Sugar mills use “XM rolls” (rolls with internal drainage) in order to reduce juice reabsorption during the milling process. Nevertheless, it is almost unavoidable that both nozzles and the internal drainage channels of the XM rolls get clogged with solids as the milling season advances. To minimize this problem, different kinds of nozzles have been designed, always keeping the basic principle of being conical and divergent. The best results were obtained with the new design that consists of nozzles with multiple holes that together maintain the same area as the original design with one-hole nozzle. Tests started in 2015 with one nozzle and were followed by several tests in different sugar mills until the XM rolls started to be used with all new-type nozzles and in all mill positions in the tandem (top, feed, delivery or fourth roll). Besides avoiding the clogging with bagasse, better and more stable milling indicators were obtained during the milling season. Currently, XM rolls with all their nozzles of the new type and of different diameters and number of holes (three to eight) are being fabricated. At the same time, different materials for the nozzle are also being tested to achieve an optimal lifespan to match of the XM roll itself.
For decades, during the extraction process in the sugar mills, high levels of juice reabsorption were almost unavoidable, increasing bagasse moisture and reducing sucrose extraction. Mill designers have been working towards new methods and devices to fight against this reabsorption phenomenon: Messchaert grooves, vacuum suction, lower grooving angles, Lotus rolls and XM rolls, for example.
Lotus rolls were developed 40 years ago and were made of low carbon cast steel. Extraction and moisture indicators were good, but their high manufacturing costs and time-consuming maintenance procedures due to bagasse clogging drainage channels limited their success (Rein 2017).
In 2007, XM rolls, with an internal drainage system and made out of cast iron, were produced (Figure 1), avoiding reabsorption mainly due to the separation of juice from the bagasse during the compression stage. These XM rolls have nozzles installed at the bottom of the teeth in order to drain the juice that will be then axially conducted through a tube system that goes from one end of the roll to the other. This way, the evacuated juice falls freely to the juice collector tray (Rein 2017). Details of the XM rolls were provided by Sanchez and Chavarro (2013).
With rolls of this type, there is a high chance of bagasse clogging of the axial tubes due to the solid particles that get into the nozzle holes. Basically, the problem exists because the axial tube or passages cannot be of a larger diameter due to the lack of space. The axial tubes are limited in terms of quantity and a maximum diameter of 64 mm.
Figure 1. Top XM roll showing the drainage system
Discharging to these axial tubes, there are more than 2000 nozzles in the more advanced versions of XM rolls (Figure 2). These nozzles have a single hole with diameters between 7 mm and 9 mm.
Figure 2. Conventional, single perforation nozzles
Use of nozzles has been increasing year after year, taking advantage of better cast iron material used for their manufacture. In 2007, XM rolls had a density of 108 to 129 nozzles per square meter surface. Today, the density can go up to 215 nozzles per square meter when the roll is made out of cast iron and up to 323 nozzles per square meter when ductile iron is used. With more nozzles feeding the axial tubes, the potential for clogging is increased.
For small mills, the bagasse clogging problem is more critical. Their drainage tubes are proportional to the general dimensions of the roll and cannot even reach 51 mm diameter and the size and shape of the bagasse fiber remains the same as in larger mills producing a limitation in the drainage capacity. Bagasse fiber shape and length are a result of the preparation process (shredder and/or cane knives) and the compression work of each mill unit. Another situation that does not help in small mills is the lack of space between the roll ends and the housings. The cane fiber is the main cause of bagasse clogging, it intertwines and forms knots that catch the rest of the solid particles present (stones, soil, sand, welding remnants, etc.).
In addition to clogging of the axial tubes, there is also clogging inside the nozzles themselves, mainly with stones and metal particles that “match” the nozzle hole diameter. Conventional nozzles have only one hole with a diameter between 6 mm to 10 mm depending on the supplier’s design and the subsequent wear caused by the milling process (from corrosion or reduction of the diameter of the roll). The nozzle hole diverges, and so its diameter increases as the roll starts losing diameter. One solid particle can block a hole, preventing drainage through that particular nozzle.
To address this clogging situation, new filtering nozzles were designed, with the main purpose to restrict the passage of all the solids, especially the sugarcane fiber of any size. The new design of nozzle has multiple holes. If one of the holes gets clogged, the others will continue to drain the juice. In the field tests conducted, very few cases of clogging were reported, with the majority of the small particles remaining with the cane flow.
