Technical Evaluation Of Side Stream Filtration For Cooling .

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FEDERAL ENERGY MANAGEMENT PROGRAMTechnical Evaluationof Side StreamFiltration forCooling TowersBackgroundCooling towers are an integral componentof many refrigeration systems, providingcomfort or process cooling across a broadrange of applications. Cooling towersrepresent the point in a cooling systemwhere heat is dissipated to the atmospherethrough evaporation. Cooling towers arecommonly used in industrial applicationsand in large commercial buildings torelease waste heat extracted from aprocess or building system throughevaporation of water.Cooling tower systems operation is mostefficient when the heat transfer surfacesare clean. However, these are dynamicsystems due in part to their operatingenvironment and because of the natureof their application. Cooling towers operate outside and therefore are open to theelements, making them susceptible to dirtand debris carried by the wind. Further,they often experience wide load variationsand their operation can be significantlyinfluenced by the quality of the water usedfor makeup in the system.The combination of process and environmental factors can contribute to four primary treatment concerns encountered inmost open-recirculating cooling systems:corrosion, scaling, fouling, and microbiological activity. As shown in Figure 1,these treatment concerns are inter-relatedsuch that reducing one can have an impacton the severity of the other three. Corrosion: Corrosion is an electro-chemical or chemical process that maylead to the premature failure of systemmetallurgy.Cooling Towers (photo from Pacific Northwest National Laboratory) Scaling: Scaling is the precipitationof dissolved mineral components thathave become saturated in solution,which can lower efficiency of thesystem. Fouling: Fouling occurs whensuspended particles or biologic growthforms an insulating film on heattransfer surfaces. Common foulantsinclude organic matter, process oils,and silt, which can also lower systemperformance. Microbiological Activity:Microbiological activity refers tomicroorganisms that live and grow inthe cooling system that can contributeto fouling and corrosion.Figure 1. Cooling Tower Primary TreatmentConcernsSide stream filtration systems reducesuspended solids and debris in the systemcooling water, which leads to less foulingin the system. Decreasing suspendedsolids can also help reduce biologicalgrowth in the system because suspendedsolids are a good source of food formicrobiological organisms. Decreasingbiological growth in turn helps to reducemicrobiologically influenced corrosion.In addition, scaling can be reduced fromside stream filtration by limiting foulingand corrosion byproducts which can alsocontribute to scale formation on the heatexchange surfaces. Effectively managingthese conditions can optimize systemperformance, often resulting in moderateto significant energy and water savings.Full flow and side stream filtration arethe two most common methods used tofilter the water that is pumped into thecirculation systems. Full flow filtrationuses a filter installed after the coolingtower on the discharge side of the pump.This filter continuously filters all of therecirculating system water in the system.Inherently, the filter must be sized tohandle the system’s design recirculationrate. Side stream filtration, on the otherhand, continuously filters a percentageof the flow instead of the entire flow. Itcan be a cost-effective alternative to fullflow filtration that can easily improve thewater quality to reduce water consumption and ensure efficiency of the coolingsystems. And unlike full flow filtration,side stream filtration systems can becleaned while the cooling systems areonline, avoiding the need for planneddowntime (BAC 2012).

