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`Euro-Pro Exhibit 1006
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`Page 1 of 89
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`Page 1 of 89
`Page 1 of 89
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`

`
`Integrating
`Electrical Heating
`
`Elements in
`
`Appliance Design
`
`Page 2 of 89
`Page 2 of 89
`
`

`
`ELECTRICAL ENGINEERING AND ELECTRONICS
`A Series of Reference Books and Textbooks
`
`EXECUTIVE EDITORS
`
`Marlin 0. Thurston
`Department of Electrical Engineering
`The Ohio State University
`Columbus, Ohio
`
`lVi'lliorn Middendorf
`Department of Electrical
`and Computer Engineering
`University of Cincinnati
`Cincinnati. Ohio
`
`EDITORIAL BOARD
`
`Maurice Betionger
`Télécornmunieations. Radioélcctriques,
`et Téléphoniqucs (TRT)
`Le Plessts~Robinson. France
`
`Norman E. Fuqrua
`Reliability Analysis Center
`Grifiiss Air Force Base, New York
`
`Pradeep Kikosia
`Camegie—Mellon University
`Pittsburgh, Pennsylvania
`
`J‘. Lewis Blackburn
`Bothcll,Washington
`
`Naim A. Kiteir
`Department of Electrical and
`Systems Engineering
`Oakland University
`Rochester, Michigan
`
`Glenn Zelniker
`Z-Systems, Inc.
`Gainesville, Florida
`
`1 . Rational Fault Analysis, edited by Richard Seeks and S. R. Liberty
`2. Nonparametric Methods in Communications. edited by P. Papanroni-
`Kazakos and Dimitn' Kezakos
`
`interactive Pattern Recognition, Yi—tzuu Chien
`Solid-State Electronics, Lawrence E. Murr
`Electronic, Magnetic, and Thermal Properties of Solid Materials.
`Kiaus Schrdder
`
`T.
`
`
`
`T4?‘-'F"?!-*’
`
`|Vlagnetic—Bubb|e Memory Technologv. Hsu Chang
`Transformer and inductor Design Handbook, Colonel Wm.
`Mclyman
`8. Electromagnetics: Classical and Modern Theory and Applications,
`Samue! Seery and Alexander 0. Fouiarikas
`9. One—Dimensional Digital Signal Processing, Chi—Tsong Chen
`10.
`Interconnected Dynamical Systems, Raymond A. Decario and Richard
`Sacks
`
`Page 3 of 89
`Page 3 of 89
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`11.
`12.
`13.
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`14.
`15.
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`16.
`17.
`18.
`19.
`20.
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`21.
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`29.
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`31.
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`32.
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`33.
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`34.
`35.
`
`36.
`
`37.
`38.
`39.
`
`Modern Digital Control Systems, Raymond G. Jacquot
`Hybrid Circuit Design and Manufacture, Roydn D. Jones
`Magnetic Core Selection for Transformers and Inductors: A User's
`Guide to Practice and Specification, Colonel‘ Wm. T. Mciyman
`Static and Rotating Electromagnetic Devices, Richard H. Engeimann
`Energy-Efficient Electric Motors: Selection and Application, John C.
`Andreas
`
`Electromagnetic Cornpossibility, Heinz M. Schiicke
`Electronics: Models, Analysis, and Systems, James G. Gotr.-ling
`Digital Filter Design Handbook, Fred J. Tayior
`Multivariable Control: An Introduction, P. K. Sinha
`Flexible Circuits: Design and Applications, Steve Guriey, with con-
`tributions by Cari A. Edstrom, Jr., Ray D. Greenway, and VVifliam P.
`Keiiy
`Circuit Interruption: Theory and Techniques, Thomas E. Browne, Jr.
`Switch Mode Power Conversion: Basic Theory and Design, K. Kit
`Sum
`
`Pattern Recognition: Applications to Large Data-Set Problems, Sing-
`Tze Bow
`
`Custon1—Specific Integrated Circuits: Design and Fabrication. Stanley
`L Hurst
`
`Digital Circuits: Logic and Design, Ronald C. Emery
`Large-Scale Control Systems: Theories and Techniques, Magdi .5‘.
