Factory Engineering LOTUS ROLL PERFORMANCE AT BAWANY SUGAR MLLS, PAKSTAN Mohammad Azam Butt 1 Bawany Sugar Mills, Pakistan ABSTRACT Over a four-year period, the capacity of Bawany sugar factory has been increased from the designed 2000 to 3200 tclday with increased extraction, principally by the gradual addition of Lotus rolls as top rollers for all mills, and the gradual addition of larger turbines for each mill. The original tandem of five 762 x 1524 mm three-roller mills plus shredder has been retained. A surprising increase in steam production and boiler efficiency is reported. This is confirmed by no need for additional boilers and the production during 84185 crop of 15% surplus bagasse over prior years. Performance data are presented year by year 1981 through 1985. NTRODUCTON 1 Sugar technologists have long recognised the necessity for a cane mill to have ample drainage provision for optimum recovery of the extracted juice. n the beginning, the rolls of a mill had plain surfaces which resulted in poor gripping of the cane and juice drainage, thereby causing frequent chokes through cane slippage at the front nip of the mill. Later, the roll surfaces were provided with circumferential grooves which improved both the cane throughput and juice extraction considerably. Things got even better with the subsequent introduction of the so-called chevron and Messchaert grooves. However, in many countries where the demand for sugar has led factories to grind at nominal or excess capacity those drainage provisions are simply not enough, resulting in 'flooding' of the mills which, in turn, causes the cane being fed to the mill to be soaked with juice, thereby rendering feeding difficult and impairing both the throughput and extraction of the mill. With the use of the Lotus roll, however, such flooding is completely eliminated, thereby making it possible for any mill to grind up to 30% more cane than before while maintaining satisfactory extraction. BREF DESCRPTON OF THE LOTUS ROLL The Lotus roll is similar in configuration to a conventional mill roll, except that the shell is made of steel (instead of cast iron) and equipped at the periphery with a plurality of radially-disposed stainless steel juice nozzles interconnecting with juice channels axially disposed within the body of the shell which readily drain a considerable amount of extracted juice which would otherwise have been trapped in the cane mass being fed to the mill. The shaft is identical to that of the conven- Keywords: Lotus roll, mill capacity
MOHAMMAD AZAM BUTT 823 tional roll which the Lotus roll is to replace, thus making the replacement possible without requiring any modification work on the mill. The Lotus roll is primarily used as a mill top roll, although it has proven to be very beneficial (in bringing down the final bagasse moisture) when used as a discharge roll on the last mill. A typical cross-sectional view of the Lotus top roll is shown in Figure 1. Of course, the flanges are not provided when the Lotus roll is to be used as a discharge roll. FGURE 1. Sectibnal diagrams of a Lotus roll THE BAWANY EXPERENCE The Bawany sugar factory is situated in the Sind Province of Pakistan and started operation in November of 1965. Designed for a normal capacity of 1500 metric tons of cane per day, the factory's milling plant included two cane knives and a 762 x 1524 mm fifteen roller mill tandem. For many years, the sugar mill nearest to Bawany was 48 km away and the latter had all the cane it could crush over as long a period as it wished (usually eight months). However, with the commissioning of another sugar mill in the area, the Sind Province attracted the attention of industrialists and it was not long before four more sugar mills were sanctioned to be erected in the 48 km radius area around Bawany. To meet the challenge of competition with those new sugar mills, the existing factories around Bawany expanded their boiling houses, added boilers and speeded up their mills in order to crush the same or more tonnage of cane in five months instead of eight. Bawany, under those circumstances, had no choice but to increase its grinding rate also to the maximum which was thought to be around 3000 tc/day. Although Bawany added a shredder which allowed the factory to increase its grinding rate by 500 tc/day, the mill drives were now operating at their maximum and the only alternative appeared to be the installation of a second (or larger) milling tandem. There was also the possibility of adding a sixth mill but it was concluded that the advantage to be derived from a sixth mill does not justify the expenditure. The installation of a second (or larger) milling tandem was going to be a c~stly affair and the arrangement of official sanction, finances and execution of the installation was going to take two to three years, which was too long.
