Improving cooling tower water and energy efficiencies based on a new analytical method

Stockholm, Sweden 2022 DEGREE PROJECT IN REFRIGERATION ENGINEERING, SECOND CYCLE, 30 CREDITS Improving Cooling Tower Water and Energy Efficiencies based on a New Analytical Method Rayan Abboud www.kth.se Master of Science Thesis Department of Energy Technology KTH 2022 Improving Cooling Tower Water and Energy Efficiencies Based on a New Analytical Method TRITA: TRITA-ITM-EX 2022:509 Rayan Abboud Approved 2022-09-12 Examiner Björn Palm Supervisor Björn Palm Industrial Supervisor Klas Berglöf Contact person Klas Berglöf Abstract Cooling towers are largely used in many industrial processes for cooling purposes. The basic concept of a cooling tower includes cooling down water by means of evaporation, thus giving it the terminology “evaporative cooling”. Air mixes with water in a cooling tower and is ejected carrying the heat that is transferred from the water. Evaporative condensers also follow the same evaporative cooling process, however, could differ in design and application. A lot of water is consumed by cooling towers to perform the cooling process and maintain the quality of the water. The common method that is used to calculate the amount of water consumed due to evaporation is by following the rule of thumb, which states that approximately 1% of the water flow rate is evaporated for a cooling range of 6.5 ℃. There are also empirical formulas that have been developed to calculate the amount of water consumed due to evaporation. However, the rule of thumb, which is followed the most, is not a very accurate method of evaluating the amount of water consumed since it does not depend on ambient conditions. This report dives into the details of the empirical formulas and the rule of thumb. In addition, a more accurate method of calculating the evaporation of water is developed to create an established benchmark that cooling tower operators can use to evaluate their water efficiencies. A study was conducted on a hypothetical cooling tower with the following design conditions: 3 500 kW cooling capacity, 3 ℃ approach, and 5 ℃ range. The motive of this study began with ClimaCheck’s desire to explore a way to define a water efficiency KPI to enhance analyses of cooling towers. ClimaCheck measures the heating load of a chiller, the inlet and outlet water temperatures of the water in a cooling tower. They also measure the ambient temperature and humidity conditions at any time required, which was vital in conducting the study. The results showed that the ambient conditions (dry bulb and wet bulb temperatures) highly impact the amount of water evaporated. The data was compared with the rule of thumb/empirical formula, which resulted in the same amount in all locations since it does not depend on the ambient conditions. In addition, to further validate the study, the analysis was performed on a site monitored by the minute by ClimaCheck that has a cooling tower. Comparing the actual water consumption with what the rule of thumb indicates showed that the latter is not accurate. The study is concluded by illustrating the importance of operating at appropriate air flow rate and water conditions, how cooling tower operators can analyze the water efficiency of the cooling tower using the developed technique, while providing some guidelines on how to maintain high cooling tower performance. Sammanfattning Kyltorn används till stor del i många industriella processer för kylningsändamål. Grundkonceptet för ett kyltorn inkluderar kylning av vatten genom avdunstning, vilket ger det terminologin "evaporativ kylning". Luft blandas med vatten i ett kyltorn och stöts ut med värmen som överförs från vattnet. Evaporativa kondensorer följer också samma evaporativa kylningsprocess, men kan skilja sig åt i design och tillämpning. Mycket vatten förbrukas av kyltorn för att utföra kylningsprocessen och bibehålla kvaliteten på vattnet. Sättet som ofta används för att beräkna mängden vatten som förbrukas på grund av avdunstning är genom att följa tumregeln, som säger att cirka 1 % av vattenflödet förångas vid 6,5 ℃ temperaturdifferens. Det finns också empiriska formlersom har utvecklats för att beräkna mängden vatten som förbrukas på grund av avdunstning. Tumregeln, som följs mest, är dock inte en särskilt exakt metod för att utvärdera mängden vatten som förbrukas eftersom den inte beror på omgivningsförhållandena. Denna rapport dyker ner i detaljerna i de empiriska formlerna och tumregeln. Dessutom utvecklas en annan metod för att beräkna avdunstning av vatten för att skapa ett etablerat riktmärke som kyltornsoperatörer kan använda för att utvärdera sin vatteneffektivitet. En studie genomfördes på ett hypotetiskt kyltorn med följande konstruktionsvillkor: 3 500 kW kylkapacitet, 3 ℃ differens mellan utgående vatten och inkommande lufts våta temperatur och 5 ℃ temperaturändring på vattnet. Motivet för denna studie började med ClimaChecks önskan att utforska ett sätt att definiera en vatteneffektivitets- KPI för att förbättra analyser av kyltorn. ClimaCheck mäter värmebelastningen för en kylare, inlopps- och utloppsvattentemperaturerna för vattnet i ett kyltorn. De mäter också de omgivande fukt- och temperaturförhållandena när som helst, vilket var avgörande för att genomföra studien. Resultaten visade att de omgivande förhållandena (torr- och våt temperatur) i hög grad påverkar mängden vatten som förångas. Data jämfördes med tumregeln/empirisk formel, som gav samma mängd på alla platser eftersom det inte beror på omgivningsförhållandena. Dessutom, för att ytterligare validera studien, utfördes analysen på en plats med kyltorn som övervakades minutvis av ClimaCheck. Att jämföra den faktiska vattenförbrukningen med vad tumregeln indikerar visade att det senare inte stämmer. Studien avslutas med att illustrera vikten av att arbeta med lämplig luftflödeshastighet och vattenförhållanden, hur med hjälp av en mjukvara/server kyltornsoperatörer kan beräkna kyltornets vatteneffektivitet med den utvecklade tekniken, samtidigt som det ger några riktlinjer för hur man upprätthåller en hög kyltornsprestanda. Acknowledgements I would like to acknowledge my supervisor from ClimaCheck Sweden AB, Klas Berglöf who gave me the opportunity to work on this wonderful topic. His support, as well as his knowledge and experience in cooling towers were crucial in the completion of this thesis. I would also like to thank my colleagues from ClimaCheck who always showed their support and availability in any assistance I required. In addition, I would like to express my sincere gratitude to my supervisor and examiner at KTH, Björn Palm. Without his guidance and insightful comments throughout this journey, the undertaking of this thesis would not have been possible. I am thankful for having a supervisor like him that was always available to address my questions. Finally, I would like to thank my family and friends for always motivating and supporting me throughout my journey. Table of Contents 1 Introduction ............................................................................................................................. 1 1.1 Cooling Towers ................................................................................................................ 1 1.1.1 Cooling Tower Fundamentals ................................................................................... 2 1.2 Problem Statement ........................................................................................................... 3 2 Literature Review.................................................................................................................... 3 2.1 Types of Cooling Towers ................................................................................................. 3 2.1.1 Mechanical Draft Cooling Towers............................................................................ 3 2.1.2 Natural Draft Cooling Towers .................................................................................. 5 2.2 Components of a Cooling Tower ..................................................................................... 6 2.3 Evaporative Condensers ................................................................................................... 8 2.3.1 Components and Operating Principles of Evaporative Condensers ......................... 8 2.4 Water as the Cooling Media ............................................................................................. 9 2.5 Performance Parameters of Cooling Towers ................................................................. 10 2.5.1 Range of a Cooling Tower ...................................................................................... 10 2.5.2 Approach ................................................................................................................. 