The measuring point is placed at x = 0 and z = 0.09.

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∆T

Temperature 800 ◦ C 593 ◦ C 648 ◦ C + 55 ◦ C 850 ◦ C 681 ◦ C 725 ◦ C + 44 ◦ C

Table 6.7: Deviation between calculated skin temperature and calculated skin thermo-couple temperature for the new skin thermothermo-couple.

Simulation 2

∆T

Temperature 50 W/m2 ◦ C 748 ◦ C 760 ◦ C + 12 ◦ C 100 W/m2 ◦ C 708 ◦ C 728 ◦ C + 20 ◦ C 800 W/m2 ◦ C 548 ◦ C 583 ◦ C + 35 ◦ C Table 6.8: Deviation between calculated skin temperature and calculated skin thermo-couple temperature for the new skin thermothermo-couple in an ambient temperature of 800

◦

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Results

6.1.4 New Skin Thermocouple – Larger Spray Area

Simulation 1

Figure 6.3: Temperature field of the surface between the tube and the skin thermocou-ple. The z axis is parallel to the length of the tube and the tube’s height is increased with decreasing z. The measuring point is placed at x = 0 and z = 0.09.

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∆T

Temperature 800 ◦ C 593 ◦ C 629 ◦ C + 36 ◦ C 850 ◦ C 681 ◦ C 709 ◦ C + 28 ◦ C

Table 6.9: Deviation between calculated skin temperature and calculated skin thermo-couple temperature for the new skin thermothermo-couple with a larger spray area.

Simulation 2

∆T

Temperature 50 W/m2 ◦ C 748 ◦ C 755 ◦ C + 7 ◦ C 100 W/m2 ◦ C 708 ◦ C 720 ◦ C + 12 ◦ C 800 W/m2 ◦ C 548 ◦ C 567 ◦ C + 19 ◦ C Table 6.10: Deviation between calculated skin temperature and calculated skin ther-mocouple temperature for the new skin therther-mocouple with a larger spray area, in an ambient temperature of 800 ◦

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Chapter 7 Discussion

7.1 Simulations

7.1.1 Simplifications

In all simulations a steady state situation has been considered. This is of course a simplification since there will be fluctuations both in the temperature on the outside of the tube due to variations of the fuel gas feed rate and quality of the fuel gas but also on the inside of the tube. The fluctuations on the inside of the tube will mainly depend on the product feed rate and the quality of the naphtha. However these variations are fairly small over time and so the approximation to a steady state is valid.

Another simplification that has been made regards the thermal conductivity. In real life the thermal conductivity is dependent on the temperature, in the simulations however it has been approximated as a constant.

In the models, the thermocouple has been excluded. This has been done for two main reasons. The first reason is simply to reduce the number of calculations that has to be done. In order to include the thermocouple in the simulations a very fine grid must be used and thus more calculations must be done. The second reason is to avoid mesh related problems. In early models, which included a thermocouple, problems with the grid occurred. In order to get a working mesh of good quality a very large number of cells were required. Unfortunately the computational power available was not enough. In short, the exclusion of the thermocouple in the models, will have an impact on the results; the extent of which has not been investigated in this thesis.

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Discussion

7.1.2 Mesh Independence

The mesh independence was controlled in the following manner. Four meshes with different number of elements were made for the current skin thermocouple. The mean temperature over outside boundary were then compared amongst the meshes. Table 7.1 presents the results.

Number of Elements Mean Value of Temperature Deviation From

on the Outside Boundary [◦

C] Densest Mesh [%]

112136 596.683 –

64816 596.701 0.003

62992 596.701 0.003

8102 596.810 0.02

Table 7.1: Results from the mesh independence check.

As table 7.1 shows, there is little difference between a mesh with about 112 000 ele-ments and one with about 8 000 eleele-ments. It is therefore quite safe to assume mesh independence. However when a mesh with few elements is used, every element cover a larger piece of the geometry and in certain regions, e.g. areas with small details, problems with the resolution might arise.

7.1.3 Material Parameters

In the simulation process it is important to try to approximate the thermal conductivity of the different materials as well as possible since it might have a large impact on the solution. To give an example consider the current skin thermocouple and in particular the verilight. In this simulation verilight has been approximated with ordinary cement and with a thermal conductivity of 1.77 W/m K. If however a higher thermal conduc-tivity is used say 35 W/m K, as the one for aluminiumoxide, the plate on which the thermocouple is fasten to will experience a 140 ◦

C change upward in temperature in an ambient surrounding of 800 ◦

C. This will mean that from measuring a lower tem-perature than the real tube temtem-perature, it will now measure a higher one. However, it should be realized that this example was a sort of worst case scenario, where the thermal conductivity changed with about 2000 %. For smaller uncertainties, e.g. the one with aluminumoxide which according to [5] is in the range of 18 – 35 W/m K, the impact on the solution will hardly be noticable. In the case with the aluminumoxide the change from 35 W/m K, which was used in the simulations, to 18 W/m K gave an over all change in temperature of less than 2◦

C in an ambient surrounding of 800◦

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7.2 Thermocouple

The life expectancy of the thermocouples are very difficult to anticipate. Although it can be said that the life expectancy will decrease with increased temperature. The life expectancy will also decrease if the thermocouple is located in an unfavorable atmo-sphere. Since the thermocouple is embedded in either cement or in two metal layers and a aluminiumoxide layer, the atmosphere is assumed to have little impact on the thermo-couple and thus the lifetime of the thermothermo-couple is more dependent on the temperature than on the atmosphere in the fired process heater. However at high temperatures some materials will be no more dense than a sieve.

