3.8 REHEATING FURNACE

One F O fired furnace has been installed for rolling mill.

Its broad specifications and operating conditions are shown at Annexure no.6/1.

Following is discussed from energy conservation view:

Chemical composition of FO – For further calculations, we would assume following

%age composition and G C V (As per ISS).

GCV Carbon Sulphur Moisture

10500 84 3.5 1

Hydrogen Oxygen Nitrogen Ash

12 0 0 0.1

Stoichiometric air (theoretical air) requirement for FO = 13.8 Kg/ kg FO

i) Specific fuel consumption:

First thing for any energy conservation is to know existing specific fuel consumption.

For this, the plant authorities are regularly monitoring it. This topic has already been

discussed in detail in earlier paras.

Efficiency is calculated by direct method as below:-

Total weight charged – MT 1203940

Specific heat of steel – 0.11 to 0.12 K Cal / Kg. / 0C 0.12

Final maximum temperature – 0C 1280

Initial temperature - 0C 30

Calorific value of F O = 10500 K Cal / kG. 10500

Theoretical FO – Kg/ ton 14.3

Actual calculated at Annexure no. G/1 after ignoring FO consumption during idle period

41.9

Furnace efficiency 34.1

Prize winning firm's and some manufacturer claim 42 to 45

ii) Flue gas analysis –

The flue gas analysis was carried out at about 2 M distance from furnace exist. Flue

gas duct is un-insulated from furnace exist to chimney from out side. But on inside

refractory bricks are installed. The results are as follows:

Sl. No. Item

During idle running

period, when burners were

off

During plant set up period, when some burners were ON

After about 2 hours of

proper furnace operation on

load

2/12/2010 3/12/10, 11:45

1 Oxygen% 20.9 9.3 9.5

2 Excess Air % Almost all air 78 81

3 CO2 0 8.7 8.5

4 CO ppm 0 25 24

5 Flue gas temp. C 105 428 436

6 Ambient temp. C 28.6 26.8 29.6

7 CO/CO2 0.0001 0.0002

8 Combustion efficiency

75 74

The results are discussed as follows:

• Excess oxygen- Thus excess oxygen in both set up and stable running

period is about 9.4 %. In reheating furnaces, with pusher side, ejector side

and discharge side openings, this is quite normal and average figure. But it is

possible to operate furnace at about 5% excess oxygen also. The pressure in

furnace is positive. So there is no infiltration of air. The furnace is quite long

and flame retention time is also sufficient. So proper burning of fuel can also

be achieved. It is suggested that efforts be made to operate it between 5 to

7% excess oxygen. Roughly 1% fuel is saved for every 1% fall in oxygen.

• Excess temperature – The temperature of flue gasses at chimney base was

4250C. It is very high. The flue gas temperature is dictated by sulphur

contents in fuel. In furnace oil, generally sulphur varies from 2 to 4%. As per

ISS, maximum permissible is 4%. Assuming it 4.5%, dew point

correspondingly is 145 0C. Based upon this, general flue gas temperature at

chimney base is 1600C. Taking worst case, maximum can be 180 degrees

centigrade. Thus 425-180 = 245 degrees centigrade should be recovered

from flue gasses. The air would be correspondingly heated. The heated air

temperature should be maximum 250 degrees centigrade.

Rough proposal is some thing like this:

Based upon above and after ignoring all other furnace losses, the auditors

calculated its combustion efficiency. The results are shown at annexure no. 6/2.

Then on the same annexure, combustion efficiency with 6% excess oxygen and 180 0C

flue gas was calculated. The summary of results is as follows:

Combustion efficiency under existing circumstances = 68.4%

Combustion efficiency after heat recovery & oxygen reduction

Approximate Saving and investment is as follows:

Sl. No. Parameter Values

1 Existing efficiency (Annexure no. 6/2) h2 0.684

2 Achievable efficiency (Annexure no.6/2) h1 0.864

3 Assume modest Efficiency - h1 0.82

4 Total Fuel consumption MT 947

5 Fuel saving (3)[h1-h2/h1]- KL 157

6 Average Cost Rs. / MT 28000

7 Total saving Rs. Lacs 43.98

8 Expenditure on recuperator, oxygen analyzer, FO pipe insulation etc Rs in Lakhs

5

9 Pay back period - Months 1

Iii) Skin temperature –

Bare furnace surfaces, connected piping & heating equipment give up heat to the

atmosphere. Proper insulation seeks to reduce these heat losses. If these are already

lagged and the temperature is still high, the insulation be properly checked & if need be

strengthened or redone. Skin temp. up to 400C is excellent , up to 550C satisfactory and

up to 700C is tolerable. But if is more, it should be reduced by providing, improving or

redoing insulation.

