FEDERAL MANAGEMENT PROGRAM

Cooling Towers: Understanding Key Components of Towers and How to Improve Efficiency Source: Paul Johnston-Knight Source:

Introduction Overview water is evaporated into the air passing Federal laws and regulations require Cooling towers are an integral component through the tower. As the water evapo- Federal agencies to reduce water use of many systems, providing rates, the air absorbs , which lowers and improve water efficiency. Namely, comfort or process cooling across a broad the of the remaining water. Executive Order 13514 Federal range of applications. They are the point This process provides significant cool- Leadership in Environmental, Energy, in the system where heat is dissipated to ing to the remaining water stream that and Economic Performance, requires an the atmosphere through the evaporative collects in the tower basin where it can be annual two percent reduction of water use process, and are common in industries pumped back into the system to extract intensity (water use per square foot of such as oil refining, chemical process- more process or building heat, thereby building space) for agency potable water ing, power plants, steel mills, and many allowing much of the water to be used consumption as as a two percent different manufacturing processes where repeatedly to meet the cooling demand. reduction of water use for industrial, process cooling is required. They are The amount of heat that can be rejected landscaping, and agricultural applica- also commonly used to provide comfort from the water to the air is directly tied tions. Cooling towers can be a significant cooling for large commercial buildings to the relative of the air. Air source of water use for both of these cat- including airports, office buildings, with a lower relative humidity has a egories of water use at Federal facilities. conference centers, hospitals, and hotels. greater ability to absorb water through To realize potential savings it is essential structures vary greatly than air with a higher relative for Federal agencies understand the key in size and design, but they all function humidity, simply because there is less components of cooling towers and how to provide the same thing: liberation of water in the air. As an example, consider to improve water efficiency of the system extracted from a process or cooling towers in two different locations– as a part of a comprehensive approach to building system through evaporation of one in Atlanta, Georgia, and another in water management. water. In technical terms, cooling towers Albuquerque, New Mexico. The ambient are engineered and designed based on air temperature at these two locations may be similar, but the relative humidity In this Fact Sheet: a specified cooling load, expressed in refrigeration tons.(1) The cooling load in Albuquerque on average will be much • Introduction is determined by the amount of heat lower than that of Atlanta’s. Therefore, • Overview that needs to be extracted from a given the cooling tower in Albuquerque will be able to extract more process or building • Structure process or peak comfort cooling demand. The cooling tower must be adequately heat and will run at a cooler temperature • Basic Cooling Tower Terms sized to reject this same amount of heat because the dry desert air has a greater • Types of Towers to the atmosphere. capacity to absorb the warm water. • Important Properties of towers are used to reject heat Cooling towers can be split into two • System Calculations through the natural process of evapora- distinct categories: open circuit (direct contact) and closed circuit (indirect) • Factors That Limit Cycles of Concentration tion. Warm recirculating water is sent to the cooling tower where a portion of the systems. In open circuit systems the recir- • System Concerns culating water returns to the tower after

• Treatment Options (1) One ton of refrigeration is equal to 12,000 Btu/hour. continued > 2 FEDERAL ENERGY MANAGEMENT PROGRAM