11 2.5.3 Effectiveness/Efficiency ......................................................................................... 13 2.5.4 Cycles of Concentration .......................................................................................... 14 2.5.5 Cooling Capacity .................................................................................................... 14 2.5.6 Liquid to Gas Ratio ................................................................................................. 15 2.6 Water Loss in Cooling Towers....................................................................................... 15 2.6.1 Evaporation of Water .............................................................................................. 15 2.6.2 Drift ......................................................................................................................... 18 2.6.3 Leaks ....................................................................................................................... 21 2.6.4 Bleed-off ................................................................................................................. 21 2.6.5 Other Types of Water Loss ..................................................................................... 22 2.6.6 Makeup Water ......................................................................................................... 23 2.6.7 Merkel’s Method ..................................................................................................... 26 3 Methodology on Defining a New Benchmarking Method ................................................... 28 3.1 Water and Air Requirements .......................................................................................... 30 3.2 Mass and Energy Balances ............................................................................................. 30 3.2.1 Mass Balance .......................................................................................................... 31 3.2.2 Energy Balance ....................................................................................................... 31 3.2.3 Air Flow Calculation............................................................................................... 32 3.2.4 Water Consumption Calculation ............................................................................. 33 3.3 MATLAB Model............................................................................................................ 33 3.3.1 Relative Humidity Calculation ............................................................................... 33 3.3.2 Psychometrics ......................................................................................................... 34 3.3.3 MATLAB Results ................................................................................................... 36 3.4 Comparing Analysis with the “Rule of Thumb” ............................................................ 39 3.4.1 Cities Analyzed ....................................................................................................... 39 3.4.2 Results ..................................................................................................................... 39 3.4.3 Scenario Analysis.................................................................................................... 44 3.4.4 More Examples ....................................................................................................... 45 4 Discussion and Water Loss Minimization ............................................................................ 48 4.1 Water Efficiency ............................................................................................................ 48 4.2 Preventing Water Losses ................................................................................................ 49 4.2.1 VFD Scenario Analysis........................................................................................... 50 4.2.2 Summary of Water Loss Prevention ....................................................................... 52 5 Conclusion ............................................................................................................................ 54 List of Figures Figure 1 Cross flow (a) Counter flow (b) cooling towers (Kim, Lee, & Jeon, 2016) .................... 4 Figure 2 Open circuit cooling system (left) & closed-circuit cooling system (right) (Hoffschmidt, et al., 2022) ..................................................................................................................................... 6 Figure 3 Components of an evaporative condenser (Harby, Gebaly, Koura, & Hassan, 2016) ..... 9 Figure 4 Relationship between range, approach, and wet-bulb temperature ................................ 12 Figure 5 Effect of approach on chosen tower size (SPX Cooling Technologies, Inc., 2009) ...... 13 Figure 6 Psychrometric representation of a cooling tower (ASHRAE, 2008) ............................. 16 Figure 7 Cooling tower components (ASHRAE, 2008) ............................................................... 25 Figure 8 Control volume of a cooling tower (Kloppers & Kröger, 2005) .................................... 26 Figure 9 Pressure-enthalpy diagram for compression refrigeration system (Baltimore Aircoil Company) ...................................................................................................................................... 30 Figure 10 Streams entering and exiting a cooling tower .............................................................. 31 Figure 11 Yearly water requirements to cool down 3500 kW at different inlet air conditions .... 37 Figure 12 Air flow requirements to cool down 3500 kW at different inlet air conditions ........... 38 Figure 13 Annual water consumption in the selected locations ................................................... 41 Figure 14 Average air flow required in the selected locations ..................................................... 42 Figure 15 Cost of water consumption in different regions ........................................................... 42 Figure 16 Difference between the rule of thumb water consumption and minimum water consumption in two separate summer weeks in Las Vegas .......................................................... 43 Figure 17 Difference between the rule of thumb water consumption and minimum water consumption in two separate days in Las Vegas .......................................................................... 44 Figure 18 Water consumption comparison of a site with 28 000 kW cooling tower in Dubai .... 46 Figure 19 Water consumption analysis of a site with 4 200 kW cooling tower in Pennsylvania . 47 4 flows induced draft, counter flow forced draft, and cross flow induced draft (ASHRAE, 2008). The difference between the three types is what their names suggest. Figure 1 shows the difference between a cross flow and a counter-flow cooling tower. As can be seen in Figure 1, the counter flow towers (b) are designed in a way in which hot water enters at the top, while the air is introduced at the bottom and exits at the top, usually through a fan. In cross flow draft cooling towers (a), hot water also enters from the top of the tower. However, the air enters at the side of the tower, either at one side which in this case is referred to as single-flow tower, or opposite sides (double-flow tower) and passes through the fill while crossing the water flow into open air (Kim, Lee, & Jeon, 2016). Moreover, mechanical draft cooling towers are most used because they are available in a wide range of capacities. Depending on the application, capacities range from around 35 kW, with 2.5 m3/hr flow rates to thousands of kilowatts and flow rates. In addition, most towers are designed in a way they can be grouped with other cooling towers, to cover a certain cooling Figure 1 Cross flow (a) Counter flow (b) cooling towers (Kim, Lee, & Jeon, 2016) 5 capacity. Therefore, when cooling towers are grouped, they are given the term “cells”. For example, if there is a group of 7 cooling towers linked together, it would be referred to as sevencell cooling tower. 