the performance of a heavy duty shredder...thirty-seventh conference the performance of a heavy duty...

THIRTY-SEVENTH CONFERENCE

THE PERFORMANCE OF A HEAVY DUTY SHREDDER

By C. D. CLARKE and R. J. McCULLOCH Farleigh Co-operative Sugar Milling Association Ltd., Farleigh, Mackay

Summary In this paper comments and observations on the first year's per-

formance of a new shredder at Farleigh have been set down, together with an assessment of its operating characteristics.

The shredder is of recent, heavy duty design having an anvil bar and breaker section covering an arc of 85" and is driven by two motors having a combined rating of 1400 horse-power.

The results of test runs taken during the 1969 crushing operations were analysed and a relationship established between cell breakage in prepared cane, fibre rate and power consumption.

Introduction With increasing factory throughputs in recent years a satisfactory

level of extraction has become increasingly difficult to attain. A decrease in the fineness of prepared cane was recognized as one of the contributing factors.

A series of modifications was made to the Searby shredder pre- viously in operation, including the use of hammers of increased effective weight and length, modification of the breaker section, and the provision of prime movers of greater horse-power. Some improvement in results was obtained, but cell breakage in the shredded cane was still estimated at below seventy per cent: and no noticeable improvement in milling extraction was evident.

In considering possible solutions to this problem, due attention was given to shredder developments reported from overseas, particularly from Hawaii. From these reports, and a comparison of our experiences with shredder operation, it was felt that a shredder of much more robust construction than that of current local designs, with power input con- siderably upgraded, would be required.

However, during 1968 season the opportunity was afforded the authors of observing a new heavy duty shredder in operation at C.S.R. Co. mills in the Ingham area. This machine had been designed to run at a maximum speed of 1100 revlmin, and was originally provided with one anvil bar and hammers up to 42 lb in weight. A breaker section covering 60"-70°, and of saw tooth design had been added. This shredder was claimed to be giving cell breakage figures of 85 per cent and over at a crushing rate of 250 tons of cane per hour with cane of 13 per cent fibre content. The prime mover was a turbine of 1,000 hp.

Description of the Shredder A machine developed from this design was installed at Farleigh for

1969 season. The shredder is shown in cross-section in Figure l .

182 THIRTY-SEVENTH CONFERENCE 1970

An anvil bar and six breaker bars cover a breaker section of approximately 85" of the arc swept by the hammers. The anvil bar is adjustable externally and breaker bars are detachable. By using bars of different thickness, setting variations can be made with each breaker. The shredder is driven by two 700 hp motors at 960 revlmin. This power requirement was based on a figure of 32 hp per ton of fibre per hour. Eighty-seven hammers having a double cutting edge, as shown in Figure 2, were used, each hammer being approximately 36 lb weight. A further setting variation was possible by having pivot rods and through rods interchangeable but on different pitch circles.

Feeding of the Shredder The shredder is mounted with the anvil bar under the vertical casing

of the top knife box as shown on Figure 1. Cane discharged under the top knives, which are at the head of the carrier, strikes the casing and falls into the shredder.

Fig. I-Cross section of the shredder.

I 1970 THIRTY-SEVENTH CONFERENCE 183

Fig. 2-Double cutting edge of hammer.

Shredder Settings Initial settings were 3 in over the anvil bar, 4 in over the first two

breakers, 3 in over the riext two and t in over the final two. These settings have been progressively reduced to 3 in over the anvil bar, + in over the first breaker and 1 in over the remainder.

Operation of the Shredder

No serious operating problems have been encountered, apart from a very bad choke when hammer wear was extremely heavy because of high levels of mud in cane.

When crushing whole-stick cane, quite heavy surges of power demand of short duration were experienced. This was apparently caused by the tendency of the top knives to drag material through unevenly into the feed shute of the shredder, and was accentuated with excessive wear on the top knives.

Loss of preparation was noticeable when this occurred, due partly, it is assumed, to reduced knifing preparation, and partly to the higher throughput of short durationato be handled by the shredder. The effect always occurred following a heavy surge in top knife amperage.

