An evaluation of cooling system water efficiencies and efficiency improvement strategies within Victoria, Australia, 2008 2011 Sven Denton, BSc App Chem (Hons), M.AIRAH. ABSTRACT A data-gathering and operator-training exercise was commissioned to review 469 cooling tower systems (>10% Victoria s total systems). Using data from the sample, water consumption and water-use efficiencies for the entire state were estimated. It was calculated Victorian cooling tower systems consume approximately 26.5GL/year; 1.7GL/year is consumed in excess of best practice. Key outcomes of the study demonstrated that efficiencies were highly variable (few systems accounting for much of the inefficiency), that system size was not an indicator of efficiency, and some correlation existed between efficiency and location. The impact of operator training was evaluated: 10% of the original sample set accounted for 30% of the excess water consumption. Simple water-conservation measures were implemented on these 51 systems, and the impact was reviewed six months and 12 months after the original survey. After 12 months, a 51% reduction in excess water consumption was observed. The study identified a range of actions that could be taken to improve water efficiency in Victorian cooling towers. A key recommendation is improved knowledge and education and to this end tools such as a cooling tower system efficiency calculator and training programme have been implemented. INTRODUCTION In 2008, anecdotal evidence suggested that cooling towers in Victoria consumed excessive amounts of water. In the context of increased concerns about Victoria s water security, a project to review water efficiency was commissioned to test this claim. Cooling towers are heat-rejection devices used in large air conditioning or process-cooling systems. Heat is rejected primarily through evaporation. Given that water supplied to a cooling tower contains small amounts of dissolved materials, these solids will concentrate in the cooling system as water evaporates. Allowing the dissolved solids to become too concentrated will impact on the cooling system s efficiency. Cooling systems are therefore bled at a controlled rate such that the dilution by fresh water limits the concentration. The rate of bleed is typically set by the water treatment service provider, cooling tower manufacturer, or similar service provider. A well-run system sets the bleed at a rate which minimises operational risks such as corrosion, scale and microbial contamination, but optimises water efficiency. Any water used in excess of this bleed rate would be considered an uncontrolled loss. Water in a cooling tower is therefore used in three ways: through evaporation, bleed and uncontrolled losses. Across the state of Victoria, about 26.5GL of water is consumed in cooling towers. Of this water, 22.3GL (84%) evaporates, and the remaining 4.2GL (16%) is lost as bleed (2.5GL) or uncontrolled losses (1.7GL) METHODOLOGY With knowledge of water chemistry, it is possible to estimate the degree of uncontrolled losses. A key metric to understanding water efficiency in cooling water systems is the concept of cycles of concentration. Cycles of concentration is by definition the volume of water entering a cooling system (makeup) divided by the nonevaporative losses of that system (bleed). By mass balance it is also known that the sum of evaporative and non-evaporative losses must equal the volume of water entering the cooling tower. The term cycles of concentration can therefore be used to relate makeup, evaporation and bleed. It can also be shown that cycles of concentration is equivalent to the concentration of some non-reactive substance in the recirculating water divided by the concentration of that substance in the makeup supply. Water chemistry can therefore be used to relate makeup, bleed and evaporation. If the make up is known, for example by water meter, or estimated based on system size and load, then the bleed can be calculated by knowledge of the water chemistry. Water efficiency is then simply the difference between actual bleed rate and the design bleed rate. The project was designed to review a representative sample of cooling tower systems, estimate the level of uncontrolled losses, understand the reasons for the losses, provide training and recommendations to the operator, and then review any performance improvements. In each case a thorough audit was carried out on the cooling tower system to give an overall picture of performance and water consumption. This included a 12-month historical review of water treatment data to capture seasonal variations in water consumption. The large number of sites evaluated meant that statistically meaningful trends and relationships could be drawn from the data set. 32
