Saving Water With Cooling Towers

This article was published in ASHRAE Journal, August 2015. Copyright 2015 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org. Saving Water With Cooling Towers PHOTO COURTESY OF NCSA, UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN BY FRANK MORRISON, MEMBER ASHRAE Saving water with cooling towers. At first glance, this statement seems counterintuitive. Cooling towers save energy but aren t they major users of water? This article will help readers understand the critical role evaporative heat transfer systems play in a sustainable environment, explore how water is consumed in such systems, and review the strategies that help minimize the use of both water and energy. The first water-cooled systems used potable water to provide heat rejection with the cooling water wasted to a drain. Cooling towers were developed to recycle more than 98% of this water, resulting in tremendous reductions in both water and energy use as these systems grew in size and popularity. Evaporative heat rejection also enables higher system efficiencies, which conserves water at the power plant and reduces emissions of greenhouse gases and other pollutants. This is because thermoelectric power generation accounts for 38% of freshwater withdrawals in the United States essentially equal to that withdrawn for irrigation. 1 By reducing the electrical energy consumed at the site, less power is required to be generated and less water is used at the power plant and in the extraction and processing of the plant s fuel source. For example, in some climates, the total water use (source and site) between air- and water-cooled chillers is almost equal. 2 The lower energy use also enables a higher percentage of renewable, clean power from solar and wind at a given facility. Energy is also required to treat and distribute water. The balance between the use of these two natural resources is often referred to as the energy/water nexus. Today, water supplies are challenged in many areas of the world, including Atlanta (recent drought) and California (current drought). Therefore, it is critical that all water consuming systems, no matter where they are located, optimize their use of this resource. Methods to conserve water include using low-flow bathroom fixtures, repairing leaks in water-distribution systems, and taking advantage of relatively simple techniques to help ensure that evaporative heat rejection systems use only the amount of water required to save energy, maintain optimized system performance, minimize system maintenance, and ensure a long system life. Frank Morrison is manager, global strategy at Baltimore Aircoil Company in Jessup, Md. He is past chair of ASHRAE TC 3.6, Water Treatment. 20 ASHRAE JOURNAL ashrae.org AUGUST 2015

Evaporative heat rejection systems encompass open circuit cooling towers, closed circuit cooling towers, and evaporative condensers. For the purposes of this article, the term cooling tower will be used synonymously for all of these devices, except as noted. Evaporation The primary consumption of water in a cooling tower is through evaporation a process that is also used by the human body to help regulate its internal temperature. In a cooling tower, the warm water from the system comes into contact with the entering air, usually over a heat transfer surface such as fill, where a small portion of the recirculating water evaporates, cooling the remaining flow. This process is very energy efficient as approximately 1,000 Btu (1055 kj) are required to evaporate 1 lb (0.454 kg) of water at standard design conditions (1,000 Btu/lb [2,326 kj/kg]). In contrast, air-cooled heat exchangers must move far more air to reject the same heat, consuming additional fan energy in the process, usually at a much higher system temperature since the dry-bulb temperature is higher than the wet-bulb temperature of the air. These higher temperatures result in greater energy use by the cooling system, often 30% or more as in the case of an air-cooled versus a water-cooled chiller. Note that the difference between the dry bulb and wet bulb of the air is known as the wet-bulb depression. In areas of high wet-bulb depression, such as the American Southwest, evaporative heat rejection offers even greater energy efficiency by enabling significantly lower system temperatures. While water is consumed by evaporation in the cooling tower, the water is not lost or destroyed, unlike what occurs when natural resources such as oil or natural gas are consumed. This pure water, sometimes visible as plume from the cooling tower discharge in cooler weather, is returned to the environment as part of the natural water cycle. Water in reservoirs, lakes, rivers, and in cooling ponds used in some industrial