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10.1.3

Notes for the Evaluation

After the material balance for a mill tandem made completely as shown above, the next step
would be the making of evaluation.
The evaluation for the performance of a mill tandem can be made in 3 (three) steps:
Overall for the whole tandem.
Individually for each unit of mill.
Comparing the Brix Curves.
The criterions used for the evaluation, for both the overall and individually of a mill tandem is
shown above (see the sheet on page-2).
The main points essentially to be evaluated for the performance of a mill tandem overall are
comparing the actual figures achieved with the planned figures made by the projection, such as:
The crushing capacity.
The imbibitions water % fiber as well as % cane.
The mixed juice % cane and its purity (HK).
The last mill bagasse for its quantity (in %cane), %pol, %Brix and the dry matter. Also the
juice in bagasse %fiber (nss) and the loss of juice %fiber (gnhs).
The results of extraction for Brix in mill #1 (HPB-I), the total Brix by the tandem (HPB-t)
and the sugar extraction (HPG).
The ratio of juice purity (PSHK).
The crystal, for obtain-ability (kt), which contain in the mixed juice (knm) and the loss
relatively in the last mill bagasse (khar).
While the main and essential items, which should be evaluated individually for each mill, among
other things are the followings:
a. The Extractions
There are 3 (three) extractions in cane milling:
The juice extraction,
The Pol extraction, and
The Brix extraction.
The value of juice extraction is good if more than 60%, and gradually decreasing for the
ensuing mills. The same criterion applies for the value of Pol and Brix extractions.
b. The Compression Ratio
The compression ratio (K) is a ratio between the volume of the incoming material and the
escribed volume of the delivery opening. The value indicates whether the pressing work of
the milling is good or not.
The value of the compression ratio depends on the ability of the respective mill rollers in
use. Mainly the material, which in turn should provides ability of forming raw surface
against acid condition of juice during the occurrence of extraction.
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For a good roller material, beside the result of a good compression ratio will also obtaining
optimal fiber index (c).
c. Is juice extracted by the feed opening ?
As it was expressed in Chapter VII, that if the value of y = positive, then juice is really
extracted by the feed opening. But likewise, if the value of y = negative, it is concluded that
the feed opening of the respective mill in average does not extract any juice.
The value of y is also indicating the presence of slip if a too high value defined. Generally
speaking a heavy slip is presence, if too many juice extracted by the feed opening when
compared to the juice extracted by the delivery opening.
Beside of no extraction made by the feed opening when the value of y = negative, it also
indicates that the feeding of the respective mill is in average lesser and not stable.
The normal value of y is between 0,10 0,30. It means the juice volume extracted by the
feed opening is between 10 30% from the volume of juice extracted by the delivery
opening. Then it is suggested that the most extraction should due by the delivery opening to
avoid the occurrence of heavy slip in a respective mill. The condition will be achieved when
the ratio between feed and delivery work opening (i) actually sets referring to the value
defined by the projection of the material balance (the Projection Program).
d. Is there any deflection made by the top roller ?
Among the 3 (three) rollers in use, the top roller is attaining the most pressing work in a unit
of mill (attaining the pressing due by the feed and delivery openings). And because the two
upper halves of the top metals support it, then a deflection will occur on the shaft and the
roller shell as well. Forged steel material made for the shafts, while cast iron or cast steel is
for the roller shells. Based upon the material, the deflection of the roller shell should be
intensely monitored to prevent the roller breakage during the course of milling.
The different between the actual work opening (calculated) and the work opening when set,
and then divided by the roller length will show the value of deflection made by the roller.
The limit value of deflection (maximum) for the roller shell made of cast iron/steel is around
0.22%. If the average value of deflection exceeding the limit, it is definitively concluded that
the roller works in danger. The occasion would not happen, if the setting of the respective
mill done precisely according to the calculation made by the projection program of the
material balance. Especially when the centers to center distances between the rollers were
not checked during the setting of mills to prove its accuracy, or, if the roller lift (hydraulic
lift) sets exceeding or higher than the dimension determined by the calculation (the material
balance).
e. Everything related to bagasse
Items evaluated in bagasse among other things are:

