Re: How Refrigerators Work


W. Watson wrote:
The following is an excerpt from "Hos Stuff Works" on the topic of
refrigerators, question at the bottom, step 3:

"The second experiment is extremely interesting if you think about it in the
following way: Imagine some creature that is able to live happily in an oven
at 400 degrees Fahrenheit. This creature thinks 400 F is just great -- the
perfect temperature (just like humans think that 70 F is just great). If the
creature is hanging out in an oven at 400 F, and there is a cup of water in
the oven boiling away at 212 F, how is the creature going to feel about that
water? It is going to think that the boiling water is REALLY cold. After
all, the boiling water is 188 degrees colder than the 400 F that this
creature thinks is comfortable. That's a big temperature difference!
(This is exactly what is happening when we humans deal with liquid nitrogen.
We feel comfortable at 70 F. Liquid nitrogen boils at -320 F. So if you had
a pot of liquid nitrogen sitting on the kitchen table, its temperature would
be -320 F, and it would be boiling away -- to you, of course, it would feel
incredibly cold.)
Butane Lighters
If you go to the local store and buy a disposable butane lighter with a
clear case (so that you can see the liquid butane inside), what you are
seeing is liquid butane stored in a high-pressure container. Butane boils at
31 degrees F at normal atmospheric pressure (14.7 PSI). By keeping butane
pressurized in a container, it remains liquid at room temperature. If you
took a cup of butane and put it on your kitchen counter, it would boil, and
the temperature of the boiling liquid would be 31 F.
The boiling point of butane, by the way, also explains why butane lighters
don't work very well on cold winter days. If it is 10 degrees Fahrenheit
outside, the butane is well below its boiling point, so it cannot vaporize.
Keeping the lighter warm in your pocket is what allows it to work in the
winter.
Modern refrigerators use a regenerating cycle to reuse the same refrigerant
over and over again. You can get an idea of how this works by again
imagining our oven creature and his cup of water. He could create a
regenerating cycle by taking the following four steps:
1. The air temperature in the oven is 400 degrees F. The water in the cup
boils away, remaining at 212 F but producing a lot of 400 F steam. Let's say
the creature collects this steam in a big bag.
2. Once all the water boils away, he pressurizes the steam into a steel
container. In the process of pressurizing it, its temperature rises to 800 F
and it remains steam. So now the steel container is "hot" to the creature
because it contains 800 F steam.
3. The steel container dissipates its excess heat to the air in the oven,
and it eventually falls back to 400 F. In the process, the *high-pressure
steam in the container condenses into pressurized water* (just like the
butane in a lighter -- see sidebar [BELOW]).
4. At this point, the creature releases the water from the steel pressurized
container into a pot, and it immediately begins to boil, its temperature
dropping to 212 F.
By repeating these four steps, the creature now has a way of reusing the
same water over and over again to provide refrigeration."

Side Bar
"Butane Lighters
If you go to the local store and buy a disposable butane lighter with a
clear case (so that you can see the liquid butane inside), what you are
seeing is liquid butane stored in a high-pressure container. Butane boils at
31 degrees F at normal atmospheric pressure (14.7 PSI). By keeping butane
pressurized in a container, it remains liquid at room temperature. If you
took a cup of butane and put it on your kitchen counter, it would boil, and
the temperature of the boiling liquid would be 31 F.
The boiling point of butane, by the way, also explains why butane lighters
don't work very well on cold winter days. If it is 10 degrees Fahrenheit
outside, the butane is well below its boiling point, so it cannot vaporize.
Keeping the lighter warm in your pocket is what allows it to work in the
winter."

Why does the steam in step 3 condense to water if the temp is still 400F in
the container?


Because it's under pressure. Boiling point rises with pressure. The
example assumes that the pressure is high enough to cause the boiling
point to be between 400 and 800 degrees.

Paul Cardinale

.

Relevant Pages

  • Re: How Refrigerators Work
    ... It is going to think that the boiling water is REALLY cold. ... (This is exactly what is happening when we humans deal with liquid nitrogen. ... clear case (so that you can see the liquid butane inside), ... 31 degrees F at normal atmospheric pressure. ...
    (sci.physics)
  • How Refrigerators Work
    ... If the creature is hanging out in an oven at 400 F, and there is a cup of water in the oven boiling away at 212 F, how is the creature going to feel about that water? ... (This is exactly what is happening when we humans deal with liquid nitrogen. ... If you go to the local store and buy a disposable butane lighter with a clear case, what you are seeing is liquid butane stored in a high-pressure container. ...
    (sci.physics)
  • Re: Dumb Question
    ... I have some vauge knowledge of vapor pressure, ... The boiling point is that temp at which the liquid's vapor pressure ... liquid, leading to interesting BLEVEs when oil storage fires are ... A liquid that expands upon freezing (water, silicon, ...
    (sci.physics)
  • Re: thinkin of you guys...
    ... Keep in mind that it's not really gas inside the propane cynlinders ... Dropping the pressure just lowers the boiling point of the liquid ...
    (uk.people.support.depression)
  • Re: Cedar Sticks: Anyone still use them?
    ... My butane specifically says NOT to shake it. ... Shaking the can raises the pressure in the can. ... liquid butane remains. ...
    (alt.smokers.cigars)