applications also evaporates; cooling towers simply put evaporation to work to efficiently cool the buildings and processes that serve our society. Evaporation is a function of the heat rejection load and the psychrometric properties of the air entering the cooling tower. Many rules of thumb exist for calculating peak evaporation, such as: FIGURE 1 Evaporation vs. wet bulb and relative humidity for a fixed, constant load; tower airflow at full speed for all cases. Evaporation (gpm or L/s) (Base = 1.0 at 78 F [25.6 C] WB and 50% RH) 1.3 1.2 1.1 1.0 0.9 0.8 0.7 30% RH 50% RH 70% RH Base = 1.0 0.6 40 45 50 55 60 65 70 75 80 85 90 Wet Bulb ( F) 2.0 gpm of water evaporated per 1,000,000 Btu/h (0.0004 L/s kw) and 3.0 gpm of water evaporated per 100 tons (0.0004 L/s kw). While these rules of thumb can be useful for calculating design makeup flow rates, sizing piping, and estimating water treatment regimens, they significantly overestimate the annual water use by a cooling tower by not taking into account load profiles and the effect of weather conditions throughout the year. Besides being proportional to the heat load, the evaporation rate is strongly influenced by both the entering air dry-bulb and wet-bulb temperature. At off peak wet-bulb temperatures, which occur the majority of the year, the evaporation rate is reduced by up to 30% or more versus design as can be seen in Figure 1. Figure 1 also illustrates that water use increases in drier climates and is reduced in more humid climates for the same heat load. Furthermore, in cooler weather, the heat load to be rejected is typically lower, especially on HVAC applications, further reducing the evaporation. Several water use calculators are available that can demonstrate the variation in evaporation rate with varying climate, load and design conditions. Blowdown When water is evaporated in a cooling tower, pure water vapor enters the air moving through the unit, leaving any dissolved solids or minerals in the remaining water. Left unchecked, the recirculating water will become increasingly saturated with mineral content, scaling heat transfer surfaces and increasing the corrosivity of the water. AUGUST 2015 ashrae.org ASHRAE JOURNAL 21

To keep dissolved solids to an acceptable level, a small amount of water must be bled from the system in proportion to the evaporation rate to achieve the desired cycles of concentration. Cycles of concentration (COC) measures the ratio of the dissolved solids, such as calcium, chlorides, or magnesium, in the recirculating water to the concentration found in the incoming makeup water supplied to the cooling tower. The COC can be calculated according to the following formula: COC = (Evaporation/Blowdown) 1 (1) Evaporation and blowdown are measured in gpm (L/s). Conversely, the formula for calculating blowdown, also known as the bleed rate, based on the desired COC is as follows: Blowdown = Evaporation/(COC 1) (2) The COC that can be achieved is dependent on the quality of the incoming makeup water, the water treatment program, and the system s construction materials (and not just those of the cooling tower). While four or five cycles is often used as a target value for a typical system, this value can range from two cycles on a system with very poor supply water quality up to 30 or more when very soft water, such as air conditioning condensate, is used as makeup water. Thus, a regulation requiring a single, fixed minimum COC can be misguided, as the COC that is achievable is dependent on site-specific factors, which can vary with each installation and over time. Even in cases where the water quality is generally good, high levels of a single constituent, such as chloride or silica, can exist, which can limit the maximum achievable COC. Several calculators, including one from the California Energy Commission, are available to assist in determining the proper COC value for a given facility. Additionally, a water treatment professional, either internal or external to the facility, should be consulted to analyze the site and help to establish an optimized, balanced water treatment program designed to control scale, fouling, corrosion, and biological growth while conserving water. In the past, a fixed bleed rate often was employed, but this method should not be used. Instead, the desired COC setting can be maintained in several ways, two of which are described below. The most common method is the use of a conductivity controlled blowdown system, consisting of a conductivity probe, a controller, and a motorized blowdown valve. The conductivity controller signals the blowdown valve to open as necessary to maintain the desired COC. Water treatment chemicals can affect the conductivity of water, and this effect needs to be considered when adjusting settings on the controller. Another technique involves the use of flow meters on the makeup and blowdown lines. In this method, the blowdown valve is opened in proper proportion to the makeup flow to maintain the desired COC. However, drift, leaks, filtration backwash, and other uncontrolled water losses can result in a lower COC than desired, in turn leading to excessive water loss, so these factors must be properly accounted for when using this method. For either system, the blowdown valve should be located before the introduction of any chemical treatment to allow the chemicals to mix thoroughly in the tower, while reducing the loss of treatment chemicals. 