ITP Metal Casting: Advanced Melting Technologies: Energy Saving ...

scrap preheating system must be of large capacity to capture the energy from combustion, this
system is also impractical for foundry operations.
4.6.2 Twin Shell System
Preheating systems have been in use for more than a decade in electric arc furnace steel melting
shops. Twin-shell furnaces are among the more recent developments, incorporating preheating as
an integral part of the melting process. At a twin-shell operation, while one part of the furnace is
making steel, the other side is being loaded with scrap. The off-gas from the operating side of the
two-furnace system is piped to the other side to heat the charge before melting. Generally, this
technology is used with a single graphite electrode DC furnace.
The off-gas from the current heat being melted or refined is passed through the scrap for the next
heat in the second furnace shell. Typically, only the first charge is preheated. Therefore, most
plants try to operate with a single charge. Twin shell systems are cheaper than having two
separate electric arc furnaces because they require only one set of electrodes and achieve similar
production rate. Electricity costs are also lower with the twin shell system. When two scrap
charges are required, the first (preheated charge) contains approximately 60% more scrap than
the second charge. Preheating in the second shell can also be supplemented by gas burners.
Nippon Steel reported energy consumption of 260 kWh/ton steel with preheating to about 900°C
(1,650°F), achieving a 30% energy reduction from conventional melting. This technology is
applicable to larger foundries’ arc furnaces (greater than 20 ton) but would be difficult to retrofit
as it requires replacement of furnaces.
4.7 Refractories
Refractories are used throughout the various processes conducted in all kinds of furnaces in the
metal casting industry, affecting approximately 90% of the direct energy consumption in these
processes. Improvements in refractory systems, such as life extension, offer high potential to
save energy up to 8.5 trillion Btu per year.
g
t
83 Ceramic linings are used in most furnaces to
minimize reaction resulting in corrosion or mechanical damage in the furnace when processin
specific metals. Although the service life of refractory linings has been extended in the las
decade, there still exists a substantial problem where thermal efficiency may be sacrificed for a
better protective refractory lining.
A recent development in insulating materials is the use of micro-porous silica. The improved
opacifiers
block heat transfer in radiation lowering thermal conductivity. The low thermal
conductivity can increase the capacity of ladles, kilns, industrial ovens and commercial
appliances while maintaining or improving thermal performance. It can reduce weight, which
in
turn, can reduce structural requirements for furnaces and other high-temperature vessels.
The development of insulation system within refractory system also ensures proper temperature
control
in melting systems (for example, maintaining a homogeneous aluminum liquid
throughout the aluminum melter). The board form of the new insulation materials can backup
33