Triple Evacuation with Nitrogen

[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