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Design of a Heater for Indirect Heating-Industry Project(Analysis Part Only)

By SinghKamalpreet Apr 16, 2013 6966 Words
DESIGN OF A HEATING ELEMENT FOR INDIRECT HEATING

A REPORT SUBMITTED
IN THE PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE COMPLETION OF MAJOR PROJECT WORK

Project Report By:
Kamalpreet Singh (753/08)
University Roll No.: 80801114047

Submitted by:Submitted to:
Kamalpreet Singh Dr. J.S Oberoi College Roll No: 753/08Project Coordinator
Univ. Roll No: 80801114047Department of
Mechanical Engineering Dept.Mechanical Engg.

PUNJAB TECHNICAL UNIVERSITY
BABA BANDA SINGH BAHADUR ENGINEERING COLLEGE
FATEHGARH SAHIB – 140407, PUNJAB (INDIA)
MECHANICAL ENGINEERING DEPARTMENT
2008-2012

Ref. No.:753/08 Date: May 2, 2012

DECLARATION
  
I hereby certify that the work which is being presented in the project report entitled, “Design of a Heating Element for Indirect Heating”, by Kamalpreet Singh, College Roll No. 753/08 (Uni. Roll No. 80801114047 ) in partial fulfilment of requirement for the completion of project report. (Major Project, Mechanical Engineering) submitted at Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, under Punjab Technical University is an authentic record of my own work carried out during a period from January 1,2012 to May 2,2012 under the supervision of Dr. J.S Oberoi, Assistant Professor, Mechanical Engineering Department. The matter presented in this report has been completely formulated by me with my project members under the supervision of Project Coordinator/Supervisor.  

Signature of the Student
 
 

This is to certify that the above statement made by the candidate is correct to the best of my/our knowledge  
 
 
Signature of the Coordinator/Supervisor
ACKNOWLEDGEMENT
It is my privilege to acknowledge the kindness and help that our respected and learned teachers showed towards us. It is because of their inspiration, constructive criticism and valuable suggestions that I have completed my major project in Mechanical Engineering. Their precious guidance and unrelenting support kept me on track through my project work. I want to thank all the project members of this project for their warm support and co-operation in successful planning of design and fabrication of project work.

I express my sincere gratitude to Dr. J.S Oberoi (Project Coordinator) for spending his precious time gathering knowledge about various aspects of Heat Treatment and its applications for design and fabrication of project work. The successful completion of project would not have been possible without his precious advice and suggestions.

A special thank awaits Dr. APS Sethi (Head, Mechanical Engineering Department) and Dr. J.S Oberoi (Project Coordinator and Supervisor) for having immense faith in me and thus providing me with a valuable major project work in Mechanical Engineering.

Kamalpreet Singh
Univ. Roll No. 80801114047
College Roll No. 753/08
B.B.S.B.E.C

ABSTRACT
The valuable opportunity provided to design a Heating Element for Indirect Heating was full of knowledge. It helped us a lot in formulating our bookish knowledge into practical. It provided us with industrial exposure, gathering information about works being performed in industries using fuels like wax, coaltar etc. We started our project work under the esteem guidance of Dr. J.S Oberoi in designing a test rig for measuring the heat transfer rates from one medium to another. The main motive was to design such a heating element which could liquefy the solid wax (fuel used in industry) during winters (as wax solidifies during winters). The heating element had to be installed in an indirect manner without affecting the initial setup.

The design of heating element was planned with the help of concepts of Heat and Mass Transfer. The project took nearly three months for its completion. The concepts of wattage, watt density, electrical and thermal conductivity are studied an applied in designing the required test rig.

LIST OF CONTENTS

Declaration 2 Acknowledgement 3 Abstract 4 S.NO.| TOPICS| Page No.|

1| OVERVIEW| 6|
2| HEAT TRANSFER| 7|
3| MECHANISM| 8|
45| PHASE CHANGESHEATING ELEMENT AND SPECIFICATIONS| 1214| 6| PROJECT DETAILS| 31|
7| BIBLIOGRAPHY| 37|

OVERVIEW

Heat is defined in physics as the transfer of thermal energy across a well-defined boundary around a thermodynamic system. It is a characteristic of a process and is not statically contained in matter. In engineering contexts, however, the term heat transfer has acquired a specific usage, despite its literal redundancy of the characterization of transfer. In these contexts, heat is taken as synonymous to thermal energy. This usage has its origin in the historical interpretation of heat as a fluid (caloric) that can be transferred by various causes, and that is also common in the language of laymen and everyday life.

