In this paper an experimental and empirical methods are proposed to estimate the heat transfer enhancement in industrial heating processes in oval furnaces. An investigation was conducted to study the suitability of inserting radiant panels of different positions and radiation surface. Two case studies were considered. The maximum energy saving was obtained for case 5: 32,89 % off from the standard experiment (with no panels). The minimum energy saving was obtained for case 10: 11,72 % off from the standard experiment (with no panels). Finally, based on the results of this study, a correlation was developed to predict the inner configuration of an oval furnace.U članku su predložene eksperimentalne i empirijske metode procjene povećanja prijenosa topline u industrijskom zagrijavanju u ovalnoj peći. Istraživanje je provođeno proučavanjem usklađivanja postavljenih radijacijskih panele na različita mjesta radijacijske površine. Dva položaja su uzeta u razmatranje. Maksimum očuvane energije je polučen za slučaj 5:32,89 % u odnosu na standardni eksperiment (bez panela). Minimum očuvane energije je polučen za slučaj 10:11,72% jednako u odnosu na standardni eksperiment (bez panela). Završno, na temelju rezultata ove studije razvijana he konfiguracija predkazivanja unutarnje konfiguracije ovalne peći

A. A. Minea

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473METALURGIJA 51 (2012) 4, 473-476A. A. MINEAEXPERIMENTAL AND EMPIRICAL TECHNIQUE TO ESTIMATE ENERGY DECREASING AT HEATING IN AN OVAL FURNACEReceived – Prispjelo: 2012-01-10Accepted – Prihvaćeno: 2012-04-20Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 51(4) 473-476 (2012)UDC – UDK 669.041:621.3.036=111A. A. Minea, Technical University Gheorghe Asachi from IASI, RomaniaIn this paper an experimental and empirical methods are proposed to estimate the heat transfer enhancement in industrial heating processes in oval furnaces. An investigation was conducted to study the suitability of inserting radiant panels of diff erent positions and radiation surface. Two case studies were considered. The maximum energy saving was obtained for case 5: 32,89 % off  from the standard experiment (with no panels). The minimum energy saving was obtained for case 10: 11,72 % off  from the standard experiment (with no panels). Finally, based on the results of this study, a correlation was developed to predict the inner confi guration of an oval furnace.Key words: oval furnace, heat transfer, experimental method, energy savingEksperimentalna i empirijska tehnika procjene smanjenja energije pri zagrijavanju u ovalnoj peći. U članku su predložene eksperimentalne i empirijske metode procjene povećanja prijenosa topline u industrijskom zagri-javanju u ovalnoj peći. Istraživanje je provođeno proučavanjem usklađivanja postavljenih radijacijskih panele na različita mjesta radijacijske površine. Dva položaja su uzeta u razmatranje. Maksimum očuvane energije je polučen za slučaj 5:32,89 %  u odnosu na standardni eksperiment (bez panela). Minimum očuvane energije je polučen za slučaj 10:11,72% jednako u odnosu na standardni eksperiment (bez panela). Završno, na temelju rezultata ove stud-ije razvijana he konfi guracija predkazivanja unutarnje konfi guracije ovalne peći.Ključne riječi: ovalna peć, prijenos topline, eksperimentalna metoda, očuvanje energijeINTRODUCTIONEnergy Effi ciency covers all changes that result in a reduction in the energy used for a given energy service or level of activity. This reduction in the energy con-sumption is not necessarily associated to technical changes, since it can also result from a better organiza-tion and management or improved economic effi ciency in the sector (e.g. overall productivity gains). Energy effi ciency is generally the largest, least expensive, most quickly deployable, least visible, least understood, and most neglected way to provide energy services [1].Process heating is vital to nearly all manufacturing processes, supplying heat needed to produce basic ma-terials and commodities. Industry’s heavy reliance on these processes creates a critical need to optimize their performance for improved energy effi ciency, and com-petitiveness [2 - 4]. Process heating systems are made up of four components including:• Heating devices that generate and supply heat;• Heat transfer devices to move heat from the source to the product;• Heat containment devices, such as furnaces, heaters, ovens, and kilns;• Heat recovery devices.For many industrial applications, 15 % -85 % of the energy