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The no-void density (da). It is best when the value gradually increasing from mill #1 up to
the last mill.
The absorption ability factor (r). Its value should gradually decreasing from mill #1 up to
the last mill.
The value of normal absorption ability factor (r) also gradually decreasing.
The bagasse percentage to cane (gat) gradually decreasing.
%pol (pa) and %Brix (ba) also gradually decreasing.
The dry matter (zk) is best when gradually increasing.
f. Everything about fiber
The evaluation should be at least focused to the followings:
The fiber index (c).
The fiber index normally expressed in kg/dm3 of the volume escribed by the delivery
work opening is to know the fiber true density when pressed or milled under the rollers.
If the value obtained from the calculation of mill material balance compared to the value
from the table (see the Java Method of Mill Setting, E. Hugot third edition 1986, page
201-203), then well know the true capability of the roller shell material in use. The
higher the value of fiber index, meaning the best material for the roller is and that it has
the character of building up raw grain surface. Therefore the value of its fiber index
obtained from the calculation showing the capability of the respective mill.
The fiber loading (q).
The fiber loading expressed in gram/dm 2 of the roller surface is to know the rate of average fiber distributed to the roller surface during the pressing of cane or bagasse under
the rollers. Besides of that, also to know the difference when compared to the value
obtained from the table (see the Java Method of Mill Setting, E. Hugot, third edition
1986, page 201-203).
For the mills driven by reciprocating steam engines, where the roller rotations decreasing
from the first to the ensuing mills, the values of the fiber loadings gradually increasing.
But it is not the case with mills driven by steam turbines or by electric motors. Steam
turbines have nominal speed, where the efficiency could be achieved optimally. Thus for
the mill tandem with steam turbine or electric motor drives, the rotation of the rollers
should be set based on the nominal speed of the steam turbines or the electric motor
respectively.
When steam turbines used for the tandem, the fiber loading should refers to the capability of the normal 30 mills or it is usually called the reduced fiber loading (q), whereas
if the value lies between:
120-130 gram/dm2 is normal. It means the respective mill is sufficient when provided
only with an ordinary feeder roller for the feeding device.
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130-140 gram/dm2, besides the feeder roller the respective mill also needs to be
provided with a Donnelly chute to perform a reasonable feeding.
140-160 gram/dm2, needs a light-duty pressure feeder.
150-170 gram/dm2, needs a heavy-duty pressure feeder.
The fiber percentage to bagasse (kf).
The value gradually increasing from mill #1 to the ensuing and up to the last mill, also
should be compared with its normal value.
10.1.4

Conclusion

From the example calculation of mill material balance and its evaluation as described above,
which based on the local condition, especially for the cane quality and the existing mill train
with their results of analyses, a brief conclusion could be made within the following items:
1. The Extraction
Generally the milling tandem of 2 CC + 5 mills was performing an in-adequate extraction.
The conditions can be seen from the following values, which were below the normal and
even lower or worst than the targets, such as:
Description

Reached

Targets

% Targets

Mixed juice % cane

85,47

100,00

85,47

Brix extraction of mill#1 (HPB-I), %

49,23

53,19

92,55

Total Brix extraction (HPB-t), %

93,34

93,67

99,65

Sugar extraction (HPG), %

94,14

97,00

97,05

Crystal in mixed juice, % cane

8,59

8,99

95,55

Uncounted the less quality of cane, all the above values of extractions were below the targets
that hopefully should be reached.
It was happen among other things, because the imbibitions water was given below the
required rate. Secondly, the mills were not set correctly to anticipate poor quality of the cane
crushed. The value of Total Brix Extraction (HPB-t) that was nearly reaches the target
(99,65%) is not fully good enough, because the important thing is the actual sugar extraction
obtained (here HPG only 97,05% from the target). Therefore, the overall crystal gained and
contained in mixed juice only 95,55% from the target of 8,99% cane.
2. The Compression
The values of compression ratios are lower than the normal, it can be seen from the
following table:

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Mill, number
Actual compression ratio, K
The normal value

