The paper describes and discusses SB Reshellers new three-piece roller in place of two-piece having shaft, sleeve and the top shell. It is fabricated with es the inherently higher strength and modulus of elasticity of SBR Alloy2. The shaft is identical to the one conventional roller. The sleeve which is shrunk fitted over the shaft, has juice channels that are entrenched during casting of the sleeve. Those can be skewed or straight i.e. parallel to axis of the shell. Both the top and bottom rollers have circumferential grooves connecting the juice channels. The geometrical arrangement of the top shell is identical to top of the roller except that it has radial nozzles fitted to extract the juice from the high pressure nip in the mill. Top shell is shrunk fitted over the sleeve having circumferential grooving and nozzles etc. The benefits of the new three-piece roller compared with conventional rollers include higher extraction efficiency, bagasse that is over 2% drier and lower cost for refurbishing shell.
Most sugar mill rollers consist of two major components, a steel shaft and a cast iron shell shrink fitted over it. The shell has circumferential grooves to prepare the cane fibers for increased grip on the fibrous blanket and to provide an area for juice drainage. Due to corrosion and erosion during crushing operation, the shell grooves wear and need to be regrooved to form the profile required. During each regrooving, the shell diameter reduces considerably. Eventually, the complete shell needs to be replaced (normally after 6% reduction on the diameter) even though the rest of the shell is intact. Replacement of the shell is known as reshelling.
Lotus style internal bleeding rollers aim to provide additional juice drainage paths. The main juice channels are drilled / embedded, normally parallel to the axis of the shaft. The diameter of these juice passages is a function of space available between the bottom of the circumferential grooving and the bore of the shell. Substantially radial passages with inlet nozzles connect the root of the circumferential grooving to the main drainage channels. The diameter of the main drainage channel decides the size and number of the substantially radial nozzles that may be fitted. Further, the conventional shell may have provision for integral juice rings at either end to reduce the likelihood of juice entering the shaft bearings. Even if the juice rings are in good condition, they normally get replaced with the shell during reshelling.
After considerable experimentation of design and shell material, S. B. Reshellers Pvt. Ltd., has patented geometry1 of a roller having three major parts. This roller has a normal steel shaft, over which an intermediate sleeve is shrunk. This fitted sleeve has a length equal to the main shaft, i.e. shell length plus the width of the juice rings at either end. A shell is shrink fitted over the intermediate sleeve similar to a normal shell, to complete the roller. This geometry, with use of stronger SBR Alloy2 for the shell opened up various options for design of internal bleeding rollers, like multiple nozzles and skewed juice drainage channels. This paper discusses the mill roller described above and the advantages and other possibilities derived through this geometry.
Background of Internal bleeding rollers
In a typical conventional three roll mill, as the cane blanket moves through the mill, part of the extracted juice comes out on the top of the cane blanket and drains under the action of gravity to collection chutes at the ends of the top roll. However, part of the juice which gets extracted, remains pressurized in the cane blanket till the cane blanket comes out of the final nip of the rollers and gets reabsorbed in the cane blanket, thus decreasing the efficiency of the mill. An internal bleeding roller is one which, while crushing sugar cane, drains this pressurized juice from inside of the roller. To collect this pressurized juice inside the cane blanket, an internal bleeding roller has radial passages leading to axial passages which empty into the juice guard basin.
Jean Bouvet3 invented the basic internal bleeding roller, and called it a Lotus roller in 1978. SB Reshellers (SBR) has modified the Lotus roller considerably to remove its inherent defects and has patented the modified design in 1988 as the Kamal4 roll.
For many years, the roll shell material of construction has essentially been open grain cast iron. Open grains allow the easy creation of necessary rough surface for gripping the cane during crushing.
Though the internal bleeding roller was invented to reduce reabsorption, significant reabsorption still occurs when the juice passages are flooded due to the limited diameter of the juice passage. It is understood that the reabsorption takes place when the top roller surface comes out of the high pressure area (Figure 1). Due to its size, juice passages are prone to be filled up as the emptying rate is relatively slow. This results in the juice reissuing out of the nozzles and reabsorbed by the cane blanket, thereby, reducing the extraction.
Figure 1: Flow of cane through Mill conventional Kamal roller
SBR has introduced an innovative concept of a semi nodular cast iron shell in place of the conventional cast iron shell, the ‘SBR Alloy’ shell, which is a slightly closer grained casting, with higher tensile strength. Advantages of the open grained structure which offers higher coefficient of friction for gripping shell can also be easily incorporated by use of hard facing. The nodular structure offers better weldability6 than cast iron and consequently better arcing. The basic mechanical strength of the shell is increased to almost double with higher wear resistance. This higher strength allows multi nozzle geometry. Instead of one nozzle per groove per juice channel, two, three or even more nozzles per groove per juice channel can be incorporated. Use of SBR Alloy also reduces shell wear and consequently reduces the number of regrooving operations for the life of the shell.
The conventional shell is a single unit having an outside diameter equal to that of the roller and a bore equal to the diameter of the steel shaft5. Hence while carrying out reshelling of the roller (replacement of the shell because of reduction of the outside diameter due to wear), the complete shell must be replaced, including that part of the shell with the juice passages which are usually intact.
The three piece roller design
As indicated in the introduction, the new concept proposes three piece roller having shaft, sleeve and the top shell. The sleeve has the juice channels that are entrenched during casting of the sleeve. Those can be skewed or straight i.e. parallel to axis of the shell. The top shell is shrunk fitted over the sleeve having circumferential grooving and nozzles etc.
Top ‘Sleeve-Kamal’ sleeve
If the juice passages are made skewed, the resulting geometry reduces the stress concentration and increases the strength of the shell – shaft combination. Figure 2 shows that the sleeve has cast passages which diverge at 140O to 175O (preferably 166O) to each other from the center.
They are skewed on both sides in a herringbone pattern with reference to the axis through the center of the shell (and not 180O as in the case of passages parallel to the axis of the shaft).
Figure 2. Top roller sleeve with herringbone juice channels
Figure 3 shows the sleeve, similar to figure 2, but with added circumferential grooves and without the herringbone pattern of the juice channels. Bottom roller sleeve may have herringbone pattern too if necessary. In the case of bottom rollers, juice collected in the juice channels either empties into juice rings or due to gravity flows through these circumferential grooves to the bottom juice channels and empties in the juice rings. In other words, these grooves act as huge Messchaert grooves inside the roller.
Figure 3. Bottom roller sleeve with circumferential grooves
The second component of the shell replacement is the top shell which is constructed of SBR alloy (Figure 4) and shrink fitted over the sleeve. Figure 4 shows a design suitable for the herringbone juice channels (drilled and fitted with radial nozzles). Since both sleeve and top shell are manufactured from SBR alloy (having the same modulus of elasticity), the problem of fretting and fretting corrosion in this shrink fit are considerably reduced. For the bottom rollers, the shell may have straight line or herringbone nozzle configurations as suitable for the bottom roller sleeve.
Figure 4. Grooved and nozzle fitted top shell with herringbone geometry
It can be observed in Figure 5 that the length of the sleeve is more than length of the shell. This additional length of sleeve gives more support to the shaft resulting in reduction of fatigue stresses in the fillet portion in the journal and consequently a decrease in the chance of premature failure. Overall, the increase in the life of the shaft and reduction in corrosion and wear due to the reduction in the number of shrinking cycles reduces the shaft cost per ton of cane. Typically sleeve life is expected to be as much as shaft life hence requiring only one shrinking on the shaft (one of the early sleeves manufactured in 2006 is still in service on its shaft).
Since most of the juice ring is replaced by the sleeve, the size of the juice ring is reduced. In some cases, only a juice guard is fitted over the sleeve ends to form the juice rings.
Figure 5. Longer sleeve length than shell
The nozzles have a standard external Morse taper and are force fitted in the drilled radial taper holes leading to the horizontal main juice channels with additional adhesion from anaerobic adhesive. The nozzles are positioned above the juice passages of the sleeve so that the crushing process forces the nozzle inward thereby ensuring a tighter fit. The cross section of the nozzle is such that it has a taper or increasing bore size in the reverse direction to outside taper (Figure 6) ensures the nozzle remains free from clogging. The cross section of the nozzle passage increases allowing possible clogging bagasse pieces to flow through it.
Figure 6. Brass nozzles having increasing bore
The nozzles are made of soft brass which has better corrosion resistance than cast-in types of nozzles. The brass is soft enough to wear off as the shell wears and the nozzles do not obstruct the scraper like stainless steel nozzles do.
Operation of the assembly
In the top roller, the skew of the juice passages is arranged so that, while in operation, the center part of the line of drainage holes reaches the incoming cane first while the two ends of the line of drainage holes are above the horizontal plane passing through the center. As the roll rotates and the line of drainage holes moves towards the nip, expressed juice fills the juice passage (Figure 7). With the discharging ends of the juice passages being above the horizontal plane of the center, only excess juice flows from the juice passage into the juice ring basin. The balance juice passage remains full of juice. However, since bagasse pressure is high in the area indicated by ‘A’ and ‘B’, re-absorption is not possible. As rotation progresses, the line of drainage holes comes out of the high pressure area (‘C’) where the ends of the juice passage are lower than the center and the slope of the juice passage allows the juice to empty into the ring basin due to gravity thus reducing the likelihood of re-absorption.
Figure 7: Flow of juice for top roller (New design Kamal roller)
For the feed and discharge rollers, the geometry is different. As indicated in figure 3. The circumferential grooves, along with the straight or skewed juice passages (Figure 8 and Figure 9) act like huge internal Messchaert grooves. In the nip high pressure area indicated by ‘A’ and ‘B’ in Figure 8, expressed juice fills the juice passages and, by virtue of gravity, trickles down via Messchaert grooves to the bottom of the roller and out through either juice passages or nozzles that empty in the juice ring/guard. This geometrical arrangement significantly supports reduction in reabsorption. It also avoids the need to have Messachaert grooves, thus reducing the possibility of the shell failure.
Figure 8: Flow of juice for bottom roller (New design Kamal roller)
Figure 9. Bottom roller sectional details
The improvement in juice drainage is expected to lead to drier bagasse. Table 1 presents results from one sugar factory showing a two unit reduction in moisture. It is expected that the improved drainage will allow higher imbibition rates and consequently reduced pol in bagasse further. Lesser moisture in any mill improves the gripping capacity of the mill, and potentially increasing the crushing rate. The reduced slippage is expected to reduce the wear.
To sum the benefits:
- High extraction efficiency, through more drainage.
- Reduced reabsorption.
- Stronger mill roller in comparison with conventional. Due to higher strength SBR Alloy.
- Reduced wear. Higher life.
- Better weldability hence better arcing life.
- Drier bagasse. Lower % moisture in bagasse. Higher burning efficiency for boiler.
- Lower recurring cost of refurbishing (reshelling) due to lesser weight shell replacement in the subsequent reshellings.
- Patented Geometry: Application No. PCT/IN2015/000453
- SBR Alloy: I) Patent Application No. 1253/MUM/2004 dated 22nd Nov. 2004
- II) ISJ October 2013 Page 698
- Jean Bouvet: Lotus Roller US Patents Patent No. 4546698
- Kamal: Indian Patent No. 167469 Dated 12th Aug.1988
- Steel Shaft: Normally 45C8 or AISI /SAE 1045 / 4140 / 4340 etc.