Psychrometry is the study of air-water vapor mixtures. It is important for air conditioning design to understand the properties of atmospheric air, which is a mixture of gases, water vapor, and pollutants. Specific humidity refers to the mass of water associated with each kilogram of dry air. Key psychrometric processes used in air conditioning include: 1) Sensible cooling, where air temperature decreases over a cooling coil while moisture content remains constant. 2) Heating and humidification, where air is first sensibly heated and then water vapor is added through steam nozzles. 3) Cooling and humidification, where air temperature and humidity both increase by spraying cool water into the air stream

1. Psychrometry
Psychrometric Properties Atmospheric air makes up the environment in
almost every type of air conditioning system. Hence a thorough
understanding of the properties of atmospheric air and the ability to
analyze various processes involving air is fundamental to air conditioning
design.
Psychrometry is the study of the properties of mixtures of air and water
vapour. Atmospheric air is a mixture of many gases plus water vapour
and a number of pollutants.
The amount of water vapour and pollutants vary from place to place. The
concentration of water vapour and pollutants decrease with altitude, and
above an altitude of about 10 km, atmospheric air consists of only dry air.
The pollutants have to be filtered out before processing the air. Hence,
what we process is essentially a mixture of various gases that constitute
air and water vapour. This mixture is known as moist air. Fig. Atmospheric
air The molecular weight of dry air is found to be 28.966 and the gas
constant R is 287.035 J/kgK. The molecular weight of water vapour is
taken as 18.015 and its gas constant R is 461.52 J/kgK.
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2. Specific Humidity or Humidity Ratio The humidity ratio (or specific
humidity) W is the mass of water associated with each kilogram
of dry air. Assuming both water vapour and dry air to be perfect
gases, the humidity ratio is given by:
Substituting the values of gas constants of water vapour and air Rv
and Ra in the above equation; the humidity ratio is given by:
Specific humidity or absolute humidity or humidity ratio (w) =
If Pa and Pv denote respectively the partial pressure of dry air and that of
water vapour in moist air, the specific humidity of air is given by
(We know Pa = Pb – Pv)
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3. Where ф is the relative humidity (in fraction). DBT and DPT are in 0C. Of course, since
from it is definition, the dew point temperature is the saturation temperature
corresponding to the vapour pressure of water vapour. 3

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7. In case the relative humidity of air is 100% (saturated air) then
• Dry bulb temperature
• wet bulb temperature
• dew point temperature
• Saturation temperature will be equal
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8. Adiabatic Saturation of Air and Adiabatic Saturation
Temperature
Adiabatic saturation temperature is defined as that
temperature at which water, by evaporating into air, can
bring the air to saturation at the same temperature
adiabatically. An adiabatic saturator is a device using which
one can measure theoretically the adiabatic saturation
temperature of air. After the adiabatic saturator has achieved
a steady-state condition, the temperature indicated by the
thermometer immersed in the water is the thermodynamic
wet-bulb temperature. The thermodynamic wet bulb
temperature will be less than the entering air DBT but
greater than the dew point temperature.
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17. Basic Processes in Conditioning of Air
The processes which effect the psychrometric properties
of air are called psychrometric processes.
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19. Sensible cooling
During this process, the moisture content of air remains constant but its temperature
decreases as it flows over a cooling coil. For moisture content to remain constant the
surface of the cooling coil should be dry and its surface temperature should be greater
than the dew point temperature of air. If the cooling coil is 100% effective, then the exit
temperature of air will be equal to the coil temperature. However, in practice, the exit air
temperature will be higher than the cooling coil temperature. Figure below shows the
sensible cooling process O–A on a psychrometric chart. The heat transfer rate during this
process is given by:
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22. Heating and Humidification (Process O-D)
During winter it is essential to heat and humidify the room air for comfort. As
shown in the figure, it is normally done by first sensibly heating the air and then
adding water vapour to the air stream through steam nozzles as shown in the
figure.
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27. Cooling and humidification (Process O–E)
As the name implies, during the process, the air temperatures drops and its
humidity increases. This process is shown in Figure show in below. As shown in
the figure, this can be achieved by spraying cool water in the air stream. The
temperature of water should be lower than the dry bulb temperature of air but
higher than its dew-point temperature to avoid condensation (TDPT < TW < TO).
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29. Air Conditioning
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