Correlation of dew point of dry air 2

[TrevorP]

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?

 

[25362]

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 -57[sup]o[/sup]C.

 

[quark]

I get around -46[sup]0[/sup]C. First, I calculated the humidity ratio (4.17x10[sup]-5[/sup], 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.

 

[25362]

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

Normal: The density of dry air at 0[sup]o[/sup]C and 1 ata is 1.3 kg/m[sup]3[/sup], thus, the mass concentration would be:

50*10[sup]-6[/sup]/1.3 = 3.8*10[sup]-5[/sup]​

Standard: Dry air at 70[sup]o[/sup]F and 101.325 kPa has a density of 1.2 kg/m[sup]3[/sup] resulting in a mass concentration of

50*10[sup]-6[/sup]/1.2 = 4.17*10[sup]-5[/sup]​

 

[25362]

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

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

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

 

[TrevorP]

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

Cheers
Trevor