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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For example, in cold climates, you can cool industrial processes or datacenters just by moving | 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 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, 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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It doesn't cool quite as quickly, but that's generally not an issue. | It doesn't cool quite as quickly, but that's generally not an issue. | ||
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===Is a flipped AC a heater?=== | |||
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In various countries, ACs are designed that way. | |||
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 (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. | |||
Function: Most ACs them will stop moving heat if the cold side is below 15C or so. | |||
Efficiency: An 'hot outside, cool inside' AC is generally not going to push for more than 15 C of difference. | |||
" When it's freezing outside, you need more than that. | |||
Efficiency/safety: This also carries into the size of the condenser and evaporator. | |||
: Would this lead to pressure issues? | |||
Efficiency: Various designs deal with cold-side condensation by vaporizing it. That doesn't make much sense when that's outside. | |||
These and other potential issues are designed out of heat pump systems. | |||
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Low-E | Low-E | ||
Glass is by nature emissive. | Glass is by nature emissive (0.8, 0.9), though multiple-pane can reduce that drastically. | ||
Double glazing tries to avoid connecting the inside temperature to the outside pane to be emitted | |||
(because there will be very little convection going on compared to a single sheet of glass), | |||
triple pane more so. | |||
Because heat loss (measured by U-value) and solar energy heat gain (may be seen measured in 'solar factor') are entirely separate concerns | |||
that apply to different climates (and, in theory, seasons), windows that care about heat may do any of: | |||
: be double or triple pane to lower convection-based loss | |||
: have a coating that that lowers trasmission - by reflecting sunlight's heat before it enters, but lets most visible light through | |||
:: this also exists in foil form, though those are often not quite as good at the job, and may peel off | |||
: have a coating that that lowers the emissivity | |||
Because low-emissivity ("Low-E") windows are a combination of all, | |||
some people incorrectly and confusing use the term to refer to windows that do only some of that. | |||
In theory you could also put reflective coating inside to keep heat in, but that's barely worth it because most heat is lost through convection, not from radiation (...in most situations. Fireplaces are not so common). | |||
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https://homeadvisor.com/r/low-e-windows/ | https://homeadvisor.com/r/low-e-windows/ | ||
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Energy label rating is often based on these figures. | Energy label rating is often based on these figures. | ||
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==More on fridges and freezers== | |||
===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 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. | |||
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. | |||
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===On frost=== | |||
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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 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 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) 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''. | |||
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. | |||
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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. | |||
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. | |||
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====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, | |||
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. | |||
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====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. | |||
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). | |||
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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.