REVIEW OF THE INSTALLATION OF A NEW SET OF COOLING TOWERS AT FARLEIGH MILL BRYAN LAVARACK, BRUCE STEVENSON, ROBERT RASMUSSEN

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1 REVIEW OF THE INSTALLATION OF A NEW SET OF COOLING TOWERS AT FARLEIGH MILL By BRYAN LAVARACK, BRUCE STEVENSON, ROBERT RASMUSSEN Mackay Sugar Limited, Mackay Queensland b.lavarack@mkysugar.com.au KEYWORDS: Cooling Tower, Injection Water, Productivity, Pan Stage. Abstract THE COOLING TOWERS at Farleigh mill were upgraded prior to the start of the 2014 season. A set of Jord cooling towers was demolished and replaced with a Breezewater installation. Other factory cooling towers including the Pritchard were not replaced in the upgrade. The condition of the Jord cooling towers at Farleigh Mill had deteriorated progressively after many years of good use. The fill in the cells of the tower started to collapse and the risk posed in operating Farleigh without sufficient cooling water capacity was of concern. The repair of the Jord tower was not considered economically viable. This paper reviews the upgrade to the cooling water towers and assesses the impact of the upgraded cooling tower on factory performance in the 2014 season. In particular, the performances of the pan stage and evaporator station in 2013 and 2014 are reviewed. The impact on the pan stage has been noticeable with only 1% of raw sugar samples failing to meet the premium grade for fine grain in the 2014 season. This compares favourably to the 2013 season where 12% of the samples failed. The season average crushing rate performance of the factory increased from 482 tonne cane per hour in 2013 to 496 in Introduction By the end of the 2012 season, both the performance and integrity of the main cooling towers at Farleigh Mill were of concern. After many years of good service, both of the main cooling towers at Farleigh Mill, namely Pritchard and Jord cooling towers, were suspected of having reached the end of their practical life. There was a major risk that one set of cooling towers could become unusable in an event similar to what occurred at the start of the 2010 season at Macknade Mill (Douglas et al., 2011). Farleigh Mill has two large cooling towers, namely Jord and Pritchard sets, along with other smaller cooling towers. The two large towers are designed to supply cooled injection water to condensers located at the pan stage and evaporator station. The other smaller cooling towers, called machinery cooling towers, are required for specific tasks with relatively small loads. Although important for factory operation, the performance of the machinery towers is not discussed in this paper. This paper reports on how sugar quality is affected by the operation of the new cooling tower. The effect of the new cooling tower on the performance of the evaporator station is also discussed. The cooling towers at Farleigh Mill At the end of the 2012 season, the Jord and Pritchard cooling towers at Farleigh Mill were tested and performance data for each are given in Table 1. These data were collected at the respective cooling towers. 335

2 The factory was functioning satisfactorily at the time of the performance tests. Crushing rates of 514 and 489 tonnes of cane per hour (tch) were recorded as weekly averages for the weeks ending 18 and 25 November 2012 respectively. Pitot tubes were used to measure water flows in the tests. Wet and dry bulb temperatures were not recorded in the 2012 tests. The Jord cooling tower was considered to be in a poorer state despite it performing better than the Pritchard tower in the tests. In particular, the fill in the cells of the Jord cooling tower had started to collapse and the repair of the tower was not considered feasible by Breezewater (Spencer, 2012). The decision was made to replace the Jord cooling tower. Table 1 Performance tests for Farleigh cooling towers on 21 November Jord cooling tower Pritchard cooling tower Pumps running 3 3 Hot well temperature (HWT), C Cold well temperature (CWT), C Range (HWT CWT), C Water flow riser 1, L/sec Water flow riser 2, L/sec Water flow total, L/sec Water flow total, t/h Water flow on cane*, % 866% 1170% *Assume nominal crushing rate of 500 tch for Farleigh Mill The rule of thumb for sizing injection water flows in Australian sugar factories is that the injection water flow requirement should be about 1200% on cane (Wright and Miller, 1999; Lavarack, 2000). The injection water requirement will reduce as the factory becomes progressively more steam efficient (Lavarack, 2001). Farleigh Mill is not considered to be highly steam efficient, therefore flows higher than the rule of thumb would be expected to apply. The pan stage at Farleigh is operated only on low pressure (LP) steam (not on vapour). The main LP steam saving measures in place at Farleigh are: (i) quintuple set of evaporators; and (ii) (ii) a combination of LP steam and vapour 1 for juice heating. Replacing either one of the cooling towers was considered sufficient to satisfy a major portion of Farleigh s requirements for cool injection water and should substantially reduce the risk to factory operations in future seasons. Effect on sugar grain size control During the 2013 season, Mackay Sugar (MSL) launched a series of profit improvement projects throughout the company as part of its program for Operational Excellence. One of these projects centred on improving raw sugar quality at Farleigh Mill. In the past, Farleigh Mill was known to have been a top performer in the Queensland industry for achieving premium grade product. However, the factory consistently failed to achieve premium grade quality sugar at the end of the 2013 season mainly because of increased levels of fine grain in the raw sugar product. This was especially disappointing because it occurred while achieving premium grade standards for the other sugar quality parameters. A profit improvement project to study the causes of poor sugar quality was undertaken by the Farleigh Mill production department in conjunction with MSL s Focussed Improvement Group. The study identified several key causes of poor sugar quality and developed a course of action to enable the factory to achieve premium levels of high sugar quality in the future. Prominent among the causes identified was the lack of sufficient supply of adequately cool injection water at the end of the season required to maintain good vacuum in the condensers of the pan stage. 336

