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Fans Key to Optimum Cooling-Tower Design - …

fans Key to Optimum Cooling-Tower Designby MonroeHudson Products Corporation, Houston, TexasBased on paper presented toCooling Tower Institute Annual MeetingNew Orleans, Louisiana1974 Hudson Products CorporationPage 2 of 9 Houston, TexasFans Key to Optimum Cooling-Tower DesignFans Key to Optimum CoolingTower DesignBy R. C. MonroeThe questions most frequently asked of a fanengineer about axial-flow fans for today's wetcooling towers generally cover: Performance Efficiency Corrosion resistance NoiseThis article reviews such fundamentals and hopefullygives new insight for Optimum tower Optimum fan performance is severalthings other than just delivery of Design -air quantity: Lowest first cost.

Hudson Products Corporation Page 3 of 9 Houston, Texas Fans Key to Optimum Cooling-Tower Design • A fan is supposed to move air and do work.

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Transcription of Fans Key to Optimum Cooling-Tower Design - …

1 fans Key to Optimum Cooling-Tower Designby MonroeHudson Products Corporation, Houston, TexasBased on paper presented toCooling Tower Institute Annual MeetingNew Orleans, Louisiana1974 Hudson Products CorporationPage 2 of 9 Houston, TexasFans Key to Optimum Cooling-Tower DesignFans Key to Optimum CoolingTower DesignBy R. C. MonroeThe questions most frequently asked of a fanengineer about axial-flow fans for today's wetcooling towers generally cover: Performance Efficiency Corrosion resistance NoiseThis article reviews such fundamentals and hopefullygives new insight for Optimum tower Optimum fan performance is severalthings other than just delivery of Design -air quantity: Lowest first cost.

2 This means optimizing the fandiameter and number of blades. Why buy extrablades if they aren't needed? Lowest horsepower requirements. The fanshould work as close to an efficient operatingpoint as possible. Lowest noise without extra cost. Since noiseemitted by the fan is a function of tip speed toalmost the 6th power, even a small reduction ofspeed has significant effect on the this can be accomplished and the required airduty attained at no extra cost, why not? Lowest maintenance costs. The Optimum choiceof fan blades and hub materials ensures againstcorrosion and subsequent going into some suggestions on how toachieve the proceeding goals, a review of basic fanprinciples may be helpful. Starting from groundzero, they are:Basic Fan EquationsFig.

3 1 TP = VP + SP (in. H2O) TP = total pressure, VP = velocity pressure, Static pressure = SP (All pressures must be relative to the same airdensity) ACFM = actual cfm Air hp = Efficiency x 6,356 ACFM x actual TP of Efficiency total = actual HP x 6,356 ACFM x actual TP Efficiency static = actual hp x 6,356 ACFM x actual SP Air velocity = fpmfan of area freeNet ACFM Velocity pressure 4,005V2 Air density std = ftcu lb Basic fan law = CFM = f (rpm)1 , TP =f (rpm)2 , HP = f (rpm)3 Solidity ratio = diameter chords tipof sum When stacks are used Velocity recovery = (VP fan VP exit) Efficiencyof recovery (Both velocity pressures must be calculated atsame density Efficiency of recovery will usually be in therange of to ) TP effective = TP std Products CorporationPage 3 of 9 Houston, TexasFans Key to Optimum Cooling-Tower Design A fan is supposed to move air and do work.

4 This air is supposed to be evenly distributed overthe entire exit area of the move air, the fan must overcome tworesistances, which are measured as pressure dropsacross the first is a parasitic loss called thevelocity-pressure loss. I see this as the energyrequired to move the required air quantity withoutdoing any work to overcome the system is done however to move the hot air awayfrom the second resistance is the static pressure loss. It isthe accumulated losses due to duct, fill, and mist-eliminator pressure drops. This would be the "work"to be accomplished and reflects the Design of thetotal system, including inlet the air is distributed evenly across the fan isprimarily a function of the blade and hub Design .

