mr. a. k. sinha - energy efficiency industrial utilities - boiler systems

7/28/2019 Mr. a. K. Sinha - Energy Efficiency Industrial Utilities - Boiler Systems

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3.0 Energy Efficiency

in Industrial Utilities:3.1 Boiler systems

A. K. SINHANATIONAL PRODUCTIVITY COUNCIL,

INDIA


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Introduction to Boiler It is an enclosed PressureVesselHeat generated byCombustion of Fuel istransferred to water to

become steamProcess: Evaporation

Steam volume increases to1,600 times from water andproduces tremendous forceCare is must to avoid explosion .

What is a boiler?


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Boiler Specification

Boiler Make & Year :XYZ & 2003

MCR(Maximum Continuous Rating) :10TPH (F & A100 oC)

Rated Working Pressure :10.54 kg/cm 2(g)

Type of Boiler : 3 Pass Fire tube

Fuel Fired : Fuel OilHeating surface : M 2


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Boiler Regulation

IBR Steam Pipe meansany pipe through whichsteam passes from aboiler to a prime mover

or other user or both, if pressure at which steampasses through suchpipes exceeds 3.5kg/cm 2 aboveatmospheric pressure or such pipe exceeds 254mm in internal diameter

Steam Boilers meansany closed vesselexceeding 22.75 litersin capacity and which isused expressively for generating steam under pressure


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Boiler Systems

Flue gas system

Water treatment system

Feed water system

Steam System

Blow down system

Fuel supply system

Air Supply system


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Boiler Types and Classifications Fire in tube or Hot gas through

tubes and boiler feed water in shellsideFire Tubes submerged inwater Application

Used for small steam capacities( upto 25T/hr and 17.5kg/cm 2

MeritsLow Capital Cost and fuelEfficient (82%)

Accepts wide & loadfluctuationsSteam pressure variation isless (Large volume of water)Packaged Boiler

Fire Tube Boiler


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Boiler Types and Classifications

Water Tube Boiler Water flow through tubesWater Tubes surroundedby hot gasApplicationUsed for Power PlantsSteam capacities rangefrom 4.5- 120 t/hr

CharacteristicsHigh Capital CostUsed for high pressurehigh capacity steam boiler Demands more controlsCalls for very stringent

water quality


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Performance Evaluation of Boilers

What are the factors for poor efficiency?Efficiency reduces with time, due to poor combustion,heat transfer fouling and poor operation andmaintenance.Deterioration of fuel and water qualityalso leads to poor performance of boiler. How Efficiency testing helps to improveperformance?

Helps us to find out how far the boiler efficiency driftsaway from the best efficiency. Any observed abnormaldeviations could therefore be investigated to pinpointthe problem area for necessary corrective action.


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Boiler EfficiencyThere are two methods of assessing boiler efficiency.

1) The Direct Method: Where the energy gain of the workingfluid (water and steam) is compared with the energy content of the

boiler fuel.

2) The Indirect Method: Where the efficiency is the difference between the losses and the energy input.

Boiler EfficiencyEvaluation Method

1. Direct Method 2. Indirect

Method


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Example:Type of boiler: Coal fired Boiler Heat input dataQty of coal consumed : 1.8 TPHGCV of coal :3200K.Cal/kg

Heat output data Qty of steam gen : 8 TPHSteam pr/temp:10 kg/cm 2 (g)/180 0 CEnthalpy of steam(sat) at 10 kg/cm 2(g) pressure

:665 K.Cal/kgFeed water temperature : 85 0 CEnthalpy of feed water : 85 K.Cal/kg

Find out the Find efficiency ?

Find out the Evaporation Ratio?

Efficiency Calculation by Direct Method


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Boiler efficiency ( ): = Q x (H

h) x 100 (q x GCV)

Where Q = Quantity of steam generated per hour (kg/hr)H = Enthalpy of saturated steam (kcal/kg)h = Enthalpy of feed water (kcal/kg)

q = Quantity of fuel used per hour (kg/hr)GCV = Gross calorific value of the fuel (kcal/kg)

Boiler efficiency ( )= 8 TPH x1000Kg/Tx (665 85) x 1001.8 TPH x 1000Kg/T x 3200

= 80.0%

Evaporation Ratio = 8 Tonne of steam/1.8 Ton of coal

= 4.4


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Boiler Flue gas

Steam Output

Efficiency = 100 (1+2+3+4+5+6+7+8)

(by In Direct Method)

Air

Fuel Input, 100%

1. Dry Flue gas loss2. H2 loss3. Moisture in fuel4. Moisture in air 5. CO loss

7. Fly ash loss

6. Surface loss

8. Bottom ash loss

What are the losses that occur in a boiler?