gathering heat and is distributed across different components of a cooling detail in the System Concerns section). the tower where the water is in direct tower structure. Development of any of these problems contact with the atmosphere as it recircu- greatly reduces the cooling efficiency First, the water is distributed evenly lates across the tower structure. Closed and in severe cases can collapse portions across the top of the cooling tower circuit systems differ in that the return of the tower fill or tower structure. To structure. Tower distribution decks can fluid (often water, or sometimes water avoid this, film fill should be inspected be a series of spray nozzles oriented up mixed with glycol) circulates through the routinely to ensure it is clean and free of or down (like a landscaping sprinkler tower structure in a coil, while cooling debris, scale, and biological activity. system) to uniformly distribute the water tower water recirculates only in the tower over the tower structure. In some cases, To minimize losses due to drift and help structure itself (see Figure 1). In this case, the distribution deck may just be a series direct airflow into the tower, and the return fluid is not exposed directly to of holes through which the water falls drift eliminators are commonly used. the air. onto the tower structure. Regardless Louvers are most often seen along the the distribution deck must uniformly sides of the tower structure, while drift apportion the recirculating water across eliminators reside in the top section of the the tower structure. Broken nozzles or tower to capture entrained water droplets plugged orifices will impede uniform that may otherwise leave through the distribution across the tower structure, stack. Damaged or incorrectly oriented negatively impacting the overall heat louvers along with damaged drift elimi- exchange capacity of the system. nators will lead to excessive losses due to drift from the tower structure. Therefore, As the water falls from the distribution louvers and drift eliminator sections deck, the surface area is further expanded should be routinely inspected and in the fill section. Older tower systems repaired to ensure optimal water usage. may feature splash bars made of , fiberglass, or wood that serve to break the After the water passes through the fill falling water into tiny droplets. In recent it cascades down to a collection basin years, many different forms of labyrinth- at the base of the tower structure. From Figure 1. Example of a Closed Circuit Cooling like packing or film fill have been the basin the cold water can be pumped Tower. incorporated. The closely packed nature back into the system to extract process of film fill causes the water to travel or comfort cooling needs and begin the Structure through this portion of the tower in thin cycle all over again. streams, improving By design, cooling towers consume large Cooling towers are the primary com- and the evaporation rate, thereby increas- volumes of water through the evapora- ponent used to exhaust heat in open ing heat rejection (see Figure 3). recirculating cooling systems. They tion process to maintain comfort cooling are designed to maximize air and water or process cooling needs, although they contact to provide as much evaporation use significantly less water than similar as possible. This is accomplished by capacity once-through cooling systems. maximizing the surface area of the water Because the evaporative loss is water as it flows over and down through the containing little to no dissolved , tower structure. Figure 2 illustrates the the water remaining in the cooling tower becomes concentrated with dissolved solids, which can lead to scaling and corrosive conditions. To combat these problems, water with high total dissolved content must be drained from the Figure 3. Relative Thermal Performance of system via “blowdown.” The associated Different Types of Fill. losses caused by blowdown, evaporation, drift, and system must be accounted There are several variations of film fill for by system make-up requirements. geometries commercially available, Figure 4 illustrates these water system and though they do greatly increase the flow locations in generic terms. heat rejection rate over splash-type fills, they are also much more susceptible to , scaling, and microbiological growth (these are discussed in greater Figure 2. Overview of Cooling Tower Structure Components. continued > FEDERAL ENERGY MANAGEMENT PROGRAM 3

Cross-Flow Towers – Cross-flow cooling towers are structured so that air flows horizontally across the falling water (see Figure 6). This design provides less resistance for the air flow, thereby reduc- ing the horsepower required to meet the cooling demand. These towers usually feature gravity-fed water distribution decks that are either open and uncovered, or that are covered to limit growth and debris from getting into the distribu- tion deck. Gravity-fed distribution decks have evenly spaced openings that the water drops through to be spread across the tower fill.

Figure 4. Illustration of Water Flow Across a Cooling Tower.

Basic Cooling Tower Terms by fans located at the base of the tower Blowdown – Water discharged to remove (referred to as forced draft), or pulled high mineral content system water, through by fans located at the top impurities, and sediment. of the tower (referred to as induced draft). Induced draft towers tend to be larger Cycles of Concentration – Technical than forced draft units. term used to describe the mass flow rela- Figure 6. Example Cross-flow Cooling Tower. tionship between the amount of system Natural Draft Towers – Natural draft feed water and the amount of blowdown towers are designed to move air up sent down the drain. Also referred to as through the structure naturally without Counter-flow Towers – Counter-flow concentration ratio, cycles of concentra- the use of fans. They use the natural law cooling towers have upward air flow that tion correlates to the effective use of of differing densities between the ambient directly opposes the downward flow of water in your system to provide heating air and warm air in the tower. The warm the water providing very good thermal or cooling needs. High cycles of concen- air will rise within the structure efficiency because the coolest air contacts tration are directly related to low levels of because of its lower density drawing cool the coolest water (in the bottom section water loss from your system. ambient air in the bottom portion. Often of the tower structure). times these towers are very tall to induce Dissolved Solids – The amount of dis- adequate air flow, and have a unique solved minerals present in the water. shape giving them the name “hyperbolic” Important Properties of Water A primary consideration for the operation Drift – Droplets of water entrained in the towers (see Figure 5). of the cooling tower system is the water air leaving the top of the tower, or blown quality of the make-up source. Differing from the side of the tower by crosswinds. sources present differing challenges. Make-up – Water supply needed to Surface water sources include , replace all losses due to evaporation, , and streams, while leaks, or discharge in cooling systems. sources consist of or aquifers. Depending on the location, surface water Types of Towers sources will have seasonal variations and can carry high levels of suspended Cooling Towers – Cooling towers are silt and debris that cause fouling if classified by the direction of air flow not removed by pre- systems. (counter-flow or cross-flow) and the Groundwater sources don’t have the type of draft (mechanical or natural). seasonal variations that surface water Mechanical Draft Towers – sources have, but depending on the Mechanical draft towers have air geology of the region, they can have high forced through the structure by a fan. Figure 5. Hyperbolic Cooling Towers levels of dissolved minerals that contrib- The air flow can be pushed through (Source: Victor Vizu). ute to scale formation or in the