`Mahmoud, Mohamed F. Hassan, and Mohamed G. Darwrsh
`Microprocessor Software Project Management. E.-‘i T. Farhi and Cedric
`V. W Armstrong {Sponsored by Ontario Centre for Microeiectronicsi
`Low Frequency Electromagnetic Design. Miehaei P. Perry
`Multidimensional Systems: Techniques and Applications. edited by
`Spyros G. Tzafestas
`AC Motors for High-Performance Applications: Analysis and Control,
`Sakae Yamamura
`
`Ceramic Motors for Electronics: Processing, Properties, and Applica-
`tions, edited by Reiva C. Buchanan
`Microcomputer Bus Structures and Bus Interface Design, Arthur L.
`Dexter
`
`End User's Guide to Innovative Flexible Circuit Packaging, Jay J.
`Miniet
`
`Reliability Engineering for Electronic Design, Norman 3. Fuqua
`Design Fundamentals for Low-Voltage Distribution and Control, Frank
`W Kussy and Jack 1.. Warren
`Encapsulation of Electronic Devices and Components, Edward R.
`Salmon
`
`Protective Relaying: Principles and Applications, J. Lewis Biackburn
`Testing Active and Passive Electronic Components, Richard F. Poweii
`Adaptive Control Systems: Techniques and Applications,
`1/.
`l/.
`Chaiam
`
`Page 4 of 89
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`41.
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`56.
`57.
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`62.
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`63.
`64.
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`65.
`66.
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`67.
`68.
`69.
`70.
`71.
`
`Computer~Aided Analysis of Power Electronic Systems, Venirarachari
`Rajagopaian
`Integrated Circuit Quality and Reliabiiity, Eugene H’. Hnatek
`Systolic Signal Processing Systems. edited by Earl E. Swarm.-iander, Jr.
`Adaptive Digital Filters and Signal Analysis, Maurice G. Beiianger
`Electronic Ceramics: Properties, Configuration,
`and Applications,
`edited by Lionei M. Levinson
`Computer Systems Engineering Management, Robert S. Afford
`Systems Modeling and Computer Simulation, edited by Naim A. Kheir
`Rigid-Flex Printed Wiring Design for Production Readiness, Waiter S.
`Fr‘r'gi.ing
`Analog Methods for Computer-Aided Circuit Analysis and Diagnosis,
`edited by Takao Ozawa
`Transformer and Inductor Design Handbook: Second Edition, Revised
`and Expanded. Colonel Wm. T. Mcllyman
`Power System Grounding and Transients: An Introduction, A. P.
`Sakis Meiiopouios
`Signal Processing Handbook, edited by C. H. Chen
`Electronic Product Design for Automated Manufacturing, H. Richard
`Stiiiweil
`
`Dynamic Models and Discrete Event Simulation,
`Erminia Vaccari
`
`I/I/wiam Deianey and
`
`FET Technology and Application: An Introduction, Edwin S. Oxner
`Digital Speech Processing, Synthesis, and Recognition, .S‘adao.-'riFurui
`VLSI RISC Architecture and Organization, Stephen B. Furber
`Surface Mount and Related Technologies, Gerald Ginsberg
`Uninterruptible Power Supplies: Power Conditioners for Critical Equip-
`ment, David C. Griffith
`Polyphase Induction Motors: Analysis, Design, and Application, Paul
`L. Cochran
`
`Battery Technology Handbook, edited by H. A. Kiehne
`Network Modeiing, Simulation, and Analysis, edited by Ricardo F.
`Garzia and Mario R. Garzia
`
`Linear Circuits, Systems, and Signal Processing: Advanced Theory
`and Applications, edited by Nobuo Nagai
`High-Voltage Engineering: Theory and Practice. edited by M. Khaiifa
`Large-Scale Systems Control and Decision Making, edited by Hiroyuki
`Tamura and Tsuneo Yoshikawa
`
`Industrial Power Distribution and Illuminating Systems, Kao Chen
`Distributed Computer Control for Industrial Automation, Dobrivoje
`Popovri: and I./day P. Bbarirar
`Computer~Aided Analysis of Active Circuits. Adrian loinovici
`Designing with Analog Switches, Steve Moore
