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Optimal cooling of rolls in hot rolling
Journal of Materials Processing Technology 125±126 (2002) 700±705
Optimal cooling of rolls in hot rolling M. Raudensky*, J. Horsky, M. Pohanka Heat Transfer and Fluid Flow Laboratory, Brno University of Technology, 616 69 Brno, Czech Republic Received 15 December 2001; accepted 12 February 2002
Abstract A laboratory experimental device was developed to allow full-scale measurements on roll cooling to be carried out. The full-scale tests use a complete con®guration of rows of nozzles as in the plant conditions or prepared by a designer. The laboratory approach allows the design to be optimised or to compare old and new solutions. The tests provide a distribution of cooling intensity (heat transfer coef®cient and heat ¯ux) at the roll surface. The second step of the optimisation process is the usage of a numerical model for computation of temperature and roll crown in a hot rolling regime. A industrial typical rolling schedule is used to check the ef®ciency of cooling. A typical application of the experimental±numerical procedure is in improvements of cooling, in intensi®cation of rolling and in design work. The paper shows how this approach can help in ``making a decision'' related to a prior plant application. Examples of the results and general recommendations for cooling are included. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Experimental methods; Cooling; Heat transfer; Rolling
1. Introduction Optimal cooling of rolls can be designed with regard to two aspects. The ®rst one is wear of a roll where high temperature decreases the durability of the surface layer. Roll producers usually prescribe demanding temperature limits. The second aspect is the thermal deformation of a roll. This is critical for shape and tolerances of ¯at products. Cooling sections at rolling mill should be designed considering both aspects. Numerical modelling of the thermal behaviour of rolls during rolling can be done with good accuracy. The level of knowledge of numerical methods for heat conduction in a roll and level of computers can make this problem a routine task. The temperature ®eld is known, so thermal deformation of rolls (roll crown) can be calculated. The only dif®cult part of this model is the knowledge of boundary conditions re¯ecting real physical reality with good accuracy. There is no analytical or a numerical method of how to predict the distribution of heat transfer coef®cient on the surface of the cooled roll knowing the spray conditions. The only possibility is to carry out the measurements. The experiments done under industrial conditions are rare and
* Corresponding author. E-mail address: [email protected] (M. Raudensky).
very expensive. This is not the testing way for new con®gurations of nozzles and in¯uence of particular parameters. Laboratory experiments are effective for ®nding description of cooling intensity. Experiments in roll cooling started at the Brno University of Technology in 1988. Initial tests were performed for a single nozzle. The test programme has continued with a row of nozzles [1]. It was increasingly obvious that the generalisation of the results for a complete cooling con®guration based on separate measurements of components of cooling sections does not bring reliable results. The only acceptable way was to prepare a full-scale experiment [2]. The full-scale experiment uses an identical con®guration of rows of nozzles as in rolling mill, the same pressures, velocities and coolant temperatures are used there. The cooling intensity described by heat transfer coef®cient distribution re¯ects the real mill conditions. An experimental method connected to numerical simulation provides a tool for cooling design and optimisation. 2. Experiment and simulation The experiments use 4 bars with nozzles as a maximum con®guration. Some experiments use a fewer number of bars. The nozzle manifold adjusts the distance between nozzles and roll surface to 100 mm. The mutual distance between bars with nozzles is 60 mm. The number of nozzles
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 3 1 0 - 2
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705
701
Fig. 1. Footprint of the impact areas at roll surface.
