CHAPTER 3 ISSUES IN MICRO TUBULAR COIL HEATER AND PROBLEM FORMULATION

Transcription

1 31 CHAPTER 3 ISSUES IN MICRO TUBULAR COIL HEATER AND PROBLEM FORMULATION 3.1 INTRODUCTION Micro tubular coil heaters convert electricity into heat through the process of Joule heating. Electrical current running through the element encounters resistance, resulting in heating of the element. Micro tubular coil heaters are adapted to a variety of applications. This is because; it can be formed into any shape. Micro tubular coil heaters are made as rectangular, square and circular sections, with or without built-in thermocouple. Micro tubular coil heaters are swaged and compacted with Magnesium Oxide and a helical or straight resistance wire element. Figure 3.1 shows the micro tubular coil heater. The main features of these heaters are 360 o heated area, readily confirms to surface, fast response and quick heat transfer, helical coil design for superior performance, good resistance to corrosion and it can easily be coupled with J or K type thermocouple. It also provides the best combination of physical strength, high emissivity and good thermal conductivity to heat hot runner bushings and nozzles, mainly for multi-cavity hot runner PET (PolyEthylene Terephthalate) performed moulds and thin wall container moulds. It also provides high durability and reliability, and excellent thermal conductivity. These are fabricated using cutting edge technology. These heaters are covered with axial clamps for offering

2 32 adjustability and loading at front end. Predefined dimensions and easy fitment covers with coiled structure helps in saving downtime hours in case of heater failure. Micro tubular coil Heaters are hygroscopic in nature due to MgO contents. If kept unused for longer period, there is moisture deposition on the terminals. Therefore the micro tubular coil heater is to be demoisturised prior to installation by heating them at C in an oven for approximately 1 to 2 hour. This will help evaporate the moisture present inside the micro tubular coil heater. There should not be air gaps between the heater and the nozzle. The inner diameter of the heater should never be opened by twisting as it will not fit tight which leads to premature heater failure. Due to high watt densities per sq.cm., micro tubular coil heaters require precise temperature controllers. Lead ends once bent should not be re-bent which will lead to breakage. Connection lead areas should be protected from combustible gases and liquids to avoid short-circuits. Stabilized Voltage supply increases the life of the heater as well as increases the wattage output. Figure 3.1 Micro tubular coil heater

3 Types Based on specific requirements and applications, micro tubular coil heaters are manufactured in different cross sections. Each one is having its own advantages and disadvantages. Basically there are three types of micro tubular coil heaters. They are Circular micro tubular coil heater Square micro tubular coil heater Rectangular micro tubular coil heater A special type of micro tubular coil heater is hermetically sealed micro tubular coil heater. The micro tubular coil heater s hermetically sealed construction prevents moisture from entering the beater, resulting in very long life. The element s nickel sheath is much more efficient for heating steel. The hermetically sealed micro tubular coil heater is available with cam operated axial clamp. One radial clamp is provided with this tubular coil heater. Figure 3.2 shows the hermetically sealed micro tubular coil heater. Figure 3.2 Hermetically sealed micro tubular coil heater

4 Specifications Micro Tubular Coil Heaters are manufactured in different diameters to suit different applications. The specification of a typical micro tubular coil heater is represented in Table 3.1. Table 3.1 Specifications of micro tubular coil heater Sheath Material Cross Section Insulation Material Chrome - Nickel Steel Circular, Square or Rectangular High Purity MgO Heating Element NiCr 80:20 Lead Wires Voltage Range Power Rating Teflon Insulated Maximum 250 Volts, standard 230 volts Depending on application Power Tolerance ± 5% to ± 2% Length Tolerance ± 2% Unheated Length Minimum Bending Diameter Minimum 25 mm plus adapter connection 6 mm Micro tubular coil heaters are formed into a coil of predefined dimension and equipped with a special cover for easy fitment. This special cover called axial clamp allows front end loading and adjustability Construction Micro tubular coil heaters are made as rectangular, square and circular sections, with or without built-in thermocouple. These heaters are

