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Transcript of Furnaces and refractories.ppt
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Imbaba Aviation Institute
Mechanical Power Department, 4th YEAR
Fall Semester 2010/2011
- Introduction to Combustion systems
- Definition of combustion efficiency andfactors affecting it.
- Methods of energy conservation incombustion systems.
- Control systems in combustion.
- Waste heat recovery.
- Performance control of various systems.
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Energy Management
What Is Energy Management?
The use of Engineering and Economicprinciples to CONTROL the cost of energy
to provide needed services in buildings andindustries.
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Energy Management
NEED FOR ENERGY MANAGEMENT
IMPORTANT REASONS:
1. ENVIRONMENTAL QUALITY
2. ECONOMIC COMPETITIVENESS3. REDUCE COSTSAND CREATE JOBS
4. ENERGY SECURITY
5. CORPORATE REQUIREMENT
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DEFINITIONS
ENERGY: the capacity of doing work
Thermal, Electromagnetic, Nuclear,
Mechanical, Chemical, etc.ENERGY CONSERVATION LAW
Energy is transformed from one form to
another and the total amount of energy
remains the same.
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DEFINITIONS
EFFICIENCY is the ratio of the output of asystem in relation with its input.
MOTORS a device that converts electrical
energy into mechanical energy.
GENERATOR converts mechanical energy into
electrical energy.
TRANSFORMER- Is a device that converts AC
electric energy at one voltage level to an AC electric energyat another voltage level. They are classified as step-up or
step-down transformers depending of the function theyare being used for.
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DEFINITIONS
POWER FACTOR : is the ratio of the total
power produced between the power used.
PF = COS
KVA
KW
KVAR
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DEFINITIONS
COGENERATION is the sequentialproduction of thermal and electric energyfrom a single fuel source.
Heat, that would normally be lost, is recoveredin the production of one form of energy. Theheat is then used to generate the secondform of energy.
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HOW& WHY ENERGY CONSERVATION
HOW?
Energy Audits
Fuel Switching
Electric Rate Structures
Electrical System Utilization
PF Correction
Lighting Improvements
Motors And Applications
Insulation HVAC Improvements
Waste Heat Recovery; Cogeneration, ETC.
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ENERGY AUDITS
ENERGY AUDITS
An Energy Audit (or Energy Survey) is a study of how energy isused in a facility and an analysis of what alternatives could beused to reduce energy costs.
This process starts by collecting information of the facilitysoperation and about its past record of utility bills. This data isthen analyzed to get a picture of how the facility uses ( andpossibly wastes) energy, and identify
ECOs (Energy Conservation Opportunities).
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COMBUSTION EFFICIENCY
In any closed combustion system suchas a boiler, we can measure preciselywhat occurred at the burner bycarefully measuring the exhaust.
The goal is to be able to carefully
control the fuel and airflow to ensurethe complete and efficient combustion.
We will see why excess air is importantand why too much excess air isexpensive.
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SAVINGS
% SAVINGS IN FUEL= (New Eff. Old Eff.)/New Eff.
Savings = (% Savings)(Fuel consumption)
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SAVINGS
Example 1 Last year a 20 x 106 BTU/HR boiler consumed
19000 MCF of natural gas at $4.00/MCF. The
boiler operates at 6% O2 and 700 F STR.What is the saving to correcting that to
3% O2 ?
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SAVINGS
As can be seen later
Eff 1. = 74.5% Eff. 2 = 77%
% Savings = (77 74.5)/77 = 3.2 %
$ Savings = (3.2%) [ 19,000 MCF][$ 4.00]
= $ 2,500 / YR
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Introduction to Furnaces
Introduction
Type of furnaces and refractory
materials
Assessment of furnaces
Energy efficiency opportunities
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MAIN COMPONENTS OF COMBUSTION SYSTEM
There are six components that may be important in industrialcombustion processes load itself, a combustor, heat recovery
device flow control system air pollution control system.
Schematic of an industrial combustion process.
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FURNACES
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COMBUSTION PRINCIPLES
Combustion chemistry
In practice, since combustion conditions are never ideal.
The actual quantitydepends on many factors, such as fueltype and composition, furnace design, firing rate, and thedesign and adjustment of the burners stoichiometricrequirement industrial processes.
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Theother speciesdepends on what oxidizer is used andwhat is the ratio of the fuel to oxidizer is air nearly 79% N2
by volume. If the combustion is fuel rich, If the combustionis fuel lean.
