Is water really?

slipss

Is water really the best coolant known? As far as does it really absorb more heat quicker than anything else? I've heard this several times from several presumably knowlegable people on the subject? If it is then why? If its not then what is and why? Also what makes something better at cooling than something else? Whats the deal there? Thanks.

DemocAnarchis

As long as you arent talking about extreme highs or lows of temperature, ie a range where water will be in its liquid state, it is an incredibly effective coolant;

The "Specific Heat Capacity" of a substance is the amount of energy required to raise the temperature of 1 Kg of the substance by 1 degree. Water happens to have a very high value, 4.184 kJ/degree. This means that water, compared to a coolant of lower heat capacity, will absorb the same amount of heat and remain cooler. This means it is an effective coolant for longer.

In addition, water has very low levels of viscosity, which means it can be pumped with a minimum of energy and will not adhere to the pipes/tubes of the system. Coupled with the fact that water is very cheap and common, water is the best effective coolant known for general use.

Im sure some sort of ideal liquid might be a better coolant, but it would be prohibitively expensive.

ApeXaviour

I think the issue is more with its availability and high heat capacity than its ability to absorb heat.

For instance if you were to pour a much more volatile liquid on your hand, say butane, it would likely make your skin freeze as it evaporates. So you could say it absorbs heat much better than water in that regard. But unlike water it would disappear very quickly. Water can absorb a lot more heat without changing much in temperature. Making it viable say for keeping machinery at a running temperature, rather than insta-freezing and boiling off.
Plus water is cheap, non-polluting, and very readily available.

Professor_Fink

slipss wrote:

Is water really the best coolant known? As far as does it really absorb more heat quicker than anything else? I've heard this several times from several presumably knowlegable people on the subject? If it is then why? If its not then what is and why? Also what makes something better at cooling than something else? Whats the deal there? Thanks.

Ammonia has a higher specific heat capacity of 4.700 (Helium gas has 5.1 or so and Hydrogen has 14.3). I have to agree with the others that this may well be a cost thing, but ammonia isn't overly expensive.

Usually we think of heat being absorbed into degrees of freedom of a molecule. Since there are never more than 6 (and always at least 3) rotational or translational degrees of freedom for any molecule, they all store within a factor of two the same amount of energy. At higher temps, vibrations within the molecule contribute. As water molecules are lighter than many others, you can pack more of them into the same space, and so you get a high heat capacity.

Actually, in many cooling systems a phase transition is used (i.e. evaporation) in which case you would need to look at the relevant latent heats.

SOL

isn't there an 2N-3 or 2N-4 formula for degrees of freedom?

Professor_Fink

SOL wrote:

isn't there an 2N-3 or 2N-4 formula for degrees of freedom?

N being what? The point is that at low temps you do not have enough energy to actually excite vibrations within the molecule.

slipss

Ahh ok, very well explained there, thanks. I've a couple of follow up questions so I guess I'll just stick them here instead of taking up space with another thread.
I searched out a table of specific heat cpacities there, this puppy http://en.wikipedia.org/wiki/Specific_heat_capacity#Table_of_specific_heat_capacities

I was just wondering whats the difference between these three formulas on it, what do they mean, cp Jg−1 K−1, Cp J mol−1 K−1, and Cv J mol−1 K−1 (I don't think there layed out right but I'm sure you know the ones). How come the substance with the highest SHC changes depending on which column there in, I'm presuming it has something to do with the mol, which I'm guessing is short for molecule? But why does gassious Hydrogen have the highest in the first row and solid diamond have it in the second row? If that makes sense.

Delphi91

slipss wrote:

Ahh ok, very well explained there, thanks. I've a couple of follow up questions so I guess I'll just stick them here instead of taking up space with another thread.
I searched out a table of specific heat cpacities there, this puppy http://en.wikipedia.org/wiki/Specific_heat_capacity#Table_of_specific_heat_capacities

I was just wondering whats the difference between these three formulas on it, what do they mean, cp Jg−1 K−1, Cp J mol−1 K−1, and Cv J mol−1 K−1 (I don't think there layed out right but I'm sure you know the ones). How come the substance with the highest SHC changes depending on which column there in, I'm presuming it has something to do with the mol, which I'm guessing is short for molecule? But why does gassious Hydrogen have the highest in the first row and solid diamond have it in the second row? If that makes sense.

A mol is short for "mole" which is defined as atomic/molecular weight expressed in grams. So for water, 1 mole of water weighs approximately 18 grams (H + H + O = 1 + 1 + 16 = 18). You'll notice that the J mol^(-1) K^(-1) figure is 18 times the J g^(-1) K^(-1) in the case of water.

Professor_Fink

slipss wrote:

I was just wondering whats the difference between these three formulas on it, what do they mean, cp Jg−1 K−1, Cp J mol−1 K−1, and Cv J mol−1 K−1 (I don't think there layed out right but I'm sure you know the ones). How come the substance with the highest SHC changes depending on which column there in, I'm presuming it has something to do with the mol, which I'm guessing is short for molecule? But why does gassious Hydrogen have the highest in the first row and solid diamond have it in the second row? If that makes sense.

Cp and Cv are the heat capacity at constant pressure and constant volume respectively. mol-1 indicates the molar value, where as g-1 indicates it per unit mass.

mol is short for mole. 1 mole is equal to an Avagadro's number (6.022*10^23) of atoms or molecules, so you are talking about fixed number of particles. If you talk about it per unit mass, then you can have far more of the light molecules than heavy ones in the same mass. This is the reason for the difference between the position of diamond and hydrogen in each column: Carbon weighs roughly twelve times as much as atomic hydrogen, and so in a gram you have only 1/12th as many particles (or 1/6th if it is molecular hydrogen).