Lebanese International University Faculty of Engineering
LIU Mechanical Department
HVAC – Chapter 3 – Moist air
properties and conditioning
processes
Part 1 of 2
Introduction
• The ability to analyze the various processes involving moist atmospheric air
is basic to the HVAC engineer.
• Atmospheric air makes up the environment in almost every design situation,
and psychrometrics deals with the properties of moist air.
• Fundamental concepts and common moist air processes are covered
followed by various combinations of processes used for space air
conditioning.
• Both design and off-design conditions are considered.
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3.1 – Moist air and the standard
Atmosphere
• Atmospheric air is a mixture of many gases plus water vapor and countless
pollutants.
• Aside from the amount of water vapor and pollutants, which may vary
considerably, the composition of the gases making up dry air is nearly
constant, varying slightly with time, location, and altitude.
• In 1949, a standard composition of dry air was fixed
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3.1 – Moist air and the standard
Atmosphere
The gas constant Ra is:
The molecular mass Ma of dry air is Ma 28.965
The molecular mass of water is 18.015 and the gas constant for water vapor is:
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3.2 – Fundamental Parameters
a. Pressure
• Moist air up to about three atmospheres pressure obeys the perfect gas law
with sufficient accuracy for most engineering calculations.
• The Dalton law for a mixture of perfect gases states that the mixture
pressure is equal to the sum of the partial pressures of the constituents.
• For moist air:
b. Humidity ratio
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3.2 – Fundamental Parameters
c. Relative humidity
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3.2 – Fundamental Parameters
d. Relation between humidity ratio and relative humidity
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3.2 – Fundamental Parameters
e. Dew point
• The dew point td is the temperature of saturated moist air at the same
pressure and humidity ratio as the given mixture.
• As a mixture is cooled at constant pressure, the temperature at which
condensation first begins is the dew point.
• At a given mixture (total) pressure, the dew point is fixed by the humidity
ratio W or by the partial pressure of the water vapor. Thus td, W, and Pv are
not independent properties.
f. Enthalpy
• The enthalpy i of a mixture of perfect gases is equal to the sum of the
enthalpies of each constituent:
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3.2 – Fundamental Parameters
f. Enthalpy (continued)
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3.2 – Fundamental Parameters
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3.4 –Psycrometrics
• A psychrometer consists of two thermometers, or other temperature-sensing
elements, one of which has a wetted cotton wick covering the bulb.
• The temperatures indicated by the psychrometer are called the wet bulb and
the dry bulb temperatures.
• To facilitate engineering computations, a graphical representation of the
properties of moist air has been developed and is known as a psychrometric
chart.
• Charts 1a, 1b, 1Ha, and 1Hb for sea level, and 5000 ft (1500 m) elevations
in English and SI units are provided in Appendix E.
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3.4 –Psycrometric chart - SI
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3.4 –Psycrometric chart - IP
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3.4 – Wet bulb temperature and the
psycrometric chart
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