Figure 18. Stoichiometric Air
Requirements for Combustion
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Unburned hydrocarbons
Fuel was not fully combusted
Fuel properties: Heating value of the fuel either the higherheating value (HHV) lower heating value (LHV)excludes the heat ofvaporization.
The stoichiometry or mixture ratio in industry is as follows:
The mixture ratio :
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Where SP is the stoichiometric ratio for theoretically perfect
combustion . fuel-rich combustion of CH4, S2 < 9.52. For thefuel-lean combustion of CH4, S2 > 9.52. Using the above definition forthe mixture ratio,
1.0 for fuel-lean flames.
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Combustion properties
Combustion products
The oxidizer composition, mixture ratio, air and fuel preheattemperatures, and fuel composition.
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An adiabatic process means that no heat is lost during the reaction,or that the reaction occurs in a perfectly insulated chamber.
An equilibrium process means that there is an infinite amount of timefor the chemical reactions to take place.
FLAME TEMPERATURE
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FLAME TEMPERATURE
The actual flame temperature is lower than the adiabaticequilibrium flame temperature due to imperfect combustion andradiation from the flame.
A highly luminous flame generally has a lower flametemperature than a highly non-luminous flame. The actual flametemperature will also be lower when the load and the walls are
more radiatively absorptive.
The flame temperature is a critical variable in determining theheat transfer from the flame to the load.
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Oxidizer and fuel composition
The flame temperature increases significantly when air isreplaced with oxygen
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Nearly all industrial combustion applications are run at fuel-leanconditions to ensure that the CO emissions are low.
NOx emissions are also maximized since NOx increasesapproximately exponentially with gas temperature.
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1. Point A:2C + O2 2 CO + heat
2. Point B:2CO + O2 2 CO2 + heat
3. Point C CO to have reached a low level.
A small amount of oxygen
4. To achieve .complete. combustion, a small amount of air
must be added over. Point D. At this point, the CO 2
level reaches a peak (typically around 15- 16 percent foroil fuels, and 11-12 percent for natural gas).
5. Point E, oxygen level builds towards 20.9 percent.
STACK GAS COMPOSITION
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BURNER TESTING:
Operating parameters, pollutant
emissions, flame dimensions,
heat flux profile, safetylimitations, and noise data heat
release range of the burner.
Turndown is defined as the ratio
of maximum heat release to
minimum heat release:
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An operator also needs to know the point at which a burnerwill become unstable if fired below the minimum heat release
absolute minimum the combustion air settings can bedetermined through testing to ensure the efficient operation.
The emissions of pollutants such asNOx ,CO, and unburnedhydrocarbons (UHC).When firing burners on a wide varietyof fuels, flame dimensions can change, depending on the fuel
fired.
Another valuable piece of data that can be collected is noise.
API 535 gives some good guidelines for specifications anddata required for burners used in fired
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Introduction to Furnaces
Materials that
Withstand high temperatures and sudden
changes Withstand action of molten slag, glass, hot
gases etc
Withstand load at service conditions
Withstand abrasive forces Conserve heat
Have low coefficient of thermal expansion
Will not contaminate the load
What are Refractories:
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Introduction to Furnaces
Refractories
Refractory lining of a
furnace arc
Refractory walls of afurnace interior with
burner blocks
(BEE India, 2005)
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Introduction to Furnaces
Melting point Temperature at which a test pyramid (cone)
fails to support its own weight
Size Affects stability of furnace structure
Bulk density Amount of refractory material within a
volume (kg/m3)
High bulk density = high volume stability,heat capacity and resistance
Properties of Refractories
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Introduction to Furnaces
Porosity Volume of open pores as % of total refractory
volume
Low porosity = less penetration of moltenmaterial
Cold crushing strength Resistance of refractory to crushing
Creep at high temperature Deformation of refractory material under
stress at given time and temperature
Properties of Refractories
