Environmental Engineering Reference
In-Depth Information
Here:
L w. is.a.sound.power.level.equal.to.102;
N.is.the.distance.from.the.turbine.tower,.in.meters;
α.is.a.sound.absorption.coeficient.α.=.0.005.dB/m
7.5.1 
Density, Temperature, and Pressure Correlation
The.density.of.dry.air.can.be.calculated.using.the.ideal.gas.law,.expressed.as.
a.function.of.temperature.and.pressure:
p
R T
ρ =
.
.
(7.11)
i
where:
ρ.is.the.air.density;
p .is.absolute.pressure;
R .is.the.speciic.gas.constant.for.dry.air;
T .is.absolute.temperature.
The.speciic.gas.constant.for.dry.air.is.287.058.J/(kg·K).in.SI.units,.and.53.35.
(ft·lbf)/(lbm·R).in.U.S..customary.and.Imperial.units. 6
Therefore:
•. At.IUPAC.standard.temperature.and.pressure.(0°C.and.100.kPa),.dry.
air.has.a.density.of.1.2754.kg/m 3 .
•. At.20°C.and.101.325.kPa,.dry.air.has.a.density.of.1.2041.kg/m 3 .
•. At.70°F.and.14.696.psia,.dry.air.has.a.density.of.0.074887.lbm/ft3.
Table  7.1. illustrates. the. air. density-temperature. relationship. at. 1. atm. or.
101.325.kPa:
7.5.2 
Water Vapor
The.addition.of.water.vapor.to.air.(making.the.air.humid).reduces.the.den-
sity.of.the.air,.which.may.at.irst.appear.contrary.to.logic. 7
This. occurs. because. the. molecular. mass. of. water. (18. g/mol). is. less. than.
the.molecular.mass.of.dry.air.(around.29.g/mol)..For.any.gas,.at.a.given.tem-
perature.and.pressure,.the.number.of.molecules.present.is.constant.for.a.par-
ticular. volume. (see. Avogadro's. law).. So,. when. water. molecules. (vapor). are.
added.to.a.given.volume.of.air,.the.dry.air.molecules.must.decrease.by.the.
same.number.to.keep.the.pressure.or.temperature.from.increasing..Hence,.
the.mass.per.unit.volume.of.the.gas.(its.density).decreases.
The.density.of.humid.air.may.be.calculated.as.a.mixture.of.ideal.gases..In.
this.case,.the.partial.pressure.of.water.vapor.is.known.as.the.vapor.pressure..
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