Title of Invention	AN IMPROVED ROLE COOLING SYSTEM WITH IMPROVED HEAT TRANSFER COEFFICIENT AND A METHOD OF ACHIEVING IMPROVED ROLL COOLING FOR MULTISTAND COLD MILL
Abstract	This invention relates to an improved roll cooling system and an improved method of roll cooling for enhancing roll life by improved heat transfer for Multi-Stand cold Mill. The improved roll cooling system with improved heat transfer coefficient comprises selective nozzle means ; means for regulated spray conditions ; means for regulated coolant flow distribution to thereby achieve maintaining substantially constant heat transfer coefficient at impact area during cooling operation. The system is directed to meet increasing requirements on the geometric quality parameters like strip thickness, width, profile and flatness of CR Strips for superior strip profile and flatness for trouble free downstream processing such as cold rolling, coating and press forming. The system would provide for improved roll cooling system which would facilitate enhancing roll life especially for multi-stand cold mills. In particular the system achieves the following advantageous feature/characteristics : I) Achieving optimum impact density of the coolant spray-impingement onto the roll for maximised heat transfer effect. Versatility of thermal control in relation to coolant quantity, flow and placement, to cater all present and future possible rolling conditions. II) To achieve optimum Heat Transfer Coefficient (HTC) and reduce the roll surface temperature as well as premature roll thermal failure. III) Optimum utilisation of coolant for effective roll cooling. The invention is also related to an improved method of roll cooling using the improved system of the present invention.

Title of Invention

AN IMPROVED ROLE COOLING SYSTEM WITH IMPROVED HEAT TRANSFER COEFFICIENT AND A METHOD OF ACHIEVING IMPROVED ROLL COOLING FOR MULTISTAND COLD MILL

Abstract

This invention relates to an improved roll cooling system and an improved method of roll cooling for enhancing roll life by improved heat transfer for Multi-Stand cold Mill. The improved roll cooling system with improved heat transfer coefficient comprises selective nozzle means ; means for regulated spray conditions ; means for regulated coolant flow distribution to thereby achieve maintaining substantially constant heat transfer coefficient at impact area during cooling operation. The system is directed to meet increasing requirements on the geometric quality parameters like strip thickness, width, profile and flatness of CR Strips for superior strip profile and flatness for trouble free downstream processing such as cold rolling, coating and press forming. The system would provide for improved roll cooling system which would facilitate enhancing roll life especially for multi-stand cold mills. In particular the system achieves the following advantageous feature/characteristics : I) Achieving optimum impact density of the coolant spray-impingement onto the roll for maximised heat transfer effect. Versatility of thermal control in relation to coolant quantity, flow and placement, to cater all present and future possible rolling conditions. II) To achieve optimum Heat Transfer Coefficient (HTC) and reduce the roll surface temperature as well as premature roll thermal failure. III) Optimum utilisation of coolant for effective roll cooling. The invention is also related to an improved method of roll cooling using the improved system of the present invention.