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Introduction to Furnaces
Pyrometric cones Used in ceramic industries
to test refractoriness ofrefractory bricks
Each cone is mix of oxidesthat melt at specifictemperatures
Properties of Refractories
Pyrometric Cone Equivalent (PCE) Temperature at which the refractory brick and
the cone bend
Refractory cannot be used above this temp
(BEE India, 2004)
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Introduction to Furnaces
Volume stability, expansion &
shrinkage
Permanent changes during refractory service
life
Occurs at high temperatures
Reversible thermal expansion
Phase transformations during heating andcooling
Properties of Refractories
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Introduction to Furnaces
Thermal conductivity Depends on composition and silica content
Increases with rising temperature
High thermal conductivity: Heat transfer through brickwork required
E.g. recuperators, regenerators
Low thermal conductivity: Heat conservation required (insulating
refractories)
E.g. heat treatment furnaces
Properties of Refractories
T f F d R f t i
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Type of Furnaces and Refractories
Classification of Refractories
Classification method Examples
Chemica l compos i t ion
ACID, which readily combines with bases Silica, Semisilica, Aluminosilicate
BASIC, which consists mainly of metallicoxides that resist the action of bases
Magnesite, Chrome-magnesite, Magnesite-chromite, Dolomite
NEUTRAL, which does not combine withacids nor bases
Fireclay bricks, Chrome, Pure Alumina
Special Carbon, Silicon Carbide, Zirconia
End use Blast furnace casting pit
Method o f manufacture Dry press process, fused cast, handmoulded, formed normal, fired or chemicallybonded, unformed (monolithics, plastics,ramming mass, gunning castable, spraying)
T f F d R f t i
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Type of Furnaces and Refractories
Common in industry: materials available andinexpensive
Consist of aluminium silicates
Decreasing melting point (PCE) with increasingimpurity and decreasing AL2O3
Fireclay Refractories
45 - 100% alumina High alumina % = high refractoriness
Applications: hearth and shaft of blast furnaces,ceramic kilns, cement kilns, glass tanks
High Alumina Refractories
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Type of Furnaces and Refractories
>93% SiO2 made from quality rocks
Iron & steel, glass industry
Advantages: no softening until fusion point isreached; high refractoriness; high resistance to
spalling, flux and slag, volume stability
Silica Brick
Chemically basic: >85% magnesium oxide
Properties depend on silicate bond concentration
High slag resistance, especially lime and iron
Magnesite
T f F d R f t i
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Type of Furnaces and Refractories
Chrome-magnesite
15-35% Cr2O3 and 42-50% MgO
Used for critical parts of high temp furnaces
Withstand corrosive slags
High refractories
Magnesite-chromite
>60% MgO and 8-18% Cr2O3 High temp resistance
Basic slags in steel melting
Better spalling resistance
Chromite Refractories
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Type of Furnaces and Refractories
Zirconium dioxide ZrO2
Stabilized with calcium, magnesium, etc.
High strength, low thermal conductivity, not
reactive, low thermal loss
Used in glass furnaces, insulating refractory
Zirconia Refractories
Aluminium oxide + alumina impurities
Chemically stable, strong, insoluble, highresistance in oxidizing and reducing atmosphere
Used in heat processing industry, crucible shaping
Oxide Refractories (Alumina)
T f F d R f t i
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Type of Furnaces and Refractories
Single piece casts in equipment shape
Replacing conventional refractories
Advantages Elimination of joints
Faster application
Heat savings
Better spalling resistance Volume stability
Easy to transport, handle, install
Reduced downtime for repairs
Monolithics
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Type of Furnaces and Refractories
Material with low heat conductivity:
keeps furnace surface temperature
low
Classification into five groups
Insulating bricks
Insulating castables and concrete
Ceramic fiber Calcium silicate
Ceramic coatings (high emissivity coatings)
Insulating Materials Classification
T pe of F rnaces and Refractories
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Type of Furnaces and Refractories
Consist of
Insulation materials used for making piece
refractories
Concretes contain Portland or high-aluminacement
Application
Monolithic linings of furnace sections
Bases of tunnel kiln cars in ceramics
industry
Castables and Concretes
Type of Furnaces and Refractories
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Type of Furnaces and Refractories
Thermal mass insulation materials
Manufactured by blending alumina