FACTORY ENGNEERNG The relevant data of Bawany mill at the beginning of the 1981-82 crop were as follows: Levelling knife set - 30 blades, 150 kw motor Heavy knife set - 60 blades, 185 kw motor Shredder - BMA, 250 kw Milling tandem - BMA, 5 mills, 250 kw turbines Process - Double carbonatation, double sulphitation, for the production of plantation white sugar. At this time the author came to know about the installation of a Lotus roll on the first mill in a neighbouring factory. The owner of Bawany and the author visited the mill in question and were convinced; a Lotus roll was ordered and installed, at the end of the 1981-82 season for trial on the first mill of the Bawany tandem. Because of severe competition for cane, Bawany was already pushing its milling tandem to the maximum speed with the hydraulic pressures reduced to minimum. The first mill hydraulic pressure (before the installation of the Lotus roll) was set at 120 kg/cm2 and the speed of that mill over 6 r/min. The Lotus roll was tried under the same conditions, except that the mill openings were reduced by 20%. mmediately upon commissioning the Lotus roll, its influence on the performance of the mill became visible through a marked reduction in the power consumed. Though the speed of the first mill was subsequently reduced to 5.6 r/min and the hydraulic pressure increased to 140 kg/cm2. t soon became apparant that 20% reduction in the openings was not good enough, as the mill speed could not be reduced further due to a lower availability of power; whereas running the mill at 5.6 r/min resulted in delivery of material at a rate which the second mill could not cope with. The full amount of cush-cush was being applied in front of the first mill; and running the latter at a slower speed was leading to stalling of the turbine. The openings of the first mill were, therefore, further reduced by 10% (with a view to avoiding the frequent chokes which were occurring at the second mill due to overloading) and a balance in the speed of the first mill and fibre loading of the second mill was finally established. Though the Lotus roll was available for only the last two months of the 1981-82 crop, the duration was long enough to convince the technical staff of Bawany that, their goal of grinding 3000 tc/day (or more) could definitely be achieved through the use of more Lotus rolls. Accordingly, purchase of two more Lotus rolls was recommended and effected for use on the second and last mills during the 1982-83 crop. ndeed, this decision proved to be correct as the 3000 tc/day barrier was broken during the crop, although milling losses were somewhat high due to the use of relatively low hydraulic pressures on the third and fourth mills which could otherwise not cope with the increased tonnage. t was therefore decided to order for the 1983-84 crop tw0,additional Lotus rolls for use on the third and fourth mills; and a 330 kw turbine to replace the existing 250 kw turbine driving the last mill in order to allow the latter to work under up to 75 kg/cm2 hydraulic pressure for better control of the final bagasse moisture.
MOHAMMAD A 1 AZAM BUTT 825 The mill openings, speeds and hydraulic pressures for the 1983-84 crop are given in Table 1. TABLE. Mill settings and other data Feed Delivery Mill opening opening Speed Hydraulic pressure mm mm rlmin M Pa kglcm2 The beginning of the 1983-84 crop saw the first-in-the-world all-lotus mill tandem in operation at the Bawany sugar factory. Other sugar factories are now operating with all-lotus mill tandems but Bawany was the first. The addition of the two Lotus rolls on the third and fourth mills and replacement of the 250 kw turbine by a 330 kw turbine for driving the last mill brought about significant improvement in the overall performance of the milling plant. Peak capacity reached 3200 tc/day and the bagasse moisture was brought down to around 46% (the lowest in Pakistan), thereby allowing the boilers to generate up to 25% more steam. Actually, this fact was verified by the factory personnel, through the use of flowmeters, after it became evident that the apparent consumption of steam per cent cane was unbelievably low, if calculation had been based on 20 ton designed capacity of each boiler, representing 43% on cane. Actually the flowmeter tests showed that each boiler was in fact generating comfortably 25 tons of steam per hour and that the steam consumption on cane was 53.9% and not 43%. t should be noted, however, that the Bawany factory's average steam consumption on cane is currently 48% to 50% Another sign that the boilers were operating more efficiently was the fact that Bawany was able to produce 15% additional surplus bagasse which was supplied to the factory's bagasse particle board plant. Unfortunately though, the boiling house was not able to sustain continuous processing of the juice at such an increased rate of crushing, and milling had to be stopped at intervals, or reduced accordingly. Another bottleneck was the shredder drive power of 250 kw which proved to be much too low for handling cane at such increased rate and had to be operated at reduced speed to the detriment of the cane preparation. But, due to its higher drive power and increased hydraulic pressure, the last mill performed so impressively that it was decided to purchase another new 330 kw steam turbine for driving the first mill under increased hydraulic pressure of 175 kg/cm2 during the 1984-85 crop, along with a new 515 kw steam turbine for operating the shredder at the correct speed.