Full Text	FIELD OF THE INVENTION This invention relates to an improved roll cooling system and an improved method of roll cooling for enhancing roll life by improved heat transfer for Multi-Stand cold Mill. To achieve desired shape of cold rolled (CR) Strips is of vital importance during cold rolling process at Multi-Stand Cold Mill (MCM). Due to increasing requirements on the geometric quality parameters like strip thickness, width, profile and flatness of CR Strips for superior strip profile and flatness for trouble free downstream processing such as cold rolling, coating and press forming. Modern Cold Mill uses technological control system to obtain close tolerances for thickness, width, profile and flatness for the strip body as well as for the entire length. BACKGROUND OF THE INVENTION Achieving desired shape of cold rolled strips is of vital importance during cold rolling process at Multi-Stand Cold Mill (MCM). It is essential to analyse and correct any flatness deviation that might occur by uneven thermal gradient of work rolls (thermal crown). It is therefore imperative that efficient roll cooling and lubrication systems are essential to control the thermal crown and improve the overall performance of the mill. It is known to install the mill with roll bending controlled through water based hydraulics system. In the previous hydraulic system 5% hydraulic oil is fixed with 95% water in lieu of 100% hydraulic oil. The system is also coupled with auxiliary drives of the mill like transfer car, carriage, guides, etc. Due to the use of water in the system leads to frequent jamming hydraulic piston cylinder and leakage of hoses results to improper control of roll bending system. Initially the mill was designed with same amount of coolant flow to the work rolls and back-up rolls of the mill and also each stands (5-stands) was having equal amount of coolant flow distribution. Initially all the headers are fitted with screw-in type flat jet nozzle are having two zone control with 3 nozzles at the edge zone and 6 nozzles at the central zone catering half of the central portion of the roll barrel length. All the five stands were catered by 4 pumps running of flow rate 5300 Ipm @ 10 bar of each pump It was resulted with higher roll surface temperature with higher thermal camber during the rolling of thinner gauge (25 mm) steel strips. It is also known to provide modified roll coolant system by putting additional 4 pumps, out of which 5 pumps adapted to run and remaining pumps kept as spare and also changing of the roll cooling header configuration. All the headers were fitted with 16 Nos. screw in type flat jet nozzle per header for more coolant discharge to the roll to control thermal camber of roll. As a result, due to the increase in number of nozzles, the coolant discharge flooded the roll with reduced impinging coolant pressure and led to poor heat transfer effect of rolls and higher premature failure (spalling) of rolls. The screw-in type nozzles also did not ensure align coolant spray onto rolls. By way of extensive studies of conventional roll cooling system presently available following deficiencies of the existing system are identified : i) Problems of uniform and equal coolant distribution system to each stand ; ii) Non-aligned coolant spray over rolls due to the use of screw-in nozzles; iii) Use of excess number of nozzles leading to flooding and wastage of coolant emulsion ; iv) Problems of equal coolant distribution for both back-up rolls and work rolls ; v) Poor heat transfer coefficient due to low coolant impinging force and pressure; vi) Higher roll surface temperature ; vii) Higher temperature difference between the centre and edge of rolls ; viii) Higher premature roll thermal failure. OBJECTS OF THE INVENTION It is thus the basic object of the present invention to provide for an improved roll cooling system which would avoid the above discussed drawbacks/limitations of the known cooling systems presently available. % Another object of the present invention is directed to provide for an improved roil cooling system which would facilitate enhancing roll life especially for multi-stand cold mills. Yet further object of the present invention is directed to providing for an improved roll cooling system which would achieve the following advantageous feature/characteristics : i) Achieving optimum impact density of the coolant spray-impingement onto the roll for maximised heat transfer effect. ii) Versatility of thermal control in relation to coolant quantity, flow and placement, to cater all present and future possible rolling conditions. iii) To achieve optimum Heat Transfer Coefficient (HTC) and reduce the roll surface temperature as well as premature roll thermal failure. iv) Optimum utilisation of coolant for effective roll cooling. Yet further object is directed to provide an improved method of roll cooling using the improved system of the present invention. SUMMARY OF THE INVENTION Thus according to the present invention there is provided an improved roll cooling system with improved heat transfer coefficient comprising : selective nozzle means ; means for regulated spray conditions ; means for regulated coolant flow distribution to thereby achieve maintaining substantially constant heat transfer coefficient at impact area during cooling operation. 3 Importantly, in the above disclosed improved roll cooling system of the invention the following modified features are incorporated : i) Variable coolant flow distribution stand-wise proportional to stand speeds. ii) Using concept of intense cooling of work rolls and mild cooling for back-up rolls. iii) Using higher number of nozzles for work roll cooling than back-up rolls. iv) Reducing number of nozzles for increased coolant pressure and higher impinging force. v) Using dove-tail type flat jet nozzles for ensuring aligned coolant spray pattern onto the roll surface. vi) Atleast 60% overlap of spray to avoid dry spotting on roll due to nozzle choking. vii) Effective header design in relation to coolant application and versatility, robustness to withstand impact, ease of maintenance and inter-changability. viii) Ease and simplicity of control of the system to achieve maximum benefits and retrofitting to the existing system. In accordance with yet further aspect of the present invention there is provided a method of achieving improved roll cooling comprising steps for effecting maximum heat transfer by selective use of nozzle means, regulating spray conditions and coolant flow distribution such as to maintain substantially constant heat transfer coefficient at impact area through cooling operations. The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to non-limiting exemplary illustrations and the accompanying figures wherein Fig. 1 illustrates heat transfer coefficient at the work roll surface with spray cooling; 4 ' Fig. 2 illustrates water flow density at the impact area of different types of nozzles; Fig. 3 is an illustration of heat transfer coefficient in the impact area as function of water flow density; Fig. 4 is an illustration of heat transfer coefficient as a function of pressure ; Fig. 5 is an illustration of heat transfer coefficient as a function of distance ; Fig. 6 is an illustration that heat transfer coefficient as a function of spray angle ; Fig. 7 is a schematic illustration of convention roll coolant header arrangement in multi stand unit. Fig. 8 is a schematic illustration of modified roll coolant header arrangement in multi stand unit. Fig. 9 is a schematic illustration of pumping scheme of emulsion system. Fig. 10 is a modified header arrangement at stands. The modified coolant system of the invention was developed incorporating the following features : a) Designing of modified coolant system for achieving higher HTC. b) Variable coolant flow distribution onto roll surface. c) Modified coolant application header configuration retrofitting to existing system and ease of maintainability. (i) Achieving maximum HTC : The important criterion for achieving maximum HTC are proper nozzle selection and the spray condition. Nozzle Selection : To maintain nearly constant HTC on the impact area (Fig. 1), the nozzle should be as large as possible and the coolant flow density at the impact area should be nearly constant. It has been identified that the flat jet nozzles are having maximum HTC as compared to square and oval nozzle (Fig. 3). The coolant flow density at the impact areas is achieved nearly equal to the 5 width of spray in flat jet compared to other nozzles (Fig. 2) and having maximum length-width ration for the impact area (15:1) in comparison to oval nozzle (5:1) and square nozzle (1:1). Thus flat jet nozzles were identified to be suitable for achieving maximum HTC with relatively low coolant pressure and has been selected for the modified roil coolant system. Spray Condition : Basically the spray condition depended upon three basic parameters like coolant pressure, spray angle and the spray height i.e. the distance between nozzle tip and roll surface. It has been observed that with increase in coolant pressure, there was increase in HTC upto a pressure limit 15 bar (Fig. 4) but in the modified system by reducing total number of nozzles, the coolant pressure has been increased from 2.1 bar to 4 2 bar, which clearly indicated the improved HTC with the modified coolant system. It has been also identified that lesser spray distance resulted in higher HTC (Fig. 5). There is a mere change in HTC, if the spray distance increases beyond 200 mm. In the modified system, the spray distance is restricted to 183 mm for top work roll cooling header and 152 mm for bottom work roll cooling header, due to space constraint. The distance of the coolant spray should be perpendicular to the roll surface. If the impinging spray angle (ß) is more than 15°, an increasing disturbance of the coolant flow at the impact area takes place, and hence cooling efficiency decreases (Fig. 6). In the modified system the impinging spray angle has been kept at 15° for top work roll (WR) compared to 18° earlier and 25° for bottom WR compared to 30° earlier (Figs. 7 & 8). (ii) Coolant flow distribution : The volume of coolant used for roll cooling varies, depending on the type of mill. However, the pressure requirements are constant and generally ranged between 3 and 7 bar. This range is perfectly acceptable for the application of coolant in cold mills, with a recommended value of approximately 5 to 6 bar. As the volume and pressure of the coolant are directly related to the impact density of the spray and in turn to the heat transfer efficiency, a minimum of 3 bar is need to achieve any thermal response to the sprays. 