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Principles of Electrical Heating Technology

September 28, 2020
By Tyler Durden
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Ipsen Furnace Electrical Heating System Introduction

Heating system is one of the most important key systems inside an industrial heat treatment furnace. The performance of the heating system directly determines the stability and quality of the thermal field provided by the furnace, and its reliability often determines the maintenance and operational cost of the whole equipment to a great extent. So let's dive in to understand the key principles and factors for selection of suitable electrical heating system for your furnace.

IPSEN atmosphere multi-purpose furnaces are available with electrical and gas heating systems. In this article we will cover electrical heating as used inside an IPSEN branded furnace. However, the main ideas also apply to other brands as well.

IPSEN Electrical Heater

Picture: Electrical Heater for IPSEN furnace.

Components of Electrical Heating System

The electric heating system of IPSEN multi-purpose furnace is composed of electric heating element, measuring element, control instrument and power controller. The power controller transmits the adjusted power to the electric heating element (usually the heating device with resistance heating wire at its core). The temperature measuring element monitors the temperature in the furnace at any time and transmits the temperature measurement to the control instrument. The control instrument adjusts the power controller according to the program setting and real-time temperature signal to ensure that the temperature in the furnace is controllable. This is the whole control process of the heating system.

In the whole system, the control instrument and power controller are long-life working elements, which usually have a long service life. As long as they are properly maintained, they can be used for a long time, however the heating and measuring elements have limited service life. Because both of them work at very high temperature and in harsh working environment, thus they experience high temperature oxidation and deformation.

Replacement cost of electrical heating elements can easily account for more than 40% of the overall maintenance expense of the furnace. And that does not include loss of production due to furnace shutdown. Therefore, systematic optimization of the service life and reliability of electric heating elements can significantly reduce the overall operating and maintenance cost of the production line.

Electrical Heater for IPSEN furnaces has two main components:

  1. Squirrel Cage Heater:It is basically a resistance wire looped through a number of ceramic plates with holes in a shape that resembles a bird cage.
  2. Radiation Tube: Is an encasing for the heating element to prevent the furnace atmosphere to get in direct contact with the heater.

The heater is placed inside a radiation tube, which is directly heated by the heater to about 1000 Degrees Celsius, and then the energy is transferred into the furnace by the radiation tube in the form of thermal radiation. The reason why the radiation tube is used as enclosure for the heater is to prevent the furnace from leaking and allowing easy replacement of heaters without requiring lengthy shutdown of the furnace.

Principle, characteristics and performance of electrical heaters

The electrical heater transforms electrical energy into heat energy by using resistance thermal effect.

When the current flows through a conductor with non-zero resistance, heat will be generated, which is proportional to the square of the current and proportional to the resistance of the conductor. This is called Joule's law. The heating body of electrical heater is usually formed by winding the wire around an insulator to separate the wires, thus allowing more compact heater design. When the current flows through the resistance wire, heat is generated, which heats the medium. Resistance heating elements can be made into many different shapes to meet the actual needs.

Various resistance heating elements

Picture:Variety of Resistance Heating Elements With Different Shapes.

The material and quality of resistance wire is the decisive factor of heater quality. There are many kinds of common industrial resistance heating media, such as traditional metal alloy materials, carbon based materials such as graphite, and new type of cermet materials (a general term for a large class of new composite materials, formed by metal and ceramic micro composite structure. It has the advantages of ceramic high temperature and chemical corrosion resistance, while preserving the metal properties of conductivity and ductility). Graphite material has superior performance, but it must be isolated from oxygen. Currently new cermet materials are mainly used in sensors. In the heat treatment industry, metal alloy is still the mainstream of heating system selection. Common metal alloy resistance heating materials include tungsten, iron chromium aluminum, nickel chromium, molybdenum, tungsten, platinum, tantalum and platinum rhodium alloy. These alloys can be divided into two types, one can be used in oxygen environment, and the other must be isolated from oxygen (such as tantalum, tungsten and molybdenum (widely used in vacuum heat treatment furnace)). Among the alloys that can be used in oxygen environment, quite a few of them are superior in performance but expensive in price (for example, platinum rhodium platinum alloy is used for S-type thermocouple with working temperature higher than 1300 ℃). Therefore, in the industrial use of atmosphere furnaces, only nickel chromium and iron chromium aluminum are the most viable options from cost and performance perspective. Heating elements made from graphite are mainly used in vacuum furnaces.