supplied is used for heating the materials. Many factors, such as process temperature, equipment design and operation, and the type of heat recovery systems used, determine the energy effi ciency of a process heating sys-tem. Hence, industrial process heating systems offer op-portunities to save signifi cant amounts of energy. There-fore, an attempt to reduce the heating time will lead to signifi cant saving of energy in the condition that the qual-ity is guaranteed. However, currently, the thermal sche-dule for the heating up process of workpieces is mainly determined by experience in industrial production [5].In this paper, the main research objective is to estab-lish an experimental method and then to develop an ef-fi cient tool for the workpieces heating.METHODOLOGYThe chamber geometry re-design or improvement can potentially lead not only to energy saving but also reduced furnaces production costs. Previous studies as well as the larger community have already highlighted such potential. However, these studies were largely based on smaller scale furnaces [6]. There is a pressing need to expand previous research to industrial scale fur-naces and deliver novel design that will lead to signifi -cant reduction in energy consumption and environment footprint [7, 8].474  METALURGIJA 51 (2012) 4, 473-476A. A. MINEA: EXPERIMENTAL AND EMPIRICAL TECHNIQUE TO ESTIMATE ENERGY DECREASING AT HEATING...In this paper simplifi ed models based both on empiri-cal and experimental methods are proposed to estimate heat transfer enhancement in heating processes. The in-vestigations were completed to understand radiation or convection enhancement when panels are introduced. These experimental panels are showed in Figure 1.Hence the present work is carried out with the fol-lowing objectives:(i) Evaluation of two radiant panels surfaces: one regular and one with symmetrical holes (see Figure 1).(ii) Comparison of energy consumptions for the 12 considered cases.(iii) Assessment of heating behaviour for panels in-clination and surface variation.(iv) Empirical estimation of energy savings for pan-els inclination and surface variation.EXPERIMENTAL WORKThe investigations were started from the geometry of an oval 3 liter Barnstead (Dubuque, Iowa, USA) manu-factured furnace and different panels are introduced in the working area [9] in order to study its heating behav-iour. Experiments were carried out in the same conditions for every case with the same initial heating conditions. In this work, all the experiments were performed involving radiation in a closed domain and were compared with the standard heating regime (with no panels).The heated enclosure includes two metallic radiant panels with the dimensions and the confi guration de-picted in Figure 1, along with the experimental set-up. It is shown in Figure 1a that the decreasing of the radia-tion surface is made by perforating the panel. The holes position is shown in Figure 1a. Figure 1b contains the panels position and the inclination rule, along with the considered measuring points (where additional thermo-couples are inserted).The temperatures were recorded automatically using two K type thermocouples inserted in the furnace and sealed to prevent any leakage. Their accuracy is of ± 1,6 K at 773 K according to USA National Institute of Stand-ards and Technology (NIST). Respectively, furnace tem-perature control accuracy is ± 0,25 K and, further, ex-perimental system accuracy goes to ± 1,25 - ± 1,85 K, according to USA NIST. The experiments have been per-formed in different days, thus maintaining the initial heating conditions for the equipment and for the charges. A data-logger (Comark, type EV) was used to collect measurements at intervals throughout each experiment.For each panel position three experiments were con-ducted, in the tables being written the arithmetic aver-age of the registered values. In these practical condi-tions, the experimental investigation uncertainty is di-minished as much as is possible.The following two sets of experiments were carried out:1.  Heating with regular panels, at different inclina-tion angles, and2.  