FEDERAL ENERGY MANAGEMENT PROGRAMTechnologyCharacterizationSide stream filtration systems continuously filter a portion of cooling waterto remove debris and particles. Filteredwater is then pumped back into the maincondenser line through a nozzle or returned to the cooling tower basin (calledthe sump). Figure 2 below shows a simplified cooling tower schematic, including the two example locations where sidestream filtration can typically be installed.These systems remove suspended solids,organics, and silt particles for a portion ofthe water system on a continuous basis,reducing the likelihood of fouling andbiological growth, which helps to controlother issues in the system such as scalingand corrosion. This improves system efficiency and often reduces the amount ofwater rejected from the system. There area variety of filter types, which generallyfall into four basic categories: screenfilters, centrifugal filters, sand filters, andmulti-media filters (WPCP 2012).Side stream filtration requires a minimumsupply pressure to account for the inherent differential pressure drop across thefilter. This typically ranges from 20 to30 psi. All side stream filters have amaximum working pressure; sand filtershave a threshold of 80 psi, whilemechanical filters, such as screen filters,can operate up to 150 psi. Systemsrequire cleaning of filters or holdingchambers to remove debris and particlesthat are trapped in filters.2Filters are rated by the size of particlesthat can be removed, measured inmicrons. Suspended solids in coolingtowers typically range in size from1 to 50 microns as shown in Table 1. Ingeneral, 90% of the particles in cooling towers are smaller than 10 microns(Bobby et al. 2001). However, formechanical filtration the smaller numbersof larger particles are of more concernthan the large number of smaller particleswhich are often bacteria removed bydisinfection rather than filtration (BAC2012), or micron and sub-micron sizedsuspended solids which can be treatedand removed by chemical treatment. Sidestream filtration systems are generallysized to filter from 3 to 10% (up to 20%)of the overall system flow. Filters areselected based on the percent of flowthat the side stream filtration system isdesigned to handle. For example, in acooling system with a recirculation rateof 1500 gpm, a filtration system sizedto handle 10% of the recirculation ratewould be sized to handle 150 gpm.Table 1. Relative Size of CommonCooling Water Contamimants(McDonald 2009)ParticleSandPollensMold SporesBacteriaMicrons100 to 2,00010 to 1,00010 to 303Side stream filtration increases waterand energy efficiency and reduces cost,as described below (Latzer 2012;BAC 2012). Reduction in water consumption:Demand for makeup water in coolingtowers is decreased with an increase inthe system’s cycles of concentration.Essentially, higher cycles of concentration mean that water is being recirculated through the system longer beforeblowdown is required. Less blowdownreduces the amount of makeup waterrequired in the system, resulting inwater savings. Reduction in energy consumption:Side stream filtration reduces the likelihood of scale and fouling on the heatexchangers. Even the smallest layerof scale or fouling on heat exchangesurfaces can reduce the rate of heatexchange, forcing the system to workharder to achieve the required cooling. Reduction in chemical use: A sidestream filtration system can removesuspended particles, reducing the needfor additional chemical treatment suchas dispersants and biocides. Lower maintenance cost:Traditionally, cooling towers arecleaned by draining the tower and having the sediment removed mechanicallyor manually from the basin or sump.Cooling systems that are cleaned viaside stream filtration routinely providelonger periods of continuous operationbefore being taken offline for requiredmaintenance. Improvement in productivity andreduction in downtime: When acooling system is fouled or has scalebuildup, production may be slowed dueto inefficient heat exchange equipment.In some cases, the cooling system andheat exchange equipment may need tobe taken offline for repairs, decreasingproduction. Control of biological growth:Biological growth control and reduction can mitigate potential healthFigure 2. Cooling Tower with Side Stream Filtration Examples

3FEDERAL ENERGY MANAGEMENT PROGRAMproblems, such as those caused byLegionella. ASHRAE Guideline12-2000 has basic treatment recommendations for control and prevention,stating that the key to success is systemcleanliness.Technology ApplicationsThe following are applications wherethe addition of a side stream filtrationsystem can improve the water and energyefficiency of the system. Systems with difficult biological Systems that are susceptible tofouling due to either the nature ofthe application or the environmentin which they operate. Systems where scaling deposits causea loss of heat transfer. Systems with high levels of solidsbuildup in the sump due to dirt anddebris deposited by windy conditions. Systems for which the primary sourceof makeup water is a surface or otherunclarified source.System Optionsproblems, even with the presenceof a good biocide program. Systems in which the heat exchangersrequire frequent mechanical cleaning.There are generally four system optionsfor side stream filtration: centrifugalseparators, screen filters, disc filters, andsand filters. For all of these options, thekey performance elements to consider inthe filter system are the particle removallevel, self-cleaning function, ease ofoperation, and water loss from backwash. These characteristics were assessedfor the four basic types of side streamfiltration systems. An overview of theirperformance characters is summarizedin Table 2 and more detailed informationon each system type is presented in thefollowing sections.Table 2. Side Stream Filtration System CharacteristicsFilter TypeCentrifugal SeparatorsParticleRemoval LevelSelf-Cleaning FeaturesMaintenance andPart Replacement40-75 microns, fine toPurge collected solidsPurge components onlyNo water loss due to backcoarse inorganics with afrom the collection– periodic inspection andwashing. Water may bespecific gravity of 1.62 orchamberservicinglost during the purginggreaterAutomatic Screen FilterPlastic Disk FilterWater Lossfrom BackwashDown to 10 micronsDown to 10 micronsof the particle chamberAutomatic backwash byRegular maintenance may Requires much lessusing a rotating suctionbe required because ofwater than other self-scanner assemblymoving parts that enablecleaning filters that utilizeautomatic backwashbackwash cyclesConsumable discsRequires much lessAutomatic backwashthrough releasing grooved can require frequentwater than other self-discs and reversing watercleaning filters that utilizereplacementflow to wash collectedbackwash cyclessolids off the discsPressure Sand FilterDown to 10 micronsAutomatic backwash,Requires regular inspec-Requires significant wateronce a day or on pressuretion of sand media andfor backwashingdrop as neededelectromechanical parts,and periodic replacementof sand mediaHigh Efficiency Sand Filter Down to 0.45 microns.Automatic backwashSand media mustRequires more backwashBest for fine, lightfeatures, requires lessbe monitored andwater than centrifugalparticles; avoid heavytime and water thanperiodically disposedseparators, automaticcoarse particleother sand filtersand replacedscreen, and disc filters;applicationsbut about eight times lesswater than other sandfilters