6 However, the benefit of increased pressure decreases rapidly above 7 bar, thereby making its use unjustified. With regard to the general aspects of roll cooling, hot rolled strip entering for stand # 1 to stand # 5 in a 5-stand cold mill, the roll cooling pattern in each of stands #1 to #4 play a vital role for controlling the strip shape. Mostly, the coolant application are neglected at these stands, and very efficient roll cooling was carried out. By optimising the aspect of the rolling process, not only with the objective of improving product quality, but for achieving significant benefits, the coolant flow distribution is optimised stand-wise in accordance with stand speed, roll force and percentage reduction pattern. In the modified system design, coolant flow distribution was varied standwise with respect to mill speed and roil force. There is a gradual increment of flow from stand #1 to stand #5 (Figs. 7 & 8). (iii) Roll coolant header configuration : As discussed above the sprays are selectively provided to impinge as precisely and accurately as possible onto the roll for maximised effectiveness, which directly depends on the positioning of the headers within the mill fixtures. This optimal location has to take into account existing mill furniture, movements of entry guides and bridles and access of services coolant piping, air and electrical hoses and ease of accessibility for maintenance. A considerable amount of effort is required to complete the flow calculations on the roll coolant system. Piping must be sized and routed so as to eliminate pressure losses. Spray headers must be chambered to accommodate these flow rates without restrictions. The spray headers are zoned into three zones - centre and edge. The number of nozzles at the centre zone is just double the number of nozzles at the edge zones to maintain balanced and uniform flow rate through each nozzles, as because the inlet connections to the centre zone and two edge zones are having equal branching pipe lines of 100 NB from main delivery pipe line of 200 NB from pump-set. Both the back-up and work roll spray headers (top and bottom) are having balanced and equal flow distribution and volume on each stands. In the modified roll coolant system, the roll coolant headers (Fig. 8) are arranged taking into following considerations : 7 i) Header location at the mill housing ii) Nozzle configuration for spray pattern iii) Zonal cooling of rolls iv) Retrofitting to existing system v) Ease of maintenance. Roll coolant headers : The top work roll coolant headers are box-type and fitted on the hold down board at the entry side of stands with three zones - centre and edge. The spray height is maintained within 183 mm. The bottom work roll coolant headers are made up from 50 NB pipe with three zones for similar flow distribution like top work roll. These headers are of fixed type having spray height of 152 mm. All the back-up roll coolant headers are made up from 50 NB pipe with three zones and are fixed type. The spray is oriented towards the bite of work roll and back-up roll to facilitate lubricity at the bite and to take out the heat from rolls. All these roll coolant headers are provided with protection plates and bottom flushing port in each zones for ease of periodic maintenance and are made retrofitting to the previous coolant supply connections. Nozzle configuration : The spray nozzles are selected of dove-tail type flat jet nozzles. The dove-tail type ensures perfectly aligned spray pattern onto the roll surface and facilitates periodic cleaning of nozzles. The nozzles are fitted on a single row to the spray header with an off-set angle of 15° in order to avoid interference of the adjacent spray and to maintain perfectly aligned herringbone spray pattern for effective cooling of rolls. The spray angle of nozzle is maintained at 60° for work rolls and 90° for the back-up rolls for more spray width coverage and helped to reduce the number of nozzles and to provide more intense cooling i.e. high flow density to work rolls. In accordance with another aspect of the invention the variable flow distribution stand-wise are being introduced with the number of nozzles which in turn depend on the inter spacing of the nozzles per header. All the back-up roll coolant header are provided with eight numbers of nozzles with 4 nozzles at centre zone and 2 nozzles at each edge zones. For stand 1 & 2, 12 number of nozzles are provided 8 on the work roll coolant header with 6 nozzles at centre zone and 3 nozzles at edge zone. For stand 3, 16 number of nozzles are provided on the work roll coolant header with 8 nozzles at the centre zone and 4 nozzles at the edge zone. For stand 4 & 5, 20 number of nozzles are provided on the work roll coolant header with 10 nozzles at the centre zone and 5 nozzles at the edge zone (Fig. 10). All the nozzles are fitted to the header with a minimum overlap of 40% to maximum 70% in order to avoid dry spot running of work rolls due to nozzle choking. All the nozzles are kept of same flow rate for ease maintenance and inventory. Preferred Nozzle Specification : Nozzle for Back-up roll coolant header: Type - Dove-tail flat jet Flow rate - 99.6 Ipm at 5 bar Spray angle - 90° Spray width - 525 mm at a height of 250 mm Bore diameter - 10.0 F mm Narrowest cross-section - 6.4 F mm Make - M/s Lechler (India) Pvt. Ltd. Part No. (Nozzle tip) - 665.126.17 Part No. (Nozzle nipple) - 066.419.17 Part No. (Threaded cap) - 065.600.17 Nozzles for Work roll coolant header: Type - Dove-tail flat jet Flow rate - 99.6 ipm at 5 bar Spray angle - 60° 9 Spray width 340 mm at a height of 250 mm Bore diameter - 10.0 F mm Narrowest cross-section - 7.4 F mm Make - M/s Lechler (India) Pvt. Ltd. Part No. (Nozzle tip) - 665.126.17 Part No. (Nozzle nipple) - 066.419.17 Part No. (Threaded cap) - 065.600.17 Tests were carried out to confirm the advantages in the modified roll coolant system (Fig. 8 & 9) vis-a-vis conventional roll cooling system at stands units. The system performance for a period of 6 months was evaluated and following results had been achieved. In the modified design coolant flow distribution was redistributed stand-wise (Fig. 8) to achieve optimum heat transfer effect and to avoid flooding and wastage of coolant. First redistribution was carried out by reducing the coolant flow at back-up rolls and saved coolant was used for intensive cooling at work rolls as per Table-1 hereunder for achieving higher HTC. Table - 1 : Emulsion flow distribution at roll cooling headers of TEM II Stand No. Roll position Before Modification at 2.1 bar pressure After Modification at 4.2 bar pressure Flow rate (Ipm) No. of Nozzles Flow rate (lpm) No. of Nozzles Stand # 1 Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll 1355 988 1355 1355 21 12 21 21 730 1095 1095 730 8 12 12 8 Stand # 2 Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll 775 988 775 775 12 12 12 12 730 1095 1095 730 8 12 12 8 10 Stand No. Roll position Before Modification at 2.1 bar pressure After Modification at 4.2 bar pressure Flow rate (Ipm) No. of Nozzles Flow rate (Ipm) No. of Nozzles Stand # 4 Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll 1355 988 1355 1355 21 12 21 21 730 1825 1825 730 8 20 20 8 Stand # 5 Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll 1355 988 1355 1355 21 12 21 21 730 1825 1825 730 8 20 20 8 The maximum heat is being generated at the roll-bite due to cold reduction of strip and the deformation work i.e. friction. This generated heat is taken by strip, coolant and the major part by the work rolls, this gradual heat input to the work roils are to be efficiently taken out by the roll cooling system or other-wise the work roll will be subjected to premature thermal failure. A nominal part of heat is being conducted to the back-up rolls and the frictional effect between the back-up roll and work roll generates comparatively low heat input to the back-up rolls, therefore the cooling of back-up roll is not so vigorous as the work roll. If the work roll temperature is controlled effectively, it will help to reduce the heat flow to the back-up rolls, therefore in the modified coolant system, the coolant flow distribution for back-up rolls are reduced drastically (to nearly half) and the curtailed coolant flow was distributed to work rolls for intense cooling. The total coolant flow distribution for work rolls before the modification was 10813 Ipm and after redistribution the total flow was 14500 Ipm. Similarly the total coolant flow for BUR cooling was 12390 Ipm which was reduced to 7300 Ipm after modification (Table-1). In the original design the total number of nozzles fitted in all stands were 343 numbers (Table -1). The total flow rate of emulsion in all stands were 23203 Ipm and having low emulsion pressure of 2:1 bar due to higher number of nozzles with 4 nos. of pump running resulting to low impact force and low heat transfer effect from work rolls. In the modified design, the total number of nozzles were reduced from 343 to 240 to enhance the emulsion pressure, which increases the impact 11 force of spray to improve HTC. In the modified design the coolant pressure was achieved of 4.2 bar with the 4 pumps running. The modified roll coolant headers for BUR and work rolls are erected at all the five stands of the multi-stand unit as per new system of stage-wise as per Table -2 hereunder: Table - 2 : Stage-wise flow distribution of emulsion at TCM-ll, BSL Stand No. Roll Position Before modification at #1,2,3,4 & 5 (at 2.1 bar) After modification at # 4 (at 2.4 bar) After modification at # 1, 2 & 4 (at 3.4 bar) After modification at #1,2,3,4 & 5 (at 4.2 bar) #1 Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll 1355 988 1355 1355 1448 1056 1448 1448 640 960 960 960 730 1095 1095 730 #2 Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll 775 988 775 775 828 1056 828 828 640 960 960 960 730 1095 1095 730 #3 Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll 1355 988 1033 1355 1448 1056 1104 1448 1672 1220 1275 1672 730 1460 1460 730 #4 Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll 1355 988 1355 1355 554 1385 1385 554 640 1600 1600 640 730 1825 1825 730 #5 Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll 1355 988 1355 1355 1448 1056 1448 1448 1672 1220 1672 1672 730 1825 1825 730 In the first stage, the modified coolant headers for work roll are installed with intense cooling, using 8 nozzles for edges cooling and 12 nozzles for centre cooling, whereas the back-up roll coolant headers are installed with single row cooling using 8 nozzles, at stand #4. With the introduction of modified system at stand #4, the performance has resulted in marginal enhancement in emulsion pressure to 2.4 bar with slight drop in work roll surface temperature by 5°C. in the second stage, all the modified coolant headers are installed at stand #1 & 2 with 12 the single row cooling, using 12 nozzles per header for work roll cooling, whereas the back-up roll coolant headers are installed with 8 nozzles per header. With the introduction of modified system at stand #1 & 2, the performance has resulted in significant enhancement in emulsion pressure to 3.4 bar with drop in work roll surface temperature by 7°C. In the final stage, the modified coolant headers are installed at stand #3 & 5 with the single row cooling. The modified coolant headers are installed at stand #3, using 16 nozzles per header for work roll cooling, whereas the back-up roll coolant headers are installed with 8 nozzles per header. The modified coolant headers are installed at stand #5, using 20 nozzles per header for work roll cooling, whereas the back-up roll coolant headers are installed with 8 nozzles per header. With the introduction of modified system at stand #3 & 5, the performance has resulted in significant enhancement in emulsion pressure to 4.2 bar with drop in work roll surface temperature by 10°C (avg.) and flattened roll thermal profile by reducing the total number of nozzles (Table - 1). Use of dove-tail type flat jet nozzles has ensured perfectly aligned herringbone spray band are improved heat transfer rate from rolls, the nozzles are fitted to the coolant header with an off-set angle of 15 to 20% to ensure no interference of two successive spray band and can take care of effective and higher heat removal rate from each spray band. The modified system has resulted in reducing average roll surface temperature from 70°C to 60°C (avg.). The temperature difference between the centre and edge of the roll has reduced from 10°C to 5°C (avg.) and resulted in flattened roll thermal profile as per Table - 3 hereunder which in turn helped to improve the strip profile/shape. 