The fundamental methods of heat transfer in engineering include conduction, convection, and radiation. Physical laws describe the behavior and characteristics of each of these methods. Real systems often exhibit a complicated combination of them. Heat transfer methods are used in numerous disciplines, such as automotive engineering, systems, climate, insulation, materials processing, and power plant engineering.

The various mathematical methods have been developed to solve or approximate the results of heat transfer in systems. Heat transfer is a path function (or process quantity), as opposed to a state quantity; therefore, the amount of heat transferred in a thermodynamic process that changes the state of a system depends on how that process occurs, not only the net difference between the initial and final states of the process. Heat flux is a quantitative, vectorial representation of the heat flow through a surface.

Heat transfer is typically studied as part of a general chemical engineering or mechanical engineering curriculum. Typically, thermodynamics is a prerequisite for heat transfer courses, as the laws of thermodynamics are essential to the mechanism of heat transfer. Other courses related to heat transfer include energy conversion, thermo fluids, and mass transfer. The transport equations for thermal energy (Fourier's law), mechanical momentum (Newton's law for fluids), and mass transfer (Fick's laws of diffusion) are similar and analogies among these three transport processes have been developed to facilitate prediction of conversion from any one to the others.

HEAT TRANSFER
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy and heat between physical systems. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system. Heat conduction, also called diffusion, is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems. When an object is at a different temperature from another body or its surroundings, heat flows so that the body and the surroundings reach the same temperature, at which point they are in thermal equilibrium. Such spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature, as required by the second law of thermodynamics. Heat convection occurs when bulk flow of a fluid (gas or liquid) carries heat along with the flow of matter in the fluid. The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume), thus influencing its own transfer. The latter process is often called "natural convection". All convective processes also move heat partly by diffusion, as well. Another form of convection is forced convection. In this case the fluid is forced to flow by use of a pump, fan or other mechanical means. The final major form of heat transfer is by radiation, which occurs in any transparent medium (solid or fluid) but may also even occur across vacuum (as when the Sun heats the Earth). Radiation is the transfer of energy through space by means of electromagnetic waves in much the same way as electromagnetic light waves transfer light. The same laws that govern the transfer of light govern the radiant transfer of heat.

MECHANISM

The fundamental modes of heat transfer are:
Conduction or diffusion:
The transfer of energy between objects that are in physical contact. Convection:
The transfer of energy between an object and its environment, due to fluid motion. Radiation:
The transfer of energy to or from a body by means of the emission or absorption of electromagnetic radiation. Advection:
The transfer of energy from one location to another as a side effect of physically moving an object containing that energy.

Conduction:
On a microscopic scale, heat conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring particles. In other words, heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from one atom to another. Conduction is the most significant means of heat transfer within a solid or between solid objects in thermal contact. Fluids—especially gases—are less conductive. Thermal contact conductance is the study of heat conduction between solid bodies in contact.

Q= Heat transferred in time = t K= Thermal conductivity of the barrier
A= area
T= Temperature
d= Thickness of barrier
Steady state conduction is a form of conduction that happens when the temperature difference driving the conduction is constant, so that after an equilibration time, the spatial distribution of temperatures in the conducting object does not change any further.In steady state conduction, the amount of heat entering a section is equal to amount of heat coming out.

Transient conduction occurs when the temperature within an object changes as a function of time. Analysis of transient systems is more complex and often calls for the application of approximation theories or numerical analysis by computer.

Figure 1 - Conduction, Convection and Radiation

Convection
Convective heat transfer, or convection, is the transfer of heat from one place to another by the movement of fluids, a process that is essentially transfer of heat via mass transfer. (In physics, the term fluid means any substance that deforms under shear stress; it includes liquids, gases, plasmas, and some plastic solids.) Bulk motion of fluid enhances heat transfer in many physical situations, such as (for example) between a solid surface and the fluid. Convection is usually the dominant form of heat transfer in liquids and gases. Although sometimes discussed as a third method of heat transfer, convection is usually used to describe the combined effects of heat conduction within the fluid (diffusion) and heat transference by bulk fluid flow streaming. The process of transport by fluid streaming is known as advection, but pure advection is a term that is generally associated only with mass transport in fluids, such as advection of pebbles in a river. In the case of heat transfer in fluids, where transport by advection in a fluid is always also accompanied by transport via heat diffusion (also known as heat conduction) the process of heat convection is understood to refer to the sum of heat transport by advection and diffusion/conduction.

Figure 2 – Conduction, Convection and Radiation
The Free, or natural, convection occurs when bulk fluid motion (steams and currents) are caused by buoyancy forces that result from density variations due to variations of temperature in the fluid. Forced convection is a term used when the streams and currents in the fluid are induced by external means—such as fans, stirrers, and pumps—creating an artificially induced convection current.

