53Analysis of fan/systemcharacteristics andapplicationsAdvanced technology and energy sensitivity in designcall for more accurate methods of matching fan tosystem other than manufacturers’ data coupled withsafety factors. This discussion, based on actual tests,outlines a way to combine a system curve with apublished fan curve for a true picture of systembalance.

229

230 X / AIR SYSTEMS, ADJUSTING, AND BALANCING

The word "system"and the phrase"systemsapproach"have been widely used in recentyears to describeany numberof conceptsinengineeringand architecture.At the risk ofjumping onto a popularbandwagon,in thischapter"system"will be usedin two differentways.

First,"system"asdefinedby thefirst meaning in Websteris "a regularly interactingorinterdependentgroup of items forming aunified whole." It is in this context thatsystems engineeringwill be discussed.Second,to paraphrasethe definition of systemasdefined in elementarythermodynamics,theterm system"is usedto designateanyportionof matterthat is separatedfrom its surroundings by eitherreal or imaginaryboundaries."It is in this contextthatsystemwill be usedtoanalyzecomponentsor componentsubassembliesthat operatewithinan overallengineeredbuilding system.

In thefield of building environmentaltechnology, HVAC systemsare, in general, designed by a team of systemsengineers.Theapproachis to selecta groupingof manufacturedproductsand integratethem togetherinsuchamannerthattheir respectiveinteractionwill achievethedesigngoalsset for the overallsystem.However,overtheyears,manufacturing technology,taking advantageof the costefficiencies of mass production versus fielderection,hasat thecostof flexibility resultedin prepackagingof various subsystemsto beincorporatedinto an overall system,in manycases,in lieu of individualmachinerycomponents.It is theintentof thischapterto addressthe subjectsof definition of boundariesinorder to define the limits of such systems,andthe resultingmathematicalformulasthatwill improve the sciencecurrently employedby an overall systemsdesigner.Eachwill bediscussedin relationship to handlingconditioned air.

Fan curve important toolMany yearsago,manufacturersof airmovingdevices, blowers, developed an extensivegrouping Of variables affecting the performanceof a blowerwhenappliedto anattacheddistribution system.A product’sperformance,

as testedin accordancewith industrytestingstandards,is registeredas a fan curve. Anexampleof a typical fancurvefor a backwardinclined or "airfoil" fanis shownin Fig. 53-1.Figure 53-2 illustrates the standard testmethodfor measuringthe respectiveinlet andoutletstatic andtotal pressuresandvolumetric flow rates. It is relevantto point out at thispoint thata fanis an assemblyof componentsconsistingof an inlet cone, wheel, shaftandbearingassembly,andscroll. The purposeofthe testarrangementshownin Fig. 53-2 andthe resulting curve and rating tables is toprovide a standard method of rating theperformance of one product compared toanother.Membersof thesystemsengineeringprofessionhavehadonlythesedatato assistinapplying a fan to a systemsincethe ratingswere established;albeit standardtest conditionsareseldom,if ever,experiencedin actualsysteminstallations.

Pitot tube /lhs and h Throttling

devicu

Fig.53-2. Thestandardtestmethodusedto measureinlet and outlet static and total pressuresandvolumetricflow rates.

Discharge rate. cfm

Fig. 53-1. A typical fan curvefora backwardinclinedor airfoil fan.

I

100F 8-112

h

Straightener

ANALYSIS OF FAN/SYSTEM CHARACTERISTICS AND APPLICATIONS 231

Understand fan laws

An additional considerationof backgrounddevelopmentare the so-called fan laws. Althoughmuchmoreextensive,all fan laws arebasedon threebasic relationships:

* Thedischargerate,cfm, variesdirectly asthe speed.

* The totalpressureincreasevariesdirectlyas the squareof the speed.

* The air powervaries as the cubeof thespeed.

Theselaws are readily indentified by avectoranalysisof a fan wheelif thesysteminwhich the fanis performingfollows theturbulent flow characteristicsof:

where

= KQ2

= headloss,K = systemconstant,

Q = dischargerate, cfm.