Implementation of the new filtering nozzles
Trials with nozzles with multiple holes and of different diameters were conducted and led to the conclusion that the risk of clogging inside the axial tubes is reduced with nozzle holes of smaller diameter. Nozzles with holes from 3 mm to 13 mm were tested. Larger diameter holes allow for higher flows of juice, but at the same time more solid particles and larger solid particles are passed through the nozzle. Cane fiber was the main solid particle present and the larger the fiber, the bigger the probability of that fiber intertwining or forming knots that can block the passages and reduce the juice flow. By reducing the hole diameter, the solid particle size that can pass through the nozzle is less and, more importantly, the amount of solid particles being released into the axial tubes that could later cause blockages is reduced.
Best results in terms of reducing internal clogging of the drainage system were obtained with holes with diameters from 3 mm to 4 mm. With every hole being conical, diverging toward the interior of the roll, the exit hole is larger than the entry hole and the number of holes that can be machined in every nozzle is limited. This new type of nozzle with multiple holes has been named filtering nozzle. The filtering nozzle not only drains the juice but it also filters out solid particles, reducing the amount of bagasse and other solids contained in the juice. In contrast to the one-hole nozzles, very few solids were found clogging the filtering nozzles.
To keep the same drainage area as the conventional one-hole nozzles, several conical holes are required. To replace the 8 mm conventional nozzle, a nozzle with an external diameter of 16 mm containing six 3-mm holes evenly distributed was utilized (Figure 3).
Figure 3. 16-mm filtering nozzles with six holes.
Following the same reasoning, eight holes were utilized in the 19-mm diameter nozzles (Figure 4), four holes were utilized in the 13-mm diameter nozzles and three holes were utilized in the 11-mm diameter nozzles.
Figure 4.19-mm diameter filtering nozzles with eight holes.
Three factors related to the roll geometry determine the external diameter of a nozzle: angle, pitch and the setting of the nozzle that connects simultaneously to the same drainage passage (Figure 5). A smaller angle and pitch between teeth will result in a smaller outside diameter in the nozzles. In order to avoid teeth breakages during milling, the diameter is less than half the height of the teeth. Currently, there are six XM roll types, depending on the nozzle configuration: singles, doubles, triples and combinations among them. Table 1 shows the types of filtering nozzles (patented) and the recommendations for their use according to the grooving. Nozzle length depends on the roll dimensions.
Figure 5. Radial nozzle distribution in a type 4 XM roll.
Filtering nozzles – some examples of results obtained
The first trial of the filtering nozzles in a sugar mill was in Colombia in 2015. A total of 80 filtering nozzles (16 mm diameter, six holes each) were installed across the roll connected to one of the 18 drainage tubes of an XM 2134 mm top roller with 51 mm pitch, installed in a sixth mill. After 3 months of crushing, the roll was disassembled to find out that, of the 18 drainage tubes, only one was free from bagasse clogging, the one connected to the filtering nozzles. The other 17 tubes were so clogged that it was not possible to see from one end of the tube to the other.
The next year, three more trials were made in different Colombian sugar mills with identical results:
- The first sugar mill had a 1219 mm roll with 38 mm pitch. All 720 nozzles were of the filtering type, with half having four holes and the other half having six holes.
- In the second sugar mill, 164 nozzles with six holes each were installed along one of the tubes of the top roller of a sixth mill with rolls of 1829 mm length and grooving of 51 mm pitch.
- In the third sugar mill, 54 nozzles with eight holes each were tested in a top roll of 51 mm pitch and 54 nozzles with six holes each were tested in the top roll of a sixth mill with 38 mm pitch. Both mills were of 2134 mm length.
The conclusions in all three sugar mills were the same: all the axial tubes connected to filtering nozzles were clean, while those with conventional nozzles were totally or partially clogged.
In 2017, trials continued in the remainder of the Colombian sugar mills and some were also conducted in other countries: Barahona in Dominican Republic (5th 2134 mm mill), Paramonga in Peru (2nd and 6th 1981 mm mills) and Ledesma in Argentina (6th 2134 mm mill). In all three sugar mills outside Colombia, six-hole nozzles were installed connecting with several axial tubes with the same satisfactory results: no bagasse clogging and no holes clogged. Eventually, one of the six holes in one nozzle of one roll revealed a small stone but the other five were draining correctly.
In the second half of 2017, manufacturing of XM rolls with all nozzles of the filtering type commenced for several Colombian sugar mills.