continued > 4 FEDERAL ENERGY MANAGEMENT PROGRAM

cooling system. Recently, water reuse (OH-). Additional alkaline components (5) Cycles of Concentration × -3 has become popular and many cooling may include phosphate (PO4 ), Blowdown = Blowdown +

systems are being supplied reclaimed (NH3), and silica (SiO2), Evaporation effluent or discharge water from other though contributions from these ions Solving for blowdown: processes. While water reuse is a wise are usually relatively small. resource option, consideration should be (6) Blowdown = Evaporation ÷ Conductivity – Conductivity is a mea- made regarding the quality of the water (Cycles of Concentration -1) surement of the water’s ability to conduct and how that will the efficient electricity. It is a relative indication of Also, if the blowdown rate and cycles of operation of the cooling system, and the total dissolved mineral content of the concentration are known, the make-up the system’s ability to meet the required water as higher conductivity levels cor- rate can be determined by solving equa- cooling demand. relate to more dissolved salts in . tion 4, and then the evaporation rate can Whether the source water is surface, Conversely, purified water has very little be determined by solving equation 6 for ground, or reuse, in nature there are a few dissolved minerals present meaning the evaporation: basic water quality considerations that conductivity will be very low. (7) Evaporation = Blowdown × should be understood. (Cycles of Concentration – 1) pH – pH is a measurement of how acidic System Calculations or how alkaline a substance is on a scale To properly operate and maintain a Factors That Limit Cycles of 0 to 14. A pH of 7.0 is neutral (the cooling tower, there needs to be a basic concentration of hydrogen ions is equal understanding of the system water’s use. of Concentration to the concentration of ions), Water use of the cooling tower is the Increasing cycles of concentration saves while measurements below 7.0 indicate relationship between make-up, evapora- water because it essentially means that acidic conditions, and measurements tion, and blowdown rates. There are a water is being recirculated longer through above 7.0 indicate basic or alkaline couple simple mathematical relationships the system before being blown down. But conditions. The pH scale is logarithmic between the blowdown rate, evaporation as levels of dissolved minerals elevate (each incremental change corresponds rate, make-up rate, and cycles of concen- with higher cycles of concentration, scal- to a ten-fold change in the concentration tration of a cooling tower that are very ing and corrosion potential also increase. of hydrogen ions), so a pH of 4.0 is ten useful to understand the principal flow All dissolved minerals have a saturation times more acidic than a pH of 5.0 and rates. The first relationship illustrates limit that, if exceeded, will lead to scale one hundred times more acidic than a pH the overall mass balance consideration formation.(2) Additionally, high levels of of 6.0. Similarly, a pH of 9.0 is ten times around a given cooling tower: dissolved minerals (high cycles of con- more basic or alkaline than a pH of 8.0 centration) increases the water’s tendency and one hundred times more alkaline than (1) Make-up = Blowdown+ Evaporation to be corrosive (corrosion is discussed a pH of 7.0. In this case, the blowdown accounts for in greater detail below). Chemical and mechanical treatment programs allow Hardness – Hardness refers to the pres- all system losses including leaks and the thresholds of scaling tendencies and ence of dissolved calcium and magne- drift, except for evaporation. corrosion to be pushed; however, limits sium in the water. These two minerals are The second principal relationship defines still persist necessitating management of particularly troublesome in heat exchange cycles of concentration in terms of make- dissolved minerals (conductivity) levels applications because they are inversely up flow and blowdown flow: through elimination of high mineral soluble – meaning they come out of solu- content water through blowdown. The tion at elevated and remain (2) Cycles of Concentration = Make-up following section discusses these soluble at cooler temperatures. For this ÷ Blowdown concerns in greater detail. reason calcium and magnesium-related This equation can be rearranged to either deposits will be evident in the warmest of the following to solve for the make-up areas of any cooling system, such as the rate or blowdown rate: System Concerns tubes or plates of heat exchangers, or in Cooling towers are dynamic systems the warm top regions of the cooling tower (3) Blowdown = Make-up ÷ Cycles of because of the nature of their operation fill where most of the evaporation occurs. Concentration and the environment they function within. Alkalinity – Alkalinity is the presence (4) Make-up = Cycles of Concentration Tower systems sit outside, open to the of neutralizing, or acid buffering × Blowdown elements, which makes them susceptible to dirt and debris carried by the wind. minerals, in the water. Primary contribu- If the evaporation rate and cycles of tors to alkalinity are carbonate (CO -2), Their structure is also popular for birds 3 concentration are known, the blowdown bicarbonate (HCO -), and hydroxide and bugs to live in or around, because 3 rate can then be determined by substitut- ing equation 4 into equation 1:

(2) Scaling will occur predominantly in the heat exchangers and in the fill-section of the tower structure, but may also occur in the piping or on the tower distribution deck. continued > FEDERAL ENERGY MANAGEMENT PROGRAM 5

of the warm, wet environment. These Scaling – Scaling is the precipitation of area, leading to lower thermal efficiency factors present a wide range of opera- dissolved minerals components that have of the system. tional concerns that must be understood become saturated in solution. Figure 9 Microbiological Activity – and managed to ensure optimal thermal illustrates scale Microbiological activity is micro- performance and asset reliability. Below collecting on a faucet head. Factors that organisms that live and grow in the cool- is a brief discussion on the four pri- contribute to scaling tendencies include ing tower and cooling system. Cooling mary cooling system treatment concerns water quality, pH, and temperature. Scale towers present the perfect environment encountered in most open-recirculating formation reduces the heat exchange abil- for biological activity due to the warm, cooling systems. ity of the system because of the insulating moist environment. There are two distinct properties of scale, making the entire Corrosion – Corrosion is an electro- categories of biological activity in the system work harder to meet the cooling chemical or chemical process that leads tower system. The first being plank- demand. An expanded discussion for to the destruction of the system metal- tonic, which is bioactivity suspended, or scaling is available at www.gewater. lurgy. Figure 7 illustrates the nature of a floating in solution. The other is sessile com/handbook/cooling_water_systems/ corrosion cell that may be encountered biogrowth, which is the category given ch_25_deposit.jsp. throughout the cooling system metal- to all biological activity, , or lurgy. Metal is lost at the anode(3) and that stick to a surface in the deposited at the cathode.(4) The process is cooling system. Biofilms are problematic enhanced by elevated dissolved mineral for multiple reasons. They have strong content in the water and the presence of insulating properties, they contribute , both of which are typical of most to fouling and corrosion, and the bi- cooling tower systems. products they create that contribute to further micro-biological activity. They can be found in and around the tower structure, or they can be found in bundles, on heat exchangers surfaces, (see Figure 10), and in the system piping. Additionally, biofilms and algae mats are problematic because they are difficult to kill. Careful monitoring of biocide Figure 7. Example of a Corrosion Cell. treatments, along with routine measure- ments of biological activity are important to ensure bio-activity is controlled and There are different types of corrosion limited throughout the cooling system.(6) encountered in cooling tower systems Figure 9. Calcium Carbonate Scale including pitting, galvanic, microbiologi- (Source: Hustvedt). cally influenced (Figure 8), and erosion corrosion, among others (expanded Fouling – Fouling occurs when sus- discussion is available at www.gewater. pended particles fall out of solution com/handbook/cooling_water_systems/ forming deposits. Common foulants ch_24_corrosion.jsp). Loss of system include organic matter, process oils, and , if pervasive enough, can silt (fine dirt particles that blow into the result in failed heat exchangers, piping, tower system, or enter in the make-up or portions of the cooling tower itself. water supply). Factors that lead to fouling are low water velocities(5), corrosion, and process leaks. Fouling deposits, similar to scale deposits, impede the heat exchange capabilities of the system by providing an insulating barrier to the system metal- lurgy. Fouling in the tower fill can plug Figure 10. Biofouled Figure 8. Microbiologically Influenced Corrosion film fill reducing the evaporative surface (Source: GmbH). (Source: Taprogge GmbH).