A systemsdesigner,working with the onlyavailableinformation,appliesa fanwithin anair systemby using the manufacturer’scomparative data in the form of fan curvesortables,assumesthe systemcurve is basedonEq. 1, plots the parabolaagainsttherespective fan curve, and thus identifies an anticipated operatingpoint. This approach,however,hasresultedin lessthanreliableresultsina vastnumberof designs.Heretofore,multiplesafetyfactorsincorporatedthroughoutdesigndevelopmenthaveallowedapproximationtocontinue.In recentyears,however,with designrefinementsrealizedby computerizedfullloadcalculations,variableair volumesystemswhereinfull load diversity is applied to fanand distribution system sizing, along with -

energy sensitivity in design,the professionisfacedwith the needfor more accuratemethods for matching a fan component to thedistribution andconditioningcomponentsofan overall system.Onemethodfor assistingasystemdesignerin improving the accuracyofapplicationdataaccountsfor systemeffectsbyassigningK-factor constantsto varioustypesof fan connectionconfigurations.

Consider total fan systemA fansystemis generallyassumedto include,as statedpreviously,an-inlet cone,wheel,andscroll asthe devicesthat relateto characteristics; i.e., thosedevicesthat if dimensionallychangedwould result in a different characteristic curve. Thus, a systemis definedby theboundariesencompassingthesecomponents.

It hasalso becomerathercommonpracticeto includewithin the boundariesof a systemvariable inlet vanes. As vanes are closed,creatingan additionalpressureor energylossat the inlet of a fan, the discharge ratedecreases.Figure 53-3 showsa typicalmethodof representingthis phenomenonon a fancurve. If two operatingpoints are selected,

FIg. 53-3. A typical method of representingthedecline in dischargerate when vanesare closed,creatinganadditionalpressureorenergyloss at theinlet of a fan. Referring to thebottom drawing, thecurvemovesupwardandto the left whenadamperisprovidedon thedischargesideandtheairflow rateisreducedfrom ConditionA to B.

1

232 X / AIR SYSTEMS, ADJUSTING, AND BALANCING

ConditionsA andB, it is readilyseenthat asthe inlet vanesclose, both the dischargerateand thepressurerise decrease.If, on theotherhand,a damperis provided on the dischargeside of afan,andthedischargerateis reducedfrom Condition A to B, this is generallyrepresentedasan increasedconstantK in Eq.I, swingingthe systemcurve upwardto theleft. In this case,the dischargeratedecreases

12 in. by16-1/2 in.

Fansingle inlet.single width Pitot tube Damper

h5 and hv

l!l i

-18 in. by12 in.

94 in.

2Ti

*1’‘-. 36 in.

115 in.I- 142 in.

Inlet vanes‘-I

Fan single inlet.single width

12 in. by16-1/2 in.

Damper

-i

Slotfororifice plates /

Pitot tubeh5 and by

butthe pressurerise increases.What explainsthe differencein thesetwo relationships?

The basic hypothesis to explain this issimply where the boundariesof a systemaredefined. In the caseof inlet vanes,a damperwas includedas part of thefan system,and inthe othercase,it was not. To substantiatethishypothesis,a teststandwasconstructed,admittedly with considerableliberties as con-cernsAMCA standardtests.The differenceswerenecessitatedby theneedfor changingtheboundariesand obtainingconsistentresults.Figure 53-4 is a schematicdiagramof the testconfiguration shown in the accompanyingphoto. The testsrevealedthat the inlet vanecurve could be duplicatedby positioning ofthe dischargedamper and vice versa. Toremovethe inlet vanesfrom the fan section,the inlet pressurewasmeasuredin the centerof thefan wheel. In eithercase,thefan systemefficiencyassumingagood selectioninitiallydecreasedwith reducedflow. However, thisreductionin efficiency is less pronouncedwiththe- inlet vane systembecauseof the directional nature of the vanes. This improved

Fig. 53-4. A schematicdiagramaboveand photographof thetest configuration.

ANALYSIS OF FAN/SYSTEM CHARACTERISTICS AND APPLICATIONS 233

reducedflow efficiencycould likely be reproduced or evenimprovedby fixed inlet vanes,which would still allow a designerto extendthe boundariesof a fan systemto includemultiple distribution zones,eachwith different flow andpressurerequirements.Thecontrol techniqueswould be identical to thoseemployedfor variableinlet vanes.

