Effects of Altitude on Psychrometric Calculations and Fan Selection ...
Effects of Altitude on Psychrometric Calculations and Fan Selection ...
Effects of Altitude on Psychrometric Calculations and Fan Selection ...
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providing insights for today’s hvac system designerEngineers Newslettervolume 39 –4<str<strong>on</strong>g>Effects</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>Altitude</str<strong>on</strong>g><strong>on</strong> psychrometric calculati<strong>on</strong>s <strong>and</strong> fan selecti<strong>on</strong>sFor this EN we're pulling from thearchives to address a subject thatstill causes c<strong>on</strong>fusi<strong>on</strong> within theindustry <strong>and</strong> c<strong>on</strong>tinues to be thesubject <str<strong>on</strong>g>of</str<strong>on</strong>g> frequently askedquesti<strong>on</strong>s. This EN investigates theeffects <str<strong>on</strong>g>of</str<strong>on</strong>g> altitude <strong>on</strong> psychrometriccalculati<strong>on</strong>s <strong>and</strong> fan selecti<strong>on</strong>s."St<strong>and</strong>ard Air"As altitude increases, the averagebarometric pressure drops <strong>and</strong> airdensity decreases."St<strong>and</strong>ard air" has historically beendefined by ASHRAE as having adensity <str<strong>on</strong>g>of</str<strong>on</strong>g> 0.075 lb/ft 3 , whichequates to air density at sea level(barometric pressure <str<strong>on</strong>g>of</str<strong>on</strong>g> 29.92 in.Hg). The 2009 ASHRAE H<strong>and</strong>book<str<strong>on</strong>g>of</str<strong>on</strong>g> Fundamentals (page 18.13) statesthat this c<strong>on</strong>diti<strong>on</strong> is represented byeither saturated air at 60°F dry bulbor dry air at 69°F dry bulb.Since the performance <str<strong>on</strong>g>of</str<strong>on</strong>g> heating,cooling, <strong>and</strong> air-moving equipment iscomm<strong>on</strong>ly rated at "st<strong>and</strong>ard air"c<strong>on</strong>diti<strong>on</strong>s, cataloged performancedata cannot be used directly forhigher altitude applicati<strong>on</strong>s. Forinstance, at a barometric pressure <str<strong>on</strong>g>of</str<strong>on</strong>g>24 in. Hg (approximately 6000 ftaltitude), cataloged data may be <str<strong>on</strong>g>of</str<strong>on</strong>g>fby as much as 20 to 40 percent.While areas above 6000 ft arestatistically limited, a number <str<strong>on</strong>g>of</str<strong>on</strong>g>states <strong>and</strong> cities have barometricpressures in the range <str<strong>on</strong>g>of</str<strong>on</strong>g> 29 to 27 in.Hg. In this range, cataloged ratings maydiffer from actual c<strong>on</strong>diti<strong>on</strong>s by 3 to 20percent.<strong>Psychrometric</strong> Calculati<strong>on</strong>sThe equati<strong>on</strong>s used in psychrometriccalculati<strong>on</strong>s remain the same for allaltitudes. However, some <str<strong>on</strong>g>of</str<strong>on</strong>g> the factorsused in these equati<strong>on</strong>s are affected byaltitude.The sensible heat gain (Q s ) equati<strong>on</strong> is<str<strong>on</strong>g>of</str<strong>on</strong>g>ten displayed as follows:Q s = 1.085 × cfm × THowever, the 1.085 in this equati<strong>on</strong> isnot a c<strong>on</strong>stant. Rather, it is the product<str<strong>on</strong>g>of</str<strong>on</strong>g> the density () <strong>and</strong> specific heat (C p )<str<strong>on</strong>g>of</str<strong>on</strong>g> the air at "st<strong>and</strong>ard air" c<strong>on</strong>diti<strong>on</strong>s,<strong>and</strong> the c<strong>on</strong>versi<strong>on</strong> factor <str<strong>on</strong>g>of</str<strong>on</strong>g> 60 minutesper hour.Q s = ( × C p × 60 min/hr) × cfm x TThe specific heat for 69°F dry air at sealevel is 0.241 Btu/lb°F. Therefore, at"st<strong>and</strong>ard air" c<strong>on</strong>diti<strong>on</strong>s, theseproperties result in the value 1.085.0.075 lb/ft 3 × 0.241 Btu/lb°F × 60 min/hr= 1.085© 2010 Trane, a business <str<strong>on</strong>g>of</str<strong>on</strong>g> Ingersoll R<strong>and</strong>. All rights reserved. 1