in 1 bar is ®ve and six, respectively. The footprint of the impact areas (Fig. 1) shows that the direct impact is concentrated only in circumferential angle from 808 to 1308. This represents the area from 57 to 227 mm in the x co-ordinate system where x 0 is surface point at horizontal axis of the roll. There are seven temperature sensors in a roll. The central sensor no. 4 is located in the centre of a roll width. The mutual distance of the thermal sensors is 55 mm. It is beyond scope of this paper to give the details about the test bench [3] and experimental procedure. Only basic information follows. The experiment starts by the heating of a test plate when roll is stationary. As soon as the initial temperature is reached the heater is removed, rotation starts and the pump is switched on with closed de¯ector. The de¯ector prevents the roll surface from spraying. The de¯ector is opened at the moment when the rotating test plate is in the top position. This mechanism ensures that the de¯ector opens and closes at exactly the same instant in all experiments. Temperatures and roll position is stored to a data logger that rotates with the roll. The data are moved to computer when the experiment has ®nished and the rotation stops. All the measured temperatures go through a standard inverse procedure [4]. Surface temperature, HTC and heat ¯ux are computed. Each data point carries information about position (angle). The data of the ``time'' order are converted to the position order. The position is connected with the geometry of the roll not with the positioning of the nozzles. A special program for interpolation of HTC by single curve has been designed. The program uses convolution with Gaussian distribution and exports vectors of HTC. The HTC distribution is used as the boundary condition for
the program CoolRoll [3]. This program simulates a rolling regime and provides temperatures ®elds and roll thermal deformation (roll crown). 3. Studied parameters of roll cooling The following questions are usually proposed when roll cooling is designed or optimised: What coolant pressure and ¯ow should be used? What type of nozzles should be used? What is an optimum con®guration of nozzles? How can good controllability of the roll crown be ensured? Will the present cooling be suf®cient when increasing production? The question concerning the in¯uence of pressure on cooling intensity is leading. This will not be discussed here, however, previous results are in [2]. All the results in this paper were obtained using nozzles with ¯at spray, spraying angle of 608, water ¯ow of 40 l/min at 5 bar. 3.1. Number of rows Some plants use either two, three or even four rows of nozzles at exit or entry side of rolls. How can adding of one row of nozzles intensify cooling? The basic experimental set-up uses four rows of nozzles (Fig. 1). There are three experiments where the number of working bars is gradually reduced. Experiments with 3, 2 and 1 bars are conducted. Fig. 2 shows the distribution of heat transfer coef®cient at the roll surface. Results from four experiments are plotted here to provide a direct comparison. The averaged values of HTC are computed in Table 1. The averaged HTC is for surface area from 250 to 1000 mm.
702
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705
Fig. 2. Influence of number of spray bars at heat transfer coefficient distribution.
It can be stated that the usage of two rows of nozzles, instead of one row, increases cooling intensity by a factor of approximately 2.5. The usage of 4 bar instead of 2 or 3 bars does not improve cooling linearly. Real in¯uence on roll crown will be even smaller because of surface sub-cooling as has been explained in part 3.4.
8000 W/m2 K. The second conclusion is more interesting. Increase of the spray distance has little effect on maximum values of HTC. The coverage of the roll by water from distant sprays is better and the HTC curve is wider. Numerical results using averaged values of HTC are shown in Table 2.
3.2. Distances
3.3. Sectional cooling
Should the nozzles be placed close to the roll surface or is a distant position better? Two ``distance'' cases are tested. The ®rst one is 100 mm from the roll surface. The second one uses the same con®guration of nozzles, but the distance from the roll surface is increased to 200 mm in horizontal direction (i.e. the manifold with all nozzles is shifted out of the roll in horizontal direction). Only the distance of group of nozzles has been changed, other parameters remains the same. Fig. 3 shows four curves. There are two pressures for distance 100 mm and two pressures for distance of 200 mm. Each distance was measured for two pressures. The ®rst conclusion can be for water pressure. Increasing this from 4.5 to 8 bar increased the maximum value of HTC by
Rolling of ¯at products requires a width strategy for cooling. Cooling can be either ®xed and independent from the width of rolled material or sectional, when the sections or nozzles are controlled by switching or by setting pressure. There is a question when is it possible to choose a technically complicated arrangement with spray control? A case study showing the importance of proper width strategy of cooling was prepared for this paper. Real cooling conditions of the HSS roll used for hot rolling of steel sheet were used. The roll has diameter of 760 mm, width of 2100 mm. The roll is cylindrical at initial temperature of 20 8C. Cooling is constant during all simulated rolling cases. It is assumed that nozzles cover 1000 mm of central part of the roll. Outside of this central part is the roll cooled only by re¯ected water and HTC is decreasing linearly to the roll edge. The rolling schedule is composed of three widths of sheet (see Table 3). It is obvious from Table 3, and above description of cooling, that the width strategy of cooling is not suitable for rolling of sheet of 2000 mm wide. Fig. 4 shows the thermal deformation of the roll (thermal crown) for an 1 h period. Rolling of wide sheet case shows
Table 1 Averaged values of heat transfer coefficient for 1, 2, 3 and 4 spray bars Number of spray bars used in experiment
Averaged HTC (W/m2 K) Averaged HTC (%)
1
2
3
4
4785 100
11600 242
14660 306
17975 375
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705
703
Fig. 3. Influence of nozzles distance at heat transfer coefficient distribution. Table 2 Averaged values of heat transfer coefficient for two pressures and two distances from roll surface Water pressure (bar) Spray distance (mm) Averaged HTC (W/m2 K) Averaged HTC (%)
0.45 0.45 0.80 0.80 100 200 100 200 17975 20150 20230 21650 100 112 100 107
Table 3 Rolling campaign for simulation of width strategy Time of rolling (h)
Width of sheet (mm)
Reduction (mm)
0±2 2±4 4±6
1000 2000 600
18 18 18
Fig. 4. Thermal deformation of roll with improper distribution of cooling.
