Operating data for two pusher-type furnaces, old and new, of approximately identical length but having different longitudinal profiles were analysed. The old furnace had a partly inclined longitudinal profile and the new one had a rectangular longitudinal profile. Results of analysis showed a higher rate of specific fuel consumption for the old furnace than for the new one. To reduce the difference a possibility was considered of building in an air-preheating recuperator instead of relocating the burners and altering the longitudinal profile of the old furnace. Results of quantification demonstrated that by building a recuperator into the old furnace specific fuel consumption would greatly diminish and would become comparable to that of the new furnace.Analizirani su pogonski proizvodni zapisi dviju potisnih peći podjednake dužine ali različitog uzdužnog profila. Starija peć ima djelomično ukošeni uzdužni profil a novija peć pravokutni uzdužni profil. Rezultati analize su pokazali da starija peć ima veću specifičnu potrošnju goriva od novije peći. U cilju smanjenja navedene razlike razmatrana je mogućnost dogradnje rekuperatora za predgrijanje zraka umjesto analognog rješenja da se premještaju gorionici i promijeni uzdužni profil starije peći. Rezultati provedene kvantifikacije pokazuju da bi se dogradnjom rekuperatora na stariju peć znatno smanjila specifična potrošnja goriva i približila onoj novije peći

J. Črnko

M. Kundak

Đ. Nikolić

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METALURGIJA 41 (2002) 4, 301-304 301M. KUNDAK at all.: THE PROFILE OF A PUSHER-TYPE FURNACE AND FUEL CONSUMPTIONReceived - Primljeno: 2002-01-08Accepted - Prihvaćeno: 2002-29-05Original Scientific Paper - Izvorni znanstveni radISSN 0543-5846METABK 41 (4) 301 (2002)UDC - UDK 621.783.223.2.004.68=20. NDAK, J. ČRNKO, Đ. NIKOLIĆM. Kundak, J. Črnko, Đ. Nikolić, Faculty of Metallurgy, University ofZagreb, Sisak, CroatiaTHE PROFILE OF A PUSHER-TYPE FURNACE AND FUEL CONSUMPTIONINTRODUCTIONAt the strip and billet rolling mill of the Sisak Steel Worksthere are two pusher-type furnaces of comparable workinglength but with different longitudinal profiles. In the oldfurnace, which has a partly inclined longitudinal profile andthe burners built into all zones at the front, the rate of fuelconsumption was considerably higher than in the new fur-nace, which has a rectangular longitudinal profile and theburners installed laterally in all zones except in the charge-preheating zone. To cut down fuel consumption the burnersin the old furnace were relocated and laterally built into allzones with the exception of the charge-preheating zone. Inaddition to relocation of the burners a minor reconstructionof the furnace ground was undertaken but its inner profileremained unchanged. There was no alteration concerningthe system of natural gas supply to the burners; it operatedin accordance with the temperature of the walls which wasmeasured by means of zonal pyrometers. Those were placedOperating data for two pusher-type furnaces, old and new, of approximately identical length but having differentlongitudinal profiles were analysed. The old furnace had a partly inclined longitudinal profile and the new one hada rectangular longitudinal profile. Results of analysis showed a higher rate of specific fuel consumption for theold furnace than for the new one. To reduce the difference a possibility was considered of building in an air-preheating recuperator instead of relocating the burners and altering the longitudinal profile of the old furnace.Results of quantification demonstrated that by building a recuperator into the old furnace specific fuel consump-tion would greatly diminish and would become comparable to that of the new furnace.Key words: pusher-type furnace, longitudinal profile, built-in recuperator, fuel consumptionProfil potisne peći i potronja goriva. Analizirani su pogonski proizvodni zapisi dviju potisnih peći podjednakeduine ali različitog uzdunog profila. Starija peć ima djelomično ukoeni uzduni profil a novija peć pravokutniuzduni profil. Rezultati analize su pokazali da starija peć ima veću specifičnu potronju goriva od novije peći. Ucilju smanjenja navedene razlike razmatrana je mogućnost dogradnje rekuperatora za predgrijanje zraka umjestoanalognog rjeenja da se premjetaju gorionici i promijeni uzduni profil starije peći. Rezultati provedenekvantifikacije pokazuju da bi se dogradnjom rekuperatora na stariju peć znatno smanjila specifična potronjagoriva i pribliila onoj novije peći.Ključne riječi: potisna peć, uzduni profil, dogradnja rekuperatora, potronja gorivaat the end of the heating zone, in its upper and lower sec-tions, and in the preheating zone [1]. On the basis of theoperating data for the old furnace recorded before and afterrelocation of the burners the relationship between specificfuel combustion and furnace productivity was