Cooling things

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Physical mechanics of cooling

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

On the technical side

Moving heat, in general, happens though:

  • conduction
a good conductor spreading heat throughout it more quickly
something might get cooled if it is in direct contact with something it can move heat into -- but that is rarely a large effect
conduction helps cooling in the sense that if heat can spread easily, it more quickly gets to areas where radiation and convection can happen
  • radiation
specifically thermal radiation - radiated as EM at the surface
which in most of our everyday cases means 'if it is hot, it will radiate heat from its surface; more surface make that go faster'
  • convection
fluid passing over something will exchange heat into that fluid
whether that fluid is air (poor exchange, but it's there anyway)
or water, or oil (better exchange, but if it's more management to keep it clean)
as a bonus, that heat exchange may set up more flow (caused by heat changing temperatures and densities - plus gravity) that sustains more exchange for a while
e.g. warming up air will create a draft


Notes:

  • In practice there's more than one of these happening, but often one that counts for most exchange.
  • Those three happen easily and all over the place.
There are a handful more, which usually require some specific design,
and more when we startnaming specific designs.
Say, phase change, often focusing on evaporation - as used in the refrigeration cycle.
  • The speed at which you can move heat is generally proportional to the amount of temperature difference.
  • conduction has some everyday intuitions - that might be distracting
you know how metal feels colder to the touch than most other materials?
that's not because it is lower temperature somehow
but because most materials heat up locally first and we are used to that happening within seconds
this sense of "metal feels cold" only happens
because the quick movement of heat means it heats up evenly
is more pronounced when you touch it when it is below body temperature - you will barely get this sense touching metal when it's 35 C outside.


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 without any help.


In most places, passive cooling doesn't move a lot of heat, in part because the temperature difference is never large.


Passive plus a little help

Heatsink plus fan

A fan on a heatsink is frequently a good idea because it often keeps the temperature difference at the point of transfer higher, so the heat transfer keeps going a faster than if warm air just sits around.

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).


Technically this is active cooling - because you're adding work, so using energy - but intuitively it feels like it hardly qualifies.

In particular because heatsink would probably stir the air a little by convection, and we're just ensuring that that happens, a little more, and even if there is a casing in the way.
Just stirring better is not going to make the world of difference. It also does not feel like not a completely different category.



Evaporative cooling

Refrigeration cycle

Refrigerants

Heat pumps

Free cooling

Dessicant based systems

Peltier effect and related effects

Theory

Peltier device behaviour and uses

Ratings

Transport

On efficiency

Using the effects - practice and products

When passive cooling works

Convection in houses; Windows and windcatchers

Device cooling

Passive device cooling

Heatsinks

Heat pipes and vapor chambers

Personal cooling

Fans / Why wind helps even when you're not sweating

Spray bottle, misting system, etc.

Some small tricks

Wrapped towel

Ice

Swamp coolers

Mini dehumidifier

"Personal ACs"

Air conditioning

One-hose and two-hose portable air conditioners

Car AC

Is a flipped AC a heater?

"Eco coolers"

Passive effects

Windows

Parts of designs

Heat exchanger

Unsorted

Cold and hot drinks

Ice packs

BTU

COP, EER

Adding some more considerations: SCOP, EER2, SEER, SEER2, CEER
Comparison?

More on fridges and freezers

Super!

On frost

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

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

In defrosting a fridge, where does the water go?