Convective heating or cooling in some circumstances may be described by Newton's law of cooling: "The rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings." However, by definition, the validity of Newton's law of cooling requires that the rate of heat loss from convection be a linear function of ("proportional to") the temperature difference that drives heat transfer, and in convective cooling this is sometimes not the case. In general, convection is not linearly dependent on temperature gradients, and in some cases is strongly nonlinear. In these cases, Newton's law does not apply. Radiation

Fig 3 - A red-hot iron object, transferring heat to the surrounding environment primarily through thermal radiation.

Thermal radiation is energy emitted by matter as electromagnetic waves due to the pool of thermal energy that all matter possesses that has a temperature above absolute zero. Thermal radiation propagates without the presence of matter through the vacuum of space.

Thermal radiation is a direct result of the random movements of atoms and molecules in matter. Since these atoms and molecules are composed of charged particles (protons and electrons), their movement results in the emission of electromagnetic radiation, which carries energy away from the surface. Unlike conductive and convective forms of heat transfer, thermal radiation can be concentrated in a small spot by using reflecting mirrors, which is exploited in concentrating solar power generation. For example, the sunlight reflected from mirrors heats the PS10 solar power tower and during the day it can heat water to 285 °C (545 °F).

PHASE CHANGES
Transfer of heat through a phase transition in the medium—such as water-to-ice, water-to-steam, steam-to-water, or ice-to-water—involves significant energy and is exploited in many ways: steam engines, refrigerators, etc. For example, the Mason equation is an approximate analytical expression for the growth of a water droplet based on the effects of heat transport on evaporation and condensation.

Boiling
Heat transfer in boiling fluids is complex, but of considerable technical importance. It is characterized by an S-shaped curve relating heat flux to surface temperature difference.
At low driving temperatures, no boiling occurs and the heat transfer rate is controlled by the usual single-phase mechanisms. As the surface temperature is increased, local boiling occurs and vapor bubbles nucleate, grow into the surrounding cooler fluid, and collapse. This is sub-cooled nucleate boiling, and is a very efficient heat transfer mechanism. At high bubble generation rates, the bubbles begin to interfere and the heat flux no longer increases rapidly with surface temperature (this is the departure from nucleate boiling, or DNB). At higher temperatures still, a maximum in the heat flux is reached (the critical heat flux, or CHF). The regime of falling heat transfer that follows is not easy to study, but is believed to be characterized by alternate periods of nucleate and film boiling. Nucleate boiling slows the heat transfer due to gas bubbles on the heater's surface; as mentioned, gas-phase thermal conductivity is much lower than liquid-phase thermal conductivity, so the outcome is a kind of "gas thermal barrier".

At higher temperatures still, the hydrodynamically-quieter regime of film boiling is reached. Heat fluxes across the stable vapor layers are low, but rise slowly with temperature. Any contact between fluid and the surface that may be seen probably leads to the extremely rapid nucleation of a fresh vapor layer ("spontaneous nucleation").

Condensation
Condensation occurs when a vapor is cooled and changes its phase to a liquid. Condensation heat transfer, like boiling, is of great significance in industry. During condensation, the latent heat of vaporization must be released. The amount of the heat is the same as that absorbed during vaporization at the same fluid pressure.

There are several types of condensation:
* Homogeneous condensation, as during a formation of fog. * Condensation in direct contact with subcooled liquid.
* Condensation on direct contact with a cooling wall of a heat exchanger: This is the most common mode used in industry: * Filmwise condensation is when a liquid film is formed on the subcooled surface, and usually occurs when the liquid wets the surface. * Dropwise condensation is when liquid drops are formed on the subcooled surface, and usually occurs when the liquid does not wet the surface. Dropwise condensation is difficult to sustain reliably; therefore, industrial equipment is normally designed to operate in filmwise condensation mode.

HEATING ELEMENT
A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element.
Most heating elements use Nichrome 80/20 (80% nickel, 20% chromium) wire, ribbon, or strip. Nichrome 80/20 is an ideal material, because it has relatively high resistance and forms an adherent layer of chromium oxide when it is heated for the first time. Material beneath this layer will not oxidize, preventing the wire from breaking or burning out. Resistance wire: may be wire or ribbon, straight or coiled. Used in common items such as toasters and hair dryers, furnaces for industrial heating, floor heating, roof heating, pathway heating to melt snow, dryers etc. Most common wires are from the following classes: Kanthal (FeCrAl) wires.

