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- 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:
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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
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for sea level to 29,000 foot altitude:
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Pb = 29.873 - 1.049A + .012 A2 ; 3% max. error |
(eq.5) |
for 30,000 to 60,000 foot altitude:
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Pb = 24.021 - 1.049A + .012 A2 ; 3% max. error |
(eq.6) |
| where |
A = altitude in thousands of feet |
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