`Contamination Effects on Electronic Products, Can’ J. Taotscber
`Computer-Operated Systems Control, Magdi S. Mahmoud
`Integrated Microwave Circuits, edited by Yoshrbiro Konr's.-"a'r'
`
`Page 5 of 89
`Page 5 of 89
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`72.
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`75.
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`81.
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`83.
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`87.
`88.
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`89.
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`90.
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`91.
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`92.
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`93.
`94.
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`95.
`96.
`97.
`
`98.
`
`Ceramic Materials for Electronics: Processing, Properties, and Appli-
`cations, Second Edition. Revised and Expanded, edited by Reiva C.
`Buchanan
`
`Electromagnetic Compatibility: Principles and Applications, David A.
`Weston
`
`Intelligent Robotic Systems. edited by Spyros G. Tzafestas
`Switching Phenomena in High-Voltage Circuit Breakers, edited by
`Kunio Nakanrshi
`
`Advances in Speech Signal Processing, edited by Sadaoki Furui and
`M. Mo.-‘Ian Sondhi
`
`Pattern Recognition and Image Preprocessing, Sing-Tze Bow
`Energy-Efficient Electric Motors: Selection and Application, Second
`Edition, John C. Andreas
`Stochastic Large-Scale Engineering Systems, edited by Spyros G.
`Tzafestas and Keigo Watanabe
`Two-Dimensional Digital Filters, Wu-Sheng Lu and Andreas Antoniou
`Computer-Aided Analysis and Design of Switch-Mode Power Supplies,
`Wm-Shu Lee
`
`Placement and Routing of Electronic Modules, edited by ilzfichaei Pecht
`Applied Control: Current Trends and Modern Methodologies, edited
`by Spyros G. Tzafestas
`Algorithms
`for Computer-Aided Design of Multivariable Control
`Systems, Stanoje Bingulac and Hugh F.
`'l/anLandingharn
`'
`Symmetrical Components for Power Systems Engineering, J. Lewis
`Blackburn
`
`Advanced Digital Signal Processing: Theory and Applications, Gienn
`Zeiniker and Fred J. Ta yior
`Neural Networks and Simulation Methods, Jian-Kang Wu
`Power Distribution Engineering: Fundamentals
`and Applications,
`James J. Burke
`
`Modern Digital Control Systems: Second Edition, Raymond G. Jac-
`quot
`Adaptive IIR Filtering in Signal Processing and Control, Pnifiip A.
`Regaiia
`Integrated Circuit Quality and Reliability: Second Edition, Revised and
`Expanded, Eugene R. Hnatek
`Handbook of Electric Motors, edited by Richard H. Engelmann and
`l/lfiiiiam H. Middendorf
`
`Power-Switching Converters, Simon 8. Any
`Systems Modeling and Computer Simulation: Second Edition, Naim
`A. Kheir
`
`EMI Filter Design, Richard Lee Ozenbaugh
`Power Hybrid Circuit Design and Manufacture, Haim Taraseiskey
`Robust Control System Design: Advanced State Space Techniques,
`Chia—Chi Tsui
`
`Spatial Electric Load Forecasting, H. Lee l/Wiis
`
`Page 6 of 89
`Page 6 of 89
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`99.
`
`100.
`
`101.
`
`102.
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`Permanent Magnet Motor Technology: Design and Applications, Jacek
`F. Gieras and Mitcheil‘ Wing
`High Voltage Circuit Breakers: Design and Applications, Ruben D.
`Garzon
`
`Integrating Electrical Heating Elements in Appliance Design, Thor
`Hegbom
`Magnetic Core Selection for Transformers and Inductors: A User's
`Guide to Practice and Specification, Second Edition, William T.
`Mciyman
`
`Additional Volumes in Preparation
`
`Statistical Methods in Control and Signal Processing. eo‘r’Ied by Tohru
`Katayama and Sueo Sugimota
`
`Page 7 of 89
`Page 7 of 89
`
`