3 The new cooling tower In the 2014 maintenance season, the Jord cooling tower was demolished. The Breezewater cooling tower was erected and commissioned in time for the start of the 2014 season. On 29 October 2014, the Breezewater cooling tower was performance tested according to the Cooling Tower Institute (CTI) Type B cooling tower thermal performance test [ The test was deliberately undertaken near to the end of the season when the factory thermal load was high and the weather warm. Pitot tubes were employed in the 2014 test to determine injection water flows in the risers of the Breezewater cooling tower. The Breezewater cooling tower at time of CTI performance testing is shown in Figure 1. The Pritchard cooling tower is to the right in the figure. Fig. 1 Breezewater cooling tower during performance testing in October Some performance data for the Pritchard cooling tower were determined by Mackay Sugar personnel during the 2014 CTI test period. The MSL data were all spot measurements. Efforts were made to measure the flows of injection water with a strap-on ultrasonic flowmeter both before and during the CTI test. No meaningful signal for the strap-on flowmeter could be obtained despite placing the flowmeter at several places on the injection water circuit. The reason for a lack of signal for the ultrasonic flowmeter remains uncertain. Likely causes include the increased level of particulates in injection water and scaling of the walls of the injection water pipes. A high loading of particulates and / or scaling of walls are likely consequences of lime added to maintain ph control. The results of the performance tests for the Breezewater and the Pritchard cooling towers on 29 October 2014 are given in Table 2. The flows from both towers are combined and circulated to condensers on the pan stage and evaporator station. 337