The latent heat gain (Q L ) equati<strong>on</strong> is<str<strong>on</strong>g>of</str<strong>on</strong>g>ten displayed as follows:Figure 1. Air density ratiosQ L = 0.69 × cfm × W (gr/lb)Elevati<strong>on</strong>Sea level1000 ft.2000 ft.3000 ft.4000 ft.5000 ft.6000 ft.7000 ft.Barometer29.92 in. Hg.28.86 in. Hg.27.82 in. Hg.26.81 in. Hg.25.84 in. Hg.24.89 in. Hg.23.98 in. Hg.23.09 in. Hg.However, the 0.69 in this equati<strong>on</strong> isnot a c<strong>on</strong>stant. Rather, it is the product<str<strong>on</strong>g>of</str<strong>on</strong>g> the density <strong>and</strong> latent heat <str<strong>on</strong>g>of</str<strong>on</strong>g>vaporizati<strong>on</strong> (h vap ) <str<strong>on</strong>g>of</str<strong>on</strong>g> the air at"st<strong>and</strong>ard air" c<strong>on</strong>diti<strong>on</strong>s, <strong>and</strong> thec<strong>on</strong>versi<strong>on</strong> factors <str<strong>on</strong>g>of</str<strong>on</strong>g> 60 minutes perhour <strong>and</strong> 7000 grains/lb.Q L = (× h vap × 60 min/hr / 7000 gr/lb)× cfm × WThe latent heat <str<strong>on</strong>g>of</str<strong>on</strong>g> vaporizati<strong>on</strong> for 69°Fdry air at sea level is 1076 Btu/lb.Therefore, at "st<strong>and</strong>ard air" c<strong>on</strong>diti<strong>on</strong>s,these properties result in the value0.69.Air Density Ratio(0.075 lb/ft 3 × 1076 Btu/lb × 60 min/hr) /7000 gr/lb = 0.69The total heat gain (Q T ) equati<strong>on</strong> is<str<strong>on</strong>g>of</str<strong>on</strong>g>ten displayed as follows:Q T = 4.5 × cfm × hHowever, the 4.5 in this equati<strong>on</strong> is nota c<strong>on</strong>stant. Rather, it is the product <str<strong>on</strong>g>of</str<strong>on</strong>g>the density <str<strong>on</strong>g>of</str<strong>on</strong>g> the air at "st<strong>and</strong>ard air"c<strong>on</strong>diti<strong>on</strong>s <strong>and</strong> the c<strong>on</strong>versi<strong>on</strong> factor <str<strong>on</strong>g>of</str<strong>on</strong>g>60 minutes per hour.Air Temperature, ºFQ T = ( × 60 min/hr) × cfm × hFor "st<strong>and</strong>ard air" density, the result isthe value 4.5.0.075 lb/ft 3 × 60 min/hr = 4.5Air at other c<strong>on</strong>diti<strong>on</strong>s <strong>and</strong> otheraltitudes will cause these factors tochange.<strong>Fan</strong>s<strong>Fan</strong>s are c<strong>on</strong>sidered to be c<strong>on</strong>stantvolumedevices. That is, a given fan willdeliver a specific volumetric flow rate(cfm) at a specific fan rotati<strong>on</strong>al speed(rpm). The mass <str<strong>on</strong>g>of</str<strong>on</strong>g> air that the fanmoves at a given speed will vary based<strong>on</strong> the density <str<strong>on</strong>g>of</str<strong>on</strong>g> the air being moved.Air density also changes the staticpressure that the fan will develop <strong>and</strong>the horsepower needed to drive it.<strong>Fan</strong> <strong>and</strong> air h<strong>and</strong>ler manufacturerstypically catalog fan performance dataat "st<strong>and</strong>ard air" c<strong>on</strong>diti<strong>on</strong>s. If theairflow requirement for a givenapplicati<strong>on</strong> is stated at n<strong>on</strong>-st<strong>and</strong>ardc<strong>on</strong>diti<strong>on</strong>s, a density correcti<strong>on</strong> mustbe made prior to selecting a fan.The procedure for selecting a fan atactual altitude (or temperatures) isoutlined in the following steps:1 Determine the actual air density<strong>and</strong> calculate the air density ratio,which is the density at actualc<strong>on</strong>diti<strong>on</strong>s divided by density atst<strong>and</strong>ard c<strong>on</strong>diti<strong>on</strong>s. Figure 1provides a useful chart fordetermining the air density ratiobased <strong>on</strong> altitude <strong>and</strong> airtemperature.Air Density Ratio =2 Divide the design static pressure atactual c<strong>on</strong>diti<strong>on</strong>s by the air densityratio determined in Step 1.SP st<strong>and</strong>ard =SP actualDensity actualDensity st<strong>and</strong>ardAir Density Ratio3 Use the actual design airflow (cfm)<strong>and</strong> the static pressure correctedfor st<strong>and</strong>ard c<strong>on</strong>diti<strong>on</strong>s (see