Cooling things

Physical mechanics of cooling

Passive cooling

Passive cooling tends to mean 'what happens with no moving parts'.

...so whatever amount of conduction, radiation, or convection would happen anyway.

Sometimes includes adding a fan.

You're stirring the air better than just convection would, so heat transfer goes a little faster than if warm air just sits around - but the difference is rarely much (if you're in gravity, convection will happen).

And you could argue that's technically active cooling (because you're adding work, so using energy), but intuitively it feels like it hardly qualifies.

On the technical side

This tends to mean

• conduction - a good conductor spreading heat throughout

if any cooling happens, conduction's spreading brings the whole down

• radiation - thermal radiation means movement of charges in materials (anything above 0 K) is radiated as EM at the surface

(black-body radiation can be seen as a "thermal radiation's real-world math becomes easier if we make some assumptions like that it's not really interacting in other ways")

• convection - fluid flow, in this context often

air,

flow caused by heat changing temperatures and densities

that flow assisting better heat interchange with that fluid, because warmer air moving up tends to draws in colder air from the sides (which technically is an effect that needs gravity)

In practice there's more than one of these happening, but often one that counts for most exchange.

Unsorted

More on fridges and freezers

Super!

Terms like SuperFrost amount to temporarily cooling more than required for a stable temperature.

Why? Well, for context, freezers are slow. Because usually they don't have to work hard - they're isolated so well that once cold, keeping it at that temperature doesn't take much effort.

So they are optimized to be efficient at cooling fairly slowly.

Yet if you put in a lot of new groceries, the average temperature will spend a few hours warmer then the target.

With Superfrost, you can get it to be maybe 10 degrees colder than usual for a while, so that chances are better that once you put the groceries, the temperture will rise from colder-than-usual to usual temperature.

It's usually not very important, because a freezer is often -18C making it unlikely the temperature the rise will be enough for anything to melt.

It's also a bit of bother, because you have to think about this hours in advance (again, the cooling is slow).

...but it's a nice option to have, for the cases where it does matter.

Most freezers switch back to regular cooling automatically.

Supercool seems to be the same idea but for fridges rather than freezers.(verify)

On frost

In the olden days, a fridge was no more complex than a cooler element in a box.

In freezers, and in fridges with freezer compartments, that means any moisture in the air will, over time, condense and freeze on those cooler elements - which is what frost frost is.

This matters more clearly to freezers, where everything inside is is below freezing point (-18° C, 0° F).

Fridges (without freezers) aim for just above freezing point (4° C, 40° F), but the evaporator (the coldest part inside, because it's the thing that draws out heat) may dip a little lower.

Bottom line, when there is anything below freezing, and you add air, you add moisture that will eventually become frost somewhere.

Frost build up is both inconvenient and makes the fridge less efficient, so there is a slew of names (Self-defrosting', auto-defrost, NoFrost, Low Frost, Smart Frost, and more) that refer to designs that try to manage this somehow.

This usually means ensuring frost happens in one main place, where we can deal with it.

Say, Self-defrosting basically means there is a heater that fairly directly warms up the evaporator every now and then. Say, every day, for twenty minutes.

Yes, this means the temperature inside the fridge would fluctuates a little.

But it means the evaporator should never be overwhelmed with ice, so you never have to deal with defrosting it, or having it defrost at unpredictable times.

Yes, self-defrosting takes more energy than not doing so in the short run -- though a freezer that is 'never' defrosted becomes less efficient, and oveer enough time this tips over to become less efficient (scale of a year or so).

Exactly where such a feature lies between convenient no-brainer and most efficient possible is hard to know.

Other designs may try to take moisture out - often in the same basic way, but not necessarily on the main evaporaor(verify).

fridges and freezers, a.k.a. ;;;"auto-defrost, have a heater on the evaporator that is occaionally turned on for a short while, to ensure the evaporator never builds up frost.

Because this may happen frequently (sometimes once a day), and because you want the evaporator to be dry before freezing again (meaning this may take ten minutes), this take a little more energy to operate. The fridge may also have shorter lifespans due to being a little more difficult to repair.

Independent of other features with "frost" in the name, some fridges will sometimes decide to defrost in the sense of just switching off for a while.

No Frost, Low Frost, and Smart Frost are variations on the idea that making air drier means much less frost, with frost being ice coming from condensation.

Explanations easily mention circulation, and are a little more coy about mentioning why exactly that makes air dry.

Because what most of these things do is extra freezing it in separate compartment, that is occasionally warmed to get rid of the ice in there.

Mentioning that clearly would make it obvious that this is a convenience at the cost of energy. Though not necessarily that much energy.

Low Frost sometimes refers to a specific variant, and sometimes to the fact that No Frost is usually a lie.

Cycle defrost

Manual defrost

Defrosting

You may wish to let it warm up completely.

Yes, doing so when it looks clean enough could save a little energy, but if the evaporator was still wet,

Note that top-loading freezers tend to lose much less cold air, so tend to have less problems with frost.

Does a lot of ice make a fridge/freezer less efficient?

In modern freezers, little. They tend to have a feature that regularly defrosts the cooling element with a heater. This means that there may be ice everywhere, it won't affect cooling much until it clogs up airflow.

In freezers without it, yes. It's not that there is ice at all, but because that will likely also be on the cooling elements, insulating it.

How much this matters, and how quickly to resport to defrosting, is a more interesting question, though, because there are a bunch of different effects together.

The cooling element will be the coldest part of the fridge - it needs to be, because physics.

The cooling element in older and/or simpler fridges was often a coil exposed directly inside. (Also, it makes it easy to damage if you're forcefully removing ice, even though it's often wrapped with another metal)

Most of the heat is moved via the air, so it may also be a separate evaporator in its own compartment. Evaporators have many fins for faster heat transfer - but also frost up much faster, so this design is often no-frost.

Ice inside the fridge may even help, because it acts as a heat buffer, that stabilizes temperature a little. This is also why you often get a few of those cooling blocks.

Why would it affect more than cooling speed?

You can argue that as long as the cool thing you put in the fridge loses all its heat inside, this might only affect the speed it cools, not the efficiency.

That's true in theory, but not so much in practice. The refrigeration cycle (which we use because it's roughly the most energy-efficient way to cool that we know) is a fairly continuous thing.

That fluid needs to go keep going around, and it spends time outside the fridge, so all heat not absorbed from inside the fridge is lost outside it.

And there is some loss in cooling it again.

Yes, you could make it flow slower, but but there is a limit due to some parts of the (phase change) refrigeration cycle, and the specific design.

It ends up being a curve depending on a few things.

My freezer causes freezer burn on food, what do I do?

That's mostly due to direct exposure to the air, which over time slowly freeze-dries food.

Freezer burn is just another name for freeze drying that we didn't intend.

Since very cold air is easily quite dry, and it's preferable to have dry air in there (less frost), the best solution is packaging.

In defrosting a fridge, where does the water go?

The same place that the small but fairly continually generated condensation water goes: out the hole in the bottom, onto a pan over the compressor, which due to being slightly warm will be slowly evaporated.

In theory, extreme amounts of frost can make that overflow. Due to positioning this should nor be an electrical risk, but during manual defrosts you may still care to catch most of the water (use towels), to not have a pool of water everywhere, including there where you can't see it.

Note that this hole is small, and sometimes clogs with food. Declog is with anything pokey (there's nothing much you can damage here), though there often is a plastic doohickey for it (that you may have thrown away not knowing what it is).