`
`Integrating
`Electrical Heating
`Elements in
`
`Appliance Design
`
`Thor Hegbom
`Haflsrahammar; Sweden
`
`C RC P ress
`Taylor 5; Francis Group
`n London NewYorIc
`Boca Plato
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`R
`
`P
`ress is an imprint of the
`0 Ex F
`rancis Group, an informa business
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`Page 8 of 89
`Page 8 of 89
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`

`
`First Published by Lawrence Erlbaum Associates, Inc.. Publishers
`10 Industrial Avenue
`Mahwah, New Jersey 07430
`Reprinted 2010 by CRC Press
`CRC Press
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`Suite 300, Boca Raton, FL 3348?
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`
`ISBN: 0-8247-9840-6
`
`The publisher offers discounts on this book when ordered in bulk quantities. For
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`Copyright © 1997 by Marcel Dekker, Inc. All Rights Reserved.
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`Page 9 of 89
`Page 9 of 89
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`

`
`3 G
`
`
`
`eneral Information About Electrical
`
`Heating Elements
`
`3.1. DEFINITION or AN ELECTRICAL HEATING
`
`ELEMENT
`
`-
`
`An electrical heating element is normally a detachable part of a furnace,
`an appliance, or a heater consisting of one or more electric circuits. In each
`circuit heat is generated by passage of electric current through a resistor
`that is joined to a terminal in each end. The two terminals are connected
`to an available voltage by means of copper leads. The terminals are dc»
`signed in such a way that the temperature at the joint between them and
`the leads is low enough to avoid oxidation of the leads or damage to their
`
`81
`
`Page 10 of 89
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`
`

`
`
`
`82
`
`Chapter 3
`
`ln electrical heating elements for appliances the assembly of
`insulation.
`resistor(s) and terminals is often mechanically supported by electric insu~
`lation that is part of the element.
`A similar definition may be found in the vocabulary of the Interna-
`tional Electrotechnical Commission [1].
`
`3.2. HEAT TRANSFER
`
`Heat is transferred by conduction, convection, or radiation or a combination
`of them.
`
`3.2.1. Conduction
`
`Through solid and opaque bodies heat is transferred by conduction only.
`The most important formula for the conduction in a thermal steady
`state is:
`
`p = —)t dTldx
`
`(3.1)
`
`where
`
`p = surface load or rating per unit cross section
`A = thermal conductivity
`T = temperature
`
`The temperature difl"erence between two parallel isothermal surfaces
`at a distance L from each other and heated by surface load p is, according
`to Equation (3.1),
`
`'1‘ = pLf)t
`
`(3.2)
`
`The temperature drop between two concentric cylinders having di~
`ameters D, > D, temperatures T, < T, and a rating per unit cylinder
`length of PIL can also be found from Equation (3.1):
`
`T -1 T2 - T, = (PIL) ln(D,J'D3).+'(2¢r)i)
`
`(3.3)
`
`If the two cylinders are eccentric in relation to each other. the temperature
`difference is smaller. -The result from Equation (3.3) can then be multiplied
`by the factor shown in Figure 3. 1. Calculation and testing indicate that the
`curve of this figure is a quarter circle. The eccentricity along the abscissa
`in the figure is defined as:
`
`Eccentricity = 2Af(D, — D2) < I
`
`-
`
`(3.4)
`
`where A is the distance between the two cylinder centers.
`The temperature difference between a cylinder and a plane parallel
`to the cylinder and outside it is:
`
`Page 11 of 89
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`
`