3 Regular inspection, calibration, and maintenance of the motorized blowdown valve and the conductivity probe or flow meters will help to sustain the desired setting over time. A filter or strainer ahead of the flow meters and valves is also good practice to protect these devices. 3 Maintaining the proper COC is important to system water and energy efficiency. Too high a COC setting for the site-specific conditions can lead to scaling and corrosion of the system, leading to poor heat transfer, higher energy use, and shortened equipment life. Too low a COC setting will generally result in better recirculating water quality, but at the price of higher than necessary water use and loss of associated treatment chemicals. Reducing Evaporation As previously stated evaporation is the largest use of water in a cooling tower and is primarily dependent on the load and the psychrometric properties of the air entering the cooling tower. So reducing the load that must be handled will not only save energy but also directly reduce the evaporation required along with the associated blowdown. Load reduction techniques include, but are not limited to, optimal building orientation, using better insulation, more energy-efficient production processes, and heat recovery. As an example, more efficient chillers can reduce the heat load on the cooling tower. A chiller with a full load efficiency rating of 0.55 kw/ton (0.16 kw/kw) (6.39 COP) will evaporate 2.4% less water per peak ton than a chiller operating at 0.65 kw/ton (0.18 kw/kw) (5.41 COP). This 22 ASHRAE JOURNAL ashrae.org AUGUST 2015

TABLE 1 Effect of reduced condenser water temperature and cooling tower fan speed on energy and water use in a water-cooled chiller system. OPERATING MODE BASE TOWER PRIORITY REDUCTION CHILLER PRIORITY REDUCTION Tower Conditions(EWT/LWT/WB) 95.0 F/85.0 F/78.0 F 95.0 F/85.0 F/58.4 F VERSUS BASE 79.7 F/70.0 F/58.4 F VERSUS BASE Tower Fan Speed (Percent of Design) 96% 47% 53.0% 96% 0.0% Evaporation (gpm) 11.34 9.60 15.3% 9.23 18.6% Blowdown (gpm) 3.78 3.20 3.08 Makeup (gpm) 15.12 12.80 12.31 Chiller Energy Consumption (kw/ton) 0.586 0.586 0.0% 0.442 24.6% Tower Energy Consumption (kw/ton) 0.041 0.005 87.8% 0.041 0.0% Chiller + Tower Energy Consumption (kw/ton) 0.627 0.591 5.7% 0.483 23.0% Note: Assumptions for all cases: Cooling tower flow 1,120 gpm; chiller at full load; COC of 4.0; relative humidity 50%. is due to the reduced amount of non-productive, waste compressor work (heat) that must be rejected to the atmosphere, in this case, 0.10 kw/ton (0.03 kw/kw) (35 COP). Furthermore, as the amount of water evaporated is directly proportional to the load and the blowdown is proportional to the evaporation, there will be a corresponding reduction in the amount of blowdown required. Assuming four cycles of concentration, for every gallon (3.79 L) of evaporation avoided there will be an additional 0.33 gallon (1.25 L) reduction in the required blowdown volume to maintain the same recirculating water quality. While many believe that load reduction is all that can be done to reduce evaporation, several other techniques can have a meaningful impact. As illustrated in Table 1, the psychrometric properties of air can be used to minimize water consumption and reduce system energy use with a water-cooled chiller system. In the first case, the cooling tower is operated at full fan speed at a wet bulb that will AUGUST 2015 ashrae.org ASHRAE JOURNAL 23

