Dew Point calcs 2

[Barhp]

Hi Everyone

I need to know how can calculate the dewpoint in a tank that was opened. We are going inyect N2 to displace the O2 or Air and the client told me that the purge end when the N2 dewpoint coming in is ~ N2 dewpoint coming out. I was told that -60F or lower (1 atm) is a better temperature.

Any help will be appreciated

 

[dcasto]

if you set up a dew point measurement on the stream coming out, you can watch it. The dew point reading will start out at say 60F. as the gases are purged, the dew point will slowly drop and then level out. That level point will be some number lower than -30F. If it goes down and bounces and is not stable, that is a goo indication that there is freewater in the vessel. Getting freewater to vapourize out is a problem. Heating the N2 to 150F is sometimes done to assure frewater is vaporized.

 

[Barhp]

Hi dcasto,

Thanks for the info. But have you done some cals before to get those numbers?

 

[dcasto]

what kind of calculations? Do you mean a table/calc that converts ppm H2O to dew point as read on the analyzer? Most analyzers will display ppmm, lbs/mmscf or temp.

 

[Barhp]

Imagine that I do not have the analyzer? and just know the temperature coming out.

 

[DRWeig]

Need to know something other than dry bulb temperature to calculate dew point....

Goober Dave

 

[dcasto]

Other than just calculating the volume of the vessel being purged and allowing for a mixing front there isn't anything to calculate. If the vessel has 1000 cubic feet, then use 3000 cubic feet of N2 to displace it.

If you think there is 100 pounds of water and N2 at 100F can pick up 300 pounds of water for every 1000 cubic feet with perfect mixing, then it would take 3000 cubic feet (assuming 10% efficency) to get all the water.

so how do you know how much water is in there.

You could add 100F N2 and measure the N2 coming out and once the temp is close to 100, you know you have exchanged all the air.

 

[mbeychok]

Barhp:

You might find it interesting to read this article in the online Citizendium encyclopedia:

Dew point

Milton Beychok
(Visit me at www.air-dispersion.com)
.