Measured the surface temperature on all sides and top. These are comparable with

most of industry but at present rate of F O on higher side.

Based upon these, calculated radiation losses at Annexure no. 6/3.

The summary of existing losses and saving after improvement is as follows:

Total losses - kCal/ hour 24626

Assume modest- KCal/hour 24000

Total FO consumption - KL 947

Average F O consumption / hour, assuming 6000 hours – liters 158

Caloric value /kg 10500

%age radiation losses 1.4

Total F O savable – KL 14

Amount savable @ Rs 28000 / KL 3.84

Approximate expenditure - Rs. Lakhs 4

Pay back period - Months 13

The insulation inside is not easy as refractory bricks are satisfactory. It is suggested

that if major renovations are not contemplated then, 3 to 4” thick ceramic fiber be

applied on cold face of all furnaces. This be cladded with aluminium sheet. The

emissivity of Aluminium is only 0.3 against 0.7 to 0.8 of mild steel. This would reduce

heat losses. Insulation thickness should ensure that skin temperature is not more

than 60 degree centigrade to start with. But this should be done after checking up

with original designer. Insulating out side would contain heat losses and cause

uneven expansion. The auditing team have taken token saving and investment of

about 25% to keep issue alive.

iv) Heat losses from openings: -

Lot of heat is lost through different openings in the furnaces. These openings may be

for inspection, feed or drawl of material. So all unnecessary doors are sealed.

Wherever needed, proper lightweight refractory doors, which are durable and heat

proof and operate with chains, pulleys, sheaves and counter weights or hinged or

swinging type should be provided at such openings. The loading should be

mechanical.

Condition of openings in this furnace is as follows:

i. Pusher side: It is minimum possible. Temperature on this side is also

about 435o C.

ii. Ejector & discharge side: Opening is about 750 mm dia. Some of it already

blocked. More of it can be blocked as illustrated by following sketch-

Calculated avoidable losses through above openings at Annexure no. 6/4.

Summary of the same is as follows:

Total avoidable losses- Kcal/ h 29610

Equivalent FO savable = liters/ hour 2.8

Annual saving potential in about 3300 hrs working – KL 9.3

Amount savable @ Rs 28000 / kl- Rs. lacs 2.61

Expenditure 0

Pay back period - Months Immediate

vi) Furnace oil temperature-

A proper heating unit is equipped with service tank & filter- heater, unit has been

provided. Thermostat control for oil temperature is also provided in both. The oil

temperature has been set as follows:

i) Service tank – 78 0C. This tank is un-insulated. The surface temperature of tank is

520C.

ii) Heater, filter & supply unit – 92 to 101 0C. The piping from this unit to burners are

mostly un-insulated. The heater body is also not properly insulated.

The auditors measured temperature of pipes surface at various places and found as

follows:

Furnace oil temperature in heating unit = 98 0C.

Pipe surface temperature immediately after oil heater = 840C.

Pipe surface temperature at feeding end in some burners = 740C.

Pipe surface temperature at feeding end in some burners = 630C.

It shows that temperature is dropping by 10 to 210C.

Heat loss due to above is as below:

Total annual FO consumption – KL 947

Specific gravity of FO 0.92

Specific heat of FO 0.28

Temperature drop - 10 to 15 10

Total heat loss - Thou Kcal/ annum 2439

Equivalent electricity loss- kwh/ annum 2837

Add 60% for insulated service tank 4539

Amount savable @ Rs 3.15/ kwh 14296

Expenditure - Considered under excess air

Pay back period = Months

The electrical loss is not much but the effect of lower temperature is much.

The FO temperature at burner tips should be 100 to 110 degrees centigrade.

vii) Idle running:

• Adjustment of rolls: Billets after heating are rolled to angle iron for

transmission line structures. As explained in introduction, about 35 different

sizes of angle iron are rolled. So stopping of rolling mill for adjustment of rolls

for changing from one section to another is very frequent.

Each adjustment takes 2 to 3 hours and some times more. Then after each

adjustment, one billet is rolled. Its size checked. If not found as per

requirement, then readjustment is carried out.

