In article ,
John Cochran wrote:
...
The Air Conditioner. This little device doesn't "generate cold". What it does
is move heat from one location to another. But heat only goes from hotter
to colder. So if it's 100 degrees outside and you want it to be 75 degrees
inside, you have a problem. You need to make something colder than 75 degrees
inside and make something hotter than 100 degrees outside. The air conditioner
works as follows:
1. Compress the refrigerant gas.
Some high school physics will show you that you now have made the
gas hotter. In an ideal world, if you now released the pressure on
this compressed gas, you then get some uncompressed gas at its
original temperature.

It's been such a long time since I had a course
in thermodynamics, and even then I didn't really
understand it.

So what I say here might be totally wrong ...

Above, given an ideal world (no friction, everything
done super-slowly, etc), are you saying that you
could compress a gas, then let it escape (through
a small hole?), and you'd be back to where you started?

Especially, that if you could get that escaping gas
to do some "work", like drive a turbine or piston
or something, that it'd all net out to zero?

Probably you're not saying that -- but if you are,
or if someone thinks you are, then I have a dim
recollection of something called a "carnot engine"
or something like that, and there's some inherent
max efficiency you can get, and never any more --
some work done to compress the gas is just plain
lost, gone, unrecoverable for useful work.

Again, I never did really understand this stuff --
maybe someone who does can expand on it, or maybe
even show that I'm just plain wrong.

David

(David Combs) wrote in news:cgh9d2$ecc$1
@reader1.panix.com:

[snip]
I have a dim recollection of something called a "carnot engine"
or something like that, and there's some inherent
max efficiency you can get, and never any more --
some work done to compress the gas is just plain
lost, gone, unrecoverable for useful work.

The perfect heat engine utilizes the Carnot Cycle (you can't do any better
than that for heat engine efficiency):

http://hyperphysics.phy-astr.gsu.edu...mo/carnot.html

Again, I never did really understand this stuff --
maybe someone who does can expand on it, or maybe
even show that I'm just plain wrong.

Here's a page that describes it in fairly simple terms:
http://oak.cats.ohiou.edu/~piccard/p...ot/carnot.html

At the end, it also shows how heat pump efficiency is calculated if the
Carnot cycle is used -- notice that the COP (Coefficient of Performance)
depends strongly on the temperatures involved. Given the right
temperatures, a heat pump is the most efficient way to heat or cool a
space.