THIRTY-FIRST CONFERENCE

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Albert White
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1 THIRTYFIRST CONFERENCE RETURN PLAT By D. S. SHANN Introduction Investigational work carried out in recent years into the milling process has confirmed that two fundamental requirements for successful mil operation are effective feeding and efficient drainage, but it will be agreed that the detailed information now available has made such matters as selection of grooving and mill peripheral speeds and the arrangement of feeding devices much more of a science than the art that it once had to be. To date, however, time has allowed very little opportunity for the investigation of the behaviour of a very important component of the three roll millthe return plate. Whilst we may still surmise that here again feedability and drainage will be the main factors affecting operation, at the moment basic knowledge is very limited. At the same time there is a great deal of general mill operational information available which represents practical attempts by operating engineers to get the best out of their plant; as such the writer considers it to be of great value, and the object of this paper is to attempt to translate these data into terms which permit basic comparisons to be drawn and rationalized designs to be made. In addition, data from some overseas factories will be introduced to see where practices may differ and to explain why such differences should occur. Previous Investigations One of the first writers to discuss return plate action was Bergmann [3] who from his assumption that the radial and tangential velocities of a bagasse particle in its motion across the plate should be constant, deduced mathematically that plate profile should take the form of a logarithmic spiral. Deerr [4] in 1912 attempted to predict the pressures existing on return plates from pressurevolume data he obtained from press tests. In 1927 in Java a report [9], issued following a comprehensive inquiry into established factory practice, detailed the "Gogelein" method of laying out return plates, one of its main criteria being the angle of contact which the scraper teeth made with the feed roll. It also concluded that a high plate with small slope or drop was to be preferred, based on the observed fact that during decompression bagasse expands only 25 to 40 per cent of its compressed volume. This information has been commented on by Behne [2] who also made comparison between settings based on the Gogelein method and on Queensland practice at the time. The Mackay Institute of Milling Engineers, in a series of papers to this Society [7] [8] made recommendations as to desirable plate settings, which, in effect, related the toe setting to the mill feed opening, the desirable ratios quoted being in the region This group of engineers, from their pooled experience, also considered the maximum requirement of drop across the plate to be 4 inch, with the proviso that Q& inch could be sufficient.

62 THIRTYFIRST CONFERENCE 1964 More recently the matter has been considered by Hugot [6] who makes recommendations based on his quantity "fibre loading" which is the quotient of fibre rate and

2 62 THIRTYFIRST CONFERENCE 1964 More recently the matter has been considered by Hugot [6] who makes recommendations based on his quantity "fibre loading" which is the quotient of fibre rate and escribed surface area per unit of time. Hugot's ideas may be extended a stage further to include plate setting or, more correctly, plate work opening, the quantity then obtained being what may be termed the "fibre rating" or pounds of fibre per unit escribed volume over the plate. This concept is by no means newit was first proposed in Java in connection with delivery roll settings many years ago, and has been used by a number of engineers in Queensland for this purpose. The writer feels that itsusein the present instance conveys the best impression of "degree of compaction" which may be of use in considering plate operation, and in fact low pressure press data [l] indicates a definite relationship between fibre rating and applied pressure. There is another factor which should be taken into account in the study of the behaviour of material entering the delivery nip, and that is the initial angle of contact which the material makes with the rolls. This quantity in the case of the top roll is rather nebulous since presumably fairly effective contact is maintained right across the plate. For present purposes then the effective angle of contact is that which the blanket would make with the delivery roll, on the assumption that the blanket thickness is equal to the work opening at the heel of the plate. It has been generally accepted in the formulation of feeding theories that where the point of contact of feed and rolls is greater than 20 degrees from the point of closest approach of a pair of rolls, external feeding force has to be applied, and that the total feeding force must increase as this angle of contact becomes progressively greater than 20 degrees. It thus seems possible that in a given mill, as the work opening over the return plate increases and the corresponding escribed volume increases with resultant decrease in compaction in the vertical or transverse plane, there could be a significant increase in pressure in the direction of flow i.e. longitudinally, with a corresponding compaction in that direction. In the absence of complete data on pressure volume relationships, feeding pressures etc., the combined result is largely a matter of conjecture. However it seems significant that in the case of the feed opening of a pressure fed mill where longitudmal and transverse compaction of material is present to pqobably a lower degree on the average than over the return plate, drainage conditions through the material are such that. quantities of juice escape through the top of the blanket, even though a significant reduction in moisture content has taken place at the pressure feeder. Bearing in mind that at the delivery opening there is no free and direct path of escape from the top of the blanket we may suspect that undercompaction could produce equally serious drainage difficulties as overcompaction if both quantities were assessed simply on a basis of fibre rating at nominal roll peripheral speeds. There thus exists the possibility of an optimum plate setting which ensures that the juice expressed during the delivery squeeze is drained away most effectively thus resulting in minimum bagasse moisture for a given degree of compression. Existing Plate In a preliminary survey [l] plate fibre ratings and the ratios between plate work openings and feed work openings were calculated from 1960 plant data. In this case and for all other such data in this paper, the means of toe and heel settings have been used to calculate average plate work