The load so induced in the shredder tended to be a limiting factor when considering close settings. Although the average hourly loading on the shredder motors at no time exceeded 75 per cent of full load, surges of short duration in excess of full load were quite frequent when crushing whole-stick cane.

Description of Test Procedure

All results were obtained under normal milling conditions, and no special alterations were made to any equipment. As a consequence it was expected that some unaccountable variations must occur in data collected through faulty sampling and analysis, unsteady conditions etc., regard- less of precautions taken. It was decided therefore to perform a large number of measurements of all the variables in order to obtain some significant results. In all 35 tests were performed on bin cane and none have been deleted from the analysis.

The measurement of operating variables was carried out as follows: 1. Crushing rate was measured by obtaining the weights of all trucks

tipped in a measured time of about twenty minutes. Care was taken to ensure that the stock of cane between the tip and top knives was approximately the same before and after the test.

2. Power consumed was measured using the Golds relay, which indicated the moving average current, as a percentage of full load current, over 14-minute intervals. With voltage constant and power factor varying slightly with load, the value so obtained, expressed as a percentage of


nominal maximum horse-power, can be regarded as a measure of horse-power. Appendix 1 explains this further.

3. A sample of prepared cane was taken through the fibre sampling door, mixed on a table by hand for about one minute, and about f removed as a subsample into a covered container. The subsample included every particle of cane down to the table in the section taken. This sample was then removed and a further sample taken, and so on until the time for the run had elapsed. About four to six subsamples were taken on each test. After collecting the subsamples the table was cleared and all cane tipped back and mixed again by hand. A final subsample was taken and removed for analysis for p01 per cent open cells (Henderson, 1970) and fibre (Blake, 1964).

4. Samples of No. 1 mill first expressed juice were taken from the sampling trough about six times per test period and mixed in a container. Samples of last expressed juice from this mill were taken from under the delivery roll using a long handled dipper.

5. A sample of first mill bagasse was taken and the p01 per cent bagasse found using disintegrater analysis.

6. The nominal shredder setting was known for each run.

Dimensional Analysis of the Operation of Shredding Before undertaking the investigation, the classical methods of

dimensional analysis were applied to the situation to help in organizing the data and to reduce the number of variables to be correlated.

The criterion by which shredder performance will be judged is P, the percentage of broken cells in the prepared cane. The physical nature of the situation demands that the following variables be taken into account. (a) fibre rate, ton/h, f (b) power consumed, ft lbf/h, p (c) a shredder setting, ft, 1 (d) cane density, lbm/ft3, d (e) cane strength, lbf/ft2, k (f) a hammer wear factor, dimensionless, W (g) conversion factor, 1.862 X 10' ft ton/lbf h', g

The derivation of the dimensionless groups is gven in Appendix 3. These groups are:

1. (g) = A

3. P 4. W

A relationship should exist whereby:

P = KA"BbWW where K, a, b and W are all constants.

At this point we now have four variables to handle, P, A, B and W, whereas there were eight before grouping.

In practice it was not possible to measure all these variables and the dimensionless groups used were modified as follows:

A was modified to


where hp is horse-power consumed and X is fibre % cane. HP is related to p by a constant factor.

The cane density d could not be measured in the time available so it was assumed to be constant, and was neglected.

The value k is a measure of the toughness of the cane, or its resistance to cell breakage, and has been given the dimensions of pressure, and presumably k is some ultimate tensile or compressive strength of the cane. This value could noto be measured but it seems reasonable to approximate it by the fibre /o of cane X.

2. The group B has been modified to p i 4 g x

The d and k factors were treated as in A. The difficulty of expressing the shredder setting as a length when

there is an anvil bar and six breaker bars each with a separate setting, was overcome in a very arbitrary manner. The influence of each bar was assumed to extend over a sector of a circle of 12.5". This value was divided by the nominal clearance in ft. between the hammers and the appropriate bar. - - -

Thus 1 is defined as C for each bar.

The dimensions of 1 have been defined as ft-l and hence 1 now appears in the numerator of B.

3. P was not modified. 4. The wear factor W was defined using the following assumptions :

(a) the observed effects of hammer wear on cell rupture would be proportional to the per cent loss in weight of the hammers.

