furnaces in power boiler
TRANSCRIPT
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4.FURNACESSyllabus
Classification
General fuel economy measures in furnaces:, excess
air, heat distribution, temperature control, draft
control,
Waste heat recovery.
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What is a Furnace?
A furnace is an equipment to melt metals forcasting or heat materials for change of
shape ( rolling, forging etc) or change of
properties (heat treatment).
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Types and class cat on o urnaces
Furnaceclassification
Recuperative
Regenerative
According
to mode ofheat transfer
According
to mode of
charging
Mode of heat
recovery
Open fire place furnace
Heated through liquid medium
Periodical
Forging
Re-rolling
(Batch / continuous
pusher)
PotContinuous
Glass tankmelting
(regenerative /
recuperative)
Based on the method of generating heat: combustion type (using fuels) and electric type
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Characteristics of an Efficient Furnace
Furnace should be designed so that in a given time, as much of
material as possible can be heated to an uniform temperature as
possible with the least possible fuel and labour.
To achieve this end, the following parameters can be considered.
Determination of the quantity of heat to be imparted to the materialor charge.
Liberation of sufficient heat within the furnace to heat the stock
and overcome all heat losses Transfer of available part of that heat from the furnace gases to the
surface of the heating stock.
Equalisation of the temperature within the stock.
Reduction of heat losses from the furnace to the minimum possible
extent.
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Furnace Energy Supply
The products offlue gases directly contact the stock,so type of fuel chosen is of importance.
For example, some materials will not tolerate sulphurin the fuel. Also use of solid fuels will generateparticulate matter, which will interfere the stock place
inside the furnace.Hence, majority of the furnaces use liquid fuel,gaseous fuel or electricity as energy input.
Melting furnaces for steel, cast iron use electricity ininduction and arc furnaces. Non-ferrous meltingutilizes oil as fuel.
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Oil Fired Furnace
Furnace oil is the major fuel used in reheating and heattreatment furnaces
LDO is used in furnaces where presence of sulphur isundesirable.(No problom with sulphur )
Furnaces operate with efficiencies as low as 7% as against
upto 90% achievable in other combustion equipment such asboiler.
This is because of the high temperature at which the furnaceshave to operate to meet the required demand. For example, a
furnace heating the stock to 1200oC will have its exhaust gasesleaving atleast at 1200oC resulting in a huge heat loss throughthe stack. However, improvements in efficiencies have been
brought about by methods such as preheating of stock,preheating of combustion air and other waste heat recoverysystems.
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Typical Furnace System
Forging Furnaces9Used for preheating billets and ingots to attain a forge
temperature.
9The furnace temperature is maintained at 1200 to 1250oC.
9 Forging furnaces, use an open fireplace system and most
of the heat is transmitted by radiation.9The typical loading in a forging furnace is 5 to 6 tones
with the furnace operating for 16 to 18 hours daily.
9The total operating cycle can be divided into (i) heat-uptime (ii) soaking time and (iii) forging time.
9Specific fuel consumption depends upon the type ofmaterial and number of reheats required.
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Rerolling Mill Furnace
Batch type furnace:Used for heating up scrap, smallingots and billets weighing 2 to
20 kg. for batch type rerolling.Charging and discharging of thematerial is done manually andthe final product is in the form ofrods, strips etc.
Operating temperature is1200oC.
Total cycle time can becategorized into heat-up time andrerolling time.
Specific fuel consumption variesfrom 180 to 280 kg. of coal /tonne of heated material.
Continuous Pusher Type:The process flow and operatingcycles of a continuous pusher
type is the same as that of thebatch furnace.
Operating temperatureis1250oC.
The material or stock recovers apart of the heat in flue gases as itmoves down the length of the
furnace.Heat absorption by the materialin the furnace is slow, steady anduniform throughout the cross-section compared with batchtype.
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Furnace efficiency Direct method:
The efficiency of furnace can be estimated by measuring the amount of fuel needed
per unit weight of material.