4 Table 2 Performance tests for Farleigh cooling towers on 29 October Breezewater cooling tower + Pritchard cooling tower # Combined flows in factory * Pumps running Hot well temperature (HWT), C Cold well temperature (CWT), C Range (HWT - CWT), C Dry bulb temperature (DBT), C Wet bulb temperature (WBT), C Approach temperature (CWT - WBT), C Water flow riser 1, L/sec 317 Water flow riser 2, L/sec 368 Water flow riser 3, L/sec 289 Water flow total, L/sec 974 Water flow on cane, % 700% + CTI performance test measurements for the Breezewater cooling tower were collected at the tower and undertaken by Breezewater. The design total flow is 1133 L/s or 816 % on cane assuming nominal crushing rate of 500 tch for the calculation. # Spot measurements by MSL for the Pritchard cooling tower include (i) temperatures of hot and cold wells by digital thermometer at the tower basin; and (ii) wet and dry bulb temperatures by Zeal wet and dry thermometer at the base of the tower. *Spot measurements by MSL for the temperatures of the combined flows were measured by digital thermometer at the hot and cold wells located at the cold well pumps inside the factory building (away from the towers). The wet and dry bulb temperatures (listed under combined flows) were measured by thermal hygrometer at the base of the Breezewater cooling tower. The installation of the new Breezewater cooling tower resulted in a significant increase in the range temperature (hot well temperature minus cold well temperature). The range temperature increased from 11.4 C (Jord) and 7.9 C (Pritchard) measured in November 2012 to 20.6 C for Breezewater tower (alone) and to 15.1 C for the combined flows from both towers in October The performance of the Pritchard cooling tower in October 2014 is poorer than 2012 as indicated by the reduction in range temperature to 6.0 C. During the 2014 test period, the Pritchard cooling tower had one of its two fans out of order and had poor distribution of hot injection water in the tower. The approach to wet bulb temperature for cooled injection water leaving the Pritchard cooling tower is poor (18.2 C). This poor approach temperature is not unexpected given the condition of the Pritchard tower. The approach temperature for the Breezewater cooling tower is, by comparison, good (4.9 C). The design flow for the Breezewater cooling tower was close to being achieved in the CTI test. The cool and hot injection water temperatures for the combined flows before and after installation of the Breezewater cooling tower are plotted in Figures 2 and 3 respectively. The plots are for the end of the 2013 and 2014 seasons when the thermal operating conditions are high. The data for the combined flows, reported in Figures 2 and 3, were collected from the Farleigh DCS and were measured at the hot and cold wells located inside the factory building. The temperatures of the cool and hot injection water recorded on the DCS in 2014 season and reported in Figure 3, were on average about 3.5 C higher and 1.4 C higher respectively than the digital thermometer measurements of the combined flows, as reported in Table 2 1. The temperatures of the cooled injection water reported in Figure 3 for the Breezewater cooling tower were measured by a datalogger located at the basin overflow. The temperature of the cool injection water increased on 18 November 2014 when the third hot water pump on the Pritchard cooling tower was turned off due to the poor condition of the flood deck of the Pritchard tower. 1 This is based on 15 check measurements undertaken at the time of the 2014 tests. No measurements were undertaken to cross check the DCS temperatures in the 2013 season (reported in Figure 2). 338

5 Fig. 2 Combined injection water temperatures in 2013 before tower installation. Fig. 3 Combined injection water temperatures in 2014 after tower installation. There was an imbalance of hot injection water in the factory. Water from the hot well overflowed into the cool well and increased the cool injection water temperature. This problem was partially alleviated when a short term measure to dump hot injection water to effluent was introduced on about 27 November Also at this time hot northerly winds started to affect the operation of the cooling towers at Farleigh Mill. 339

6 The impact of cooling tower on production Pan stage operation and raw sugar quality One of the concerns for Farleigh Mill in the 2013 season was the failure to consistently meet premium quality targets for raw sugar. This became evident in a profit improvement program undertaken at the end of the season when raw sugar quality became a focus of attention. One of the main reasons for not achieving premium was the increased levels of fine grain in raw sugar product. The Queensland Sugar Limited (QSL) specification requires a maximum of 19% fine grain. Figure 4 shows the frequency of results for the fine grain analysis in the 2012, 2013 and 2014 seasons. All fine grain analyses, reported in this paper, were undertaken by Gateway Laboratories in Brisbane. About 12% of raw sugar samples from Farleigh Mill failed to meet the fine grain specification in the 2013 season. In the 2012 and 2014 seasons, 0% and 1% of samples failed to meet the fine grain specification respectively. The one failed sample for fine grain in the 2014 season occurred when the third pump of the Prichard tower was out of service and the injection temperature increased significantly. Fig. 4 Fine grain analyses for Farleigh raw sugar samples in the 2012, 2013 and 2014 seasons. All samples not achieving the QSL specification for fine grain in 2013 were produced at the end of season (in a period between 26 October 2013 and 30 November 2013). Figure 5 plots: (i) the crushing rate; (ii) the cool injection water temperature; (iii) fine grain analysis; and (iv) QSL fine grain maximum specification against time. Investigation into the issue showed the following. The season average crushing rate reduced from 491 tch in 2012 to 482 tch in This was restored to 496 tch in 2014 following commissioning of the Breezewater cooling tower at the start of the season. At the end of the 2013 season, the pans were not able to pull the required vacuums to achieve the required factory throughput rate. This was particularly noticeable for the low grade continuous pan. 340