2.1.2 Natural Draft Cooling Towers Unlike the mechanical draft cooling towers, natural draft cooling towers are mainly used for power plants that are located near the load centers. These cooling towers work without any fans, thereby reducing the power costs (Basu & Debnath, 2015). In this case, the air flow rate through a natural draft cooling tower depends on the density difference between ambient air and the air inside the tower. However, the best conditions for this type of tower to operate is in humid ambient conditions. Comparing between summer and winter, the effectiveness of the cooling tower was higher in winter when the humidity was high, reaching up to 80% (Dansena & Desai, 2021). If operating in a region where the ambient temperature is considerably high and low relative humidity, the density difference would not be adequate for an efficient performance and thus mechanical draft cooling towers would be preferred. The natural draft cooling tower is also available in two types: counterflow and crossflow. Both types have similar functions and rely on the buoyancy effect to flow air through the tower. Moreover, the natural draft cooling tower can either function with a closed circuit or open circuit cooling system. Figure 2 shows the different process of both systems (Hoffschmidt, et al., 2022). Both systems operate in a similar mechanism, however the heat transfer mechanism functions differently. As can be seen in Figure 2, the opencircuit cooling system (left) uses a heat exchanger to produce the current by buoyancy. On the other hand, the closed-circuit cooling system (right) contains a fill material instead of a heat exchanger, thereby altering the heat transfer mechanism (Hoffschmidt, et al., 2022). 6 Figure 2 Open circuit cooling system (left) & closed-circuit cooling system (right) (Hoffschmidt, et al., 2022) 2.2 Components of a Cooling Tower The components that shape up a cooling tower might differ in design from one type to another. For example, the location of the fan could either be in the bottom or the top of the tower, while crossflow and counter flow cooling towers have different inlet air and water locations. However, all cooling towers are equipped with similar components, which are listed in Table 2. 7 Table 2 Components of a cooling tower (Hill, Pring, & Osborn, 1990) Component Description Casing The structure that encloses the cooling process where heat and mass transfer occur between the air and water (usually made of steel or fiber reinforced polyester) Air inlet and outlet The entry and exit positions of cool and warm air which can vary depending on type of cooling tower. Fan The correct selection of the fan is essential since it controls the volumetric air flow rate. Can be fixed speed fans, or variable speed fans which are usually preferred. Water inlet The inlet of warm water to be mixed with drawn air Water distribution system Usually spray nozzles are installed at the top of the cooling tower to spray down the water entering the cooling tower to increase the surface area of spread water. Fill As shown in Figure 1, the fill is a material that maximizes surface area to increase contact between water droplets and air. The fill can be designed from different materials, it is usually made of PVC or PP plain material. There are two basic types of fill that are used, the splash fill and film fill. As the names suggest, the splash fill is designed in a way in which water is splashed, while the film fill is designed to act as a channel in which water passes through (Midwest Cooling Towers Blog, 2013). Basin Can also be referred to as tank or sump. Located at the bottom of the tower to collect the cooled water that is exiting the cooling tower Cold water outlet The water exit of the cooling tower, which is connected to the basin 8 2.3 Evaporative Condensers Similar to cooling towers, evaporative condensers reject heat by the evaporation of water. Even though an evaporative condenser has a similar cooling concept to cooling towers, it functions differently. The basic function of an evaporative condenser is to convert a refrigerant from heated vapor to cooled liquid. The refrigerant flows inside a cooling tube where water splashes on the outside of the tube, thereby cooling down the refrigerant by means of evaporation of water (Harby, Gebaly, Koura, & Hassan, 2016). Evaporative condensers are useful in counties where temperatures are high. In many middle eastern countries for example, temperatures reach around 40 ℃ in the summer, or sometimes higher. During these conditions, a compressor of an air conditioner runs almost all the time and consumes high amounts of electrical power and might be at risk of getting damaged if not operated correctly. To reduce the risks of that occurring, the temperature and pressure of the compressor must be decreased. This is where an evaporative condenser plays an important role, since it reduces the temperature of the ambient air as it enters the condenser from its dry bulb temperature to its saturated condition, which is close to the ambient wet bulb temperature. 2.3.1 Components and Operating Principles of Evaporative Condensers Because an evaporative condenser behaves similar to a cooling tower, most of its components are also similar. A condensing coil, a fan, water spray, a pump to circulate water, a water eliminator, an outer casing, and controls are all components that make up an evaporative condenser. Figure 3 shows a descriptive process of how an evaporative condenser works. As can be seen in Figure 3, water is pumped from the basin to the top of the tower where it is sprayed down on the tubes that the refrigerant passes through. Air is usually controlled by a fan that enters at ambient conditions and exits carrying the heat rejected. As air is drawn, a part of the water evaporates, and heat is absorbed through the pipes that the refrigerant passes through, which leads to the cooling and condensation of the refrigerant gas. The remaining water falls back to the water basin where it is 9 pumped back to the top of the tower. Like the cooling tower, an ideal evaporative condenser would lose water only through evaporation of water (Harby, Gebaly, Koura, & Hassan, 2016). 2.4 Water as the Cooling Media After understanding the fundamentals of a cooling tower and the different steps of the evaporative cooling process, an intriguing question would be why is water used as a cooling media and where did this idea come from? In terms of thermodynamics, many applications where cooling is required use water as a media since water has high boiling point, high specific heat, and high heat of evaporation (Sensorex, 2018). In addition, water being easily accessible is another advantage. Historically, Newcomen injected water into a cylinder to condense steam to retract a piston and develop as he called it his “fire engine”. The engine became a commercial success, and it needed some means of condensing steam, water was found in abundancy and there was no other substitute. Since water worked as a coolant media, it became more common to use it in heat engines (Baker, 1984). Today, there are many heat engines in practice and therefore the amount of water to be used Figure 3 Components of an evaporative condenser (Harby, Gebaly, Koura, & Hassan, 2016) B D C 10 has increased significantly. To that end, it remains highly important to operate a cooling tower at the appropriate design conditions to utilize the use of water efficiently. 2.5 Performance Parameters of Cooling Towers There are several factors measured or calculated to ensure that a cooling tower is performing at a good level. As mentioned in chapter 1, there are some importance criteria that a cooling tower is designed upon to ensure that the tower performs as efficient as possible. In this section, the factors that affect the performance of a cooling tower are explained in more detail. 2.5.1 Range of a Cooling Tower The Range of a cooling tower is the difference in temperature between the inlet water and the water exiting the tower (Schwedler, 2014). In other terms, it is the temperature difference between the hot and cold-water streams at the tower. At the same capacity, increasing the range of a cooling tower reduces its capital and energy costs. Additionally, it would also reduce the capital and energy costs of the condenser water system since less water would be required for a specific capacity. However, to achieve a larger difference in temperature between the inlet and exiting water depends on the chiller and the load that is produced by it. Higher water temperatures exiting the chiller would produce a higher range in the cooling tower to keep the system stable. Although that would benefit the cooling tower performance, chillers with warmer exiting condensing water consume more energy and may also be more expensive than chillers with lower condensing water temperatures (Trane Engineers Newsletter volume 34-1, 2005). To that end, choosing good operational conditions becomes a tradeoff between cooling tower range and the chiller. This is the reason why manufacturers specify the range and flow rate that the cooling tower is designed to achieve. For example, on Baltimore’s Series 3000 Single Cell data sheet, the nominal tonnage of cooling represents the capability of a cooling tower to cool 0.68 m3/hr (3 USGPM) of water from 35 ℃ (95 ℉) to ≈30 ℃ (85 ℉) at a 25.5 ℃ (78 ℉) entering wet bulb temperature (Baltimore Aircoil Company). In this case, the range is around 5 ℃ which is identified as a good range parameter. The importance of understanding the entering wet bulb temperature and how it influences the performance of a cooling tower is discussed in the next section. The formula used to calculate the range is shown below. 