Regarding the differences between the type ’K’ and type ’N’ thermocouple, it is hard to draw any good conclusions. Information obtained in articles, e.g. [11, 20, 12] points to an slight advantage for type ’N’. One big advantage with the type ’N’ is that it does not age in the same manner as type ’K’ and thus the output signal from a type ’N’ will be more reliable. Furthermore, in todays industry more and more companies are replacing their old type ’K’ thermocouples with new type ’N’.

Thermocouples can be designed in various ways. One example of this is the placement of the measuring point, which can be connected to ground through the sheathing or not. However, in this thesis the effects on the thermocouple due to the placement of the measuring point has not been investigated.

7.3 Skin Thermocouples

As can be viewed in chapter 6 the current skin thermocouple presents a 20 – 40 ◦

C lower temperature with respect to the actual tube temperature, given the set conditions in the first and second case in the first simulation set. The boundary condition on the inside of the tube is however, relatively harsh. If this condition is replaced with a more gentle one, as that in cases 1 – 3 in the second simulation set, the skin thermocouple temperature will differ downwards with about 10 – 15 ◦

C from the actual tube tem-perature. If this information is used in a lifetime analysis of the tubes it will result in an overestimation of the lifetime of the tubes. Having the thermocouple in a lower temperature however, is favorable for the thermocouple itself since it will last longer.

Regarding the new skin thermocouple it is important to realize that it will work as the opposite of a cooling flange. If only a small area is utilized when fixating the ther-mocouple to the tube, it will mean that a large outside area will be connected to a small area on to the tube and thus a hot-spot will be formed. This however is heavily dependent on which type of material used in the fixating-, i.e. spray-, process.

In a comparison between the new skin thermocouple and the current one, the simula-tions shows that the deviation from the tube skin temperature is about the same for the two. However the new skin thermocouple will, according to the simulations, show a too high temperature meanwhile the current one will show a too low temperature

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Discussion

with respect to the actual tube temperature. Figure 7.1 shows the deviation between the calculated skin thermocouple temperature and the calculated skin temperature. It should be noted that the heat transfer coefficient will most likely be in the range of 100 – 800 W/m2 ◦

C in the real case.

Tamb = 800 °C Tamb = 850 °C h = 50 W/m K h = 100 W/m K h = 800 W/m K 0 10 20 30 40 50 60 [°C]

Difference in temperature − Calculated Measured Temperature Vs Calculated Tube Temperature (Absolute Values)

Current Skin Thermocouple New Skin Thermocouple

New Skin Thermocouple − Larger Spray Area

Figure 7.1: The absolute deviance between calculated skin thermocouple temperature and calculated skin temperature for the various simulations. It should be noted that the current skin thermocouple shows a lower temperature than the actual skin temperature.

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7.4 Long Time Operation of the Skin Thermocouple

One important thing to have in mind is that once the skin thermocouple is in place it will run continuously for, at least, five years and without any means of calibration. In order to validate the thermocouple, a reference system should be implemented. It is not important that the reference method used gives a correct reading of the temperature, what is important is that the error in the reference system is constant.

As stated before, there is no means of replacing or calibrating the skin thermocouples today. Once they are installed they will be in use at least five years. If however a solution with a sheathing through the ceiling or the floor of the fired process heater was designed, it would mean that the thermocouple could be replaced and or calibrated. It is important to realize that the thermocouple is just a measuring device and like all measuring devices it is in need of regular calibration in order to be reliable.

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Chapter 8 Conclusions and Recommendations

It is hard to draw any conclusions regarding which type of thermocouple to use. The type ’N’ thermocouple has the advantage that it will not drift in output signal in the same manner as type ’K’. Furthermore, many industries are making the switch from type ’K’ to type ’N’. An educated guess would then be that, since they are not changing back, the type ’N’ thermocouple is better or at least just as good as the type ’K’. In order to validate this, it is recommended that a survey is carried out on companies that have gone from type ’K’ to type ’N’.

According to the simulations made, there is little difference between the two different skin thermocouples, in a measured temperature point of view. The current skin ther-mocouple will show a lower temperature than the skin temperature on the tube and the new skin thermocouple will show a higher one. The difference between measured and wanted temperature is however almost the same. It is therefore recommended that the decision on which skin thermocouple to use should be based more on other parameters, e.g. the usage of welding or not.

The biggest drawback with the skin thermocouples is that it is not possible to replace or remove them for calibration during operation. It is therefore highly recommended that, if not a removable solution is implemented, a reference system is utilized.

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Chapter 9 Nomenclature

Symbol Description Unit

σ Stefan-Boltzmann constant W/m2 K4 ǫ Emissivity – Θ Debye temperature ◦ C A Area m2

h Convection heat transfer coefficient W/m2

K k Thermal conductivity W/m K L Length m Q Heat flux W R Thermal resistance K/W ro Outer radius m ri Inner radius m

SAB Resulting Seebeck coefficient V/◦C

SA Seebeck coefficient for material A V/◦C

SB Seebeck coefficient for material B V/◦C

t Thickness m T1 Lower temperature ◦C T2 Higher temperature ◦C Ts Surface temperature ◦C T∞ Ambient temperature ◦ C V Voltage V

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Bibliography

[1] The Columbia Electronic Encyclopedia. Columbia University Press, sixth edition, 2003.

[2] The American Heritage Dictionary of the English Language. Houghton MifflinR

Company, fourth edition, 2004.

[3] Encyclopedia of Science and Technology. McGraw-Hill Companies, Inc., 2005. [4] Yunus A.Çengel and Robert H. Turner. Fundamentals of Thermal-Fluid Sciences.

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Simulation and Evaluation of Two Different Skin Thermocouples : A Comparison made with Respect to Measured Temperature