A Colombian sugar mill with an XM top roll of its sixth mill with 38 mm pitch and 1458 nozzles of six holes each achieved one of the best results ever during the crushing period from January to May 2018:
- Bagasse moisture: 46.2%
- Bagasse pol: 1.42%
- Reduced extraction: 97.4%
- Imbibition water: 26.2% on cane
- Cane fiber: 16%
- Average crushing rate: 10,000 t/d.
Azucarera Nacional in Panamá with an XM top roll of its sixth mill with 51 mm pitch and 1280 nozzles of six holes each in the 2018 crushing season achieved:
- Bagasse moisture: 46.2%
- Bagasse pol: 1.93%
- Reduced extraction: 96.2%
- Imbibition water: 29.9% on cane
- Cane fiber: 15.2%
- Average crushing rate: 6500 t/d.
XM rolls with filtering nozzles have been manufactured for sugar mills in Guatemala, Costa Rica, Honduras, Panamá, Dominican Republic, Peru, Ecuador, Bolivia, Argentina, México, Belize and the USA (Louisiana and Florida).
A special application was developed in 2018 when a mill with all its rolls of the XM type, 25 mm pitch, was used to dry the pith (residue of the depithing process of the bagasse in a paper production plant) in order to use this pith as fuel for the boilers. This pith has a very high moisture content (75%) and the mill was able to reduce it to 50%. Filtering nozzles of 11 mm diameter and three holes each were used. After the first 3 months of operation, no clogging was reported.
In two Colombian sugar mills, the filtering nozzles were not able to drain all the juice observed over the top roll, a situation that was not the case when the conventional one-hole type of nozzle was used. This situation was reported during the first weeks of the milling season due to the high amount of mud entering the mill from the cane field. In Colombia, the milling season is more than 310 days long and crushing continues even when it rains. When there is mud and dirt present, the juice flow is slower due to the small holes. Nevertheless, the holes never clogged and their drainage efficiency recovered once the rainy season ended.
Something similar happened in a sugar mill in Florida (SCGC). At this mill, it is normal to crush the cane with a high mud content. In contrast with Colombia, in Florida the milling season lasts 5 months and during this time, the cane enters the mill with a mud content of around 8% due to the type of soil and the mechanical process. There is no cane table or any dry or wet cleaning system in place. To overcome this condition, the XM top roller drainage capacity must be increased. There are several paths: more nozzles or holes or better cleaning routines with high pressure water. The use of filtering nozzles has helped to address this problem.
Among other new developments, the possibility of manufacturing XM rolls with some nozzles of each type (conventional and filtering) is under evaluation, in order to adapt the roll to changing crushing conditions, like winter and raining season when a lot of mud and soil enters the milling tandem.
Discussion and conclusions
Nozzles with holes of smaller diameter have been proved successfully to avoid clogging of the axial tubes by not letting the bagasse fibers enter. Filtering nozzles limit the size and amount of solids entering the axial tubes of the XM rolls. Only a few of the small stones or metallic remnants in the cane get trapped in the entry side of the holes. In the event of one of the holes getting clogged by a stone, the rest will continue to drain.
Filtering nozzles of 19 mm diameter with eight holes generate 33% more juice draining area when compared with conventional nozzles with only one 8 mm hole. They are very useful in 76 mm pitch grooving where it is not possible to install a large number of nozzles.
Filtering nozzles helps the draining process in mills with very little space between the housings and the rolls. They also allow for the design of rolls with reduced diameter of the draining passages due to the sealed system used and small rolls.
By controlling the passage of solids through the axial tubes, it is possible now to design XM rolls for the 4th roll position and for bagasse rolls, both with a strong tendency to clog with the conventional nozzles.
Maintenance times and costs are both reduced with this new type of nozzle. The nozzles and axial tubes remain clean and open for longer periods of time.
Small holes (3 mm to 4 mm in diameter) reduce damage from scraper tips hitting the nozzles.
Mills have been able to achieve better and more stable average performance indicators (extraction and pol and moisture in bagasse) after fitting filtering nozzles.
It is important to select the right filtering nozzle type taking into account the expected mud content of the cane.
* Paper presented at the XXXth Congress of the International Society of Sugar Cane Technologists, Tucuman, 2-5 September 2019 and published here with the agreement of the Society.
Rein P. 2017. Cane Sugar Engineering, 2nd edition. Bartens, Berlin.
Sánchez JJ, Chavarro S. 2013. Reducing sugarcane juice reabsorption in sugar mills using mill rolls with internal drains: greater sucrose extraction and lesser moisture in bagasse. Proceedings of the International Society of Sugar Cane Technologists 28: 1710-1722.