Material classification of resistance heating elements

Picture:Material classification of resistance heating elements

The advantages and disadvantages of Fe Cr Al and Ni Cr alloys are as follows:

The advantages of Fe Cr Al alloy are as follows:

  • High Working Temperature

    The average maximum service temperature of ferrite alloy Fe Cr Al can reach 1400 ° C, while that of Austenitic Alloy Ni Cr is only about 1200 ° C.

  • High Resistivity

    The resistivity of Fe Cr Al alloy is higher than that of Ni Cr alloy. This makes it possible to select materials with larger cross-sections, thus prolonging the life of the heating element. Especially in the application of thin wire, the weight can be greatly reduced. The higher the resistivity, the less material is used. Moreover, compared with Ni Cr alloy, the resistivity of Fe Cr Al alloy is less affected by cold working and heat treatment.

  • Higher Surface Load and Longer Service Life

    The higher operating temperature and longer service life of Fe Cr Al alloy ensure the ability to withstand high surface loads. The service life is 2 to 4 times longer than that of Ni Cr alloy operating in the atmosphere at the same temperature.

  • Light Weight and Low Cost

    The weight of ferro chromium aluminum alloy is lower than that of Ni Cr alloy for the same resistivity value, hence the weight and component cost can be saved significantly by using ferro chromium aluminum in comparison to Ni Cr Alloys.

  • Perfect Oxidation Resistance

    Alumina (Al2O3) formed on the surface of Fe Cr Al alloy has better adhesion, so it has less corrosion. It is an excellent antioxidant, diffusion barrier layer and electrical insulator. It has better carburizing resistance than chromium oxide (Cr2O3) formed on the surface of Ni Cr alloy.

The advantages of Ni Cr alloy are as follows:

  • It has perfect shape stability at high temperature;

    Because of its higher thermal and creep strength than Fe Cr Al alloy, Ni Cr alloy does not easily deform at high temperatures, thus keeps very good shape stability.

  • Non-magnetic

    Ni Cr alloy is a non-magnetic material that can be used in low temperature applications. At the same time, Fe Cr Al alloy only starts being non-magnetic at temperatures above 600 ° C.

  • Good Ductility After Long Term Use;

    After long-term use, the nickel chromium alloy maintains its toughness. This feature makes the heating element more durable and allows it to be repaired in case of damage.

  • High Emissivity

    Under the condition of complete oxidation, the emissivity of Ni Cr alloy is higher than that of Fe Cr Al alloy. With the same surface load, the temperature of Ni Cr alloy is lower than that of Fe Cr Al alloy.

  • Corrosion Resistance

    Generally, the corrosion resistance of Ni Cr alloy is better than that of non oxidized Fe Cr Al alloy at room temperature (except for sulfur environment and controllable atmosphere).

 

The maximum working temperature of Kanthal Alloys

Picture:Comparison of maximum operating temperatures between Fe Cr Al and Ni Cr Alloys (data source: Kanthal) - where Kanthal series is Fe Cr Al alloy and Nikrothal series is Ni Cr alloy

Resistivity comparison of Kantal heating alloys

Picture:Resistivity comparison of Fe Cr Al and Ni Cr Alloys (data source: Kanthal) - where Kanthal series is Fe Cr Al alloy and Nikrothal series is Ni Cr alloy

In view of the working conditions of the heat treatment equipment, most of the electric heating elements adopt Fe Cr Al resistance wire as heating material. The reasons are mainly longer service life, lower cost and the higher working temperature.