704
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705 Table 4 Temperatures at roll after 2 h of rolling in dependence on position of spraying
Temperature in roll axis (%) Maximum surface temperature at entry to the rolling gap (%) Temperature in depth of 100 mm (%)
Cooling at exit
Cooling at entry
100 100
114.3 123.2
100
120.6
Fig. 5. HTC distribution in scheme of cooling: (a) cooling at entry side; (b) cooling at exit side.
overheating of the edge parts of the roll. It should be mentioned that similar cooling defect could occur when some nozzles are clogged. 3.4. Distribution of water on roll surface Even if the limits for water pressure and ¯ow rate are ®xed the thermal balance could be seriously in¯uenced by the positioning of sprays on the roll circumference. Heat transfer intensity depends on both, heat transfer coef®cient magnitude and difference between temperatures of surface and coolant. The surface temperature is highly transient and it is important when the spray hits the surface [5]. This can be a problem of the most effective positioning of several rows of nozzles on the roll circumference. Fig. 5 shows two con®gurations for this case study.
Both con®gurations use identical distribution of HTC measured for a single row of nozzles but the cooling is for the ®rst case, located at the entry side and, for the second case, at exit side of the upper roll. Two simulations of rolling of aluminium were done. The simulations used realistic rolling schedule recorded at a mill. The results are summarised in Table 4 and Fig. 6. Table 4 shows temperature differences after 2 h of rolling. The values are valid for the roll centre. It can be seen that positioning of cooling plays the major role in thermal balance. Cooling near to the rolling gap is much intensive due to much higher surface temperature of the roll. Difference in roll crown is even more instructive. The results in Fig. 6 are for roll diameter of 900 mm, roll width of 3000 mm and width of rolled material of 1500 mm. Even though the example in this paragraph is far from rolling practice, the results demonstrate the major role of spray distribution.
Fig. 6. Roll crown for two positions of bar of nozzles.
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705
4. Conclusion Full-scale experiments on roll cooling in combination with numerical simulation of rolling provide a tool for cooling design and optimisation. Older experiments con®rmed that usage of superposition for complex sprays composed from simple units (single nozzle or single bar of nozzles) is not possible. Only experiments with complete cooling con®guration, spraying of a test roll rotating by proper speed, could get a suf®ciently precise description of heat transfer. Results based on experiments with a different number of rows of nozzles showed how the cooling effect could increase with an increasing number of rows. The experiments with the different distances of nozzles from the roll surface con®rm more that there is more ef®cient cooling for distant sprays. This observation was con®rmed for both, low and medium coolant pressures. Numerical simulations of rolling show high sensitivity of the roll crown to the distribution of cooling intensity along the length of roll. Simulations show the sensitivity of roll crown to the clogging of nozzles. It has been shown that the identical cooling, located at different positions of roll, has
705
completely different impact on thermal balance and roll crown. All the presented results should support the conclusion that cooling could not be optimised using only industrial or laboratory experiments or using only numerical models. Both factors must be taken into consideration to gain the cooling optimisation. References [1] M. RaudenskyÂ, J. HorskyÂ, Experimental study of roll cooling by means of flat and cone water jets, in: Proceedings of the First International Conference on Modelling of Metal Rolling Processes, London, UK, pp. 660±667. [2] L. Bendig, M. RaudenskyÂ, J. HorskyÂ, Measurement of heat transfer coefficients of spray nozzles for roll cooling and their application in mathematical modelling, ROLLS 2000, Advances in Mill Roll Technology, Birmingham, 1999, pp. 257±266. [3] Experimental data for rolling technology. http://ktermo41.fme.vutbr.cz/heatlab/. [4] M. RaudenskyÂ, Heat transfer coefficient estimation by inverse conduction algorithm, Int. J. Num. Meth. Heat Fluid Flow 3 (1993) 257±266. [5] M. Carnogurska, Temperature field of locally heated rod with discrete changes of diameter, Acta Mech. Slovaca 1 (2000) 69±76.
