Cooling things: Difference between revisions
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Passive cooling tends to mean 'what happens with no moving parts'. | Passive cooling tends to mean 'what happens with no moving parts'. | ||
...so whatever amount of conduction, radiation, or convection would happen ''anyway''. | ...so whatever amount of conduction, radiation, and/or convection would happen ''anyway''. | ||
Sometimes includes adding a fan. | Sometimes includes adding a fan, to add to the convection. | ||
You're stirring the air better than just convection would, so heat transfer goes a | You're stirring the air better than just convection would, so heat transfer goes a faster than if warm air just sits around - but the difference is rarely much -- convection always does this at least a little when there is temperature difference (if you're in gravity; this is about density differences). | ||
And you could argue that's ''technically'' active cooling (because you're adding work, so using energy), | And you could argue that's ''technically'' active cooling (because you're adding work, so using energy), | ||
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(If you have a datacenter in the arctic, you might actually get a lot from just having a fan outside | (If you have a datacenter in the arctic, you might actually get a lot from just having a fan outside -- if you didn't care about moisture anyway; you'd actually use a heat exchanger but this could be passive beyond the pumps | ||
In most places, passive cooling doesn't move a lot of heat, though) | |||
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It turns out | It turns out to not make a lot of difference ''why'' evaporation happens - it will take away heat regardless. | ||
So when evaporation happens without adding heat, it cools somewhat. | |||
{{comment|(If you want a technical term, you could call it the [https://en.wikipedia.org/wiki/Latent_heat latent cooling] of vaporization)}} | |||
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==Free cooling== | ==Free cooling== | ||
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Free cooling means "use the fact that it is cold outside to cool inside". | |||
For example, in cold climates, you can cool industrial processes or datacenters just by ''moving'' water or air around. | |||
Yes, you could open the window, but more controlled in terms of humidity but also the actual temperature, | |||
so this often amounts to heat exchangers to cool an otherwise closed system. | |||
If temperature difference are seasonal, you may prefer heat pumps - putting in a little more energy | |||
for a lot more over-the-year guarantees, so just mean part of the system becomes more efficient in some seasons. | |||
This is no longer ''free'' cooling, but still ery useful. | |||
If there is a significant day-night differences in temperature, | If there is a significant day-night differences in temperature, | ||
it may make sense to have reasonably-sided buffers. | it may make sense to have reasonably-sided buffers. | ||
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As in, a little 12V device rated at a few watts, to at most a few dozen watts. | As in, a little 12V device rated at a few watts, to ''at most'' a few dozen watts. | ||
They can eventually pull moisture out of a sealed-enough room. | |||
These are often basically a peltier with two | These are often basically a [[peltier element]] with two heat sinks: | ||
: one large one on the warm side, to drive off heat (just to keep the peltier working), | |||
: one small one to drip condensated water from. | |||
So they will gently heat the air while dehumidifying. | So they will gently heat the air while dehumidifying, but that is an even subtler by-effect | ||
because that will happen at a little less than the watt rating it has. | |||
There are somewhat chunkier mains-powered variants, | |||
which are effectively small ACs | |||
''without'' managing the airflow that would cool one room at the cost of the temperature of another, as normal ACs do. | |||
In fact, they will intentally flow air slowly. | |||
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In various countries, ACs are designed that way. | In various countries, ACs are designed that way. | ||
And yes, | |||
And yes, this setup is more efficient than a basic resisive heater. | |||
But not as efficient as a variation of this that is ''designed'' to do this. | |||
When they are not, you could still mount them in reverse, but it's not going to be nearly as efficient | When they are not, you could still mount them in reverse, but | ||
* it's not going to be nearly as efficient (if its CoP is still >1 like this, then it is still better than your basic resistive heater). | |||
* there may be reasons it will not work like that, or isn't the safest thing to do. | |||
For various reasons. | For various reasons. | ||
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==More on fridges and freezers== | ==More on fridges and freezers== | ||
===Super!=== | ===Super!=== | ||
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Terms like '''SuperFrost''' amount to temporarily cooling more. | |||
More than would normally be required for a stable temperature. | |||
Why? Well, for context, freezers are ''slow''. Once they're cold, they don't have to work hard at all, | |||
because they're insulated well. Mostly they're 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 than the target. | |||
With Superfrost ('''Supercool''' seems to be the same idea but for fridges rather than freezers, but these terms are sometimes brand specific so eh), | |||
you make it cool harder for a while. | |||
Still without measuring temperature much, so if you do this without a reason, you might get it to be maybe 10 degrees colder than usual for a while. | |||
So | So should you do this before you put the groceries in, or after? | ||
Before means the average temperature should now go between colder-than-usual to usual temperature, | |||
though also that some things may freeze more than you want. | |||
At the same time or after means less time of other things being warmer, | |||
In a freezer the same may apply, but is usually less important because a freezer is often -18C | |||
making it unlikely the temperature the rise will be enough for anything to melt. | making it unlikely the temperature the rise will be enough for anything to melt. | ||
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===On frost=== | ===On frost=== | ||
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In the olden days, a fridge was no more complex than a cooler element in a box. | 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, | 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 | that means any moisture in the air will, over time, condense and freeze on those noticeably-cooler elements. | ||
That is what [https://en.wikipedia.org/wiki/Frost frost] ''is''. | |||
This matters more clearly to freezers, where ''everything'' inside is is below freezing point (-18° C, 0° F). | This matters more clearly to freezers, where ''everything'' inside is is below water's 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) | 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) is often still a little colder. | ||
Bottom line, when there is ''anything'' below freezing, and you add air, you add moisture that will eventually become frost somewhere. | Bottom line, when there is ''anything'' below freezing, and you add air, you add moisture that will eventually become frost ''somewhere''. | ||
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so tend to have less problems with frost. | so tend to have less problems with frost. | ||
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====Does a lot of ice make a fridge/freezer less efficient?==== | |||
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Yes, but modern freezers 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. | |||
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It ends up being a curve depending on a few things. | It ends up being a curve depending on a few things. | ||
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====My freezer causes freezer burn on food, what do I do?==== | ====My freezer causes freezer burn on food, what do I do?==== | ||
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That's mostly due to direct exposure to the air, | That's mostly due to direct exposure to the air, | ||
which over time slowly freeze-dries food. | which over time slowly freeze-dries food. | ||
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and it's ''preferable'' to have dry air in there (less frost), | and it's ''preferable'' to have dry air in there (less frost), | ||
the best solution is packaging. | the best solution is packaging. | ||
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====In defrosting a fridge, where does the water go?==== | ====In defrosting a fridge, where does the water go?==== | ||
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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. | 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. | ||
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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). | 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). | ||
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Latest revision as of 15:55, 31 January 2024
Physical mechanics of cooling
Passive cooling
Passive cooling tends to mean 'what happens with no moving parts'.
...so whatever amount of conduction, radiation, and/or convection would happen anyway.
Sometimes includes adding a fan, to add to the convection.
You're stirring the air better than just convection would, so heat transfer goes a faster than if warm air just sits around - but the difference is rarely much -- convection always does this at least a little when there is temperature difference (if you're in gravity; this is about density differences).
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.