Nichrome 80/20 wire and strip.
Cupronickel (CuNi) alloys for low temperature heating.
Molybdenum disilicide (MoSi2, molybdenum silicide, or MOSI2), an intermetallic compound, a silicide of molybdenum, is a refractory ceramic with primary use in heating elements. It has moderate density, melting point 2030 °C, and is electrically conductive. At high temperatures it forms a passivation layer of silicon dioxide, protecting it from further oxidation. Application area is Glass Industry, Ceramic sintering, heat treatment furnaces, semiconductor diffusion furnace

Molybdenum disilicide doped with Al or Mo(Si,Al)2, an intermetallic compound, a silicide of molybdenum, is a refractory ceramic with primary use in heating elements. At high temperatures it forms a passivation layer of alumina (Al2O3) protecting it from corrosion or further oxidation. Application area is Glass Industry, Ceramic sintering, heat treatment furnaces, semiconductor diffusion furnace. Working 3000C higher in reducing atmospheres than MoSi2.

Screen-printed metal–ceramic tracks deposited on ceramic insulated metal (generally steel) plates. These elements have found widespread application for kettles and other domestic appliances since the mid-1990s. Etched Foil: elements are generally made from the same alloys as Resistance wire elements, but are produced with a subtractive photo-etching process that starts with a continuous sheet of metal foil and ends with a complex resistance pattern. These elements are commonly found in precision heating applications such as Medical Diagnostics, Satellite, and Aerospace. Tubular (sealed element, often known by the trademark "Calrod"): a fine coil of Nickel chrome wire in a ceramic insulating binder (MgO, alumina powder), sealed inside a tube made of stainless steel or brass. These can be a straight rod (as in toaster ovens) or curved to span an area to be heated (such as in electric stoves, ovens, and coffee makers). Heat lamp: a high-powered incandescent lamp usually run at less than maximum power to radiate mostly infrared instead of visible light. These are usually found in radiant space heaters and food warmers, taking either a long, tubular form or an R40 reflector-lamp form. The reflector lamp style is often tinted red to minimize the visible light produced; the tubular form is always clear. PTC ceramic: This material is named for its Positive Thermal Coefficient of resistance. Most ceramics have a negative coefficient; most metals, a positive one. While metals do become slightly more resistant at higher temperatures, this class of ceramics (often barium titanate and lead titanate composites) has a highly nonlinear thermal response, so that it becomes extremely resistant above a composition-dependent threshold temperature. This behavior causes the material to act as its own thermostat, since current passes when it is cool, and does not when it is hot. Thin films of this material are used in automotive rear-window defrost heaters, and honeycomb-shaped elements are used in more expensive hair dryers and space heaters. Thick film technology:

Heating elements for high-temperature furnaces are often made of exotic materials, including platinum, molybdenum disilicide, molybdenum (vacuum furnaces) and silicon carbide. Silicon carbide igniters are common in gas ovens.

Overview -: Heating Element Design and calculation heating element design
To perform as a heating element the tape or wire must resist the flow of electricity. This resistance converts the electrical energy into heat which is related to the electrical resistivity of the metal and is defined as the resistance of a unit of unit cross-sectional area. The linear resistance of a length of tape or wire may be calculated from its electrical resistivity. Where:

Ρ| Electrical Resistivity (microhm.cm)|
R| Element Resistance at 20oC (ohms)|
D| Wire diameter (mm)|
T| Tape thickness (mm)|
B| Tape width (mm)|
L| Tape or wire length (m)|
A| Tape or wire cross-sectional area (mm2)|

For Round wire

For Tape

As a heating element , tape offers a large surface area and therefore , a greater effective heat radiation in a preferred direction making deal for many industrial applications such as injection mould band heaters.

An important characteristics of these electrical resistance to heat and corrosion which is due to the formation of oxide surface layers that retard further reaction with oxygen in air. When selecting the alloy operating temperature, the material and atmosphere with which comes into contact must be considered. As there are so many types of applications variables in element design and different operating conditions the following equations for element design are given as a guide only. Electrical resistance at operating temperature:

With very few exceptions the resistance of a metal will change with temperature which must be allowed for when designing an element. As the resistance of an element is calculated at operating temperature, the resistance of the element at room temperature, divide the resistance at operating temperature by the temperature resistance factor shown below Where

F = Temperature resistance factor
Rt= Element resistance at operating temperature(ohms)
R = Element resistance at 200C(ohms)

Alloy| Temperature-Resistance Factor (F) at:|
| 20oC| 100oC| 200oC| 300oC| 400oC| 500oC| 600oC| 700oC| 800oC| 900oC| 1000oC| 1100oC| 1200oC| RW80| 1.00| 1.006| 1.015| 1.028| 1.045| 1.065| 1.068| 1.057| 1.051| 1.052| 1.062| 1.071| 1.080|

