Heat Capacity of Water Vapor in Atmosphere?

[brandontlee]

The atmosphere consists of two components; dry air and water vapor, the pressure of a typical room is the sum of these individual constituent's pressures. The room condition is at 22 celcius with a relative humidity of 50% at 101.325 KPa. The room in this condition will contain dry air and water vapor, but this water vapor will be superheated because the partial pressure of the liquid constituent (water) is less than the saturation pressure of water at the specified room conditions.

So the water vapor exists in the room as superheated steam. How can I find the Heat Capacity of Water Vapor at 22 celcius and 101.325 KPa? I seem to only have tables for super heated steam above 100 celcius at atmospheric pressure.

Any insight would be appreciated, Thanks!

 

[pmover]

your observations are good.

for most analysis of moist air, use Cp=0.24 Btu/lbm-°F.

-pmover

 

[kmpillai]

Dear friend,
Specific heat(Heat Capacity) is function of temperature and very weak function of pressure. Some literaures are avaiable for the specific heat values of water vapour at low temperature ranges you mentioned. I am just giving the values of specific heat of water vapour at constant pressure (Cp) for the temperatures of your interest.
Cp at 200 K = 33.34 kJ/kmol K
Cp at 293 K = 33.56 kJ/kmol K
Cp at 298 K = 33.58 kJ/kmol K
For other temperatures in between these temperature limits the above values can be interpolated, but variation is small in these ranges.

Regards,
KMPillai

 

[quark]

Saturation vapor pressure at 22⁰C is 0.0264534 bar. With 50% RH, actual vapor pressure will be 0.0264534/2 = 0.0132267 bar.

At 22⁰C temp. and 0.132267 bar pre. the specific heat(Cp) is 1.87935 kJ/Kg⁰C and Cv is 1.41409kJ/Kg⁰C .

Download the free Steamtab companion from the link below. It is a very useful tool which will give you good lot of info on water and steam properties at saturated, subcooled and superheated conditions.

http://www.chemicalogic.com/steamtab/companion/default.htm

If it for sensible heat calculation of air, you can use the value given by pmover or the simplified version of the equation Q(Btu/hr) = 1.08 x cfm x dT(⁰F)

Regards,

Believe it or not : Eratosthenes, a 3rd century BC true philologist, calculated circumference of earth with the help of a stick and it's shadow. The error was just 4% to the present day calcuated value.