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EnergyConservationOpportunities

in Boilers


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1. Reduce Stack Temperature

Stack temperatures greater than 200 Cindicates potential for recovery of waste

heat.It also indicate the scaling of heat

transfer/recovery equipment and hence

the urgency of taking an early shut downfor water / flue side cleaning.

22o C reduction in flue gas temperature

increases boiler efficiency by 1%


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2. Feed Water Preheating usingEconomiser

For an older shell

boiler, with a flue gasexit temperature of 260 oC, an economizer could be used to reduce

it to 200oC, Increase inoverall thermal

efficiency would be inthe order of 3%.

Condensingeconomizer(N.Gas)Flue gas reduction upto 65 oC

6oC raise in feed water temperature, by economiser/condensate recovery,corresponds to a 1% saving in fuel consumption


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3. Combustion Air

Preheating Combustion air preheating is analternative to feedwater heating.

In order to improve thermal efficiency by1%, the combustion air temperature mustbe raised by 20 oC.


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4. Incomplete Combustion(c c c c c + co co co co)

Incomplete combustion can arise from a shortage of air or surplus of fuel or poor distribution of fuel. In the case of oil and gas fired systems, CO or smokewith normal or high excess air indicates burner system

problems.Example: Poor mixing of fuel and air at the burner. Poor oilfires can result from improper viscosity, worn tips,carbonization on tips and deterioration of diffusers.With coal firing : Loss occurs as grit carry-over or carbon-in-ash (2% loss).Example : In chain grate stokers, large lumps will not burnout completely, while small pieces and fines may block theair passage, thus causing poor air distribution.

Increase in the fines in pulverized coal also increasescarbon loss.


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5. Control excess air for every 1% reduction in excess air ,0.6% rise inefficiency.The optimum excess air level varies with furnace design, type of burner,fuel and process variables.. Install oxygen trim system

TABLE 2.5 EXCESS AIR LEVELS FOR DIFFERENT FUELS

Fuel Type of Furnace or Burners Excess Air(% by wt)

Completely water-cooled furnace for slag-tap or dry-ash removal

15-20Pulverised coal

Partially water-cooled furnace for dry-ashremoval

15-40

Spreader stoker 30-60Water-cooler vibrating-grate stokers 30-60Chain-grate and traveling-grate stokers 15-50

Coal

Underfeed stoker 20-50Fuel oil Oil burners, register type 15-20

Multi-fuel burners and flat-flame 20-30 Natural gas High pressure burner 5-7Wood Dutch over (10-23% through grates) and

Hofft type20-25

Bagasse All furnaces 25-35Black liquor Recovery furnaces for draft and soda-

pulping processes30-40

6 R d i f S li


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6. Reduction of Scalingand Soot LossesIn oil and coal-fired boilers, soot buildup on tubes acts asan insulator against heat transfer. Any such depositsshould be removed on a regular basis. Elevated stacktemperatures may indicate excessive soot buildup. Alsosame result will occur due to scaling on the water side.High exit gas temperatures at normal excess air indicatepoor heat transfer performance. This condition can resultfrom a gradual build-up of gas-side or waterside deposits.Waterside deposits require a review of water treatment

procedures and tube cleaning to remove deposits.Stack temperature should be checked and recordedregularly as an indicator of soot deposits. When the fluegas temperature rises about 20 oC above the temperaturefor a newly cleaned boiler, it is time to remove the soot

deposits


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7. Effect of Boiler Loading onEfficiency

As the load falls, so does the value of the massflow rate of the flue gases through the tubes. Thisreduction in flow rate for the same heat transfer

area, reduced the exit flue gas temperatures by asmall extent, reducing the sensible heat loss.Below half load, most combustion appliancesneed more excess air to burn the fuel completelyand increases the sensible heat loss.Operation of boiler below 25% should be avoidedOptimum efficiency occurs at 65-85% of full loads


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8. Boiler Replacement

if the existing boiler is :Old and inefficient, not capable of firing cheaper substitution fuel, over or under-sized for presentrequirements, not designed for ideal loadingconditions replacement option should beexplored.

