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Chapter 11 Gas-Vapor Mixtures and Air Conditioning

Chapter 11 Gas-Vapor Mixtures and Air Conditioning. 11-1 Dry and Atmospheric Air. 11-1-1 Atmospheric air. The air in the atmosphere normally contains some water vapor and is referred to atmospheric air. Air that contains no water is called dry air.

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Chapter 11 Gas-Vapor Mixtures and Air Conditioning

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  1. Chapter 11Gas-Vapor Mixtures and Air Conditioning

  2. 11-1 Dry and Atmospheric Air 11-1-1 Atmospheric air The air in the atmosphere normally contains some water vapor and is referred to atmospheric air. Air that contains no water is called dry air. Although the amount of water vapor in the air is small, it plays a major role in human comfort. Therefore it is an important consideration in air conditioning applications The dry air and vapor of atmospheric air in air conditioning application range (temperature changes from -10 ℃ to 50 ℃) can be treated as ideal gas

  3. 11-1-2 The pressure of atmospheric air The total pressure of atmospheric air The partial pressure of vapor, increases with the amount of vapor in air The partial pressure of dry air

  4. T p2 pmax p1 T1 s

  5. T p1 1 T1 Dew- point 11-1-3 Dew-point Temperature As to vapor state 1, decrease the temperature , then pressure of vapor won’t change

  6. The dew-point Tdp is defined as the temperature at which condensation begins if the air is cooled at constant pressure

  7. 11-2 The properties of air 11-2-1 Specific humidity of air The ratio of the mass of water vapor present in a unit mass of dry air, denoted by ω Ra= 287.1 J/kg.K Rv= 461.4 J/kg.K Since Vv=Va and Tv=Ta

  8. 11-2-2 Relative humidity of air The comfort level depends more on the amount of moisture the air holds(mv) relative to the maximum amount of moisture the air can hold at the same temperature(mg). The ratio of these two quantities is called the relative humidity The saturated pressure under the temperature of atmospheric air

  9. 11-2-3 Enthalpy of air Specific enthalpy:

  10. 1.863kJ/kg. ℃ is the average specific heat of dry air In the field of air-conditioning, the reference temperature is 0℃. Then: ha= 1.005t kJ/kg.℃ hv= 2501 + 1.863t kJ/kg.℃ 2501kJ/kg means the enthalpy at 0 ℃ 1.863kJ/kg. ℃ is the average specific heat of vapor h= 1.005t + ω( 2501 + 1.863t) kJ/kg.℃

  11. 11-3 Adiabatic Saturation and Wet-Bulb Temperature Unsaturated air Saturated air t1, ω1, f1 t2, ω2, f2=100% hf 11-3-1 Adiabatic saturation

  12. Conservation of mass Conservation of energy

  13. Dividing by gives:

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