and silica
Bulk wool to make insulation
products
Blankets, strips, paper, ropes, wet felt etc
Produced in two temperature grades
Ceramic Fibers
Type of Furnaces and Refractories
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Type of Furnaces and Refractories
Low thermal conductivity
Light weight
Lower heat storage
Thermal shock resistant
Chemical resistance
Mechanical resilience
Low installation costs
Ease of maintenance
Ease of handling
Thermal efficiency
Ceramic Fibers
Remarkable properties and benefits
Lightweight furnace
Simple steel fabrication
work
Low down time
Increased productivity
Additional capacity
Low maintenance costs Longer service life
High thermal efficiency
Faster response
Type of Furnaces and Refractories
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Type of Furnaces and Refractories
Emissivity: ability to absorb and
radiate heat
Coatings applied to interior furnacesurface:
emissivity stays constant
Increase emissivity from 0.3 to 0.8
Uniform heating and extended refractory life Fuel reduction by up to 25-45%
High Emissivity Coatings
Type of Furnaces and Refractories
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Type of Furnaces and Refractories
High Emissivity Coatings
Assessment of Furnaces
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Assessment of Furnaces
IntroductionType of furnaces and refractory
materials
Assessment of furnaces
Energy efficiency opportunities
Assessment of Furnaces
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Assessment of Furnaces
Heat Losses Affecting Furnace Performance
FURNACE
Flueg
as
Moist
ureinfuel
Openingsinfurnace
Furna
cesurface/skin
Other
losses
Heat inputHeat in stock
Hydro
geninfuel
FURNACE
Flueg
as
Moist
ureinfuel
Openingsinfurnace
Furna
cesurface/skin
Other
losses
Heat inputHeat in stock
Hydro
geninfuel
Assessment of Furnaces
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Assessment of Furnaces
Instruments to Assess Furnace Performance
Parameters
to be measured
Location of
measurement
Instrument
required
Required
Value
Furnace soaking zonetemperature (reheatingfurnaces)
Soaking zone and side wall Pt/Pt-Rh thermocouple withindicator and recorder
1200-1300oC
Flue gas temperature In duct near the dischargeend, and entry to recuperator
Chromel AlummelThermocouple with indicator
700oC max.
Flue gas temperature After recuperator Hg in steel thermometer 300oC (max)
Furnace hearth pressure inthe heating zone
Near charging end and sidewall over the hearth
Low pressure ring gauge +0.1 mm of Wc
Oxygen in flue gas In duct near the dischargeend
Fuel efficiency monitor foroxygen and temperature
5% O2
Billet temperature Portable Infrared pyrometer or opticalpyrometer
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Assessment of Furnaces
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Assessment of Furnaces
Direct Method
Thermal efficiency of furnace
= Heat in the stock / Heat in fuel consumed
for heating the stock
Heat in the stock Q:
Q = m x Cp (t1 t2)
Calculating Furnace Performance
Q = Quantity of heat of stock in kCalm = Weight of the stock in kgCp= Mean specific heat of stock in kCal/kg oCt1 = Final temperature of stock in oCt2 = Initial temperature of the stock before it enters thefurnace in oC
Assessment of Furnaces
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Assessment of Furnaces
Direct Method - example
Heat in the stock Q =
m x Cp (t1 t2)
6000 kg X 0.12 X (1340 40) 936000 kCal
Efficiency =
(heat input / heat output) x 100
[936000 / (368 x 10000) x 100 =25.43%
Heat loss = 100% - 25% = 75%
Calculating Furnace Performance
m = Weight of thestock = 6000 kgCp= Mean specificheat of stock = 0.12kCal/kg oCt1 = Final temperatureof stock = 1340 oC
t2 = Initial temperatureof the stock = 40 oCCalorific value of oil =10000 kCal/kgFuel consumption =368 kg/hr
Assessment of Furnaces
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Assessment of Furnaces
Indirect Method
Heat lossesa) Flue gas loss = 57.29 %
b) Loss due to moisture in fuel = 1.36 %
c) Loss due to H2 in fuel = 9.13 %
d) Loss due to openings in furnace = 5.56 %
e) Loss through furnace skin = 2.64 %
Total losses = 75.98 %
Furnace efficiency = Heat supply minus total heat loss 100% 76% = 24%
Calculating Furnace Performance
Assessment of Furnaces
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Assessment of Furnaces
Typical efficiencies for industrial furnaces
Calculating Furnace Performance
Furnace type Thermal efficiencies (%)
1)Low Temperature furnaces
a. 540 980 oC (Batch type) 20-30
b. 540 980o
C (Continous type) 15-25c. Coil Anneal (Bell) radiant type 5-7
d. Strip Anneal Muffle 7-12
2) High temperature furnaces
a. Pusher, Rotary 7-15
b. Batch forge 5-10
3) Continuous Kiln
a. Hoffman 25-90
b. Tunnel 20-80
4) Ovens
a. Indirect fired ovens (20 oC370 oC) 35-40
b. Direct fired ovens (20o
C370o
C) 35-40