II

III

IV

2,20

2,73

2,49

2,38

2,30

2,5-3,3

2,6-3,5

2,6-3,3

2,5-3,2

2,43,0

Beside the quality of the raw material, which the fiber content = 16,07% (hard cane), also the
cane knives in use doesnt work efficiently. See the value of no-void density of bagasse
when pressed, mainly for mill #1 da = 1,15826 kg/dm3 which means the 2 (two) cane knives
works inefficiently, therefore it can be judged that the value of IP (index of preparation) is
low (< 70%).
The deflection number of mill#1 shows a negative result (l = -0,03), this also means that the
feeding of cane to mill#1 was unstable.
3. Is there any juice extracted by the feed opening ?
For mill#1 the value of y = negative. The feed opening of mill #1 in average doesnt extract
any juice during the course of milling, in this regard proved the conclusion that the feeding
for mill#1 was not stable (see point 2 above).
On the other hand for the other mills juice was extracted by the respective feed opening (the
value of y = positive). And those are good, but remember a too high value of y could mean
the occurrence of heavy slip such as shown for mill#2 and mill#3.
4. Is there any deflection made by the top rollers ?
Except mill#1 and mill#2, the other mills have the deflection number of more than 0.22%; the
percentage number permissibly for the roller made of cast iron/steel.
The deflection numbers for mill#3, mill#4 and mill#5 are above the permissible one. Then it
is concluded that the respective mills were working dangerously.
And it was right all the top rollers of the respective mills found cracks in its shells during the
inspection made in the off season, including cracks of several gears of the high speed gear
box of mill#4.
5. The Fiber Index (c).
The other factor influenced the less good extraction was also the material of the roller shells,
which have no raw grain sized surface essentially needed for the milling. The value of fiber
index (c) also emphasized the reasoning, mainly for mill #1, which has only 0.40 kg/dm3
while it should be best if the value lies between 0.46 and 0.48 kg/dm3.
The value of fiber index for mill#2 should be more than 0.51 kg/dm3 as shown on the
calculation. Also for the other mills, its values should be more than what they have reached.
For mill#3 the best figure is between 0.63 0.67 kg/dm3; mill#4 between 0.70 0.74 kg/dm3
and for mill#5 is between 0.75 0.80 kg/dm3, which sometimes more.
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Thus, the value of fiber index should be gradually increasing from mill #1 to the ensuing up
to the last mill. But it is to be noted that, the value of fiber index also depends on the
accuracy of all the recording of mill speeds and the setting of each mill.
6. The Fiber Loading (q).
The value of fiber loading for each mill in average is more than 160-grams/dm 2-escribed
roller surface, even further for mill#4 and mill#5, each exceeding 200 grams/dm2.
It is anticipated that for the fiber loading, the same condition applies for the accurateness of
the recording of mill speeds and the mill setting as well. The speed of each mill should be set
based on the prime mover nominal speed, especially for steam turbine driven mills to obtain
its optimal efficiency.
Based on the actual speed of each mill recorded (see the input data sheet), it may be
concluded that each steam turbine was not operated in its nominal speed. In practice the mill
speed will always accelerates with the actual operating work opening. Thus the final
conclusion may be made; that all the mills generally were not set correctly and not based
upon the quality of the cane crushed.
7. The Brix Curve.
It was mentioned in point 1 and point 2 above, that in general an in-adequate extraction
performed by the tandem of the example, or shall we say less than good. The actual curve of
Brix made from the average of juice analyses was above the projected curve (see the graph).
Such actual Brix curve obtained has proved, besides the mills were set incorrectly also less
rate of imbibition water was applied. The best is when the actual Brix curve runs
approximately the same (in line) with the projection curve.
10.1.5

Individual Evaluation

Mill#1:
The unstable feeding of cane and the low value of IP has also the result of unstable feeding
to mill#1, although the mill was provided with feeding device but not well in function (see
the low value of compression ratio = 2,20; which means low extraction).
The setting of work opening resulted the feed opening in average doesnt extract any juice.
The fiber index (c) of 0.40 kg/dm3 is low. The said unstable feeding and the quality of
roller material caused this.
The reduced fiber loading (q) = 123.18 grams/dm2 is normal.
The number of deflection l = - 0.03% indicating the roller works safely, but the minus
value indicates unstable feeding for the respective mill.

