Aerospace and Defense - Altitude Operation

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.
Altivar fans use "high-slip" induction motors that slip or sag in speed under the increased load of high density air.

DENSITY CALCULATION:

	r = .075 ( Pb / 29.92) X (530 / t)	(eq.4)
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 + .012 A2 ; 3% max. error

(eq.5)

for 30,000 to 60,000 foot altitude:

	Pb = 24.021 - 1.049A + .012 A2 ; 3% max. error	(eq.6)
where	A = altitude in thousands of feet