`
`236
`
`Chapter 6
`
`varies along the circumference of each wire turn. The part of the wire
`which is situated closest to the element surface will reach a lower tempera-
`ture than the part that is farthest away. The biggest imaginable tempera-
`ture difference along one l1l.lIT1 of wire is presented in Equation (6.38) of
`Section 6.10.
`
`In embedded heating elements the heat-generating conductor is en-
`closed in insulating materials and not easily available for temperature mea-
`surements. in Section 3.l0 it has been explained how such a measurement
`can be performed. The temperature of this conductor should not exceed a
`certain limit in order to secure a long element life and satisfactory electrical
`insulation. Equation (3.100) in Section 3.10 shows the connection between
`the temperature of the heat-generating conductor, the element surface load,
`and the element surface temperature, which is often easy to measure. It
`follows from this equation that for embedded elements the maximum per-
`missible element surface load and the maximum permissible element sur-
`face temperature cannot be applied simultaneously. When the element sur-
`face load is close to the permissible maximum the element surface
`temperature has to be low, and when the surface temperature is close to
`its permissible maximum the surface load has to be low.
`In the embedded elements the resistance heating conductor is locked
`and can in most elements not elongate permanently as it does for at least
`Fe-Cr-Al alloys above 700°C in suspended and supported elements. This
`implies that the rating remains more constant in embedded elements op-
`erating above 700°C.
`
`6.1. METAL-SHEATHED TUBULAR ELEMENT
`
`6.1.1. General Description
`
`A metal-sheathed tubular element usually consists of a coil of a resistance
`heating alloy that is connected at each end to a terminal pin and is elec-
`trically insulated from a concentric metallic tube. The construction is
`shown in Figure 6.1. The insulation between coil and tube normally con-
`sists of ceramic material, often fused MgO powder. The tube ends are
`frequently equipped with beads of insulating material such as ceramic or
`silicone rubber. The aim is to increase the creepage distance (see Section
`3.8.2.) between the tube and the terminal pin. If a ceramic bead is applied
`there may in addition be a seal between the bead and the insulation in
`order to avoid penetration of moisture into the element.
`Metal-sheathed tubular elements with both or all terminals at the
`same tube end may be classified as cartridge elements. A cartridge element
`in this book is, however, a metal-sheathed tubular element that has all
`
`Page 12 of 89
`Page 12 of 89
`
`_
`
`i l
`
`

`
`Embedded Elements
`
`237
`
`
`
`FIGURE 6.1 Metal-sheathed tubular element with one coil having one ter-
`minal at each tube end.
`
`terminals at the same tube end. a straight tube. and a round cross section.
`Powder-filled cartridge elements are treated in Section 6.2.
`The cross section of the tubular element is usually round but may
`instead be oval, rectangular. triangular, or square.‘-' [f the cross section de-
`viates from the round shape it maybecome more difficult to achieve a high
`powder density and a high thermal conductivity of the embedding powder.
`When it is important to keep a small temperature drop between wire and
`tube, for instance when the tube surface load is very high and the heat
`dissipation conditions at the tube surface are very good, the round cross
`section may be best.
`ln some cases, however. the temperature difference between tube and
`fluid outside the tube becomes more important for the element design than
`the temperature drop between coil and tube. and in such cases it may
`sometimes be better to choose an oval or flat cross section than a round
`one. At free convection, for instance, a horizontal tubular element with an
`oval or flat cross section and with its width pointing in vertical direction
`has. for the same tube surface load and the same circumference. a lower
`tube temperature than a round tube.
`The tubular element may be equipped with two or three parallel coils
`instead of only one. These coils may have one terminal in each tube end,
`or all the terminals may be at the same tube end. In the latter case the
`other tube end is closed by welding. The cross section of such an element
`may be flat, oval, triangular, or round. There may, for instance. be three
`parallel coils joined together at the closed tube end, or there may be two
`parallel coils and one terminal running all along the two coils. The terminal
`is joined to the two coils at the closed tube end. In Figure 6.2 the. two
`alternatives are shown for a tubular elements with a flat or oval cross
`
`at
`
`Page 1330f 89
`Page 13 of 89
`
`

`
`238
`
`Chapter 6
`
`_
`
`_mr-
`
`.
`
`-
`
`. "nevi. ~. ... ‘
`
`_
`
`'-
`
`as“
`
`FIGURE 6.2 Metal-sheathed tubular element with all terminals at the same
`tube end.
`'
`
`section. Also, the alternative that has both terminals of one coil placed at
`the same tube end is shown. The manufacture of such an element is de-
`scribed in Section 6.2.1.
`
`Tubular range-top heaters having two coils and a return terminal in-
`side the tube have been made. The element with an originally round cross
`section was, after filling, bent to spiral shape and subsequently pressed to
`a triangular cross section in order to increase the density of the powder
`and achieve good thermal contact between element and pan. The tube
`temperature was about 800°C and the tube surface load 4 to 5 Wfcmz [I].
`In elements having more than one coil or with one coil bent in a
`hairpin shape the insulation between the coils or coil parts reaches a high
`temperature and may be more exposed to an insulation failure than in
`tubular elements with just one coil having one terminal at each tube end.
`Tubular elements of the former type should consequently not be used for
`very high temperatures.
`Soft annealed tubular elements having two terminals at one end,
`a hairpin—shaped coil between them. and a tube outer diameter less than
`5 mm may be called cable heaters. They can easily be formed to coils,
`porcupines,
`flat spirals, or a sinuous pattern. They are dealt with in
`Section 6.1.7.
`Tubular elements exposed to free or forced convection are frequently
`equipped with fins to improve the heat emission and enable the use of a
`higher tube surface load.
`
`5
`
`l
`Page 14 of 89
`Page 14 of 89
`
`
`
`

