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Correlation of dew point of dry air
2

Correlation of dew point of dry air

Correlation of dew point of dry air

(OP)
I have a plant with a quoted ideal loading of 50mg/Nm3 of moisture in air post a drying tower.  Any idea what this is in terms of the air dew point?

RE: Correlation of dew point of dry air


Tranform it into a mol fraction. Multiply by the total pressure to get the moisture's partial pressure. Look for temperatures in tables of water/ice vapor pressures. I may be wrong, but my estimates for atmospheric conditions indicate a dew point of about -57oC.

RE: Correlation of dew point of dry air

I get around -460C. First, I calculated the humidity ratio (4.17x10-5, mass of water vapor to the mass of dry air) and then read from psychrometric tables. Alternately, I checked by 25362's method and getting the same result.

However, this link Dew Point says something else.

RE: Correlation of dew point of dry air


It all depends on what is "standard" or "normal" air.

Normal: The density of dry air at 0oC and 1 ata is 1.3 kg/m3, thus, the mass concentration would be:

50*10-6/1.3 = 3.8*10-5

Standard: Dry air at 70oF and 101.325 kPa has a density of 1.2 kg/m3 resulting in a mass concentration of

50*10-6/1.2 = 4.17*10-5


RE: Correlation of dew point of dry air


Quark, sorry I wrongly read the vapor pressure table, and your finding is right. Then,

4.17*10-5 mass basis *29/18 → 67.2 ppm by vol, corresponding to a dew point of ~ -50F or ~ -46oC in the given link.

I assumed 29 and 18 for the MW of air and water, respectively.

RE: Correlation of dew point of dry air

(OP)
Thanks folks.  I have calc'd using 25362's method and using Antoine vp for ice also get -46°C.

Cheers
Trevor

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