5 Aproperly designed blade will have adequate chordwidth and "twist" to ensure an even distribution ofvelocity pressure over its entire properly designed hub will include a centerair-seal disk which prevents negative air flow at thecenter of the basic fan equations are shown in Fig. Following are two cases that most everyonehas faced: Designing a new tower from scratch, attemptingto get the best fan Design possible. Replacing a fan on an old tower wherepractically nothing is for new tower designFig. 2 Total pressureDensity ratio: air saturated = = = (read from 28ft fan curve orcalculated using NFA of fan.)TPStd. = + = in. H2 OSpeed FactorSpeed factor (flow) = 000,10000,12 = 12 Speed factor (pressure) = ( )2 = factor (hp) = ( )3 = corrected = 1,100,000 x =1,320,000 ACFMTP corrected = x = in H20 The check list for a brand new Design includes:1.

6 Fan-diameter or cell-size Actual cfm of Actual static Air temperature and elevation (density).Comparison of SelectionsTable 1 FanDiameterftNo. ofbladesPitchAngleFan totalEfficiencyCurvehorse-powerActualhor sepowerFancost($)28920 ,621281216 ,737261119 ,074301017 ,345 Hudson Products CorporationPage 4 of 9 Houston, TexasFans Key to Optimum Cooling-Tower DesignEffect of velocity recovery stackTable Savingsofhorsepower26 Limitations on tip speed other than standardgearbox Should a velocity-recovery stack (VR) be used?7. Are there noise-level specifications involved?8. Are any unusual corrosion problems expected?Factors that must be known when replacing a fan onan existing installation are:1. Fan Installed Gearbox Shaft size or gearbox Some estimate of elevation above sea level course what we are seeking in Case 1 is anoptimum fan diameter, number and type of blades,required pitch angle, fan rpm, and some estimate ofhorsepower.

7 In some cases, we are looking for anestimated sound pressure level or possibly a soundpower level to satisfy OSHA requirements forworking-area noise levels or more importantly noiselevels at a plant Case 2 we are looking for a fan that at least willbe an adequate replacement for the original diameter. The fan diameter bears onperformance primarily because it affects: (a)magnitude of the velocity pressure, which is aparasitic loss; and (b) the pressure capability of our estimation, velocity pressure should fall in therange of to in. for Optimum course, other factors influence the choice of fandiameter, such as cell-size limitations due to plotplan, economics, or selection of a fan for an high velocity pressures are the result ofhigh flow requirements for the fan size required,resulting in a waste of may require adding blades just to cope with a highvelocity-pressure requirement.

8 However, thispresents a good case for converting velocitypressure into useful static-pressure capability byadding a velocity-recovery solidity ratio is a way to compare a fan'spressure capability. The higher the r a t i o, usuallythe more work the fan can do. Still another aspect ofoptimum fan diameter is cost. Nonstandard sizesmean special handling by the fan manufacturer atadditional factors. Besides fan diameter we must alsoconsider: Actual cfm, or the Design -air quantity at the fanrequired to do a specified job. Actual static pressure, usually stated at theactual exit-air temperature to the fan, and at theelevation of the cooling tower. This must beconverted to static pressure at standardconditions before being used with thefan-performance curves.

9 Air temperature, elevation. These factors areimportant in fan selection because of their effecton fan-pressure capability and horsepowerrequirements through changes in air factors are combined into a density 's fan catalog has a nomograph for thedensity of dry air which is accurate enough inmost cases to use for saturated air as well. ThisHudson Products CorporationPage 5 of 9 Houston, TexasFans Key to Optimum Cooling-Tower Designnomograph gives the reciprocal of the tower. Let's look at a typical case of fanselection for a new tower Design . Considered will bea 26-ft, 28ft, or 30-ft-diameter fan. Dutyrequirements are million actual cfm, static pressure, 95 F. outlet-air temperatureat sea level.

10 It is to be restricted to a maximum tipspeed of 10,000 be determined are diameter, number of blades,pitch angle, and estimated all fan curves are based on standard conditions(70 F., sea level or lb/cu ft density) let's tookat the standard total pressure, considering28-ft-diameter fans (Fig. 2).Then we must consider the effect of the reduced tipspeed by calculating a speed factor (Fig. 2).Since the fan loses capability as it slows down, wenow increase our requirements to include theselosses before we enter the fan curves. Thesecorrections are shown in Fig. you can see, the speed limitation has imposed a20% penalty on flow and a 44% penalty on select a 28-ft-diameter fan, we can go to theperformance curves and enter them for by 101,actual cfm and in (H,O).


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