Optimal cooling of rolls in hot rolling M. Raudensky*, J. Horsky, M. Pohanka Heat Transfer and Fluid Flow Laboratory, Brno University of Technology, 616 69 Brno, Czech Republic Received 15 December 2001; accepted 12 February 2002
Abstract A laboratory experimental device was developed to allow full-scale measurements on roll cooling to be carried out. The full-scale tests use a complete con®guration of rows of nozzles as in the plant conditions or prepared by a designer. The laboratory approach allows the design to be optimised or to compare old and new solutions. The tests provide a distribution of cooling intensity (heat transfer coef®cient and heat ¯ux) at the roll surface. The second step of the optimisation process is the usage of a numerical model for computation of temperature and roll crown in a hot rolling regime. A industrial typical rolling schedule is used to check the ef®ciency of cooling. A typical application of the experimental±numerical procedure is in improvements of cooling, in intensi®cation of rolling and in design work. The paper shows how this approach can help in ``making a decision'' related to a prior plant application. Examples of the results and general recommendations for cooling are included. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Experimental methods; Cooling; Heat transfer; Rolling
1. Introduction Optimal cooling of rolls can be designed with regard to two aspects. The ®rst one is wear of a roll where high temperature decreases the durability of the surface layer. Roll producers usually prescribe demanding temperature limits. The second aspect is the thermal deformation of a roll. This is critical for shape and tolerances of ¯at products. Cooling sections at rolling mill should be designed considering both aspects. Numerical modelling of the thermal behaviour of rolls during rolling can be done with good accuracy. The level of knowledge of numerical methods for heat conduction in a roll and level of computers can make this problem a routine task. The temperature ®eld is known, so thermal deformation of rolls (roll crown) can be calculated. The only dif®cult part of this model is the knowledge of boundary conditions re¯ecting real physical reality with good accuracy. There is no analytical or a numerical method of how to predict the distribution of heat transfer coef®cient on the surface of the cooled roll knowing the spray conditions. The only possibility is to carry out the measurements. The experiments done under industrial conditions are rare and
* Corresponding author. E-mail address: [email protected] (M. Raudensky).
very expensive. This is not the testing way for new con®gurations of nozzles and in¯uence of particular parameters. Laboratory experiments are effective for ®nding description of cooling intensity. Experiments in roll cooling started at the Brno University of Technology in 1988. Initial tests were performed for a single nozzle. The test programme has continued with a row of nozzles [1]. It was increasingly obvious that the generalisation of the results for a complete cooling con®guration based on separate measurements of components of cooling sections does not bring reliable results. The only acceptable way was to prepare a full-scale experiment [2]. The full-scale experiment uses an identical con®guration of rows of nozzles as in rolling mill, the same pressures, velocities and coolant temperatures are used there. The cooling intensity described by heat transfer coef®cient distribution re¯ects the real mill conditions. An experimental method connected to numerical simulation provides a tool for cooling design and optimisation. 2. Experiment and simulation The experiments use 4 bars with nozzles as a maximum con®guration. Some experiments use a fewer number of bars. The nozzle manifold adjusts the distance between nozzles and roll surface to 100 mm. The mutual distance between bars with nozzles is 60 mm. The number of nozzles
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 3 1 0 - 2
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705
701
Fig. 1. Footprint of the impact areas at roll surface.