• Lunch & tea breaks: The stoppage due to this is as follows:

Lunch breaks- 2 times each ½ hours = 1 hour

Tea breaks - 4 times each 1/4hours = 1 hour

Due to above, actual utilization of plant is 53% as explained at Annexure no. G/3.

During this period, auxiliaries continue running as explained below:

� Furnace blower:

It also continues running except on long shut down. About 20 to 30% air

is by-passed via standby blower and some dampers to individual burners

closed. Normally, dampers of front 2 burners are closed 15% and those

of sides 50%.

� F O feeding:

Oil valves to burners are only 15 to 20% opened. On longer shut down,

oil supply is also stopped.

It is suggested that more care to reduce loss during idle period be taken.

viii) Furnace temperature –

For every type of heating operation, there is an optimum temperature of furnaces.

Excess temperature causes: -

i). Unnecessary wastage of fuel.

ii). Over heating or longer heating of stock, resulting in excess scale losses.

iii). Over stressing of refractoriness.

ix) Material losses:

The average yield in the plant is 93 %. It means material losses are about 7%.

The matter was discussed with plant engineers, who informed that roughly, these are

as follows:

1

Material Yield

93 %

2

Mis Roll Losses ( Due to power cut etc )

1 %

3

End Cut material wastage

2 %

4.

Scale Losses

1.2 %

5.

Rolling Losses ( Scales during rolling process )

1.2 %

6.

Straightening Loss

0.5 %

7.

Invisible Losses ( Unable to account )

1.1 %

This still leaves 1.1 %. The auditing team observed hot billets. The thickness of scale

falling out of billets on discharge side was higher than normally observed. The

auditing team noted the scale losses are 1.2 % + 1.2 % .( Refer Column 4 and 5 )

From the above table it may kindly noted that the end cut loss is 2 %

This can be avoided by incorporating Bar Cutting Machine and eliminating gas

cutting process of Steel Ingots .

We can achieve the following .

( 1 ) Normally Gas cutting process material loss is 0.05 % per cut .

We are taking minimum 8 cut pieces to max 10 cut pieces from one

Bar steel .

Hence the losses will be = 0.05 x 8 = 0.4 % .

Incorporating bar Cutting machine will help us to save material of approx

0.5 % considering the whole volume of operation .

( 2 ) After incorporating bar cutting machine process the Ingot end will be

straight as well as square and the quality as well as square ness will be far

better than the Gas cutting

This will definitely ensure reduction of End cut which at present is 2 %

Also from the above table it may kindly noted that the scale loss is 2.4 %

The major reasons are:

i) Due to long period of delays due to system constraints, scale losses increases.

ii) The soaking zone temperature remains 12800C to 13000C. It should be reduced to

11900C.

iii) The excess oxygen remains 9.3%. It may be reduced to 6%.

Due to delays and idle running of furnace, plant authorities may target 1.5%. Some

reduction can be achieved by reducing soaking temperature and some by excess air.

Some reduced temperature will cause more load on rolling motors. As discussed in

chapter 2 on motors, they are capable for the same.

Summary of this paragraph is as follows:

Location Suggested Average in this plant Remarks

Soaking zone 1190 1280 to 1350. Average 1280

Reduce

Heating zone 950 1150 Reduce

Preheating zone 600 Gauge defective. Replace Gauge

Scale loss % 1 >3. Take 3 Reduce

The auditing team has observed that there is general tendency with workers in rolling

and forging mills to operate at higher temperature. There is generally a great

resistance to its lowering. We had been suggesting offering some incentive to

workers in rolling mill section to operate at about 1190 0C.

Saving and investment for the same is as follows:

Average ingots heated @ 2000 MT/ month- MT/ Year 24000

Desired temperature 1200

Actual temperature 1280

Specific heat of steel 0.12

Extra heat - Kcal*1000 230400

Furnace efficiency 34%

Equivalent oil – KL 67.8

Amount @ Rs.28000/ KL = Rs. Lacs 19

Running expenditure in form of incentive etc - Say Rs. Lacs 5

Net saving in furnace oil - Rs. lacs 14

Expenditure in the form of still better instruments= Rs. Lacs 3

Pay back period = Months 3

Additional advantage

Steel savable @ 0.5 % - MT 120

Cost @ Rs. 20/ kg -Rs. lacs 24

x) Proper heat distribution –

The furnace should be designed to heat maximum material uniformly. For this

following precautious are needed: -

• The flame should not touch the material or refractory. The obstruction de-

atomizes the fuel particles; increases scale losses & effects combustion

efficiency. For this, proper angle of burner should be kept. If flame touches

any part of furnace, it leaches the refractory.