produce 70 F (21.1 C) leaving water temperature. In the second case using the same wet bulb, the cooling tower fan speed is modulated to maintain a fixed leaving water temperature to the condenser. In both cases, system energy is significantly reduced, as is water use. Each method saves resources at offpeak conditions and results in a winwin situation in terms of both energy and water savings for the operator. However, in the case shown previously, the lower condenser water temperature has the overall advantage in terms of more significant energy savings and additional water savings on the majority of installations. While these two full-load cases are illustrative, actual control strategies should seek to operate at the optimum chiller speed, cooling tower fan speed, and condenser flow resulting in the lowest system energy consumption for the given load and ambient conditions. Blowdown Reduction After evaporation, blowdown is the next largest use of water in cooling towers. As the blowdown is proportional to the evaporation rate, the blowdown can be reduced simply by using the techniques for reducing the evaporation rate described above. Beyond this, the maximum COC for a given cooling tower system should be established, which is a function of the makeup water quality, the water treatment program used, and the construction materials of the tower and of the remainder of the system. Increasing COC can reduce the required blowdown (and the amount of water used) considerably, though the volume saved decreases dramatically at higher cycles as can be seen in Figure 2. However, the risk of scaling and/or corrosion can increase significantly at higher cycles. Consequently, operators and water treatment professionals must weigh the benefit from smaller and smaller water savings versus risk when setting an aggressive COC target for a given system. Higher cycles call for closer monitoring of system parameters. To enable the cooling tower to tolerate higher cycles and/or the challenges of unconventional water sources, more corrosion resistant construction materials can be used, ranging from a stainless steel or polyurethane-lined cold water basin up to a complete unit fabricated from Type 304 or even Type 316 stainless steel. The materials used in the remainder of the system, including piping, heat exchangers, and valves, must also be taken into consideration since these components also come into contact with the same recirculating water. A best practice for the system designer is to evaluate the composition and quality of the makeup water source(s) early in the design stage and consult with water treatment professionals and equipment manufacturers on the most appropriate construction materials and water treatment strategies for the site. Systems operated at a high COC can often benefit from filtration to help minimize the concentration of suspended solids from airborne contamination. Keeping suspended solids low reduces fouling of heat transfer surfaces, helps keep microbiological growth under control, and improves the effectiveness of water treatment programs. A cyclonic type separator or sand filter is typically used to remove solids from the water, in conjunction with a sump sweeper 24 ASHRAE JOURNAL ashrae.org AUGUST 2015

package in the tower basin to help keep particles in suspension. These systems are backwashed periodically, which can be a hidden use of water. Ideally, backwash cycles should be run only when necessary and for only as long as required to keep the filtration system operating properly. Blowdown typically is discharged into the sewer for disposal. The sewer charge is often based on the makeup flow rate and in some cases can be as much as or more than the expenditure for water. However, the blowdown is only a small fraction of the makeup flow. To account for the portion of the makeup flow that will not have to be handled by the water treatment plant, many localities will allow an evaporation credit that substantially reduces the sewer charge. Depending on the utility, the blowdown may need to be metered or a calculation can be applied based on the target COC to earn this credit. In some cases, blowdown can also be recovered and used for other applications though the higher mineral and chemical content of this water must be taken into account. Finally, on some sites, pretreatment of the makeup water by softeners or reverse osmosis (RO) has been used to increase the COC to conserve water. However, most softeners use a brine solution for regeneration and RO systems generate RO reject water, both of which must be disposed of properly, offsetting some of the benefits. Restrictions on the use of such systems are appearing in many areas of the country for this reason. Drift and Carryover Drift is defined as the relatively small droplets of water that leave the cooling tower, while carryover AUGUST 2015 ashrae.org ASHRAE JOURNAL 25