DATA GATHERING Between January 2008 and August 2009, 481 cooling tower systems (comprising 698 individual towers) at 193 sites were audited to establish the potential for reducing water consumption in cooling towers in the state of Victoria. The first 50 systems audited were selected from the Victorian cooling tower registry. Variables such as system type, size, location and industry type were examined to ensure a broad and inclusive sample was taken in this first group. The remaining 431 systems audited were volunteered by their owners, who either responded to invitations by their water company or were already in contact with the auditors. For each system, 12 months of data was examined. This data history allowed a complete picture of the system s water consumption to be drawn, taking into account seasonal factors and other variables such as local water quality. It also allowed a best-practice baseline to be established for each system, to which its actual water consumption was compared to give an indication of excess water usage in each system. On completion of each audit, site personnel responsible for the system were presented with a confidential report outlining their system s performance, its potential for water savings and how these savings could be realised. Where appropriate, or requested, additional training was also provided. Meaningful data was available for 469 of the 481 cooling tower systems audited over 10% of the state s total of 4128 systems at that time. The number of systems audited, combined with their geographical and operational diversity, means that this sample set is statistically robust, and can be used to draw conclusions about the performance of cooling tower systems across the state. FINDINGS AND ANALYSIS Results showed that the 469 systems for which data was available used a combined total of 3.0GL of water per year. Of that 3.0GL, 0.2GL or 7.9% was found to be water consumed in excess of best practice. Extrapolating the review of these 469 systems over the 4128 cooling tower systems operating in Victoria (at the time of the study): Estimated total water usage by cooling towers in Victoria: 26.5 GL/year Estimated water consumption in excess of best practice: 1.7 GL/year In the context of this project, excess consumption is defined as the difference between actual non-evaporative loss and the theoretical bleed rate based on the recommendations given by the cooling tower service provider. Figure 1 examines the relationship between total water supplied to a cooling tower and the calculated uncontrolled loss for each of the 469 systems examined. The following observations were made: Most systems performed efficiently. A small number of systems performed very inefficiently and accounted for a large proportion of excess water consumption. Size of system was a poor indicator of inefficiency. This in many respects is a counter-intuitive outcome given that the expectation would be for large, and well-resourced systems to be under closer control than smaller systems. Other observations that can be made were as follows: Only a small number of systems were observed to be leaking or overflowing; however, those systems were consuming water far in excess of best practice. Overall, systems located outside the Melbourne metropolitan area exhibited higher excess water consumption. Systems without automated bleed control typically had high excess water consumption; however, less than 4% of the systems audited lacked automated control. No statistically valid relationship was observed between excess water consumption and system size, age, type, tower manufacturer, water treatment service provider or the presence of side-stream filtration. REVISITING THE 51 WORST PERFORMERS Fifty-one systems with either a high percentage of excess water use or very high gross excess water use were identified. Between September 2009 and May 2010, each of these cooling systems was revisited approximately six, and then 12 months after the initial audit to assess the extent to which the recommendations presented to the system owners had been able to reduce, and maintain a reduced water consumption. Originally these 51 systems were consuming 454ML/year of water, representing 25.9% uncontrolled loss. After the second revisit these systems were using 332ML/year, equivalent to 10.7% uncontrolled loss. It was clear that excess water usage had been significantly reduced, and the reduction had been maintained for the 12 months. APPLICABILITY TO WIDER COOLING TOWER SYSTEM POPULATION Using data from the original survey, 11% of systems accounted for approximately 30% of total excess water consumption in the original sample. Implementing water efficiency measures at these sites reduced excess water usage to 32ML, which is a 51% reduction in excess consumption. It is estimated that Victoria s 4,128 cooling tower systems are responsible for 1.7GL of excess water consumption every year. Assuming the whole state has a similar usage profile to the original sample group, it can be expected that the 11% of systems with the highest excess water use are responsible for 500ML (i.e. 30%) of excess consumption every year. Across the state of Victoria, if these 11% (i.e. the 449 worstperforming systems) applied the same level of diligence to water saving as those revisited (i.e. achieving a 51% reduction), by extrapolation, it can be calculated that water savings of 254ML per year would be expected. 1. As measured after the second revisit, allowing approximately 12 months of operation under the new conditions. 34
