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LATEST DEVELOPMENTS IN INDIRECTLY HEATED ELECTRIC RESISTANCE FURNACE DESIGN SivadasAniyan T.S.1 and Pramod V.R. 2 1 Mechanical Engineering, Karpagam University Mechanical Engineering, NSS College of Engineering, Palakkad 2 Abstract-Heat treatment is a manufacturing process to control the mechanical properties of metallic components. The thermal history of each part and the temperature distribution in the whole load directly determine the final quality of parts. The thermal exposure a component under gone depends on the design of the furnace load, location of the component with in the furnace, furnace configuration, thermal schedule and control energy. Key words- Heat treatment, Control System, Refractory, Furnace configuration, heating coil. I. INTRODUCTION Process heating is essential in the Manufacture of consumer and industrial products. Efficient systems, manufacture a product at the required quality level at the lowest cost. Efficiency of indirectly heated furnace can reach close to 100% and temperature can exceed 19820 C. In indirect heating method,one of those three methods to heat an element, which transfers the heat to work piece by either conduction, convection, radiation or a combination of these three, With controllability, rapid heat up qualities resistance heating is used in many applications from melting metals to heating food products. Furnaces commonly used in heating are classified in two broad categories, batch furnaces and continues furnaces. Electrical furnaces are commonly found in all temperature ranges from low temperature (150-6750C) and higher temperature (675-9550C).In high temperature furnaces heat transfer is mainly by radiation and low temperature are convective furnaces. The basic objectives of furnace designs are 1. Obtain satisfactory product 2. Use minimum energy to achieve the product 3. Construct the furnace for lowest capital cost 4. Operate with the lowest possible manning levels. 5. Achieve a satisfactory long life with low maintenance cost. Control system is an important part of the heat treatment furnace. Accurate and repeatable temperature control is at the heart of most heat treatment process. It is important that special measures are taken to control the process to the metallurgical requirements. II. INNOVATIONS IN FURNACES Electric Resistance heating In Electric heating good, clean and easily controllable source of heat is used. The work can be heated indirectly by exposing it to an electrically heated element. The heat transfer is taking place by conduction and radiation. 2.1 Materials for heaters 1. MetallicAlloys Fe-Cr-Al or Ni-Cr – Fe alloys can be used in the range 1000-14000C 2. Refractory materials Tungsten, Molybdenum and Tantalum can be used from 1500-20000C. 3. Noble metals and alloys Platinum or Platinum Rhodium alloys used in the range 120018000C @IJMTER-2016, All rights Reserved 251 International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 03, Issue 06, [June– 2016] ISSN (Online):2349–9745; ISSN (Print):2393-8161 4. Non-metallic elements. Elements made from silicon carbide or molybdenum disilicide are useful in the 1200-17500C 5. For even higher temperature above 22000C graphite is useful. Heating elements require support which must no conducting and capable of withstanding maximum temperature without any reaction with the element. These are usually mullite Al 2O3SiO2or pure alumina shape. Commonly available refractory supports can be used up to 18000C 2.2 Material Selection The selection of open or indirect elements is a choice also determine by the need to protect against mechanical damage from parts being heated from accumulation of metallic scale or from broken refractories. In furnaces were bottom heat is mandatory and scale can be formed readily on the parts and electric elements should be protected in radiant tubes below the hearth. Watt loadings for these classes of elements Metallic – 8 to 12 – W/cm2 SiC – 10 to 15 – W/cm2 MoSi2 20-30 W/cm2 2.3 Refractory Fire clay refractories Silica SiO2and Alumina Al2O3 TridyniteSiO2870 – 14700C Crystodalite – Silica mineral 1470 – 17230C Almost all the above grades are also available as insulating bricks. These have similar properties but due to higher porosity, they have low thermal conductivity and low strength. High Alumina Refractories (48-99% Alumina) – 18500C In industrial heating where high temperatures are as high as 1500-18000C, high alumina refractories are the only choice. Silica Refractories (94-98% Silica), they are useful up to17000C . Carbon and Graphite refractories contain 98% or more carbon. These refractories can be used where there is no direct access to oxygen, Up to 20000C. Silicon carbide Sic and Carborandum Major special refractories and is made by fusing Silica and Coke at 20000C. Temperature range (1800-20000C) Zircon Refractories – Zirconium Silicate Zirconia Refractories – pure Zirconium Oxide (ZrO2) melting point 26800C 2.3.1 Refractory Fiber or filaments A synthetic or mixed composition of the refractory is melted at a high temperature. The melt is then poured in the form of stream. Air is blown into stream so that it breaks the stream in to fibres which solidify. By using textile manufacturing technique, the fibres are converted in to cloth and blankets of various thickness and length. By adding the addition of resins and other bonding materials the fibres are converted to rigid bonds. Properties of refractories Room temperature properties Chemical composition mineralogical constituents Bulk density or porosity Aberration resistance Cold crushing strength @IJMTER-2016, All rights Reserved 252 International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 03, Issue 06, [June– 2016] ISSN (Online):2349–9745; ISSN (Print):2393-8161 High temperature properties Refractoriness Dimensional stability Thermal expansion and creep under load Thermal conductivity Thermal spalling Specific heat Selection of refractories Type of furnace Maximum temperature Size and shape of the furnace Thermal gradient across walls and general permitted loss to atmosphere Operation cycle – continous or batch type Heat stored in the walls and structure Total heat input, type of fuel, impact of flakes on the walls, placement of electrical heaters Load exerted by the charge 2.3.2 Ceramic fiber lining It has replaced conventional brick work in many batches and continous heat treatment furnaces. Ceramic linings reduce heat loss by conduction through the wall and decrease furnace heat up time because of their law thermal conductivity and law heat storage mass. Their low mass, reduce heat capacity and good insulation properties are ideal for intermittent furnace operation. Ceramic fibre linings are Ceramic fiber blanketing, Ceramic fiber veneer or tiles and sprayed on ceramic fibre. Each of these products forms is based on the use of Alumina & Silica fibres containing a small amount of reducible metal oxide. 2.3.3 Insulating refractories and materials The heat loss between hot and cold face can be reduced by using a material with low thermal conductivity for furnace construction, are known as insulating materials. Porous fibre clay bricks are good insulators and are reasonably strong up to about 10000C. Ceramic and mineral materials are also used as insulating materials. Walls or roof can be constructed from a number of layers of different materials suitable for temperature range of that layer this is called a composite wall. 2.4 Metals and alloys for high temperature applications Several furnace parts such as belts, conveyors, buckets, containers, boxes, gas type muffles etc. require strength, ductility and easy formability. These properties are available only in metals and alloys. Mechanical properties of metals at high temperatures. 1. Creep 2. Ultimate tensile strength at high temperature 3. Oxidation and Corrosion 4. Corrosion by other gases. 2.4.1 Selection of Metals or high temperature application 1. Aluminium - up 3000C 2. Plain Carbon steel – 400 to 5000C 3. Low alloy steel containing 1 to 3% Cr, small amount of molybdenum and Vanadium 600-7000C 4. Nickel and Nickel Chromium alloys – 600 to 11000C 5. Beyond 12000C, the only materials available are refractory material – Tungsten, Molybdenum and tantalum. The maximum useful temperature when properly protected are Tungsten – 25600C, Molybdenum 19000C and Tantalum – 24000C @IJMTER-2016, All rights Reserved 253 International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 03, Issue 06, [June– 2016] ISSN (Online):2349–9745; ISSN (Print):2393-8161 III. LATEST DEVELOPMENTS 3.1 Furnace Design Placement of heatingcoils Usually designed length of the heating coil is quiet large and poses - accommodation problem with in the space available in the muffle. The majority of the furnaces the heating elements are located in vertical walls. The best way to obtain good heat transfer is to freely hang the wire or strip on the wall. To make maximum use of available accommodation area, heating elements are often design for series or parallel combination. This enables applications to be operated on high or low heat modes. 3.1.1 Design of Metallic elements Power radiated per unit area W=P/A W/cm2 Some heat will directly strike the lining; some of this will be reflected by it to the work. This is the second useful component and will depend on lining geometry and surface properties. The lining will absorb some heat, which will rise the lining temperature and more heat will be lost through the lining and insulation and to the surrounding. The power radiated by unit area of heater to the object is ( ) Once the power is known, it is necessary to decide the heater size (e.g.: Wire diameter and length). This heater has to withstand the surface loading (W/cm2) the melting temperature limit the maximum working temperature. The working temperature is 100-2000C less than the melting point. Energy efficiency is also affected by wall losses, opening loss and conveyor system loss. Once the furnace reaches a study state operating condition, the furnace loss from the furnace walls opening and conveyor system remains constant provided that the operating conditions are unchanged. IV. FUTURE TRENDS 4.1 Control system for heat treatment process Temperature control loop is an integral part of heat treating operations which generally consists of separate functions. 4.1.1 Furness Sequence Control Work piece transfer Furnace door action Furnace hearth and hood movement Furnace transfer mechanism Quench transfer Gas / Fan quench control Heat shelf door action Cooling water control 4.1.2 Temperature control and thermal profiling Accurate and repeatable temperature control is at the heart of most heat treatment process. It is important that special measures are taken to control the processes to the metallurgical requirements of the component. This requirements accurate control for both the programming set points and study state temperature control over a wide range of temperature set points and furnace loading. 4.1.3 Furnace testing and audit To achieve maximum Furnace loading and equipment utilization, temperature in a defined work load region must be within a given tolerance limit. Uniformity surveys are often carried out to determine the compliance. In many processes where there is considerable delay between process and @IJMTER-2016, All rights Reserved 254 International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 03, Issue 06, [June– 2016] ISSN (Online):2349–9745; ISSN (Print):2393-8161 Furnace, it may be required to control the Furnace from work piece sensors. Having strategies for cascade or over ride control and being able to implement effective routines for generalised soak time and hold back on thermal profile is a necessary requirement of a control system design in these applications. 