FEDERAL ENERGY MANAGEMENT PROGRAMCentrifugal SeparatorsCentrifugal separators remove solidsfrom water by the centrifugal force developed as water passes through the device.The technology is simple in design.Separators are fed high-velocity rawwater to develop the circular flow patternthat produces the centrifugal action. Thiscentrifugal action causes heavy solidsthat are suspended in the water to migratetoward the separator’s sidewalls anddownward, into a solids holding chamber.Cleansed water rises through the vortexand is returned to the system through anoutlet at the top of the separator. Solidscollected in the holding chamber areeither periodically or continuously purgedfrom the collection chamber (Figure 3).to clean itself. Cooling water entersthe filter through an inlet, then passesthrough a rigid cylindrical screen fromthe inside out, causing particles largerthan the openings of the screen to accumulate on the inside surface and form afilter cake. Filtered water leaves the filterbody through the outlet. The buildup ofthe filter cake inside the screen causesa difference in pressure between theinlet and outlet of the filter. A controllermonitors the pressure in the filter andopens a flush valve when it senses adifferential pressure threshold has beenexceeded. When the flush valve opens toatmosphere, the difference between thehigher pressure of water inside the filterand the atmosphere outside the filter bodycauses high suction forces at the openingsof each of the suction scanner nozzles.The suction force causes water to flowbackward through the screen in a smallarea at very high velocity at each nozzle,pulling the filter cake off the screen andforcing it into the suction scanner and outthe exhaust valve to drain (Figure 4).Figure 3. Centrifugal Separator SchematicThe capacity for solids removal is a function of particle density, size, and shape,and device design. Centrifugal separators are best used for and most efficientat separating large, heavy particles.A centrifugal separator requires littlemaintenance and infrequent replacementbecause it does not trap particles thatclog or damage its system. Therefore,centrifugal separators tend to be moreeconomical than other filtering systemswith the same filtration efficiency, but arejust as effective at removing suspendedsolids. (Griswold Filtration 2008)Automatic Screen FiltersAn automatic screen filter, also knownas a self-cleaning screen filter, is a typefiltering system that uses system pressureFigure 4. Automatic Screen Filter SchematicThe driving mechanism of the filterrotates the suction scanner assembly at aslow, fixed rotation while simultaneouslymoving the scanner linearly at a fixedspeed. The combination of the rotationand the linear movement gives each suction scanner nozzle a spiral path along theinside surface of the filter screen, whichallows the nozzles to remove the filtercake from every square inch of the filterscreen. The cleaning cycle usually takesless than 1 minute. The total volume ofwater used for cleaning is small, usuallyless than 1% of the total flow.2Automatic screen filters are unique inthat the self-cleaning cycle does notrequire the entire system flow to stopand reverse. Therefore, unlike manyother types of filters, the self-cleaningcycle of these filters does not interruptsystem flow during the rinse cycle. Inaddition, automatic screen filters providea two-dimensional, discrete openingthat positively removes particles that arelarger than the pore size of the screenbased on size alone, regardless of othercharacteristics such as particle density,shape, or particle materials. Self-cleaningscreen filters are used in a variety ofapplications where continuous water flowis crucial, including accelerator or reactorcooling, hospitals, power generation,climate controlled research facilities,and manufacturing processes that requirecontinuous cooling. This technology isrelatively inexpensive for the high flowrates it offers (BAC 2012).Plastic Disc FiltersThis technology uses plastic discsmade of polypropylene that are stackedtogether under pressure and grooved tofilter particles of specific micron sizes.Each disc has etched grooves in a slightlydifferent pattern array between the topand bottom of the disc. When multiplediscs are stacked and centered arounda skeletal cylindrical structure, called a“spine,” the discs form a hollow cylinderwith the ends of the grooves exposed toboth the inside and the outside surfacesof the cylinder (Figure 5). The differentgroove patterns of the stacked discs create intersections of different sizes to trapparticles when cooling water passes fromthe outside to the inside of the hollowFigure 5. Plastic Disc Filters