statisticallyanalysed. Results of analysis showed that following the re-location of the burners in the old furnace the rate of specificheat energy consumption increased instead of decreasing[1-2]. To solve the problem, instead of relocating the burn-ers and substituting the partly inclined longitudinal profilefor a rectangular longitudinal one, a possibility was consid-ered of building in an air-preheating recuperator thus re-ducing specific heat energy consumption.The amount of energy required for heating steel semi-products in pusher-type furnaces is part of overall energyconsumption in a rolling mill production. A relationshipbetween the rate of specific energy consumption and fur-nace productivity was determined by regression analysisusing the operating data. Specific fuel consumption basedon its heating value was recalculated in terms of specificheat energy consumption. Furnace productivity varied withthe pressure of natural gas, whereas distribution of heatMETALURGIJA 41 (2002) 4, 301-304302M. KUNDAK at all.: THE PROFILE OF A PUSHER-TYPE FURNACE AND FUEL CONSUMPTIONenergy by furnace zones remained stable. A change in therate of specific heat energy consumption in the course ofresumed operation following overhaul was assessed fromstatistical data collected for the period between two over-hauls. By structure, the heated semiproducts were slabshaving a thickness of 190 mm (65 %), and slabs and bloomswith a thickness exceeding 190 mm (35 %). In terms ofquality, they were made of low-carbon steel. The relation-ship between heat power, that is specific heat energy con-sumption, and furnace productivity was described by theauthors earlier [2, 3].RESULTS AND DISCUSSIONFor analysis of operation of pusher-type furnaces theoperating data from the strip and billet rolling mill wereused. The data pertained to the quality of semiproducts andtheir dimensions, to the charge heating temperature and tothe amount of natural gas of known heating value that wasused up. All data for a furnace and its period of operation(e. g. before and after reconstruction) were placed intogroups. Each group comprised only the pairs of data (xi, yi)referring to identical operating conditions in regard to chargedimensions, steel quality, heating temperature and durationof shift operation of the rolling mill stand. The selectedgroups of data pairs are shown as clusters of dots in Figure1. The dependence of specific heat energy consumption (y)on productivity (x) was determined by regression analysisusing the computer programs Statistica and Excel and isshown in the form of regression curves also in Figure 1.The regression curve C1 refers to the old furnace beforereconstruction and is defined by the equation:Y = 2895.5 e-0.0057 X (1)The regression curve C2 refers to the old furnace afterits reconstruction and is defined by the equation:Y = 3274.4 e-0.0068 X (2)The regression curve C3 refers to the new furnace, whereno changes were made, and is defined by the equation:Y = 2282.3 e-0.0047 X (3)The regression curve C4 refers to the old pusher-typefurnace before its reconstruction assuming that a recuperatorfor preheating air to a temperature of 350 °C has been builtin. In that way the heat effect was expected to increase by14.8 % and to match that of the new furnace [4].The regression curve C4 is based on the pairs of datathat were computed from those for the old furnace beforeits reconstruction and is defined by the equation:Y = 2467 e-0.0057 X (4)Figure 2. shows an increase in specific heat energy con-sumption in the period between two overhauls of the old(S1) and new (S2) furnaces. There is a continuous rise in therate of consumption (y) with time (x) i. e. with the approachof overhaul. It should be pointed out that pairs of data forgroups in Figure 1. were obtained from the operating datafor the first 150 working days after the overhaul of the oldand new furnaces. A rise in specific heat energy consump-tion in the period between two overhauls (360 days) wasdue to the falling of the refractory lining off the verticalFigure 1.Slika 1.Empirical regression curves as a result of linear correla-tion of measured pairs of data for the old furnace (C ), for the old furnace after reconstruction (C ), for the new fur-nace (C ) and for the old furnace with a built-in recupe-rator (C )Empirijske krivulje regresije kao rezultat linearne kore-lacije mjerenih parova podataka za stariju peć (C ), stariju peć poslije rekonstrukcije (C ), noviju peć (C ) i stariju peć s dograđenim rekuperatorom (C )123412 34Figure 2.Slika 2.Empirical regression curves as a result of linear corre-lation of measured pairs of data in the period between two overhauls for the old furnace after reconstruction (S ) and for the new furnace (S )Empirijske krivulje regresije kao rezultat linearne kore-lacije mjerenih parova podataka u periodu između dva remonta za stariju peć poslije rekonstrukcije (S ) i noviju peć (S )1212METALURGIJA 41 (2002) 4, 301-304 303M. KUNDAK at all.: THE PROFILE OF A PUSHER-TYPE FURNACE AND FUEL CONSUMPTIONstand pipes of the skid pipes in the first place and to thefalling of a major amount of scale off the surface of theheated steel in the second place [5-6]. Pairs of data given inFigure 2. demonstrate that a rise in the rate of specific heatenergy consumption in the old furnace following its recon-struction was higher than in the new furnace. The regres-sion line S1 which pertains to the old furnace after its recon-struction is defined by the equation:Y = 0.3149 X - 561.88 (5)The regression line S2 referring to the new furnace isdefined by the equation:Y = 0.3858 X - 510.92 (6)The correlation characteristics can be assessed bymeans of the criterion R2( )( )10  ,1 212122 ≤≤−−−=∑∑== RyyyyR niinitii(7)wheren - number of pairs of data,yi - measured value,yti - computed value,y - mean value.If R  is close to one, a link between X and Y is takento be very tight, whereas when R  is close to zero the linkis either very weak or non-existent. R  is equivalent to thecorrelation coefficient r , which is a measure of strengthof the link between X and Y (Table 1.).For groups of data referring to charges of variable thick-ness and to different heating capacities and inadequate tem-perature regimes the strength of the link between X and Yis satisfactory. With the help of equations (1) - (4) it isnow possible to evaluate the effect of realized heating ca-pacity on specific heat energy consumption. A rise in con-sumption along with a diminished realized heating capac-ity of the old furnace after reconstruction could be ex-pected. One of the reasons for that was a drop in heatingefficacity caused by an increased ratio between the heat-ing surface of a charge and the inside surface of the fur-nace walls (Y, [7]), which was due to the rising up of thefurnace ground level. The latter was the result of intensi-fied falling off to the ground of the refractory mass fromthe skid pipes and of the scale forming on the surface ofthe steel charge owing to inadequate temperature regimein the furnace [1]. The position of the regression curve C2above the regression curve C1  as shown in Figure 1. al-lows to conclude that the furnace reconstruction failedbecause the rate of specific heat energy consumption in-creased over the entire working space of the furnace. Thiswas taken to confirm the starting hypothesis that the spe-cific heat energy consumption in the old furnace couldbecome comparable to the one in the new furnace pro-vided an air-preheating recuperator, to serve for fuel com-bustion in the furnace working area, were built into theold furnace. In that case the regression curve C4 would lieimmediately above the regression curve C3 (Figure 1.).Each rise in the air-preheating temperature above 350 °Cwould reduce the distance between the regression curvesC4 and C3. This shows that the problem of reducing spe-cific heat energy consumption by furnace reconstructionwas not examined in sufficient detail and that not all pos-sibilities at disposal were analysed.In socialist countries, before the introduction of mar-ket economy, no serious consideration used to be given tothe costs of production. Failures have been known to oc-cur many of which are still waiting to be put right. Theconstruction of the above mentioned new pusher-type fur-nace stands as an example. The furnace was built for thepurpose of heating a larger amount of steel charge thanpreviously, because the capacity of the rolling mill standincreased in the meantime owing to reconstruction. Fromthe technological and economic standpoints the construc-tion of a new furnace was unnecessary, because it waspossible, by building a recuperator into the old furnace, toincrease its heating capacity by heating a continuously castcharge of appropriate thickness and by inserting it in thefurnace in a hot state. According to literature data [8] slabsof 150 mm thickness were more acceptable than those thatwere used in the old furnace until then. Inserting 150 mmthick slabs into the furnace would raise its heating capac-ity from the current 70 t/h to over 100 t/h. Specific heatenergy consumption would become lower by about 35 %as computed from the difference in consumption ratesobtained using equation (4) by inserting first one numeri-cal value for productivity and then the other. The requiredtemperature of the slabs before their processing by rollingwould be lower than the temperature for rolling thickerslabs, and that would additionally reduce specific heatenergy consumption. Apart from a drop in heat energy con-sumption in the course of heating, the power consumptionTable 1.Tablica 1.Numerical values of the criteria R  and R for regression curves defined by equations (1) to (6)2Brojčane vrijednosti kriterija  za krivulje regresije koje su definirane jednadbama (1) do (6)R  