FIGURE 2 Bleed rate ratio versus cycles of concentration. 6 5 Bleed Ratio Base = 4.0 COC Bleed Rate Ratio 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Cycles of Concentration is generally considered larger droplets that are carried over into the leaving airstream and may originate from condensation on cold surfaces inside the tower. Drift eliminators located in the leaving airstream help to keep this entrained water in the cooling tower. With current eliminator technology, this loss is quite small, on the order of 0.005% or less of the recirculating flow. For a 100 ton (352 kw) cooling tower, this water loss is less than 1 gph (1.10 ml/s) at full flow and heat load, which is quite small compared to the evaporation and bleed rate. Eliminators with a maximum drift rate of 0.005% or less should be specified on new cooling towers and retrofitted whenever possible on older units. Eliminators, or combinations of eliminators, with drift rates as low as 0.001% are available. However, the higher airside pressure drop of many of these designs must be taken into account as the higher airflow restriction can negatively impact thermal performance, which forces the cooling tower to use more energy to perform the same cooling duty. Eliminators, typically PVC or other plastic material, must be kept in good condition, spaced properly, and inspected routinely per manufacturer guidelines (Photo 1). Proper eliminator maintenance will help minimize drift and eliminate blow through, especially in high velocity areas, which can dwarf the stated drift rate. Warped or damaged eliminators should be replaced. Only when properly installed and maintained can eliminators achieve their stated drift rate. Important reasons to minimize drift include limiting particulate emissions, eliminating spotting on cars near the cooling tower, and most importantly, minimizing the risk of Legionnaires disease. However, water savings, 26 ASHRAE JOURNAL ashrae.org AUGUST 2015

even with the most efficient eliminators, is typically not a benefit. Besides being a very small volume compared to evaporation and bleed, any drift leaving the cooling tower only serves to reduce the amount of water required to be bled from the unit when using a conductivity controlled blowdown to maintain the desired cycles. Two exceptions to this would be where makeup and blowdown flow meters are used to maintain the water quality, or where the blowdown is captured and used for another purpose such as toilet flushing. Thus, claims of water saving drift eliminators can often be misleading. Low drift rates can also be very difficult, expensive, and time consuming to accurately verify. Splashout, Leaks, and Overflows Splashout can occur at the air inlet faces of the cooling tower where the falling water can splash out of the unit itself. Splashout must be minimized with good air inlet louvers that capture any water from the fill or plenum and return it to the basin. As with eliminators, the louver sections, whether slat type or cellular, must be maintained properly and operators must ensure they are tight fitting with the proper spacing. Spray nozzles should also be kept clean and free flowing over the heat transfer surfaces, not only to minimize scaling and maximize thermal performance but also to reduce splashout. Clogged nozzles can also cause water to overflow hot water-distribution basins or spray out through the top of the PHOTO 1 Proper inspection and maintenance of drift eliminators is critical. cooling tower, resulting in unnecessary water losses. Leaks can occur in a variety of areas, but most often from the cold water basin seams. Obviously, any leaks in AUGUST 2015 ashrae.org ASHRAE JOURNAL 27

the cooling tower or system pipework should be corrected immediately. The use of welded stainless steel or polyurethane lined cold water basins can also help minimize leaks by eliminating seams in the basin. Loss of treated water can be especially costly as it includes both the cost of the water and the water treatment chemicals. Another method to reduce water use and lower treatment costs is by minimizing the volume in a water-cooled system, including the design of the system pipework. These savings take place from the initial fill to every time the system is drained for cleaning. Closed circuit cooling towers keep the process flow in a clean, closed loop and the volume of the open spray water loop is limited to the internal volume of the unit, which can be considerably less than a system using an open circuit cooling tower. Note that upon shutdown of a cooling tower, all the water above the operating level will flow back into the cold water basin, which must have volume available to accommodate this water without overflowing the cold water basin. Such overflows can be a significant, yet often hidden, water use, occurring every time the system pump shuts down. Designers must ensure that the model selected has an adequately sized cold water basin or remote sump for the project. Once installed, the water levels in the cold water basin of the cooling tower must be properly set for unit startup and to avoid wasting water upon shutdown through the overflow connection. Using the makeup valve arrangement, whether mechanical or electronic, the operating level of the cold water basin should be set close to the lowest possible level, such that air is not drawn into the cooling tower pump to avoid system flow issues and noise, while allowing the maximum basin volume to accommodate the shutdown water. An electric water level control can also be used to provide finer control of the level in the cold water basin. To assist operators, a low level alarm can be used to help protect the tower pump, and a high level alarm can be added to alert the operator of possible overflow conditions (which is now required by certain codes, such as California Title 24). These sensors can be separate devices or incorporated into the electric water level control 28 ASHRAE JOURNAL ashrae.org AUGUST 2015

