Understanding Enthalpy Change in Cooled, Moist Air

[TSN79]

Genrerally it's so that hot air has larger enthalpy value than cold air. But is it also so that if you cool air and add water to it at the same time, the enthalpy will stay the same? How will the temp of the water affect the situation?

 

[Astronuc]

Using the following references:
1) http://en.wikipedia.org/wiki/Specific_heat_capacity#Table_of_specific_heat_capacities for specific heat capacities of N₂ and O₂
2) http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/shegas.html
3) http://en.wikipedia.org/wiki/Water_vapor for eat capacity of H₂O

I found, specific heat capacities as follows:

N₂, 1.04 kJ/kg-K
O₂, 0.92 kJ/kg-K
H₂O, 1.84 kJ/kg-K

Dry air, 1.005 kJ/kg-K
Moist (100% humid) air, 1.03 kJ/kg-K

Water is polyatomic and so has three axes of rotation, and therefore higher specific heat (ref 2), approximately twice that of oxygen.

So since water has higher specific heat, adding a little moisture to air would increase the enthalpy, or maintain constant ethalpy if the temperature decreased somewhat. This however depends on the amount, or rather, partial pressure (mole fraction) of the moisture.

 

[charng]

I can confirm that the statement about hot air having a larger enthalpy value than cold air is generally true. Enthalpy is a measure of the total energy of a system, and hot air has more energy than cold air due to its higher temperature.

However, when it comes to cooling air and adding water at the same time, the enthalpy change will depend on several factors. The temperature of the water is one important factor that can affect the situation.

If the water is at a lower temperature than the air, it will absorb heat from the air and decrease its enthalpy. This is because the water molecules will have a higher energy level than the air molecules, causing the transfer of heat from the air to the water. In this case, the enthalpy of the air will decrease due to the cooling effect of the water.

On the other hand, if the water is at a higher temperature than the air, it will release heat to the air and increase its enthalpy. This is because the water molecules will have a lower energy level than the air molecules, causing the transfer of heat from the water to the air. In this case, the enthalpy of the air will increase due to the heating effect of the water.

Therefore, the temperature of the water does play a significant role in determining the enthalpy change of cooled, moist air. Other factors such as the humidity and pressure of the air and the amount of water added will also impact the enthalpy change. It is important to take all of these factors into consideration when studying and understanding enthalpy changes in cooled, moist air.