Lately I’ve become slightly addicted to videos where a very hot metal ball is placed on something and a reaction occurs. It led me to wonder-what would be the quickest way to cool down this object?
Liquid helium?
Quelching to metal ball by dropping it into a drum of liquid nitrogen.
Water has just about the highest specific heat of commonly available materials. Meaning that immersion in flowing cold water is about the ideal way to cool anything.
Not exactly relevant to your goals, but note that cooling metal too quickly will result in microscopic structural changes in the bulk material and / or warping of the finished part. This is both a useful fact used deliberately in some manufacturing, and the bane of some other useful processes.
ETA: cooling anything is a matter of the temperature difference and of the specific heat of the colder item. Ref @penultima_thule I wonder how much the greater delta-T of liquid N2 compares to the greater specific heat of ordinary water at ~33F / ~1C ?
Temperature variance for a ball heated to 1000C (bright yellow) is 1,000 degrees for water, 1,200 for LN and 1270 for LHe. I doubt that the extra 20-27% temperature variance gets you more cooling performance than the water based cooling.
That was my intuition as well, without bothering to look up the specific heat of LN2. Even chilling the LN2 to just above its freeze point, analogous to using +1C water, only buys another 14C ~= 1.5% increase in available delta-T.
Now looking at heat capacity, liquid water has ~2.5x the capacity of LN2. No comparison; 1C / 274K almost-frozen water beats the pants off -208C/64K almost-frozen nitrogen.
Both will work better in a fast flow to avoid local heating and indeed vaporization of the coolant. See also Leidenfrost effect.
A further factor is that real quickly the rate limiting factor for an iron or steel sphere will be the rate of conduction within the sphere. A sphere, of course, is the worst-case shape for max volume holding heat versus min surface area through which to shed that heat.
There will be some temperature where you can cool the ball down faster by first heating it to a liquid state and then pouring the liquid into a cooling solution where it will have much more area exposed to the coolant. Or do anything to turn the ball into smithereens or just more than one piece before cooling.
As @LSLGuy notes above, any change to the shape of the ball will also make it cool faster by exposing more surface area.
True, if it doesn’t have to be a ball when we’re done, this would get that mass all below some threshold temperature faster.
A faster way to cool molten metal is to spray it onto the polished edge of a rapidly spinning copper or silver disk. It freezes so fast it can’t form crystalline regions, and winds up a glassy sort of tinsel. I visited a lab where this was done in the late 70’s.
If we need to keep it as a ball, a faster heat transfer medium than water would be a hydrogen or helium jet. Water will not work that well, if there’s boiling at the interface that creates a gas layer. This is why you can dip your hand into liquid nitrogen without much effect. Gas jets, especially with high conductivity gasses like these, can be cranked up as high as you like.
Whether faster heat transfer would make a big difference or a small one depends on the heat transfer coefficient, and the radius. There’s a dimensionless number, the Biot number, which is the heat transfer coefficient on the exterior divided by the heat transfer coefficient on the interior (which latter you might consider as the conductivity of steel divided by the ball radius). When Bi >> 1, additional heat transfer makes little difference.
To get a truly factual answer, you need to supply more specific information.
For example, how hot is “very hot?”
What is the ball made out of (metal could be anything from mercury to tungsten).
What temperature are you trying to cool it to?
In general, you will get the fastest heat transfer by submerging it is a liquid. But, if the ball is significantly higher in temperature than the boiling point of the liquid, then the ball will develop a vapor blanket around it which, in effect, insulates the ball from the liquid. So, if the ball is made of, say, carbon steel, and heated to, say, 1600F, it will cool off much faster if it is dropped into a bucket of 70F water than a bucket of LN2, since with the LN2, just the radiant heat from the ball at 70F will create a vapor blanket.
In heat treating steel, one operation is called quenching and it is performed to cool the material as rapidly as possible. As has been noted, sometimes cooling too fast can cause problems with distortion and/or cracking, so some processes with use oil, or perhaps even molten salt, instead of water for this operation (and in today’s world, they have developed synthetics that can be used as well) depending on what temperature they are wanting to cool to and how fast they want to cool it to.
The fastest cooling rates are commonly obtained with a water spray, which is used to blast through the vapor blanket and cool the surface to water temperature almost immediately. Then it is just a matter of keeping enough water flowing over the surface to remove enough heat to prevent the heat from the interior from being conducted to the surface causing the surface to rise in temperature.
In specialized cases, specific salt brines are used which have a higher boiling point than plain water which can produce faster cooling rates, but they don’t beat the spray cooling, if done correctly.
Make the ball out of sodium and then liquid nitrogen is the safer choice. Of course, that wouldn’t be as interesting as water. But at 1,000 degrees F, the metal would be liquid. So let’s drop ice into liquid sodium.
This was my first thought; shatter then cool if “speed of cooling” is all that matters.
At the limit case, an ordinary grinding wheel produces white hot sparks while grinding an iron / steel workpiece. Which sparks are dust to fine sand-sized bits of metal that will cool from white-hot incandescence to room temperature in just a few seconds while exposed to only room temperature air. Which air sucks as a coolant.
It’s all about surface area to volume ratio.
I… would not have even dared to hand drop the ice like that. Gloved or not. WTH was this guy thinking? Does he often show callous disregard for his own safety? Was he wearing breathing protection at least? Face mask? What an idiot…
The moron says “… it was completely unexpected how fast this actually reacted …”. Which suggests that he has never opened a chemistry textbook in his life. IANA expert, but this was deeply into “No shit Sherlock” territory before he dropped the cube.
Dropping water ice into a bath of liquid metal at just about the boiling temp of water, the outcome is obvious. The ice will flash to water and some steam. The H2O surface area will expand very rapidly, be well-mixed with the liquid Na and the water-sodium reaction will proceed apace.
Some of his videos showed up on my suggestions column when he first started. There is a minimum of one major stupidity in all of them.