𝑅𝑎𝑛𝑔𝑒= 𝑇𝑤 𝑖𝑛−𝑇𝑤 𝑜𝑢𝑡 (2.1) 11 Where: 𝑻𝒘 𝒊𝒏 = 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑤𝑎𝑡𝑒𝑟 𝑒𝑛𝑡𝑒𝑟𝑖𝑛𝑔 𝑡ℎ𝑒 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑡𝑜𝑤𝑒𝑟 (℃ 𝑜𝑟 ℉) 𝑻𝒘 𝒐𝒖𝒕 =𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑤𝑎𝑡𝑒𝑟 𝑒𝑥𝑖𝑡𝑖𝑛𝑔 𝑡ℎ𝑒 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑡𝑜𝑤𝑒𝑟 (℃ 𝑜𝑟 ℉) 2.5.2 Approach The Approach is another important factor and an indicator that is used to analyze the performance of a cooling tower (Chemical Engineering Site, 2020). The approach is the temperature difference between the product and the “power source”. In cooling tower calculations, the approach is evaluated by calculating the difference between the temperature of the water exiting the cooling tower and the inlet wet bulb temperature of the air (Trane Engineers Newsletter volume 34-1, 2005). The wet bulb temperature of the air is referred to as the heat source since it is the driving force that influences the outlet temperature of the water. For example, in the example obtained from Baltimore’s design conditions, the approach is 4.5 ℃. The wet bulb temperature of the air is a more relevant indicator than the dry bulb temperature since it is related to the evaporation of water. To get a better grasp of this idea, The psychrometric chart of an evaporative cooling process is found in Appendix A (SPX Cooling Technologies, Inc., 2009). Consider air is entering at point 1 and after mixing with water, leaves the cooling tower at saturated conditions (relative humidity = 100%) with a significant increase in its enthalpy as it carries the heat from the water. Now consider air entering at point 1’ and exiting at point 2. The dry bulb decreases as point 1’ has a 100% relative humidity and therefore the wet bulb temperature is similar to point 1 at 18.3 ℃ (65 ℉). As can be seen from the chart, the dry bulb temperature, which is the measured temperature changed while the wet bulb temperature remains the same as point 1. Moreover, the enthalpy exchange between points 1’ and 2 remains equivalent to the difference between points 1 and 2. This is the reason why the wet bulb temperature is the primary basis for the design of a cooling tower. Equation 2.2 shows the formula that can be used to calculate the approach of a cooling tower. 𝐴𝑝𝑝𝑟𝑜𝑎𝑐ℎ= 𝑇𝑤 𝑜𝑢𝑡 −𝑇𝑎𝑤𝑏 (2.2) Where: 𝑻𝒘 𝒐𝒖𝒕 = 𝑤𝑎𝑡𝑒𝑟 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑙𝑒𝑎𝑣𝑖𝑛𝑔 𝑡ℎ𝑒 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑡𝑜𝑤𝑒𝑟 (℃ 𝑜𝑟 ℉) 12 𝑻𝒂 𝒘𝒃 = 𝑤𝑒𝑡 𝑏𝑢𝑙𝑏 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑎𝑖𝑟 (℃ 𝑜𝑟 ℉) Understanding the importance of each parameter individually aids in understanding the relation between them. Figure 4 shows an example to illustrate the relationship between the range, approach, and the wet bulb temperature. Suffice it here to say that the lower the approach, the higher the efficiency of the cooling tower. In addition, Figure 5 shows the relationship between approach and the cooling tower size. Note how the decreasing approach in the figure begins its asymptotic movement as it decreases. For this reason, manufactures would almost never guarantee a cooling tower with an approach below 3 ℃, not because it is impossible to achieve, but because any errors in measurement become very significant when calculations of performance are evaluated at the design point (SPX Cooling Technologies, Inc., 2009). Figure 4 Relationship between range, approach, and wet-bulb temperature Hot water inlet Range Approach Wet-bulb temperature Cold water outlet 30 ℃ 35 ℃ 26 ℃ 13 Figure 5 Effect of approach on chosen tower size (SPX Cooling Technologies, Inc., 2009) 2.5.3 Effectiveness/Efficiency After understanding the difference between range and approach, and what they are used for, one can comprehend what the effectiveness is and what it is used for. In basic terms, the effectiveness of a cooling tower is a measure of its efficiency, to determine how well the cooling tower is operating. Calculating the cooling tower effectiveness, or in other terms its efficiency depends on the range and approach (Chemical Engineering Site, 2020). A cooling tower performing at a good level should have an effectiveness between 70-75% (Hall, 2012). Equation 2.3 shows how the range and approach shape out the formula to calculate the effectiveness of a cooling tower. 𝜀= 𝑇𝑤 𝑖𝑛−𝑇𝑤 𝑜𝑢𝑡 𝑇𝑤 𝑖𝑛−𝑇𝑎 𝑤𝑏 × 100 (2.3) In other terms, 𝜀= 𝑅𝑎𝑛𝑔𝑒 𝑅𝑎𝑛𝑔𝑒+𝐴𝑝𝑝𝑟𝑜𝑎𝑐ℎ ×100 As can be seen from equation 2.3, as the approach decreases, the effectiveness increases since they are inversely proportional. However, the performance of the plant as a whole depends on the range and approach of a cooling tower and the COP of the chiller in a refrigeration plant. The main idea is to operate all equipment properly to remain in the limits of the desirable efficiency parameters. Choosing a cooling tower with a close design approach is desirable since it will supply the chiller with cooler water. However, it will probably also result in a significant increase in capital cost and 20 Table 3 Drift Measurement Techniques (Roffman & Van Vleck, 1974) Techniques Description Sensitive paper Chemically treated filter paper. The droplets form insoluble blue stains when they splash onto the paper. Coated slide A coating material (a mixture of oil and vaseline, magnesium oxide, gelatin, and sensitive indicators such as methyl red or naphthol green B mixed with gelatin) acts as a droplet capturer. The captured droplets leave rings whose size and number can be observed Impaction sampling method A glass tube filled with small glass beads collects the drift droplets and particles. The tube is surrounded by an electric resistance wire that provides the tube enough heat to completely evaporate the droplets entering the tube Cyclone separator The entering liquid or solid particles are separated from the air stream by means of centrifugal force and are drained and collected by a jar located at the bottom of the cyclone. Light scattering method The system consists of a laser diode and other suitable components that can result in an output as drift rate and droplet size distribution. The light scattered by the droplets is detected by a photo detector. High volume sampling method Dissolved solids from the droplets are collected on a filter. The filter is heated by infrared lamps to remain dry and collect efficiently. Chemical balance A method that involves measuring the rate of decrease in concentration of a tracer chemical that is added. Time yield is used in this method to estimate the drift rate. Calorimetric method A method that involves drift droplets passing the throttle point in the calorimeter that evaporate due to the difference in pressure. The temperature change in this case is taken as an indication of drift rate. On the other hand, some countries-imposed guidelines to limit the drift that is ejected from cooling towers. For example, In Australia and New Zealand, cooling tower drift is required to remain below 0.002% by AS/NZS 3666.1, which is a percentage that cannot be achieved without efficient drift eliminators (The Australian Institute of Refrigeration, Air Conditioning, and Heating, 2009). It is important to note that drift eliminators guarantee an output that remains below a specific percentage but does not accurately measure the drift. Therefore, it remains highly important to constantly measure the amount of drift droplets that are exiting a cooling tower. 21 2.6.3 Leaks Leaks are usually minimal and the water that is lost through leaks is usually insignificant if the cooling tower is well maintained and is thus not included in makeup water calculations (Bhatia, 2001). However, leaks can result in being a major type of water loss if not taken care of directly when identified. They can be linked to weary material of construction, or irregular treatment of water. After operating for a long time, the quality of the constructive material decreases, and corrosion can negatively impact the cooling process by producing leaks. A combination of internal and external corrosion in piping can create leaks, thereby increasing the amount of water lost (Jena, 2020). Cooling towers constructed from steel develop leaks over time, which can be treated by galvanized dip or stainless-steel options. In addition, due to movement that occurs between a cooling tower and the other components that it is connected to, poor pipe connections can result in major leaks (Jena, 2020). The best way to treat leaks would be to conduct a visual survey periodically. Moreover, water meters and consumption levels should be looked at frequently to witness any change in water consumption (The Australian Institute of Refrigeration, Air Conditioning, and Heating, 2009). Therefore, the first steps that should be taken to avoid leaks is installing water meters and measuring the water consumption. 