3 1964 THIRTYFIRST CONFERENCE 63 openings and fibre ratings. This may not be ideal from the viewpoint of pressure, since relationship does not appear to be hear with fibre rating, but in view of the wide variation in plate profile in practice it seems a reasonable initial approximation. This survey showed that both fibre rating and plate to feed opening *work ratio varied over wide limits, there being no standardization apparent. The inquiry has now been extended further to an analysis of information from 1962 plant data and this is summarized in Table I. Due to the lengthy calculations involved choice of data has been restricted generally to those factories having the highest fibre rate as it is felt that this would be more in keeping with future required milling capacities. As the trains considered constitute a mixture of 12, 15 and 18 rolls it has been found necessary to include other mills, in order of fibre capacity, in the fourth intermediate mill section, and all final mills are included in one section, irrespective of lengths of tandems. It is evident that there are quite wide variations in both fibre rating and contact angle for any given position in the train, and that differences in general levels occur through the train, the most striking variation being observed in final mills, particularly with respect to fibre ratings. One of the main causes of variation is the wide range of peripheral speeds for given fibre rates, or, more accurately the variation in pounds of fibre crushed per square foot of escribed surface. In general this has been determined by the efficacy of feeding devices and in final mills for example the wide use of pressure feeders had certainly meant that in order to maintain a reasonably normal contact angle on the delivery roll, the height has been limited to a figure which has resulted in a comparatively low value of plate escribed volume and thus in a high value of nominal fibre rating. The average value of plate fibre rating for other mills along the train are rather lower, one reason being that pressure feeders have not been made as widely here as on final mills. Another most interesting effect of peripheral speed may be seen by comparing figures for Fairymead with those of other factories. The adoption of higher speed milling techniques at this factory has resulted in both low nominal plate fibre ratings and small contact angles, a combination which would appear to be very desirable. A further interesting comparison may be made for No. 1 mill at Plane Creek, with and without crusher feeder. The fibre rating and contact angle for 1962, following the installation of the crusher, were 10.4 lb/ft3 and 37 degrees respectively. In 1961, with a less effective feeding device these values were 8.1 lb/ft3 and 37 degrees. The marked increase in fibre rate coupled with a decrease in peripheral speed is responsible for the change in plate fibre rating. It is important to note however that the extraction performance of the mill was outstanding, and perhaps we may conclude that, where pressure feeders are employed, higher plate fibre ratings may not be disadvantageous. This should be sound theoretically if the provision of a drier feed to the mill proper results in a lower moisture content of the material leaving the feed squeeze. In order to obtain an idea of operating practice in some overseas factories, data for Caribbean and Philippines factories are also given in Table I. This information has some practical significance, as in the mills concerned, fibre rates are similar to present or projected rates in Queens

4 THIRTYFIRST CONFERENCE IReturn late data QUEENSLAND Proserpine.. Farleigh.... Racecourse.. Pleystowe.... Plane Creek.. Fairymead I Proserpine.. Farleigh.... Racecourse.. Pleystowe.... Plane Creek.. Fairymead.... Proserpine.. Farleigh.. Racecourse Pleystowe... Plane Creek Fairymead.... abinda.. lnkerman Marian.... lsis.... Pleystowe. Fairymead...: Babinda lnkerman.. Proserpine Farleigh.. Racecourse Pleystowe Marian.. Plane Creek Fairymead lsis.... Mill No Fin Peripheral speed ftlmin Work ratio. toe of plate to feed opening b. fib1 ft3 OVZ plate Eontat angle on del roll PHILIPPINE IS Fin Factor *2 Factories only. :I Factory on