Energy Efficiency Opportunities
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Energy Efficiency Opportunities
IntroductionType of furnaces and refractory
materials
Assessment of furnaces
Energy efficiency opportunities
Energy Efficiency Opportunities
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Energy Efficiency Opportunities
1. Complete combustion with minimum excess air
2. Proper heat distribution
3. Operation at the optimum furnace temperature
4. Reducing heat losses from furnace openings
5. Maintaining correct amount of furnace draft
6. Optimum capacity utilization
7. Waste heat recovery from the flue gases
8. Minimize furnace skin losses
9. Use of ceramic coatings10.Selecting the right refractories
Energy Efficiency Opportunities
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Energy Efficiency Opportunities
Importance of excess air Too much: reduced flame temp, furnace
temp, heating rate Too little: unburnt in flue gases, scale losses
Indication of excess air: actual air /theoretical combustion air
Optimizing excess air Control air infiltration
Maintain pressure of combustion air
Ensure high fuel quality
Monitor excess air
1. Complete Combustion with
Minimum Excess Air
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Energy Efficiency Opportunities
When using burners
Flame should not touch or be obstructed
No intersecting flames from different burners Burner in small furnace should face upwards
but not hit roof
More burners with less capacity (not one big
burner) in large furnaces
Burner with long flame to improve uniform
heating in small furnace
2. Proper Heat Distribution
Energy Efficiency Opportunities
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Energy Efficiency Opportunities
Operating at too high temperature: heat loss,
oxidation, decarbonization, refractory stress
Automatic controls eliminate human error
3. Operate at Optimum Furnace
Temperature
Slab Reheating furnaces 1200oC
Rolling Mill furnaces 1200oC
Bar furnace for Sheet Mill 800oC
Bogie type annealing furnaces 650oC750oC
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Energy Efficiency Opportunities
Heat loss through openings
Direct radiation through openings
Combustion gases leaking through the openings
Biggest loss: air infiltration into the furnace
Energy saving measures
Keep opening small
Seal openings
Open furnace doors less frequent and shorter
4. Reduce Heat Loss from Furnace
Openings
Energy Efficiency Opportunities
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Energy Efficiency Opportunities
Negative pressure in furnace: air
infiltration
Maintain slight positive pressure
Not too high pressure difference: air
ex-filtration
Heat loss on ly abou t 1% if furnace
pressure is con tro l led proper ly !
5. Correct Amount of Furnace Draft
Energy Efficiency Opportunities
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Energy Efficiency Opportunities
Optimum load
Underloading: lower efficiency
Overloading: load not heated to right temp
Optimum load arrangement Load receives maximum radiation
Hot gases are efficiently circulated
Stock not placed in burner path, blocking flue
system, close to openings
Optimum residence time
Coordination between personnel
Planning at design and installation stage
6. Optimum Capacity Utilization
Energy Efficiency Opportunities
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Energy Efficiency Opportunities
Charge/Load pre-heating
Reduced fuel needed to heat them in furnace
Pre-heating of combustion air
Applied to compact industrial furnaces
Equipment used: recuperator, self-
recuperative burner
Up to 30% energy savings
Heat source for other processes
Install waste heat boiler to produce steam
Heating in other equipment (with care!)
7. Waste Heat Recovery from Flue Gases
Energy Efficiency Opportunities
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Energy Efficiency Opportunities
Choosing appropriate refractories
Increasing wall thickness
Installing insulation bricks (= lowerconductivity)
Planning furnace operating times
24 hrs in 3 days: 100% heat in refractorieslost
8 hrs/day for 3 days: 55% heat lost
8. Minimum Furnace Skin Loss
Energy Efficiency Opportunities
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gy y pp
High emissivity coatings
Long life at temp up to 1350 oC
Most important benefits Rapid efficient heat transfer
Uniform heating and extended refractory life
Emissivity stays constant
Energy savings: 8 20%
9. Use of Ceramic Coatings
Energy Efficiency Opportunities
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gy y pp
Selection criteria
Type of furnace
Type of metal charge Presence of slag
Area of application
Working temperatures
Extent of abrasion
and impact
10. Selecting the Right Refractory
Structural load of
furnace
Stress due to temp
gradient & fluctuations
Chemical compatibility
Heat transfer & fuelconservation
Costs