15 Table - 3 : Average Temperature of work roll surface Stand No. Roll Position Temp. Diff. ?T (°C) Avg. Roll Temp. (°C) Roll Temperature (Along Barrel Length) (X) #5 Before modification TWR BWR TWR BWR 13 11 4 4 67 66 61 60 56 56 60 58 63 66 61 59 69 67 63 61 67 65 61 60 58 60 59 57 After Modification #4 Before modification TWR BWR TWR BWR 13 9 5 4 74 69 60 59 63 63 57 56 74 67 59 58 76 70 62 60 72 68 60 59 63 61 58 57 After Modification #3 Before Modification TWR BWR TWR BWR 10 10 5 5 66 66 60 59 59 59 56 55 63 64 58 56 69 68 61 60 66 63 60 59 60 58 57 56 After Modification #2 Before Modification TWR BWR TWR BWR 9 7 5 4 67 65 59 58 59 61 56 55 65 64 58 57 68 67 61 59 67 62 59 58 60 60 58 57 After Modification #1 Before Modification TWR BWR TWR BWR 6 4 3 2 59 58 56 54 54 55 54 54 57 57 54 54 60 59 57 55 58 58 56 54 56 56 55 53 After Modification The spray angle of nozzles for work roll cooling header was kept at 60° for higher impact force of the coolant. The nozzles are interspaced in such a manner such that the overlap of the two adjacent spray can have atleast more than 50% in order to avoid dry spot rolling due to nozzle choking. Whereas the nozzles for back-up roll cooling header was designed with higher spray angle of 90° for the use roll cooling with lesser number of nozzles and with an overlap of atleast 40% of spray width. The number of nozzles selected for each stand were selected with respect to stand-wise speed and roll force. This redistribution of coolant has also resulted to reduce the premature work roll thermal failure from 57 nos. to 27 nos. per annum. The effective modified roll cooling system also helped to reduce the finishing coil temperature from 130°C to 110°C (avg) and also reduced the specific roll consumption from 1.98 to 1.36 Kg/T and specific rolling oil consumption from 1.33 to 0.85 lit/T as per Table - 4 hereunder: 14 Table - 4 : Specific consumption of rolls and oil Period Number of Roll Spalled Specific Roll Consumption (Kg/T) Specific Oil Consumption (Lit/T) Jan '99 6 1.80 0.6 Feb '99 3 1.45 0.67 Mar '99 5 1.73 0.69 Apr '99 1 1.83 0.78 May '99 2 2.07 1.31 Jun '99 1 0.84 1.01 Jul '99 5 1.34 0.97 Aug '99 3 2.0 0.96 Sep '99 5 1.44 0.66 Oct '99 2 1.36 0.61 Nov '99 1 1.44 0.90 Dec '99 0 0.88 0.75 Jan '00 2 0.97 0.68 Feb '00 4 1.16 0.98 Mar '00 1 1.1 0.85 1998-99 57 1.98 1.33 1999-2000 27 1.36 0.85 It is thus possible by way of the modified roll coolant of the present invention to achieve the following : a) Improved heat transfer effect with the help of perfectly aligned spray band onto the rolls ; b) Possible use of lesser number of nozzles to enhance the coolant pressure at the nozzle tip from 2.1 bar to 4.2 bar with the use of 4 pump running and improved HTC effect. c) The redistribution of coolant flow distribution stand-wise which avoided flooding and wastage of coolant and made effective cooling of rolls and has enhanced intensive cooling of the work rolls. d) Reduced roll surface temperature from 70°C to 60°C (avg.) and also reduced the temperature difference between centre to edge from 10°C to 5°C (avg) Table-3) which has resulted in improved flattened roll thermal profile and improved strip shape/flatness on account of roll thermal crown. 15 e) Improved heat transfer removal rate from work rolls and has resulted to reduced premature roll thermal profile from 57 numbers to 27 numbers per annum. f) Reduce the strip cooling temperature from 130°c to 110°C (avg) with reduced specific roll consumption by 35% and specific rolling oil consumption by 30%. g) Improve the overall mill performance with respect to productivity and quality. 16 WE CLAIM 1. An improved roll cooling system with improved heat transfer coefficient for multi stand cold mill comprising: selective nozzle means ; means for regulated spray conditions ; means for regulated coolant flow distribution to thereby achieve maintaining substantially constant heat transfer coefficient at impact area during cooling operation. 2. An improved roll cooling system as claimed in claim 1 wherein said selective nozzle means comprise large nozzle adapted for maintaining substantially constant coolant flow density. 3. An improved roll cooling system as claimed in anyone of claims 1 or 2 wherein said selective nozzle comprise flat jet nozzles. 4. An improved roll cooling system as claimed in anyone of claims 1 to 3 wherein said coolant flow density at the impact area is achieved nearly equal to the width of spray in flat jet and having a maximum length - width ratio for the impact area of 15 : 1. 5. An improved roll cooling system as claimed in anyone of claims 1 to 4 wherein said means for regulated spray conditions comprise means for monitoring basic parameters comprising coolant pressure, spray angle and spray height. 6. An improved roll cooling system as claimed in anyone of claims 1 to 5 wherein the spray distance is maintained upto 183 mm for top work roll cooling header and upto 152 mm for bottom work roll cooling header. 17 7. An improved roll cooling system as claimed in anyone of claims 1 to 6 wherein the impinging spray angle is maintained upto 15° for top work roll and upto 25° for bottom work roll. 8. An improved roll cooling system as claimed in anyone of claims 1 to 7 wherein said means for coolant flow distribution comprise means for maintaining the, coolant pressure in the range of 3 to 7 bar. 9. An improved roll cooling system as claimed in anyone of claims 1 to 8 wherein said spray headers are provided at optimum location based on existing mill infrastructure, movement of entry guides and bridles and access of services cooling piping, air and electrical hoses and ease of accessibility for maintenance. 10. An improved roll cooling system as claimed in anyone of claims 1 to 9 wherein said spray headers are chambered to accommodate the flow rates without restriction. 11. An improved roll cooling system as claimed in anyone of claims 1 to 10 wherein said spray headers are provided in zones comprising a central zone and two edge zones on either sides of the central zone wherein the number of nozzles at the central zone is double the number of nozzles at the edge zones such as to maintain balanced and uniform flow rate through each nozzle. 12.An improved roll cooling system as claimed in anyone of claims 1 to 11 wherein said central zone and edge zone nozzles are provided with equal branching pipe lines from a main delivery pipe line operatively connected to a pump set. 13.An improved roll cooling system as claimed in anyone of claims 1 to 12 wherein the back up and work roll spray headers have balanced and equal flow distribution and volume on each stand. 18 14.An improved roll cooling system as claimed in anyone of claims 1 to 13 wherein the roll coolant headers art 18 d based on header location at the mill housing, nozzle configuration for spray patterns, zonal cooling of rolls, retrofitting to existing system and ease of maintenance. 15. An improved roll cooling system as claimed in anyone of claims 1 to 14 wherein the top work roll cooling header are box type and fitted on the hold down board at the entry side of stands with said three zones. 16.An improved roll cooling system as claimed in anyone of claims 1 to 15 wherein the spray is adapted to be oriented towards the bite of roll works and back up roll to facilitate lubricity at the bite and to take out the heat from rolls. 17. An improved roll cooling system as claimed in anyone of claims 1 to 16 wherein said roll coolant headers are provided with protection plates and bottom flushing port in each zones for ease of periodic maintenance and are made retrofitting to the previous coolant supply connections. 18. An improved roll cooling system as claimed in anyone of claims 1 to 17 wherein said spray nozzles are selected of dove-tail type flat jet nozzles. 19.An improved roll cooling system as claimed in anyone of claims 1 to 18 wherein said nozzles are fitted on a single row to the spray header with an off set angle of 15° to thereby avoid interference of the adjacent spray and to maintain perfectly aligned herringbone spray pattern for effective cooling of rolls. 20.An improved roll cooling system as claimed in anyone of claims 1 to 19 wherein the spray angle of nozzle is maintained at 60° for work rolls and 90° for the back-up rolls for more spray width coverage. 19 21.An improved roll cooling system as claimed in anyone of claims 1 to 20 comprising means for variable flow distribution stand-wise comprising varying number of nozzles and on the inter spacing of the nozzles per header. 22.An improved roll cooling system as claimed in anyone of claims 1 to 21 wherein said nozzles are fitted to the header with a minimum overlap of 40% to maximum 70% in order to avoid dry spot running of work rolls due to nozzle choking. 23.An improved roll cooling system as claimed in anyone of claims 1 to 22 wherein all said nozzles are of same flow rate. 24, A method of achieving improved roll cooling for multi stand cold mill comprising steps for effecting maximum heat transfer by selective use of nozzle means, regulating spray conditions and coolant flow distribution such as to maintain substantially constant heat transfer coefficient at impact area through cooling operations. 25. A method as claimed in claim 24 wherein the coolant flow density at the impact area is maintained substantially constant. 26.A method as claimed in anyone of claims 24 or 25 wherein the coolant pressure is maintained in the range of 4.0 to 4.5 bar preferably 4.2 bar, the spray distance is maintained in the range of upto 183 mm for top work roll cooling header and upto 152 mm for bottom work roll cooling header and the distance of the coolant spray is maintained to the roll surface. 27.A method as claimed in anyone of claims 24 to 26 wherein the impinging spray angle is kept upto 15° for top work roll and 22° to 26° preferably 25° for bottom work roll. 28.A method as claimed in anyone of claims 24 to 27 wherein the coolant pressure is maintained in the range of 3 to 7 bar. 20 29. A method as claimed in anyone of claims 25 to 28 wherein the roll surface temperature is maintained in the range of 58°C to 62°C preferably 60°C and the temperature difference between centre to edges is maintained in the range of 6°C to 3°C preferably 5°C. 30. An improved roll cooling system with improved heat transfer coefficient for multi stand cold mill substantially as herein described and illustrated with reference to the accompanying examples and figures. 31.A method for achieving improved roll for multi stand cold mill cooling substantially as herein described and illustrated with reference to the accompanying examples and figures. Dated this 26th day of March 2001. 21 This invention relates to an improved roll cooling system and an improved method of roll cooling for enhancing roll life by improved heat transfer for Multi-Stand cold Mill. The improved roll cooling system with improved heat transfer coefficient comprises selective nozzle means ; means for regulated spray conditions ; means for regulated coolant flow distribution to thereby achieve maintaining substantially constant heat transfer coefficient at impact area during cooling operation. The system is directed to meet increasing requirements on the geometric quality parameters like strip thickness, width, profile and flatness of CR Strips for superior strip profile and flatness for trouble free downstream processing such as cold rolling, coating and press forming. The system would provide for improved roll cooling system which would facilitate enhancing roll life especially for multi-stand cold mills. In particular the system achieves the following advantageous feature/characteristics : I) Achieving optimum impact density of the coolant spray-impingement onto the roll for maximised heat transfer effect. Versatility of thermal control in relation to coolant quantity, flow and placement, to cater all present and future possible rolling conditions. II) To achieve optimum Heat Transfer Coefficient (HTC) and reduce the roll surface temperature as well as premature roll thermal failure. III) Optimum utilisation of coolant for effective roll cooling. The invention is also related to an improved method of roll cooling using the improved system of the present invention.