(b) the weight loss is a linear function of elapsed time. Measure- ments gave an average weight loss of 1.25 per cent over 130 hours operation.

:. W = hours x 1.25 x 10 = 0.0962 x elapsed hours 130

Thus W is really a measure of elapsed time.

Results

Abbreviated primary data and the resultant dimensionless groups are shown in Table I.

The fitting of the data to an equation was done using multiple regression techniques, recourse being made to the S.R.I. computer because of the complexity and volume of the calculations involved. The mass of data produced by the computer is too large to present here, but the equations of best fit, with their standard deviations and multiple correlation coefficients, are given in Table 11.

In Table I11 the values of P found by measurement are compared with the values of P predicted by equation (3) in Table 11. This relationship has the lowest standard deviation.

In Figure 3 the relationship between P and first mill extraction is shown graphically.

Discussion Some comments on the various aspects of shredder operation and

results of the tests carried out are set out below.


Effects of Fine Preparation on Factory Operations: Figures for crushing rate appeared to indicate that with a reasonably effective feeding device on the crusher feeder at the first mill, throughput varied as the bulk density of the feed material.

Mill feeding was not affected by the more finely prepared bagasse, although increased loading in the drive for the feeding device on No. 1 mill was noted.

The finer nature of final bagasse did result in some minor problems

TABLE I-Primary data

No.

TABLE Il-Correlation equations

THIRTY-SEVENTH CONFERENCE

Fig. 3-Relationship between degree of preparation and first mill extraction.

TABLE Ill--Predicted P values

* Rounded to nearest whole number.


in the furnaces of a hard pressed boiler station. The levels of fuel beds on t h e flat grates had to be more closely controlled, and the draft pressure difference between furnace and combustion chambers of the sloping grate furnaces had to be reduced to prevent bagasse carryover.

The finer bagasse tended to compact into the juice grooves of the final mill rollers and difficulties were experienced with worn juice grooves in delivery rollers not being cleaned by new apron plates. The apron plate teeth wore heavily on the tips in these mills which have 3 in pitch grooving on the delivery rollers.

Actual Worse-power Usage: The figures given in Table 1 for hp, have not been corrected for power factor. As shown in Appendix 1 the power factor is expected to be about 0.83 and so the power input is about 22 hp per ton of fibre per hour. As explained in Appendix 2, the fibre figures taken are likely to be low so a more realistic value would be about 20. This compares with the value of 17 given by Nicklin (1967).

The Influence of Extraction: The overall extraction for 1969 showed an increase of 0.3 units over that of the previous year. A number of influences come in here, and it is difficult to say how much of an increase in extraction was due to cane preparation. Certainly 1969 will not be remembered as a vintage year for cane quality and low fibre. For a meaningful comparison we would have to know what extraction would have resulted if the preparation this year had been the same as last year's.

The data in Figure 3 show that considerable scatter has clouded any relationship which may exist. Work done under controlled conditions by the Sugar Research Institute (1958) has shown that first mill extraction should increase with degree of preparation because the onset of reab- sorption is delayed.

There is a tendency towards this relationship in Figure 3, but the scatter makes a formal regression analysis seem not worthwhile.

Variations in Crushing Rates: The crushing rates measured over the relatively short duration of the tests showed surprising variations. A high value of 294 tons per hour, with average hourly rates from 250 to 260, was regarded with suspicion, but it fits into the correlation quite well. This was test number 20.

Over all the tests, which refer to bin cane only, the average rate was 246 tons per hour, with a standard deviation of 23 tons per hour.

Interpretation of Dimensionless Groups Used: (a) The group A, which is k f pi could be written as p/, and could be interpreted as the potential kf d

for breaking cells, i.e. the energy input divided by the capacity per volume for absorbing this energy, i.e. the resistwee to breakage. The value of this group should be highly significant in determining cell breakage. P

(b) The group B, which is 1 4 ~ kd' is a little more obscure, and a

little licence has been used to try t; interpret its physical significance. B can be written as

(A}, ($), (' , disregarding which is only a dimensional

constant.

