Hacker News Reader

I observed the phenomenon growing up when I filled buckets of hot and cold water and left them outside in freezing temperatures.

I think it might have something to do with the convection current of the hot water. Perhaps the higher initial velocity allows the ice crystals to nucleate faster?

Hot water melts snow, and the ground is much colder than the snow which isolates heat. So in your case the simple explanation is just that, the hot buckets melted the snow/ice and therefore stopped being isolated against the cold ground so froze faster.

The assumption being they were placed on snow, which I would argue is an unlikely assumption to hold up. We have much more days with freezing temperatures than days with snow lying around and even then with lots of snow there are enough places that don't have any (like a balcony).

They were set on concrete.

Some more ideas. Cold water holds more dissolved air in it than hot water, and the hot water heater would have recently rendered most of those gases out. That dissolved air could negatively impact the thermal conductivity of the water.

Near boiling water is about 4% less dense than near freezing water. If you filled the buckets right to the top, there would be less water in the hot bucket.

How about this theory. The hot water causes the freezer to run its compressor sooner.

This may happen but, in a normal freezer, the action of the compressor is meaningless.

My feezer is 70% full, with probably 30kg of stuff in it. This stuff is mostly at -22C (I have a thermometer), and so, this mass is what cools new things added to the freezer, fast.

(Why -22C instead of -18C? Salt water fish is in there, and I also want a little headroom.)

Unless the new things are physically touching the other mass, then my guess is that the primary cooling effect is due to 1.) convection through the air in the freezer, followed by the 2.) conduction through the surface the new things are sitting on, followed by the 3.) IR radiation of the new thing. The existing thermal mass in the freezer interacts with the new thing via 1 (indirectly) and 3 (by absorption), but I suspect that the compressor cooling the air is a bigger effect.

All documentation I've read, and personal experience, says otherwise.

For example, most manuals urge one to not have an empty freezer. That cold mass is one reason why.

When I was a kid, we had a fridge from the 50s in a cottage. When I started drinking, we'd put 5 cases of beer (24*5) in glass bottles in there, and 2 hours, yes hours later they'd be at 1C.

Now, modern fridges/freezers literally do not have that degree of cooling power. This is on purpose, for it is more efficient to power a small compressor all the time, than a massive compressor for 10 minutes.

You may want to argue this point, and that's fine, but I am merely providing info both from the manuals of modern freezers and fridges, which I have read, and from online when looking at why they are so bloody slow to cool things.

My LG fridge manual actually says not to put warm meat in my freezer, unless the thing has loads of frozen stuff in it.

Otherwise the meat could go bad before freezing.

Oh, another wonder of modern fridges. If you buy one and put it outside, or in an unheated garage, the freezer becomes useless.

This is because many fridges have no thermometer in the freezer part, and only get a reading when the compressor comes on, to cool the fridge.

As the fridge is always cool when it is 3C outside, or cooler, the freezer never keeps stuff frozen.

You're putting a lot of faith into manuals for mass produced household appliances. All that holds true to a first order, but the actual thermodynamics going on is certainly way more complex than what's captured in a consumer facing manual. When discussing the relative freeze times of a modest amount of water for the purposes of a science experiment, the transient higher order effects are going to be an important factor.

Oh, I totally get the argument to keep it full. Both the colder thermal mass and less cold air to escape make it considerably more efficient. I just doubt that the thermal transfer from the mass to the newer items is as significant as the compressor kicking in. After all, the compressor is going to flood the internal radiator with liquid far below the ambient temperature of the freezer.

> My LG fridge manual actually says not to put warm meat in my freezer, unless the thing has loads of frozen stuff in it.

That sounds silly to me. In my experience ice cubes freeze within an hour and meat within a couple hours.

The compressor is moving the air around in the freezer, which can have an impact on how much heat is transferred. You can sit comfortably in a sauna, but your arms will heat up quickly if you wave them around. Similarly, a convection oven heats things faster than a conventional oven.

The compressor doesn't move any air inside the refrigerator. There is a separate fan in the freezer compartment that does that.

True, but I meant that fan runs when the compressor is running to blow air over the coils. That movement of air will allow more transfer of heat from the stuff in the freezer.

You've got a good point, which is that moving air greatly improves heat transfer, but your errors in terminology are confusing people. The fan that blows cold air is the "evaporator fan". The compressor fan (if there is one) blows hot air on the outside of the freezer. Here's a diagram: https://home.howstuffworks.com/freezer2.htm.

I love that theory. I would assume they use big enough freezer or with enough ice inside for that not to matter, but that would be a hilarious solution.

> the excess energy stored in the hot water is trivial compared to the capability of the heat pump

I don't know if I'd call it trivial, bringing 1 kg of water from 85C to 0C involves about 100W for a duration of one hour whereas a typical freezer removes heat at, what, 200W or 300W?

But a 100ml beaker would only require 10W

>bringing 1 kg of water from 85C to 0C involves about 100W

That looks like the amount of energy needed to heat that water.

But cooling things doesn't work the same way. Instead you want to extract that energy/move it somewhere else. The efficiency of some approaches, unlike when heating something, can hugely depend on ambient temperature. It's possible you'll be able to extract more useful energy cooling something than you'll have to expend!