Full Text

FIELD OF THE INVENTION
This invention relates to an improved roll cooling system and an improved method of roll cooling for enhancing roll life by improved heat transfer for Multi-Stand cold Mill. To achieve desired shape of cold rolled (CR) Strips is of vital importance during cold rolling process at Multi-Stand Cold Mill (MCM). Due to increasing requirements on the geometric quality parameters like strip thickness, width, profile and flatness of CR Strips for superior strip profile and flatness for trouble free downstream processing such as cold rolling, coating and press forming. Modern Cold Mill uses technological control system to obtain close tolerances for thickness, width, profile and flatness for the strip body as well as for the entire length.
BACKGROUND OF THE INVENTION
Achieving desired shape of cold rolled strips is of vital importance during cold rolling process at Multi-Stand Cold Mill (MCM). It is essential to analyse and correct any flatness deviation that might occur by uneven thermal gradient of work rolls (thermal crown). It is therefore imperative that efficient roll cooling and lubrication systems are essential to control the thermal crown and improve the overall performance of the mill. It is known to install the mill with roll bending controlled through water based hydraulics system. In the previous hydraulic system 5% hydraulic oil is fixed with 95% water in lieu of 100% hydraulic oil. The system is also coupled with auxiliary drives of the mill like transfer car, carriage, guides, etc. Due to the use of water in the system leads to frequent jamming hydraulic piston cylinder and leakage of hoses results to improper control of roll bending system.
Initially the mill was designed with same amount of coolant flow to the work rolls and back-up rolls of the mill and also each stands (5-stands) was having equal amount of coolant flow distribution. Initially all the headers are fitted with screw-in type flat jet nozzle are having two zone control with 3 nozzles at the edge zone and 6 nozzles at the central zone catering half of the central portion of the roll

barrel length. All the five stands were catered by 4 pumps running of flow rate 5300 Ipm @ 10 bar of each pump It was resulted with higher roll surface temperature with higher thermal camber during the rolling of thinner gauge (25 mm) steel strips.
It is also known to provide modified roll coolant system by putting additional 4 pumps, out of which 5 pumps adapted to run and remaining pumps kept as spare and also changing of the roll cooling header configuration. All the headers were fitted with 16 Nos. screw in type flat jet nozzle per header for more coolant discharge to the roll to control thermal camber of roll. As a result, due to the increase in number of nozzles, the coolant discharge flooded the roll with reduced impinging coolant pressure and led to poor heat transfer effect of rolls and higher premature failure (spalling) of rolls. The screw-in type nozzles also did not ensure align coolant spray onto rolls.
By way of extensive studies of conventional roll cooling system presently available
following deficiencies of the existing system are identified :
i) Problems of uniform and equal coolant distribution system to each stand ;
ii) Non-aligned coolant spray over rolls due to the use of screw-in nozzles; iii) Use of excess number of nozzles leading to flooding and wastage of
coolant emulsion ; iv) Problems of equal coolant distribution for both back-up rolls and work rolls ; v) Poor heat transfer coefficient due to low coolant impinging force and
pressure; vi) Higher roll surface temperature ;
vii) Higher temperature difference between the centre and edge of rolls ; viii) Higher premature roll thermal failure.
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide for an improved roll cooling system which would avoid the above discussed drawbacks/limitations of the known cooling systems presently available.
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Another object of the present invention is directed to provide for an improved roil cooling system which would facilitate enhancing roll life especially for multi-stand cold mills.
Yet further object of the present invention is directed to providing for an improved roll cooling system which would achieve the following advantageous feature/characteristics :
i) Achieving optimum impact density of the coolant spray-impingement onto the roll for maximised heat transfer effect.
ii) Versatility of thermal control in relation to coolant quantity, flow and placement, to cater all present and future possible rolling conditions.
iii) To achieve optimum Heat Transfer Coefficient (HTC) and reduce the roll surface temperature as well as premature roll thermal failure.
iv) Optimum utilisation of coolant for effective roll cooling.
Yet further object is directed to provide an improved method of roll cooling using the improved system of the present invention.
SUMMARY OF THE INVENTION
Thus according to the present invention there is provided an improved roll cooling
system with improved heat transfer coefficient comprising :
selective nozzle means ;
means for regulated spray conditions ;
means for regulated coolant flow distribution to thereby achieve maintaining substantially constant heat transfer coefficient at impact area during cooling operation.
3

Importantly, in the above disclosed improved roll cooling system of the invention the following modified features are incorporated :
i) Variable coolant flow distribution stand-wise proportional to stand speeds.
ii) Using concept of intense cooling of work rolls and mild cooling for back-up rolls.
iii) Using higher number of nozzles for work roll cooling than back-up rolls.
iv) Reducing number of nozzles for increased coolant pressure and higher impinging force.
v) Using dove-tail type flat jet nozzles for ensuring aligned coolant spray pattern onto the roll surface.
vi) Atleast 60% overlap of spray to avoid dry spotting on roll due to nozzle choking.
vii) Effective header design in relation to coolant application and versatility, robustness to withstand impact, ease of maintenance and inter-changability.
viii) Ease and simplicity of control of the system to achieve maximum benefits and retrofitting to the existing system.
In accordance with yet further aspect of the present invention there is provided a method of achieving improved roll cooling comprising steps for effecting maximum heat transfer by selective use of nozzle means, regulating spray conditions and coolant flow distribution such as to maintain substantially constant heat transfer coefficient at impact area through cooling operations.
The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to non-limiting exemplary illustrations and the accompanying figures wherein
Fig. 1 illustrates heat transfer coefficient at the work roll surface with spray cooling;
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' Fig. 2 illustrates water flow density at the impact area of different types of nozzles;
Fig. 3 is an illustration of heat transfer coefficient in the impact area as function of water flow density;
Fig. 4 is an illustration of heat transfer coefficient as a function of pressure ;
Fig. 5 is an illustration of heat transfer coefficient as a function of distance ;
Fig. 6 is an illustration that heat transfer coefficient as a function of spray angle ;
Fig. 7 is a schematic illustration of convention roll coolant header arrangement in multi stand unit.
Fig. 8 is a schematic illustration of modified roll coolant header arrangement in multi stand unit.
Fig. 9 is a schematic illustration of pumping scheme of emulsion system.
Fig. 10 is a modified header arrangement at stands.
The modified coolant system of the invention was developed incorporating the following features :
a) Designing of modified coolant system for achieving higher HTC.
b) Variable coolant flow distribution onto roll surface.
c) Modified coolant application header configuration retrofitting to existing system and ease of maintainability.
(i) Achieving maximum HTC : The important criterion for achieving maximum HTC are proper nozzle selection and the spray condition.
Nozzle Selection : To maintain nearly constant HTC on the impact area (Fig. 1), the nozzle should be as large as possible and the coolant flow density at the impact area should be nearly constant. It has been identified that the flat jet nozzles are having maximum HTC as compared to square and oval nozzle (Fig. 3). The coolant flow density at the impact areas is achieved nearly equal to the
5