Since boiler plants traditionally have a useful lifeof well over 25 years, replacement must becarefully studied.


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Requirement for Evaluation of

Boiler Performance To assess the existing efficiency of theboilers by both direct and direct methods.

To compare the efficiency with design /PG test values.Suggest ways to improve boiler efficiencyTo check general health of the equipment.


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INSTRUMENTS REQUIRED

Flue gas analysersPortable temperature indicator On-line instruments of boiler control room.Facilities of the chemistry lab or outside lab for coal /ash / water analysis. (coal proximate or ultimateanalysis, un-burnt in bottom and fly ash, TDS, pH of feed water / blow-down / condensate.

Power analyser for power measurement of ID fan, FDfan, ESP, crushers, BFP (boiler feed water pump) coolhandling plant/ash handling plant, etc.


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AUDIT PROCEDURE

Activity - I Select the boiler for which the ENERGY audit to becarried out.

Collect specifications, design and performanceguarantee (PG) test data.Collect maintenance history, previous boiler inspectiondetails, running hours and any problems existing in thesystem.

Check availability and working condition of various on-line and portable instruments required for measurementsduring the trials.Observations should be made by running the boiler for aminimum of six hours on fairly constant load, with

readings being logged at an interval of half an hour.


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AUDIT PROCEDURE

Activity - II Take overall view of the boiler system comprising of fuelhandling plant, burners, ID fans, FD fans,

dampers/valves, steam piping, thermal insulation, etc.,with a view to check general health of variousinstallations i.e., to observe noticeable leakages of coal/oil/gas/air/steam/feedwater/ condensate and also tocheck condition of thermal insulation, steam traps etc.

During the trial run, take samples of coal/oil, bottom ashand fly ash, feed water, boiler drum water, returncondensate and get them analysed (if possible sample of each of the above should also be sent to a reputed andreliable outside laboratory (to facilitate counter-checkingof in-house lab results).


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AUDIT PROCEDURE

Activity - II The following parameters are to be tested :-a) Raw Coal / Solid Fuel GCV, ash content, volatile

matter, fixed carbon, total moisture and HGI value.b) Oil GCV, carbon, hydrogen, sulphur, moisture.c) Gaseous Fuel GCV, carbon, hydrogen, sulphur.d) Fly Ash & Bottom Ash quantity.

Combustibles in fly and bottom ash and GCV.Carry out power measurements of FD fan, ID fan, boiler feed water pump, crushers, ash slurry pumps, etc.


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AUDIT PROCEDURE

Activity - III Collect sample of flue gas and measure temperature, CO,CO2 and O2, at different locations, i.e., before economiser,after economiser, before air preheater and after air preheater,before ESP and after ESP. Using on line analysis ,portableanalysers, orsat apparatus.During trials, also take note of key unit parameters like boiler steam flow, steam pressure, steam temperature, etc.Measure furnace draft at combustion chamber and inlet to ID

fan.Fuel flow rate and fuel consumed during the trial duration is tobe measured.Various other parameters observed should be logged on thelog sheet provided in Annexure-I.


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AUDIT PROCEDURE

Activity - IV Calculation may be performed as per formula given inauditors tools.

1) Efficiency of the boiler by direct method andevaporation ratio.

2) Blow-down percentage calculation.List out scope for improvement.

List out recommendations and actions to be taken for improvement.Cost benefit analysis with savings potential for initiatingimprovement measures.


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AUDIT GUIDELINES FOR BOILERS

Storage and preparation of fuel :For Solid Fuel

Stack coal in neat heaps not exceeding 1.5 metres in heightand limit the individual heaps to 200 MT.Stack coal on hard ground and ensure a tightly packedheap to subdue ventilation which can be lead tospontaneous combustion.Size coal properly as per requirements.Wet coal carefully according to the content of fines in thecoal.

For Liquid Fuel Pre-heat the oil as per the manufacturers recommendation.Water draining periodically from storage tanks.Oil lines heat tracing.