2000 Make up and excess water by tower 1800 1600 1400 m 3 /year 1200 1000 800 600 400 200 0 1 14 27 40 53 66 79 92 105 118 131 144 157 170 183 196 209 222 235 248 261 274 287 300 313 326 339 352 365 378 391 404 417 430 443 Tower Excess water m 3 Total makeup m 3 Figure 1: Total water usage and excess water usage by system. The savings can be summarised as follows: Total number of cooling tower systems in Victoria: Excess water consumption across all systems: Excess consumption of worstperforming 449 systems across the state of Victoria: Saving if these systems could reduce excess by 51%: Further savings could be achieved if action was taken by the owners of the remaining 89% of systems. Hypothetically, if all cooling tower systems in Victoria were able to achieve improvements such that the same 51% reduction in excess water consumption were achieved statewide, the water saving would be 51% of 1.7GL, or 860ML per year. It should be reiterated, however, that this last scenario has not been tested, because the revisit program only focused on the top excess-water-using systems, and it may not be possible or practical for all systems to achieve a 51% reduction. CONCLUSIONS 4,128 1.7GL/annum 500ML/annum (extrapolated) 253ML/annum The project vindicated the original anecdotal evidence for excess water consumption in cooling water systems. Overall, nearly 8% of the water consumed by a cooling system could be saved if the cooling tower bleed was operated strictly to specification. Extrapolating over the state of Victoria, this equates to 1.7GL of water, equivalent to nearly two days water supply to the city of Melbourne. Most systems performed efficiently. A small number of systems were responsible for a large proportion of excess water consumption. For example, based on the data gathered, 11% of systems are responsible for 30% of the excess water consumed. Unfortunately, identifying these systems is problematic. There are very few characteristics of a system that will assist in targeting resources and help. Some correlation was noted between high water loss and: Systems without automated bleed control. In reality, not many systems were found without some form of automated control (<4%). Overall, systems located outside the Melbourne metropolitan area exhibited higher excess water consumption. The correlation was, however, weak. Where a system was seen to be overflowing, it was common for this system to be using excess water. Observing the overflow in many cooling systems was not possible. No statistically valid relationship was observed between excess water consumption and system age, type, tower manufacturer, water treatment service provider or the presence of side-stream filtration. Interestingly, the size of a system was also a poor indicator for inefficiency. What was evident was the poor understanding by stakeholders of the concept of cycles of concentration as a primary indicator of water efficiency. Many sites had invested in water meter monitoring. The legitimate variability of water meter consumptions due to seasonal and environmental factors did, however, blunt the effectiveness of this as a tool for measuring excess water consumption. There was strong evidence that a robust training program and increased visibility of the potential for water losses associated 36
with cooling systems was effective in driving change within an organisation. The top 11% of poorly performing systems identified in the project saw a reduction of nearly 50% after 12 months. These observations have driven such initiatives as the cooling tower system water efficiency training course, the web-based cooling tower water efficiency calculator www.mycoolingtower. com.au and the revision and creation of various policy and bestpractice documents such as AIRAH s Water Conservation in Cooling Towers Best Practice Guidelines. ACKNOWLEDGMENTS The author would like to acknowledge the support and assistance of the Office of Water Victorian Department of Sustainability and Environment, AIRAH, and City West Water: the primary stakeholders in the project. NOMENCLATURE Bleed Bleed is the process of removing small amounts of water from a cooling tower system on a continuous or periodic basis to prevent the over-concentration of dissolved solids in the system. This process is automatically controlled in most cooling tower systems. Cycle of concentration A system s cycles of concentration is the ratio of water entering the system (through makeup) to the nonevaporative losses from the system (bleed). Heat rejection Heat rejection is the process whereby heat is taken from one medium and rejected to another. Within a cooling tower, heat rejection is primarily via the evaporation of water. This is different to an air-cooled heat-rejection device, where heat rejection is primarily through heat transfer to air. Makeup Water that is introduced to cooling tower system to replace water consumed by evaporation, bleed or uncontrolled losses. Overflow Overflow is an uncontrolled water loss in a cooling tower system where too much makeup water is allowed into the system and some is lost to spillage. Scaling Scaling is a process whereby salts, oxides or other dissolved solids in a water system crystalise on the system s internal surfaces. Scaling on the internal surfaces of a system can interfere with its function and reduce its efficiency. Side-stream filtration Water treatment is used in cooling towers to balance water chemistry and address issues with microbiological contaminants. Some cooling towers also use filtration systems to remove larger contaminants from the water. In a side-stream filtration system some, but not all, of the water flowing through the cooling tower is passed through a filtration system on a continuous basis.
