Drying Vessel to -40oF DewPoint

[Welding1927]

This is something we have never done before, and we are struggling to achieve.
We are trying to dry a vessel to -40F by blowing warm nitrogen through it. However, as the temperature rises, the Dew Point is not lowering. The best we can achieve right now is 5% RH, and a Dew Point of 14. As we heat it more, the dew point rises. What are we missing here? How does the temperature relate to the Dew Point? Can we keep heating the nitrogen, and expect the Dew Point to lower?

 

[EdStainless]

The heating will speed the process. Get it as warm as you can and keep flowing.
You have to drive off all of the surface moisture.
Are you sure that the entire surface is getting warm?
Do you have good turbulence (mixing) inside?

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P.E. Metallurgy, consulting work welcomed

 

[sshep]

Probably there was a pocket of free water which was holding you up. That will take a long time to evaporate with a steady purge, especially if it is not in the N2 flow path.

Pressure purging is a more effective way remove free water because it creates internal flow at all points in the system. Pressure up the whole system and then rapidly depressure from a low point using a large valve. Dropping the outlet spool from a control valve can make this easier, but you may need to barricade the discharge point. If free water comes out then you know you are doing good. When all the free water is gone the dew point will usually come down quickly.

good luck!
sshep

 

[pierreick]

Hi,
If the size of the tank is big enough ,I would suggest to bring someone in , with all the safety precautions in place (permits, life line, O2 continuous monitoring,..) to sweep the walls and bottom to remove the free water and then apply N2 pressure process to reach the dew point .
This should save a lot of time and some money (N2 costs) .
Pierre