General Operating and Maintenance Instructions for Heating Elements 1. During initial start-up and after extended shut downs (maintenance periods, breakdowns, or holidays, etc...), the control instrument must be manually controlled through the High Limit thermocouple within the radiant tube or element chamber. Maximum allowable increase per hour as sensed by the heating element control thermocouple should not exceed 500 F. per hour up to 1000 F, and only 100 F per hour thereafter until furnace operating temperature is obtained. Without careful control of element temperature, over-temperature conditions will occur, causing sagging, distortion, and rapid fatigue leading to melting. 2. Use a high quality pyrometer, and be sure safe operating conditions are NOT exceeded. Make sure that the T/C's are functioning and in proper position at all times the equipment is in operation.3. The thermocouple used to control the Firebar should be within the radiant tube, minimum of 1/3 third down the effective heater length. Remember the greater the distance the T/C is from the element, the greater the lag time in temperature readings. 3. Sometimes excessive temperature is caused by furnace control couple calling for heat too rapidly. Batch loaded furnaces with doors are the most likely type to have this condition. Controller must be set to specific rate to prevent overheating of elements. 4. When a burnout occurs, either due to normal use or excessive temperature, the metal in the center of the wire melts first and may run out and the end will be hollow, this is not a defect, at the point of breakage there will always be a sign of fusion. 5. Turn off and secure disconnects for all power to the electrical devices being worked on. Wear safety glasses and insulating gloves. 6. Heat generated by electric heating elements will cause bolted terminal connections to expand and loosen. Retighten connections after initial furnace start-ups or installation of new elements. Periodic tightening of buss bars, transformer lugs, and element connections will prevent downtime for equipment repair. Do NOT continue to operate the furnace if the terminal connectors are red hot. Tighten clamp and/clean contact. 7. Proper grounding guards against shock when working on part of the system. It also prevent damage to sensitive electrical and computer equipment. We recommend ground connections be re-torqued once a year. Electrical ground connection is a MUST. 8. The furnace manufacturer is solely responsible for handling, installation, and control of heating elements, their cycling and furnace heat up time. 9. Maintenance schedules for thermocouples is required for proper control of elements, we recommend they be replaced once a month minimum, depending on the type being used. 10. Procedures mentioned are merely recommendations to follow during installation and are not intended or implied as operating instructions regarding furnace equipment. Refer to furnace manufacturers operation manual. 11. KEEP ELECTRICITY SAFE, always cut power to elements before removing any safety cover. Terminal covers should be tagged DANGER HIGH VOLTAGE. "Do's & Don'ts" in Designing Heating Elements

1. If accurate wattage is important, test the finished device to determine proper allowance for rise in resistance with temperature. 2. Always remember that 1% excess voltage will result in 2% excess wattage. 3. If you want tthe device to draw more power you should reduce the length of the element or increase the diameter of the wire. 4. Try to design elements so that they'll be free to expand and contract. If an element must be anchored in any way between terminals, test it out to make sure warpage or "creeping" will not cause trouble in the finished device. 5. Welded, brazed or spot-welded joints between elements and terminals are most satisfactory. Pressed or pressure joints may also be used if they are thoroughly tested prior to production. 6. If a pressure joint is made, be sure to avoid poor contact caused by stretching or yielding of the clamping device. Remember that it is better to place the element between two nuts rather than clamp it under just one. 7. If an element is wrapped around a terminal, do so in a clockwise direction and without over lapping as pressure of the clamping means may be sufficient to cut part way through it. 8. Don't design an element for a 115 volt device and then expect to use it for 230 volts, too. Instead, carry on the design with the 230 volt unit in mind from the start. Remember-it is quite difficult to work out a good 230 volt heating element for a small device. 9. Don't design a helical coiled element whose ratio of arbor size to wire size is over 10-to-1or less than 2-to-1 unless absolutely necessary. Such coils are very difficult to make. 10. If twisted tails are to be used to connect the element to the device, don't cut off the loop at the end because current may flow from the terminal into the loose strand-then into the element, and cause unnecessary trouble. 11. Don't make a pressure joint between a heating element and brass-if the brass is apt to be heated up to 300 F. This will cause the zinc to oxidize and form an insulating film. 12. Don't expose hot heating elements to dripping oil, oil spray, or to contaminating gases

