Filetype PDF | Posted on 30 Jan 2023 | 2 years ago
Authentication
digital resources
143x Filetype PDF File size 0.06 MB Source: staff.futminna.edu.ng
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
no reviews yet
Please Login to review.