7 Fig. 5 Farleigh production data for the end of the 2013 season. Magma produced in the low grade continuous pan was considered by the pan boilers to be undersized at times in the 2013 season. No measurements of the size for magma and for crystal in massecuite were undertaken in the study. Size was indicated by microscopes on the pan stage. Farleigh Mill produced small sized raw sugar product when pushed to achieve rate at the end of the 2013 season when the temperature of the cooling water increased. When the factory slowed down because of stoppages at the end of the 2013 season, the correct grain size was produced by the pan stage since the stage can often run through the stoppages. Other factors have contributed to the improvement in raw sugar quality in the 2014 season at Farleigh. These include minor changes to the seeding procedure for C seed, better training of operators and a focus on continual improvement of operational practices. Evaporator stage There are many factors that influence the performance of the evaporator stage including cane quality, crushing rate, scaling rates, addition of anti-scalants, frequency and methods of cleaning, steam supply, factory operating conditions and vacuum in the final (no.5) effect. Seasonal data for the vacuum achieved in no.5 evaporator are not routinely kept for past seasons at Farleigh Mill. However the temperatures of the juice exiting the final effect are recorded enabling the vacuum data to be determined. The temperature data for the injection water and torri water are measured at regular intervals and are paired readings for the evaporator station. These data do not correspond to other data reported in this paper since the latter are measured at different time intervals. Other data for the evaporator stage are available and are presented in Table

8 Table 3 Season average performance data for the Farleigh evaporator stage. Season Liquor dry substance, % No.5 effet vacuum, kpa(a) + Injection water temperature [CWT], C Torri water temperature [TWT], C Injection - Torri water temp. [CWT TWT], C Vacuum data calculated from juice temperature data. The 2014 season average temperature of the cool injection water has decreased by more than 2 C from the previous two seasons allowing both a slightly improved vacuum and an increased crushing rate of 496 tch to be achieved. The evaporator station performed well throughout the 2014 season and was not a major rate limiting factor compared to previous seasons (Lavarack et al., 2009). The cleaning regime, increased addition rate of antiscalant and caustic recycling have continued to aid performance for the station in the 2014 season (Lavarack et al., 2014). There are no condensers for the mud filters at Farleigh Mill so their performances are not influenced by the Breezewater cooling tower. Conclusions A new Breezewater designed cooling tower has been installed at Farleigh Mill and operated in the 2014 season. The approach to wet bulb temperature achieved during CTI type B performance testing of the Breezewater cooling tower on 29 October 2014 was 4.9 C. The effect of the lower temperature injection water has assisted in achieving the fine grain specification for raw sugar product. The effect of the lower temperature injection water on the evaporator station has permitted the season average cane throughput to increase to 496 tch compared to the previous season. Acknowledgements The management of Mackay Sugar Limited is thanked for allowing the authors to present this paper. Mr Babu Ramanathan of Mackay Sugar project department is thanked for his assistance in providing reports and other information. The management and staff of Breezewater Pty Ltd are thanked for the information provided during discussions and for allowing the authors to reproduce selected data from the performance testing undertaken at various times at Farleigh Mill. REFERENCES Douglas A, Broadfoot R, Craig A (2011) Effects of loss of cooling tower capacity on factory operations at Macknade Mill. Proceedings of the Australian Society of Sugar Cane Technologists 33, (electronic format), 5 pp. Lavarack B (2000) Determining the water balance in a raw sugar factory. In Sustainable environmental solutions for industry and government: Proceedings of the 3 rd Queensland Environment Conference May, Brisbane. pp (Environmental Engineering Society: Brisbane). Lavarack B (2001) Reducing process steam usage? What will happen to the water balance. Proceedings of the Australian Society of Sugar Cane Technologists 23, Lavarack B, Lloyd M, Giannangelo M, O Hara M (2009) An update on the Farleigh evaporators. Proceedings of the Australian Society of Sugar Cane Technologists 31, Lavarack B, Stevenson B, Schneider S, Bampton B (2014) Spent caustic recycle at Farleigh Mill. Proceedings of the Australian Society of Sugar Cane Technologists 36 (electronic format), 9 pp. Spencer G (2012) Cooling tower inspection report Jord 4 cell Mackay Sugar Limited Farleigh Mill. Breezewater Confidential report no /Jord. Wright PG, Miller KF, (1999) Minimising the effluent flow from a cane sugar factory. Proceedings of the International Society Sugar Cane Technologists 23,