Thermal efficiency of the furnace = Heat in the stockHeat in the fuel consumed for heating the stock
The quantity of heat to be imparted (Q) to the stock can be found from
Q = m x Cp (t1 t2)
WhereQ = Quantity of heat of stock in Kcal
m = Weight of the stock in Kg
Cp = Mean specific heat of stock in kCal/kgoC
t1
= Final temperature of stock desired, oC
t2 = Initial temperature of the stock before it entersoC
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Indirect Method
Method: Similar to the method of evaluating boiler
efficiency by indirect method. Furnace efficiency is
calculated after subtracting sensible heat loss in flue gas,loss due to moisture in flue gas, heat loss due to
openings in furnace, heat loss through furnace skin and
other unaccounted losses Parameters : hourly furnace oil consumption, material
output, excess air quantity, temperature of flue gas,
temperature of furnace at various zones, skin
temperature and hot combustion air temperature.
Instruments - infrared thermometer, fuel efficiencymonitor, surface thermocouple.
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Thermal efficiencies for common
industrial furnaces
Furnace Type Typical thermal efficiencies(%)
1) Low Temperature furnaces
a. 540 980 oC (Batch type) 20-30
b. 540 980oC (Continous type) 15-25
c. Coil Anneal (Bell) radiant type 4-7
d. Strip Anneal Muffle 7-12
2) High temperature furnaces
a. Slot forge 5-12
b. Pusher, Roll down or Rotary 7-14
c. Batch forge 5-10
d. Car Bottom 7-12
3) Continuous Kilna. Hoffman 25-93
b. Tunnel 21-82
c. Transverse-arch Annular 26-96
4) Ovensa. Indirect fired ovens (20oC-370oC) 35-40
b. Direct fired ovens (20oC-370oC) 35-40
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Furnace Efficiency Calculation: Example
An oil-fired reheating furnace has an operating temperature ofaround 1340oC. Average fuel consumption is 400 litres/hour. The
flue gas exit temperature is 750 oC. Air is preheated from ambient
temperature of 40 oC to 190 oC through an air pre-heater. The
furnace has 460 mm thick wall (x) on the billet extraction outlet
side, which is 1 m high (D) and 1 m wide. The other data are as
given below. Find out the efficiency of the furnace by both
indirect and direct method.
Exit flue gas temperature = 750oC
Ambient temperature = 40
o
CPreheated air temperature = 190oC
Specific gravity of oil = 0.92
Average fuel oil consumption = 400 Litres / hr
= 400x0.92 =368 kg/hrCalorific value of oil = 10000 kCal/kg
Average O2percentage in flue gas = 12%
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Furnace Efficiency (Direct Method)
Furnace Efficiency (Direct M ethod)
Heat input = 400 litres / hr
= 368 kg/hr
Heat output = m x Cp x
T= 6000 kg x 0.12 x (1340 40)
= 936000 kCal
Efficiency = 936000 x 100 / (368 x 10000)= 25.43 %
= 25% (app)
Losses = 75% (app)
F Effi i (I di t M th d)
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Furnace Efficiency (Indirect Method)
1. Sensible Heat Loss in Flue Gas:
Corresponding excess air = (O2 x 100) / (21 O2)
= 133% excess air
Theoretical air required to burn 1 kg of oil = 14 kg
Total air supplied = 14 x 2.33 kg / kg of oil
= 32.62 kg / kg of oil
Sensible heat loss = m x Cp x Tm = Weight of flue gas
= 32.62 + 1.0 = 33.62 kg / kg of oil.
Cp = Specific heat
T = Temperature difference
Heat loss = 33.62 x 0.24 x (750 40)= 5729 kCal / kg of oil
% Loss = 5729 /1000 = 57.29%
% Heat gain by combustion air = 32.62 x 0.24 x (190 40)
--------------------------------x 100=1174%
10000
Net % sensible heat loss in flue gas = (57.29 11.74) %= 45.55%
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2. Loss Due to Evaporation of Moisture Present in Fuel
% Loss = M {584 + 0.45 (Tfg-Tamb)}------------------------------ x 100
GCV of Fuel=1.36%
3. Loss Due to Evaporation of Water Formed due to Hydrogen in Fuel
% Loss = 9 x H2 {584 + 0.45 (Tfg-Tamb)}
---------------------------------------- x 100
GCV of Fuel=9.13%
4. Heat Loss due to Openings: Heat loss due to openings can be calculated by
computing black body radiation at furnace temperature, and multiplying these
values with emissivity (usually 0.8 for furnace brick work), and the factor ofradiation through openings.