The working life of electrical resistance wire is affected by the following factors:

  • Alloy Composition

    Different Fe Cr Al alloys with different resistivity (from 1.25 to 1.45) can be obtained by micro adjusting the content of Cr and Al in Fe Cr Al alloy. The highest working temperature of these alloys is between 1300 ℃ and 1400 ℃. Under the same temperature, the continuous working time is slightly different.

  • Working Temperature

    In addition to the above factors, the working temperature plays a decisive role in the working life of the heater. The data show that the service life of the ferro chromium aluminum alloy is greatly affected by the working temperature when it is close to the limit working temperature. When the temperature is higher than 1100 ℃, the working life of the alloy wire will decrease to 25% of the original value with each 100 degrees in higher temperature.

Fe Cr Al alloy resistance wire

Picture:Fe Cr Al Alloy Resistance Wire

NESDRA - Electrical Heating Elements

"Squirrel Cage Heater" for IPSEN multi-purpose atmosphere furnace made with resistance wire out of iron chromium aluminum heating wire wound into bird cage or squirrel cage shape on a series of ceramic plates with spacing holes to prevent the wires touching each other.

Picture: Squirrel Cage Electrical Heater

NESDRA provides three kinds of ferro chromium aluminum resistance wire heaters with different specifications to allow users to pick the most optimal electrical heater most suitable for their working conditions:

  1. NESDRA - Standard Heater

    Uses 0CR21AL6NB or Fe Cr AL 140 resistance wire. It is suitable for customers with low working temperature, low loading capacity and low temperature drop once the load is pushed inside the furnace.

  2. NESDRA - Premium Heater

    Uses Shougang HRE heating wire, equivalent to ferro chromium aluminum 145 resistance wire, is suitable for customers with moderate working temperature, high loading capacity and high overall electrical load (difference in temperature drop, thus required ramp up to get the hot zone to needed working temperature after load is introduced into furnace.)

  3. KANTHAL AFTM Heater

    Uses Kanthal AFTM heating wire, which is made in Sweden. It has a higher level of surface quality control, thus suitable for customers with higher working temperature, higher loading capacity and overall load on the heaters.

Principle, Structure and Performance of Radiation Tube

After looking at the heating elements, let's look at the radiation tubes.

The main task of a radiation tube is to absorb high temperature, radiate it inside the furnace and ensure the furnace is not leaking at places where the heaters are mounted. In fact, it is a heat-resistant steel pipe sealed at one end, which goes deep into the furnace hotzone from the outside, and then holds the heater inside the tube. When the heater is energized, a large amount of heat is generated inside the radiation tube. After the radiation tube is heated to the red hot state, the radiation tube itself becomes an infrared radiator and radiates the heat generated by the heater into the furnace hotzone. The inner side of the radiation tube faces the high temperature from the electrical heater, and the outer side is in direct contact with the furnace atmosphere. Hence, it is exposed to very challenging working environment.

In addition, removal and installation process of radiation tubes are more complicated than that of an electrical heater. Radiation tube also serves the function of protecting the heaters. Once the radiation tube is damaged, the heaters will definitely need to be scrapped. If the heaters are deformed and short circuited, it is often not necessary to replace the radiation tubes. Therefore, furnace design requirements call for radiation tubes to be of sufficient quality to outlast the heaters. This can reduce the frequency of replacing radiation tubes and greatly reduce the operational and maintenance cost.

Several factors have great influence on the stability and expected service life of the radiation tube.

  1. Oxidation Resistance and Creep Resistance

    At high temperature, the wall of radiation tube will be oxidized continuously, so it will becomes thinner and thinner. In order to achieve high thermal conductivity, the wall of radiation tube should be thin, but at the same time a thin wall will be oxidized earlier than the thick wall, resulting in faster perforation. Oxidation is like rust, and rust will gradually spread from the surface to the interior of the material. If the material does not have sufficient oxidation resistance, it will rust through quickly. At the same time, it will also produce oxide skin falling off from the inner side of the radiation tube. If that fine dust sufficiently accumulates on the spacing isolation disks of the electrical heater, it will short circuit resistance wires and burn the heater.

