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AU J.T. 9(2): 83-88 (Oct. 2005) Design Analysis of an Electric Induction Furnace for Melting Aluminum Scrap K. C. Bala Mechanical Engineering Department, Federal University of Technology Minna, Niger State, Nigeria Abstract The advancement of any nation technologically has been influenced and elevated by the extent to which it can usefully harness and convert its mineral resources. The productions of metal in foundries and in all human lives have also become a general practice. Different melting techniques are in practice with different energy sources. The cleanliness and availability of electrical energy sources in Nigeria is of paramount importance to its use in foundries, hence the need for this design. This paper deals principally with the mechanical and electrical requirements for induction furnace production. The mechanical aspect gives consideration to the geometrical components, cooling system, and the tilting mechanism. The electrical aspect deals with the furnace power requirement to make it functional. The design was achieved through consideration of relevant theories and their practical application. Keywords: Electrical, Mechanical, Induction, Furnace, Aluminum, Heat energy, Charge, Melting Introduction display a marked decrease in performance level after some years of service and have to be In the production of mineral resources, discarded. the melting of metals has become one of the The re-melting of these scraps product of tremendous industrial practices in the forefront. aluminum will go a long way to enhance the This is because metals are versatile elements availability of the product without over reliance whose fields of application are very wide in on the foreign market, and thereby improving human lives. the foreign reserve. Similarly, the acquisition Of all metals, iron production has of melting equipment for this purpose has also developed substantially, such that different become a very difficult thing such that there is types of furnaces ranging from blast furnaces, a need to look inward for fabrication of some open-hearth furnaces, to converters and electric vital components for our technological growth. furnaces for steel production are in use today It is in view of this, that different methods of worldwide. Here in Nigeria, Ajaokuta Steel melting aluminum are being used in the Company and Delta Steel Company are country, such as crucible furnaces, either on examples of steel making companies that use industrial or local small scale, by burning of these types of furnaces. fossil or organic fuels. These have the Aluminum being the most abundant disadvantage of producing low quality products metallic element, forming about 8% of the as a result of the impurities present in the fuel. solid portion of the earth’s crust, is rarely In recognition of these facts, and available as rich ores. Hence most countries are considering the availability of electricity - a dependent on supplies of it being imported. cleaner source of power in Nigeria, the design Nigeria, for instance, uses aluminum in all of an Electric Induction Furnace for Aluminum aspects of human endeavor (Abubakre 2001), scrap melting and indeed any metal is in the be it transportation, machine components, right direction worth undertaking (Mastrukov cooking utensils alloying etc. these components 1986). 83 AU J.T. 9(2): 83-88 (Oct. 2005) A furnace is an apparatus in which heat is Basic Components liberated and transferred directly to solid or fluid charge mass, for the purpose of effecting The induction furnace consists basically a physical or chemical change, through cycle of a crucible, inductor coil, and shell, cooling involving temperature in excess of 400°C. system and tilting mechanism. There exist various classifications of furnaces The crucible is formed from refractory based on the purpose and energy source. material, which the furnace coils is lined with. In the early nineteenth century, the This crucible holds the charge material and phenomenon of induction heating was applied subsequently the melt. The choice of refractory to the experimental melting of metals. The material depends on the type of charge, i.e. early furnace consisted of circular hearth or acidic, basic or neutral. In this design a neutral trough, which contained the molten metal of an refractory is use and based on effectiveness, annular ring. This formed a short circuited availability and practical application in single turn