Actually i heard it was relatively cheap ... like a few dollars a liter

I don't know hwere you learned chemistry, but it sure is an endothermic reaction. Physicial changes can have energy changes - some things when they dissolve are endothermic/exothermic.

I've been thinking - it would be cold if it is a liquid under high pressure. Take fog horns for example - its a highly pressurized liquid that when you press the button ... the gas goes out rELALly fast. But if you press the button while tipping it upside down, the liquid comes out and its REALLY cold. All those things are the same like that.

Yeah, you're right on with that. I remember turning over one of those dusters and spraying it on my jeans - its extremely cold and turns a big spot into solid ice!

It's not an endothermic reaction. It's an endothermic phase change.

Pouring liquid N2 on something room temperature will cause the N2 to immediately evaporate. When things evaporate, they absorb heat. Thus, the object the N2 is being poured on gets very cold very fast.

These cans of compressed gas for blasting away dust usually contain some sort of flouroethane. When you spray it, the gas quickly escapes because it is under pressure. A little bit of liquid evaporates to make up for this loss of pressure, and it doesn't take much of that to get the can very, very cold.


I should point out that people would need cryogenic gloves. Latex gloves, or even Arctic expedition gloves, won't cut it.

And the dewar you mentioned is basically a thermos. LN2 won't evaporate as long as no heat gets to it, so you can keep a liter of it in a good dewar for a good day or two before you loose it all. Dewars are unsealed to allow a little bit of LN2 to evaporate, since heat will still get through the thermos, and if it was sealed it would pressurize until it eventually exploded.

I used nitrogen gas and LN2 in some of my research labs, and it's some fun stuff. It's very cheap - less than $50 for an 80 gallon tank, I belive. But that's only because we own the tanks and receive daily shipments of hundreds of dollars worth of compressed and liquefied gases. If I bought LN2 I wouldn't need to pay for delivery, plus, I get a massive discount simply due to the amount of business the supply company gets from us.

Of course, if you don't receive large daily shipments of compressed and liquified gases and a loading dock facility to accept them *and* you do not have a proper LN2 storage tank (*very* expensive, and dewars won't work if you expect to run your system for more than a few minutes) *and* you do not have the facilities to pump the LN2 (it's at a very low pressure and won't escape by itself) *and* you don't have an adequate ventilation system . . .

And that's just the stuff I could think up off the top of my head. Unless someone had access to all these facilities, building a LN2 cooling system would cost tens of thousands of dollars. If one intended to run a system continously, we'd be talking operating costs approaching a million dollars a year. This is *not* the sort of budget for a hobby.

BTW, the term is displacement, not dissociation. You got the idea right, though.

Alchemy

Eh? I would call a phase change a reaction . . . after all you loose one substance and gain another. Not to mention theres a change in enthalpy and entrophy.

A chemical reaction is when a one chemical changes to another chemical.

Nitrogen is still nitrogen whether it's liquid or gas.

Alchemy

he's considering it a physical reaction... but its usually called a physical change. reaction doesn't seem to be applied to things like that very often.

Liquid nitrogen is alot of fun. Its neat how if you spill it on a table or something it scatters all over in small beads because of the fast change to a gas actually holds the LN2 off the table.

You can actually take some, and put it in your mouth and just blow out smoke. It works on exactly the same principal.

(don't do it with lots, just a small amount, and definatly don't swallow)

oooooo thank goodness.... there is still hope for me..... I am so relieved now, there I thought I was as crazy as can be.... :D

I feel better now..... :D

N(g) + energy ===> N(l)

Though I think you're right.

How the hell did these people afford two gallons of Fluorinert?

I'm tempted to call & get pricing for FC-40 for a similar project I'm thinking up. Perhaps it's a bit more affordable here in the States. Maybe I can ship it to my department without my advisor noticing . . .

Alchemy

i wouldnt do that simply because its not a permanent solution that you can use to cool your computer for more than a day.

Then why are you DIYing yourself a nice N2 compressor. It requires 2000psi correct? Well use a biggun to cool down the entire neighborhood.

I'm sure you could cool down quite a few computers subzero with that type of hardware.

Does Fluorinert become conductive after such a short time?

Alchemy

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

I see.

To be more specific, I was thinking of using Fluorinert chilled with some sort of phase change apparatus or a peltier device. I wanted to know if fluorinert had a metal solubility such that it would lose its dielectric properties after a certain amount of time in contact with fine circuitry.

Alchemy

Its actually very, very, cheap, just the containers aren't. Here at school they'll give it out to anyone who is competant not to burn themselves with it.