a course used for describing the gas vapor mixture in thermodynamics

1. PART TWO
Air Conditioning
GAS VAPOR MIXTURES

2. Gas Vapor mixtures
• Gas vapor mixtures involve gases close to
condensation. Previous sections involved gases that are
above their critical temperatures and, thus, no
possibility of condensation during processes.
• Dealing with gas vapor mixtures is different from the
simple analysis we have been dealing with so far.
• It is important in different air conditioning concepts
and analysis.

3. Dry and Atmospheric Air
• Air is a mixture of Nitrogen, Oxygen and small amount of
other gases.
• In this context, air is referred to as dry air if it does not
contain moisture and as atmospheric air if it contains
moisture.
• Thus, atmospheric air is a mixture of dry air and moisture. In
this regard, moisture is water vapor, i.e. superheated steam.
• During processes, the amount of moisture in the air may
reduce or increase while the amount of dry air remains
constant. This is due to condensation and evaporation.

4. Dry and Atmospheric Air
• The amount of moisture in air significantly affects human
comfort. Thus, air-water vapor mixture analysis is
important in air conditioning applications.
• In air conditioning analysis, both dry air and moisture in
air can be treated as ideal gases.
• PONDER POINT: Can we consider both dry air and
moisture as ideal gas? When can we consider water
vapor as ideal gas?

5. Dry and Atmospheric Air
• From Dalton’s law,
• The total pressure of a mixture of gases is the sum of the
partial pressures.
• Thus, for air-water vapor mixtures,
• P = Pa + Pv where Pv is the partial pressure of water vapor referred
to as vapor pressure.

6. Specific and Relative Humidity
• Due to the importance of the amount of water vapor in air, it has to be
quantified.
• Absolute or Specific Humidity (Humidity Ratio)
• Is the ratio of the mass of water vapor present in a unit mass of dry air.
𝜔 =
𝑚𝑣
𝑚𝑎
=
0.622𝑃𝑣
𝑃 − 𝑃𝑣
• If water is added to dry air continually, there comes a point where more water
can not be carried by the air. At this point, the air is referred to as saturated air.
More introduction of water vapor into this saturated air results in condensation.
• PONDER POINT: Derive the absolute humidity relation using its definition and the
ideal gas equation.

7. Specific and Relative Humidity
• As importance as specific humidity is, relative humidity play a
greater role in deciding human comfort.
• Relative Humidity is the ratio of the mass of water vapor in the air
to the mass of water vapor of saturated air at the same
temperature and pressure.
• Mathematically,
𝜙 =
𝑚𝑣
𝑚𝑔
=
𝑃𝑣
𝑃
𝑔
• Ponder Point: Derive the relative humidity relation based on its
definition and ideal gas relation.

8. Specific and Relative Humidity
• The specific humidity and relative humidity relations can be
combined into:
𝜔 =
0.622𝜙𝑃𝑔
𝑃 −𝜙𝑃𝑔
and
𝜙 =
𝜔𝑃
(0.622 − 𝜔)𝑃
𝑔
• Relative humidity ranges from 0 to 1.
• The relative humidity can change with temperature while the
specific humidity remains the same.

9. Specific and Relative Humidity
• The enthalpy of atmospheric air is expressed as the
sum of the enthalpies of dry air and water vapor.
𝐻 = 𝐻𝑎 + 𝐻𝑣 = 𝑚𝑎ℎ𝑎+𝑚𝑣ℎ𝑣
• Expressing total enthalpy in terms of enthalpy per
unit dry air (because the amount of dry air remains
constant), gives
ℎ = ℎ𝑎+
𝑚𝑣
𝑚𝑎
ℎ𝑣 = ℎ𝑎+𝜔ℎ𝑣

10. Dew Point Temperature
• It is the temperature at which condensation starts
when air is cooled at constant pressure. Thus, it’s the
saturation temperature at the vapor pressure.
• Further cooling of the air results in condensation.
This process follows a 100% relative humidity.

11. Adiabatic Saturation Temperature
• The wet-bulb temperature can be measured by a
thermometer whose bulb has been covered by a wet
cotton wick.
𝜔1 =
𝐶𝑝 𝑇2 − 𝑇1 − 𝜔2ℎ𝑓𝑔2
ℎ𝑔1 − ℎ𝑓2

12. Psychrometer

13. Psychrometry
• What is it?
• The psychrometric chart
• Psychrometric processes

14. Psychrometric chart
• The state of air at a specified pressure is completely
specified by two independent intensive properties.
• The rest can be easily calculated or read from charts.
These charts are known as psychrometric charts.