2.6.4 Bleed-off Cycles of concentration has a direct role in calculating the blowdown, which is also referred to as bleed-off (also blowdown or bleed). When the concentration of dissolved solids in the water of the cooling tower increases above a certain threshold, some of the water that is found in the basin is ejected to retain appropriate concentration levels. (Baker, 1984). Adequate bleed-off control is important to avoid scale in the cooling tower. Scale is formed from minerals such as calcium carbonate, calcium phosphate, calcium sulfate, and silica. Scale is formed on surfaces in contact with water and is the result of poor water treatment. To that end, a conductivity sensor is crucial to measure the total dissolved solids in the water and maintain it at a level close to the maximum solubility level, which minimizes the bleed-off (Guyer, 2014). Additionally, it is important to maintain adequate water flow to avoid the settling of suspended solids. A dispersant is used to prevent particulate solids from settling or attaching to heat transfer surfaces. The suspended solids are removed from the tower through the bleed-off, it is therefore important to occasionally clean 22 the tower sump. Equation 2.8 shows the formula that is used to calculate the bleed-off, which as mentioned earlier, depends on the cycles of concentration and the evaporation of water which will be explained in the next section (Guyer, 2014). To ensure that the concentration of solids does not exceed the design conditions, a sensor is installed in the basin of the cooling tower that measures the conductivity of the water. Conductivity is a parameter that is easily measured and is directly related to the total dissolved solids level, therefore by measuring the conductivity of the cooling tower water and make up water, the cycles of concentration can be determined (Weimar, 2009). Table 4 shows some of the standards that are created to maintain a good quality of water and minimize the risks of having corrosion and fouling. 𝐵𝑙𝑒𝑒𝑑−𝑜𝑓𝑓=𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 𝐶.𝑂.𝐶−1 (2.8) Table 4 Standards to maintain the quality of water in "normal" conditions (SPX Cooling Technologies, Inc., 2009) Parameter Value pH 6-8 Chloride content (NaCl) <750 ppm Sulfate content (SO4) <1200 ppm Sodium bicarbonate (NaHCO3) <200 ppm Temperature <49 ℃ Chlorine if used, should be added intermittently, with a free residual not to exceed 1 ppm, for short periods 2.6.5 Other Types of Water Loss As explained in previous sections, there is no doubt that there are different types of water losses that take place in during the cooling process in a cooling tower. However, it is important to understand that some loss types can be classified as controllable, while others are not. For example, the water lost through evaporation is inevitable and must take place for the water to be cooled. On the other hand, leaks and improper water treatment are losses that can be treated. Table 5 summarizes all possible water losses that could occur and how they can be reduced. 23 Table 5 Water Loss Types and Possible Reduction Methods (The Australian Institute of Refrigeration, Air Conditioning, and Heating, 2009) Water loss type Description Reduction method Evaporation Water that is evaporated as the water cools down Reducing cooling load Upgrading or redesigning the system Bleed Water that is drained to maintain a level of concentration of water Installing a bleed valve which is linked to a conductivity sensor in the basin Overflows Caused by water flowing back into the basin once the circulating pump has stopped Ensure the makeup level is set correctly Can occur due to poor pipework design and installation, or incorrectly set makeup level Consider redesigning pipework and installation Drift Droplets of water that exit with the evaporated water Regulate fan speeds Preventing ambient wind speeds from impacting the tower Splash-out Water leaving the tower through the air inlet stream or other openings Install anti splash louvres Install a splash deck Regulate fan speed Windage Similar to drift, but occurs due to strong wind Protect the tower from wind by screening Leaks Water loss due to corrosive material or weary construction Visually check for leaks frequently Filter backwashing Water consumed through cleaning the filters Use recycled water for backwash purposes 2.6.6 Makeup Water After the water is lost through any of the previous types, fresh water must be added to ensure proper functionality of the cooling tower. As mentioned earlier, the water evaporates leaving behind the dissolved solids which increases corrosion tendencies. Therefore, water is added to the cooling tower directly to the basin and is referred to as makeup water (Bhatia, 2001). 24 Calculating the makeup water is simple in theory since it is equivalent to all the water that is lost during the cooling process. Therefore, it is essential to measure how much water is consumed and released through evaporated, bleed-off, drift, and leaks. Equation 2.9 shows the formula used to calculate the makeup water. However, it is very difficult to know exactly how much water is lost through leaks and drift. It is therefore important to measure the water consumed through evaporation and bleed-off and analyze it with the “ideal” water consumption that is explained in further chapters. Figure 7 depicts the major components of a cooling tower, while showing the areas where a cooling tower loses water from. 𝑀=𝐸+𝐵+𝐷+𝐿 (2.9) Where: 𝑴= 𝑚𝑎𝑘𝑒𝑢𝑝 𝑤𝑎𝑡𝑒𝑟 (𝐿) 𝑬= 𝑤𝑎𝑡𝑒𝑟 𝑙𝑜𝑠𝑡 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 (𝐿) 𝑩= 𝑤𝑎𝑡𝑒𝑟 𝑙𝑜𝑠𝑡 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑏𝑙𝑒𝑒𝑑 (𝐿) 𝑫= 𝑤𝑎𝑡𝑒𝑟 𝑙𝑜𝑠𝑡 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑑𝑟𝑖𝑓𝑡 (𝐿) 𝑳= 𝑤𝑎𝑡𝑒𝑟 𝑙𝑜𝑠𝑡 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑙𝑒𝑎𝑘𝑠 (𝐿) 25 Figure 7 Cooling tower components (ASHRAE, 2008) 1: Fan stack, 2: Discharge, 3: Sound attenuator, 4: Fan, 5: Drift eliminator, 6: Water distribution system/nozzles, 7: plenum, 8: Fill packing, 9: plenum, 10: Air inlet louvres, 11: Filters, 12: Makeup water inlet, 13: Overflow, 14: Water basin, 15: Basin sweeper piping, 16: Blowdown (bleed), 17: Tower outlet with screen strainer. 26 2.6.7 Merkel’s Method The basis of the evaporative cooling technique began with Merkel’s theory that was created in 1925. However, it was not given much attention or applied until it was translated to English in 1941 (Kloppers & Kröger, 2005). Merkel’s theory also gave birth to Poppe’s formula that is shown in equation 2.5. The theory is commonly used for thermal evaluation of cooling towers and the method is used in cooling tower design (Shah & Tailor, 2015). The theory begins with a mass and energy balance on a control volume of an ideal interface between air and water as shown in Figure 8. The mass and energy balances are shown in equations 2.10 and 2.11. Merkel assumed that the amount of water evaporated is negligible, thereby not affecting the flow rate of water across the cooling tower (𝑑𝑚𝑤=0). Equation 2.10 describes the change in the enthalpy of the air-water vapor mixture while equation 2.11 resembles the change in water temperature as air flows through the cooling tower. Combining equations 2.10 and 2.11 would yield Equation 2.12, which is Merkel’s equation (Kloppers & Kröger, 2005). Figure 8 Control volume of a cooling tower (Kloppers & Kröger, 2005) 27 𝑑𝑖𝑚𝑎 𝑑𝑧 =ℎ𝑑𝑎𝑓𝑖𝐴𝑓𝑟 𝑚𝑎(𝑖𝑚𝑎𝑠𝑤 −𝑖𝑚𝑎) (2.10) Where: 𝒉𝒅 𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 (𝑘𝑔 𝑚2𝑠) 𝒂𝒇𝒊 =𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙 𝑝𝑒𝑟 𝑢𝑛𝑖𝑡 𝑣𝑜𝑙𝑢𝑚𝑒 (𝑚−1) 𝑨𝒇𝒓 =𝑓𝑟𝑜𝑛𝑡𝑎𝑙 𝑎𝑟𝑒𝑎 (𝑚2) 𝒎𝒂=𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑎𝑖𝑟 (𝑘𝑔 𝑠) 𝒊𝒎𝒂𝒔𝒘 =𝑒𝑛𝑡ℎ𝑎𝑙𝑝𝑦 𝑜𝑓 𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 𝑎𝑖𝑟 𝑎𝑡 𝑡ℎ𝑒 𝑏𝑢𝑙𝑘 𝑤𝑎𝑡𝑒𝑟 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 ( 𝐽 𝑘𝑔) 𝒊𝒎𝒂 = 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑒𝑛𝑡ℎ𝑎𝑙𝑝𝑦 𝑜𝑓 𝑚𝑜𝑖𝑠𝑡 𝑎𝑖𝑟 ( 𝐽 𝑘𝑔) 𝑑𝑇𝑤 𝑑𝑧 =𝑚𝑎 𝑚𝑤1 𝑐𝑝𝑤𝑑𝑖𝑚𝑎 𝑑𝑧 (2.11) 𝑀𝑒𝑀=ℎ𝑑𝑎𝑓𝑖𝐴𝑓𝑟𝐿𝑓𝑖 𝑚𝑤=ℎ𝑑𝑎𝑓𝑖𝐿𝑓𝑖 𝐺𝑤=∫𝑐𝑝𝑤𝑑𝑇𝑤 (𝑖𝑚𝑎𝑠𝑤−𝑖𝑚𝑎) 𝑇𝑤𝑖 𝑇𝑤𝑜 (2.12) Where: 𝑴𝒆𝑴=𝑀𝑒𝑟𝑘𝑒𝑙′𝑠 𝑛𝑢𝑚𝑏𝑒𝑟 𝑳𝒇𝒊 =𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙 (𝑚) 𝑮𝒘=𝑚𝑎𝑠𝑠 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 (𝑘𝑔 𝑚2𝑠) 𝒎𝒘=𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 (𝑘𝑔 𝑠) 𝒄𝒑𝒘 =𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 ( 𝐽 𝑘𝑔𝐾) 𝑻𝒘𝒊 & 𝑻𝒘𝒐 =𝑖𝑛𝑙𝑒𝑡 𝑎𝑛𝑑 𝑜𝑢𝑡𝑙𝑒𝑡 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 (℃) Merkel’s number is used to measure the degree of difficulty of the mass transfer processes taken place in the cooling tower (Navarro, Ruiz, Hernandez, Kaiser, & Lucas, 2022). Another crucial assumption that Merkel considered is that the exiting air is saturated with water vapor, which simplifies the calculation of the exiting air temperature. To calculate the heat rejected using Merkel’s approach, equation 2.13 is used. 28 𝑄= 𝑚𝑤𝑐𝑝𝑤(𝑇𝑤𝑖 −𝑇𝑤𝑜) (2.13) Where: 𝑸=ℎ𝑒𝑎𝑡 𝑟𝑒𝑗𝑒𝑐𝑡𝑒𝑑 (𝑘𝑊) However, the equation excludes the effect of the change in the mass flow rate of water in the energy balance equation. Nevertheless, assuming that air is saturated at the outlet, the flow rate of the evaporated water can be calculated using equation 2.14. 