in 1 bar is ®ve and six, respectively. The footprint of the impact areas (Fig. 1) shows that the direct impact is concentrated only in circumferential angle from 808 to 1308. This represents the area from 57 to 227 mm in the x co-ordinate system where x 0 is surface point at horizontal axis of the roll. There are seven temperature sensors in a roll. The central sensor no. 4 is located in the centre of a roll width. The mutual distance of the thermal sensors is 55 mm. It is beyond scope of this paper to give the details about the test bench [3] and experimental procedure. Only basic information follows. The experiment starts by the heating of a test plate when roll is stationary. As soon as the initial temperature is reached the heater is removed, rotation starts and the pump is switched on with closed de¯ector. The de¯ector prevents the roll surface from spraying. The de¯ector is opened at the moment when the rotating test plate is in the top position. This mechanism ensures that the de¯ector opens and closes at exactly the same instant in all experiments. Temperatures and roll position is stored to a data logger that rotates with the roll. The data are moved to computer when the experiment has ®nished and the rotation stops. All the measured temperatures go through a standard inverse procedure [4]. Surface temperature, HTC and heat ¯ux are computed. Each data point carries information about position (angle). The data of the ``time'' order are converted to the position order. The position is connected with the geometry of the roll not with the positioning of the nozzles. A special program for interpolation of HTC by single curve has been designed. The program uses convolution with Gaussian distribution and exports vectors of HTC. The HTC distribution is used as the boundary condition for
the program CoolRoll [3]. This program simulates a rolling regime and provides temperatures ®elds and roll thermal deformation (roll crown). 3. Studied parameters of roll cooling The following questions are usually proposed when roll cooling is designed or optimised: What coolant pressure and ¯ow should be used? What type of nozzles should be used? What is an optimum con®guration of nozzles? How can good controllability of the roll crown be ensured? Will the present cooling be suf®cient when increasing production? The question concerning the in¯uence of pressure on cooling intensity is leading. This will not be discussed here, however, previous results are in [2]. All the results in this paper were obtained using nozzles with ¯at spray, spraying angle of 608, water ¯ow of 40 l/min at 5 bar. 3.1. Number of rows Some plants use either two, three or even four rows of nozzles at exit or entry side of rolls. How can adding of one row of nozzles intensify cooling? The basic experimental set-up uses four rows of nozzles (Fig. 1). There are three experiments where the number of working bars is gradually reduced. Experiments with 3, 2 and 1 bars are conducted. Fig. 2 shows the distribution of heat transfer coef®cient at the roll surface. Results from four experiments are plotted here to provide a direct comparison. The averaged values of HTC are computed in Table 1. The averaged HTC is for surface area from 250 to 1000 mm.
702
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705
Fig. 2. Influence of number of spray bars at heat transfer coefficient distribution.
It can be stated that the usage of two rows of nozzles, instead of one row, increases cooling intensity by a factor of approximately 2.5. The usage of 4 bar instead of 2 or 3 bars does not improve cooling linearly. Real in¯uence on roll crown will be even smaller because of surface sub-cooling as has been explained in part 3.4.
8000 W/m2 K. The second conclusion is more interesting. Increase of the spray distance has little effect on maximum values of HTC. The coverage of the roll by water from distant sprays is better and the HTC curve is wider. Numerical results using averaged values of HTC are shown in Table 2.
3.2. Distances
3.3. Sectional cooling
Should the nozzles be placed close to the roll surface or is a distant position better? Two ``distance'' cases are tested. The ®rst one is 100 mm from the roll surface. The second one uses the same con®guration of nozzles, but the distance from the roll surface is increased to 200 mm in horizontal direction (i.e. the manifold with all nozzles is shifted out of the roll in horizontal direction). Only the distance of group of nozzles has been changed, other parameters remains the same. Fig. 3 shows four curves. There are two pressures for distance 100 mm and two pressures for distance of 200 mm. Each distance was measured for two pressures. The ®rst conclusion can be for water pressure. Increasing this from 4.5 to 8 bar increased the maximum value of HTC by
Rolling of ¯at products requires a width strategy for cooling. Cooling can be either ®xed and independent from the width of rolled material or sectional, when the sections or nozzles are controlled by switching or by setting pressure. There is a question when is it possible to choose a technically complicated arrangement with spray control? A case study showing the importance of proper width strategy of cooling was prepared for this paper. Real cooling conditions of the HSS roll used for hot rolling of steel sheet were used. The roll has diameter of 760 mm, width of 2100 mm. The roll is cylindrical at initial temperature of 20 8C. Cooling is constant during all simulated rolling cases. It is assumed that nozzles cover 1000 mm of central part of the roll. Outside of this central part is the roll cooled only by re¯ected water and HTC is decreasing linearly to the roll edge. The rolling schedule is composed of three widths of sheet (see Table 3). It is obvious from Table 3, and above description of cooling, that the width strategy of cooling is not suitable for rolling of sheet of 2000 mm wide. Fig. 4 shows the thermal deformation of the roll (thermal crown) for an 1 h period. Rolling of wide sheet case shows
Table 1 Averaged values of heat transfer coefficient for 1, 2, 3 and 4 spray bars Number of spray bars used in experiment
Averaged HTC (W/m2 K) Averaged HTC (%)
1
2
3
4
4785 100
11600 242
14660 306
17975 375
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705
703
Fig. 3. Influence of nozzles distance at heat transfer coefficient distribution. Table 2 Averaged values of heat transfer coefficient for two pressures and two distances from roll surface Water pressure (bar) Spray distance (mm) Averaged HTC (W/m2 K) Averaged HTC (%)
0.45 0.45 0.80 0.80 100 200 100 200 17975 20150 20230 21650 100 112 100 107
Table 3 Rolling campaign for simulation of width strategy Time of rolling (h)
Width of sheet (mm)
Reduction (mm)
0±2 2±4 4±6
1000 2000 600
18 18 18
Fig. 4. Thermal deformation of roll with improper distribution of cooling.