device. Finally, to avoid overflowing the hot water basins on cross-flow designs, the gravity flow spray nozzles must be properly sized for all expected flows and kept clean. Alternative Sources of Water Alternative sources of water can also be used to reduce the use of potable water in cooling towers. Any water that can meet the cooling tower manufacturer s guidelines for the construction material can be used. The most common is reclaim, or recycled water, which is water that has processed through a treatment plant. Rather than return this water to a lake or river, the reclaim water is used for other uses, including irrigation or makeup water for cooling towers. Purple pipe, along with appropriate signage, is used to distinguish such distribution systems from potable lines. This water is often good quality, although the concentration of minerals is usually higher than potable water, having been cycled through the system at least once. As a result, usually the COC cannot be set as high when using recycled water. Rainwater can also be harvested from roof surfaces or parking lots, filtered, disinfected, and stored for use in cooling towers. Another popular alternative source of water is air-conditioning condensate. As both rainwater and condensate are pure water with few minerals, they often must be blended with other water sources so they are not overly aggressive. Such systems are frequently operated at higher cycles of concentration since the water is soft, even after blending. The most critical aspect of using any of these alternative sources of water is to monitor biological activity and minimize the introduction into the cooling tower of possible nutrient sources for biological growth. Such contaminants increase the risk in operating the cooling tower and increase the water treatment requirements. For instance, the use of untreated sink water would not be suitable, as soap would serve as a nutrient source for biological growth as well as foul heat transfer surfaces. Hybrid Wet/Dry Designs Closed circuit cooling towers and evaporative condensers use a coil to contain the heat transfer medium in a closed system. As such, the spray pump can be shut off and these units operated in an air-cooled or dry mode in colder weather. The amount of water saved with this technique will depend on the sensible heat transfer surface area available, the local climate, and the load profile. PHOTO 2 Hybrid closed circuit cooling towers with dry and wet heat exchange sections. However, any water savings is usually more than offset by the increase in unit fan energy in the dry mode, especially when considering the reduced loads and water consumption typically experienced in colder weather (Figure 1). By definition, the switch point for dry operation is at 100% fan speed and power draw, while the fan speed for the unit operating in wet mode at this same point is only a small fraction of the design speed and power draw. Because of their higher first cost and the energy penalty, standard closed circuit cooling towers and evaporative condensers are not used to replace open circuit cooling towers on the basis of cold weather water savings. For areas where water is in short supply and/or expensive, cooling tower manufacturers have developed hybrid, wet/dry designs that can save significant amounts of water, yet still offer the low system temperatures critical to efficient operation. These units incorporate a wet heat exchange section along with a dry cooling section (Photo 2). Such systems are equipped with controls that balance the use of water and energy to provide the desired level of system efficiency. By handling a portion of the load dry, typically ranging from a minimum of 20% all the way up to 100% in colder weather, evaporation and the associated blowdown can be reduced proportionally while also reducing or eliminating plume, which can be an advantage on certain projects. Many of these hybrid units are also closed circuit cooling towers, which have the added benefit of keeping the process fluid in a clean, closed loop that is separate from the external spray water loop. Keeping the process fluid isolated helps maintain system efficiency and reduce equipment cleaning and maintenance over time. Adiabatic designs precool the air with wetted pads before the air enters a dry finned heat exchanger enabling reduced condensing or fluid temperatures and lower system energy use compared to air-cooled heat 30 ASHRAE JOURNAL ashrae.org AUGUST 2015