(3) The anode in a corrosion cell is defined as the site where metal is lost from the system structure and goes into solution. (4) The cathode in a corrosion cell is defined as the site where the metal lost at the anode is deposited. (5) Low water velocities may occur in poorly designed or improperly operated heat exchangers, in the cooling system piping, or in locations across the tower fill where uniform distribution is not maintained. (6) Beyond the operational and mechanical problems bioactivity causes in cooling tower systems, there is a human health issue if the system develops a specific bacterium known as Legionella. For more information regarding Legionella and Legionnaires’ disease go to www.cdc.gov/legionella/patient_facts.htm. continued > 6 FEDERAL ENERGY MANAGEMENT PROGRAM

Treatment Options Examples of “green” chemistry programs blowdown control and the other showing Traditional programs are include polysilicate corrosion inhibitors blowdown controlled with a conductivity designed and implemented to account (used for many years in potable water controller. The charts illustrate the impact for the system concerns outlined above. systems), polyaspartic acid , of implementing blowdown control- This ensures the tower system operates and hydrogen peroxide for biocide appli- lers, revealing conductivity rates that optimally and achieves the needed cool- cations. Another biocide that received the stay much closer to the ideal set point ing requirement. These programs consist 1997 Environmental Protection Agency compared to manual control. (EPA) Green Chemistry Award is THPS of chemical additives including corrosion More robust automation platforms are – tetrakis (hydroymethyl) phosphonium inhibitors, dispersants, scale inhibitors, also available from several manufacturers sulfate.(7) and biocides that function to protect the that provide system-wide monitoring cooling system and keep heat exchange Automation – Automation systems are and dosing. These platforms are scalable surfaces clean and free of deposits or available providing a broad range of depending on the need, but offer bio-films. When this is accomplished, capacities to control single or multiple conductivity/blowdown control, pH con- maximum cycles of concentration can be parameters in the cooling system such as trol, real-time chemical monitoring and achieved, and the cooling system can be conductivity and blowdown control, pH dosing, continuous corrosion monitoring, operated at peak efficiency both in terms control, and real-time chemical monitor- web-enabled reporting, and alarm relays. of water use and energy use. Beyond ing and dosing. The benefit of these systems is tightened traditional water treatment programs control of the various control points of Blowdown controllers are available from there are options to build upon the current the water treatment program, not only several different commercial suppliers program, improve the current program, eliminating excessive water use and high and offer a range of control points from or replace the current program. cycle conditions, but also controlling simple conductivity/blowdown control, chemical residuals and treatment dosing Water Modeling – Software platforms to timed or meter relay chemical dosing. based on real-time corrosion and scaling provide the ability to model the system’s Many of them incorporate water meter indices. In trend terms, similar results to scaling tendencies, corrosion characteris- inputs and alarm relays if threshold the conductivity improvements shown tics, and view optimal chemical applica- measurements are exceeded. tion (also referred to as “dosing”). by Figures 11 and 12 can be achieved This can be a powerful tool to better Blowdown controllers offer continuous on chemical treatment residuals, pH set understand if the system is operating at monitoring and control of the blowdown point and acid feed, biocide dosing, and the maximum cycles of concentration of the tower system. This ensures high corrosion monitoring. Figures 13 and 14 possible, to see problem points in the sys- conductivity is avoided, minimizing scal- illustrate the performance improvement tem (low flow velocity, high temperature ing and corrosive conditions and mini- real-time dosing achieves on chemical heat exchangers, for example), and to see mizes excessive blowdown which wastes residuals, ensuring the proper dosage of impacts of varying water characteristics. water. Figures 11 and 12 provide two corrosion and scale inhibitors at all times Additionally, these modeling platforms opposing trends – one showing manual and eliminating overfeed or underfeed of allow a facility to review the potential impact of integrating water reuse resources, altering chemical treatment programs, or variations to certain opera- tional parameters such as pH set-point. In some cases, local and regional water treatment professionals may have the capability to supply similar modeling results as a part of your existing water treatment program or for a consulting fee. Green Chemistries – “Green” chemistry programs exist primarily to replace tradi- tional treatments that have been deemed harmful for environmental reasons. “Green” chemistries often don’t result in improved thermal efficiency or reduced water consumption, but provide environ- mental compliance and reduced dis- Figure 11. Conductivity Trend for Manual or Timed Blowdown Control. charge of harmful or illegal substances.

