Altitude and Air Temperature Correction Factors

Manufacturer HVAC product data reflects performance at standard air conditions.  Standard air conditions are considered to be dry air to 70 degrees Fahrenheit and at sea level atmospheric pressure (.0750 density).  To correct for altitude and/or temperature in Colorado and Wyoming, refer to the correction factors shown in the table below.
Fan Principles

Fan Performance Calculation Adjustments: To adjust manufacturer’s fan performance data, divide static pressure against which the fan will operate by the altitude and air temperature correction factor above.  To determine required power, multiply the indicated brake horsepower by the correction factor to find the power needed.  Note: since a fan is a constant volume mechanism, a fan with move a specific volume of air at a given rotational speed an static pressure independent of air density.

  • Fan Static Efficiency = [Air Volume (CFM) x Static Pressure (inches w.c)]/[6356 x Brake Horsepower (BHP)]
  • Fan Total Efficiency = [Air Volume (CFM) x Total Pressure (inches w.c)]/[6356 x Brake Horsepower (BHP)]

AC Electric Motor Principles

  • Motor RPM = [Electric frequency (Hertz) x 120]/[Number of Poles – Slip]
    • Slip is 2 to 3% in partially to fully loaded motors. With 60 Hertz power, a 4-pole motor operates at 1750 RPM.
  • Efficiency = [746 x Output (Horsepower)]/Input (Watts)
  • Three-Phase Watts = [Volts x Amps x Power Factor x 1.732] (divide answer by 1000 for kilowatts)
  • Single-Phase Watts = [Volts x Amps x Power Factor] (divide answer by 1000 for kilowatts)
  • Horsepower (3-Phase) = [Watts x Efficiency]/746
  • Horsepower (Single Phase) = [Watts x Efficiency/ 746

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