ELECTRIC ARC FURNACE AC (PART 1) - Layout & Components (steel - steelmaking - electric arc furnace)

1
ELECTRIC ARC
FURNACE AC (PT 1)
Layout & Components

INTERMEDIATE LEVEL
Author: Eng. Matteo Sporchia

Copyright – P4S – PRODUCTS 4 STEELMAKING
All Rights Reserved

2 Copyright – P4S – PRODUCTS 4 STEELMAKING – All Rights Reserved – Author: Eng. Matteo Sporchia
ELECTRIC ARC FURNACE AC (PT 1) – Layout & Components

SUMMARY

INTRODUCTION ………………………………………………………………………………….. PAG. 3

FURNACE STRUCTURE ………………………………………………………………..………. PAG. 5

FURNACE MOVEMENTS ………………………..………………………………………….. PAG. 10

PRIMARY ELECTRICAL SYSTEM ………………………………………………………….. PAG. 11

SECONDARY ELECTRICAL SYSTEM ………………………………………………..…... PAG. 14

AUXILIARY SYSTEM …………………………………………………………………….…….. PAG. 19

WATER COOLING SYSTEM ………………………………………………………………... PAG. 24


INTRODUCTION

In this article we're going to make a detailed description of the design features of an
AC (alternative current) Electric Arc Furnace (EAF). The layout of an EAF can be
divided into the following parts:
1. FURNACE STRUCTURE
2. FURNACE MOVEMENTS
3. PRIMARY ELECTRICAL SYSTEM
4. SECONDARY ELECTRICAL SYSTEM
5. AUXILIARY SYSTEM
6. WATER COOLING SYSTEM


1. FURNACE STRUCTURE

1.1 LOWER BOTTOM SHELL
A typical EAF has a cylindrical shape. It's composed by the lowest part, the bottom,
that has a spherical shape like a dish. It's connected to a cradle arm which has a
curved segment with geared teeth that sits on a rocker rail. This system is needed in
order to tilt the furnace for tapping (spout or EBT system) at one side and for
removing slag from the other side (slag door). The bottom is covered by a refractory
lining which generally is composed by a safety lining (normally bricks) and working
lining at the top (normally ramming mass). In the EBT EAF the furnace bottom


contains the tapping mechanism (eccentric bottom) and sometimes also a gas
stirring elements (well blocks purging plugs).
1.2 UPPER SIDEWALL SHELL
The upper part is composed by a cylindrical shell. In most of the EAF this part can be
easily removed from the bottom. Once the shell is removed, the furnace bottom can
also be changed out. The upper sidewall shell is also covered by a refractory lining
(bricks).


1.3 SIDEWALL WATER COOLED PANELS

The furnace sidewall above the slag line usually consists of water cooled panels which
are required to withstand high thermal and mechanical loads.


1.4 ROOF
The upper part, called roof, of the furnace is a flattened sphere. The roof consists of a
water cooled roof ring which forms the outer perimeter. This is a kind of cage where
water cooled panels are inserted and form a cylindrical opening at the center. The
refractory delta section is inserted to fill this opening. This delta section has
minimum opening around the electrodes without risk of arcing between the
electrodes and the water cooled panels. Generally, roof and electrode supports can
be moved together or independently.


1.5 OPENING AREAS
Delta refractory roof is equipped with three opening areas where the graphite electrode
columns can go inside the furnace. Another hole (called "fourth hole") is used for the
extraction of the furnace fumes (suction system). An additional hole (called "fifth hole")
may be provided for other operations such as the continuous feeding of DRI and HBI or
injections of coal and lime. The sidewall of the furnace is equipped with a slag door for
slag removal operations and also for additional operations such as the using of manual
oxygen lances. In the side tapping EAF a tapping spout is installed at the sidewall of the
furnace while in the bottom tapping EAF an EBT system (tap hole) is installed at the
bottom part. Other several ports could be installed around the furnace shell to be
equipped with burners. Opening areas may also be provided low in the furnace sidewall
for injection of oxygen, lime or carbon.


2. FURNACE MOVEMENTS
To arrange all the EAF operations several movements are needed as follows:
Ø ROOF RAISE/ROTATION to allow the basket to charge the scrap into the furnace;
Ø ELECTRODES RAISING/LOWERING/SWINGING to allow the scrap charging and
manage the arc regulation;
Ø ELECTRODES CLAMP OPENING/CLOSING to add/remove the electrode columns;
Ø FURNACE TILTING for tapping and slag removal. For traditional spout system the
furnace can move with a tilt angle of about 40 deg. to fully tap the furnace. For EBT
system furnace, the maximum tilt angle is usually about 20 deg.;
Ø MOVING OF AUXILIARY SYSTEMS such as the burner lances;
Normally furnace movements are made using a central hydraulic system which
provides motive power.


3. PRIMARY ELECTRICAL SYSTEM
3.1 STEP-DOWN TRANSFORMER

The EAF electrical system is generally
composed by a primary system consisting
of a step-down transformer which
receives power from the national grid
system for feeding the power after
stepping down to the EAF transformer.
This first transformer steps the voltage
down from the high voltage line to
a medium voltage level (usually 33 kV).