Heating with perforated panels, at different incli-nation angles.In this context, two variables were established: the panel inclination (α) and the panel surface (S). The ex-periment has been rigorously conducted in order to as-sure its repeatability. As a study charge, an AlMgSi Φ 0,024 x 0,1 m cylindrical part has been used. Table 1 depicts the furnace heating regime for all cases.Table 1  Furnace heating regime (up to 773 K) forall the experimental casespanel inc-lina-tion /degreepanel typecharge heating time / sec.furnace heating time / secenergy / Whenergy variation / Whwithout panels 1800 1459 7440 regular 1233 1121 509,6 113,7perfored 1508 1255 623,33,5 regular 1390 1070 574,5 41,3perfored 1490 1305 615,87 regular 1208 1092 499,3 123,5perfored 1507 1280 622,810,5 regular 1250 1140 516,6 137,7perfored 1583 1370 654,314 regular 1355 1030 560,0 96,7perfored 1589 1360 656,717,5 regular 1260 1116 520,8 132,2perfored 1580 1310 653,0DATA ANALYSISIn Table 2 the results for the centralized experiment are presented. The studied parameters were heating rate Figure 1  Experimental set-up: a. perforated panels; b. measuring points, panels and workpiece position in the furnacea.b.475METALURGIJA 51 (2012) 4, 473-476A. A. MINEA: EXPERIMENTAL AND EMPIRICAL TECHNIQUE TO ESTIMATE ENERGY DECREASING AT HEATING...and energy consumption. Heating rate was obtained em-pirical using the total heating time (provided in Table 1) and considering the fi nal temperature imposed at 773K.The results interpretation purpose was to fi nd an em-pirical model that can describe as precisely as possible the physical processes that take place in that situation. Thus, for fi tting the experimental and simulation data, a Table Curve 3D (Systat Software Inc. [SSI], San Jose, California, USA) commercial code was used. This pro-gram have wide capabilities to fi t the experimental data with different equations and to calculate all the numeri-cal data needed in order to establish its’ accuracy. Data analysis starts with the most trustable equations. Final results are presented on each diagram.Table 2 Centralized results of inserted panels experimentCase num-berpanel inclina-tion / degreespanel radiation surface / m2energy con-sumption / Wh1 0 0,029 509,642 0 0,026 623,313 3,5 0,029 574,534 3,5 0,026 615,875 7 0,029 499,316 7 0,026 622,897 10,5 0,029 516,678 10,5 0,026 654,319 14 0,029 560,0710 14 0,026 656,7911 17,5 0,029 520,8012 17,5 0,026 653,07Figure 2 contains all the experimental points consid-ered for the study and a comparison for the two consid-ered case studies. So, one can notice the differences in energy consumptions underlying the benefi ts of using the standard panels.Furthermore in Figures 3 and 4 is the statistical anal-ysis. Figure 3 represents the 3D representation of the energy consumption experimental data, which stood at the bases of this work, along with the fi tted surface. In Figure 4 are the heating rate experimental points and surface along with the polynomial fi tted surfaces. The points from Figure 4 are obtained by an empirical calcu-lus based on the heating time and the fi nal temperature imposed (773 K), as it was mentioned before.Figure 2 Experimental directions Figure 3 3D data fi tting for energy consumptionFigure 4 3D data fi tting for heating rateFigure 5 Energy consumption: Figure 3 extensionFurther comments will be made just on Figure 3 that is extended in Figure 5. So, it can notice an important energy decreasing if the panel inclination is increasing. These results are very useful but it has to consider the decreasing of heating space inside the furnace chamber.The hypothesis of inclining panels at angles larger than 25° have to consider the real heating space needed along with the number of heated workpieces. Also, good results it can be noticed for angles around 7° and this inclination do not minimize the heating available space inside the furnace, if it refers to Figure 5. Another ob-servation related to Figure 5 is about panel radiation surface, which can be correlated with the total energy consumption: it is optimum to use panels with low ra-diation surface or regular panels. But, if it refers on how 476  METALURGIJA 51 (2012) 4, 473-476A. A. MINEA: EXPERIMENTAL AND EMPIRICAL TECHNIQUE TO ESTIMATE ENERGY DECREASING AT HEATING...the radiation surface was decreased (see Figure 1) an important question arise about the panel resistance in service. A surface of about 0,016 m2 (if it consider re-sults from Figure 5) it means nearly half of the regular surface. This case will be considered for future studies.The fi tting surfaces were obtained by polynomial fi t-ting method and represent a correlation between panel position and heating rate and energy consumptions re-spectively. From statistical analysis on each graph it will consider the experimental curves:E= -1416,51-85,96α+11,23α2-0,56α3+ -0,009 α4+193761,35 S-4,13 106 S2 (1)v= 2,77-0,03 