Page 61 - Fluid Power Engineering

Properties of W ind 39

Molecule Percent in Dry Air, % Molecular Weight
78.1 28
N 2
20.95 32
O 2
Ar 0.934 18
0.038 44
CO 2
TABLE 3-7 Composition of Air and the Molecular Weight of the Components

proportional to air density, there will bea2or17.8% drop in power
depending on elevation of the site. The change in density measured
at the ground level versus density at a 100 m rotor hub is less than 1%.

Density of Air as a Function of Humidity
The composition of dry air is in Table 3-7. The average molecular
weight of dry air is 28.9644 g/mole. When moisture is added to air,
H 2 O of molecular weight 18 displaces heavier molecules in dry air.
Therefore, moist air is lighter and has lower density—a counterintu-
itive conclusion. So, moist air delivers less power.
The density of mixture of dry air and water vapor is: 5,6

p d p v p 0.378p v
ρ = + = 1 − (3-19)
R d T R v T R d T p

Relative Humidity, % Density, kg/m 3
0 1.225012
10 1.224233
20 1.223454
30 1.222674
40 1.221895
50 1.221116
60 1.220337
70 1.219557
80 1.218778
90 1.217999
100 1.217219
TABLE 3-8 Air Density as a Function of Relative Humidity for p = 101,325
Pa, T c = 15 C
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