Step 2)to select the fan from theperformance tables/charts <strong>and</strong> todetermine the speed (rpm) <strong>and</strong>horsepower requirement <str<strong>on</strong>g>of</str<strong>on</strong>g> the fanat st<strong>and</strong>ard c<strong>on</strong>diti<strong>on</strong>s.4 The fan speed (rpm) is the same atboth actual <strong>and</strong> st<strong>and</strong>ardc<strong>on</strong>diti<strong>on</strong>s.RPM actual = RPM st<strong>and</strong>ard5 Multiply the input powerrequirement by the air density ratioto determine the actual inputpower required. Power actual = Air Density Ratio X Power st<strong>and</strong>ard2 Trane Engineers Newsletter volume 39–4 providing insights for today’s HVAC system designer
It is important to note that mostpressure-loss charts for othersystem comp<strong>on</strong>ents (such asducts, filters, coils, <strong>and</strong> dampers)are also based <strong>on</strong> st<strong>and</strong>ard airc<strong>on</strong>diti<strong>on</strong>s.SummaryAlthough the wide-scale use <str<strong>on</strong>g>of</str<strong>on</strong>g>computer s<str<strong>on</strong>g>of</str<strong>on</strong>g>tware to select HVACequipment has made the process <str<strong>on</strong>g>of</str<strong>on</strong>g>correcting for altitude simpler, afundamental underst<strong>and</strong>ing is stillimportant to prevent mistakes <strong>and</strong>troubleshoot problems.By Trane Applicati<strong>on</strong>s Engineering. You can findthis <strong>and</strong> previous issues <str<strong>on</strong>g>of</str<strong>on</strong>g> the EngineersNewsletter at www.trane.com/engineersnewsletter. To comment, e-mail us atcomfort@trane.com.View the latest <strong>on</strong>-dem<strong>and</strong>courses:ASHRAE St<strong>and</strong>ard 90.1-2010.Energy-Saving Strategies forRo<str<strong>on</strong>g>of</str<strong>on</strong>g>top VAV Systems.ASHRAE St<strong>and</strong>ard 62.1: Ventilati<strong>on</strong>Rate Procedure.LEED 2009 Modeling <strong>and</strong> EnergySavings.The courses were developed <strong>and</strong> are<str<strong>on</strong>g>of</str<strong>on</strong>g>fered free <str<strong>on</strong>g>of</str<strong>on</strong>g> charge to dem<strong>on</strong>strateTrane’s commitment to sustainabledesign. LEED Accredited Pr<str<strong>on</strong>g>of</str<strong>on</strong>g>essi<strong>on</strong>als(APs) <strong>and</strong> AIA members can participate<strong>and</strong> earn an average <str<strong>on</strong>g>of</str<strong>on</strong>g> 1.5 C<strong>on</strong>tinuingEducati<strong>on</strong> (CE) hours per program.Visit www.trane.com/c<strong>on</strong>tinuingeducati<strong>on</strong> to view allcurrent courses <strong>and</strong> details.3 Trane Engineers Newsletter volume 39–4 providing insights for today’s HVAC system designer
New applicati<strong>on</strong> manualsnow availableCentral Geothermal Design <strong>and</strong>C<strong>on</strong>trol. (SYS-APM009-EN, April 2010)Chilled-Water VAV Systems. (SYS-APM008-EN, August 2009)Chiller System Design <strong>and</strong> C<strong>on</strong>trol.(SYS-APM001-EN, May 2009)Visit www.trane.com/bookstore toorder <strong>and</strong> view a complete list <str<strong>on</strong>g>of</str<strong>on</strong>g>resources.EngineersNewsletterLIVE!To register, c<strong>on</strong>tact yourlocal Trane <str<strong>on</strong>g>of</str<strong>on</strong>g>fice.March 2011Upgrading ExistingChilled-WaterSystemsJune 2011High-PerformanceVAV SystemsOctober 2011DedicatedOutdoor Air UnitsTrane,A business <str<strong>on</strong>g>of</str<strong>on</strong>g> Ingersoll R<strong>and</strong>For more informati<strong>on</strong>, c<strong>on</strong>tact your local Trane<str<strong>on</strong>g>of</str<strong>on</strong>g>fice or e-mail us at comfort@trane.comTrane believes the facts <strong>and</strong> suggesti<strong>on</strong>s presented here to be accurate. However, final design <strong>and</strong>applicati<strong>on</strong> decisi<strong>on</strong>s are your resp<strong>on</strong>sibility. Trane disclaims any resp<strong>on</strong>sibility for acti<strong>on</strong>s taken <strong>on</strong>the material presented.4 Trane Engineers Newsletter volume 39–4 ADM-APN039-EN (December 2010)