Cooling things: Difference between revisions
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"What is a BTU?" | "What is a BTU?" | ||
A British Thermal Unit (BTU, Btu) was is defined as the heat required to heat a specific mass of water by 1 degree Farenheit (there is an analogous definition for Joules) | A British Thermal Unit (BTU, Btu) was is defined as the heat required to heat a specific mass of water by 1 degree Farenheit (there is an analogous definition for Joules). | ||
Though there are varying definitions, they differ less than 0.5%. | |||
In terms of SI, a BTU equals about 1055 Joules | In terms of SI, a BTU equals about 1055 Joules | ||
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BTU is useful as a measure of | BTU is useful as a measure of how much heat we can consistently move. | ||
For example, in an AC, it works out as an indication of how well it will keep a specific volume of air down. | For example, in an AC, it works out as an indication of how well it will keep a specific volume of air down. | ||
It's imperfect at that (because | It's imperfect at that (because that's also related to the amount of temperature difference, how well insulated something is, how well ventilated), yet it's still great for estimation - you might need 5000 BTU for a small room, 10000 for a larger room, and 50000 for a small house. | ||
A window AC may be 10000 BTU, a small portable variant may be half that. | A window AC may be 10000 BTU, a small portable variant may be half that. | ||
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It looks like it's just convention. | It looks like it's just convention. | ||
People aren't used to thinking in Joules. | |||
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===COP, EER=== | ===COP, EER=== | ||
Revision as of 13:17, 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.