5 THIRTYFIRST CONFERENCE 65 land. It is obvious from figures of lb fibre!ft2 escribed area that these factories have higher peripheral speeds for a given size and fibre rate than Queensland mills, and the difference is apparent in generally lower levels of plate fibre rating especially in final mills. There is a general tendency as in Queensland for fibre ratings to increase and contact angles to decrease from first to last mills. It would appear that the range of contact angle is not markedly different from those in our own factories. In the case of the Philippines factories the averag* are rather deflated by the figures for one factory which has unusually high plate settings. It is worthwhile at this stage to refer to authorities from overseas and test their recommendations against our known practice. IIugot 's figures converted to fibre rating and mean plate opening would yield values varying approximately from lb/ft3 from first to final mills. His figures when applied to the Fairymad milling train would yield the values shown in Table 11. The most marked difference from Fairymead practice occurs in No. 5 mill, and this is due to peripheral speed as described earlier. Apart from this the other comparison worthy of note is in No. 4 mill. Here the Fairymead contact angle is lower and plate fibre rating 1 1 IlFairymead settings based on Hugot method.. I I I I Mill plate W.O. I calculated on Ib. fibre/ft3 No. "Hugot" basis over plate delivery roll considerably higher than Hugot's recommendation for even final mills. It is known that this unit consistently gives higher moistures than the preceding one for approximately the same hydraulic load; perhaps a trial _ with the lower recommended settings may pay dividends here. FarrelBirmingham have also issued recommendations [5] for the setting of return plates in mills of their manufacture. These recommendations are based on an empirical relationship between plate and delivery opening, but can be converted to average fibre ratings. On the basis of an example given, the ratings would vary as follows:8.0, 10.1, 12.0, 13.2, 13.5,13.5 in a six mill train. It is difficult to compute contact angles but it would appear that these would all be within 30 degrees. These data * are of interest in that they also pertain to trains handling quite high fibre rates. Observations In the main it would appear that Queensland practice employs delivery roll contact angles not much different to those used in other parts of the world. The main difference is in the fibre ratings we use over our return plates; especially where our factories employ pressure feeders these values are considerably higher than in overseas practice. There is no reason to suspect at this stage that there is anything wrong with our practicejudged on results it must be considered sound. However, with the very large milling units now being installed and the increase in crushing rates about to occur, there will be a tendency for higher fibre rates per square foot escribed roll surface. Whilst data from the experimental mills indicate that lowest possible peripheral speeds are desirable from the point of

66 THIRTYFIRST CONFERENCE 1964 view of keeping reabsorption to a minimum, in practical terms of the quantity of fluid to be drained however we are getting further and further away from actual

6 66 THIRTYFIRST CONFERENCE 1964 view of keeping reabsorption to a minimum, in practical terms of the quantity of fluid to be drained however we are getting further and further away from actual experimental conditions, and if a stage is reached where increase in compaction on both longitudinal and transverse directions over the plate adversely affects drainage, the beneficial effects of low peripheral speed may be completely nullified. The best method of countering the above condition would seem to be by reasonable increase in roll speed, and it is suggested that in laying out return plates the following procedure be used: 1. Decide on the approximate peripheral speed to be used for the coming season and calculate the average plate work opening which will give a fibre rating equal to the average shown in Table I. 2. Add & inch to this average work opening (assuming total of + inch drop across plate) to give work opening at heel and then calculate distance from tip of top roll grooving to heel of plate. Using this measurement determine the nominal angle of contact on delivery roll. Should this be significantly greater than the average figures given in Table I, increase roll peripheral speed and recalculate (1) and (2) until both average values are approximated. 3. Check that the work ratio between toeof plate and feed opening is within the range indicated in Table I (see also ref. [2] for plate to feed roll contact angie range). Experience in compiling the data given in this paper only serves to emphasize that fundamental knowledge of the factors having greatest effect on return plate operation is lacking. Until such information is available it is hoped the average operating data shown and application method suggested may assist in at least maintaining the status quo as far as return plate operation is concerned. Finally since drainage seems to be a vital matter it is suggested that close thought should be given to any modifications which may increase this factor. Simple modification such as deep juice grooves in the back of the plate or even through drilling may make a significant difference to overall extraction, remembering that for every half per cent reduction in moisture in ha1 bagasse in a factory crushing 600,000 tons of cane per season in, say, the Mackay district, approximately 50 extra tons of pol would be available to protess from the milling station. Anon.: "Return Plate Settings". S.R.I. Techn. Circular. Behne, E. R.: "Notes on Mill Settings". Proc. Q.S.S.C.T. Twelfth Conference, p Bergmann, R. F.: "De Theorie van den Ampasstooter". Archief v.d. Java Suikerind, Vol. N, p eerr, Noel: "The Milling of Canc Considered in Relation to the Volume Occupied by the Fiber". H.S.P.A. Exp. Station Report. Farrel Birmingham Go. Inc. Ansonia, Connecticut, U.S.A. Bulletin 312, "Farrel Sugar Mills". ugot, E.: "Handbook of Cane Sugar Engineering", p Mackay Institute of Milling Engineers: "Mill Settings and Feed S S.C.T. Eighth Conference, p. 17. ling Engineers: "Further Notes on Milling". Proc. Q.S.S.C.T. Ninth Conference, p Verslagen Adviseurs Vergaderingen: L927. "De Stelling van de Ampassplat", p. 177.

CALCULATION OF MILL SETTINGS

Proceedings ofthe South African Sugar Technologists' Association-April 1963 101 CALCULATION OF MILL SETTINGS By G. G. ASHE " The following is a simplified method of getting approximate mill settings, using