α+0,01 α2-0,001 α3+5,83 10-5 α4--197,18 S+3989,1 S2 (2)with the notations:E –  energy consumption for heating up to 773 K, Wh;v – heating rate, K/sS – panel surface, m2;α – panel inclination, degrees.Equation (1) gets a standard deviation R2 = 0,906 (obtained by computer fi tting through Gauss elimina-tion), and is very important because it gets a correlation between the consumed energy and panel’s position. This equation can be used according to technological needs refl ected in maximum heating rate and working parts dimensions. For heating rate a discussion was not included hence it was not measured, it was calculated based on the experimental data obtained. Moreover, if it refers to Figure 2 for every panel position, the lower energy consumption was observed for the regular pan-els, and the differences in some cases are quite high. For example, a medium value of 21,1 % energy saving was obtained for the regular panels compared to perforated ones. The physical explanation of this phenomenon is that by introducing the radiant panels inside the heating chamber, radiation heat transfer is more intense. But, when the panel surface is decreasing, the refl ected heat is getting lower, depending on the radiation surface. Also, if panel surface is getting very low, convection enhancement may interfere along with increased panel inclination mainly because air rate is increasing in the favor of air circulation through the depicted holes. This accounted for the effect of the fl ow-channel space around and through the panels which allows the heated air to by-pass the load, thus increasing the value of the mean air velocity inside the furnace.Furthermore, if refers to Figure 3, an optimum posi-tion can be estimated at 10 degrees inclination of the panel and S = 0,029 m2, value that corresponds to regu-lar panels. This graphical position corresponds to ener-gy consumption (E) lower than 500 Wh, if it refers to Figure 3, and a total energy reduction of 33 % from the non-panel situation. The same results it can be observed on the surface from Figure 4.CONCLUSIONSBoth, an empirical and an experimental method for heat transfer enhancement in the heating process in an oval furnace are presented.Two case studies of experimental panels are carried out and the effects of panels position and their radiation surface are evaluated. If it refers to Figure 2 for every panel position, the lower energy consumption was ob-served for the regular panels, and the differences in some cases are quite high. For example, a medium value of 21,1 % energy saving was obtained for the regular panels compared to perforated ones. The most suitable case, based on experimental, is for the inclined regular panels, obtaining an average of 21,1 % energy reduction. The maximum energy saving was obtained for case 5: 32,89 % off from the standard experiment (no panels). The minimum energy saving was obtained for case 10: 11,72 % off from the standard experiment (no panels).As a fi nal conclusion, a correlation was developed to predict the optimum chamber confi guration for heat transfer enhancement in an oval furnace.REFERENCES[1] E. Worrell, Encyclopedia of Energy: Industrial Energy Use, Status and Trends, Elsevier, 2004, USA[2] N. Depree, J. Sneyd, S. Taylor, M.P. Taylor, J.J. Chen, S. Wang, M. O’Connor, Development and validation of mo-dels for annealing furnace control from heat transfer fun-damentals, Computers and Chemical Engineering 34 (2010) 1849-1853[3] W. Eichhammer, Encyclopedia of Energy, Volume 3: Indu-strial Energy Effi ciency, Elsevier Inc, 2004, USA[4] M. K. White, S. D. Probert, Heat Transfers in Treatment Furnaces, Applied Energy 38 (1991) 293-323[5] J. Kang, Y. Rong, Modeling and simulation of load heating in heat treatment furnaces, Journal of Materials Processing Technology 174 (2006) 109–114[6] J.Yi, Z.H. Liu, J. Wang, Heat transfer characteristics of the evaporator section using small helical coiled pipe in a loo-ped heat pipe, Appl. Therm. Eng. 23 (2003) 89-99[7] A.A. Minea, Experimental and numerical analysis of heat transfer in a closed enclosure, Metalurgija 51 (2012) 2, 199-202[8] A. A. Minea, An Experimental Method to Decrease Hea-ting Time in a Commercial Furnace, Experimental Heat Transfer, Taylor and Francis, 23 (2010) 175-184Note:  Responsible translator: lecturer from Technical University Gh Asachi Iasi, Romania.

Eksperimentalna i empirijska tehnika procjene smanjenja energije pri zagrijavanju u ovalnoj peći

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