furnaces in power boiler

7/31/2019 Furnaces in power boiler

1/34

4.FURNACESSyllabus

Classification

General fuel economy measures in furnaces:, excess

air, heat distribution, temperature control, draft

control,

Waste heat recovery.


2/34

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).

T d l


3/34

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


4/34

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.

F E S l


5/34

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.


6/34

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.

T i l F S


7/34

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.


8/34

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.


9/34

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


10/34

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.

Th l ffi i i f


11/34

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


12/34

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%


13/34

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)


14/34

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%


15/34

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


16/34

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


17/34

Figure 4.3 Factor for determining the equivalent of heat release from

openings to the quality of heat release from perfect black

body


18/34

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 %


19/34

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


20/34

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.


21/34

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


22/34

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


23/34

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


24/34

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


25/34

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


26/34

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


27/34

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.



28/34


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


29/34

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


30/34

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


31/34

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


32/34

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


33/34

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


34/34

Analysis in Furnaces

furnaces in power boiler

Documents