THE DIRECT DETERMINATION OF FIBRE CONTENT OF INDIVIDUAL CANE CONSIGNMENTS

By A. F. MCCULLOCH.

INTRODUCTION.

The m e t h o d of c o m p u t i n g p a y m e n t for cane in the South African s u g a r i n d u s t r y , based as it. is upon the value of t h e J a v a r a t i o , does n o t always yield results which are r e g a r d e d as satisfactory by t h e cane grower. The J a v a r a t i o is influenced by several factors, n o t a b l y d i r t y cane, rainfall a n d t h e fibre content, b u t it is only on t h e effect of t h e l a t t e r t h a t we are concerned here. W h e r e consignments are crushed at a factory in which high fibre cane usually predominates, t h e effect of i n t r o d u c i n g a proportion of low fibre cane, such as m a y occur during a transfer, will lead to a lower price being p a i d for t h e l a t t e r than would be t h e case if crushing were done at a factory where low fibre cane p r e d o m i n a t e s . This anomaly is well k n o w n , a n d h a s been c o m m e n t e d on previously in t h e proceedings of this Association.1

The anomaly would d i s a p p e a r provided t h a t t h e weight of fibre yielded by each consignment of cane could be established within t h e limits of a commer- cially acceptable a c c u r a c y , since t h e relationship between fibre c o n t e n t a n d J a v a r a t i o is d e t e r m i n a t e .3

2. In order to be able to assign t h e fibre weights yielded by successive consignments of cane, t h e requirements a r e : —

(a) to weigh t h e bagasse continuously as it is discharged ;

(b) to identify t h e bagasse yielded by each consignment; a n d

Weighing materials on a continuously-moving conveyor belt is a well established industrial practice and has been u s e d for several years, n o t a b l y for mineral ores, coal, coke, p h o s p h a t e s a n d wood chips, and there are m a n y designs available, e.g. the Adequate Weigher, t h e Blake-Denison Weigher, a n d the Fairbanks-Morse Conveyor Scale, to n a m e b u t a few. Since bagasse h a s a low d e n s i t y (5—8 lbs./ft.3), the size of t h e weigher required is relatively large, for instance, a weigher handling, say, 50 t o n s per hour bagasse would be similar in size to a machine for weighing several times t h a t q u a n t i t y of coal per hour. For this reason, a t t e n t i o n m u s t be directed to space requirements w h e r e t h e application of a continuous weigher is being considered.

3. In continuous weighing it is usually sufficient to establish t h e weight of m a t e r i a l passed at hourly intervals, b u t in t h e case of bagasse weighing it is necessary to h a v e t h e weights established at much

shorter intervals, since t h e bagasse yielded by an individual consignment of, say, 3 tons of cane would pass through the weigher in. 1.2 m i n u t e s where t h e mill t a n d e m h a d a throughput of 150 t o n s cane per hour. Moreover, it is usually easy to leave gaps between successive consignments of discrete m a t e - rials for identification at t h e weigher. B u t since t h i s cannot be done in cane milling w i t h o u t reducing efficiency a n d t h r o u g h p u t , other means h a v e to be provided to identify the position of t h e b r e a k between consignments.

4. One method of continuous weighing a n d record- ing is shown in Fig. 1. Here the conveyor belt A rests on two rollers, B and C, spaced at a fixed distance a p a r t . The load across the span of the rollers is s u p - ported by links BD and EC, which t r a n s m i t t h e load through lever EF a n d link FG to a balance lever H K . At K counterweights are a t t a c h e d to t h e balance lever to balance the tare weight of t h e system con- nected to H K . When a load is placed on A, HK will t e n d to r o t a t e counter clockwise a r o u n d pivot J.

The movement of HK displaces a float L, which is buoyed in mercury. As the float lifts it becomes effectively heavier, a n d an equilibrium position is reached where t h e load on A is balanced. T h e m o t i o n of HK is communicated to a totalising indicator M, which has a cyclometer dial a n d pointer scale suit- ably calibrated to enable the t o t a l weight of bagasse which has passed across BC to be observed. T h e operation of M is initiated at intervals d e t e r m i n e d by successive equal segments of A coinciding with the span BC. Initiation is achieved by h a v i n g s n u b s a t t a c h e d to A equally spaced at a pitch of t h e span BC. W h e n a s n u b coincides with C the displacement of HK (which is proportional to t h e load on t h e span BC) is communicated to M, a n d t h e indicator pointer a n d t h e cyclometer dial r o t a t e to correspond to t h e displacement. Similarly, when t h e n e x t s n u b reaches C t h e cycle is repeated. T h u s t h e load passing along A is t r a n s m i t t e d in steps to M a n d totalised.