5 35 swaged and compacted with Magnesium Oxide and a helical or straight resistance element. These are fabricated using high grade raw material that backed with cutting edge technology. These heaters are covered with axial clamps that act as special cover, for offering adjustability and loading at front end. Predefined dimensions and easy fitment covers with coiled structure helps in saving downtime hours in cases of heater failure. The helical wound heating wire is made of a high temperature resistant Ni/Cr alloy and the insulation consists of superior grade of Magnesium Oxide. As the heaters are swaged, they have excellent electrical insulation and high heat transfer even at high sheath temperature. The connection ends are sealed with sealing components to prevent the heater from moisture. Figure 3.3 shows the components of micro tubular coil heater. Figure 3.3 Components of micro tubular coil heater Magnesium Oxide insulation ensures superior heat transfer and the resistance wire is precision-wound for long heater life. Tubular heaters can be used in most applications. Straight tubular heaters can be clamped to metal surface or inserted in machined groves for conductive heat transfer or use a formed tubular to provide consistent heat in any type of special application Applications Micro tubular coil heaters are mostly used in thin walled container moulds, PET preformed moulds, hot sprue bushings, sealing bars, hot metal

6 36 forming punches and hot runner nozzles and bushings. The general area of applications of these heaters are plastic processing machineries, packaging machineries, shoe making machineries, foundry equipments and radiant surface heating equipments. The most important application of the micro tubular coil heaters is hot runner system. The hot runner system is one kind of new injection molding system, it controls and maintains the melt plastic to arrive at the die space directly from the injection molding machine. The hot runner system is composed generally by three parts: main feed inlet, hot divergence board, and hot spray nozzle. Preform injection mold adopts specially designed hot runner system to achieve ideal and prompt heating effect for preform production. Larger flowing channels of such hot runner system ensures the mold to produce preform with lower injection pressure, which reduces mold s each part s wear and tear and maintenance cost, and also saves electric energy. High performance hot runner system ensures uniform melt flow and pressure in all cavities. Figure 3.4 shows the hot runner system and Figure 3.5 shows the injection moulding with micro tubular coil heater. Figure 3.4 Hot runner system

7 37 Figure 3.5 Injection moulding with micro tubular coil heater 3.2 MATERIALS AND DESIGN CONSIDERATIONS Generally Heaters are designed in two conditions: Elastic design (Lower temperature) Rupture design (Higher temperature) The micro tubular coil heaters are designed using rupture design as these are used at high temperatures. The primary requirements of materials used for micro tubular coil heating elements are (Furjes et al 2002) High melting point High electrical resistivity Reproducible temperature coefficient of resistance Good oxidation resistance Absence of volatile components Resistance to contamination

8 38 Good elevated temperature creep strength High emissivity Low thermal expansion, and low modulus to minimize thermal fatigue Good resistance to thermal shock Good strength and ductility of fabrication temperatures The basic problem in designing an electrical heating element for any application is selecting a type and size of resistance wire, ribbon or strip, which will operate satisfactorily at the desired temperature under specified environmental condition. Materials used for resistance heating applications are Nickel-Chromium alloys (80Ni-20Cr and 70Ni-30Cr) Iron-Nickel-Chromium alloys Iron-Chromium-Aluminum alloys Pure metals (e.g., Platinum, Molybdenum and Tungsten) Nonmetallic materials (e.g., Silicon Carbide and Graphite) Of these five material groups, Nickel-Chromium and Iron-Nickel- Chromium alloys are the most widely used heating materials. Most heating elements use Nickel-Chrome (NiCr) wire or ribbon as the conductor. Nickel- Chrome (NiCr) is an ideal material, as it is inexpensive, has relatively high resistance, and does not break down or oxidize in air in its working temperature range. The 80Ni-20Cr alloys permit a wider range of operating temperatures because they have the greatest resistance to oxidation, therefore they can be used at higher temperatures than other Nickel-Chromium and