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CombustionPeriodic sampling of exit flues gases andcorresponding adjustment of air supply by dampers

are important to establish optimum level of CO2/O2percentage (CO2 at 12 -14% and O2 at 3 - 6%) &excess air Check for black smoke and adjust damper to get lightbrown smoke.10 % additional Excess Air Reduces furnace efficiencyby 1.45 %


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Waste Heat RecoveryPre-heat combustion air by air pre-heater. For every22 0C rise in air temperature, there is 1% decrease in

fuel consumption.

Pre-heat feed water by economiser (for every 7 0C risein feed water temperature, 1% decrease in fuel

consumption).

Study possibility of blow-down heat recovery.


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Reducing Heat LossesCheck for effective insulation on the boiler surfaces.

Boiler furnace pressure to be kept slightly positive toavoid ingress of cold air.

Boiler doors, peepholes / openings to be kept closed toavoid radiation losses.


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BOILERS- DOs& DONTs

1. Soot-blowing regularly

2. Clean Blow-down gauge glass once a shift3. Check safety valves, weekly once4. Blow-down in each shift as per requirement5. Keep furnace doors all closed6. Control furnace draughts7. Clear discharge ash hoppers every shift8. Watch chimney smoke and control fires


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BOILERS- DOs& DONTs (contd.)

9. Check auto controls periodically10. Attend to leakages periodically

11. Check all valves, damper, etc. for correctoperation once a week12. Lubricate all mechanisms for smooth working13. Keep switch-boards neat and clean and

indication systems in working order 14. Keep area clean and dust-free


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15. Keep fire-fighting arrangements in readinessalways. Mock rehearsals should be carried out

once a month.16 . All log-sheets must be properly.17. Check FD/ID inter locks wherever available18. CO2 and O2 recorder must be checked and

calibrated.


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19. Quality of steam, water should bechecked as applicable

20. Quality of fuel should be checked once aweek

21. Keep air cocks open during start andclose


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BOILERS- DOs& DONTs

1. Dont light up torches immediately after a fire -out (purge)

2. Dont blowdown unnecessarily 3. Dont keep furnace doors open unnecessarily 4. Dont blow safety valves frequently(control

operation)

5. Dont overflow ash hoppers 6. Dont increase firing rate than permitted


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7. Dont over -load boiler as a practice8. Dont keep water level too high or too low

9. Dont operate soot blowers at high loads 10. Dont trip the ID fan while in operation 11. Dont look at fire in furnace directly, use tinted

glass.

12. Avoid thick fuel bed.13. Dont leave boiler to untrained operators /

technicians


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14. Dont overlook unusual observation(sound change, change in performance

control difficulties), rather investigate.15. Dont skip over annual maintenance


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What is a Furnace?

A furnace is an equipment to meltmetals for casting or heat materialsfor change of shape ( rolling,forging etc ) or change of properties ( heat treatment ).

3.2.FURNACES

T d l ifi i f f


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Types and classification of furnaces

Furnaceclassification

Recuperative

Regenerative

Accordingto mode of

heat transfer

Accordingto mode of charging

Mode of heatrecovery

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

Characteristics of an Efficient Furnace


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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 uniformtemperature as possible with the least

possible fuel and labour .

Furnace Energy Supply


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Furnace Energy Supply

The products of flue gases directly contact thestock , so type of fuel chosen is of importance.For example, some materials will not toleratesulphur in the fuel. Also use of solid fuels will

generate particulate matter , which will interferethe 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 electricityin induction and arc furnaces. Non-ferrousmelting utilizes oil as fuel.

Oil Fired Furnace


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Oil Fired Furnace

Furnace oil is the major fuel used in reheatingand heat treatment furnacesLDO is used in furnaces where presence of sulphur is undesirable.(No problom with sulphur )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 whichthe furnaces have to operate to meet therequired demand. For example, a furnaceheating the stock to 1200 oC will have its exhaustgases leaving atleast at 1200 oC resulting in a


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Heat Transfer in Furnaces

Figure 4.3 : Heat Transfer in furnace

Radiation from theflame,hot combustion

products and the

furnace walls androof; Convection due to

the movement of hotgases over the stock surface.


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Performance Evaluation of a TypicalFurnace

Figure 4.10 Heat losses in industrial heating Furnaces

Wh h f


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What are the furnacelosses ?

Figure 4.11 wall losses

Wall losses:

Figure 4.12. Radiation loss

Figure 4.13. Air infiltration from furnace opening.