704
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705 Table 4 Temperatures at roll after 2 h of rolling in dependence on position of spraying
Temperature in roll axis (%) Maximum surface temperature at entry to the rolling gap (%) Temperature in depth of 100 mm (%)
Cooling at exit
Cooling at entry
100 100
114.3 123.2
100
120.6
Fig. 5. HTC distribution in scheme of cooling: (a) cooling at entry side; (b) cooling at exit side.
overheating of the edge parts of the roll. It should be mentioned that similar cooling defect could occur when some nozzles are clogged. 3.4. Distribution of water on roll surface Even if the limits for water pressure and ¯ow rate are ®xed the thermal balance could be seriously in¯uenced by the positioning of sprays on the roll circumference. Heat transfer intensity depends on both, heat transfer coef®cient magnitude and difference between temperatures of surface and coolant. The surface temperature is highly transient and it is important when the spray hits the surface [5]. This can be a problem of the most effective positioning of several rows of nozzles on the roll circumference. Fig. 5 shows two con®gurations for this case study.
Both con®gurations use identical distribution of HTC measured for a single row of nozzles but the cooling is for the ®rst case, located at the entry side and, for the second case, at exit side of the upper roll. Two simulations of rolling of aluminium were done. The simulations used realistic rolling schedule recorded at a mill. The results are summarised in Table 4 and Fig. 6. Table 4 shows temperature differences after 2 h of rolling. The values are valid for the roll centre. It can be seen that positioning of cooling plays the major role in thermal balance. Cooling near to the rolling gap is much intensive due to much higher surface temperature of the roll. Difference in roll crown is even more instructive. The results in Fig. 6 are for roll diameter of 900 mm, roll width of 3000 mm and width of rolled material of 1500 mm. Even though the example in this paragraph is far from rolling practice, the results demonstrate the major role of spray distribution.
Fig. 6. Roll crown for two positions of bar of nozzles.
M. Raudensky et al. / Journal of Materials Processing Technology 125±126 (2002) 700±705
4. Conclusion Full-scale experiments on roll cooling in combination with numerical simulation of rolling provide a tool for cooling design and optimisation. Older experiments con®rmed that usage of superposition for complex sprays composed from simple units (single nozzle or single bar of nozzles) is not possible. Only experiments with complete cooling con®guration, spraying of a test roll rotating by proper speed, could get a suf®ciently precise description of heat transfer. Results based on experiments with a different number of rows of nozzles showed how the cooling effect could increase with an increasing number of rows. The experiments with the different distances of nozzles from the roll surface con®rm more that there is more ef®cient cooling for distant sprays. This observation was con®rmed for both, low and medium coolant pressures. Numerical simulations of rolling show high sensitivity of the roll crown to the distribution of cooling intensity along the length of roll. Simulations show the sensitivity of roll crown to the clogging of nozzles. It has been shown that the identical cooling, located at different positions of roll, has
705
completely different impact on thermal balance and roll crown. All the presented results should support the conclusion that cooling could not be optimised using only industrial or laboratory experiments or using only numerical models. Both factors must be taken into consideration to gain the cooling optimisation. References [1] M. RaudenskyÂ, J. HorskyÂ, Experimental study of roll cooling by means of flat and cone water jets, in: Proceedings of the First International Conference on Modelling of Metal Rolling Processes, London, UK, pp. 660±667. [2] L. Bendig, M. RaudenskyÂ, J. HorskyÂ, Measurement of heat transfer coefficients of spray nozzles for roll cooling and their application in mathematical modelling, ROLLS 2000, Advances in Mill Roll Technology, Birmingham, 1999, pp. 257±266. [3] Experimental data for rolling technology. http://ktermo41.fme.vutbr.cz/heatlab/. [4] M. RaudenskyÂ, Heat transfer coefficient estimation by inverse conduction algorithm, Int. J. Num. Meth. Heat Fluid Flow 3 (1993) 257±266. [5] M. Carnogurska, Temperature field of locally heated rod with discrete changes of diameter, Acta Mech. Slovaca 1 (2000) 69±76.