SHAPES AND TYPES OF HEATING ELEMENTS
An electrical heating element consists usually of one or more tubular heating rods, connecting means for electrical connection, and heads or flanges for mechanical fixing. The heating rods can be of circular or flat-oval cross section. The sheath of the heating rod consists of a metal tube of specific dimensions that is made of a suitable material, according to the given application. The surface of this metal sheath can be further treated so that to increase its resistance against surrounding environment. To improve the heat transfer of air and gas heating elements, these can be provided with ribbing made from a steel tape wound perpendicularly around the heating rod. Some heating elements intended for special purposes can be casted into a casting of required shape. For this purpose, mostly aluminium is used. The resistive heating wire with output pins or output stranded wires is inserted inside the metal tube into an insulating material. The tube is then sealed and closed so that its inside active parts are thoroughly protected from all influences from the environment. The heating rods, as the basic semi-product, can be processed in variety of shapes, meeting the requirements of the customer, depending on the final equipment where they are applied (up to the maximum operating pressure of 6.4 MPa). An electrical heating element can be optionally equipped with various types of heads or flanges for mechanical fixing at assembly. The heating element can also be equipped with suitable connecting means for connection to the electrical circuit. All electrical heating elements manufactured by Backer Elektro CZ are designed and produced in accordance with standard EN 60335-1. At Backer Elektro CZ, we are proud to have mastered the technology of thick film heating elements. This advanced technology process provides perspective opportunity to use heating elements of this kind in common as well as in completely new areas of application. The heating material is applied in a form of a paste on a suitable ceramics or stainless steel substrate (plate) of specific quality. The heating part is isolated from the substrate and from the surrounding environment by thin layers of special insulating material that is applied in a form of a film.

Circular profiles
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Fig7 - Cross-section of a circular rod
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Fig 8 - Bending conditions of circular profiles

Flat-oval profiles

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Fig 9 - Cross-section of flat- oval rod
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Fig 10 - Bending conditions of flat-oval profile

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Fig 11 Schematic arrangement of resistive wire spirals in rods of flat-oval cross-section SHEATH MATERIAL, CHARACTERISTIC FEATURES, FIELD OF APPLICATION| Material| Acc. to ČSN
Acc. to DIN
Acc. to ASTM| Max. surface
temperature (°C)| Characteristic
features| Field of application|
Steel| 11343
St 34-2
-| 400|  | heating of air up to 400 °C, heating of oil, alkaline bathes, contact heating| Stainless Steel| 17240
1.4301
304| 750|  | heating of air, gases, contact heating|
Stainless Steel| 17246
1.4541
321| 750| good corrosion
resistance| heating of liquids, gases, contact heating, infra heaters, grills| Stainless Steel| 17349
1.4404
316L| 750| very good corrosion resistance| generating of steam, wet - dry cycling, heating of highly chlorinated water| Stainless Steel| -
1.4876 (Incloy 800)
-| 900| high corrosion resistance, heat resistance| storage heaters, aggressive liquids| Stainless Steel| 17251
1.4828
302B| 900| heat resistance| storage heaters|
Copper, nickel plated copper| -
Cu-DHP
-| 200|  | water heating, water heaters, dishwashers, washing machines| Flat-oval profiles can be used up to maximum surface temperature of 550 °C and they are, among others, also suitable for contact heating.|

SPECIFICATION OF HEATING RODS|
Profiles (mm)| Production lengths (mm)| Sheath material| Bend inner radius R| ø 6.4| 250 - 2300| steel ČSN 11343 stainless steel DIN 1.4301, 1.4404. 1.4541, 1.4828, 1.4876| minimum R 10| ø 8.5| 250 - 8300| steel ČSN 11343 stainless steel DIN 1.4301, 1.4404. 1.4541, 1.4828, 1.4876 copper Cu-DHP| minimum R 10 (depends on wall thickness)|

6.2 x 11.5| 150 - 4500| steel ČSN 11343 stainless steel DIN 1.4541 copper Cu-DHP| minimum R 10, flat bending R 20|
5.5 x 13,0| 150 - 4500| steel ČSN 11343 stainless steel DIN 1.4541 copper Cu-DHP| minimum R 10, flat bending R 20|
6.0 x 15| 150 - 4500| steel ČSN 11343 stainless steel ČSN 17248 copper Cu-DHP| minimum R 10, flat bending R 20|
ø 10.0 *| 60 - 160| stainless steel DIN 1.4541| cannot be bent| ø 12.5 *| 60 - 160| stainless steel DIN 1.4541| cannot be bent| ø 12.0 **| 400 - 1100| stainless steel DIN 1.4541| cannot be bent| * Only heating cartridges for heating of plastics moulds, heating of machines and their accessories ** Only for heating of water in radiators and bathroom dryers.|