width of spray in flat jet compared to other nozzles (Fig. 2) and having maximum length-width ration for the impact area (15:1) in comparison to oval nozzle (5:1) and square nozzle (1:1). Thus flat jet nozzles were identified to be suitable for achieving maximum HTC with relatively low coolant pressure and has been selected for the modified roil coolant system.
Spray Condition : Basically the spray condition depended upon three basic parameters like coolant pressure, spray angle and the spray height i.e. the distance between nozzle tip and roll surface.
It has been observed that with increase in coolant pressure, there was increase in HTC upto a pressure limit 15 bar (Fig. 4) but in the modified system by reducing total number of nozzles, the coolant pressure has been increased from 2.1 bar to 4 2 bar, which clearly indicated the improved HTC with the modified coolant system. It has been also identified that lesser spray distance resulted in higher HTC (Fig. 5). There is a mere change in HTC, if the spray distance increases beyond 200 mm.
In the modified system, the spray distance is restricted to 183 mm for top work roll cooling header and 152 mm for bottom work roll cooling header, due to space constraint.
The distance of the coolant spray should be perpendicular to the roll surface. If the impinging spray angle (ß) is more than 15°, an increasing disturbance of the coolant flow at the impact area takes place, and hence cooling efficiency decreases (Fig. 6). In the modified system the impinging spray angle has been kept at 15° for top work roll (WR) compared to 18° earlier and 25° for bottom WR compared to 30° earlier (Figs. 7 & 8).
(ii) Coolant flow distribution : The volume of coolant used for roll cooling varies, depending on the type of mill. However, the pressure requirements are constant and generally ranged between 3 and 7 bar. This range is perfectly acceptable for the application of coolant in cold mills, with a recommended value of approximately 5 to 6 bar. As the volume and pressure of the coolant are directly related to the impact density of the spray and in turn to the heat transfer efficiency, a minimum of 3 bar is need to achieve any thermal response to the sprays.
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However, the benefit of increased pressure decreases rapidly above 7 bar, thereby making its use unjustified.
With regard to the general aspects of roll cooling, hot rolled strip entering for stand # 1 to stand # 5 in a 5-stand cold mill, the roll cooling pattern in each of stands #1 to #4 play a vital role for controlling the strip shape. Mostly, the coolant application are neglected at these stands, and very efficient roll cooling was carried out.
By optimising the aspect of the rolling process, not only with the objective of improving product quality, but for achieving significant benefits, the coolant flow distribution is optimised stand-wise in accordance with stand speed, roll force and percentage reduction pattern. In the modified system design, coolant flow distribution was varied standwise with respect to mill speed and roil force. There is a gradual increment of flow from stand #1 to stand #5 (Figs. 7 & 8).
(iii) Roll coolant header configuration : As discussed above the sprays are selectively provided to impinge as precisely and accurately as possible onto the roll for maximised effectiveness, which directly depends on the positioning of the headers within the mill fixtures. This optimal location has to take into account existing mill furniture, movements of entry guides and bridles and access of services coolant piping, air and electrical hoses and ease of accessibility for maintenance. A considerable amount of effort is required to complete the flow calculations on the roll coolant system. Piping must be sized and routed so as to eliminate pressure losses. Spray headers must be chambered to accommodate these flow rates without restrictions. The spray headers are zoned into three zones - centre and edge. The number of nozzles at the centre zone is just double the number of nozzles at the edge zones to maintain balanced and uniform flow rate through each nozzles, as because the inlet connections to the centre zone and two edge zones are having equal branching pipe lines of 100 NB from main delivery pipe line of 200 NB from pump-set. Both the back-up and work roll spray headers (top and bottom) are having balanced and equal flow distribution and volume on each stands.
In the modified roll coolant system, the roll coolant headers (Fig. 8) are arranged taking into following considerations :
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i) Header location at the mill housing
ii) Nozzle configuration for spray pattern
iii) Zonal cooling of rolls
iv) Retrofitting to existing system
v) Ease of maintenance.
Roll coolant headers : The top work roll coolant headers are box-type and fitted on the hold down board at the entry side of stands with three zones - centre and edge. The spray height is maintained within 183 mm. The bottom work roll coolant headers are made up from 50 NB pipe with three zones for similar flow distribution like top work roll. These headers are of fixed type having spray height of 152 mm. All the back-up roll coolant headers are made up from 50 NB pipe with three zones and are fixed type. The spray is oriented towards the bite of work roll and back-up roll to facilitate lubricity at the bite and to take out the heat from rolls.
All these roll coolant headers are provided with protection plates and bottom flushing port in each zones for ease of periodic maintenance and are made retrofitting to the previous coolant supply connections.
Nozzle configuration : The spray nozzles are selected of dove-tail type flat jet nozzles. The dove-tail type ensures perfectly aligned spray pattern onto the roll surface and facilitates periodic cleaning of nozzles. The nozzles are fitted on a single row to the spray header with an off-set angle of 15° in order to avoid interference of the adjacent spray and to maintain perfectly aligned herringbone spray pattern for effective cooling of rolls. The spray angle of nozzle is maintained at 60° for work rolls and 90° for the back-up rolls for more spray width coverage and helped to reduce the number of nozzles and to provide more intense cooling i.e. high flow density to work rolls.
In accordance with another aspect of the invention the variable flow distribution stand-wise are being introduced with the number of nozzles which in turn depend on the inter spacing of the nozzles per header. All the back-up roll coolant header are provided with eight numbers of nozzles with 4 nozzles at centre zone and 2 nozzles at each edge zones. For stand 1 & 2, 12 number of nozzles are provided
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on the work roll coolant header with 6 nozzles at centre zone and 3 nozzles at edge zone. For stand 3, 16 number of nozzles are provided on the work roll coolant header with 8 nozzles at the centre zone and 4 nozzles at the edge zone. For stand 4 & 5, 20 number of nozzles are provided on the work roll coolant header with 10 nozzles at the centre zone and 5 nozzles at the edge zone (Fig. 10). All the nozzles are fitted to the header with a minimum overlap of 40% to maximum 70% in order to avoid dry spot running of work rolls due to nozzle choking. All the nozzles are kept of same flow rate for ease maintenance and inventory.
Preferred Nozzle Specification :
Nozzle for Back-up roll coolant header:
Type - Dove-tail flat jet
Flow rate - 99.6 Ipm at 5 bar
Spray angle - 90°
Spray width - 525 mm at a height of 250 mm
Bore diameter - 10.0 F mm
Narrowest cross-section - 6.4 F mm
Make - M/s Lechler (India) Pvt. Ltd.
Part No. (Nozzle tip) - 665.126.17
Part No. (Nozzle nipple) - 066.419.17
Part No. (Threaded cap) - 065.600.17
Nozzles for Work roll coolant header:
Type - Dove-tail flat jet
Flow rate - 99.6 ipm at 5 bar
Spray angle - 60°
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Spray width

340 mm at a height of 250 mm

Bore diameter - 10.0 F mm
Narrowest cross-section - 7.4 F mm
Make - M/s Lechler (India) Pvt. Ltd.
Part No. (Nozzle tip) - 665.126.17
Part No. (Nozzle nipple) - 066.419.17
Part No. (Threaded cap) - 065.600.17
Tests were carried out to confirm the advantages in the modified roll coolant system (Fig. 8 & 9) vis-a-vis conventional roll cooling system at stands units. The system performance for a period of 6 months was evaluated and following results had been achieved.
In the modified design coolant flow distribution was redistributed stand-wise (Fig. 8) to achieve optimum heat transfer effect and to avoid flooding and wastage of coolant. First redistribution was carried out by reducing the coolant flow at back-up rolls and saved coolant was used for intensive cooling at work rolls as per Table-1 hereunder for achieving higher HTC.
Table - 1 : Emulsion flow distribution at roll cooling headers of TEM II

Stand No.
Roll position
Before Modification at 2.1 bar pressure
After Modification at 4.2 bar pressure

Flow rate (Ipm)
No. of Nozzles
Flow rate (lpm)
No. of Nozzles
Stand # 1
Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll
1355 988 1355 1355
21 12 21 21
730 1095 1095 730
8 12
12 8
Stand # 2
Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll
775 988 775 775
12 12 12 12
730 1095 1095 730
8 12 12 8
10

Stand No.
Roll position
Before Modification at 2.1 bar pressure
After Modification at 4.2 bar pressure