(7) To view the details of the THPS Green Chemistry Award go to www.epa.gov/gcc/pubs/pgcc/winners/dgca97.html. The complete list of recipients for the EPA’s Green Chemistry Awards is available at www.epa.gov/gcc/pubs/pgcc/past.html. continued > FEDERAL ENERGY MANAGEMENT PROGRAM 7

these products. These platforms offer web-enabled reporting interfaces so operators and maintenance staff can view tower performance remotely. Additionally, alarms for out-of- compliance measurements can be sent via email or text message alerting maintenance or operations personnel so corrective actions can be taken. Filtration – Filter systems are nothing new to industrial water systems, and have been used as pre-treatment in many different applications for many years. In recent years, side-stream filtration systems have become popular among many water treatment professionals. They function to remove suspended Figure 12. Conductivity Trend for a System Using a Blowdown Controller. solids, organics, and silt particles down to 0.45 microns from a portion or all of the system water on a continual basis, thereby reducing fouling, scaling and microbiological activity. This allows the cooling system to work more efficiently and often reduces the amount of water blown down. However, the net impact on water consumption must consider the fact that these platforms require back-washing to clean the filter system. The amount of water used to regenerate the filter system should be added to the water lost due to evaporation and blowdown. Softening – Softeners function to remove hardness (calcium and magnesium) from the make-up water, or can be incorpo- rated as a side-stream system to soften a portion of the water continuously. This Figure 13. Chemical Residuals Trend for Timed Feed. effectively manages (or eliminates) the amount of calcium and magnesium in the tower bulk water, thereby reducing the scaling potential of calcium and magnesium related deposits. By reducing or eliminating the scaling potential of calcium and magnesium, higher attention should be given to corrosion monitoring and the corrosive characteristics of the water. Calcium and magnesium function naturally as corrosion inhibitors, so if they are minimized or removed from the water, the corrosive conditions will increase putting more importance on the corrosion management program and tighter requirements on pH and alkalinity control. Figure 14. Chemical Residuals with Real-Time Dosing.