𝑚𝑤(𝑒𝑣𝑎𝑝)=𝑚𝑎(𝑤𝑜−𝑤𝑖) (2.14) 𝒎𝒂=𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑎𝑖𝑟 (𝑘𝑔 𝑠) 𝒘𝒐 & 𝒘𝒊=ℎ𝑢𝑚𝑖𝑑𝑖𝑡𝑦 𝑟𝑎𝑡𝑖𝑜 𝑎𝑡 𝑡ℎ𝑒 𝑜𝑢𝑡𝑙𝑒𝑡 𝑎𝑛𝑑 𝑖𝑛𝑙𝑒𝑡(𝑘𝑔𝑤𝑎𝑡𝑒𝑟 𝑘𝑔𝐷𝐴) Since Merkel’s approach neglects the water evaporated in the heat balance equation, the heat rejection calculations differ when compared to other methods such as the Poppe approach (Kloppers & Kröger, 2005). 3 Methodology on Defining a New Benchmarking Method Looking at how the state of air changes on a psychrometric graph as it flows through a cooling tower as shown in Appendix A, it is obvious that the water content changes depending on the inlet air properties. In previous sections, the theory used to evaluate cooling tower design and the evaporation of water has been discussed. In this chapter, the emphasis will be more on defining a new benchmarking method that cooling tower operators can use to ensure that the cooling towers are operating efficiently. Is important to note the assumptions that were considered in performing the calculations of this section. The assumptions can be found in Table 6. 29 Table 6 Cooling tower application assumptions Assumption (in example) Basis 3 ℃ approach Typical efficient design conditions (SPX Cooling Technologies, Inc., 2009) 5 ℃ range Typical efficient design conditions (SPX Cooling Technologies, Inc., 2009) Exiting air is at saturated conditions (Relative humidity is 100%) If the cooling tower is operating correctly, the exiting air is saturated with water vapor as it carries the heat from the water (ASHRAE, 2008) Exiting air is 8 ℃ greater than the inlet wet bulb temperature Design conditions (5 ℃ range and 3 ℃ approach) Exiting air is equivalent to condensing temperature in an evaporative condenser Design conditions (an evaporative condenser is designed to operate at such conditions) A cooling tower with 3500 kW cooling capacity is analyzed Medium sized cooling tower that is commonly used Figure 9 shows the pressure-enthalpy diagram for compression refrigeration systems. The stream shows the different states that the refrigerant passes through in the whole system. As can be seen in the diagram, the refrigerant condenses at point C. Referring to Figure 3, the refrigerant condenses at that point after spraying water on the pipe at which it is passing through. At the same point, air travels to the top of the condenser where it is discharged. To that end, the assumption of exiting air is equivalent to the refrigerant condensing temperature is made. Moreover, it is important to note that the developed analytical method works on any cooling capacity and not only the assumed capacity found in Table 6. 3500 kW was chosen for this example as it would resemble a cooling tower that is not small nor large. 36 Finally, the last steps would include calculating the differences in the enthalpy and water content of the exiting air and the air entering the cooling tower (∆𝐻=𝐻2−𝐻1 & ∆𝑥=𝑥2−𝑥1). After that, the minimum air flow required to cool down a certain heat load can be evaluated from equation 3.6. The minimum air flow required also influences the water consumption, which is then calculated from equation 3.7. 3.3.3 MATLAB Results After calculating the air and water requirements for a specific temperature, it is crucial to analyze how the air and water requirements vary according to the ambient temperature and humidity. The main idea is to show that in different locations, there are different air and water requirements. To that end, the analysis was extended over a set of temperatures consisting of dry bulb temperatures ranging from 10-40 ℃ and wet bulb temperatures ranging 5-35 ℃. The ranges were chosen depending on temperatures where cooling towers are usually found. The code was developed on MATLAB and the results are shown in Figure 11 and Figure 12. The same assumptions that are listed in Table 6 were considered for all calculations. 37 Figure 11 Yearly water requirements to cool down 3500 kW at different inlet air conditions Typical summer in Las Vegas Typical summer in Berlin 38 Figure 12 Air flow requirements to cool down 3500 kW at different inlet air conditions As can be seen in Figures 11 & 12, the water consumed due to evaporation and the required air flow rate varies significantly at different ambient temperatures. For instance, consider an example of a typical summer day in Las Vegas (Dry-bulb temperature: 37 ℃, wet-bulb temperature: 21 ℃) and a typical summer day in Berlin (Dry-bulb temperature: 21 ℃, wet-bulb temperature: 17 ℃). Looking at Figure 11, the cooling tower would consume more water to cool down 3500 kW. Moreover, Figure 12 shows that a cooling tower in Berlin would require a higher air flow. The point main outcome of this comparison is to show that the operating conditions vary according to the ambient conditions and can never be constant, unlike what the rule of thumb proposes. In addition, operating at the right air flow is crucial to ensure that the required water evaporates. The graph can also act as a basis for cooling tower operators to measure and compare their water consumption. Typical summer in Las Vegas Typical summer in Berlin 39 3.4 Comparing Analysis with the “Rule of Thumb” Identifying the required air flow and water consumption to cool down a certain process is essential for many reasons. However, it is also important to see how the results compare with the rule of thumb keeping in mind that the rule of thumb provides an approximation based on specific operating conditions, therefore it would not be applicable to any weather condition. This section explains the different analyses that are made to compare the results with the rule of thumb. 3.4.1 Cities Analyzed A comparative analysis was made on 9 different cities that were chosen to represent different markets with a significant use of cooling towers with varying climates. The cities are listed in Table 7, while the hourly weather data of each city was obtained from Meteonorm 7, which provides the hourly weather data of a normal year in a specified location. It is important to mention that the same assumptions that are listed in Table 6 are also used for this analysis. Table 7 Cities selected for the comparative analysis Country City Australia Sydney United States of America Las Vegas Kenya Nairobi Singapore Singapore United States of America Houston South Africa Cape Town United Arab Emirates Dubai Germany Berlin 3.4.2 Results Then, the air flow requirements and water consumption for every hour along a Normal Year were calculated to obtain an average air flow rate required, and the total water consumption. Figure 13 and Figure 14 show the results of the previous explained calculations. Note how Berlin, the coldest city between the 9 selected, requires the highest air flow on average to cool down 3500 kW. Moreover, as can be seen in Figure 13, the rule of thumb, or the empirical formula used (equation 2.7) appears constant in all locations since it does not depend on the ambient conditions. 40 Additionally, the rule of thumb appears to be close to the calculations that are done, which validates the theory developed in earlier sections. It is important to note that in extreme weather conditions, such as Las Vegas and Dubai where temperatures could reach up to 40 ℃, the water consumption is more than what the rule of thumb indicates. In addition, the analysis was performed over a whole “normal year” only, the amount of water consumed will show a higher variation for the shorter periods required for early detection of inefficient operations. In such cases, assigning the rule of thumb as an efficient indicator of water consumption would be insufficient and high cost will accumulate before a fault in the tower is detected. The reason to that is because the rule of thumb calculated an approximate value of the water consumed during the evaporation and therefore cannot be considered as an accurate value. Inaccurate measuring of the water evaporated also influences the amount of bleed-off which impacts the amount of makeup water which influences the balance of the cooling system leading to insufficient operation. Additionally, it is of interest to see how the different water consumptions would affect the water costs since the cost of water varies drastically from one region to another. Furthermore, it is highly crucial to preserve water in countries where water is scarce. Addressing that, Figure 15 compares the costs of water consumption in the different regions which were determined depending on the amount of water consumed, and the price of water in each country. The cost of utility water for each chosen region are listed in Table 8. Looking at Berlin, there is a significant difference between what the rule of thumb indicates and what the actual annual costs would be. Therefore, it is important to optimize by operating at the prevailing conditions to preserve water and reduce costs. In other cases, a cooling tower in Dubai requires more water than what the rule of thumb indicates. If the rule of thumb is followed, the system would alert for over consumption of water. 