Drying to -20C Dew Point

[PerryMan]

Does anyone have a procedure for drying a pressure vessel to a -20C dew point and then purging with nitrogen?. What pressure is required for a -20C dew point? Is there a table of dew points and pressures required to meet them? Thanks

 

[EdStainless]

I have seen it done by purging with warm, dry (-40 DP) gas. I have seen closed loop systems temperately set up for this.

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P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

By "pressure" do you mean a vacuum?

minus 20C is about 0.1 Kpa(a)
Otherwise it doesn't make sense. Best way is simply to blow warm dry (-20 @ ambit) air through the vessel.

There are lots of people who do this, e.g.

http://www.h-t-c.co.uk/pipeline-drying.php

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[PerryMan]

Thanks for your responses. I did mean vacuum. My information is also that we need to pull a sustained vacuum of .1 kPa. We can pull a vacuum to .5 Torr which is about .0667 kPa. I have also reviewed the pipeline drying information and it was very helpful in pointing me in the right direction. Our company manufactures shell and tube heat exchangers and pressure vessels. In our business customers usually require formal written procedures for many tasks such as welding, hydro testing, shipping, etc. We frequently nitrogen purge pressure vessels and have a procedure for nitrogen purging. However, we need a detailed procedure for the process of using a vacuum to dry to a given dew point and then purging with nitrogen. It helps to review procedures written by outside sources in order to avoid potential pitfalls. Again, thanks.

 

[LittleInch]

Usually the key point is monitoring the exhaust gas for few point or pressure if you're going to use a vacuum. You can get a long period where pressure won't fall while the waste boils off then it finally drops to what you want. Them just make sure you fill with dry nitrogen.

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[georgeverghese]

If the pressure vessel temp is say a min of 10degC at this location while the vac pump is in operation, we have saturation vap pressure for water to be 9.2mmHg = 1.2kpa abs. So if the vac pump reading drops below this pressure (or some safe value below 1.2kpa abs), then this is a sign that there is no more liquid water in the vessel. This would be a convenient time to purge out the remaining vessel vapor with dry N2 which has a water content corresponding to a sat vap temp of -20degC.

Dont see the purpose of dropping the vessel pressure further from say 0.8kpa abs to 0.1 kpa abs with the vac pump.

 

[Afox88]

We are doing a vacuum dry for a customer to -20C and we reached a pressure of 0.576 torr (below target of 0.782 torr). Great. They are asking what does this translate to in temperature. Obviously, this is below -20C but how do I calculate the actual value. Is there a simple calculation to get a +/- 0.1C result?

 

[LittleInch]

Try converting then extrapolating this. It looks like the temp changes rapidly as you get down to low vacuum levels.

http://www.engineersedge.com/h2o_boil_pressure.htm

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[Afox88]

Thanks. I had a similar chart that I was interpolating based on but I was wondering if I could get a little more precise since the numbers are logarithmic at these pressures.

 

[LittleInch]

If you search there are a few online calculators about.

Probably far more important to know what the dew point was of whatever gas you filled it with??

normally pure N2 from a bottle is well below minus 40 and will swamp any air or moisture left.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[heatexguy]

This may help.