The weight of the bagasse arising from successive consignments is established by m e a n s of t w o recorders, R1, and R2, which are coupled to M t h r o u g h clutches N¹ a n d N² respectively. T h e clutches are arranged so t h a t when R1, is coupled R2 is uncoupled, and vice versa. The operation of N1 a n d N2 m a y be achieved either by m a n u a l operation of t h e push b u t t o n on solenoid S, or by a u t o m a t i c operation. R1

a n d R2 are each fitted with an adding register, a n d a recorder paper. T h e register in t h e coupled recorder is displaced by M, b u t t h e uncoupled register remains

86 stationary. When the solenoid operates the following sequence of events occurs:—

(a) the weight summated in the coupled register is stamped on the recorder paper, and the register is then cleared automatically, and (b) the uncoupled recorder is coupled for initiation

byM.

These events occur in reversed sequence at the next operation of S. The figures stamped on the recorder paper will show in sequence the weights of bagasse passed through the weigher between each successive operation of the solenoid, and since this is the same as the sequence of milling cane consignments, the weight of bagasse and the cane consignment may be correlated.

In passing, it is noted that two sources of error arise in a weigher of this description, and the effects of the errors on the accuracy of weighing are dealt with in paragraph 5 (iii). Firstly, the equilibrium position of HK is influenced by inertia and vibration and the displacement transmitted to M may not be identical with the displacement corresponding to the load. Secondly, the register should stamp the recorder at the instant the break between consignments coincides with C, but this operation may occur when the break is not coincident, due to time lag and inertia effects.

5. Discussion of methods for identifying the position of the break and experimental results. Four methods have been considered for establishing the position of the break on the weigher so that solenoid S can be operated at the correct instant:—

(a) Staining the cane which forms the beginning of each consignment.

(b) A time delay method.

(d) A method in which the rate of milling and the rate of bagasse production are compared for each consignment.

Method (a). Experiments were made by pouring two buckets of milk of lime on to cane loaded on the carrier. When the cane passed through the knives a yellow stain developed which was clearly visible as the cane passed through the crusher and the first mill, but the stain was covered on the second mill carrier by the cush-cush. By stopping the cush-cush as the stained cane passed, it was possible to track the course of the stain from the cane carrier to the final bagasse elevator. This method is satisfactory for experimental requirements, but it is not considered to be satisfactory for process work, since—

(i) operators are required to watch continuously for the stain at the final bagasse discharge, and for stopping and starting the cush-cush fre- quently, and

(ii) care is required to decide the correct position of the break as the stain may be spread over a length of 6 to 8 feet of bagasse.

Method (b). This method utilises the time interval which elapses between the cane being loaded on the carrier and emerging as final bagasse from the last mill. If the tandem operated at constant speed and there was no slip the time interval would be constant and it would be easy to arrange for the solenoid con- trolling the recorder clutches to operate at any determined instant after the cane had been loaded on the carrier. Where the speed of the drive was varied to correspond with varying throughputs, arrangements could be made for the time interval to vary automatically with the speed of the drive.

Observations were made to study the extent of the variation in time interval at the same time as the tests made in connection with method (c), and the data are shown in Appendix I. The mean variations achieved in the first two factories, viz. 1.57 per cent.

and 1.81 per cent., would serve as a fairly satisfactory basis for time interval initiation, but the results yielded in the last two factories indicate that the arrangement would be unsatisfactory. For instance, the mean time variation of DL (i) is 91 seconds, which corresponds to an error of 1.4 tons of cane, and the magnitude of such an error is unacceptable.

Method (c) (i). It is the object of this method to identify automatically the position of the break between consignments. The method is an extension of that already in use for juice sampling and has already been suggested by Waddel.2 Referring to Fig. 2, at the instant of starting a new consignment on the cane carrier push-button A is pressed by an oprcator which energises solenoid B1 and releases a ball. The ball rolls into one of the slots cut in the rim of wheel C1 and is carried round by the rotation of C1 until it rolls out through port D1. C1 is driven by the carrier engine and the displacement of the ball between entry and exit, is devised so that it (the displacement) coincides with the travel of the cane from the point of loading on the carrier to the top of the crusher feed plate. Upon emerging from D1 the ball closes a contact which energises solenoid B2, but a time delay is imposed here before B2 operates to allow for the time required by the cane to descend the crusher feed plate, and a ball controlled by B2 is then released at the instant the cane enters the crusher. Wheel C2

is driven by the crusher, and the ball emerges from D2 as the crushed cane is discharged from the crusher rolls. The contact operating B3 is then closed, and a ball enters C3 after a delay corresponding to the time interval required for the cane to travel across the crusher discharge plate and on to the first mill carrier. The cycle is repeated in turn at each mill until a ball emerging from D5 closes a contact which energises solenoid S, and after a time delay corresponding to the displacement of the break from the

87 last mill to the roller C (Fig. 2) the bagasse weight is stamped on the recorder and clutch gear operated.