41 Table 8 Cost of Water in Each Region Region Cost of Water ($/1000 Liters) Source Houston 1.625 (2022 Harris County Municipal Utility District 501, n.d.) Dubai 2.09 (DEWA, 2021) Singapore 1.21 (PUB, n.d.) Cape Town 0.18 (City of Cape Town, 2022) Lisbon 1.84 (EPAL, 2022) Berlin 1.81 (Berliner Wasserbetriebe, n.d.) Sydney 1.47 (Sydney Water, 2022) Nairobi 0.53 (Nairobi Water Bill Calculator, 2021) Las Vegas 0.98 (Las Vegas Valley Water District, 2022) 18 628 5 000 10 000 15 000 20 000 25 000 30 000 Sydney Las Vegas Lisbon Nairobi Singapore Houstoun Cape Town Dubai Berlin Tonnes Water Minimum Annual Water Consumption in Different Regions Water Consumption Rule of Thumb Figure 13 Annual water consumption in the selected locations 42 50 000 100 000 150 000 200 000 250 000 Sydney Las Vegas Lisbon Nairobi Singapore Houstoun Cape Town Dubai Berlin Air Flow (m3/hr) Average Air Flow Required in Different Regions Required Air $29 488 $44 663 $22 231 $3 189 $32 139 $28 263 $26 639 $9 955 $23 564 $ 0 $5 000 $10 000 $15 000 $20 000 $25 000 $30 000 $35 000 $40 000 $45 000 $50 000 Houston Dubai Singapore Cape town Lisbon Berlin Sydney Nairobi Las Vegas Cost of water consumption per month Cost of Water Consumption in Different Regions Winter Rule of Thumb Figure 14 Average air flow required in the selected locations Figure 15 Cost of water consumption in different regions 43 It is interesting to see the difference between what the rule of thumb indicates and the evaluated required annual air flow rate and water consumption. The analysis was done over a “normal” year, however it is also of interest to reduce the time and perform the analysis over a week or a day. Las Vegas was chosen as the location to perform the analysis over two separate weeks, one in summer and the other in winter. Figure 16 shows the results that are obtained and the difference in water consumption between the second and first weeks of July. As can be seen, the rule of thumb indicates the same water consumption for both weeks, while the minimum required water consumption was significantly different between the two weeks (around 180 tons of water). Figure 16 Difference between the rule of thumb water consumption and minimum water consumption in two separate summer weeks in Las Vegas Moreover, the analysis was further extended and performed over the first two days of the year. As shown in Figure 17, there is still a significant difference in a period of 48 hours. There is also still 0 100 200 300 400 500 600 Las Vegas Tonnes Water Minimum Water Consumption in Different Weeks of Summer and Winter in Las Vegas 1st week of January 2nd week of July Rule of Thumb 44 a difference between what the rule of thumb indicates and the evaluated required water consumption. This analysis further validates the inaccuracy of the rule of thumb and the need for a developed benchmark that cooling tower operators can use for optimization. Figure 17 Difference between the rule of thumb water consumption and minimum water consumption in two separate days in Las Vegas 3.4.3 Scenario Analysis After witnessing the difference in air flow requirements in different regions and how it affects the water consumption, it becomes more interesting to see what happens when a cooling tower is not operating at the required air flow conditions. The air flow can be regulated by the fan speed; however, many fans operate at fixed speeds which could therefore have deteriorating effects. To perform this analysis, first a cooling tower fan with a nominal power higher than what is required should be selected. Another condition is that the selected cooling tower fans are required to have a nominal power above the required maximum, which is in Las Vegas in this case. Therefore, a cooling tower manufactured by Evapco was selected, having the following configurations: 3 528 0 20 40 60 80 Las Vegas Tonnes Water Minimum Water Consumption of Different Close Days in Las Vegas 1st day of the year 2nd day of the year Rule of Thumb 45 kW, 2 fans with 14.9 kW nominal power each, model No. AT 114-5K26. The analysis was then performed on the two extremes: Berlin and Singapore. A random day was first chosen, and a code was performed on MATLAB to analyze how the system in Singapore would change if the cooling tower was operating at Berlin’s required air flow. As expected, the electricity costs increased drastically, but the water consumption increased as well. The reason to that is because higher air flow would increase the evaporation rate and thus consume more water. On the other hand, if the same water consumption was maintained, the increasing air flow results in exiting air conditions that are unsaturated, thus limiting the cooling process. Therefore, it can be concluded that operating at the required air flow and water requirements remains essential to reduce electricity costs, consume less water, and decrease the chances of damaging the cooling system. 3.4.4 More Examples Other analyses were performed to compare the difference in water consumption between what the rule of thumb indicates and the actual water consumption. The first case is of a large mall located in Dubai, United Arab Emirates. The mall is assumed to contain 10 cooling towers that have a total cooling capacity of 28000 kW. The other parameters required to calculate the water consumption were obtained from Table 6. The annual required water consumption versus the amount that the rule of thumb indicates can be seen in Figure 18. In this case, the required water consumption was around 14% more than what the rule of thumb indicates. The importance of measuring the water consumption and operating conditions here is crucial to maintain the cooling system. 52 used costs $10 000, which is a close assumption based on VFDs available in the market since the actual cost is unavailable. Moreover, the cost of the cooling tower is $1 500 000, which is evaluated by applying the same assumptions that were used to calculate the cost of the variable speed drive. The calculated difference in NPV after 20 years between the two systems is a bit less than $2 000 000 as shown in Figure 21, considering that both systems consume the same amount of water. In reality, the water consumption of the system without a VFD would be larger, therefore accounting for more costs. Installing a cooling tower without a VFD is a cheaper option, which is why it is more favored. However, the benefits of having a VFD outweigh the costs and allows much greater savings as shown in Figure 21. 4.2.2 Summary of Water Loss Prevention Adjusting the air flow rate is essential to preserve water and energy. In addition, there are other techniques that can be applied to ensure that the cooling towers are operating efficiently. Since it is now possible to determine the required water consumption, cooling tower manufacturers can Figure 21 NPV of a cooling tower with VFD over a 20-year period $ 10 000 $1 500 000 $1 500 000 $ 78 908 $ 161 736 $2 181 790 $2 876 946 $ $ 500 000 $1 000 000 $1 500 000 $2 000 000 $2 500 000 $3 000 000 $3 500 000 With VFD Without VFD Costs Costs of a cooling tower operating in Berlin without VFD versus VFD over a 20 year timeline NPV after 20 years Annual Operational Costs Cooling tower VFD 53 measure their water consumption and compare it with the required water consumption. If there is a difference in the numbers, then water is lost through other means, such as drift, leaks, blowdown. Table 9 includes suggestions to minimize the water lost through means other than evaporation, while also summarizing the necessary actions that should be regularly completed to maintain high cooling tower performance. Table 9 Improving cooling tower performance Goal Action Minimize Drift Installing efficient drift eliminators that can reduce the drift down to 0.001% of water flow rate Minimize leaks and overflows Visually check for leaks frequently Ensure makeup level is set correctly Fix leaking pipes immediately Maintain good quality of water Have a proper calibration routine for the conductivity sensor Have a proper calibration routine for the temperature sensors Optimize the cycles of concentration Reduce maintenance & operational costs Operate at minimum required air flow rate and water consumption Measuring based predictive maintenance that included monitoring the cooling tower and chiller efficiencies as well as water efficiency to reduce the