Refrigerators
Last updated: December 9, 2009.
Now here's a cool idea: a metal box
that helps your food last longer! Have you ever stopped to think how a
refrigerator keeps cool, calm, and collected even in the blistering
heat of summer? Food goes bad because bacteria breed inside it. But
bacteria grow less quickly at lower temperatures, so the cooler you can
keep food, the longer it will last. A refrigerator is a machine that keeps food cool with some very clever
science. All the time your refrigerator is humming away, liquids are turning
into gases, water is turning into ice, and your food is staying
deliciously fresh. Let's take a closer look at how a refrigerator works!
Photo: A typical domestic refrigerator or "fridge." Freezers work in a similar way,
but cool down to a much lower temperature.
How to move something you can't even see
Suppose your chore for today is to empty a stable full of rank
smelling horse manure. Not the nicest of jobs, so you'll want to do it
as quickly as possible. You won't be able to move it all at once,
because there's too much of it. To get the job done fast, you need to
move as much manure as you can in one go. The best thing to do is use a
wheelbarrow. Pile the manure up into the barrow, wheel the barrow
outside, and then empty the manure into a pile in the stable yard. With
a few of these trips, you can shift the manure from inside the stable
to outside.
Moving something you can see is easy. But now let's give you a
harder chore. Your new task is to move the heat from the inside of a
refrigerator to the outside to keep your food fresh. How can you move
something you can't see? You can't use a wheelbarrow this time. Not
only that, but you can't open the door to get at the heat inside, or
you'll let the heat straight back in again. Your mission is to remove
the heat, continually, without opening the door even once. Tricky
problem, eh? But it's not impossible—at least not if you understand the
science of gases.
How to move heat with a gas
Let's step sideways a moment and look at how gases behave. If you've
ever pumped up the tires on a bicycle,
you'll know that a bicycle pump
soon gets quite warm. The reason is that gases heat up when you compress (squeeze) them. To make the tire support
the weight of the bicycle and your body, you have to squeeze air into
it at a high pressure. Pumping makes the air
(and the pump it passes through) a little bit hotter. Why? As you
squeeze the air, you have to work quite hard with the pump. The
energy you use in pumping is converted into potential
energy in the compressed gas: the gas in the tire is at a higher
pressure and higher temperature than the cool air around you. If you
squeeze a gas into half the volume, the heat energy its molecules
contain fills only half as much space, so the temperature of the gas
rises (it gets hotter).
Left: Gases get hotter when you compress into less volume them because you have to work to
push their energetic molecules closer together.
What happens if you release a gas that's stored at high pressure?
When you spray an aerosol air freshener, you've probably noticed that
the spray is really cold—for exactly the opposite reason that a bicycle
pump gets hot. When you release the gas, it is suddenly able to expand and occupy much more volume. The heat
energy its molecules contain is now divided over a much bigger volume
of space, so the temperature of the gas falls (it gets cooler).
Photo: Right: Liquids can turn to gases (and gases cool down) when you let them expand into more volume. That's why aerosol sprays feel so cold.
The heating and cooling cycle
By compressing gases, we make them hotter; by letting them expand,
we make them cooler. How can we use this handy bit of physics to shift
heat from the inside of a refrigerator? Suppose we made a pipe that was
partly inside a refrigerator and partly outside it, and sealed so it
was a continuous loop. And suppose we filled the pipe with a gas.
Inside the refrigerator, we could make the pipe gradually get wider, so
the gas would expand and cool as it flowed through it. Outside the
refrigerator, we could have something like a bicycle pump to compress
the gas and release its heat. If the gas flowed round and round the
loop, expanding when it was inside the refrigerator and compressing
when it was outside, it would constantly pick up heat from the inside
and carry it to the outside like a heat conveyor belt.
And, surprise surprise, this is almost exactly how a refrigerator
works. There are some extra details worth noting. Inside the
refrigerator, the pipe expands through a nozzle known as an expansion valve. As the gas passes through it, it
cools dramatically. This bit of science is sometimes known as the
Joule-Thomson (or Joule-Kelvin) effect for the physicists who
discovered it, James Prescott Joule (1818–1889) and William Thomson
(Lord Kelvin, 1824–1907). You won't be surprised to discover that the compressor outside the refrigerator is not really
a bicycle pump! It's actually an electrically powered pump. It's the
thing that makes a refrigerator hum every so often. The compressor is
attached to a grill-like device called a condenser
(a kind of thin radiator behind the refrigerator) that expels the
unwanted heat. Finally, the gas that circulates round the pipe is
actually a specially designed chemical that alternates between being a
cool liquid and a hot gas. This chemical is known as the coolant or refrigerant.
Photo: Humid air inside your fridge contains
water vapor. When the refrigerator cools, this water turns to ice. The
coldest part of your fridge is the icebox at the top. That's because
the expansion valve is placed right next to it.
