Dry air and water vapor combine to form atmospheric air. The pressure of the dry air and water vapor pressure together make up the total pressure of the air mixture. Specific humidity is the ratio of mass of water vapor to total mass of air, while relative humidity is the ratio of actual water vapor pressure to saturation water vapor pressure. The enthalpy of atmospheric air depends on the enthalpies of dry air and water vapor as well as their relative quantities based on specific and relative humidity.

1. THERMODYNAMICS II
LECTURE 15 (PART a)
VAPOR-GAS MIXTURES AND AIR
CONDITIONING
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2. Dry and Atmospheric Air
• Dry Air + Water vapours = Atmospheric Air
• In range of -10 – 50 OC, water vapour is ideal gas
• Pressure of dry air (Pa) and water vapour
pressure (Pv) = Pressure of mixture
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3. Dry and Atmospheric Air
• In this temperature range,
• Therefore enthalpy of water vapour is
taken as enthalpy of saturated vapour :
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4. Dry and Atmospheric Air
• Average Cp value in range of -
10 to 50 oC is 1.82 KJ/kg-K
• Therefore
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5. Specific and Relative Humidity Of Air
• Specific or absolute humidity :
• Saturated air!
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6. Specific and Relative Humidity Of Air
• Relative Humidity
• Saturated air, RH = 100%
• Combing SH and RH, We have:
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7. Enthalpy of Atmospheric Air
• In absolute terms:
• Diving by mass of dry air
• We have Enthalpy per unit mass of
dry air:
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