Quote:
Originally posted by Alchemy
Does Fluorinert become conductive after such a short time?
Alchemy
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I think he is talking about using LN2 to cool fluorinert.
Fluorinert or not, anything using LN2 is not a long term solution.
Crafty,
The liquid in pressurised canisters is NOT cold in the canister.
Take Liquid nitrogen for example, and I will try and explain this as thoroughly as possible.
Firstly, you need to understand that nitrogen can be a liquid even at room temperature. You just have to compress it.
Although I can't find one on the web, if you saw a Nitrogen phase diagram, you would see that N2 can be solid liquid or gas depending on the temperature and pressure.
At room temperature and pressure, nitrogen is a gas.
There are two ways of liquifying it.
Cool the N2 down to its boiling temperature at standard pressure, around -197 I think, or pressurise it, several thousand psi at room temperature.
Obviously, you can use a combination of cooling and compressing.
So on to the two ways of storing liquid nitrogen.
1. Dewar, liquid nitrogen is decanted into a dewar. Liquid nitrogen is at -197 degrees at room pressure. In order to keep it liquid, you need to insulate it. Think of a dewar as a very expensive thermos flask. It can maintain the LN2 at -197 at room pressure, so it will not evaporate.
2. Pressurised canister. N2 is compressed until it liquifies at room temperature. This liquid is transferred into a large canister which can hold the LN2 at the correct temperature.
It is important to note that these canisters are metal, ie good thermal conductors. If the LN2 were cold when it was transferred into the cylinder, then heat will be conducted into the canister. The temperature of the LN2 will rise until it reaches the same temp as the outside of the canister. The pressure in the canister will rise accordingly to apoint were the N2 is stable as a liquid at room temp, or the canister bursts.
Dewars are used for transporting LN2 and for short term storage, however, a perfectly insulated storage solution is hard to come by, thus, it will slowly heat up. Provided that the LN2 is used up relatively quickly, this won't be a problem. That is why large scale storage of LN2 is under pressure.
When the pressurised LN2 is released, ie, the pressure drops to room pressure, the effective boiling point will drop from room temperature (at the storage pressure) to -197 (at atmospheric pressure). However, the LN2 will only really boil at the surface, though this boiling LN2 will absorb the latent heat of evaporation from the bulk of the remaining liquid. This will continue to happen until the liquid reaches roughly -197. It will continue to boil because heat will continue to conduct from the surrounding air into the LN2, thus boiling off more LN2, which takes more latent heat, keeping the temp at -197.
The point I'm getting at, is that LN2 and similar high pressure liquids, such as your fog horn example only become cold once they are released into atmospheric pressure. This is why LN2 is not a long term cooling solution, as it will continue to evaporate until it is all gaseous.
As for pressurised cans feeling cooler when you shake them, the only solution I can think of is that shaking them encourages vapour to be released from the liquid, in a similar way that shaking a can of fizzy drink forces the gas out of solution. This gas will again absorb latent heat fro m the liquid, cooling it down.
If left to settle, the gas will liquify again, releasing latent heat back into the liquid.
As for using a pressurised can, as the contents are released, the total amount in the can will be reduce, thus more of it can evaporate. Again, this will absorb latent heat.
I hope this clears things up.
8-ball