LN2 vs CO2 for emergency cooling

[pierhogunn2]

got a question for the brain trust...

which would make a more efficient/ better, in terms of the amount of heat to be absorbed vs. amount of material to carry..

wondering if LN2 or CO2 could be used to provide sufficient cooling to protect a firefighter for 30 - 60 minutes while working in a blaze

and provide some sort of spot supression ability...

have some friends looking into making a core/head cooling aparatus for their friends in a local department...

[Kristofor]

The latent heat of vaporization for N2 is 200kJ/kg @ -196C
The latent heat of fusion for N2 is 25.7kJ/kg @ -210C

The latent heat of vaporization for CO2 is 574kJ/kg @ -57C
The latent heat of fusion for CO2 is 184kJ/kg @-78C

Ammonia would have double the capacity of CO2, but if you're looking to vent to atmosphere then it's probably not a great choice

Oh, how about water... 2260kJ/kg @100C

[jdon]

From a purely chemical/thermodynamic viewpoint, dry ice is the "more efficient" than LN2. For LN2, the energy needed to vaporize is 397 kJ/kg. For CO2, the energy (enthalpy) of sublimation (solid to vapor) is 571 kJ/kg.

The heat energy is transferred from whatever the substance is in contact with, which is cooled in the process. So, on a weight basis, dry ice provides about 44% greater "cooling". Since dry ice is about twice the density of of LN2, it is even more "efficient" on a volume basis. If my calculations are correct, a liter of dry ice will cool 857 kJ, and a liter of LN2 will cool 322 kJ.

You don't say how the cooling would occur. There obviously would have to be some intermediate heat transfer mechanism- you wouldn't want direct contact with either substance.

Also, neither is very efficient compared to, for example, a small window air conditioner. 857 kJ is only 812 BTUs, and small AC's will cool 6,000 - 12,000 BTU's/hour.

Disclaimer for purists- these are rough calculations- I didn't include the heat needed to warm either substance from its boiling/sublimation temperature to body temperature, but that's a small factor (<1%).

[LCHIEN]

there are much better coolants than LN or CO2 but they require closed systems.
Freon or ammonia, but it appears that the application requires single usage of the coolant and then its to be released into the environment.

How is the cooling to be applied? Liquid N vs solid CO2 might make a serious difference as to how its to be transported to the end location (e.g. the head?) since liquids can be piped and pumped.

[pierhogunn2]

the idea was to route the working fluid through a vest and cap of some sort, and then vent out of the back of the helmet. does liquid CO2 have the same thermal properties as dry ice?

So I was leaning towards ln2 or even compressed nitrogen gas, or shoot for that matter compressed CO2... something that you could shoot out of a small nozel on the back of a fire fighting glove in a pinch to snuff out a small fire

[atgcpaul]

I'm going to throw this out there. How about using a technology like this?

http://en.wikipedia.org/wiki/Thermoelectric_effect

This technology was at the core of the DNA research I was involved in. By
applying current, you can vary the temperature from 4C to over 100C. In
your application, maybe this could be molded into the firefighter's helmet and
then connected to a battery pack. Maybe even sew it into the linings of their
jackets and pants. I haven't kept up to date with building computers, but I
think you can buy these to cool your CPU.

I have no idea how much power or peltier material you would need or how
much it would weigh, though.

Would using LN2 or CO2 pose a health risk to the user? Possibility of suffocation or explosion if the tank were punctured?

Paul

[master53yoda]

In order to limit the cooling effect of LN2 to it must be maintained at a fairly high pressure.(over 150 PSI) You would then need to maintain it in a liquid form or cool another medium to have cooling at any where near body temperature. If the liquid chamber where breached it would take the temperature to well below -200f turning the occupant in the suit into a brick of ice very quickly.

Just as an aside Liquid nitrogen can not be stored in most containers due to its ability to cool the container to a level that the container becomes porous and the LN2 goes through the container wall. I have seen this happen with a system that was piped in copper piping. Everyone around me thought that the fluid running of the pipe was condensate. It was not it was Liquid nitrogen that went right through the pipe wall. When it dripped the cement floor the floor blew chips because of instantly freezing and expanding the moisture in the cement.

You would have similar problems with LCo2. Dry ice is not affected by the pressure issue as it is a solid at atmospheric pressure.

[atgcpaul]

You would have similar problems with LCo2. Dry ice is not affected by the pressure issue as it is a solid at atmospheric pressure.

It can be a solid but dry ice cannot be bottled or the container will explode. It must be allowed to constantly vent or be packed in something porous like styrofoam.

[LCHIEN]

the idea was to route the working fluid through a vest and cap of some sort, and then vent out of the back of the helmet. does liquid CO2 have the same thermal properties as dry ice?

So I was leaning towards ln2 or even compressed nitrogen gas, or shoot for that matter compressed CO2... something that you could shoot out of a small nozel on the back of a fire fighting glove in a pinch to snuff out a small fire

materials have three phases: liquid solid and gas.
Every material has the so called phase diagram with pressure and temperature on the vertical and horizontal axes. The three phases are diagrammed on this chart, above, for CO2. In the case of CO2 the answer is yes it has a liquid phase but if you look on the chart it does not exist below about 5 Atmospheres (~75 PSI) at any temperature. Only solid and gas can exist below 75 psi. And at one ATMosphere pressure where you and I live and breathe, it says that CO2 solid (dry ice) exists below -78C and gas exists above -78C. A hunk of dry ice will hang around for a short time sublimating (going from solid to gas state) unitl it all warms up.