i R2METALURGIJA 41 (2002) 4, 301-304304M. KUNDAK at all.: THE PROFILE OF A PUSHER-TYPE FURNACE AND FUEL CONSUMPTIONduring the rolling operations would also diminish as a re-sult of fewer passes through a blooming mill. In general, theentire rolling mill production calls for optimum control ofenergy consumption as essential to maximum energy sav-ing [9-10]. A saving of heat energy of about 2.4 % reportedin literature [11] was a result of control of fuel combustionby means of continuous analysis of oxygen in waste fluegases. Depending on the temperature of waste flue gases upto 5 % of energy can be saved. Today, optimization of agiven production is facilitated with the use of computingtechnique. From the regression curves S1 and S2 (Figure 2.)which show the rate of specific heat energy consumption asrelated to its cost, it is possible to make an assessment of theprofitability of furnace overhauls that are aimed at remov-ing major amounts of scale and refractory lining [6], as wellas of the profitability of its reconstruction.CONCLUSIONThe justifiability of relocating the burners in the oldpusher-type furnace in order to reduce heat energy con-sumption was assessed. The burners were moved from thefront of the heating zone to the sides of the heating andpreheating zones, following the example of the new pusher-type furnace. Results of assessment were opposite to thoseexpected. Instead of decreasing the rate of specific heatenergy consumption increased. Investigation showed thatan air-preheating recuperator should have been built intothe old furnace. In that case specific heat energy consump-tion would have been comparable to the one in the newfurnace. A relationship between the length of time passingbetween two overhauls and the rate of specific heat en-ergy consumption was also investigated. Results showedthat specific heat energy consumption during operationfollowing overhaul increased as a function of time. Theincrease was mainly a result of falling of the refractorylining off the vertical stand pipes of the skid pipes in thefurnace heating zone owing to the smelting of scale and itsreaction with the lining. The amount of scale falling onthe furnace ground grew larger as a result of inadequatetemperature regime and of a greater length of time a chargewas held in the furnace due to unexpected interruptions ofREFERENCES1. J. Črnko, M. Kundak: Dependence of Heat Energy Consumptionon Location and Arrangment of Burners on Pusher-type Furnace,Kovine, zlitine in tehnologije, 29 (1995) 5-6, 549-553.2. J. Črnko, M. Kundak, Đ. Nikolić: The Change of Working Caracte-ristic of a Pusher Furnace in Conditions of New Location of the Burn-ers, Zbornik prispevkov z konferencie «Energeticke premeny vpriemysle», Technicka univerzita v Koicah, Koice, 1996,  99-104.3. J. Črnko, M. Kundak: Empiric Determination of the ConnectionBetween Heat Power and Productivity of a Pusher-type Furnace,Metalurgija, 36 (1997) 4, 211-214.4. M. Kundak, . Acs: Analysis of Heat Regime in Strip, Billet andSeamless Steel Tube Rolling Mills, Metalurgija, 15 (1976) 3-4, 17-22.5. M. Kundak, J. Črnko: Influence of the Scaling Upon the HeatingProcess of Steel Slabs in a Pusher-type Furnace, Kovine, zlitine intehnologije, 28 (1994) 4, 619-622.6. J. Črnko, M. Kundak, M. Gojić, . Acs: Structure and Composi-tion of Scale on Heated Blooms in a Pusher-type Furnace, Metalur-gija, 4 (2001) 4, 201-205.7. W. Heiliegenstaedt: Wärmetechnische rechnung für industrieöfen,Düsseldorf, 1966.8. M. Čauević: Metal Processing by Rolling, Veselin Maslea, Sara-jevo 1983.9. M. Kundak, Đ. Nikolić: Analysis of the State of Production of Seam-less Steel Tubes at the Sisak Steel Works, Metalurgija, 41 (2002) 1,43-46.10. P. Fleissig, F. N. Feet: Optimirung des Lastmanagements in einemHüttenwerk mit Hilfe eines Energiemodells, Stahl und Eisen, 118(1998) 2, 73-79.11. M. Calon, B. Dohm, R. Klima, D. Stroo: Energieeinsparung undQualitätsverbesserung durch Regelung der Verbrennungsgasatmo-sphäre in Wärmöfen, Stahl und Eisen, 121 (2001) 2, 75-80.12. M. Kundak, J. Črnko: Study of Adverse Effect of Steel Scaling onHeating and on Fuel Combustion Control, Kovine, zlitine in tehno-logije, 29 (1995) 5-6, 555-558.operation. Verification of the duration of the period be-tween two overhauls in terms of cost effectiveness is there-fore called for. By heating up a thin and hot charge in onefurnace instead of a thicker and cold charge in two fur-naces the amount of scale would be more than doubly re-duced [5, 12], because the slab temperature before rollingwould be lower and the heating time more than twiceshorter. In this manner a rise in the rate of specific heatenergy consumption as a function of time elapsing betweentwo overhauls would become lesser as would charge lossesdue to extensive scaling of iron from steel.

Profil potisne peći i potrošnja goriva

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