Triple Evacuation with Nitrogen

[DTLarca]

Pure nitrogen and OFN, are the same thing, you purchase OFN as a method of purity.
Triple evacuation, if you have free moisture (water droplets), when you go into a deep vac, you go below the triple point. Some of the water vapourize and some will turn into ICE. Continue vacuum, the ice will sublime (like dry ice "solid CO2") The density of the vapour can be many 1000M3 per Kg, so your vac pump has little or no effect. You now add nitrogen basically to add energy to the system, raising the boiling point above the triple point.

The specific heat of nitrogen is 1kJ/kg.K

The heat required to vapourise, or sublime if necessary - if you have been vacuuming for too long before the trip-vac process, water is 2.5kJ/gram.

So if the nitrogen is 1K warmer than the free water and all of that 1kg comes into contact with the free water then it would vapourise less than half a gram.

The problem is the nitrogen expands into the system causing a temperature there actually colder than the water or ice is.

So how does it all work?

[mad fridgie]

The specific heat of nitrogen is 1kJ/kg.K

The heat required to vapourise, or sublime if necessary - if you have been vacuuming for too long before the trip-vac process, water is 2.5kJ/gram.

So if the nitrogen is 1K warmer than the free water and all of that 1kg comes into contact with the free water then it would vapourise less than half a gram.

The problem is the nitrogen expands into the system causing a temperature there actually colder than the water or ice is.

So how does it all work?

The expansion of the nitrogen is the joule thompson effect,
How ever whilst the nitrogen is being introduced it is picking energy up from the thermal mass of the system. thus adding energy to the system.
If you are adding 1Kg of nitrogen, approx 0.8m3, you are looking a an system thermal mass of in excess of 100kg, so the nitrogen would warm close to the mass temperature, which inturn would add energy to the free H2O. On a system like this we should be talking about removing grams of water, not KGs of water. (that then becomes a different kettle of fish), then I would use heat and velocity pressure to remove large excesses

[DTLarca]

The expansion of the nitrogen is the joule thompson effect,
How ever whilst the nitrogen is being introduced it is picking energy up from the thermal mass of the system. thus adding energy to the system.
If you are adding 1Kg of nitrogen, approx 0.8m3, you are looking a an system thermal mass of in excess of 100kg, so the nitrogen would warm close to the mass temperature, which inturn would add energy to the free H2O. On a system like this we should be talking about removing grams of water, not KGs of water. (that then becomes a different kettle of fish), then I would use heat and velocity pressure to remove large excesses

Okay, say the 1kg of nitrogen warms up to 20°C being the ambient temperature. And the free water in the system is at -11°C (frozen) and is 1 gram in weight.

The 1kg of nitrogen would be able to pass 31kJ of heat to that gram of moisture. But how much actually comes in contact with the 1 gram of moisture? How much does the ice actually subcool on account of the rise in system pressure? Does the heat cause more vaporisation than the pressure causes subcooling?

[mad fridgie]

Okay, say the 1kg of nitrogen warms up to 20°C being the ambient temperature. And the free water in the system is at -11°C (frozen) and is 1 gram in weight.

The 1kg of nitrogen would be able to pass 31kJ of heat to that gram of moisture. But how much actually comes in contact with the 1 gram of moisture? How much does the ice actually subcool on account of the rise in system pressure? Does the heat cause more vaporisation than the pressure causes subcooling?

The nitrogen is acting as a heat transfer medium, as only radiation is transfered in a vacuum. Therefore, it is not just the energy of the nitrogen but the continual energy pick up from the system due to convection/conduction.

[DTLarca]

The nitrogen is acting as a heat transfer medium, as only radiation is transfered in a vacuum. Therefore, it is not just the energy of the nitrogen but the continual energy pick up from the system due to convection/conduction.

And how much heat does it transfer

[mad fridgie]

And how much heat does it transfer

It transfers enough energy until equalibrium is reached in both temperature and vapour pressure.
Your next question should now really involve "Time"

[DTLarca]

It transfers enough energy until equalibrium is reached in both temperature and vapour pressure.
Your next question should now really involve "Time"

Yes, time is a part of the question - but a fundamental part of the question. We know that the vacuum pump on it's own - with multiple oil changes and/or a cold-trap will work well - given time - but we want it all speeded up - so we come along with this nitrogen idea. The question is "Is it more effective in removing free water than is simply vacuuming with heat and time?"

My conclusions from recent experiences is that simply time and heat are required and not any interference with nitrogen.

On larger systems blowing hot air through the the unit works best.

Here's a picture from my trip to the Carrier factory in Lyon.