Download latest developments in indirectly heated electric resistance

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
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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
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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
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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
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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.
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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
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