secondary winding of a transformer Nigerian foundries, zirconium oxide (ZrO ) is which was energized by a supply of alternating 2 current at normal line frequency. This design implored. The durability of the crucible has inherent defects, such as mechanical force depends on the grain size, ramming technique, set up by the current flowing in the molten charge analysis and rate of heating and cooling metal which tended to cause contraction and the furnace. could result in the interruption of the current, The inductor coil is a tubular copper coil thereby posing operational difficulties. This with specific number of turns. An alternating effect was called ‘pinch effect’ (Shrets et al. current (A.C) passes through it and magnetic 1987), and a lot of attempts to solve it were not flux is generated within the conductor. The successful until in the early 1900’s, when Ajax magnetic flux generated induces eddy currents Wyatt removed the difficulty by placing the that enable the heating and subsequently the secondary channel in the vertical plane. The melting process in the crucible. In order to weight of the metal in the bath was then eliminate electrical breakdown the turns are sufficient to overcome the forces, which caused insulated by wrapping with mica tape, this the pinch effect. serve as a good insulator. It was later that a new approach was The shell is the outer part of the furnace. made by Dr. E. F. Northrup, who substituted a This houses the crucible and the inductor coils, crucible containing the metal charge in place of and has higher thermal capacity. It is made of the channel (Hammond 1978) surrounded with rectangular parallelepiped with low carbon a multi-turn coil through which current was steel plate and joined at the corners by edge passed at high frequency. The development of carriers from angular pieces and strips of non- these types of furnaces, the core-type and the magnetic metal. core-less type, the former for brass and the The cooling system is a through-one- latter for steel were extremely rapid, and many way- flow system with the tubular copper coils hundreds of thousands of kilowatts of capacity connected to water source through flexible are installed throughout the world today. rubber hoses. The inlet is from the top while The poor development of foundries in the outlet is at the bottom. The cooling process Nigeria today reported in (Bala 1998) extends is important because the circuit of the furnace to the fact that science and engineering infra- appears resistive, and the real power is not only structure was not provided at the beginning of consumed in the charged material but also in its national independence. However, today the resistance of the coil. This coil loss as well there is a good thrust to foundry technology as the loss of heat conducted from the charge and the trend of induction furnace application through the refractory crucible requires the coil is just in its prime age. Its application is mostly to be cooled with water as the cooling medium in smaller foundries for iron melting. to prevent undue temperature rise of the copper coils. Tilting of the furnace is to effect pouring of the melt as a last operational activity before 84 AU J.T. 9(2): 83-88 (Oct. 2005) casting. Since this furnace is of small capacity, b = thickness of bottom refractory t a manually operated tilting mechanism is lining, (b = 25.5mm for 10kg capacity). t adopted. The furnace is hinged on at the spout edge with a shaft and bearings. At one side to The slag height is calculated thus: the bearing is pinion and gear system to give a 4 h = Vs ..............................................7 gear reduction, so that when the handle is s πd2 turned clockwise, the furnace is tilted to m where, V = volume of slag in one heat, taken achieve a maximum angle of 90 degrees for s 3 complete pouring of the molten metal. as 8% of total charge, m . Design Analysis Height of inductor holding poles: Hp = Hin + 2Tf .....................................8 where, T = flange thickness, taken as 3mm. Geometrical parameters f The analysis is based on a 10kg capacity. Heat Energy and Electrical Parameters The shape of the crucible is cylindrical. The internal diameter of the crucible and the height The required theoretical heat energy of melt is determined by the furnace capacity (Ilori 1991), consumed during the first period (melt volume), with considerations that the of melt is given by: ratio: Qth =Qm +Qsh +Qs +Qen −Qex ............9 Hm =(1.6−2.0)........................... 