1970 THIRTY-SEVENTH CONFERENCE l89 f

The term can be regarded as a measure of the volume rate passing throughthe area between hammers and bars, or a velocity of cane through the shredder opening. At a given rotor speed, this term is inversely proportional to the number of hammer blows a given piece of material would receive in its passage through the shredder.

I The term is the mass flux per unit area through the shredder.

Thus (h) 1;) is prooprtional to quantity of materialjnumber of hammer blows, and it would be expected that degree of preparation would be inversely proportional to this. Thus B is made up of two

1 opposing influences, the other being -, where degree of preparation 1 k would be proportional to ,. The coeflicient or exponent in the equations K

will depend on which effect is the greater. (c) The interpretation of W is obvious, but it should be pointed out

that any influence due to top knife wear with time will also be included here.

Comments on the Derivd Equations: In interpreting the equations, the contribution of each term to the final value of P is important. If we take some typical values of the variables as

A = 2.0 B = 1000 W = 8 hp = 800 f = 30

the contribution of each term can be calculated. With equation (1) in Table 11, A increases the value of P by 7 units,

B by 7 units and W decreases P by one unit. With equation (3), A increases P by 13 units, B by 3.5 units and W

decreases P by 4 units. Equation (3), the linear combination of dimensionless groups, gave

the best set of predicted values of P, Table 111, so could perhaps be regarded as a better equation.

On the basis of these two equations, it could be concluded that A has the most effect on P, with an appreciable contribution from B and W. There is a tendency for the preparation to drop off towards the end of the week, perhaps helped by deterioration in knife condition.

Equation (4) shows that a reasonable estimate can be made based just on hp although commonsense indicates that horse-power per unit of fibre rate would be more logical. This is done in equation (7) and a better correlation results. Equations (5) and (6) give further correlations with hp and f, together with the setting number. The hp terms contribute about three times as much to the value of P as do the f terms. The setting number has negligible effect.

The equations in Table 11 can be compared using the standard deviation, which is a measure of the difference .between the observed value of P and the predicted value using the equation.

The standard deviation i

The best estimate, equation 3, indicates that at the 95 per cent confidence limit, a range of 1 5 units will include the true value of P,


when calculated from the measured variables. The multiple correlation coefficient shown in Table I1 is a measure

of the degree of excellence of fit of the observed data to the derived equation. A value of 1.0 indicates a perfect fit and a value over 0.7 indicates a fairly strong dependence.

The fact that a good correlation can be found using only hp/f as the independent variable means that the dimensionless groups are not independent. If a value is given for fibre rate, the shredder setting really determines the horse-power input. It could therefore be agreed that only group A is pertinent. The justification for using all the dimensionless groups is that they can have some interpretation relating to the mechanism of shredding, and this can be instructive.

The interpretation of group B as depending on the number of hammer blows per unit of material suggests that preparation could be improved with a higher rotor speed.

The Influence of Fibre per cent Cane: If the crushing rate, per cent fibre in cane, time of the week and cane properties were constant, the value of A would depend on the shredder setting. Therefore values of A and B cannot be chosen independently. From Table I, the average value of A for each value of setting number was found, and a graph was drawn up. A linear relationship seemed justified and the line of best fit was found to be A = 0.993 + 0.463 1. Details are set out in Appendix 4.

Using this relationship as a basis, values of 1 and hence A were assumed, and at a constant cane crushing rate the value of B was found for different values of the per cent fibre in cane, x.

Then, hp = A.f.x. and p = 62.6 ~0.163 ~0.013

A graph of P against hp for various values of X is shown in Figure 4. Note that the horse-power figures here are not corrected for power

factor, and fibre figures are low. These inaccuracies do not affect the conclusion that the fibre content has a very big effect on cell breakage. It was noticed a number of times that a high degree of preparation at an average hp value was associated with a fibre of about 11 per cent when crushing well grown plant cane.

Using Figure 4 and assuming the fibre curves are one unit too low

,e 5%. 'Le "LZ +m *g F,- _/.- I' ,-

,/.- __r"

/'

- A L" K,sri gbo ,A

Fig. 4--Effect of cane fibre on degree of re aration using the relationship P = 62 .6 AO.lgJ $0".


and that power factor is about 0.83, and engaging in a bit of doubtful extrapolation of the curves, a target of 90 per cent open a l ls would be reached on the average with this shredder and fibre of about 14 per cent if the horse-power was about 1400 average.