Flow rate (Ipm)
No. of Nozzles
Flow rate (Ipm)
No. of Nozzles
Stand # 4
Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll
1355 988 1355 1355
21 12 21 21
730 1825 1825 730
8
20 20
8
Stand # 5
Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll
1355
988
1355
1355
21 12 21 21
730 1825 1825 730
8
20 20
8
The maximum heat is being generated at the roll-bite due to cold reduction of strip and the deformation work i.e. friction. This generated heat is taken by strip, coolant and the major part by the work rolls, this gradual heat input to the work roils are to be efficiently taken out by the roll cooling system or other-wise the work roll will be subjected to premature thermal failure. A nominal part of heat is being conducted to the back-up rolls and the frictional effect between the back-up roll and work roll generates comparatively low heat input to the back-up rolls, therefore the cooling of back-up roll is not so vigorous as the work roll. If the work roll temperature is controlled effectively, it will help to reduce the heat flow to the back-up rolls, therefore in the modified coolant system, the coolant flow distribution for back-up rolls are reduced drastically (to nearly half) and the curtailed coolant flow was distributed to work rolls for intense cooling. The total coolant flow distribution for work rolls before the modification was 10813 Ipm and after redistribution the total flow was 14500 Ipm. Similarly the total coolant flow for BUR cooling was 12390 Ipm which was reduced to 7300 Ipm after modification (Table-1).
In the original design the total number of nozzles fitted in all stands were 343 numbers (Table -1). The total flow rate of emulsion in all stands were 23203 Ipm and having low emulsion pressure of 2:1 bar due to higher number of nozzles with 4 nos. of pump running resulting to low impact force and low heat transfer effect from work rolls. In the modified design, the total number of nozzles were reduced from 343 to 240 to enhance the emulsion pressure, which increases the impact
11

force of spray to improve HTC. In the modified design the coolant pressure was achieved of 4.2 bar with the 4 pumps running.
The modified roll coolant headers for BUR and work rolls are erected at all the five stands of the multi-stand unit as per new system of stage-wise as per Table -2 hereunder:
Table - 2 : Stage-wise flow distribution of emulsion at TCM-ll, BSL

Stand No.
Roll Position
Before
modification
at #1,2,3,4 &
5 (at 2.1 bar)
After modification at # 4 (at 2.4
bar)
After modification at # 1, 2 & 4 (at 3.4 bar)
After
modification at
#1,2,3,4 & 5 (at
4.2 bar)
#1
Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll
1355 988 1355 1355
1448 1056 1448 1448
640 960 960 960
730 1095 1095 730
#2
Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll
775 988 775 775
828 1056 828 828
640 960 960 960
730
1095
1095
730
#3
Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll
1355 988 1033 1355
1448 1056 1104 1448
1672 1220 1275 1672
730 1460 1460 730
#4
Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll
1355 988 1355 1355
554 1385 1385 554
640 1600 1600 640
730 1825 1825 730
#5
Top Back-up Roll Top Work Roll Bottom Work Roll Bottom Back-up Roll
1355 988 1355 1355
1448 1056 1448 1448
1672 1220 1672 1672
730 1825 1825 730
In the first stage, the modified coolant headers for work roll are installed with intense cooling, using 8 nozzles for edges cooling and 12 nozzles for centre cooling, whereas the back-up roll coolant headers are installed with single row cooling using 8 nozzles, at stand #4. With the introduction of modified system at stand #4, the performance has resulted in marginal enhancement in emulsion pressure to 2.4 bar with slight drop in work roll surface temperature by 5°C. in the second stage, all the modified coolant headers are installed at stand #1 & 2 with
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the single row cooling, using 12 nozzles per header for work roll cooling, whereas the back-up roll coolant headers are installed with 8 nozzles per header. With the introduction of modified system at stand #1 & 2, the performance has resulted in significant enhancement in emulsion pressure to 3.4 bar with drop in work roll surface temperature by 7°C. In the final stage, the modified coolant headers are installed at stand #3 & 5 with the single row cooling. The modified coolant headers are installed at stand #3, using 16 nozzles per header for work roll cooling, whereas the back-up roll coolant headers are installed with 8 nozzles per header. The modified coolant headers are installed at stand #5, using 20 nozzles per header for work roll cooling, whereas the back-up roll coolant headers are installed with 8 nozzles per header. With the introduction of modified system at stand #3 & 5, the performance has resulted in significant enhancement in emulsion pressure to 4.2 bar with drop in work roll surface temperature by 10°C (avg.) and flattened roll thermal profile by reducing the total number of nozzles (Table - 1). Use of dove-tail type flat jet nozzles has ensured perfectly aligned herringbone spray band are improved heat transfer rate from rolls, the nozzles are fitted to the coolant header with an off-set angle of 15 to 20% to ensure no interference of two successive spray band and can take care of effective and higher heat removal rate from each spray band. The modified system has resulted in reducing average roll surface temperature from 70°C to 60°C (avg.). The temperature difference between the centre and edge of the roll has reduced from 10°C to 5°C (avg.) and resulted in flattened roll thermal profile as per Table - 3 hereunder which in turn helped to improve the strip profile/shape.
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Table - 3 : Average Temperature of work roll surface

Stand No.

Roll Position
Temp. Diff. ?T (°C)
Avg. Roll Temp. (°C)
Roll Temperature (Along Barrel Length) (X)
#5
Before modification
TWR BWR TWR BWR
13 11 4 4
67 66 61 60
56 56 60 58
63 66 61 59
69 67 63 61
67 65 61 60
58 60 59 57

After Modification

#4
Before modification
TWR BWR TWR BWR
13 9 5 4
74 69 60 59
63 63 57 56
74 67 59 58
76 70 62 60
72 68 60 59
63 61 58 57

After Modification

#3
Before Modification
TWR BWR TWR BWR
10 10 5 5
66 66 60 59
59 59 56 55
63 64 58 56
69 68 61 60
66 63 60 59
60 58 57 56

After Modification

#2
Before Modification
TWR BWR TWR BWR
9 7 5 4
67 65 59 58
59 61 56 55
65 64 58 57
68 67 61 59
67 62 59 58
60 60 58 57

After Modification

#1
Before Modification
TWR BWR TWR BWR
6 4 3 2
59 58 56 54
54 55 54 54
57 57 54 54
60 59 57 55
58 58 56 54
56 56 55 53

After Modification

The spray angle of nozzles for work roll cooling header was kept at 60° for higher impact force of the coolant. The nozzles are interspaced in such a manner such that the overlap of the two adjacent spray can have atleast more than 50% in order to avoid dry spot rolling due to nozzle choking. Whereas the nozzles for back-up roll cooling header was designed with higher spray angle of 90° for the use roll cooling with lesser number of nozzles and with an overlap of atleast 40% of spray width. The number of nozzles selected for each stand were selected with respect to stand-wise speed and roll force. This redistribution of coolant has also resulted to reduce the premature work roll thermal failure from 57 nos. to 27 nos. per annum. The effective modified roll cooling system also helped to reduce the finishing coil temperature from 130°C to 110°C (avg) and also reduced the specific roll consumption from 1.98 to 1.36 Kg/T and specific rolling oil consumption from 1.33 to 0.85 lit/T as per Table - 4 hereunder:
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Table - 4 : Specific consumption of rolls and oil