This method has the supreme advantage of being both simple and automatic, but it depends for its accuracy upon the correlation of bagasse displacement through the tandem and the revolutions of the driving units; in other words, that there is no slip, or if there is, that the slip remains constant.

(ii) Experimental. Tests were made at four factories to check the correlation by staining the bagasse on the first mill carrier with milk of lime and tracking the course of the stain through the tandem. The revolutions of each driver were counted in turn on a signal being given by an observer watching the movement of the stain; the time intervals from the instant of staining to the emergence of the bagasse from each mill were recorded; the number of slats bearing stained bagasse on the final bagasse carrier and the speed of this carrier were also recorded. A typical set of observations is tabulated in Appendix II, and a summary of the data obtained at the factories is tabulated in Appendix III.

The least variation was obtained at factory CK (1.78 per cent. on the mean total revolutions), and the greatest variation was obtained at factory DL (8.91 per cent.). Since the latter variation would yield an error of large magnitude it was decided to make further tests at the same factory as a check, and it was also decided to track the course of the bagasse from the point where the cane was loaded on to the carrier to the point of discharge from the last mill. Data from one test are shown in Appendix IV, and a summary of all the tests in Appendix V. The data in Appendix IV show that the mean variation in total revolutions, viz. 9.15 per cent., confirms the results obsreved on the tandem only, viz. 8.91 per rent. An explanation to account for such relatively large variations is fluctuation of engine speed.

Observations were made during the tests, and showed a variation of 4.21 per cent. on the mean speed of engine No. 1 and 1.59 per cent. on engine No. 2.

Since speed variations of this order are insufficient to account for the observed total variation, it is considered that the cause is due to variable slip effects in the mill, but no explanation can be offered why the slip should be so much greater at one factory than at another.

(iii) Calculation of the effect of the variations on the accuracy of bagasse weights. Calculations have been made utilising the data obtained at factories CK and DL as representing the extreme differences which would occur, and the results have been plotted

111 Figs. 6 and 7. The assumptions on which the calculations have been based a r e -

(a) the maximum weighing error does not exceed 1 per cent. (i.e. referring to paragraph 4 and Fig. 1, the position of HK is always within 1 per cent. of the correct equilibrium position);

(b) the maximum error due to a break between consignments and the snub not being coincident at the instant of actuating the recorder, is 80 lbs. of bagasse or (further assuming a bagasse moisture content of 50 per cent.) 40 lbs.

of dry matter;

(c) the maximum error due to variation of the time interval to descend the crusher feed plate is proportional to the mean time variation observed experimentally;

(d) the cane mixes to the extent experimentally indicated by the number of slats bearing stained bagasse, and the fibre content of ad- jacent consignments differs by 5 per cent.;

(c) the maximum identification of break error is equal to the mean variation of the total revolutions of all driven units. Thus, referring to Appendix II, the mean value of the total revolutions made by the driving units whilst the bagasse passes through the tandem is 1,620:

the mean value of the sum of the differences of the actual number of revolutions made during each test is 114.5, and the mean variation is 114.5/1,620 = 7.1 percent.

It is improbable that the maximum values of the errors would occur simultaneously; it is also improbable that they would occur simultaneously with the same algebraic sign. A practical approach would be to adjust the values of each error to realistic levels, instead of using maximum values, and this has also been clone in the calculations. Thus the maximum values are reduced by 50 per cent. to yield the average expected error.

The curves show that the error varies rapidly with the weight of consignment, and in Fig. 6 the expected error in the fibre/cane ratio would be ±1.4 per cent.

for a 5-ton consighment, and ±0.92 per cent. for a 20-ton consignment. The expected error in the measured weight of fibre would be ±8.1 per cent. for a 5-ton consignment, and ±5.56 per cent. for a 20- ton consignment. In Fig. 7 the errors are greater, and show ±2.1 per cent. expected error for a 5-ton consignment, and ±1.52 per cent. for a 20-ton consignment in the fibre/cane ratio. The expected errors in the measured weight of fibre would be ±12.1 per cent. for a 5-ton consignment, and ±8.7 per cent.

for a 20-ton consignment. The magnitude of the components of the total error are also plotted in Fig. 7 and show the dominant effect of the identification of break error. For instance, in a 5-ton consignment 60 per cent. of the total error is due to the identification error, and in a 20-ton consignment 82 per cent.—proportions which indicate the need of improving the accuracy of identification.

