MadSci Network: Engineering

Re: explain in detail, the physics of refrigeration

Date: Tue Apr 21 02:10:26 1998
Posted By: Don Pettibone, Other (pls. specify below), Ph.D. in Applied Physics, Quadlux Inc.
Area of science: Engineering
ID: 892569444.Eg

Scientists use many different types of refrigerators, depending upon the 
temperature that is needed.  Some really exotic refrigerators have been in 
the news lately.  These use laser cooling (after a bunch of other 
refrigerators have gotten things fairly cool) to obtain temperatures of 
about 10 nanokelvin, only 10 billionths of a degree above absolute zero!  
At these temperatures some strange properties of matter are being explored.  
No one knows what this work might lead to, so scientists are very excited 
about it.

I think your question is asking about the refrigerators we use in our homes 
for keeping food cold.  This same basic type of refrigerator is used in air 
conditioners to supply cool air to keep homes and offices cool.  
Refrigerators work by taking energy and using it to pump heat from a low 
temperature reservoir, the thing you are trying to cool, to a high 
temperature reservoir.  On your refrigerator at home you might have noticed 
that the coils in back, or underneath, the refrigerator get very hot.  This 
heat is dumped into the room, which serves as the high temperature 
reservoir.  Incidentally, this means that you can’t cool your kitchen on a 
hot day by leaving the refrigerator door open.  The extra heat the coils 
pass to the room air will more than make up for the cool air coming out of 
the refrigerator.  In order to cool your room you need to hang the high 
temperatures coils outside, as an air conditioner does.  This means that 
you are actually heating up the outdoors a bit when you run your air 

Now we will consider the details of how a refrigerator cools something.  
Home refrigerators use the fact that a phase change from liquid to gas 
requires the input of a lot of energy.  You have probably noticed that if 
you are wet and it is windy you can get cold very quickly.  This is due to 
the evaporation of water from your skin, which cools your skin a lot.  This 
is also the basis for how sweating allows us to cool ourselves on hot days.  
If you have a droplet of liquid and some of the droplet evaporates 
(changing from a liquid to a gas), it gets the extra energy needed to boost 
the molecules from the liquid to gaseous phase from the rest of the 
droplet.  This loss of energy shows up as a reduction in temperature of the 
droplet.  The cold liquid can cool the surrounding material, which in turn 
cools the gas.   Inside the refrigerator the refrigerant starts out as a 
liquid.  In order for it to be a liquid at room temperature it needs to be 
pressurized.  The liquid refrigerant is pumped into another section where 
its pressure is reduced and it then evaporates.  It is at this stage that 
the refrigerant becomes very cold, due to the effect described above.  This 
cold gas is pumped through coils that run through the freezer and 
refrigerator sections.  Next a compressor pressurizes the gas, which heats 
it up.  The hot gas circulates through a condenser.  The condenser is a 
coiled set of tubes that are outside the refrigerator.  The condenser gives 
up the heat of the gas to room air which circulates over the condenser 
tubes.  When the temperature in the condenser is low enough the gas 
condenses back to the liquid phase.  Then the cycle can start all over.  
Refrigerators that have perfect insulation would only need to run long 
enough to cool the food down to the set temperature, then it could turn 
off.  However, refrigerator insulation is imperfect, so heat leaks into the 
food chamber necessitating that the refrigerator run from time to time to 
maintain the desired temperature.

Next we will talk about why it takes energy to boost a molecule from the 
liquid phase into the gaseous phase.  In a liquid there are weak bonds 
between neighboring molecules.  These bonds are not strong enough to hold 
the molecules rigidly in place, as in a solid.  The molecules can sort of 
roll around one another, but always keeping some nearby neighbors.  If you 
try to separate a single molecule from a bunch of its buddies and let it 
wander around as a single molecule, in the gaseous phase, it will take 
energy to pull it away from the other molecules.  This energy can only come 
from the other molecules, and they all wind up slowing down just a little, 
which is equivalent to a reduction in temperature.  Think of it as a sort 
of lottery.  Say it takes $100.00 in order to buy your way out of a 
particularly bad class.  All of the people in the class contribute a dollar 
and then the lucky winner is chosen.  They take the money and run, leaving 
the rest of the class slightly poorer.  Every time a student evaporates 
from class they leave the students that are left somewhat poorer, analogous 
to the liquid molecules being at at lower temperature.

There is another effect known as the Joule-Thomson effect which I think 
contributes slightly to the cooling of the gas.  If gas is pressurized and 
then allowed to expand so that the gas stream has some bulk velocity, then 
the gas cools down.  You may have noticed this when you let the air out of 
a tire through a valve very quickly.  The escaping gas is so cold that 
frost can form on the valve.  If you can get the gas to perform some net 
work while it expands, all the better, and the gas will cool further.

There is more to be said about equilibrium states where vapor and liquid 
are sealed up together in a container.  In such a situation evaporation is 
balanced by condensation, and there is no net cooling.  For this reason, 
sweating does not work on a hot day if the humidity is 100%, since water 
will not show any net evaporation.  I’ll close by saying that it is the 
energy that you pump into the system that keeps the system from being in a 
state of equilibrium.  The playwright Tom Stoppard paraphrased the Second 
Law of Thermodynamics as saying, roughly, that in a sealed system, 
everything comes to room temperature.  Fortunately in our universe there is 
a lot of energy available that is not in thermal equilibrium with us, or 
things would get very boring.

“The Feynman Lectures on Physics” (look up evaporation in Volume I),

“The Second Law”, P.M. Atkins, Scientific American Books, 1984.

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