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  • Home
    • What's New
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  • SketchUp Models
    • SketchUp Resources
    • Bureaucratic Affairs Building HHW System Logic Exercise
    • Bureaucratic Affairs Building Scavenger Hunt
    • Chilled Water Plant System Diagram Exercise
    • The HIjend Hotel >
      • Chilled Water Plant Scoping Exercise
      • Ballroom AHU Scoping Exercise
      • Cooling Tower Scoping Exercise
  • Tools
    • Altitude Correction Factor
    • Economizer Evaluation Checklist
    • Eikon for Educators and WindLGC
    • Effective Duct Length Tool
    • Excel Third Axis Tool
    • Excel Time Value Conversion
    • Insulation Savings Tools
    • Logic Diagram Tool >
      • Logic Diagram Example - Discharge Air
      • Logic Diagram Example - CHW Plant 01
      • Logic Diagram Example - VAV AHU
    • Mixed Air Calculations
    • Monitoring Plan Spreadsheet
    • Pipe Friction Chart
    • Plot Digitizer >
      • Plot Digitizer Pump Curve Example
    • Square Law Spreadsheet
    • System Diagram Symbols
    • Thermodynamic Diagrams Spreadsheet
    • Universal Translator Data Analysis Tool
    • y = (m * x) + b Spreadsheet
  • Useful Formulas
    • Affinity Laws
    • Circular Equivalent Duct
    • Effective Duct Length
    • FT Guide Appendix C
    • HVAC Equations and Concepts
    • Hydraulic Diameter
    • Pump Power and Energy
    • "Square Law"
  • What's That Thing?
    • Cooling Towers
    • Pumps
    • Valves
  • Resources
    • Bill Coad's Writings
    • Data Logging Resources
    • EBCx Skills Guidebook
    • Energy Design Resources
    • Fisher Controls Valve Cavitation Bulletin
    • Functional Testing Guide
    • Honeywell Gray Manual
    • MCC Powers Bulletins
    • NBCIP Reports
    • PEC Tool Lending Library
    • PID Resources
    • Pneumatic Control Resources
    • Resource List
    • Scoping Resources
    • TAB Resources
    • Vintage Carrier Design Manual
    • VRF Systems
    • Williams' Wisdom
  • Videos
    • Bureaucratic Affairs HHW Logic Answers
    • Cooling Tower Flow Variation
    • Economizer Stratification
    • Functional Testing
    • Induction Principles
    • Monitoring Plans
    • CW Pump Deadheaded by Tower Lift
    • Ripple Effects
    • Testing a Pump
    • The Garden of Low Entropy
    • Variable Flow Systems
  • Training
    • Materials from Classes and Presentations >
      • ACEEE
      • ASHRAE
      • ASHRAE - Engineers Notebook
      • BCxA and NCBC
      • BEST Institute
      • Case Studies
      • ERDC 2020 RCx Academy
      • ELPNW
      • ICEBO
      • Marriott AEP
      • Magazine Articles
      • NAVFAC
      • Pacific Energy Center Design, Performance and Commissioning Issues Classes
      • Pacific Energy Center EBCx Project Review
      • Pacific Energy Center EBCx Workshop Series >
        • EBCx Workshop Series 12
        • EBCx Workshop Series 13
        • EBCx Workshop Series 14
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        • EBCx Workshop Series 16
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      • Portland General Electric
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    • On Demand Training >
      • Benchmarking UCA and Scoping
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      • Induction Motor Principles
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Altitude Correction Factor Spreadsheet

Many of the equations we use in HVAC calculations involve air flow and virtually all of the equations we use in HVAC calculations have a units conversion constant in them.   For instance, in the equation for sensible load ...
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... the 1.08 is the units conversion constant, which allows us to multiply a flow rate with units of cubic feet per minute by a temperature difference in °F and come up with Btu's per hour.
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The problem is that the units conversion constant in this case includes a density term (that's part of how we convert cubic feet per minute to pounds per hour), and a specific heat term (that's how we convert a temperature difference into a quantity of energy per pound of air).  And, as you can see from the chart above, both of those metrics vary with temperature.

In other words, units conversion constants are not always constant.  
If you are interested in how the 1.08 in the equation above is derived and the impact changes in the properties of air can have on it, that is discussed in a post on our blog titled Unit Conversion Constants; Not Always Constant. 

The point here is that the constant isn't constant and for air, most of the time, the factor that impacts the units conversion constant the most in HVAC applications is the variation in density with altitude.  Obviously, as you can see from the graph, temperature impacts the density of air, especially moist air, even at a constant altitude.  But the impact at the temperatures typically seen in HVAC systems is less significant than the impact of altitude.
 

Towards that end, this spreadsheet has a calculator in it that creates a correction factor to be used with common HVAC equations when you apply them at higher elevations;  Denver, for instance.  The spreadsheet assumes the profile associated with a "standard atmosphere" and also includes:
  • The data behind the chart above as well as the chart
  • The derivation of the units conversion constant in the equation above for a number of different conditions
  • A table with the composition of dry air 
​Note that when you download the spreadsheet, you will need to strip out the descriptive name and parenthesis around the file name to get it to run properly on some systems.  For instance, for this file, you would need to eliminate the parenthesis and the words Air Density Correction with Altitude Spreadsheet from the file name.  You can also change the file name to something that works better for you if you want as long as you keep the .xlsx file extension.
Altitude Correction Factor Spreadsheet (standard_atmosphere_vblog.xlsx)
File Size: 102 kb
File Type: xlsx
Download File

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