rejection, with a lower volume of water evaporated compared to a typical evaporative condenser or closed circuit cooling tower. To minimize water treatment needs, the sumps are typically drained once per day, but the sump is designed to keep the volume low to minimize this water loss. When considering these hybrid designs, it is important to weigh the potential savings in water, water treatment, sewage, energy, and maintenance costs, along with such factors as the availability of water on the site, against the higher initial cost of such equipment. Systems with high, constant year-round loads typically benefit most from these technologies. An accurate assessment of potential water savings must be made, taking into account both load profiles and ambient conditions, to properly calculate the potential payback for such investments. Other drivers may also influence the decision to use hybrid technologies, such as reduced water availability to a facility, a desire to limit plume in colder weather, or a critical need to be able to operate dry in the event of a loss of the water supply to the site. Tracking Water Use Tracking cooling tower water use through makeup and/or blowdown flow meters can be a wise investment. This data provides useful information on how well the system and water treatment program are operating over time so system parameters and maintenance schedules can be adjusted for peak overall performance. For instance, by metering the makeup and blowdown flows, the COC for the system can be tracked and the savings and benefits of water conservation efforts can be more easily established, as well as helping earn a sewage credit for evaporation. Several codes and standards, such as ASHRAE Standard 189.1-2013 and California Title 24-2013, have incorporated requirements for such flow meters. Some water utilities also offer rebates on energy and water saving equipment, which can offset the cost of such monitoring. Finally, efforts to reduce water and energy use can help earn LEED points through direct energy and water savings and possibly innovation credits. 32 ASHRAE JOURNAL ashrae.org AUGUST 2015

Summary Cooling towers play an integral role in the conservation of both energy and water and as such are a key part of a sustainable future. This article describes many relatively simple techniques and best practices necessary to reduce the use of water in cooling towers, along with consideration of alternative makeup water sources and hybrid wet/dry designs. No matter what the choice of equipment or techniques used, the manufacturers operating and maintenance guidelines must be followed to ensure optimal performance and service life. The items listed in the Bibliography, including Saving Energy with Cooling Towers, the companion piece to this article, can also be consulted for further guidance. Finally, the services of a water treatment professional should always be used when treating evaporative heat rejection equipment to maximize thermal performance, conserve water, and extend equipment lifetimes, while effectively controlling scale, corrosion, and biological growth. References 1. Maupin, Molly A., et al. 2014. Estimated Use of Water in the United States in 2010. Circular 1405, U.S. Department of the Interior. U.S. Geographical Survey. 2. Hydeman, Mark. 2008. A Comprehensive Comparison of Airand Water-Cooled Chillers Over a Range of Climates. Seminar 48 at the ASHRAE Annual Conference. 3. Aherne, A.J. 2015. Best Practice Guidelines: Water Conservation In Cooling Towers. Australian Institute of Refrigeration, Air Conditioning, and Heating. Bibliography ANSI/ASHRAE/IES Standard 90.1-2013, Energy Standard for Buildings Except Low-Rise Residential Buildings. ANSI/ASHRAE/IES/USGBC Standard 189.1-2014, Standard for the Design of High-Performance Green Buildings. California Title 24 2013, Building Energy Efficiency Standards for Residential and Nonresidential Buildings. Hamilton, J., T. Bugler, J. Lane. 2010. Water/Energy nexus, comparing the relative value of water versus energy resources. Cooling Technology Institute Technical Paper. TP10-16. Morrison, F. 2014. Saving Energy with Cooling Towers. ASHRAE Journal (1). Torcellini, P., N. Long, R. Judkoff, D. 2003. Consumptive Water Use for U.S. power production. National Renewable Energy Lab. NREL/ TP-550-33905. AUGUST 2015 ashrae.org ASHRAE JOURNAL 33