Use figure 4.2 for black body radiation losses
Use figure 4.3. For Factor of radiation through openings
Figure 4 2 Graph for determining black body radiation at
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Figure 4.2 Graph for determining black body radiation at
a particular temperature
Fi 4 3 F t f d t i i th i l t f h t l f
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Figure 4.3 Factor for determining the equivalent of heat release from
openings to the quality of heat release from perfect black
body
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4. Heat Loss due to Openings:
The shape of the opening is square and D/X = 1/0.46 = 2.17
The factor of radiation (Refer Figure 4.3) = 0.71
Black body radiation corresponding to 1340oC = 36.00 Kcal/cm
2/hr
(Refer Figure 4.2 On black body radiation)Area of opening = 100 cm x 100 cm
= 10000 cm2
Emissivity = 0.8
Total heat loss = 36 x 10000 x 0.71 x 0.8= 204480 Kcal/hr
= 20.45 kg/hr
% of heat loss = 20.45 /368 x 100
= 5.56 %
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5. Heat Loss through Skin:
5. Heat Loss through Skin:
a. Heat loss through roof and sidewalls:
Total average surface temperature = 122oC
Heat loss at 122 oC = 1252 Kcals / m2/ hr
Total area of heating + soaking zone = 70.18 m2
Equivalent oil loss = 1252 kCal / m2
/ hr x 70.18 m2
= 87865 kCal/hr
= 8.78 litres / hr= 8.08 kg/hr
b. Total average surface temperature of
area other than heating and soaking zone = 80oC
Heat loss at 80
o
C = 740 kCal / m
2
/ hrTotal area = 12.6 m2
Equivalent loss of fuel oil = 740 kCal / m2/ hr x 12.6 m
2
= 9324 kCal/hr
= 0.93 litres / hr
= 0.86 kg/hr
c) Heat loss through burner walls and changing end cover= 1.19 kg/h
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Total loss of fuel oil = a + b + c = 10.12 kg/hr
Total percentage skin loss = 10.12 / 368
= 2.75%
6. Unaccounted Loss
These losses comprises of heat storage loss, loss of furnace gases around charging door
and opening, heat loss by incomplete combustion, loss of heat by conduction throughhearth, loss due to formation of scales.
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Furnace Efficiency (Indirect Method)
1. Sensible Heat Loss in flue gas = 45.55%
2. Loss due to evaporation of moisture in fuel = 1.36 %
3. Loss due to evaporation of water
formed from H2 in fuel = 9.13 %4. Heat loss due to openings = 5.56 %
5. Heat loss through skin = 2.75%
6. Unaccounted losses = 10.65%
( Assessed by subtracting summation of losses 1 to 5 from the losses worked out by
direct method i.e. [75 (45.55+1.36+9.13+5.56+2.75) ] )
Total losses = 75%
Furnace Efficiency = 100 75
= 25 %
General Fuel Economy Measures in
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General Fuel Economy Measures in
Furnaces1) Complete combustion with minimum
excess air
2) Correct heat distribution
3) Operating at the desired temperature
4) Reducing heat losses from furnaceopenings
5) Maintaining correct amount of furnacedraught
6) Optimum capacity utilization
7) Waste heat recovery from the fluegases
1) Complete Combustion with
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1) Complete Combustion with
Minimum Excess AirThe amount of heat lost in the flue gases depends
upon amount of excess air. In the case of a furnace
carrying away flue gases at 900oC, % heat lost isshown in table :
Table Heat Loss in Flue Gas Based on Excess Air Level
Excess Air % of total heat in the fuel carried away
by waste gases (flue gas temp. 900oC)
25 48
50 55
75 63
100 71
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2)Correct Heat Distribution
Heat distribution in furnace
Alignment of burners in furnace
Prevent flameimpingement.To avoid high flame
temperature,damage ofrefractory and forbetter atomization
Align burnerproperly to avoid
touching thematerialTo reduce scaleloss
3)Operating at Desired Temperature
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3)Operating at Desired Temperature
Slab Reheating furnaces 1200oC
Rolling Mill furnaces 1200o
CBar furnace for Sheet Mill 800oC
Bogey type annealing furnaces- 650oC -750oC
CORRECTTEMPERATUREENSURES GOOD
QUALITYPRODUCTS.