The use of inlet vanes for either initialbalanceof fan to systemcurve or for operational reductionofflow is notdiscouragedasasound practice if it is applied with care.However, an applicationengineershouldbecarefulin his decisionto employ inlet vanes.The idea that reducedflow can be achievedwith a moreeffectivereductionin horsepowerthanpossibleby the samefan with dischargedampercontrol is not alwaystrue. The basicreasonisthat becauseof the restrictivenatureof inlet vanehardwareat a very critical pointin the fan system,the testsrevealeda significant reductionin fancapacitywith thevanescompletelyopen.This restrictiondroppedtheeffectivefan curve in the testapparatusby apressure.rise reductionof 15 percentwith adischargerate of 0.75 free delivery.

Many devices in air systemA compositeair-handlingsystemconsistsofmultiple devicesarrangedin serieswith oneanother.Consideringthesimplesystemshowndiagrammaticallyin Fig. 53-5, thedevicesare:returnair inlet, returnairduct, filters,coolingcoil, fan,supplyairduct,supplyairgrille, andthe conditionedspace.

In order to clearly understandthe behavioral aspectsof eachdevice, the componentsare groupedinto various systemsby simplydefining the respectiveboundaries.These

Fig. 53-5. A schematic diagram of the devicesincluded in the boundariesof thetestsystem.

boundariesas illustrated in Fig. 53-5 are:ducting and grilles, filters, coil, and fan.

The reasonfor grouping in this manneristhat the pressureandflow ratecharacteristicsof eachdevice probably differ from those ofthe others.

Thesystemincludingtheductingandgrillesis found to be describedquite accuratelybyEq. 1. The sourceof this equation is theDarcy-Weisbachequation:

where

h =fl/DV2/2g 2

f friction factor,1 = length,

D = diameter,V = velocity,g = gravitationalconstant.

In this equation,thefriction factor is obtainedfrom the Moody diagram and is relativelyconstant at the higher Reynolds numbersfound in air-distribution systems.

This phenomenonof constantfriction factor doesnotnecessarilyholdtruefo-r the flowrate friction loss characteristics of other systems,i.e., thecoilsand filters. Thesignificanceof this deviationfrom the form of Eq. 1 isbecomingincreasingly important in systemswhereinthe major contributorsto headlossare these two components.This situationexistswith evenmoderatelyefficient filtrationand deep chilled water cooling coils. It is inthis type of systemthat the classicalsystemcurvebasedon Eq.1 andevenapplicationofthe basicfan laws àan be misleadingduringbothdesignandbalancing.

Cooling coil pressure dropThepublishedcatalogdatafor pressuredropsthroughcooling coils revealthat they do notbehavein accordancewith Eq. 1. The coilsproducedby onemanufacturerwereanalyzed.Theresults obtainedfor wet coilswere foundto closely follow the relation:

where

= KQxc 3

= pressuredrop throughcooling coil,K = coil constant.

234 X / AIR SYSTEMS, ADJUSTING, AND BALANCING

The exponent,X rangedfrom 1.66 to 1.81ratherthan the factor 2 as found in Eq. 1.For greateraccuracy,the coil pressuredropshouldbe expressedas:

Lh = K1Q + K2Qxr2.

However,the simplerform of the equationshould suffice. Thus, if coil manufacturerswould ratecooling coils simply by providingK valuesandX valuesforeachseriesof coils,a designercould correct the available fancurve to account for coil pressuredrop asdiscussed.

Filter air flow resistanceFilters manufacturedof relativelyhigh-resistance tightly woven media havecome intocommon use in large building systems inrecentyears. Thesefilters and cooling coilsoften representthe major air flow resistanceelements.If a filter andits holding or mounting assembly are considered as the filtersystem,it is found that therearetwo distinctcontributorsto air flow resistance:configuration resistanceand mediaresistance.In general, configurationresistancebehavesin accordancewith Eq. 1, and mediaresistancefollows thelaminar relationshipof the HagenPoiseuillelaw:

where

f= 64/Re 5

f = friction factor,Re = Reynoldsnumber.

When combined with the Darcy-Weisbachequation,weobtain:

Lh = K11Q. 6

If thesetwo relationshipswerecoupled,flowresistancefor a filter systemwould be expressedby the equation:

= K11Q + K12Q2. 7

However, with a reasonabledegreeof accuracy and within the discharge rate limitsnormally applied,the relationshipfor filterscanbe simplified to:

Again, a literature searchof currently availablefiltration systemswith efficiencies ranging from 95 to 35 percentrevealedan exponentX valueof from 1.49to 1.70with the

4 higher exponentrelating to filters in whichconfiguration loss predominated over media

loss.Filter manufacturersdo not catalogfilter

pressuredrop versusflow characteristicsforother than cleanfilters-a most unfortunateshortcomingfrom the standpointof applyinga filter to a system.