    The higher the creep resistance of radiation tube is, the better it will keep its original shape without creep at high temperature. Once the radiation tube is deformed, even if it is slightly bent or twisted, it will become extremely difficult to remove and maintain it. Once the deformation is serious, it may contact the electrical heater, causing the risk of short circuit it.

    The radiation tube made of Fe Cr Al has excellent oxidation resistance

    Picture:Oxide Formation Inside The Radiantion Tube After A Period of Use and the Deformation of the Radiation Tube (Fe Cr Al alloy on the left and Fe Cr Ni alloy on the right) photo source: Kanthal

  2. Manufacturing Process of Radiation Tube

    There are great differences in material uniformity, internal stress distribution and micro alloy composition of radiation tubes made by different manufacturing processes. The non-uniformity or stress concentration inside the tube will be released significantly at high temperature, which will change the shape of the radiation tube and cause failure. The ideal manufacturing process should be able to ensure that the micro composition and internal stress distribution of the radiation tube are uniform, so as to prevent later deformation at high temperature.

  3. Wall Thickness of Radiation Tube

    The lower the thickness of the tube wall, the faster the speed of heat conduction, that in turn better radiates heat energy. The animation below shows the speed of heat conduction under different wall thickness conditions. As can be seen from the figure, if the wall thickness is doubled, the heat conduction speed will be reduced to 1 / 4 of the original value.

    Picture: Click the triangle (play) button at the bottom of the above graphic to see the effect of different wall thickness on the heat conduction velocity (data source: Concord)

  4. Distance Between Resistance Wire and Radiation Tube Inner Wall

    If the heating resistance wire is too close to the radiation tube inner wall, it will increase the probability of short circuit caused by oxide scale from the radiation tube. Hence, higher the oxidation resistance of the radiation tube material allows for more compact build of the radiation tube and electrical heater package.

Material of Radiation Tube

The high temperature oxidation resistance and creep resistance of radiation tube are mainly guaranteed by material. In the field of industrial heat treatment furnace, although there are some non-metallic radiation tubes (mainly silicon carbide or alumina ceramics), most of the mainstay radiation tubes are made of heat-resistant steel. The common heat-resistant steel grades for radiation tubes are 2520 and 2535. The difference between them lies in the nickel content. Because the nickel content of 2535 is much higher than that of 2520, 2535 has a longer expected service life and higher temperature resistance.

Fe Cr Ni alloy material

The standard radiation tube provided by NESDRA contains 0.5% Niobium (Nb) added to the base 2535 alloy. The melting point of Niobium is very high, reaching 2468 ℃. It’s content on earth is quite rare, but only a small amount of it is added to steel, which can greatly improve the strength of steel at high temperature. Therefore, it is widely used in steam turbine blades, turbine blades and other occasions where metal high temperature strength is required. The radiation tube with Niobium (Nb) can have better creep resistance.

Niobium metal

Picture:Niobium Metal

Fe Cr Al Alloy Material

In addition, Kanthal creatively uses iron chromium aluminum material to make radiation tubes, which can reach temperatures above 1300 degrees. The radiation tube with long service life can be made by using Fe Cr Al alloy with good oxidation resistance and high temperature resistance.

Manufacturing Process of a Radiation Tube

The manufacturing process of radiation tube mainly includes casting and rolling plate welding. For the radiation tube body, the so-called "centrifugal casting" process is usually used. As shown in the figure below, after the liquid metal is poured into a high-speed rotating roller shaped mold, the liquid metal is thrown to the inner wall of the drum by centrifugal force, and a seamless steel pipe is formed after solidification.