However, if the fibre percentage were to drop to 12, i.e. the curve actually shown as 11 per cent, 90 per cent open cells could be reached with about 1000 hp input.

A paper submitted to this Society's 1964 Conference (Mackay Institute of Sugar Milling Engineers, 1964) indicated that a 20 per cent increase in power input occurred, with shredders in the Mackay area, at the end of the week. This was not noticed in the course of this work. Average values of A, with standard deviations, are set out in Table IV.

TABLE IV-Variation in power during the week

Except for some inherent change in design of the shredder the difference in this case is hard to explain.

A Comment on Test No. 4: Test No. 4 was done on the day before the shredder choked. This test stands out as having a high power usage and good preparation. The hammers were in a very worn state. It is thought that the high power input and good preparation were caused by the blunt hammers retarding the continuous flow of feed through the shredder, resulting in the cane receiving more blows from the hammers before discharge. This is an unstable condition and choking is likely at any time. The practical procedure is to rely on close settings to give a high power input and sharp hammers to give clean discharge.

Some Thoughts on Automatic Control: The strong dependence of degree of preparation on horse-power input suggests that it would be sound practice to try to keep the horse-power at a constant value and as high as practicable. This would mean that the degree of preparation would vary depending on crushing rate and cane properties. Obviously the best that can be done in any given situation is to put in as much power as possible.

Practically this could take the form of a moveable anvil bar and grid bar system capable of infinite adjustment, within the range required, by some operating medium such as a hydraulic ram with position control. An averaged horse-power figure, such as that given by a Golds relay current averaging meter, over a period of about three minutes, could provide a signal on which the ram could act to close in the settings very slowly if power is too low, and vice versa. It may also be feasible to incorporate a fast release of settings at a predetermined high average peak current value to prevent a choke and heavy motor overloads.

Period uf Oscillation and Swing Back of Hammers: Towards the end of the 1969 season, closer adjustment of shredder settings had not been accompanied by the anticipated increase in power consumption or improved preparation. It was therefore decided to investigate hammer


characteristics using formulas suggested by Shann and Cuilen (1968) and Crawford (1969).

It has been suggested that the frictional resistance between the eye of the hammer and the pivot rod could dampen out hammer swing before the hammer again came into contact with the cane. However, the periodic time of oscillation of the hammer used was checked. Using the formula for period of oscillation given by Crawford it was found that the hammer would be swinging back as it again came into contact with the feed when approaching the anvil bar.

Although four alternative hammer shapes were considered this undesirable characteristic could not be significantly improved owing to the limitation imposed on the dimension "h" (the distance from the centre of gravity of the hammer to the centre of the pivot rod) by the internal dimension of the shredder from pivot rod to the inside of the shredder casing at the breaker section.

Swing Back Angle of Hammer As most hammer wear had been confined to the leading wear face of

the hammers in use, with little or no wear on the second face, it was inferred that hammer swing-back was affording some measure of protection from wear to this face. An angle of swing-back of 15" was calculated using the formula of Shann and Cullen and about four degrees (neglecting friction) using Crawford's formula.

The geometry of hammer swing in relation to any given breaker bar increased the setting by one eighth of an inch for a swing-back angle of eight degrees. These considerations when viewed in the light of shredder performance at the closer settings would seem to indicate that the hammer swing-back angle may have been as great at eight degrees.

To concentrate the hammer mass as far as possible towards the tip to reduce the swing-back angle and to minimize the effect of increase in setting produced by swing-back, a hammer profile as shown in Figure 5 has been devised for future trials.

From considerations of power imported per hammer, the swing- back angle must be reduced by doubling the number of hammers fitted to the rotor, and the shredder is to be so assembled in the coming season.

Fig. 5-Hammer profile design for concentrating hammer mass as far as possible towards the tip.


