AMETEK Aerospace and Defense
Technical Corner

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Altitude Operation
  • Cooling Effect is directly proportional to air density,
    I.e. number of air molecules available.
  • Air flow must be increased at altitude to provide the necessary molecules, in direct proportion to loss of density.

The selection process:

  • Determine required air flow at the altitude density.
  • Determine your system impedance for this air flow at sea level conditions.
  • Select air mover from sea level test data using 1. & 2. Above
  • Evaluate the density range of operation, as it may be possible to reduce motor size, due to decreased load at altitude.

Under constant speed conditions:

  • Air flow is constant between sea level and any altitude.
  • Static Pressure is also directly proportional to air density.
  • Static Pressure at altitude will be less than at sea level, but flow remains constant because system resistance is decreased by the the same density ratio.
  • Power consumption at altitude is reduced in direct proportion to density, due to the direct reduction of static pressure.
  • Selecting an air mover to meet the altitude cooling needs, always results in excess performance at sea level.
  • Excess weight
  • Excess power consumption
  • Excess acoustic noise
  • Excess wear and tear, reduced life
  • Consider a variable speed or Altivar fan to reduce speed at lower altitudes.
  • Variable speed fans utilize electronic drive and control.
Tech Corner - 5

Density Calculation

r = .75 (Pb / 29.92) X (530 / t)
Where r = density in lb. per cubic foot

t = Rankine temperature of (°F + 460)

Pb = barometric pressure in inches of Hg

For sea level to 29,000 foot altitude:

Pb = 29.873 – 1.049A ­+ .012A2 3% max. error

Where A = altitude in thousands of feet

For 30,000 to 60,000 foot altitude:

Pb = 24.021 – 1.049A + .012A2 3% max. error