Stack loss (Waste-gas loss)

Air infiltration

Material handling lossCooling media losses

Radiation (opening) loss

Stored Heat Loss:

Wall Loss:


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Furnace Efficiency Calculation

Example: by direct Method An oil-fired reheating furnace has an operatingtemperature of around 1340 oC. Average fuelconsumption is 400 litres/hour. The flue gas exittemperature after air preheater is 750 oC. Air is

preheated from ambient temperature of 40 oC to 190 oCthrough an air pre-heater. The furnace has 460 mm thick wall (x) on the billet extraction outlet side, which is 1 mhigh (D) and 1 m wide. The other data are as given

below. Find out the efficiency of the furnace by directmethod.


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Trial DataFlue gas temperature after air preheater =750 oC

Ambient temperature =40 oCPreheated air temperature =190 oC

Specific gravity of oil =0.92 Average fuel oil consumption =400 Litres / hr

=400 x 0.92 =368 kg/hr Calorific value of oil =10000 kCal/kg

Average O 2 percentage in flue gas =12%

Weight of stock =6000 kg/hr Specific heat of Billet =0.12 kCal/kg/ 0CSurface temperature of roof and side walls= 122 oCSurface temperature other than heating and soaking zone = 85 oC


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Furnace Efficiency (Direct Method)

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

InputEfficiency = 936000 x 1003680000

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


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Theoretical Heat

Example of melting one tonne of steel from an ambient temperature of 20 oC . Specificheat of steel = 0.186 Wh/kg/ 0C, latent heat for melting of steel = 40 Wh/kg/ 0C. Meltingpoint 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 20 o C = 334 kWh.

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

Efficiency = 334 kWh x 100 = 48%700 kwh

(Electrical Furnace)

General Fuel Economy Measures in


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General Fuel Economy Measures inFurnaces

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

1) Complete Combustion with


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1) Complete Combustion withMinimum Excess Air

The amount of heat lost in the flue gasesdepends upon amount of excess air. In thecase of a furnace carrying away flue gases at900 oC, % 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 awayby waste gases (flue gas temp. 900 oC)

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 flametemperature,damage of refractory and for better atomization

Align burner properly to avoid

touching thematerialTo reduce scaleloss

3)Operating at Desired Temperature


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3)Operating at Desired Temperature

Slab Reheating furnaces 1200 oC

Rolling Mill furnaces 1200 oC

Bar furnace for Sheet Mill 800 oC

Bogey type annealing furnaces- 650 oC -750 oC

CORRECT

TEMPERATUREENSURES GOODQUALITYPRODUCTS.

TEMPERATURE

HIGHER THANREQUIREDWOULD ONLYUSE UP MOREFUEL

Temperature for Different Furnaces

4) Reducing Heat Loss from Furnace


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4) Reducing Heat Loss from FurnaceOpenings

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 radiation

A: area of opening,H: time Hr

Heat loss through openings consists of direct radiation andcombustion gas that leaks through openings.

Keeping the doors unnecessarily open leads to wastage of fuel

Inspection doors should not kept open during operationBroken and damaged doors should be repaired

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 atmaximum thermal efficiency.Best method of loading is generally obtained by trial-noting the weightof material put in at each charge, the time it takes to reach temperatureand the amount of fuel used.

Mismatching of furnace dimension with respect to charge and productionschedule.

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


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7) Waste heat recovery from the fluegases

Charge (stock) preheating,

Preheating of combustion air,

Utilizing waste heat for other process

8 Mi i i i W ll L


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8. Minimizing Wall Losses

About 30% of the fuel input to the furnace generally goes tomake up for heat losses in intermittent or continuous furnaces.

The appropriate choice of refractory and insulation materials isneeded 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 orintermittently

Radiation Heat Loss from Surface of


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Radiation Heat Loss from Surface of Furnace

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

wherea : 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

424

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21 100

273

100

273

88.4)(

t t

x E t t xaQ

9 U f C i C i


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9.Use of Ceramic Coatings

The benefits of applying a high-emissivity ceramiccoating:-

Rapid heat-upReduction of thermal massIncreased heat transfer at steady state

Improved temperature uniformityIncreased refractory lifeElimination of refractory dust.


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Thank you

mr. a. k. sinha - energy efficiency industrial utilities - boiler systems

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