`
`Embedded Elements
`
`239
`
`An aluminum profile consisting of a round aluminum tube with fins
`may be filled with'a hairpin-shaped coil and MgO powder. After filling.
`the tube may be flattened in order to increase the powder density. Such
`elements are used for convection space heating.
`Tubular elements for defrosting in refrigerators may consist of a heat-
`ing cable (see Section 6.8) inside an aluminum tube.
`Normally only one resistance material type is used inside the element.
`In ignition plug elements that are straight. and where the tube is used as
`the return path for the current, there are often two coils of different ma-
`terials welded together. This will be explained in detail in Section 6.1.6.
`Special tubularelements may be equipped with an internal thermo-
`couple. A fuse inside the terminal, consisting of memory metal or a plastic
`that shrinks or melts, has also been suggested [2].
`For tube diameters down to 1 mm and below a straight wire is used
`instead of a coil. Such elements can be used only for low voltages. They
`are used for special purposes and are treated in Section 6.1.9.
`A tubular element can be made by leaving out the coil and letting
`the current go through the metallic tube instead. Such an element is shown
`in Figure 6.3. The terminals are welded to the tube ends, and the insulating
`powder inside becomes hermetically enclosed [3]. "The tube may have a
`round, square, oval. or rectangular cross section. Because the tube ends are
`hermetically closed the tube is not oxidized from the inside. Such an ele-
`ment is said to be applicable up to 1150°C.
`Another possibility for leaving out the resistance wire consists of a
`rod in the center of a round element and a second tube inside the element
`tube. Both the rod and the additional tube are made of a material of a low
`
`resistivity. Between them is a mixture of an insulating ceramic powder like
`MgO and powdered carbon. and between the inner tube and the outer
`element tube normal insulating powder, for instance, MgO is used. The
`rod and the inner tube are the electrodes. The construction is shown at the
`bottom in Figure 6.3. Other shapes of the electrodes are also possible. The
`element is compressed in a normal manner [4]. Such an element can be
`cut to the desired length after its manufacture.
`
`6.1.2. Design Figures
`
`In the following it is assumed that the element construction is as shown in
`Figure 6.1.
`The rating of a tubular element is within the wide range 10 to 8000
`W and the voltage normally between 6 and 500 V. The current is usually
`below 15 A. The resistance is between I and 2000 ohm and the resistance
`
`_.é
`'/
`
`Page 15 of 89
`Page 15 of 89
`
`

`
`240
`
`Chapter 6
`
`
`
`‘ Terminal
`
`Metallic lube
`
`{Top} Tubular element using the tube as a heating conductor.
`FIGURE 6.3
`(Bottom) Tubular element where part of the insulating powder has been
`made conductive and used for generation of heat.
`
`per meter tube length between 3 and 4000 ohm! m, depending on the tube
`diameter. It is usually between 22 and I'.*‘0 ohm! m for a tube diameter of
`5 mm and between l0 and 450 ohmfm for a tube diameter of 3 mm [5].
`
`See also Figure 6.28.
`The tube surface load varies mostly between 0.1 to 25 Wfcmz. It
`may occasionally be as high as 40 Wfcmz. Its size depends on the heat-
`dissipating conditions outside the tube and the ambient temperature. The
`tube temperature is usually below 850°C. and is hardly ever above 1000°C.
`The tube diameter is normally between 3 and 20 mm. (Occasionally
`tube diameters down to I m and up to 30 mm occur.) D[N standard tube
`diameters are 6.5 and 8.5 mm [6]. but in English-speaking countries 6.3
`and 8.0 mm are more common. The use of elements having a diameter
`between 4 and 6.5 mm is increasing. The tube length is normally 0.3 to S
`m. Considerably longer elements are made for special applications. Shorter
`elements are occasionally also made——~l'or instance. for ignition plugs.
`For the smallest tube diameters a straight wire is used instead of a
`coil. See Section 6.|.9.
`
`6.1.3. Raw Materials
`
`Tube
`
`is
`It
`The tube is normally the most costly pan of the tubular element.
`therefore important to choose the correct material and the correct dimen-
`sions for it.
`
`The most important properties of frequently used tube materials for
`tubular elements are listed in Table 6.]. Table 6.2 shows the composition
`of some standard tube materials [79] and their highest recommended tem-
`
`Page 16 of 89
`Page 15 of 89
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`