826 FACTORY ENGNEERNG Note: A 405 kw turbine for driving the last mill would have been more useful but, unfortunately, the reduction gears were found to be not strong enough for transmitting more than 330 kw. Thus, the 1984-85 crop started with more power available to the shredder and first mill and, as expected, this brought about further significant improvement in the overall performance of the milling plant right from the start of the crop, in spite of the fact that the second, third and fourth mills were still being driven by the original 250 kw turbines. TABLE 11. Mill data for 3000 tclday Cane Bagasse Mill opening opening Speed Hydraulic pressure mm mm rlm i n M Pa kglcm2 n Table 11, are given the mill openings, speeds and hydraulic pressures for a grinding rate of 3000 tc/day during the 1984/85 crop, while Tables 11 and V show respectively the average milling results obtained during part of the months of November 1984 and January 1985. t is to be noted that the lower amount of imbibition applied during the month of January 1985 should actually have been higher than that applied during the month of December 1984 (due to the subsequent rise in the cane fibre content). However, this was not possible, due to constraints in the boiling house. n any case, though working at more than double the designed capacity of its milling plant, Bawany was the factory in Pakistan that finished the 1984-85 crop with the lowest final bagasse moisture. For the sake of comparison, are given in Table V the average figures obtained by Bawany during the month of January 1985 and another mill X identical to Bawany which is still working with its two cane knives followed by a 15-roll milling tandem of size 762 x 1524 mm originally provided. Table V also shows a comparison against mill Y which is equipped with six mills of the same size as at Bawany. t might be of interest to mention here that there is, reportedly, still more advantage to be derived from the installation of a Lotus roll when the latter is adapted to work on a mill in combination with a pressure feed roll and g;avity chute. Such a combination is shown in Figure 2 and comprises a three-roll mill, a pressure feed roll 7 (of relatively large diameter) mounted in front thereof and a gravity chute 4, the mill being equipped with a Lotus (top) roll 11, a feed roll 9 and a discharge
MOHAMMAD AZAM BUTT 827 1 TABLE ll. Milling results, November 1984 Fibre % cane - 11.52 mbibition % cane - 13.76 Date Cane Pol Pol in Moisture in (Nov. '84) crushed extraction bagasse bagasse t O/o O/o O/o TABLE V. Millin'g results, January 1985 Fibre % cane - 13.35 mbibition % cane - 12.56 Date Cane Pol Pol in Moisture in (Jan. '85) crushed extraction bagasse bagasse t Yo Yo Yo roll 10, so arranged as to form a pressure-feed nip 8, a pressure chamber 14, a mill feed nip 12 and a discharge nip 13. The gravity chute compacts the prepared cane 6 which is then delivered to the mill through nip 8 and chamber 14. A considerable percentage (typically 20%) of the juice contained in the cane is thus extracted through nip 8, and readily drains through the nozzles 16 and channels 17 of Lotus roll 11, leaving a drier and further compacted cane in chamber 14 which effectively feeds into the mill proper. Where more juice is extracted and drains partly through the Lotus roll and partly in the conventional way. Had there been no Lotus roll in the system, the juice being extracted through nip 8 would be forced upwardly (due to the higher pressure in chamber 14 below)