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Mill#2:
Although mill#2 works better (see the compression ratio K = 2.73), but because the cane
has high content of fiber and the rollers doesnt have raw grain sized surface, then a rather
heavy slip occurred with results of fair extraction only for juice, Pol and the Brix as well.
Much juice extracted by the feed opening (y = + 0.59), this helps the course of milling as
well, but the value is rather too high and concluded heavy slip was occurred in mill#2.
The fiber index of 0.51 kg/dm 3 is rather low, but the reduced fiber loading (q) of 129.80
grams/dm2 is normal.
The average of deflection number l = - 0.09% indicating mill #2 works safely, but then also
has the feeding less stable.
Mill#3:
Mill#3 has to work harder to help the extraction due in mill #2. But as it was described
above for the same mill roller material, it has no raw grain sized surface; therefore a rather
heavy slip occurred and extractions of juice, Pol and Brix were also low (see also the
compression ratio K = 2.49).
Much juice was extracted by the feed opening (y = + 0.32), this was really helps the
extraction. But the value of y is too much; hence this is also proved the above conclusion
that sometimes a rather heavy slip occurred.
The fiber index = 0.59 kg/dm3 is rather low, but the reduced fiber loading (q) = 147.20
grams/dm2 is a rather too much; hence lowering the extraction caused by too low mill roller
speed.
The roller deflection = + 0.27% is higher than 0.22% (the limit), together with the value of
reduced fiber loading of 147.20 grams/dm 2 proved that this mill #3 was really works
dangerously.
Mill#4:
Mill#4 should have been helping the extraction of mill #3 better. But then, here also happen
the course of non-existence of raw grain sized surface of the roller material, which results
low extraction occurred for the juice, Pol and the Brix as well (low compression ratio K =
2.38).
The value of y is positive (y = + 0.19), this means the feed opening was in average
extracting enough juice.
The fiber index c = 0.66 kg/dm3 was too low and the fiber loading q = 166.80 grams/dm2
was too high. The result of low extraction was influenced by those values and the low mill
speed (rotation) set during the operation.
The roller deflection of + 0.37%, far above the limit of 0.22%, together with the high value
of fiber loading gives a strong indication that mill#4 was really works dangerously.
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Mill#5:
In fact mill#5 is working for the de-watering of the final bagasse. But the extraction result
was not good enough (see the compression ratio K = 2.30), hence it could reach juice
extraction of 56.49% only.
The feed opening also extracting sufficient juice (y = + 0.11).
The fiber index was also low (c = 0.73 kg/dm3), but the reduced fiber loading was too
much (q = 160.63 grams/dm2). Those were caused by the mill rotation sets too low.
The roller deflection was + 0.25%, just above the limit of 0.22%; therefore this mill was
merely works dangerously.
10.1.6

Recommendations

Based upon the data and the results of performance described above, the following main
recommendations forwarded for a better operation of the subject mill tandem in the future:
1. Increase the value of IP (index of preparation) resulted by the two units of cane cutter as to
increase the extraction of the mills as well with the following options:
Change the knives rotor regularly every certain number of cane crushed (normally
between 100,000 150,000 tons of cane)
Replace the second knife with a bigger knife set and bigger power.
Or, install a new set of knife, such as Unigrator or Heavy Duty Shredder.
2. Make the entire surface of the rollers more rough welded with special welding rods.
3. For the next season the mill settings should refers to the calculation from its projection of
material balance.
4. Increase the rotation / mill speed of each mill to obtain adequate fibers loading. And the
rotation of each mill should be based on the nominal speed of the prime mover respectively.
But first try to adjust each delivery opening becomes narrower and if the prime mover speed
doesnt reach its nominal, then refers to the criterion of limits for the reduced fiber loading,
that the mills have to be provided with the help of forced device for its feeding.
5. The mill settings for the next season should also change the ratio between the feed and the
delivery opening refers to the calculation of its projection of the material balance. This is
important as to maintain an adequate and constant feeding to each mill.
6. Maintain the cane feeds to the cane cutters and also to the first mill always within the
adequate thickness of its layer.
7. And the most important thing is always maintaining good overall extraction for the tandem,
with an optimal water imbibition rate and re-arranges correctly the application; not only on
the bagasse before the last mill, but also on the ultimate mills as well.
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