Centrifugal Casting Process

Picture:Centrifugal Casting Process Diagram

Centrifugal Casting Process

Centrifugal casting process has the advantages of uniform composition, small internal stress and little deformation. However, it also has obvious disadvantages, that is, it is difficult to reduce the thickness of casting layer due to defects such as sand hole, thus the thin casting pipe is easy to cause air leakage due to sand hole and other defects. Therefore, the conventional centrifugal casting radiant tube is cast to 7mm wall thickness, and then the wall thickness is reduced to 5mm by inner wall machining. However, this will also cause another problem. During machining, the protective oxide layer formed on the inner surface of the radiation tube during casting will be cut away, and that in turn will reduce the oxidation resistance of the inner wall of the radiation tube and have a negative impact on the life of the radiation tube.

A new radiation tube manufacturing process is non-machining centrifugal casting process . Through the improvement of centrifugal casting process and die, the occurrence rate of porosity and other defects can be reduced. The wall thickness of centrifugal casting seamless pipe can be reduced to 4.5-5 mm directly. The seamless tube is directly used to make radiation tube. Since there is no damage to the oxide layer on the inner wall of the tube by machining, the radiation tube with this new technology has stronger oxidation resistance at high temperature.

Rolled Plate Welding

In addition to casting seamless steel pipe, radiation tube can also be made of steel plate rolled plate welding . The advantage of this method is that the wall thickness of the radiation tube can be selected more flexibly, and faster heat conduction velocity can be achieved by adopting lower wall thickness. However, the welding method and welding process of radiation tube have a great influence on its performance. The traditional welding pipe adopts the form of axial straight weld, which will form large uneven welding stress inside the radiation tube body. At the same time, due to the uneven composition of welding material and base metal, it is easy to cause the radiation tube deformation after use. The thinner the wall thickness is, the greater the deformation will be, which objectively hinders the efforts to further reduce the wall thickness and improve the heat transfer efficiency.

The performance of radiant tube with straight welding seam and coiled plate welding can not meet the requirements

Picture:The radiation tube with straight welding seam is easy to deform in use and its performance cannot meet the requirements of long service life.

Spiral Weld Rolling Plate Welding Technology

A new manufacturing method skilfully avoids such a problem by using spiral weld: by properly selecting the splicing mode of the tube wall, the weld of such radiant tube extends along a spiral track along the tube body. Therefore, although this results in longer welds per unit length of radiant tube, the weld is uniform from any angle of the circumference. In this way, the welding material is evenly distributed in all directions of the radiation tube, so that a thinner tube wall (2 - 3mm) can be used and while deformation with use can be avoided.

Steel pipe with rolling plate welding process

Picture:Spiral Welded Steel Pipe

NESDRA Radiation Tubes

NESDRA provides two kinds of radiation tubes for IPSEN furnaces. However, contrary to other market participants we do not provide the most basic radiation tube specification (2520), as we believe the material cost savings from that low specification tube is illusionary, as due to much shorter service life the user will need to more frequently replace those cheaper radiation tubes, increasing overall operations and maintenance cost due to furnace shutdowns and possibly also unnecessary heater replacements (as we mentioned above a broken tube will most certainly also result in scraping of the electrical heater.).

  • NESDRA Premium Radiation Tube

    Cr25ni35nb is used to obtain higher creep resistance at high temperature by adding Niobium. The centrifugal casting process is used to achieve a wall thickness of 5mm. Overall this radiation tube has a long service life.

  • KANTHAL AFTM Radiation Tube

    It is made of AF material from KANTHAL and welded with spiral weld. The thickness is only 2mm. It is manufactured in Sweden and directly imported to China. Extremely light weight and very long service life.

How to Choose the Right Heating System Components for You?

As mentioned above, the components of electric heating system have various forms, and each has its own advantages and disadvantages. At the same time, the cost can be significant. So how can we determine the most appropriate heating system component according to the specific situation of users? How can we find an "optimal" solution to reduce the overall maintenance cost of the heat treatment line?

First of all, the scheme with the lowest total life time cost is not necessarily the one with the lowest unit price, because the components with the lowest unit price can not cope with the severe working conditions, which will lead to frequent damage and failure, which will lead to frequent spare parts procurement and aggravate the loss of shutdown and maintenance. Of course, we can't simply think that the scheme with the highest unit price is the one with the lowest cost either, because under less stringent working conditions, the products with lower unit price (lower specification) can also be used for a long time. Although the theoretical life of products with high unit price is far longer, in practice, timely preventive maintenance and careful operation will also play a big role in achieving long service life.

Therefore, the selection of appropriate heating system components for heat treatment equipment needs to be considered comprehensively according to the users current process, maintenance status, working temperature, charging type and so on, in order to get the most appropriate optimization scheme.

Based on our experience in the heat treatment industry, we propose the following decision criteria to help users choose the solution that can achieve the lowest maintenance cost and optimize the operational cost. This principle is related to the following aspects:

  • Operating Temperature of the Equipment

    The higher the operating temperature of the equipment, the higher the specification of the heater is needed, otherwise the expected service life will be short and replacement very frequent.

  • The main process of the equipment, for carburizing, especially super carburizing, a large amount of carbon black build up on the radiation tube outter wall can easily cause radiation tube deformation, so it is recommended to use high specification radiation tube. On the other hand for quenching, normalizing, quenching and tempering processes, low specification radiation tube and electrical heaters could be used.
  • Normal loading capacity of the equipment (80% of the maximum loading)

    The larger the loading amount, the longer the heater will be in high power output mode, thus higher specification heater should be selected.

  • Temperature drop after inserting the charge into the hotzone (with or without preheating furnace)

    After the equipment is loaded, the temperature of the heating chamber will be reduced. The more the temperature is reduced, the greater the thermal impact on the heater and radiation tube will be. Therefore, high specification heater should be selected.

  • Equipment maintenance status

    Proper maintenance of the equipment can give full play to the life potential of high specification heaters. If the maintenance is not proper, it is not recommended to use high specification heating elements.

    Users can assess their own conditions according to the above scoring standards and determine the most appropriate combination of heaters and radiation tubes as a result. For users with multiple equipment, appropriate differentiation measures can be adopted, and different heating element combinations of different specifications can be used for different equipment, which can also reduce the overall cost.

Maintenance of the Heating System

The maintenance of the heating system is an essential part of the annual equipment maintenance work. In order to make the heating system of the equipment work normally and reliably, the daily spot inspection and annual maintenance should be done well.

Annual maintenance and after-sales service of IPSEN atmosphere batch type furnace

Picture:It is very important to do a good job in the daily inspection and annual maintenance of the equipment

Daily spot check:

  • Pay close attention to the power supply current. Under normal conditions, the three-phase current of the electrical heater should be equal, and the deviation should not be greater than 3%. If greater deviation is spotted, then reasons should be investigated and fixed.

Annual Maintenance

The annual maintenance of the equipment is also important. It is recommended to follow the instructions provided by us Annual maintenance specification. Do the maintenance work one by one and make records. The maintenance of the heating system includes:

  • Inspection of temperature monitoring system

    The temperature monitoring system is an important sensing system to ensure the normal operation of the heating system. If the monitoring system fails, it often leads to product quality problems or heating system failure. The function of thermocouple and over temperature monitoring instrument should be checked every year according to the requirements of the article.

  • Connection reliability check

    Check the connection reliability of the heater connection strips, and check the tightness of the fastening bolts of the electrical components in the electric control cabinet

  • Inspection of heater and radiant tube

    Clean the oxide scale inside the radiation tube and check its deformation degree. Focus on the extent of heater elongation and radiant tube bending; since the heater will become longer after use, measure the length of heater and radiant tube to ensure that there is at least 50 mm gap between the bottom of heater and the bottom of radiation tube.

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