To have liquid CO2 at room temperature takes even more pressure, like about 300 PSI. In a building on fire, with surrounding air temperatures over 120F, probably ~700 PSI is required to keep CO2 liquid.
Every material also has a heat of conversion through the various phases. That's the energy requires to sublimate, boil or melt a specified mass. There's also the specific heat of the material which is how much energy it takes to raise a given mass a certain temperature, or how much heat/cold it can absorb per mass unit. High specific heat and high heat of conversion from gas to liquid make Ammonia and CFCs (Freon family) very good refirgerants.

Not to offend you but what you don't seem to understand about thermodynamics and properties of materials makes this a very difficult task.

And if you plan to snuff out the fire with C02 or N2 then you have to worry about volumes of gas, and about keeping the firefighter breathing. He's probably going to pass out before the fire extinguishes and will not awaken before the fire reignites.

[master53yoda]

It can be a solid but dry ice cannot be bottled or the container will explode. It must be allowed to constantly vent or be packed in something porous like styrofoam.

totally agreed on the sublimation pressurizing a closed envirnmnet,If I'm looking @ LCHIEN's chart correctly there would be a pressure of about 6 or atmospheres atmospheres or about 100 psi in the container at room temp if there was any dry-ice present. It also may be over the critical pressure/ temperature.

[LCHIEN]

I'm going to throw this out there. How about using a technology like this?

http://en.wikipedia.org/wiki/Thermoelectric_effect

This technology was at the core of the DNA research I was involved in. By
applying current, you can vary the temperature from 4C to over 100C. In
your application, maybe this could be molded into the firefighter's helmet and
then connected to a battery pack. Maybe even sew it into the linings of their
jackets and pants. I haven't kept up to date with building computers, but I
think you can buy these to cool your CPU.

I have no idea how much power or peltier material you would need or how
much it would weigh, though.

Would using LN2 or CO2 pose a health risk to the user? Possibility of suffocation or explosion if the tank were punctured?

Paul

The really cool thing (pun) about peltier devices is that by reversing the polarity of the electric field it will reverse the flow of thermal energy.
Making a compact device for both heating and cooling. This is not an advantage for a firefighter who usually wants to go just one way.

The bad thing about peltier devices is that the device needs several watts to move thermal energy - it depends upon the device geometry but a 2x2" device i played with recently required 6 Amps and maybe 12VDC to cool a small device to below freezing. The bad part is that you have to add the power consumed by the device to the heat load you are trying to remove.- in this case an additional 72 watts. Thus, they are not very energy efficient in the cooling direction.

The other problem is that they need a solid temperature reference to operate against--- the peltier device has two plates a hot plate and a cold plate. (reversible of course). If you don't have a big heat sink on the hot side then the dold side wont get much cooler than the ambient. You're going to have to have a largish heat sink (several pounds) to reference against. Otherwise its like a car in quicksand - plenty of HP but nothing to push against. You need a heat sink several times the thermal mass of the device being hated/cooled.

IN my business (optical sensor systems) we see them frequently used to stabilize small solid state lasers (about .5" x .25" x 1") at some operating temperature to minimize the effect of thermal drift. Usually they're operated well above room temperature and a thermal controller heats or cools the peltier device to keep it at that temperature.

[Kristofor]

The other problem is that they need a solid temperature reference to operate against--- the peltier device has two plates a hot plate and a cold plate. (reversible of course). If you don't have a big heat sink on the hot side then the dold side wont get much cooler than the ambient. You're going to have to have a largish heat sink (several pounds) to reference against. Otherwise its like a car in quicksand - plenty of HP but nothing to push against. You need a heat sink several times the thermal mass of the device being hated/cooled.

Loring is on the ball as usual.

We use peltier coolers to maintain reagent temps on our instruments, but it's done for other design considerations, not thermal efficiency...

In a firefighting environment, where the original problem was a high ambient temperature you'd have to deal with driving energy uphill across a temperature difference of hundreds of degrees. With a sufficiently large sink you would "only" have to overcome the difference in temp between the firefighter and the fire plus a smidge. With a smaller sink you need to go even hotter to make a larger temp difference for it dump energy into the environment at faster pace.

Since peltier devices are a smallish fraction of the efficiency of an average compressor driven evaporation cooler you'd need a ton of power to make this workable....

Running out of power would be, bad. Those really big heat sinks you were dumping into will gladly keep working when the power is turned off, but now the heat will flow the other direction to quickly cook your firefighter. As a bonus, you'd slightly recharge your batteries while that was happening... :S

[pierhogunn2]

Not to offend you but what you don't seem to understand about thermodynamics and properties of materials makes this a very difficult task.

.

no offense taken, I really don't, it was just an idea talked about over cheap beer... and from the looks of things, it will stay just that...

fun to talk about, until you get into the meat of things...

thanks for helping me play my little thought exercise