2.4 furnaces.pdf

PREPARED BY K. MANOKARAN, LEAD AUDITOR, DNVPREPARED BY K. MANOKARAN, LEAD AUDITOR, DNV Page: 1

ENERGY MANAGEMENT


ENERGY MANAGEMENT

A furnace is an equipment to melt metals

for casting or for heating materials or for change of shape (rolling,

forging etc) or for change of properties (heat treatment).


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Furnace classification

Recuperative

Regenerative

According to mode of heat 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 tank melting

(regenerative /

recuperative)

Based on the method of generating heat: combustion type (using fuels) and electric type


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Furnace should be designed so that in a given time, as much of material as possiblecan be heated to an uniform temperature as possible with the least possible fuel and labour.


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Since flue gases directly contact the stock, the type of fuel chosen is very important. For example, some stock will not tolerate sulphur in the fuel. Also use of solid fuels will release particulate matter (dust), which will interfere with the stock placed inside the furnace. Hence, majority of the furnaces use liquid fuel, gaseous fuel or electricityas energy input.

Ferrous (steel, cast iron) melting furnaces such as induction and arc furnaces use electricity

Non-ferrous melting furnaces use oil.


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Furnace oil is the major fuel used in reheating and heat treatment furnaces

LDO is used in furnaces where presence of sulphur is undesirable.

Furnaces operate with efficiencies as low as 7% as against upto 90% achievable in other combustion equipment such as boiler.This is because of the high temperature at which the furnaces operate to meet the required demand. For example, a furnace heating the stock to 1200oC will have its exhaust gases leaving atleast at 1200oC resulting in a high heat loss through the stack.


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Used for preheating billets and ingotsto attain a forge temperature.

The furnace temperature is maintained at 1200 to 1250oC.

Forging furnaces, use an open fireplace system and most of the heat is transmitted by radiation.

The typical loading in a forging furnace is 5 to 6 tones with the furnace operating for 16 to 18 hours daily.

The total operating cycle can be divided into (i) heat-up time (ii) soaking time and (iii) forging time.

Specific fuel consumption depends upon the type of material and number of reheats required.


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Batch type furnace: Used for heating up

scrap, small ingots and billets weighing 2 to 20 kg. for batch type re-rolling.

Charging and discharging of the material is done manually and the final product is in the form of rods and strips.

Operating temperature is 1200oC.

Total cycle time can be categorized into heat-up time and re-rolling time.

Continuous Pusher Type:

The process flow and operating cycles of a continuous pusher type is the same as that of the batch furnace.

The material or stock recovers a part of the heat in flue gases as it moves down the length of the furnace.

Operating temperature is 1250oC.

Heat absorption by the material in the furnace is slow, steady and uniform throughout the cross-section compared with batch type.


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Heat Transfer in furnace

Radiation from the flame, hot combustion products and the furnace walls and roof

Convection due to the movement of hot gases over the stock surface.


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Based on method by which stock moves through the furnace Stocks placed side by side to form

stream of material which is moved through the furnace

Stocks placed on hearth or supporting structure which moves the stock

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Rotary hearth type furnace

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Heat losses in industrial heating Furnaces


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wall losses

Wall losses:

Radiation loss

Air infiltration from furnace opening.

Stack loss (Waste-gas loss)

Air infiltration

Material handling loss Cooling media losses

Radiation (opening) loss

Stored Heat Loss:

Wall Loss:


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An oil-fired reheating furnace has an operating temperature of around 1340oC. Average fuel consumption is 400 litres/hour. The flue gas exit temperature after air pre-heater 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.


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Furnace Efficiency (Direct Method)

Fuel input = 400 litres / hr = 368 kg/hr Heat Input =368x10,000=3680000 kCal

Heat output = m x Cp x T = 6000 kg x 0.12 x (1340 40) = 936000 kCal

Efficiency = Output x 100 Input

Efficiency = 936000 x 100 3680000

= 25.43 % = 25% (app) Losses = 75% (app)


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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 T m = Weight of flue gas = 32.62 + 1.0 = 33.62 kg / kg of oil. Cp = Specific heat T Heat loss 33.62 x 0.24 x (750 40) = 5729 kCal / kg of oil

% Loss = 5729 x 100 = 57.29%

10000


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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

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10000

M { 5 8 4 + 0 .4 5 ( T f g -T a m b ) } 1 0 0G C V o f F u e l

x=

- / . 0 1 ' ( ) * ) + ) , . - / 1

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! " # $ % & ' ( ' $ ( # ) * + % # $ % & ! ) , )

