Submitted by Red_Panagiotis t3_11l99pi in askscience
Comments
CrateDane t1_jbfss5s wrote
> > > > > Water is a actually a very good coolant because it has a very high specific heat capacity of 4.2 J/(K g). This is typically twice as large as organic solvents and at least 4-5 times, sometimes more than 10 times larger than most solids (relative to mass). Since the thermal energy or heat is conserved during the transfer, this means that water can reduce the temperature of the material to be cooled by a certain degree, while its own temperature is increasing only by a fraction of that (considering somewhat similar mass).
Alternatively, you can rely on the very high enthalpy of vaporization of water, by using some form of evaporative cooling. Some animals rely on sweating for thermoregulation, while a lot of computer hardware relies on evaporative cooling in the form of heatpipes or vapor chambers.
[deleted] t1_jbguhnr wrote
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Crimbobimbobippitybo t1_jbfhht6 wrote
Plus water has a very favorable (for this purpose) enthalpy of evaporation.
Brain_Hawk t1_jbfrv88 wrote
Aside from the other answers better than I could say, it's cheap abd plentiful!
MRI used liquid helium for supercooling. It's very expensive. Some helium loss is natural and replacement is an ongoing expense.
Likewise if you need to turn thr magnet off you need to dump the helium (MRI magnet is always on), and replacement is $15-25,000.
If water works, it's basically or nearly free! This is a very non trivial concern for.most uses. Obviously for super cooling you need something more.
luckyluke193 t1_jbgsgxe wrote
> if you need to turn thr magnet off you need to dump the helium
I've never worked with an MRI magnet, but in general, superconducting magnet systems can be ramped down to virtually zero without evaporating all the helium. You just need a power supply for the magnet.
I don't know if the magnet power supply comes with the MRI system though. I think in NMR spectroscopy, magnet systems are often sold without power supply. The technician who installs the system charges it with their power supply, and takes it back with them. As long as the users don't somehow empty out the helium or damage the magnet, it will hold its field indefinitely.
mesouschrist t1_jbko5ad wrote
IDK what hospitals are doing, but I work in a physics lab, and nobody is ever letting their liquid helium just boil into the atmosphere unless something has gone catastrophically wrong. There's a whole infrastructure for recovering boiled off helium and sending it back to the liquefaction plant.
luckyluke193 t1_jblh3o3 wrote
Sure, but usually the liquefication plant fills it into dewars, and the magnet system needs to be refilled manually.
CocktailChemist t1_jby03y8 wrote
Can’t say I’ve ever seen that for an NMR. A standard 400 MHz instrument has to be topped up with 70-100 L of liquid helium every 6-12 months.
[deleted] t1_jbgsr2o wrote
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ReceptionDry2755 t1_jbfhwnp wrote
water doesn't have to conduct heat. It is very fluid so it will convect, and it has a great specific heat so it can get rid of a lot of energy without a huge temperature rise. Also it is liquid at all room temperatures. If necessary it can evaporate and take advantage of the kind of specific enthalpy that made steam engines possible. Water is a great cooler
Yujiro_hunma t1_jbgv54m wrote
Water is actually a good conductor of heat compared to gases, but not as effective as metals. However, it's still used as a coolant due to its high specific heat capacity, which allows it to absorb large amounts of heat energy without a significant temperature increase. It's also readily available and cost-effective for many applications.
KingoPants t1_jbo07ai wrote
If you need super heavy-duty cooling, then phase change systems for water can transfer literally gigawatts of heat power because of how much energy it takes to turn water into steam.
This is effectively what most steam turbine based power plants do after all. A few kg/s through a pipe of steam is many megawatts of enthalpy. If you look at h_fg on a steam table, you can see it's like more than 2 megajoules per kg or so.
For more familiar numbers divide everything by a thousand. Just half a gram of steam per second is over a kilowatt.
[deleted] t1_jbfp8lo wrote
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[deleted] t1_jblrky3 wrote
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UnclePat79 t1_jbffn3i wrote
Since water is a liquid, the thermal conductivity is not limiting the efficiency as coolant. Instead of relying on internal heat conduction away from the surface to be cooled into the water bulk, water is a fluid, thus the heat will be transferred by convection (natural flow due to temperature gradients) or forced flow of water by a pump, for example. If the water flow is strong enough, the limiting factor is the actual transfer of heat over the phase boundary between the solid to be cooled and the liquid.
Water is a actually a very good coolant because it has a very high specific heat capacity of 4.2 J/(K g). This is typically twice as large as organic solvents and at least 4-5 times, sometimes more than 10 times larger than most solids (relative to mass). Since the thermal energy or heat is conserved during the transfer, this means that water can reduce the temperature of the material to be cooled by a certain degree, while its own temperature is increasing only by a fraction of that (considering somewhat similar mass).
The very large heat capacity in combination with the ability to transport the warm water quickly away by forced flow makes water such a good coolant, which is furthermore easy to obtain and very cheap. One of its downsides is the relatively large corrosivity and its tendency to expand when (accidentally) freezing.
To expand a bit on the difference between heat conductivity and heat capacity: Metals are very good conductors of heat, but have poor heat capacity. This makes them feel cold, because they can quickly transport the heat away from your skin, but will also quickly warm up.