Could someone explain why the Specific Heat Capacity (SHC) of dry air is 1.005kJ/kg.K whereas the accepted SHC for ventilation air is quoted 1.300kJ/kg.K? Assuming the air for the ventilation calc’s is humid say 60RH @20°C it is estimated that there would be about 8.8g/kg of moisture in the air. [Using the psychrometric chart from the British Standards Institute] The SHC of water is 4.200kJ/kg.K = 4.2J/g The SHC of water vapour is 1.86kJ/kg.K = 1.86J/g If there are about 9g of moisture (water vapour) in the air; 9 x 1.86 = 16.74J of energy will be required. The water molecules in vapour form will displace the dry air molecules therefore the dry air SHC will be less than 1.005kJ/kg.K. If dry air is about 1kg/m3 and the water vapour has displaced some of the dry air molecules then the SHC of the humid air (dry air + water vapour) is going to be less than 1.300kJ/kg.K. To convert H2O from liquid to gas (water to water vapour) requires 2.3kJ/g of latent heat which is more energy than is required to raise 1kg of water 1K.
Also is there any relationship between atomic mass and SHC? As air is mainly Nitrogen (14 atomic number) @ 78%, and Oxygen (16) 21% [+ 1% of various other gases]. Both the main gases are diatomic: Dry Air = (0.78 x (2x14)) +( 0.21 x (2x16)) = 28.56 + Argon and CO2 = 28.946 atomic mass per mole. Water = Hydrogen (2x1) + Oxygen (16) = 18.00 or there about due to impurities per mole. The argument that humid air is less dense than dry air can be deduced from the above.
If a mole of a gas at standard conditions has a volume of 22.4ltrs and 1000 ltrs = 1m3 therefore 1000/22.4 = 44.64. (moles in 1m3) 44.64 x 28.95 = 1292 which is close to the 1.300kJ/kg.K [ventilation air however it would be dry and not 60RH]. Deduct 1 mole from dry air (1000 – 22.4 = 978ltrs) 978/22.4 = 43.66 moles. (43.66 x 28.95) + (1 x 18.00) = 1.281kJ/kg.K. The SHC of humid air approx’ 1m3? Virtually the same as dry air.
Or is it coincidental? Any help would be appreciated - thanks