I have a 4700 gallon tank and am looking to create a table of the water content by mass in the tank given relative humidity %. The tank is held at 70 degC and 18 psig, and the only thing inside the tank is normal air at varying humidity %.

I believe this has something to do with partial pressures and steam tables.

  • $\begingroup$ Is the tank holding liquid water and you want to calculate the amount of liquid in the tank or do you want to calculate the amount of water vapor in the head space? $\endgroup$
    – J. Ari
    Jun 22 '17 at 20:41

Partial pressures: From wikipedia

partial volume equals total volume times partial pressure over total pressure.

At 70 degrees, saturation vapour pressure is 4.5psi (from tables). Absolute pressure of tank is 18 + 14.7psi = 32.7.

Partial volume comes out as 646.8 US Gallons, 2.45m^3. Density of water vapour is 0.2 kg/m^3 (from tables) at 70C, which comes out to a maximum of 490g of water in the air.

As relative humidity is just actual over saturation density, your going to have a straight line graph http://www.engineeringtoolbox.com/density-air-d_680.html


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