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`Page17of89o
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`Page18of898.
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`52mama
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`242
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`Chapter 6
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`

`
`Embedded Elements
`
`243
`
`peratures [8]. If the element will operate in air at a tube temperature below
`400°C, plain carbon steel is normally chosen. It is often copper plated to
`avoid corrosion. When the element is used for space heating, the hum from
`it (see Section L6) may be disturbing, and it requires nonmagnetic stainless
`steel tubing instead of tubes of plain carbon steel. or placing the terminals
`at the same tube end. to reduce the magnetic field strength in the tube.
`Ln UL 1030 [8] 760°C is considered the highest permissible temper-
`ature for tubes made of AIS} 302. 303. 304. 316, 321, and 34?. Tubes of
`AISI 403, 405. 410. 416. and 50] should not be used above 650°C. AISI
`SOI contains 46% Cr. The other tubes in this group have 1 l—l4% Cr [9].
`They have no Ni and are consequently ferritic. If tubes from this latter
`group are used for making tubular elements, the relative short elongation
`at rupture of the ferritic material has to be kept in mind. The tubes are
`magnetic and will generate hum (see Section L6 and the paragraph above).
`NiCr 20/ I2 with Si addition can be applied to 850°C. If Ce is also
`added the temperature can be increased to lO00°C. In the tube temperature
`range 700 to 900°C NiCr 20!25 is often used. If there is no danger of
`corrosion it is not necessary to have a higher nickel content than that in
`order to achieve satisfactory oxidation resistance.
`lnconel (NiCr 75;‘ 14)
`and lncoloy (NiCr 33;’20) tubing with a higher nickel content are chosen
`where corrosion is a problem, such as for tubular range-top heaters.
`The creep strength of alloyed steel tubes can be improved by adding
`nitrogen.
`is normally made of copper or
`If the tube will operate in water it
`stainless steel. Corrosion in water depends on the pH, the temperature. and
`the CI content of the water. The resistance of the tube against general
`corrosion is strongly affected by its Cr, Mo. and N content. NiCrMo 18!
`2016 is a good choice for severe corrosion conditions. Stress corrosion
`increases with increasing temperature. A chalk deposit on the tube may
`promote this type of corrosion because it increases the tube temperature.
`A high Ni content is advisable in such cases.
`In order to_ protect the tube against corrosion in water it has been
`suggested that a plastic coating be applied on the surface of the finished
`element [10]. After sandblasting a 0.1- to 0.2~mm~thicl< coating of fluor-
`polymer is applied and subsequently baked at 400°C. The method is sug-
`gested for immersion heaters.
`After element manufacture the copper tube is plated with chromium,
`nickel, or tin. Both aluminium and copper tubing are used in the half-hard
`condition. In acid solutions such as solutions of boric acid and chromic
`
`acid tubes of titanium may be used.
`
`a_‘0
`
`Page 19§of 89
`Page 19 of 89
`
`