Stainless steel and its types
Stainless steel types1.4301 and 1.4307 are also known as grades 304 and 304L respectively. Type 304 is the most versatile and widely used stainless steel. It is still sometimes referred to by its old name 18/8 which is derived from the nominal composition of type 304 being 18% chromium and 8% nickel.304 Stainless SteelStainless steel 304 is an austenitic grade that can be severely deep drawn. This property has resulted in 304 being the dominant grade used in applications like sinks and saucepans.304L Stainless SteelType 304L is the low carbon version of Stainless steel 304. It is used in heavy gauge components for improved weldability. Some products such as plate and pipe may be available as “dual certified” material that meets the criteria for both 304 and 304L.304H Stainless Steel304H, a high carbon content variant, is also available for use at high temperatures.Property data given in this document is typical for flat rolled products covered by ASTM A240/A240M. ASTM, EN or other standards may cover products sold by Aalco. It is reasonable to expect specifications in these standards to be similar but not necessarily identical to those given in this datasheet.| Chemical Composition of Stainless Steel 304|

Table 1. Typical chemical composition for 304 stainless steel alloys %| 304| 304L| 304H| C| 0-0.07| 0-0.03| 0.04-1|
Mn| 0-2.0| 0-2.0| 0-2.0|
Si| 0-1| 0-1| 0-1|
P| 0-0.05| 0-0.05| 0-0.05|
S| 0-0.02| 0-0.02| 0-0.02|
Cr| 17.5-19.5| 17.5-19.5| 17.5-19.5|
Ni| 8-10.5| 8-10.5| 8-10.5|
Fe| Balance| Balance| Balance|
|
|
Mechanical Properties of Stainless Steel 304Table 2. Typical mechanical properties for 304 stainless steel alloys Grade| 304| 304L| 304H| Tensile Strength (MPa)| 520-720| 500-6070| 520-720|

Compression Strength (MPa)| 210| -| -|
Proof Stress 0.2% (MPa)| 210| 200| 210|
Elongation A5 (%)| 45 Min| 45 Min| 40 Min|
Hardness Rockwell B| 92| -| -|
Physical Properties of Stainless Steel 304Table 3. Typical physical properties for 304 stainless steel alloys Property| Value| Density| 8.00 g/cm3|
Melting Point| 1450°C|
Modulus of Elasticity| 193 GPa|
Electrical Resistivity| 0.072x10-6 Ù.m|
Thermal Conductivity| 16.2 W/m.K |
Thermal Expansion| 17.2x10-6 /K |
Alloy DesignationsStainless steel 304 also corresponds to the following standard designations and specifications: Euronorm| UNS| BS| En| Grade| 1.4301| S30400| 304S15304S16304S31| 58E| 304|