9 39 Iron-Nickel-Chromium alloys. Small Silicon additions (0.75 to 1.75%) will improve oxidation resistance. In general, Nickel-Chromium heating elements are unsuitable above C, because the oxidation rate in air is too great and the operating temperature is too close to the melting point of the alloy. As shown in Figure 3.7, the strength of 80Ni-20Cr drops significantly when temperature exceeds C. The diameter and length of the micro tubular coil heater is calculated using the equation (3.1) and (3.2) under certain capacity and voltage. d P 2 2 V W 5 2 (3.1) L 2 2 V d 4 P 10 3 (3.2) where d - Diameter of heating wire, mm - Resistivity of heating wire -m P - Power per phase, Kilo Watts V - Voltage, Volts W - Surface loading, W/cm 2 L - Length of heating wire, m Factors of Material Selection Melting point : The melting point of a substance is the temperature at which the solid phase converts to the liquid phase less than 1 atmosphere of pressure. The melting point is one of the physical properties of a substance

10 40 that is useful for characterizing and identifying the substance. The micro tubular coil heater requires a material with high melting point as the operating temperature is high. Electrical resistivity: Electrical resistivity is the electrical resistance offered by a homogeneous unit cube of material to the flow of a direct current of uniform density between opposite faces of the cube. The resistance can be calculated by the equation (3.3). R L A (3.3) where R - Resistance - Density L - Length of the heater A - Cross sectional area of the heater Electrical resistivity should be high enough for the material to produce heating effect. Oxidation resistance : Oxidation resistance is the ability of metallic materials to resist chemical degradation of the surface, caused by the action of air or other gaseous mediums at high temperatures. The oxidation resistance of a metal or alloy in an oxidizing atmosphere is determined by the properties of the oxide layer that forms on the surface of the metal and inhibits the diffusion of gas into the metal, thus reducing the development of gaseous corrosion. Along with heat resistance, oxidation resistance is a basic criterion of the suitability of a given material for high-temperature service. Creep strength : Creep strength is the constant nominal stress that causes a specified quantity of creep in a given time at constant temperature.

11 41 Creep strength is expressed as the stress necessary to produce 0.1% strain in 1000 hours. A material should have high creep strength at elevated temperatures. Thermal expansion: Most materials are subject to thermal expansion which is a tendency to expand when heated, and to contract when cooled. It is the tendency of matter to change in volume in response to a change in temperature. The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature. The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure. Thermal expansion should be low for heating elements. Resistance to thermal shock: It is the sudden stress produced in a body or in a material as a result of a sudden temperature change. Thermal shock is the extreme temperature difference (gradient) across an object, which can result in cracking and/or breaking. Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts. This differential expansion can be understood in terms of stress or of strain, equivalently. At some point, this stress can exceed the strength of the material, causing a crack to form. A heating coil material must have high resistance to thermal shock Factors to be Considered for Heater Design The rupture temperature mainly depends on resistance of the material and there are four important factors, which affect the resistance (Inderjit Singh et al 2005).

12 42 Length of the element: Length can affect resistance because number of metal atoms is different as the length changes, making the rate of energy transfer differ. Cross sectional area of the element: The increase or decrease in cross section area, gives more or less space for the atoms to collide with each other. Temperature: If the wire is heated up, the atoms in the wire will start to vibrate because of their increase in energy. This causes more collisions between the atoms and this increase in collisions will lead to increase in resistance. Material: The type of material will affect the amount of free electrons which are able to flow through the wire. The number of electrons depend on the amount of electrons in the outer energy shell on the atoms. If there are more atoms, then there will be more number of electrons, causing a lower resistance Nickel - Chrome Alloys The Nickel-Chromium system shows that Chromium is quite soluble in Nickel. This is a maximum at 47% at the eutectic temperature and drops off to about 30% at room temperature. A range of commercial alloys is based on this solid solution. Such alloys have excellent resistance to high temperature oxidation and corrosion and good wear resistance. The introduction of small amounts (<7%) of Chromium to Nickel increase the sensitivity of the alloy to oxidation. This is because, the diffusion rate of oxygen in the scale is increased. This trend reverses after addition levels increase above 7% Chromium and increases up to an addition level of approximately 30%. Above this level, there is little change.