need for scheduled maintenance. Early detection of small deviations would discard downtime and emergency call-out since repairs would be completed long before a catastrophic failure occurs Meet the minimum required air flow rate and water consumption Install variable speed fans and pumps Measure water consumption to analyze the water efficiency 54 5 Conclusion Cooling towers are found in abundancy around the world in many cooling applications since they decrease energy consumption of chillers and refrigeration plants. Cooling towers are also used in power plants and industrial sites unrelated to chillers and refrigeration plants, but these applications have not been the focus of this thesis. Nevertheless, the same principles and efficiency parameters would apply to cooling towers in all applications. There are different types of cooling towers, evaporative, and adiabatic condensers, but they all follow the same cooling concept, which is the evaporative cooling process. The rule of thumb states that 1% of the water flow rate is evaporated for every 6.5 ℃ of water cooled. The rule of thumb does not consider the ambient conditions and is recommended to all weather conditions, as can be seen in Chapter 3. To demonstrate the limitations of the rule of thumb, the study was performed to evaluate the air flow and water consumption requirements to cool down water by taking into account the variation in ambient conditions. To conclude, the results show explicitly that the rule of thumb has clear limitations since it offers inaccurate results that could vary significantly, as can be seen in Figure 13. By measuring the total water that a cooling tower is consuming, operators can compare the values with the required minimum water consumption according to their operating conditions and determine the water efficiency. Optimization of cooling towers through determining the water efficiency and early fault detection is made possible by following the developed benchmark that cooling tower operators can use. 55 References 2022 Harris County Municipal Utility District 501. (n.d.). Retrieved from https://www.hcmud501.org ASHRAE. (2008). Cooling Towers. In 2008 ASHRAE Handbook (pp. 39.1-39.20). American Society of Heating, Refrigerating and Air-Conditioning Engineers. Baker, D. (1984). Cooling Tower Performance. New York: Chemical Publishing Co., Inc. Baltimore Aircoil Company. (n.d.). Evaporative Condenser Engineering Manual. In Product & Application Handbook Volume V. Baltimore Aircoil Company. Baltimore Aircoil Company. (n.d.). Series 3000 Single Cell Data. In Product & Application Handbook Volume V. Baltimore Aircoil Company. Basu, S., & Debnath, A. K. (2015). Chapter II - Main Equipment. Power Plant Instrumentation and Control Handbook, 39-146. Bell, A. A. (2007). HVAC Equations, Data, and Rules of Thumb, 2nd Ed. McGraw-Hill Professional. Berliner Wasserbetriebe. (n.d.). Retrieved from bwb.de Bhatia, A. J. (2001). Cooling Towers. Continuing Education and Development, Inc. Chemical Engineering Site. (2020, October 27). Retrieved from Cooling Tower Efficiency Calculations: chemicalengineeringsite.in City of Cape Town. (2022). Water and sanitation services and costs in formal housing. Retrieved from capetown.gov.za Cooling Towers. (2017, July 12). Retrieved from Centers for Disease Control and Prevention: www.cdc.gov Dansena, J., & Desai, M. S. (2021). Performance analysis of a natural draft cooling tower in summer season & winter season. International Research Journal of Engineering and Technology, 1029-1035. Department of Energy Technology. (n.d.). Psychometrics. KTH. 56 DEWA. (2021, December 9). Slab Tariff. Retrieved from dewa.gov.ae EPAL. (2022). Sales Price of Water in Lisbon. Retrieved from epal.pt Furlong, D. B. (1974). The cooling tower business today. ACS Publications. Guyer, J. P. (2014). An Introduction to Water Cooling Towers. New Jersey: Continuing Education and Development, Inc. Hall, S. (2012). Cooling Towers. In S. Hall, Branan's Rules of Thumb for Chemical Engineers (pp. 182-188). Elsevier Inc. Harby, K., Gebaly, D. R., Koura, N. S., & Hassan, M. S. (2016). Performance improvement of vapor compression cooling systems using evaporative condenser: An overview. Renewable and Sustainable Energy Reviews, 347-360. Hill, G. B., Pring, E. J., & Osborn, P. D. (1990). Cooling Towers: Principles and Practice. Butterworth-Heinemann Ltd. Hoffschmidt, B., Alexopoulos, S., Rau, C., Sattler, J., Anthrakidis, A., Boura, C., . . . Hilger, P. (2022). Concentrating Solar Power. Comprehensive Renewable Energy, 670-724. Huang, Y., Zhang, K., Yang, S., & Jin, Y. (2013). A Method to Measure Humidity Based on Dry- Bulb and Wet-Bulb Temperatures. Research Journal of Applied Sciences, Engineering and Technology, 2984-2987. Jena, J. (2020, August 4). Chills And Spills: What You Need To Know About Cooling Tower Leaks. Retrieved from Thermatec: https://www.thermatecinc.com Kim, Y., Lee, J., & Jeon, S. (2016). Hybrid ground-source heat pump systems. Advances in Ground-Source Heat Pump Systems, 331-357. Kloppers, J. C., & Kröger, D. G. (2005). Cooling Tower Performance Evaluation: Merkel, Poppe, and e-NTU Methods of Analysis. J. Eng. Gas Turbines Power, 1-7. Koepke, J. (2009). The Benefits of VFDs on Cooling Towers. Penton Media, Inc. Lappierre, P., Nazarian, E., Wroblewski, D., Saylors, A., Passaretti, T., Hughes, S., . . . Musser, K. (2017). Legionnaires’ Disease Outbreak Caused by Endemic Strain of Legionella 57 pneumophila, New York, New York, USA, 2015. Emerging Infectious Diseases, 1784- 1791. Las Vegas Valley Water District. (2022). Retrieved from lvwd.com Lucas, M., Martinez, P., Ruiz, J., Kaiser, A., & Viedma, A. (2010). On the influence of psychrometric ambient conditions on cooling tower drift deposition. International Journal of Heat and Mass Transfer, 594-604. Midwest Cooling Towers Blog. (2013, September 17). Retrieved from Midwest Cooling Towers: midwesttowers.com Nairobi Water Bill Calculator. (2021). Retrieved from calculator.co.ke Navarro, P., Ruiz, J., Hernandez, M., Kaiser, A., & Lucas, M. (2022). Critical evaluation of the thermal performance analysis of a new cooling tower prototype. Applied Thermal Engineering, 118719. Perry, R. H., & Green, D. W. (1997). Perry's Chemical Engineering Handbook, Seventh Edition. New York: McGraw-Hill. PUB. (n.d.). Water Price. Retrieved from Singapore's National Water Agency: pub.gov.sg Qureshi, B. A., & Zubair, S. M. (2006). Prediciton of Evaporation Losses in Wet Cooling Towers. Heat Transfer Engineering, 86-92. Rahmani, K. (2017). Reducing water consumption by increasing the cycles of concentration and Considerations of corrosion and scaling in a cooling system. Applied Thermal Engineering, 849-856. Roffman, A., & Van Vleck, L. D. (1974). The State-of-the-Art of Measuring and Predicting. Journal of the Air Pollution Control Association, 855-859. Ruiz, J., Kaiser, A., & Lucas, M. (2017). Experimental determination of drift and PM10 cooling tower emissions: Influence of components and operating conditions. Environmental Pollution, 422-431. Schwedler, M. (2014). Effect of Heat Rejection Load and Wet Bulb on Cooling Tower Performance. ASHRAE, 16-22. 58 Sensorex. (2018, February 21). 3 Types of Cooling Systems and How They Work. Retrieved from www.sensorex.com Shah, P., & Tailor, N. (2015). Merkel's Method for Designing Induced Draft Cooling Tower. International Journal of Advanced Research in Engineering and Technology (IJARET), 63-70. SPX Cooling Technologies, Inc. (2009). Cooling Tower Fundamentals. Overland Park, Kansas: SPX Cooling Technologies, Inc. SPX Corporation. (2016). Cooling Tower Energy and its Management. New York: Marley Cooling Towers: SPX Cooling. Sydney Water. (2022). Retrieved from sydneywater.com.au The Australian Institute of Refrigeration, Air Conditioning, and Heating. (2009). Water Conservation in Cooling Towers. The Australian Institute of Refrigeration, Air Conditioning, and Heating. Tomas, A., Araujo, S., Paes, M. D., Primo, A. R., Da Costa, J., & Ochoa, A. (2018). Experimental analysis of the performance of new alternative materials for cooling tower fill. Applied Thermal Engineering, 444-456. Trane Engineers Newsletter volume 34-1. (2005, January). Selecting cooling towers for efficiency: range or approach? Vishwakarma, K. S., Bhoyar, A. S., Larokar, S. K., Hote, V. V., & Bhudhbaware, S. (2015). Study the factors on which efficiency of cooling tower can be critically acclaimed (a case study). International Journal of Engineering Research and Applications, 73-79. Weimar, D. (2009). Practical Water Treatment for Evaporative Condensers. Chem-Aqua, Inc. Zargar, A., Kodkani, A., Peris, A., Clare, E., Cook, J., Karupothula, P., . . . Secanell, M. (2022). Numerical analysis of a counter-flow wet cooling tower and its plume. International Journal of Thermofluids. Appendix A Psychrometric Chart Psychrometric Chart in SI units