Cooling things: Difference between revisions
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In freezers, and in fridges with freezer compartments, | In freezers, and in fridges with freezer compartments, | ||
that means any moisture in the air will | that means any moisture in the air will condense and freeze on those noticeably-cooler elements. | ||
That is what [https://en.wikipedia.org/wiki/Frost frost] ''is''. | That is what [https://en.wikipedia.org/wiki/Frost frost] ''is''. | ||
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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. | 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''. | ||
And over time, there will be a lot. | |||
Frost buildup 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. | |||
Some designs may try to take moisture out. | |||
But that only postpones the problem. | |||
It more usually means ensuring frost happens in one main place, where we can deal with it more easily. | |||
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. | 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. | Yes, this means the temperature inside the fridge would fluctuates a little. | ||
But it means the evaporator should never be overwhelmed with ice, | But it also 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. | so you never have to deal with defrosting it, or having it defrost at unpredictable times. | ||
Without this, the evaporator would slowly be insulated and become less and less effective. | |||
Yes, self-defrosting takes more energy than not doing so -- in the short run. | |||
Again, enough ice on the evaporator would make it inefficient over time. | |||
After a year or so (order of magnitude), chances are it's doing badly. | |||
Exactly where such defrosting a feature lies between 'convenient no-brainer' and 'where is the most efficient choice possible' is hard to know. | |||
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. | 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. | ||
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An even cheaper solution may be that fridges just ''just switch off for a while''. | |||
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Low Frost sometimes refers to a specific variant, | Low Frost sometimes refers to a specific variant, | ||
and sometimes to the fact that No Frost is usually a lie. | and sometimes to the fact that No Frost is usually a lie. | ||
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'''Manual defrost''' | '''Manual defrost''' | ||
You may wish to let it warm up completely. | You may wish to let it warm up completely. | ||
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Yes | Yes. | ||
But modern freezers tend to have a feature that regularly defrosts the cooling element with a heater - see the previous section. | |||
This means that there may be ice | This may means that there may be ice in the compartment, | ||
it may not affect cooling much until it starts clogging up airflow to where the actual cooler is. | |||
Revision as of 13:30, 30 June 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.