13 43 A marked increase in electrical resistivity is observed with increasing Chromium additions. An addition of 20% Chromium is considered the optimum for electrical resistance wires suitable for heating elements. This composition combines good electrical properties with good strength and ductility. Figure 3.6 shows the electrical resistivity as a function of Chromium content for Nickel-Chromium alloys. Figure 3.6 Electrical resistivity as a function of Chromium content for Nickel-Chromium alloys When the compositional changes have a negligible effect on mechanical properties, higher additions of reactive elements tend to prevent flaking of the scale during cyclic heating and cooling. This effect is less of an issue with continuously operating heating elements, so addition levels do not need to be as high. The 80/20 Ni/Cr alloy is often used for wrought and cast parts for high temperature applications, as it has better oxidation and high corrosion resistance compared to cheaper Iron-Nickel-Chromium alloys. This alloy is highly suited to applications that are subject to oxidation. Figure 3.7 shows the effect of temperature on the tensile properties of an annealed 80Ni- 20Cr resistance heating element alloy.

14 44 Figure 3.7 Effect of temperature on the tensile properties of an annealed 80Ni-20Cr resistance heating element alloy Nichrome is a non-magnetic alloy of Nickel, Chromium, and often Iron, usually used as a resistance wire. A common alloy is 80% Nickel and 20% Chromium, by mass, but there are many others to accommodate various applications. It is silvery-grey in colour, corrosion-resistant, and has a high melting point of about 1400 C (2552 F). Due to its relatively high electrical resistivity and resistance to oxidation at high temperatures, it is widely used in electric heating elements, such as hair dryers, electric ovens, soldering iron, toasters, and even electronic cigarettes. Typically, Nichrome is wound in coils to a certain electrical resistance, and current is passed through to produce heat. Nichrome 80/20 is especially suitable for applications in the electrical appliance industry due to its ductility and strength at high temperature. In industrial furnace use, Nichrome 80/20 has many advantages because of its excellent mechanical properties in the hot state. Nichrome

15 45 80/20 has superior life compared to competitive NiCr alloys because of the extremely good adhesion properties of the surface oxide. Typical applications for Nichrome 80/20 are ironing machines, water heaters, plastic moulding dies, soldering irons, metal sheathed tubular elements, cartridge elements and so on. Table 3.2 shows the composition of NiCr 80/20. Table 3.2 Composition of NiCr 80/20 Alloy Ni% Mn% Fe% Si% Cr% C% NiCr 80/20 Bal. Max 1.0 Max Max Titanium Nitride Titanium Nitride has been extensively investigated because of its many potential applications (Akio Kato et al 1975). It is widely used as a wear resistant coating on tools, as a gold substitute in decorative articles, and in the fabrication of thin film resistors. The Integrated Circuits industry has shown increasing interest in TiN because of its good diffusion barrier properties, and thermal stability. It has also been explored as a solar energy absorber and transparent heat mirror because of its optical properties in the visible and IR regions. It has a melting point of 2930 ºC and its thermal conductivity is 19.2 W/mºC. It has a resistance of ohm-m under normal conditions, which is an important parameter in micro tubular coil heater design. Titanium Nitride is a material, exhibiting good wear resistance and is potentially suitable for a wide range of high-temperature applications.