The main breaker at the primary system isolates the electrical systems
at the EAF from the grid. On the secondary side of the primary electrical system,
a vacuum switch and motorized disconnect switch are usually provided to isolate the
secondary furnace transformer from the primary power supply.
3.2 EAF TRANSFORMER
A heavy duty EAF transformer receives the primary low current, high voltage power
and transforms this to a high current, low voltage power to be used in the EAF. The
mentioned transformer is designed in order to allow operation of the arcs at the
desired range of arc voltages and currents. During EAF operation, there are varying
needs of arc voltage/current configuration through and for this reason it's necessary


to have the correct choice of the
secondary voltages. For this purpose
the transformer is equipped with
a tap-changer that allows a choice of
different combinations of voltages
and currents through the different
stages of the heat. This is done by
changing the number of turns of
primary coil.


4. SECONDARY ELECTRICAL SYSTEM
4.1 DELTA CLOSURE
Secondary circuit of the EAF transformer terminates at low voltage bushings, which
are attached to the delta closure, which consists of a series of copper plates, pipes or
both. These are used so that the secondary windings of the transformer are joined to
form a closed circuit. Most of this equipment is located within the transformer room
to assure a secure and clean environment. The delta closure protrudes through the
wall of the room adjacent to the EAF and connectors are provided to attach to one
end of the furnace power cables; the other end being attached to either the current
conducting arms or the bus tubes.


4.2 FLEXIBLE POWER CABLES
The flexible cables are generally water
cooled cables, generally made by
copper ropes (parallel connected).
They're used to establish electrical
continuity between fixed blocks and
electrodes holding arms. These cables
must be flexible to permit movement of
the electrode arms up and down and to
allow swinging the electrode arms and
roof when charging of the furnace


4.3 CURRENT CONDUCTING ARM / BUS TUBES
Bus tubes provide the electrical connection between the power cables and the
electrode holder (contact pads) and usually consists of water cooled copper or
aluminum pipes, placed on the electrodes supporting arms. Many modern furnaces
utilize current conducting arms in which the arm itself transmits electricity to the
electrode holder and contact pad. Current conducting arms are usually fabricated
from copper clad steel or aluminum alloys.


4.4 ELECTRODE HOLDERS & CONTACT PADS
Electrode holders and contact pads provide the final connection between the bus
tubes or conducting arms and the graphite electrode columns. They are exposed to
extreme mechanical and thermal conditions and for this reason are considered as the
weakest part in the secondary circuit. Electrode holders are either cast or fabricated
from copper plates. Contact pads are water cooled plates made of copper, which are
pressed against the electrodes lateral surface by the clamping rings. Any dirt build up
in this area causes resistance to current flow and over-heating and damage to the
electrode holder/contact pad.


4.5 GRAPHITE ELECTRODES

The graphite electrodes deliver power
to furnace in the form of an electric arc
between the electrode and the furnace
charge. The arc itself is a plasma of hot,
ionic gasses with temperature about
3000 °C. The electrode/arm/mast/cable
assembly is quite heavy and is moved
vertically for control purposes generally
by a hydraulic cylinder incorporated in
the mast.


Since the arc length is dependent, amongst other things, on the ever
changing level of scrap or liquid under the electrode, it is necessary to have an
automatic control over electrode position known as the regulation system.
5. AUXILIARY SYSTEM
There're different auxiliary systems that are adopted to guarantee the operations and
performance of the EAF.
Here below are described the most common auxiliary systems:


5.1 OXY-FUEL BURNERS
Oxy-fuel burners are now commonly used into the EAF operations as they provide
an additional source of chemical energy needed to ensure a rapid melting of the
scrap (especially in the cold spot areas) guaranteeing a uniform melting of the
charging. Some burners are mounted directly in the water cooled panel while others
are mounted in a copper block.
5.2 OXYGEN LANCES
In the recent EAFs automatic lances and non consumable, water cooled lance for
injecting oxygen into the steel are adopted. Many of these lances also have the
capability to inject carbon as well.


5.3 CARBON INJECTORS
Carbon injectors are commonly used in the modern EAFs as carbon is needed for the
slag foaming practice.


5.4 ELECTRODES WATER COOLING SYSTEM
Electrodes water spray cooling system is adopted to reduce the electrodes lateral
oxidation. The rings spray the water just below the electrode clamps and the water
comes down the electrodes lateral surface, cooling them.

5.5 FUME EXTRACTION SYSTEM
A suction fume systems is installed to reduce pollution and gases around the furnace
guaranteeing the correct visibility for the operators. Modern EAFs shops now use the
fourth hole for direct furnace shell evacuation system (DES). DES systems consist of
water cooled duct, spray cooling, dry duct, and usually a dedicated booster fan.


5.6 TEMPERSTURE SAMPLING
A simple thermocouple is used for temperature tracking throughout the
heat. Expendable probes are also used for tracking bath carbon content and dissolved
oxygen levels in the steel.

6. WATER COOLING SYSTEM
The water cooling system is an essential part in the EAF structure. There are different
cooling systems such as transformer cooling, delta closure cooling, bus tube cooling
and electrode holder cooling. These systems usually consist of a closed loop circuit,
which conducts water through these sensitive pieces of equipment. The water in the


closed loop circuit passes through a heat exchanger to remove heat.
There are other water cooled systems, such as furnace side panels, roof panels, off
gas system and so on. Sensitive pieces of equipment normally have instrumentation
installed for measuring and monitoring the cooling water flow rate and temperature.
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#metallurgy #industrialengineering #furnace

ELECTRIC ARC FURNACE AC (PART 1) - Layout & Components (steel - steelmaking - electric arc furnace)

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