Period
Number of Roll Spalled
Specific Roll Consumption (Kg/T)
Specific Oil Consumption (Lit/T)
Jan '99
6
1.80
0.6
Feb '99
3
1.45
0.67
Mar '99
5
1.73
0.69
Apr '99
1
1.83
0.78
May '99
2
2.07
1.31
Jun '99
1
0.84
1.01
Jul '99
5
1.34
0.97
Aug '99
3
2.0
0.96
Sep '99
5
1.44
0.66
Oct '99
2
1.36
0.61
Nov '99
1
1.44
0.90
Dec '99
0
0.88
0.75
Jan '00
2
0.97
0.68
Feb '00
4
1.16
0.98
Mar '00
1
1.1
0.85
1998-99
57
1.98
1.33
1999-2000
27
1.36
0.85
It is thus possible by way of the modified roll coolant of the present invention to achieve the following :
a) Improved heat transfer effect with the help of perfectly aligned spray band onto the rolls ;
b) Possible use of lesser number of nozzles to enhance the coolant pressure at the nozzle tip from 2.1 bar to 4.2 bar with the use of 4 pump running and improved HTC effect.
c) The redistribution of coolant flow distribution stand-wise which avoided flooding and wastage of coolant and made effective cooling of rolls and has enhanced intensive cooling of the work rolls.
d) Reduced roll surface temperature from 70°C to 60°C (avg.) and also reduced the temperature difference between centre to edge from 10°C to 5°C (avg) Table-3) which has resulted in improved flattened roll thermal profile and improved strip shape/flatness on account of roll thermal crown.
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e) Improved heat transfer removal rate from work rolls and has resulted to reduced premature roll thermal profile from 57 numbers to 27 numbers per annum.
f) Reduce the strip cooling temperature from 130°c to 110°C (avg) with reduced specific roll consumption by 35% and specific rolling oil consumption by 30%.
g) Improve the overall mill performance with respect to productivity and quality.
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WE CLAIM
1. An improved roll cooling system with improved heat transfer coefficient for
multi stand cold mill comprising:
selective nozzle means ;
means for regulated spray conditions ;
means for regulated coolant flow distribution to thereby achieve maintaining
substantially constant heat transfer coefficient at impact area during cooling
operation.
2. An improved roll cooling system as claimed in claim 1 wherein said selective nozzle means comprise large nozzle adapted for maintaining substantially constant coolant flow density.
3. An improved roll cooling system as claimed in anyone of claims 1 or 2 wherein said selective nozzle comprise flat jet nozzles.
4. An improved roll cooling system as claimed in anyone of claims 1 to 3 wherein said coolant flow density at the impact area is achieved nearly equal to the width of spray in flat jet and having a maximum length - width ratio for the impact area of 15 : 1.
5. An improved roll cooling system as claimed in anyone of claims 1 to 4 wherein said means for regulated spray conditions comprise means for monitoring basic parameters comprising coolant pressure, spray angle and spray height.
6. An improved roll cooling system as claimed in anyone of claims 1 to 5 wherein the spray distance is maintained upto 183 mm for top work roll cooling header and upto 152 mm for bottom work roll cooling header.
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7. An improved roll cooling system as claimed in anyone of claims 1 to 6 wherein the impinging spray angle is maintained upto 15° for top work roll and upto 25° for bottom work roll.
8. An improved roll cooling system as claimed in anyone of claims 1 to 7 wherein said means for coolant flow distribution comprise means for maintaining the, coolant pressure in the range of 3 to 7 bar.
9. An improved roll cooling system as claimed in anyone of claims 1 to 8 wherein said spray headers are provided at optimum location based on existing mill infrastructure, movement of entry guides and bridles and access of services cooling piping, air and electrical hoses and ease of accessibility for maintenance.
10. An improved roll cooling system as claimed in anyone of claims 1 to 9 wherein said spray headers are chambered to accommodate the flow rates without restriction.
11. An improved roll cooling system as claimed in anyone of claims 1 to 10 wherein said spray headers are provided in zones comprising a central zone and two edge zones on either sides of the central zone wherein the number of nozzles at the central zone is double the number of nozzles at the edge zones such as to maintain balanced and uniform flow rate through each nozzle.
12.An improved roll cooling system as claimed in anyone of claims 1 to 11 wherein said central zone and edge zone nozzles are provided with equal branching pipe lines from a main delivery pipe line operatively connected to a pump set.
13.An improved roll cooling system as claimed in anyone of claims 1 to 12 wherein the back up and work roll spray headers have balanced and equal flow distribution and volume on each stand.
18

14.An improved roll cooling system as claimed in anyone of claims 1 to 13 wherein the roll coolant headers art 18 d based on header location at the mill housing, nozzle configuration for spray patterns, zonal cooling of rolls, retrofitting to existing system and ease of maintenance.
15. An improved roll cooling system as claimed in anyone of claims 1 to 14 wherein the top work roll cooling header are box type and fitted on the hold down board at the entry side of stands with said three zones.
16.An improved roll cooling system as claimed in anyone of claims 1 to 15 wherein the spray is adapted to be oriented towards the bite of roll works and back up roll to facilitate lubricity at the bite and to take out the heat from rolls.
17. An improved roll cooling system as claimed in anyone of claims 1 to 16 wherein said roll coolant headers are provided with protection plates and bottom flushing port in each zones for ease of periodic maintenance and are made retrofitting to the previous coolant supply connections.
18. An improved roll cooling system as claimed in anyone of claims 1 to 17 wherein said spray nozzles are selected of dove-tail type flat jet nozzles.
19.An improved roll cooling system as claimed in anyone of claims 1 to 18 wherein said nozzles are fitted on a single row to the spray header with an off set angle of 15° to thereby avoid interference of the adjacent spray and to maintain perfectly aligned herringbone spray pattern for effective cooling of rolls.
20.An improved roll cooling system as claimed in anyone of claims 1 to 19 wherein the spray angle of nozzle is maintained at 60° for work rolls and 90° for the back-up rolls for more spray width coverage.
19

21.An improved roll cooling system as claimed in anyone of claims 1 to 20 comprising means for variable flow distribution stand-wise comprising varying number of nozzles and on the inter spacing of the nozzles per header.
22.An improved roll cooling system as claimed in anyone of claims 1 to 21 wherein said nozzles are fitted to the header with a minimum overlap of 40% to maximum 70% in order to avoid dry spot running of work rolls due to nozzle choking.
23.An improved roll cooling system as claimed in anyone of claims 1 to 22 wherein all said nozzles are of same flow rate.
24, A method of achieving improved roll cooling for multi stand cold mill
comprising steps for effecting maximum heat transfer by selective use of
nozzle means, regulating spray conditions and coolant flow distribution such
as to maintain substantially constant heat transfer coefficient at
impact area through cooling operations.
25. A method as claimed in claim 24 wherein the coolant flow density at the impact area is maintained substantially constant.
26.A method as claimed in anyone of claims 24 or 25 wherein the coolant pressure is maintained in the range of 4.0 to 4.5 bar preferably 4.2 bar, the spray distance is maintained in the range of upto 183 mm for top work roll cooling header and upto 152 mm for bottom work roll cooling header and the distance of the coolant spray is maintained to the roll surface.
27.A method as claimed in anyone of claims 24 to 26 wherein the impinging spray angle is kept upto 15° for top work roll and 22° to 26° preferably 25° for bottom work roll.
28.A method as claimed in anyone of claims 24 to 27 wherein the coolant pressure is maintained in the range of 3 to 7 bar.
20

29. A method as claimed in anyone of claims 25 to 28 wherein the roll surface temperature is maintained in the range of 58°C to 62°C preferably 60°C and the temperature difference between centre to edges is maintained in the range of 6°C to 3°C preferably 5°C.
30. An improved roll cooling system with improved heat transfer coefficient for multi stand cold mill substantially as herein described and illustrated with reference to the accompanying examples and figures.
31.A method for achieving improved roll for multi stand cold mill cooling substantially as herein described and illustrated with reference to the accompanying examples and figures.
Dated this 26th day of March 2001.

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This invention relates to an improved roll cooling system and an improved method of roll cooling for enhancing roll life by improved heat transfer for Multi-Stand cold Mill. The improved roll cooling system with improved heat transfer coefficient comprises selective nozzle means ; means for regulated spray conditions ; means for regulated coolant flow distribution to thereby achieve maintaining substantially constant heat transfer coefficient at impact area during cooling operation. The system is directed to meet increasing requirements on the geometric quality parameters like strip thickness, width, profile and flatness of CR Strips for superior strip profile and flatness for trouble free downstream processing such as cold rolling, coating and press forming.
The system would provide for improved roll cooling system which would facilitate enhancing roll life especially for multi-stand cold mills. In particular the system achieves the following advantageous feature/characteristics :
I) Achieving optimum impact density of the coolant spray-impingement onto
the roll for maximised heat transfer effect. Versatility of thermal control in
relation to coolant quantity, flow and placement, to cater all present and
future possible rolling conditions.
II) To achieve optimum Heat Transfer Coefficient (HTC) and reduce the roll surface temperature as well as premature roll thermal failure.
III) Optimum utilisation of coolant for effective roll cooling.
The invention is also related to an improved method of roll cooling using the improved system of the present invention.

Documents:

00178-cal-2001-abstract.pdf

00178-cal-2001-claims.pdf

00178-cal-2001-correspondence.pdf

00178-cal-2001-description (complete).pdf

00178-cal-2001-drawings.pdf

00178-cal-2001-form-1.pdf

00178-cal-2001-form-19.pdf

00178-cal-2001-form-2.pdf

00178-cal-2001-form-3.pdf

00178-cal-2001-pa.pdf

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Patent Number

194466

Indian Patent Application Number

178/CAL/2001

PG Journal Number

30/2009

Publication Date

24-Jul-2009

Grant Date

24-Jun-2005

Date of Filing

26-May-2001

Name of Patentee

STEEL AUTHORITY OF INDIA LIMITED

Applicant Address

RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI.

Inventors:

#	Inventor's Name	Inventor's Address
1	SENGUPTA PARTHA PRATIM	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002.
2	PATHAK ASHISH	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002.
3	KRISHNA BINOD	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002.
4	SINGH SURENDRA PRASAD	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002.
5	NAFDE KISHORE	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002.
6	GANTI MAHAPATRUNI DAKSHINA MURTY	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002.
7	JHA SUDHAKER	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002.
8	MARIK APURBA KUMAR	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002.

PCT International Classification Number

B21B27/10,B21B37/32

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA