2004_M_14

Schembri, M.G. Proc. Aust. Soc. Sugar Cane Technol., Vol. 26, 2004

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IMPROVING THE UNDERSTANDING OF CANE PREPARATION BY MEASURING THE PROGRESS OF POWER USAGE

AND CANE SIZE REDUCTION THROUGH THE SHREDDER

By

M.G. SCHEMBRI

Sugar Research Institute

[email protected] KEYWORDS: Cane Preparation, Shredder, Power, Grid Bar, Cane Size Reduction.

Abstract

MODERN shredders produce high levels of preparation (greater than 85 POC) but use considerable energy, generally 30% of steam turbine-generated power used in the factory. There is considerable interest in reducing the power consumption of the shredder. Part of the process of improving the shredder design/operation is to understand the shredding action in terms of power usage and cane preparation. This paper describes work using the SRI small-scale shredder (900 mm diameter) to obtain measurements on the progress of preparation in the shredder. The progressive power consumption and preparation through the shredder was investigated by sequentially removing components of the shredder (grid bar, anvil bar, front wall). At each stage, measurements of power consumption and preparation were recorded. For the three varieties used in the test program, the average findings were: hammers only 73 POC @ 2.63 kW.h/t; add front wall/anvil bar 81 POC @ 3.56 kW.h/t; add grid bar (0 mm) 90 POC @ 5.80 kW.h/t. The average results of power/preparation measured through the shredder were compared to typical factory shredder measurements and those from impact tests. The comparisons indicated that the front wall, anvil bar and grid bar sections were essential for the shredder to allow the shredder to achieve high levels of preparation, given the restriction of operating the shredder at 100 m/s tip speed, and that those shredder components represented a reasonably efficient method of greatly increasing the preparation which can be achieved by the shredder. The paper also details a series of trials that were conducted aimed at increasing the cane preparation in the low power consumption, hammer only configuration. The options investigated were different launch speeds of the billets into the shredder (2.5 m/s. 5.0 m/s, 8.3 m/s and 10.6 m/s), variations in billet lengths (70 mm, 220 mm and 700 mm), and recycle of a portion of the prepared cane. The various launch speeds were found to produce relatively similar preparation levels. The smaller billet lengths resulted in higher preparation, which was expected. It was noted that the energy required to cut the cane into short billets would detract from the apparent power/preparation benefits of short billets. Recycling the coarse components was found to raise the overall preparation from the 73 POC region typical of hammers only to 85 POC, but the total power usage was only slightly less than from the conventional shredder (i.e. with the front wall, anvil bar and grid bar in place).


________________________________________________________________________________________________ Introduction

The performance of factory milling trains and diffuser plants is strongly dependent on the preparation of the cane achieved in the shredder. Modern shredders produce high levels of preparation (greater than 85 POC) but use considerable energy, generally 30% of steam turbine-generated power used in the factory. For many factories, the shredder restricts capacity or extraction because extra power cannot be applied or the cost of a larger power plant cannot be justified. Therefore, there is considerable interest in reducing the power consumption of the shredder. Part of the process of improving the shredder design/operation is to understand the shredding action in terms of power consumption and cane preparation.

Comprehensive shredder studies were conducted at SRI in the 1970s (Cullen and McGinn, 1974); these empirical investigations resulted in guidelines for the operation of the shredder. The current, industry shredder is well understood in terms of set-up configuration, operational conditions and resultant cane preparation. Cullen and McGinns investigation also included high speed photographic analyses of the shredding process. The images showed that a significant portion of the cane preparation occurred at the shredder entry, where the cane billets were impacted by the hammers. The smashed cane pieces were thrown against the front wall, and under gravity flowed into the grid bar section, where the material was subsequently further prepared. The high-speed imaging provided qualitative information on the progression of preparation in the shredder. No quantitative measurements of the internal progression of preparation were undertaken.

The work reported in this paper was aimed at quantifying preparation and absorbed power as cane progresses through a conventional shredder. This information will add to our knowledge of the current shredder and cane preparation in general, and ultimately to improving cane preparation.

Methodology

This section details the shredder used in the test program, the technique used to measure power usage, and changes in the feeding arrangement to improve the flexibility in test conditions. The procedure used for each test is also described.

Shredder power measurement

The SRI small-scale shredder was selected as the shredder to be used for the power usage and cane preparation measurements. Such measurements would be very difficult in a factory shredder given the crucial role a factory shredder has in the milling process. SRI technical staff investigated measuring power (accurately) from the variable frequency drive used to govern the speed of the shredder motor. It was found that the drive had the inbuilt capability to output actual power. This power signal was found to be reliable and sufficiently responsive to be suitable for the relatively short batch tests typically conducted using the SRI shredder. The power for a trial was taken as the average of the steady state power readings.


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A sample of the power and speed measurements is presented in Figure 1.

Fig. 1Plots of power and shredder speed versus time for a shredding test; cane

flow commenced at 1 s (approximately) and ceased at 12 s.

Shredder feed system

The original feed system for the SRI shredder (refer Figure 2) consisted of a conveyor of length 2 m, mounted approximately 1.3 m above the shredder. The cane was poured into the shredder where, under gravity, the cane was accelerated to 5 m/s prior to impacting with the high-speed hammers of the shredder.

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Fig. 2Schematic diagram of the SRI shredder and its original feeding system.

Referring to Figure 2, the cane enters the shredder at the impact zone (entry section of the

shredder where contact is made with the hammers). The smashed cane then travels down the front wall, and is processed through the grid bar section, where further size reduction occurs.

Modifications carried out on the feeding system were:

The feed conveyor was moved from above the shredder to in-line with the shredder (refer Figure 3).

A cane elevator was installed to supply cane to the shredder feed conveyor (refer

Figure 4). At minimum, this extended the batch test time from 5 s to a possible 14 s.

The modification of moving the feed conveyor belt from above the shredder to in-line with the shredder allowed the cane supply to be presented to the shredder under different feed injection speeds. The feed conveyor could be varied from 1 m/s to 10 m/s, thereby allowing the examination of the effect of cane inlet speed on preparation. This feature was desirable in the investigation to identify means of increasing preparation without the grid bar. It was anticipated that the impact conditions at the hammers would improve as the cane injection speed was increased.

FEED CONVEYOR


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The other factor to note with this change to the feeding system was that the change did not alter the shredder operation. Even with the modified feed, the cane still enters the impact zone where contact is made with the hammers. The smashed cane is thrown against the front wall where it subsequently flows into the grid bar section, and is further reduced in particle size.

Fig. 3Schematic diagram of the SRI shredder and the modified feeding system.

Fig. 4Cane elevator used to supply the SRI shredder.

FEED CONVEYOR

FEED DRUM

CANE BILLETS ENTER


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Shredder dismantled

The front wall, anvil bar and grid sections were removed from the shredder in order to commence the test program. The only modification required to the shredder to operate without these sections was a deflector plate to direct the shredded material down toward ground level, rather than onto neighbouring properties. The deflector plate was positioned well away from the hammers so as not to influence the preparation/power measured under hammer-only operating conditions. The no-load shredder power changed from 18 kW to 21 kW when these sections were removed, most likely due to the shredder acting like a fan.

Test procedure

The procedure used for each test was as follows:

Clean stalks were cut into billet lengths (approximately 220 mm) with the top, growing section of the stalk discarded.

36 kg of billets were evenly spread along the length of the cane elevator. The speed of the elevator was set so as to pour the cane onto the feed conveyor of the shredder in 13 s, thereby giving a pour rate of 10 t/h. (Note: dropping the cane onto the relatively fast feed belt would have resulted in most billets having their longitudinal axes perpendicular to the rotating hammers).

The shredder was brought to the selected test speed, in this case 2100 r/min or a hammer tip speed of 98 m/s, typical of modern factory shredders.

The cane elevator was operated until the shredder processed all of the cane. Power was recorded during the test event.

The prepared cane was collected, mixed, sampled and analysed using Method 5 of the Laboratory Manual for Australian Sugar Mills (Anon., 1991).

The test program consisted of replicated trials of three shredder conditions (see Table 1) for three varieties of cane. The varieties (and fibre content in brackets) were Q124 (11.4%), Q138 (14.8%) and Q190A (14.1%).

Table 1Shredder configurations used for the test program.

Shredder condition Description

Hammers Front wall and grid bar section removed. The shredder consisted of the hammers only.

Front wall Front wall installed, no grid bar. Leading edge of front wall (anvil bar) set approximately 45 mm from hammers.

Grid bar Front wall and grid bar installed, i.e. standard shredder configuration. Grid bar set at 0 mm. Results

Progression of power and preparation through the shredder

The results of the test program are presented in Table 2 and in Figures 5 and 6.


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Table 2Preparation and power usage for the shredder configurations for the three cane varieties used in the test program.

Cane variety Shredder configuration Preparation

(POC) Specific power

(kW.h/t) Q124 Hammers 75.9 2.73

Front wall 87.2 4.07 Grid bar 90.1 5.76

Q138 Hammers 71.2 2.67 Front wall 75.5 3.64 Grid bar 90.3 5.82

Q190A Hammers 73.0 2.5 Front wall 80.0 2.97 Grid bar 89.0 5.81

Average Hammers 73.4 2.63 Front wall 80.9 3.56 Grid bar 89.8 5.80

Fig. 5Preparation results for the shredder configurations for the three test cane varieties.

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Fig. 6Power usage results for the shredder configurations

for the three test cane varieties.

The results in Table 2 were analysed using a two-way ANOVA to take into consideration the varietal and shredder configuration effects. The analyses showed that differences between the shredder configurations were statistically significant (p


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Low power cane preparation

As mentioned above the SRI shredder without the front wall, anvil bar and grid bars resulted in a 54.6% reduction in the power consumption (albeit with a 16.4 POC fall in preparation). Several trials were conducted aimed at increasing the cane preparation in this hammer only configuration. The options investigated were:

Different launch speeds of the billets into the shredder; inlet billet speeds trialled were 2.5 m/s, 5.0 m/s, 8.3 m/s and 10.6 m/s.

Variations in billet lengths (70 mm, 220 mm and 700 mm). Recycle of a portion of the prepared cane. A test sample of cane was processed

through the shredder in its hammer-only state. The smashed cane was collected and passed through a 7.5 mm sieve, whereupon the coarse material (the material remaining in the sieve) was re-presented to the shredder.

The results for the launch speed and billet length trials are presented in Figure 7. For all these trials, the power usage was similar, in the range 2.52.7 kW.h/t, and corresponded to that of the hammers only trials.

Fig. 7Levels of preparation achieved for the launch speed and billet length trials.

The effect of launch speed of the billets into the shredder hammers on preparation proved to be negligible. (Note that the pour rate was constant throughout the launch speed trialsin other words, pour rate effects did not confound the results). It was expected that the higher launch speeds would produce greater levels of preparation. The process at the hammers is impact; therefore, by optimising the impact event, the amount of damage should increase. At operating speed, the time between successive hammers sets is 7.5 ms, thus by increasing the speed of the billets into the hammers, more of the billets will be hit by the full face of the hammers, thereby increasing the

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________________________________________________________________________________________________ efficiency of the impact loadings. The tests showed that this was not the case; that is, increasing the launch speed 4 fold (from 2.5 m/s to 10.6 m/s) did not lead to improved preparation.

Billet length effect on cane preparation was considered because:

Billet lengths less than the typical 200220 mm length may improve cane preparation because in a sense the shorter length represents further preparation anyway.

Longer lengths than typical would mean heavier billets, such billets have more

inertia and, potentially, could assist the impact event to cause damage. The tests showed that the shorter lengths gave higher cane preparation. It must be

remembered of course that shorter billet length requires more power (every cut consumes energy); this additional power was not measured.

Combining the measurements of the recycle tests, the overall cane preparation was approximately 85 POC at 4.0 kW.h/t. This latter result was interesting given that the level of preparation was achieved using only the hammers of the shredder; that is, the front wall, anvil bar and grid bars were removed. This option was not further investigated because the saving in power was not sufficient to justify the efforts involved in screening the smashed cane and re-processing the coarser components. For the SRI shredder, in full configuration (front wall plus grid bar), 85 POC would be achieved at 44.5 kW.h/t. Therefore, in terms of power usage and preparation, recycling the coarse components would not be advantageous over using the shredder in its standard configuration; the only benefit of the recycle option in hammers only configuration would be the reduced maintenance associated with the absence of the front wall/anvil bar and grid bar sections.

Improving the understanding of cane preparation

The previous sections have presented the results for preparation and power usage in the shredder, and examined methods to increase preparation at the relatively low power usage which occurs in the hammers only configuration. This section shows how these results have improved the understanding of cane preparation.

Figure 8 presents the average results for preparation and power usage displayed in Table 3, and the results from previous shredder and sugar cane impact investigations.


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Fig. 8Comparisons between relationships of preparation and power usage.

The shredder tests curve was measured by Cullen and McGinn (1974) using a 1450 mm (swept diameter) shredder. The data points for the curve were obtained by varying the grid bar setting. Note that the average results from the present test program fell reasonably closely to the shredder curve measured by Cullen and McGinn.

The impact tests curve was measured by Schembri and Harris (1996). The impact tests were a series of single impact tests: in each single impact test, one section of cane was hit by a projectile whose diameter was the same as the length of the cane segment. The cane segment was positioned so as to be uniformly impacted by the projectile so as to prevent energy being wasted rotating the cane segment.

The velocity of the projectile was varied in order to change the level of preparation whereas, in the shredder tests (including those of the present study), hammer tip speed was held constant, and the position (or absence) of the grid bar was used to vary the levels of preparation. This impact test curve represents the maximum preparation that can be achieved for a given power input, under impact loading conditions.

The impact tests (Schembri and Harris, 1996) showed:

The logarithmic nature of the power/preparation relation for cane. This is understandable given the structural nature of the cane plant: the pith region is mainly storage cells, and easily broken apart. The outer 1.52 mm of the cane stalk is the rind region that has few storage cells and is dominated by fibrovascular bundles. The rind region has higher strength and provides support and protection for the mass of storage cells in the pith. While the pith region is easily broken apart, the rind requires significantly higher levels of loading. Consequently, preparation to 7375 POC can be

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attained at low energy levels, but to move into higher preparation required size reduction of the rind and therefore high energy.

Higher impact velocities produced greater levels of preparation, which was

expected.

As mentioned above, for the impact tests, the velocity of the projectile was varied in order to change the level of preparation, whereas for the shredder set at 100 m/s tip speed, the grid bar setting was used to adjust the preparation. The interesting feature is the similarity of the shredder and impact test curves (assuming the impact test curve when extrapolated remains relatively parallel to the shredder curve). The similarity suggests that the grid bar is a reasonably efficient method of achieving higher preparation while maintaining impact speed. If the grid bar was energy inefficient, then the curves would be noticeably divergent. Therefore, in comparing the impact test and shredder curves:

In order to restrict the tip speed to 100 m/s, the front wall and grid bar are very important in order to allow the shredder to achieve high levels of preparation.

The front wall and grid bar are a convenient and reasonably efficient method to

boost the preparation capacity of the shredder. Common industry thinking was that the grid bar caused the shredder to absorb excessive energy. But what we have seen is that while the grid bar does cause high power usage, it also facilitates large increases in preparation, and still relatively in line with the expected power/preparation curve for sugar cane. This latter observation suggests that the front wall/anvil bar and grid bar assembly is a reasonably efficient method of allowing a constant speed machine to gain high levels of preparation.

Conclusion

The measurements of the progression of power consumption and preparation in the shredder showed that the inclusion of the grid bar assembly (set at 0 mm) doubled the power usage but significantly improved cane preparation. Cane preparation without the grid bar was relatively coarse (less than 75 POC) but, with the grid bar, the preparation was high, in the region of 90 POC.

Trials without the grid bar showed that it was difficult to improve preparation beyond the 75 POC region, unless significant extra energy was used such as cutting the cane into small pieces prior to the shredder, or recycling the coarse component of the prepared cane.

The comparison of the power usage/preparation measurements with previous impact test results showed that, in order to restrict the shredder tip speed to 100 m/s, the front wall and grid bar were essential for the shredder to achieve high levels of preparation.

Acknowledgements

Financial support for this work was provided by Sugar Research and Development Corporation and the Australian raw sugar factories. The author wishes to thank SRI staff members Neil McKenzie and John Williams for their work in the test program.


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REFERENCES Anon. (1991). The Standard Laboratory Manual for Australian Sugar Mills. Volume 2: Analytical

methods and tables. Bureau of Sugar Experiment Stations Publications, Brisbane. Cullen, R.N. (1986). The influence of shredder design on cane preparation, Proc. Int. Soc Sugar

Cane Technol., 19: 831838. Cullen, R.N. and McGinn, J.A. (1974). Shredder Performance and its Effect on Milling. Sugar

Research Institute, Technical Report 124. Schembri, M.G. and Harris, H.D. (1996). Measuring the preparation of sugar cane under single

impact loading. Proc. Aust. Soc. Sugar Cane Technol., 18: 242248.

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2004_M_14