`
`244
`
`Chapter 6
`
`Alloyed steel strips are are welded to tubes in relatively simple weld«
`ers by the large manufacturers of tubular elements. Seamless steel, copper,
`and aluminum tubes are purchased from metal plants.
`if the tube will operate at red heat in air. its radiation emissivity will
`influence its temperature. After the annealing of the element an alloyed
`steel tube has an emissivity in the range 0.5 to 0.85. depending on the
`exact composition of the tube. the atmosphere. the temperature. and the
`time of annealing.
`lf the element is operating at red heat the tube will
`oxidize further and may reach an emissivity in the range 0.9 to 0.98 at the
`end of the element life. This emissivity increase results in a corresponding
`temperature decrease at constant rating.
`The tube wall thickness is normally 0.4 to 1 mm. It usually decreases
`with decreasing tube diameter and is smaller for alloyed steel tubing than
`for Al. Cu. and Fe. In UL 1030 [8] the smallest tube wall thickness is
`specified as 0.4] mm for range~top heaters, stationary oven elements, and
`space—heating appliances. For other applications it can be down to 0.33
`mm according to the same standard. These values refer to the tube wall
`thickness of the finished element.
`
`In order to avoid difficulties when filling. the tube has to be straight
`and the tube ends free from burr. The maximum bow of a l—m~long tube
`may not exceed 10 mm. The tube outer diameter should have a narrow
`tolerance because tube diameter variations result in tube length variations
`after rolling. The hardness of the tube should be uniform for the same
`reason.
`
`When the tube is to be used above 400°C. the cleanliness of the inside
`
`tube wall becomes important for the insulation and the life of the element.
`
`Wire
`
`For tube temperatures below '.+'00°C a wire of Fe-Cr-Al may be used. For
`higher temperatures Ni—Cr and Ni~Cr~Fe wires are recommended. Even
`other less oxidation-resistant wire materials can be used, beating in mind
`that the atmosphere inside the tube has a low oxygen content. A copper-
`nickel wire that had a life less than I min at 800°C in open air had it
`extended to several hundred hours at the same temperature when operating
`inside a well-sealed tubular element [I]. However. the atmosphere inside
`the element may vary from one element to another. and it may also change
`during the element’s life.
`in spite of their poor oxi-
`Pure metals such as iron and nickel can,
`dation resistance. also be used provided the elements are well sealed. As
`mentioned earlier, pure metals have a high temperature factor of resistance,
`which results in fast heating up and a rating that varies with the temperature
`and therefore also with the heat dissipation from the tube surface. In dish-
`
`I
`Page 20 of 89
`Page 20 of 89
`
`

`
`Embedded Elements
`
`245
`
`washers. where the element sometimes operates in water and sometimes
`in air. a resistance fnaterial having a high temperature factor may be ad—
`vantageous. Also. for water-heating elements where a chalk deposit can be
`feared the use of such wire appears interesting. As explained in Section
`3.2.7. a very high wire temperature may be reached locally if there are
`differences in the heat-dissipatin g conditions along such elements. This will
`be discussed further in Section 6.l.5.
`
`to 1 mm. The cal-
`The wire diameter is normally in the range 0.|
`culation of the wire diameter will be dealt with in Section 6.1.8.
`
`Ribbon has been suggested instead of a round wire. Coiling and cut—
`ting ribbon are difficult, and two or sometimes three parallel wires are
`preferred instead. (See also Section 3.4.9.)
`
`insulation material
`
`The insulation material between the tube and the coil should have high
`resistivity, high insulation strength, and high thermal conductivity. and it
`should not corrode the wire at operating temperature. ll" powder is used it
`must be easy to fill. The most common insulation material is fused M g0
`crossed to powder. MgO is found in seawater as Mg(OH)2 or more fre-
`quently in rocks as MgCO3. In the temperature range 350 to 580°C the
`latter is decomposed to MgO and CO2. In order to reduce n-ansport weight
`the MgCO2 material is annealed at the mine to form Mg0_and then shipped
`to the place where it is melted. This takes place in submerged arc furnaces
`where the highest temperature is near the electrodes. Here also the purest
`MgO is found. The outer zone. where no complete melting takes place. is
`removed. The melted Mg0 is then crossed to powder. sieved. and mixed
`to a suitable grain size distribution. The normal grain size distribution is
`shown in Figure 6.4. The curves show. for instance. that at grain size 0.25
`mm the average is 35%. This means that 35% of the powder will not go
`through a sieve having an opening width. mesh. of 0.25 mm. The curves
`indicate that the amount of powder that will not go through this part