1 where, Q = amount of heat energy to D m c melt 10kg of charge material, J; where H = height of molten metal, m; m Q = amount of heat energy to D = diameter of crucible, m; sh c superheat the melt to temperature of superheat, J; Volume of metal charge is given by: Q = heat required to melt slag πd2H s V = m m ............................ 2 forming materials, J; m 4 Q = energy required for en where d = diameter of molten metal = D . endothermic process, J; m c Q = amount of heat energy ex The thickness of the refractory lining liberated to the surroundings as a result (Voskoboinikov, et al. 1985), of the crucible in of exothermic reactions, J. the middle of the crucible can determine from the relation Theoretically Q = Q . en ex B =0.084 T ........................................ 3 Where r Q =Q +Q +Q ........................... 10 where T = furnace capacity in tonnes. th m sh s The internal diameter of the inductor can and, be calculated from the equation: Qm = MC(θ1 −θ0)+ Lpt ......................11 Din = Dc + 2(Br + Bins) ...........................4 where BB = thickness of refractory lining, m; where, M = mass of charge, kg; r C = specific heat capacity of charge B = thickness of insulation layer. ins material, (for aluminum, C = 1100J/kg K); (B is such that 5 ≤ B ≤ 6 [mm]). ins ins L = amount of heat to accomplish phase Height of inductor coil is given by: pt transformation, (for pure aluminum L = 0, no Hin =(1.1−1.2)Hm ..................................5 pt The height of furnace from bottom of the bath phase transformation); to the pouring spout is: θ1 = melting temperature of charge, (for H =H +h +b ..................................6 aluminum θ1 = 660°C); f m s t θ0 = ambient temperature, 25°C; where, h = height of slag formed, m; s 85 AU J.T. 9(2): 83-88 (Oct. 2005) Similarly, L = H = length of coil in metres, m; in μ = permeability of free space = 4 π x Q =MC θ .................................12 sh m sh o -7 -1 where, C = average heat capacity of molten 10 Hm ; m μ = relative permeability of charge Aluminum, (= 992J/kg K); r material, (for non-magnetic material μ = 1). θsh = amount of superheat temperature, r taken as 40°C. Therefore, and, N = BmaxL ........................................... 19 Qs = KsGs .......................................13 μoI where, K = quantity of slag formed in (kg), s taken as 8% of furnace capacity; The resistance of the copper coil inductor G = heat energy for slag = 18kJ/kg. at ambient temperature is given by: s R =ρcl .......................................... 20 Total heat energy induced (Hammond, θo A 1978), in charge due to eddy current is given t by: π3 f 2H B2 d4 where, ρ = resistivity of copper Q = m max m .................. 14 c -8 ec 8ρ =1.72 x 10 Ωm at 25°C; where, f = frequency of power supply, l = total length of copper tube, m; = πD N 50Hz; in B = maximum flux density, H; max Resistance at any temperature θ is given ρ = resistivity of charge metal, ( for as: -8 aluminum, ρ = 2.83 x 10 Ωm). R =R [1+αθ (θ −θ )] ..................21 Therefore, θ θo o o 8ρQ B = ec ....................15 max π3f 2d4H where, αθo = temperature coefficient of copper m m at 25°C; Also -3 -1 Q = 3.9 x 10 K . Q = th ........................................ 16 ec t Coil loss due to resistance is: where, t = time in seconds to attain P =I2R ...........................................22 maximum flux. c θ The allowable current density in the inductor is given by: Heat loss through conduction (Shrets et I al. 1987), from furnace walls to copper coil: J = ............................................... 17 H ( ) A π m θ2 −θ t 2 QL = 1 1 d 1 D 1 d (J ranges from 20 to 40A/mm ). [ ln 2 + ln in + ln 3 ] where, I = current in inductor in amperes, A; 2 λzi Dc λas d2 λcu Din A = cross sectional area of conducting t 2 ....................................... 23 tube (mm ), take external diameter of inductor coil, d = 8mm and internal t2 where, λ = thermal conductivity, with diameter of inductor coil, d = 6mm. t1 subscripts for zircon, asbestos, and copper respectively; The number of turns of the inductor can λ = 2.093w/m K; λ = 0.117w/ m K; be determined from: zi as and λcu = 380w/m K; μ μ NI d = outer diameter of crucible = D + B = r o ..................................... 18 2 c max 2B, m; L r d = inductor diameter surrounding where, N = number of turns of inductor coil; 3 I = current in coil in amperes, A; crucible + 2 thickness of coil, m; 86
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