However, if a systemengineeracceptsagivenpressurelossincreaseas thecriterionforreplacingfilters, this fixed differential canbeconsideredin matchinga filter systemto a fansystemcurve.Sucha curvecouldbedevelopedthat takes these conceptsinto account andprovidesa more accuratepicture.

Combining the dataThe parabolic curve developedfrom Eq. 1that representsthe pressureloss characteristics found in duct systemsSystem I, Fig.53-5 is shownin Fig. 53-6.Thesignificanceingroupingthedatain this manneris asfollows:

* A fansystemmanufacturercould includein catalogdataall aspectsof asystemprovidedasa product.For example,if afanis furnishedwithin a cabinet, the resultantlossescan berepresentedasa depressionin thefancurve.Ifthe coolingcoil is included,this depressioninthe curve can also be shown,and so forth.

Total fanpressure risn

-

Filter 2fouling

ance/

Published fan curve

AhcKc{Qlxc

Fan curveoç-less coil Ahc

= K1 Ic Of

Fun curve less5he and filter

loss h1

FIg. 53-6. A corrected fan curve that reflects thevariablefactorsof coil pressuredrop, filter loss,and

8 fouling allowance.lXhf = KfQXf.

ANALYSIS OF FAN/SYSTEM CHARACTERISTICS AND APPLICATIONS 235

Thus,for the so-calledrooftop units that arecurrently so widely used, a manufacturercould publishan effectivefansystemcurve towhich a systemsengineerneedonly apply thedistribution curve to accurately identify theoperatingpoint.

* With this approach,the common fanlawscould beappliedmoreaccuratelyto assistin systembalancing,since the resulting externalsystemfollows the secondpower parabolic relationshipstatedin Eq. 1.

Consider energy use in designThesensitivityto energyeconomicsin systemsdesignhascreateda major thrustto minimizefan power and resulting energy that hasresultedin lowerfansystempressuresandanincreasing use of variable volume systems.The use of lower system pressurescreatessituationswherein the major pressurelossesare in thecooling coils and the filters, neitherof which characterizesin accordancewith Eq.I, leading to error and misunderstandingboth in the design and balance phases ofsystemdesign. The use of variable volumesystems results in the need for a clearerunderstandingof the behavior of the fanwithin thesystem-bothfor reasonsof energycontrol and systemperformanceundervarying needs.

Into this lattervoid hascomea groundswellof literature and products portraying theenergyconsumptionand control advantagesof controlled variable inlet vanes. In someapplications,these devicesare well applied;however, a designershould be awareof thelimitations of such devices. For example,insteadof comparingthereducedhorsepowerrequirementsfor airflow of an inlet vanedfanat reducedflow to that at full design flow, adesignershould first comparethe full designflow horsepowerof a fan with inlet vanestothe same fan without inlet vanes. In many

cases particularly with smaller fans, thevariableinlet vanesystemmay proveto consumemoreenergyperyear thanthe samefanwith simple dampercontrol. However,well-designedfixed inlet vanesmay prove mostbeneficial from an annualenergystandpointalthoughtheseare very rare in the air conditioning productsmarket.

Standardize product ratingsUnfortunately,theindustryratingsystemsforfan productshave been developedfor purposesof fair comparisonof one product toanother-not for the specific purpose ofmatchinga fan to a distribution and conditioning system.The first effort at the latter isthe AR! cabinetfan ratings, a systemthatshouldbeextendedto theentirefield of manufacturedsubsystems.

Manufacturersof coils, filters, and othersystemcomponentsshouldstandardizeproduct dataon flow versuspressurecharacteristics in terms of systemconstantsK andexponentsX, so that an application orsystemsengineercould eitherdevelopa corrected fan curve to which the systemcurvecould be applied,or an accuratesystemcurveof the form:

= KQ’c + KjQ’ir + KdQ2 9

where

= total systemheadloss,= cooling coil constant,

Kj = filter constant,Kd = distribution systemconstant.

This equationcould then be plotted againstthe publishedfan curve.

By combiningthis actualsystemcurve,thepublishedfan curve, and the mathematicalrelationshipof Eq. 1 with the fan laws, thephenomenonof component matching andsystembalancewould be betterunderstood.