TEMPERATURE
HIGHER THANREQUIREDWOULD ONLYUSE UP MORE
FUEL
Temperature for Different Furnaces
4) Reducing Heat Loss from Furnace
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4) Reducing Heat Loss from FurnaceOpenings
Heat loss through openings consists of direct radiation and
combustion gas that leaks through openings.
Keeping the doors unnecessarily open leads to wastage of fuel
Inspection doors should not kept open during operation
Broken and damaged doors should be repaired
The heat loss from an opening can be calculated using the formula:
Q=4.88 x ( T )4 x a x A x H k.Cal/hr100
T: absolute temperature (K),
a: factor for total radiationA: area of opening,
H: time Hr
5)Maintaining correct amount of
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5)Maintaining correct amount of
furnace draught
Negative pressures : air infiltration- affecting air-fuel ratio control,
problems of cold metal and non-uniform metal temperatures,
Positive Pressure: Ex-filtration -Problems of leaping out of flames,overheating of refractories,burning out of ducts etc.
6) Optimum capacity utilization
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6) Optimum capacity utilization
There is a particular loading at which the furnace will operate at
maximum thermal efficiency.
Best method of loading is generally obtained by trial-noting the weight
of material put in at each charge, the time it takes to reach temperatureand the amount of fuel used.
One reason for low loading is the mismatching of furnace dimension
with respect to charge and production schedule.
Rate of oxidation is depends on time, temperature, and free oxygen
content. The possible increase in surface defects can lead to rejection of
the product. It is therefore essential that coordination between the
furnace operator, production and planning personnel be maintained.
Loading of the charge should be arranged so that it receives the
maximum radiation and hot gases are efficiently circulated around.
7) Waste heat recovery from the flue gases
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7) Waste heat recovery from the flue gases
Sensible heat in flue gases can be used following
Charge preheating
Preheating of combustion air
Utilizing waste heat for other process to generate steam/hot water
Charge Pre-heating
Since raw materials are usually at room temperature, they can beheated sufficiently using high-temperature gas to reduce fuel
consumption.
Preheating of Combustion AirFor a long time, the preheating of combustion air was not used except for large
boilers, metal-heating furnaces and high-temperature kilns. This method is now
being employed in compact boilers and compact industrial furnaces as well.
Details of Air Preheaters
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Type Exhaust
gas temp
limitation
Preheated
air tempObject
furnace
Recuperat
ive
Metallic
recupera
tor
Flue
installati
on
Convectiv
e:
multitubu
lar; other
1,000 oC or
below
300-600oC Heating, heat
treatment,
industrial
&furnaces
Chimne
y
installation
Radiative
(radiative
&convectiv
e)
1,000-1,300oC
-do -do
Ceramic (tile)
recuperator
1,200-1,400oC
400-700oC Soaking pit
& glass
Regenerat
ive
General 1,000-1,600oC
600-
1,300oC
Coke oven,
hot blast
stove & glass
kilnRotary regenerative 600 oC or
below
100-300 oC Boiler, hot
blast stove
8 Minimizing Wall Losses
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8. Minimizing Wall Losses
About 30% of the fuel input to the furnace generally goes to make
up for heat losses in intermittent or continuous furnaces. The
appropriate choice of refractory and insulation materials is
needed for high fuel savings in industrial furnaces.
The extent of wall losses depend on:
Emissivity of wallThermal conductivity of refractories
Wall thickness
Whether furnace is operated continuously or
intermittently
& Minimum refractory
losses
Radiation Heat Loss from
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Surface of FurnaceThe quantity (Q) of heat release from a reheating furnace is
calculated with the following formula:
Q = a x ( tl - t2 )5/4 + 4.88 E [ ( t1 + 273 )
4- ( t2 + 273 )4 ]
100 100
where
a : factor regarding direction of the surface of natural convection
ceiling = 2.8, side walls = 2.2, hearth = 1.5
tl : temperature of external wall surface of the furnace (C)
t2 : temperature of air around the furnace (C)E: emissivity of external wall surface of the furnace
9 Use of Ceramic Coatings
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9.Use of Ceramic Coatings
The benefits of applying a high-emissivity ceramiccoating:-
9 Rapid heat-up9 Increased heat transfer at steady state
9 Improved temperature uniformity
9 Increased refractory life9 Elimination of refractory dust.
10. Fish Bone Diagram for Energy Conservation
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Analysis in Furnaces