Effects of Altitude on Psychrometric Calculations and Fan Selection ...

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The latent heat gain (Q L ) equation is<strong>of</strong>ten displayed as follows:Figure 1. Air density ratiosQ L = 0.69 × cfm × W (gr/lb)ElevationSea 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 equation isnot a constant. Rather, it is the product<strong>of</strong> the density and latent heat <strong>of</strong>vaporization (h vap ) <strong>of</strong> the air at"standard air" conditions, and theconversion factors <strong>of</strong> 60 minutes perhour and 7000 grains/lb.Q L = (× h vap × 60 min/hr / 7000 gr/lb)× cfm × WThe latent heat <strong>of</strong> vaporization for 69°Fdry air at sea level is 1076 Btu/lb.Therefore, at "standard air" conditions,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 ) equation is<strong>of</strong>ten displayed as follows:Q T = 4.5 × cfm × hHowever, the 4.5 in this equation is nota constant. Rather, it is the product <strong>of</strong>the density <strong>of</strong> the air at "standard air"conditions and the conversion factor <strong>of</strong>60 minutes per hour.Air Temperature, ºFQ T = ( × 60 min/hr) × cfm × hFor "standard air" density, the result isthe value 4.5.0.075 lb/ft 3 × 60 min/hr = 4.5Air at other conditions and otheraltitudes will cause these factors tochange.FansFans are considered to be constantvolumedevices. That is, a given fan willdeliver a specific volumetric flow rate(cfm) at a specific fan rotational speed(rpm). The mass <strong>of</strong> air that the fanmoves at a given speed will vary basedon the density <strong>of</strong> the air being moved.Air density also changes the staticpressure that the fan will develop andthe horsepower needed to drive it.Fan and air handler manufacturerstypically catalog fan performance dataat "standard air" conditions. If theairflow requirement for a givenapplication is stated at non-standardconditions, a density correction 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 densityand calculate the air density ratio,which is the density at actualconditions divided by density atstandard conditions. Figure 1provides a useful chart fordetermining the air density ratiobased on altitude and airtemperature.Air Density Ratio =2 Divide the design static pressure atactual conditions by the air densityratio determined in Step 1.SP standard =SP actualDensity actualDensity standardAir Density Ratio3 Use the actual design airflow (cfm)and the static pressure correctedfor standard conditions (see Step 2)to select the fan from theperformance tables/charts and todetermine the speed (rpm) andhorsepower requirement <strong>of</strong> the fanat standard conditions.4 The fan speed (rpm) is the same atboth actual and standardconditions.RPM actual = RPM standard5 Multiply the input powerrequirement by the air density ratioto determine the actual inputpower required. Power actual = Air Density Ratio X Power standard2 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 components (such asducts, filters, coils, and dampers)are also based on standard airconditions.SummaryAlthough the wide-scale use <strong>of</strong>computer s<strong>of</strong>tware to select HVACequipment has made the process <strong>of</strong>correcting for altitude simpler, afundamental understanding is stillimportant to prevent mistakes andtroubleshoot problems.By Trane Applications Engineering. You can findthis and previous issues <strong>of</strong> the EngineersNewsletter at www.trane.com/engineersnewsletter. To comment, e-mail us atcomfort@trane.com.View the latest on-demandcourses:ASHRAE Standard 90.1-2010.Energy-Saving Strategies forRo<strong>of</strong>top VAV Systems.ASHRAE Standard 62.1: VentilationRate Procedure.LEED 2009 Modeling and EnergySavings.The courses were developed and are<strong>of</strong>fered free <strong>of</strong> charge to demonstrateTrane’s commitment to sustainabledesign. LEED Accredited Pr<strong>of</strong>essionals(APs) and AIA members can participateand earn an average <strong>of</strong> 1.5 ContinuingEducation (CE) hours per program.Visit www.trane.com/continuingeducation to view allcurrent courses and details.3 Trane Engineers Newsletter volume 39–4 providing insights for today’s HVAC system designer
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Effects of Altitude on Psychrometric Calculations and Fan Selection ...

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