~ Thus, FACTORY ENGNEERNG TABLE V. Comparison of milling results All mills 762 mm x 1524 mm Number Fibre% mbibition Pol Bagasse Factory of mills Rate cane % cane extraction moisture tclday YO YO January 1985* X 5 1510 11.80 20.60 92.19 48.00 Bawany 5 3100 13.30 12.60 93.33 46.06 December l984* 1 Y 6 1929 15.56 19.11 93.11 49.75 Bawany 5 3001 13.35 11.25 93.92 46.59 * Average results as published by the Pakistan Society of Sugar CaneTechnologists. $ to cause 'flooding' of the mill as mentioned earlier. Another advantage of the system lies in the fact that the forces developed, as the cane is squeezed within nip 8, tend to move roll 11 horizontally toward the discharge side of the mill, thereby relieving the upper brasses of roll, 11 (whose feed-side faces thrust heavily against their housings during operation) and, thus, allowing the latter to float more freely. No foundations are required for the mounting of roll 7 which is supported by the mill headstocks and chain-driven off the main feed roll 9. the Lotus roll is adapted in the system to work much more effectively than before, in that it acts not only as a pressure feeder in combination with a pressure feed roll, but also extracts and drains a considerable amount of the juice contained in the cane before the latter even enters the mill proper. n the case of a first mill, it is recommended to have the prepared cane delivered into the gravity chute by means of a high-speed, rubber conveyor (with or without a magnetic separator), for reducing to a minimum the layer of incoming cane thereby eliminating the risk of chokes at the top of the gravity chute. n the case of the last mill, it is reported that the final bagasse moisture can be decreased drastically (down to around 43%) through additionally replacing the discharge roll by a Lotus roll to work in conjunction with the Lotus (top) roll. The idea is to position the Lotus (discharge) roll in such a way that its juice channels alternate with those of the Lotus (top) roll, thereby doubling the number of juice nozzles passing the discharge nip during each revolution of the mill rolls. CONCLUSON The experience at the Bawany Factory has revealed the following facts: 1. With all the mills equipped with Lotus rolls, a tandem can operate effec-
FGURE 2. PRESSURE-FED LOTUS MLL tively at a much higher throughput than that for which it was originally designed. 2. The Lotus roll eliminates 'flooding' thereby improving feedability which makes it possible for any mill to grind up to 30% more cane than before while maintaining excellent extraction. t is much more economical to upgrade the capacity and/or extraction of a milling tandem through the installation of Lotus rolls than through additional milling units. No modification work is required for the installation of a Lotus roll and experience has proved that two Lotus (top) rolls can be considered as being the equivalent of an additional mill.
830 FACTORY ENGNEERNG The final bagasse moisture can be reduced drastically by the installation of a Lotus (top) roll on the last mill of a milling tandem thereby making it possible for the boilers to generate more steam while using less bagasse than before. * No additional power is required for a mill to grind at the higher capacity obtainable through the installation of a Lotus roll. n Bawany, the mill turbines were replaced by stronger ones for the simple reason that the management was determined to optimise the performance of the milling tandem, i.e. to get the maximum extraction possible when grinding at maximum capacity. The actual costs of maintaining all of the five Lotus rolls of the Bawany factory amounted to $US 3093 for the whole of the 1984-85 crop, while costs incurred during the off-season for reshaping the shells for use during the 1985-86 crop amounted to $US 5125. ncidentally, the total cost of maintaining a Lotus roll shell is not more than that of maintaining a conventional cast iron roll shell, because of the fact that the Lotus roll shell is made of steel and, as such, can be maintained to have a much longer life than a conventional cast iron roll shell. While a cast iron roll shell might require replacement every two to three years, the life of a Lotus roll would be eight to ten years. Actually, by properly maintaining the shape of its grooves and replacing the juice nozzles every three to four years, a Lotus roll shell can be made to