4.1.4 Gas atmosphere control Many of the treatment and surface chemistry processes relay on being performed in a gaseous atmosphere. In surface treatments it is necessary to control the flow of enrichment gas or dilution air in to the Furnace against a defined set point for a defined period. Enrichment gasses are used to provide gaseous atmosphere for processes such as carburizing and nitriding. In these cases, feedback control is used and special algorithms are required to convert the output from Zirconia Probes in to application specific function. 4.1.5 Vacuum atmosphere control VacuumFinesses are widely used in heat treatment, particularly in Aerospace and automotive industry. Chambers operate below 10-9 Millibars. Almost all conventional heat treatment cycles carried out in vacuum. 4.1.6 Quenching System Many of the phase structure changes that occur in the heat treatment of alloys do so during quenching. Control systems are needed to accommodate the rapid transition from temperature control to quenching, since the rate of change of temperature during cooling greatly determine the micro grain structure of the component. 4.1.7 Data Management Temperature instrumentation and control system include temperature system, controllers, and final control elements for controlling energy flow, measurement instruments and set point programmers. Programmable logic controllers have been used to hand in material movement, analyse variables have been controlled by single loop controllers and set point programmers processes supervision or data management has been done by strip chart recorders or data loggers. This separation of function requires communication among the logic controllers, loop controllers and data acquisition equipment. Finally supervision computer to handle such factors such as scheduling, parts purchasing and statistical process control. 4.1.8 Power control and energy management solutions Reducing energy cost remains a key area of focus. The Continues use of phase angle (Cycle chopping) simple heaters including modern Silicon Carbide causesdisadvantages to uses through poor power factor, harmonic disturbance on the supply and RF interference around the installation. 4.1.9 Algorithms and routines to minimise the furnace energy use A recent advance in online power monitoring for batch type heat treatment cycle has produce a furnace power optimisation solution.In this application the set point at which the temperature with in the stock has become homogeneous is recognise by study state control algorithms acting on the derivative of power consumption. Clients are using this control system intelligence to recognise the point at which equilibrium is reached to shorten the processes time and improve planned utilization. 4.1.10 Load sharing and lord shedding Where heat treatment shop has large installed base of electrically heated thermal processing equipment’s, it is often desirable to sequence the firing of individual furnace zones or independent furnace to minimise the supply fluctuation. By using the intelligent thyristor firm method, it is possible to limit the power surge and instantaneous supply loading associated with any installation through a selectable combination of firing patterns. @IJMTER-2016, All rights Reserved 255 International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 03, Issue 06, [June– 2016] ISSN (Online):2349–9745; ISSN (Print):2393-8161 4.1.11 Out of Hours Furnace set point Using the intelligence of modern control systems it is possible to automate out of hour’s set point control for thermal processing equipment. 4.1.12 Furnace energy optimization Uses the steady state algorithm to shorten process cycle times on batch type furnaces running process that transform the micro structure of the work piece. By calculating that this work piece reaches a uniform temperature, the sock period can be started with confidence at the earliest possible time. This results in the integrity of the process being maintain whilst the process time and energy consumption are reduced. 4.1.13 Use of Fans Forced gas flow can be created in enclosures by using a fan. Due to possible damage by temperature and gas attack, the motor and drive components are maintained outside and only a part of the drive shaft and propeller are inside. The fans are centrifugal, propeller type and rotated at a low speed (200-400RPM). The parts in conduct with gases and high temperature (>4000C) are made of Nickel alloys. For lover temperatures they can be made of stainless steel. The main purpose of fan is to distribute the gas evenly so as to reduce concentration gradients V. CONCLUSION Even materials with normally similar specification can vary in their in service performance owing to different source of raw materials,In the case of refractories and different heat treatment processes in the case of sophisticated high performance alloys. To achieve optimum furnace performance and long life between repairs, it is essential to fully understand the duties required from both refractories and high temperature metals. The furnace design is coupled to the design of expert process control system improve the furnace compliance for heat treatments. REFERENCE [1] ASM Handbook (Revised Vol.4) [2] Improving processes heating system performance, industrial heating equipment association Industrial and process furnaces. Peter Mullinger&Berrie Jenkins. Industrial heating. Yeshvant.V. Deshmukh. Eurotherm heat treatment catalogue @IJMTER-2016, All rights Reserved 256