Method (d). The basis of this method is to use the rate of bagasse production and the rate of cane throughput on the cane carrier to identify the break

between consignments. If the cane throughput is constant, and the fibre content of successive consignments varies, the rate of bagasse production will also vary. For instance, if a consignment having a fibre content of 15 per cent. is followed by a consignment having a fibre content of 20 per cent., the rate of production of bagasse would be changed by 33 per cent. (assuming constant moisture plus brix per cent.); or taking another view, and assuming that the rate of bagasse production remains constant, then the rate of cane throughput would vary with fibre content. Now, if the instant of time at which the rate of change occurs is established, it would be possible to correlate the quantity of bagasse recorded at the weigher with the appropriate cane consignment.

The manner in which this correlation can be achieved is as follows: referring to Fig. 1, assume that the coupled recorder R1 stamps on the paper the weight of bagasse which passes through the weigher at one-minute intervals, say, so that a typical record would appear as Fig. 4. Further, that a button is pressed manually when the loading of a new consignment commences on the cane carrier; the effect of pressing the button is to stamp the record with a letter—C. Thus in Fig. 4 the first consignment commences loading at A, the second consignment at B, etc., and the problem is now to establish on the record where the bagasse from each successive consignment started and finished. Imagine the mill to be empty at first, then the weight of bagasse from A will start to appear on the record after an interval such as AEl, and thereafter the rate of bagasse production will increase until a steady state is attained.

If consignment. B has a different fibre content than A (say larger) and the rate of cane throughput is constant, the successive group of figures on the record will change systematically at some point such as Gl, and the point where this change occurs repre- sents the break between the consignments. To establish the position of the break accurately a distance E2G2 equal to AB is marked on the record. The weight of bagasse from the consignment A is thus the sum of the figures between the lines E1E2 and G1G2, but since G1G2 does not coincide with a set of stamped figures an estimate is made of the correction required. Thus in the figure the position of G1G2 is approximately one-quarter of the interval between the figures 1,740 and 1,805, and the correction to be added is obviously 1/4 x 1,805 = 451 lbs. Similarly, distance BD = G2H2 is marked off, and the weight of bagasse from consignment B is established by summating the figures between G1G2 and H1H2. Since the weights of the consignments A, B, D , . . . are known the ratio bagasse/cane for each is thus determinate.

If a change in rate of cane throughput occurs, the lengths AB, BD, DF, etc., will change likewise,

and the method outlined above will establish accurately the position of the break, but it is possible for this rate to change after a consignment has been completely loaded, and while the bagasse from this consignment is still passing through the tandem. In such a case a correction is applied to establish accurately the position of the break on the record Referring to Fig. 5, the first step is to establish the rates of cane throughput by dividing the weight of the consignment by the loading time shown on the record. If the rates are constant it may be inferred that the tandem has operated at constant speed. On the other hand, if a change of rate is indicated it may be inferred that the tandem speed has been changed at some instant during the loading of the consignment. The next step is then to establish the position of the point K, which is done by noting where there is a systematic change in the bagasse figures, e.g. as indicated in Fig. 5 between 1,753 and 1,820. The method of establishing the correction required is shown in Appendix VI. It is necessary to use judg- ment in deciding the position of K: for instance, in Fig. 5 the position has been taken as coinciding with the figure 1.753, and this has been done since the following figures 1820, 1840, 1830, 1815, etc., show that steady conditions had been attained during a single interval. If, however, the figure 1820 had been 1790 instead, the point K would have been established at 1790, since the figure sequence 1753, 1790, 1840, etc., shows that the changed rate of throughput had occurred in the interval between the figures 1753 and 1840.

The figures shown in the record are notional, and represent the following average conditions for consignment A:—

Cane throughput—tons per hour 150.

Fibre/cane—per cent. 17.5.

Moisture plus brix/bagasse—per cent. 50.

Bagasse per minute—lbs. 1,750.

For a 1 per cent. change of fibre content the rale of bagasse production would change by 100 lbs. per minute, and a systematic change of this amount would be obvious on inspection of the record, provided that large fluctuations do not occur in the rate of bagasse discharge during each successive time interval; thus an accurate identification of the start and finish of the consignments should be achieved by experienced personnel. The method does not elimin- ate the weighing error, the integrating error, nor the mixing error, but it would yield more accurate data than Method (c) in cases where slip occurs in the tandem. Two recorders are needed to enable records to be removed for inspection without breaking con- tinuity.

The accuracy of this method can be checked whenever it is desired to do so by the staining routine 88

Dalam dokumen SASTA Proceedings of the 26th Annual Congress (Halaman 90-103)