16 Aluminium Nitride Aluminium Nitride is a Nitride of Aluminium. Aluminum Nitride has a hexagonal crystal structure and is a covalent bonded material. The use of sintering aids and hot pressing is required to produce a dense technical grade material. The material is stable to very high temperatures in inert atmospheres. In air, surface oxidation occurs above 700 C (Glen A Slack et al 1976). A layer of Aluminum Oxide forms which protects the material up to 1370 C. Above this temperature bulk oxidation occurs. Aluminum Nitride is stable in Hydrogen and Carbon Dioxide atmospheres up to 980 C. The material dissolves slowly in mineral acids through grain boundary attack, and in strong alkalis through attack on the Aluminum Nitride grains. The material hydrolyzes slowly in water. Most current applications are in the electronics area where heat removal is important. Aluminium Nitride is having good dielectric properties, high thermal conductivity, low thermal expansion coefficient, close to that of Silicon and non-reactive with normal semiconductor process chemicals and gases. The important applications of Aluminium Nitride includes substrates for electronic packages, heat sinks, IC packages, microwave device packages, material processing kiln furniture, semiconductor processing chamber fixtures and insulators and molten metal handling components. Aluminium Nitride has a melting point of 2200 ºC and thermal conductivity of 28.5 W/mºC. Its resistance is under normal operating conditions, which is an important parameter for micro tubular coil heater design. Table 3.3 shows the properties of Nichrome, Titanium Nitride and Aluminium Nitride materials.

17 47 Table 3.3 Properties of materials Material / Properties Nichrome TiN AlN Melting Point, ºC Resistance at room temperature, ohm-m Thermal Conductivity, W/mºC Density, kg/m Modulus of elasticity, N/m Thermal Expansion Coefficient, K Operating temperature, ºC (As micro tubular heating element) LIMITATIONS OF PRESENT MICRO TUBULAR COIL HEATER Presently, the micro tubular coil heater wires are manufactured using Nickel-Chrome alloys, which are resistant to high temperatures. 80/20 Nickel-Chrome alloys containing long life additions make it eminently suitable for applications such as Nichrome resistance wire, subject to, frequent switching and wide temperature fluctuations. A relatively low temperature coefficient of resistance with a high resistivity makes it suitable for control resistors. Nichrome wire is almost 100% efficient in converting electrical energy into heat. The main disadvantage of Nichrome wire is that it cannot be soft soldered due to the operating temperature and must therefore be crimped or silver soldered. Another limitation of micro tubular coil heater and all types

18 48 of heaters is the change of resistance with the change in dimensions. The resistance increases with the increase in length and decreases with increase in cross sectional area i.e., diameter. Table 3.4 shows the percentage change of resistance of Nichrome heating element at different temperatures. Table 3.4 Change of resistance with temperature of Nichrome heater wire ºC % Increase in resistance It is evident from Table 3.4 that the resistance of the heater material increases with its temperature. Its operating temperature is limited to 900 ºC, which is very low for most of the heating applications, because of the increase in resistivity with the reduction in cross sectional area. In order to use micro tubular coil heaters more than 900ºC, a new material is to be identified which is stable even at high temperatures. 3.4 ELECTRO - THERMAL BEHAVIOUR Joule heating, also known as resistive heating, is the process, by which the passage of an electric current through a conductor releases heat. Heat produced is proportional to the square of the current, multiplied by the electrical resistance of the wire, which is given in equation (3.4). Q I 2 R (3.4) This relationship is known as Joule's First Law. Joule heating is caused by interactions between the moving particles that form the current and the atomic ions that make up the body of the conductor. When a potential

19 49 difference is applied across the ends of a conductor, the free electrons are accelerated and acquire kinetic energy. As the electrons move through, they collide with the positive ions and atoms of the conductor and transfer their kinetic energy to them. Between two collisions, the electrons again pick up kinetic energy from the electric field. As a result, the kinetic energy of vibration of these lattice ions or atoms increases. This increases the thermal energy of the lattice, which means that the temperature of the conductor increases. Since the source of emf is maintaining current in the conductor, the electric energy supplied by the battery is converted into heat in the conductor. Electrical energy is the input for the micro tubular coil heater. Micro tubular coil heaters are made for different wattages starting from 400 W to 1100W depending upon the required surface temperature of the heater. 3.5 CONCLUSION The micro tubular coil heater elements are manufactured using Nichrome alloy, which is a suitable material for all types of heating elements. A relatively low temperature coefficient of resistance with a high electrical resistivity makes it suitable for control resistors. Nichrome wire is almost 100% efficient in converting electrical energy into heat. The major drawback of the present micro tubular coil heater is low operating or working temperature. Therefore there is a need for determining materials which are stable even at high temperatures.