Water treatment Water treatment is the process of treating the water used in cooling towers to discourage corrosion, scaling and microbial growth. Usually chemicals such as biocides are used; however, some non-chemical alternatives also exist. Water treatment service provider In almost all cases, system owners contract specialist third parties to monitor and carry out water treatment on their cooling towers. These third parties are known as water treatment service providers. REFERENCES AIRAH s Water Conservation in Cooling Towers Best Practice Guidelines, published by the Australian Institute of Refrigeration, Air Conditioning and Heating, 2009, ISBN 978-0-949436-47-4 DA17 Cooling Towers application manual, third edition, published by the Australian Institute of Refrigeration, Air Conditioning and Heating, 2009, ISBN 978-0-949436-46-7 APPENDIX Cycles of concentration A cooling tower s cycles of concentration (n) is the ratio of water entering the system (through makeup) to the non-evaporative losses from the system (through bleed). This can be expressed as follows: E + D + B Total make-up n = = D + B Total losses where: E = Evaporation rate (L/s) D = Uncontrolled losses, such as drift (water blown out of the tower), splashout and leaks (L/s) B = Controlled losses, usually as bleed (L/s) Typically a cooling tower system is considered to be operating at best practice, and therefore consuming the minimum possible amount of water, when it is running at the maximum possible cycles of concentration. This means the water is being recycled through the system the maximum possible number of times without causing corrosion, scaling or microbial control problems. It can be shown that n is also related to the water chemistry as follows: Dissolved substances in circulating water n = Dissolved substances in makeup water Water chemistry can therefore be used to relate makeup, bleed and evaporation rates. Very few systems simultaneously measure both the water entering a cooling tower through makeup and exiting it through bleed. Both values can be estimated as long as water chemistry and heat-rejection/water-meter data is available. With this data, actual and best-practice cycles of concentration can be calculated and non-evaporative flow rates estimated. The best-practice cycles of concentration will vary from tower to tower. This is because the maximum number of cycles is governed by the supply water quality, and supply water quality varies from location to location. A cooling tower in a location with higher levels of dissolved solids in its supply water, for example, won t be able to recycle its water as many times as a location with low levels of dissolved solids because overall levels in the system will build up much more quickly. Best-practice cycles of concentration are typically calculated by the system s water treatment service provider or the cooling tower manufacturer. The owner or operator of the system, however, is ultimately responsible for setting the actual cycles of concentration. It should be noted that the relationship between cycles of concentration and water savings is non-linear (see Figure 2). Increasing cycles from 10 to 15 may only have a small effect, for example, but increasing cycles from 2 to 5 may have a very large effect. Water usage (Normalised to 2 cycles = 100 units) 200 180 160 140 120 100 80 60 40 20 0 5.0 1 15.0 2 Cycles of concentration Figure 2: Relationship between cycles of concentration and water usage. Measuring cycles of concentration and water consumption at various points over time allows an annual efficiency to be estimated. An example of such an analysis is given in Figure 3. Cycles Cycles of concentration and Excess water loss by date 12.0 50 45 1 40 35 8.0 30 6.0 25 20 4.0 15 10 2.0 5 Apr 07 May 07 Jun 07 Jul 07 Aug 07 Sep 07 Oct 07 Nov 07 Dec 07 Jan 08 Feb 08 Mar 08 Apr 08 May 08 Jun 08 Actual cycles Max cycles Excess water used (m 3 ) totalilsed excess wat4r loss (m 3 ) Figure 3: Example of efficiency tracking over 12 months. About the author Sven Denton, M.AIRAH, has been involved in the industrial water treatment industry for over 20 years. He specialises in water audits of cooling towers, boiler and waste-water plant, risk-management plans and reviews, chemical and physical water treatment, field water analysis quality, statistical analysis processes and computer programming. 38