REPORT ON A TOUR OF THE QUEENSLAND SUGAR INDUSTRY
H. P. ON MILL ENGINES AS GIVEN BY MILL ENGINEERS
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Mr. Gunn (in the Chair) said he fell the authors of the two papers had found it difficult to get all they wanted to publish in their reports. The Mutual Milling Control Project had undoubtedly stimulated much more interest in milling than had been shown in the past.
He commented that in 1956 Mulgrave Mill in Australia had installed a juice weigher and claimed that no difference was found between the extraction calculated by its use as compared with the method based on the usual assumptions employed.
He asked if Messrs. van Hengel and Kramer had visited any factories which ground cane harvested by the Massey Ferguson harvesters. He also wished to know if the automatic vacuum pan. controls seen were of the Kelso type. He related that as far as woolly top-rollers were concerned, at Tongaat the top roller of the last mill, which was a pressure fed mill, the dirty top roller wore more than the discharge roller, which appeared to indicate that the bagasse in the grooves did move as the roller went over the trash plate. Although, it was thought by some people in South Africa preferential to have rollers of different diameters in a mill to obtain, a grinding action as well as a squeezing action, it was noticeable that in Aus- tralia special effort was made to get the roller speeds exactly the same.
Hydraulic loading of mill rollers in Australia, where fixed mills were favoured, was used merely as a safety device. Chevrons were not used in the pressure fed mill at Maidstone and no ill effects were noticeable.
Mr. van Hengel said a number of factories in Australia weighed the juice and ran a separate factory control as well as the prescribed one. This indicated that differences were found as otherwise it would not be necessary to run two different methods of control.
Two factories dealing with Massey Ferguson har- vested cane were visited and both factories were pleased with these reapers, which topped and cut the cane into short lengths and loaded it into containers.
In Australia he had seen apparatus for pan control which he was told was the "normal" conductivity- electrode type using a commercial controller.
He felt that in Australia the preference was for high pressures, low speeds, big openings and small grooves on the last mill. Hydraulics, as Mr. Gunn had pointed out, were still largely a safety device, but many people thought they should be used as they were designed for, that is to say to keep a constant pressure under varying thicknesses of the layer of bagasse.
Mr. Kramer said that in certain factories it was found that allowing the top roller to become woolly did not lead to extra wear.
Mr. Grant recalled that Dr. Kerr, when he was in South Africa, said gravity chutes installed in Australia were eliminating the old pusher gear. He wished to know if the pusher gear was still in use in certain mills and in the case of gravity chutes were feeder rollers also used? Dr. Kerr considered feeder rollers
were not necessary in this case but South. African opinion differed from this view.
He noticed from the report that in Australia most or all mills had pressure feeders on the last mill.
Some of the intermediate mills had pressure feeders and some had gravity chutes. The moisture content of bagasse varied considerably at the intermediate mills and he asked if this indicated that the pressure feeder was more effected than the gravity chute.
When pressure feeders were installed, was the load on the prime movers increased or reduced?
Mr. Kramer said that in many cases an extra roller, situated anywhere on the intermediate carriers, was used in. conjunction with the pressure chutes. In Australia more emphasis was placed on the perfor- mances of the first and the last mills than on that of the intermediate mills. It had not been found necessary to increase the horsepower of the prime movers when feeder rollers were installed unless the crushing rate was increased when more powerful turbine units were put in to replace the older engines. The old pusher gear was still being used in. places and he had seen, a special type which pushed from the top and bottom alternately but this was not greatly favoured, most engineers prefering the pressure type of feeder.
Mr. van Hengel pointed out that many intermediate mills in Australia were old mills which, did say 60 tons per hour previously while the throughput had now been increased by installing very big first and last mills.
Mr. F. Johnson said the report stated that the quality of Australian sugar was better while the clarity of juice was not as good as that in South Africa. He asked if this was due to the extensive use of coil vacuum pans as he considered that coil pans pro- duced a more regular grain than did calandria pans.
Mr. van Hengel said that even newly installed pans were often of the coil type and it was apparent that in Australia this type was favoured. However, he thought the better sugar produced was mainly due to a more favourable juice to deal with.
Mr. Hill asked if the use of hot maceration water in Australia did not cause the mills to choke as had been found in South Africa. He was surprised to learn that in some Australian mills the juice had been seen to run over the top rollers as Dr. Kerr had advocated a dirty top roller to prevent this.
As to the Milling Control Project paper, the figure quoted of hydraulic weight compared with the unit of fibre passing through the last mill appeared to show no correlation and the figures quoted for Queensland of 90 lbs. of fibre per cubic foot of bagasse showed that terrific pressures were used.
Mr. Kramer replying to Mr. Hill said that in many instances juice flowing over the top roller from the discharge roller was seen, whether the top roller was woolly or not.