• The flame should be contained within the furnace walls. Firing of excess

fuel, only marginally increases the production. The flames entering

chimney is wastage of fuel.

The position in this plant is as follows:

i. Yellowish orange or golden,, Bushy in shape ; Continuous. – The flame is

yellowish. It should be slightly bluish.

ii. Presence of dark streaks indicates incomplete combustion—There are no

dark streaks.

iii. Sparks in flame indicates that oil is in sufficiently heated - There are no

sparks.

iv. Incandescent, white & transparent flame indicates excess air. - The flame is

transparent indicating excess air

v. Flame should not strike material or refractory - It is not striking

xi) Heat absorbed by furnace walls –

In a batch type furnace, 30-40% heat input is wasted in furnace walls. This plant is

works continuously but due to system requirements, it has to run idle for 47% time.

So insulation should be more. This may be discussed with designers.

xii) Control of furnace draught –

The furnace should be kept air tight & at slightly higher pressure so that no infiltration

of out side air takes place. Studies carried out by PCRA on apparently airtight

furnace have shown air infiltration up to the extent of 40%. The disadvantages of air

infiltration are: -

i) Uncontrolled & excess air up to 40%

ii) Cold metal, where fresh out side air comes in contact with hot metal.

iii) Non -uniform metal temperature, which effect subsequent forging & rolling.

iv) Excess air causes more loose scale.

The furnace pressure in different parts should be constant. Minimum fuel

consumption occurs at slightly higher pressure. Ex filtration is less serious than

infiltration. However with much greater than atmosphere pressure, flames & hot

gasses would burst out of all openings, resulting in poor life of the doors and

refractoriness. It may be about 0.1 mmwg. The flue gas duct should be neither too

large nor too small. Large duct area may create negative pressure in the furnace

causing air infiltration from opening.

It is illustrated by following sketch-

In this plant, air pressure is controlled manually. But no infiltration was observed.

Exfiltration appeared on higher side. No pressure gauge is installed. The auditing

team has already suggested installation of ON line oxygen analyzer.

xiii Transfer of heat from flame to material-

Furnaces are designed for transfer of heat (%) as below:

Temperature 0C Mode of heat transfer

1250 900 400

By radiation 70 24.7 3.6

By convection 30 75.3 96.4

By conduction 0 0 1 to 2%

In this furnace, flames are not touching material. It is correct method is illustrated

below-

xiv) Alternate sources of energy: A comparative cost of different types of fuels is

given below:

Fuel Unit Kcal/Unit

Combustion Eff.%

Price/ Unit- Rs. KCal/Rs Remarks

Coal - Gassifier

l Kg 6000

60 (It includes gassifier

efficiency)

8.5 424

Cheapest mode. Many industries have installed gassifiers. Additional investment of ~ Rs. 1.5 crores

Natural Gas

M3 9500 75 17 419 Depends upon gas availability in area.

FO Lit 9660 75 28 259 Presently being used

Electrical heating

kWh 860 96 3.2 258 It would also save scale losses of about Rs. 72 lacs.

LPG Kg 11500 75 35 246 Costliest at present rates.

As illustrated above, coal gasification and use is cheapest. A working sketch of

gassifier is as follows:

xv) Miscellaneous –

No spacers are put between ingots. Heat at high temperature is transferred by

radiation. The areas touching each other do not get radiated heat.

Summary of saving potential

Saving potentials

F O Saving Investment P B P Sl.No. Item

KL Rupees Lakhs

1 Reducing excess air & heat recovery etc

157 43.98 5 1

2 Reducing furnace surface temperature

3.4 0.96 1 13

3

More closing of discharge and ejector doors by existing properly designed doors

9.3 2.61 0 0

4 Reducing furnace temperature 67.8 14 3 3

Gross Total 237.5 61.5 9

Since all will not happen simultaneously, assume 60% saving potential

142 40 9 3

Electrical saving potential

Saving potentials

KWH Saving Investment P B P Sl.No. Item

Rupees Lacs

1

Insulation of FO piping and service

tank

4539 0.142