3FEDERAL ENERGY MANAGEMENT PROGRAMcylinder. As particles are captured withinthe depth of the disc stack, a pressure differential is created. Backwash is initiatedwhen the preset pressure differential isachieved. The stack pressure is relievedand the filtered water is forced throughthe disc stack in reverse through severalnozzles within the disc stack spine. Thesenozzles create a tremendous amount ofturbulence that cleans the discs veryeffectively in 10 to 20 seconds(Prochaska 2002).Disc filters can remove both solids andorganic particles effectively. These filtersalso use much less water than other typesof self-cleaning filters for backwashcycles, and tend to have relatively lowerinstallation and operating costs comparedto other filters with equivalent filtrationrates. Disc filters can backflush multiplefilters sequentially, and because the backflush cycle is sequential, the filtrationprocess is seldom interrupted. Triggeredby differential pressures or timingintervals, or a combination of both, theself-cleaning process is fully automatic,requiring little maintenance.Sand FiltersSand filters are a common type of sidestream filtration system. Sand filtersdirect fluid into the top of their tank(s)and onto the surface of a bed of specifiedsand and/or other media. As the cooling water flows through the bed ofsand media, suspended solids and otherparticles are captured within the upperlayer of media. Sand filters are usuallyvery efficient at removing the extremelyfine and low density particles that coolingtowers scrub from the air. Therefore,they generally have very high filtration rates. Sand filters tend to be moreexpensive and larger than other types sidestream filtration systems with equivalentfiltration rates (Melancon 2004). Thereare typically three types of sand filters,pressure sand filters, high efficiency sandfilters, and gravity sand filters. However,gravity sand filters are rarely used forcooling tower systems; they are thereforenot discussed in this fact sheet.A feature of sand filter design that shouldbe considered is the capability of supplemental chlorination during backwash orroutine maintenance. The filter mediumin many types of sand filters coupled withthe increased temperature of the recirculating cooling water can support biological activity. Supplemental chlorination isan effective strategy to reduce increasedbiological growth in the filter medium,and therefore in the cooling water aswell. In short, sand filters provide excellent removal of suspended solids, butsize, expense, and maintenance concernsare considerations when selecting thistechnology.Pressure Sand FiltersPressure sand filters are one of the mostcommon side stream filtration systemsand are used in many facilities. A pressure sand filter consists of a pressurevessel and several layers of multi-mediafilters. A coarser filter media is located onthe top layer, with layers of decreasinglygranulated material down to the finemedia at the bottom. A layer of gravel isincluded on the bottom layer to preventfiner sand media material from migratingthrough the drain (Figure 6). Typically,these systems are effective at filteringparticles of sizes between 15 and20 microns.Figure 6. Pressure Sand Filters SchematicFor pressure sand filters, the backwashrequirement is relatively high and anexternal source of backwash water isneeded. Clean, treated city water or clarified, chlorinated water are the preferredwater source for backwash, whichtypically takes 10 to 15 minutes perbackwash. Backwash is initiated by eithera pressure differential switch measuringthe incoming and outgoing pressures oran adjustable timer. A backwash with aclean, chlorinated source is recommendedat least once per day to maintain mediaefficiency and to prevent microbiologicalactivity.High Efficiency Sand FiltersThis type of filter is similar to the pressure sand filter in that sand is used asthe filtration media. However, the medialayer order is reversed, with extra finesand as the top layer and layers of gradually coarser sand down

separators, screen filters, disc filters, and sand filters. For all of these options, the key performance elements to consider in the filter system are the particle removal level, self-cleaning function, ease of operation, and water loss from back wash. These characteristics were assessed for the four basic types of side stream filtration systems.

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