Conclusions

1. The factors which have the greatest bearing on the degree of preparation achieved are :

(a) the energy absorbed per ton of fibre (b) the nature of the cane. 2. The degree of preparation achieved at a given energy consumption

falls off quite markedly as the fibre content increases. 3. To keep the shredder operating in a stable and eficient manner,

and to maintain a high horse-power input to optimise cell breakage, attention must be given to the follovving:~

(a) the shape and number of hammers. (b) provision of a steady feed to the shredder. (c) provision of some form of automatic control of shredder settings.

The authors wish to thank the management of the Farleigh Co- operative Sugar Milling Association for permission to publish this paper, the Sugar Research Institute for the use of their computer, Mr. P. G. Wright for his work in processing the data and those others who have helped by their discussion and comments.

REFERENCES Blake, H. J. (1964). Fibre in cane by dry substance method. Proc. Qd. Soc. Sug. Cane

Technol., thirty-first Conf. Crawford, W. R. (1969). Mechanics of swing hammer design. Proc. Qd. Soc. Sug. Cane

Technol., thirty-sixth Conf. Henderson, C. (1970). Pol per cent open cells in prepared cane. Proc. Qd. Soc. Sug. Cane

Technol., thirty-seventh Conf. Mackay Institute of Sugar Milling Engineers (1964). Some notes on preparation plant in

Mackay and district mills in 1963. Proc. Qd. Soc. Sug. Cane Technol., thirty-first Gonf.

Nicklin, J ~ H . (1967). Power and energy requirements for cane preparation. Proc. Qd. Sug. Cane Technol., thirty-fourth Conf.

Shann, D. and Cullen, R. (1968). Some aspects of shredder hammer design. Proc. Qd. Soc. Sug. Cane ~echnol. , thirty-fifth-conf.

Sugar Research Institute (1958). The influence of degree of preparation on No. 1 mill performance. Technical Report No. 49.

Appendix I Motor Power Factor

At the time of compilation of this paper exact data on power factor were not available, but the value should be about 0.8 at very low loads rising to about 0.9 at high loads. Therefore a value of 0.83 was assumed in the calculations performed for average conditions.

Appendix 2 (A) Analysis of Fibre

This was carried out by the method described by Blake (1964). For speed of analysis the first expressed juice was not filtered. It is thought that the fibre figures so produced, while low, are a measure of the changes in fibre content. The drying of the cane was done in a Spencer oven in duplicate.

(B) Analysis of Prepared Cane

This was done by the method described by Henderson (1970).


Appendix 3 Derivation of Dimensionless Groups

The eight variables given above can be expressed in general terms as a functional relationship

P = F ( f , p , l , d , k , W , g ) . If the form of the relationship is a product of powers of the variables,

dimensional analysis will allow a logical grouping of the variables. Therefore, P = K f a pb lc dX kY wt g :

Since P is dimensionless and K is a dimensionless constant, the product of all the dimensions on the right hand side is zero when the dimension equation is written.

Note that W is dimensionless by -definition. Each dimension must appear to the power zero. For the dimension, ton a + X t z = 0 For the dimension, h - a - b - 2 z = 0 For the dimension, lbf b - I - - y - z = O For the dimension, ft b + c - - 3 x - 2y 4 - z - = O These four equations with six unknowns can be solved in terms of a

and b. After solving, and rewriting in terms of a and b, like terms can be

grouped to give

where K, a, b and c are constants.

Appendix 4 Table V sets out the average values of A associated with each shred-

der setting number, and the number of observations associated with each average of A.

TABLE V-Dependence of A and 1

Number A / of poinv

If the regression equation is of the form A = a + bl, where 1 is shredder setting X 10--3, the values of a and b

for best fit are -- b =

' A where 6 denotes average value of A. El2 - nT2

The averages are weighted averages. From the above figures,

1970 THIRTY-SEVENTH f ONFERENCE

T =1.872 A =1.860 .Zp = 125.178 Z1A = 121.237.

Hence b - 0.463 a = 0.993

:. A = 0.993 + 0.463 1 is the line of best fit. ~igure' 6 shows the scatter of each point about this line.

Fig. 6-Regression equation of A vs shredder setting.

the performance of a heavy duty shredder...thirty-seventh conference the performance of a heavy duty...

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