continued > 8 FEDERAL ENERGY MANAGEMENT PROGRAM

Chemical-free Platforms – In recent bioactivity by damaging cell walls of Water Reuse – Water reuse options vary years many innovations have been made potential microbes that pass through the depending on the nature of water uses in systems that provide chemical-free treatment zone. Corrosion is controlled from site to site along with a broad range treatment replacements to traditional naturally by these systems as they allow of other considerations, including local treatment platforms. These include the water to run at elevated pH levels, or and regional water reuse laws and the systems that are singular in their target alkaline conditions. availability of an adequate reuse resource. treatment, such as (8) and ultraviolet The concept is relatively simple: take Similarly, systems employing hydro- biocide systems. These systems are discharge water from one application, dynamic cavitation impact the water’s very effective disrupting and eliminat- apply sufficient treatment to it if needed, characteristics to control scaling, corro- ing biological activity in the treatment and then use it as a make-up water sion, and biological activity. Instead of zone, are flexible across a broad pH resource for the cooling tower system. imparting electromagnetic principles to range, have low operating costs, and no Two examples of reuse are collecting the accomplish these ends, hydrodynamic undesirable byproducts. These systems from air-handlers and then cavitation impacts the water mechani- treat an isolated point in the system, so pumping them back to the tower system cally by rapid changes in and far-reaching parts of the cooling system make-up, or implementing a reverse mechanical collision in the treatment may have bioactivity due to extended osmosis system on the cooling tower zone. Again, scaling is controlled through distances from the treatment application blowdown. The condensed water that the seeding mechanism of calcium point. Additionally, sessile bioactivity in collects in air handlers is an excellent carbonate () formation, which in the system beyond the point treatment resource of high quality make-up water turn collects any other dissolved minerals may not be affected by the ozone and for the tower system. Because it is con- nearing their respective saturation points ultraviolet treatments. densation, by nature it will be relatively and removes them through in-line filtra- pure, and therefore will not require much There are also other types of compre- tion. Low pressure zones strip the passing additional treatment to make it sufficient hensive chemical-free platforms and the water of CO , maintaining naturally 2 for using as tower make-up water.(10) market has evolved, offering many dif- alkaline conditions to control corrosion. These systems are not difficult to engi- ferent types of platforms, which currently Lastly, the cavitation process ruptures neer, nor are they typically very capital have varying degrees of success in the cell walls of microbes passing through intensive.(11) commercial marketplace. Several have the treatment zone, thereby managing been available for many years, but the bioactivity in the cooling systems. (RO) systems (similar evolution of the differing to Figure 15) are common in many These systems and many others like them has allowed for many more platforms to have case studies and testimonials posted be developed.(9) The exact mechanisms to demonstrate successful replacement of used to alter the water’s characteristics standard treatment options, including vary from platform to platform, includ- energy savings and water efficiency ing electromagnetic and hydrodynamic gains. They offer the advantage of principles as point treatments to control reduced or eliminated need for treatment scaling and corrosion along with micro- chemicals and have had many successful biological activity. Systems that employ installations with documented improve- electromagnetic principles feature a treat- ments. However, some installations have ment zone where an electric field courses had difficulty depending on the water through the flow of the recirculating quality, especially where the natural ten- water creating a “seeding” mechanism for dencies of the source water lean toward dissolved minerals, principally calcium corrosive conditions. Furthermore, point carbonate (also known as calcite). The treatments may have some difficulty with seeding process creates a location for systems that are extensive and have far other dissolved minerals to agglomerate, reaching- runs or heat exchangers or come out of solution, and then be that are a substantial distance from the removed via in-line filtration systems that treatment system itself. follow the treatment system. The pulses of electricity also behave to counter Figure 15. Plant Using Reverse Osmosis (Source: James Grellier).

(8) Ozone is a reactive form of oxygen that has strong oxidizing characteristics making it very effective at controlling microbiological activity. (9) This document features mechanisms of a few different available systems and is not intended to be a comprehensive overview of all commercially available chemical-free water treatment systems. (10) The purity of the water will increase its natural abilities as a solvent and stagnant water systems are ideal for bioactivity. Care should be taken when considering the materials of construction for the collection and distribution system and if needed a small amount of biocide may be needed to minimize bioactivity. Additionally, the corrosion program for the tower system will need to reviewed or modeled to ensure proper corrosion control steps are taken to protect system metallurgy. (11) For more information on condensate recovery systems a case study is available at www.femp.energy.gov/program/waterefficiency_csstudies.html.

continued > FEDERAL ENERGY MANAGEMENT PROGRAM 9

applications including desalination, stream (the concentrate flow) that would industrial pre-treatment platforms, and be discharged from the system. These Where to Get More Information applications where the purity require- systems can be difficult to operate FEMP Water Efficiency: ments for water are very high. In the case correctly, and are expensive to purchase www.femp.energy.gov/program/ of water reuse from a tower system, the and operate. However, locations where waterefficiency.html blowdown water would be sent directly are limited or discharge into an RO system, which would purify permits prohibit excessive cooling tower Cooling Technology Institute: a portion of the water (the permeate blowdown, RO platforms are a consider- www.cti.org/ flow) and concentrate the bulk of the ation to minimize the overall losses from American Society of Heating, dissolved minerals into a smaller waste a cooling system. Refrigeration, and Air-Conditioning : www..org/ American Society of Mechanical Engineers: www.asme.org/ If questions remain, contact: Will Lintner Federal Energy Management Program (202) 586-3120 [email protected] Brian Boyd Pacific Northwest National Laboratory (509) 371-6724 [email protected]

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