1.4306| S30403| 304S11| -| 304L|
1.4307| -| 304S11| -| 304L|
1.4311| -| 304S11| -| 304L|
1.4948| S30409| 304S51| -| 304H|
Corrosion Resistance of Stainless Steel 304Stainless steel 304 has excellent corrosion resistance in a wide variety of environments and when in contact with different corrosive media. Pitting and crevice corrosion can occur in environments containing chlorides. Stress corrosion cracking can occur at temperatures over 60°C.Heat Resistance of Stainless Steel 304Stainless steel 304 has good resistance to oxidation in intermittent service up to 870°C and in continuous service to 925°C. However, continuous use at 425-860°C is not recommended if corrosion resistance in water is required. In this instance 304L is recommended due to its resistance to carbide precipitation.Where high strength is required at temperatures above 500°C and up to 800°C, grade 304H is recommended. This material will retain aqueous corrosion resistance.Fabrication of Stainless Steel 304Fabrication of all stainless steels should be done only with tools dedicated to stainless steel materials. Tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steel by easily corroded metals that may discolour the surface of the fabricated product.Cold Working of Stainless Steel 304Stainless steel 304 readily work hardens. Fabrication methods involving cold working may require an intermediate annealing stage to alleviate work hardening and avoid tearing or cracking. At the completion of fabrication a full annealing operation should be employed to reduce internal stresses and optimize corrosion resistance.Hot Working of Stainless Steel 304Fabrication methods, like forging, that involve hot working should occur after uniform heating to 1149-1260°C. The fabricated components should then be rapidly cooled to ensure maximum corrosion resistance.Heat Treatment of Stainless Steel 304Stainless steel 304 cannot be hardened by heat treatment.Solution treatment or annealing can be done by rapid cooling after heating to 1010-1120°C.Machinability:Stainless steel 304 has good machinability. Machining can be enhanced by using the following rules: * Cutting edges must be kept sharp. Dull edges cause excess work hardening. * Cuts should be light but deep enough to prevent work hardening by riding on the surface of the material. * Chip breakers should be employed to assist in ensuring swarf remains clear of the work. * Low thermal conductivity of austenitic alloys results in heat concentrating at the cutting edges. This means coolants and lubricants are necessary and must be used in large quantities.Welding of Stainless Steel 304Fusion welding performance for Stainless steel 304 is excellent both with and without fillers. Recommended filler rods and electrodes for stainless steel 304 is grade 308 stainless steel. For 304L the recommended filler is 308L. Heavy welded sections may require post-weld annealing. This step is not required for 304L. Grade 321 may be used if post-weld heat treatment is not possible.Applications of Stainless Steel 304Stainless steel 304 is typically used in: * Sinks and splash backs * Saucepans * Cutlery and flatware * Architectural panelling * Sanitary ware and troughs * Tubing * Springs, nuts, bolts and screws.Project DetailsObjective: To design a test rig (a heating element) for indirect heating.Requirements: A heating element, protective covering for heating element, conductive oil (transformer oil), a container, thermostat, wax, electric supply and wires, supports for heating element in container, nut and bolt arrangement, arc and gas welding.Need for designing the HEATING ELEMENT?It was observed that the industries using wax, coaltar etc as fuel find it difficult to use these fuels efficiently during winter season. The fuels like wax and coaltar usually solidify during winters and hence it becomes inconvenient to use it to serve the purpose. Therefore it was needed to design such a test rig that could be used to liquefy the solid fuel without making any drastic change in the initial industry setup.Many attempts have already been made to design such a test rig but the tests didn’t turn successful because of many reasons. The element didn’t last longer because of its free exposure to air. This resulted in frequent over heating of heating element which would further result in either damage to the element or burning of fuel due to excessive rise in temperature.The other problem that was observed was that the initial setup of the industry could not be changed. An attempt was needed to design such an element which could be installed in an indirect manner so that the initial setup is not affected. The other requirement was protecting the heating element by providing it a protective covering such that the element is not exposed to air.Major Activities Performed in Designing the Heating Element: * Arc Welding * Gas Welding * Circuit design for element using a thermostat. * Other mechanical processes like drilling, machining, etc.Specifications of Various Parts in Design of Heating Element: 1) Heating ElementFig.1 Heating ElementWattage = 1600 wattDiameter of Element = 0.0085mMaterial of Element = Stainless steel(304)Length of Element = 14 inch (0.35m) 2) Heating Tube (outer covering) Fig 2 Heating Tube 3) Flange Fig 3 FlangeDiameter = 2 inch(0.05m)Material = Brass 4) Fuel * Wax * Water 5) Oil used:Transformer oil. Flash point = 2100C approximately. Density = 895-900 Kg/m3 at 200C Conductivity = 0.110 Watt/mK 6) SocketFig 4 SocketDiameter = 2 inch(0.05m)Material - Cast iron 7) Heating Element Fig 5 Heating Element 8) ContainerFig 6 ContainerDiameter of container = 42 cm (0.42 m)Height of container = 48 cm (0.48 m)Shape – Cylindrical.Operating Performance and ConclusionWatt density is the rated wattage of an element per unit of surface heated area (usually square inches) and indicates the potential to transmit heat. The formula is as followsWatt density = _Rated wattage_ Heated surface areaAs the definition indicates, the higher the watt density, the greater the possibilities for excessive sheath temperature when designing the system spreading the wattage requirement over more or larger heaters will reduce the operating temperature. Sheath temperature will be reduced, increasing the heater’s length of serviceCertain materials such as water, vegetables oils and metals have high conductivity rates. The heat generated travels quickly from the element and through the medium, allowing these materials to be heated at relatively high watt densities. Fuel oils, lubricating oils, hydraulic fluid and other materials with low conductivity rates such as sugar and syrups and most gases must be heated at low watt densities. A major concern is to dissipate the heat generated by the element. If attention is not paid to guidelines for both the heater and the material being heated watt densities too high will result in failure of the elements and possible damage to the material and equipment.Watt density = ____ Rated Wattage______ Diameter x Heated Element Length x π Here,Diameter of heating element =8.5 mm (0.34 inch) (0.0085 m)Length of heating element = 14 inch (0.35 m)Wattage of heating element = 1600 wattSo,Watt density = __ 1600____ 0.0085 x 0.35 x 3.14 Watt density = 171278 watts/sq m Or 108.76 watts/sq inchBibliography * Heat and Mass Transfer by D.S. kumar * www.wikipedia.org * Holman, J. P. (1990) Heat Transfer. 7th ed. McGraw-Hill * Incropera, Frank P. and DeWitt, David P. (1996) Fundamentals of Heat and Mass Transfer. 4th ed. Willey * Lienhard IV, John H. and Lienhard V, John H. (2002) Heat Transfer Textbook. 3rd ed. Lienhard IV, John H. and Lienhard V, John H. * www.google.com * Engineering of Materials and Metallurgy by O.P Khanna * Engineering Metallurgy by Y Lakhtin. * Manufacturing Processes by P.C Sharma.| |

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