As far as hot maceration was concerned, the Australian engineers averred that the use of cold maceration resulted in the mill not slipping sufficiently and the factories became full up with juice.
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Mr. Buchanan did not think the relationship be- tween hydraulic load and weight of fibre per cubic foot of bagasse was a very useful one. An increase in the roller pressure would result in higher density of bagasse but of more importance was the feeding of the mill.
Mr. Fourmond enquired why no figures of boiling house performance or even boiling house recovery were shown in the Report on Australia. He also enquired if it were possible for a quintuple effect evaporator which was being bled could average an evaporation, rate of 7.5 lbs. per square foot per hour.
He wanted to know if there was a great difference between the method of calculating the extraction of the first mill as advocated by the Mutual Milling Control and that normally used in performing milling tests.
Mr. van Hengel replied that to include factory figures in the Report would have made it too bulky but such figures were obtained. It would require an additional Report to be written to cover that aspect.
The figure of 7.5 lbs. evaporation per square foot per hour attained by a quintuple evaporator was quoted because it was remarkable.
The method advocated of calculating the extraction of the first mill gave a fibre per cent cane one per cent different from that obtained by normal method and large differences noted should be investigated jointly by the S.M.R.I, and Mr. Fourmond.
Mr. Main said he welcomed the Mutual Milling Control Project which must result in improving milling. He recalled that some years ago in Hawaai sucrose per cent bagasse had averaged 0.8% for a season which showed that improvement was possible.
He would like the figure of fibre stress ratio to be introduced. In India this figure had explained a re- markable and otherwise unexplainable difference in milling efficiency as between factories with similar plant and milling the same variety. In Natal we were now milling canes with a fibre stress ratio similar to those crushed in other countries. The highest extrac- tions he had come across were on high fibre stress canes.
He had carried out fibre stress tests on different varieties grown under identical conditions and of the same age and these showed a large variation in fibre stress ratio.
Mr. Buchanan said that the milling efficiency figures obtained so far at different factories did not reveal any great variation which could be attributed to cane quality.
Mr. Royston regretted that owing to certain mechanical factors Gledhow had not been able to share in the Mutual Milling Control Project up to now.
However, recommendation in the Report, of which he had prior information, such as smaller grooving at the last mill, deeper grooves and a narrower angle, had been incorporated in the Gledhow milling plant this year as far as possible.
To compare Australia with South Africa was per- haps unfair because the Australian calculations were based largely on assumptions and the chemical control
was not as strict as here, while the cane was also more easily milled.
Mr. van Hengel said he was satisfied that, for example, moisture per cent bagasse was determined with the greatest possible care in Australia.
Sucrose per cent cane figures were avoided in making comparisons by adopting the absolute juice loss per cent fibre figure.
Mr. Lenferna asked if third massecuites were cured in Australia for as long as forty-five minutes.
Mr. van Hengel said that while this might be true under certain circumstances, this had not been wit- nessed and the usual time was from 10 to 15 minutes.
Curing time in Hawaai however could be as long as one hour and a quarter.
Mr. Rault said that the new method of fibre deter- mination in 1st mill bagasse did not present any special advantage over the one commonly used for individual units control on lines similar to the last mill bagasse.
He had also noted the record low bagasse moisture content reached by Darnall's first crushing unit and consequently excellent extraction at that stage.
Together with liberal use of imbibition this factor had contributed to the total extraction which topped that of other up-to-date South African factories, not- withstanding Darnall's shortage of one milling unit.
Mr. van Hengel said about 65 % of the juice in the cane was extracted by the first mill at Darnall at a high purity. This meant that the purity of the mixed juice was also high while that of the juice left in the final bagasse was as low as 75 deg.
Dr. Douwes Dekker said it was hoped to find a better method of determining the fibre content of first mill bagasse but the difficulty so far was to get a representative sample. Now, however, a machine was being obtained from Australia which enabled one to obtain a small sub-sample for analysis after taking a big sample.
Mr. Hibbert asked if the trash-plate described was used generally in Australia and if it was cast in that particular shape or built up.
Mr. Kramer said that particular shape was not generally used in Australia. The method of obtaining the shape could be done either by casting or building up by welding.
Mr. Gunn remarked that there had been a theory in Australia for years that better extraction at the first mill led to a lower purity of the juice resulting in a lower price being paid for cane.
As far as the horsepower required when pressure feeders were installed, he had visited a factory in Australia which studied this aspect and it was found that the horsepower required was less than before the pressure feeder was installed and that while the speed of the engine was reduced by about one half, the through-put of the mill increased about 10 per cent.
He went on to remark that as they had found in Australia that cold maceration caused less slip and the engines would pull up. This probably accounted for the roller wear being so remarkably low in that country.