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# # " # $ % & , , , ) / &

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2 # * ' 0 , = 5.56 %

Heat loss due to openings can be calculated by computing black body radiation at furnace temperature, and multiplying these values with emissivityand the factor of radiation through openings.Use fig. for black body radiation loss and Factor of radiation through openings

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Body

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at a Particular Temperature

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5a). Heat loss through roof and sidewalls:Total average surface temperature = 122oCHeat loss at 122 oC = 1252 kCal / m2 / hrTotal area of heating + soaking zone = 70.18 m2Heat loss = 1252 kCal / m2 / hr x 70.18 m2

= 87865 kCal/hrEquivalent oil loss (a) = 8.78 kg / hr

5b). Total average surface temperature of area other than heating and soaking zone= 85oCHeat loss at 85oC = 740 kCal / m2 / hrTotal area = 12.6 m2Heat loss = 740 kCal / m2 / hr x 12.6 m2

= 9324 kCal/hrEquivalent oil loss (b) = 0.93 kg / hr

Total loss of fuel oil = a + b = 9.71 kg/hrTotal percentage loss = 9.71 / 368 = 2.64%

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1 3 4 ! 6 3 7 - / 1 ) 7 3 ! - 5

7 - 0 5 )

Total losses = 75.98 %

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Theoretical Heat

Example of melting one tonne of steel from an ambient temperature of 20oC . Specific heat of steel = 0.186 Wh/kg/0C, latent heat for melting of steel = 40 Wh/kg/0C. Melting point of steel = 1600 oC.

Theoretical Total heat = Sensible heat + Latent heat

Sensible Heat = 1000 kg x 0.186 Wh /kg oC x (1600-20)oC = 294 kWh

Latent heat = 40 Wh/ kg x 1000 kg = 40 kWh

Total Heat = 294 + 40 = 334 kWh.

So the theoretical energy needed to melt one tonne of steel from 20o C = 334 kWh.

Actual Energy used to melt to 1600o C is 700 kWh

Efficiency = 334 kWh x 100 = 48% 700 kwh


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1) Complete combustion with minimum excess air

2) Correct heat distribution3) Operating at the desired temperature 4) Reducing heat losses from furnace

openings5) Maintaining correct amount of

furnace draught6) Optimum capacity utilization7) Waste heat recovery from the flue

gases8) Minimum refractory losses9) Use of Ceramic Coatings


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The 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 is shown 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 flame impingement. To avoid high flame temperature,damage of refractory and for better atomization

Align burner properly to avoid touching the materialTo reduce scale loss


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3) Operating at Desired Temperature

Slab Reheating furnaces1200oC

Rolling Mill furnaces1200oC

Bar furnace for Sheet Mill800oC

Bogey type annealing furnaces- 650oC -750oC

CORRECT TEMPERATURE ENSURES GOOD QUALITY PRODUCTS.

TEMPERATURE HIGHER THAN REQUIRED WOULD ONLY USE UP MORE FUEL

Temperature for Different Furnaces

For maintaining temperature, do not leave it to operator judgment, Use ON/OFF controls


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4) Reducing Heat Loss from Furnace Openings

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)

Heat loss through openings consists of direct radiation and combustion gas that leaks through openings. Keeping the doors unnecessarily open leads to wastage of fuelInspection doors should not kept open during operationBroken and damaged doors should be repaired


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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.


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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 temperature and the amount of fuel used.

Mismatching of furnace dimension with respect to charge and production schedule.

Coordination between the furnace operator, production and planning personnel is needed.


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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 refractoriesWall thicknessWhether furnace is operated continuously or intermittently


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The quantity (Q) of heat release from a reheating furnace is calculated with the following formula:

where.,a : factor regarding direction of the surface of natural

convection ceiling = 2.8, side walls = 2.2, hearth = 1.5tl : 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

+

++=

42

414/5

21 100273

10027388.4)( ttxEttxaQ


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10. Use of Ceramic Coatings

The benefits of applying a high-emissivity ceramic coating:-

Rapid heat-up Increased heat transfer at steady

state Improved temperature uniformity Increased refractory life Elimination of refractory dust.


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The emissivity of the surface depends on the colour of the paint. Silver paint with metallic base is radiating the minimum heat. Hence it is recommended to repaint the oven with silver paint. Energy saving potential is calculated as follows;

4 monthsSimple payback period

Rs. 0.60Estimated investment for painting

26895 x8640/(0.8 x 11000) x (13.5/0.8)Rs. 2.28 Lakhs

Annual energy savings

26895Energy Saving in Kcal/hr

31274Energy Saving in Watt

15411640612.650.545750.65With Al paint

18539040615.210.545750.95With cream paint

Heat loss (Watt)AreahvtatheParticulars

2.4 furnaces.pdf

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