In the kitchen of nearly every home in America
there is a refrigerator. Every 15 minutes or so you hear the
turn on, and it magically keeps things cold. Without
refrigeration, we'd be throwing out our leftovers instead of
saving them for another meal.
The refrigerator is one of those miracles of modern living
that totally changes life. Prior to refrigeration, the only
way to preserve
meat was to salt it, and iced beverages in the summer were a
In this edition of HowStuffWorks,
you'll find out how your refrigerator performs its magic.
We'll also look at cold packs, electronic coolers and the
propane refrigerators found in RVs.
The Purpose of Refrigeration
The fundamental reason for having a refrigerator
is to keep food cold. Cold temperatures help food stay fresh
longer. The basic idea behind refrigeration is to slow down
the activity of bacteria (which all food contains) so
that it takes longer for the bacteria to spoil the food.
For example, bacteria will spoil milk in two or three hours
if the milk is left out on the kitchen counter at room
temperature. However, by reducing the temperature of the milk,
it will stay fresh for a week or two -- The cold temperature
inside the refrigerator decreases the activity of the bacteria
that much. By freezing the milk you can stop the bacteria
altogether, and the milk can last for months (until effects
like freezer burn begin to spoil the milk in non-bacterial
Refrigeration and freezing are two of the most common forms
preservation used today. For more information on other
ways to preserve food, see How
Food Preservation Works.
Parts of a Refrigerator
The basic idea
behind a refrigerator is very simple: It uses the evaporation
of a liquid to absorb heat. You probably know that when you
put water on your skin it makes you feel cool. As the water
evaporates, it absorbs heat, creating that cool feeling.
Rubbing alcohol feels even cooler because it evaporates at a
lower temperature. The liquid, or refrigerant, used in
a refrigerator evaporates at an extremely low temperature, so
it can create freezing temperatures inside the refrigerator.
If you place your refrigerator's refrigerant on your skin (not
a good idea), it will freeze your skin as it evaporates.
There are five basic parts to any refrigerator (or air-conditioning
mechanism of a refrigerator works like this:
- The compressor compresses the refrigerant gas. This
raises the refrigerant's pressure and temperature (orange),
so the heat-exchanging coils outside the refrigerator allow
the refrigerant to dissipate the heat of pressurization.
- As it cools, the refrigerant condenses into liquid form
(dark blue) and flows through the expansion valve.
- When it flows through the expansion valve, the liquid
refrigerant is allowed to move from a high-pressure zone to
a low-pressure zone, so it expands and evaporates (light
blue). In evaporating, it absorbs heat, making it cold.
- The coils inside the refrigerator allow the refrigerant
to absorb heat, making the inside of the refrigerator cold.
The cycle then repeats.
This is a fairly standard -- and somewhat unsatisfying --
explanation of how a refrigerator works. So let's look at
refrigeration using several real-world examples to understand
what is truly happening.
what is happening inside your refrigerator, it is helpful to
understand refrigerants a little better. Here are two
experiments that help you see what is happening.
ExperimentsThese experiments can
help you understand the properties of gases and their
role in refrigeration.
You will need:
Put the pot of water on the stove,
stick the thermometer
in it and turn on the burner. You will see (if you are
at sea level) that the temperature of the water rises
until it hits 212 F. At that point, it will start
boiling, but will remain at 212 F -- this is the boiling
point of water at sea level. If you live in the
mountains, where the air pressure is lower than it is at
sea level, the boiling point will be lower -- perhaps
between 190 and 200 F. This is why many foods have
"high-altitude cooking directions" printed on the box.
You have to cook foods longer at high altitudes.
- A pot of water
- A thermometer
that can measure up to at least 250 degrees F
- A stove
You will need:
Put the thermometer
in your container of water, put the container in the
oven and turn it to 400 F.
- An oven-safe glass bowl
- A thermometer that can measure up to at least 450
- An oven
As the oven heats up, the temperature of the water
will again rise until it hits 212 F, and then start
boiling. The water's temperature will stay at 212 F even
though it is completely surrounded by an environment
that is at 400 F. If you let all of the water boil away
(and if the thermometer has the range to handle it), as
soon as the water is gone the temperature of the
thermometer will shoot up to 400 F.
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
(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.)
LightersIf 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:
By repeating these four steps, the creature now has
a way of reusing the same water over and over again to provide
- 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.
- 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.
- 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).
- 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.
Now let's take a look at how these four steps apply to your
The Refrigeration Cycle
The refrigerator in
your kitchen uses a cycle that is similar to the one described
in the previous section. But in your refrigerator, the cycle
is continuous. In the following example, we will assume that
the refrigerant being used is pure ammonia, which boils at -27
degrees F. This is what happens to keep the refrigerator cool:
- The compressor compresses the ammonia gas. The
compressed gas heats up as it is pressurized (orange).
- The coils on the back of the refrigerator let the
hot ammonia gas dissipate its heat. The ammonia gas
condenses into ammonia liquid (dark blue) at high pressure.
- The high-pressure ammonia liquid flows through the
You can think of the expansion valve as a small hole. On
one side of the hole is high-pressure ammonia liquid. On the
other side of the hole is a low-pressure area (because the
compressor is sucking gas out of that side).
- The liquid ammonia immediately boils and vaporizes
(light blue), its temperature dropping to -27 F. This makes
the inside of the refrigerator cold.
- The cold ammonia gas is sucked up by the
compressor, and the cycle repeats.
By the way, if you have ever turned your car off on a hot
summer day when you have had the air conditioner running, you
may have heard a hissing noise under the hood. That noise is
the sound of high-pressure liquid refrigerant flowing through
the expansion valve.
Pure ammonia gas is highly toxic to people and would pose a
threat if the refrigerator were to leak, so all home
refrigerators don't use pure ammonia. You may have heard of
refrigerants know as CFCs (chlorofluorocarbons),
originally developed by Du Pont in the 1930s as a non-toxic
replacement for ammonia. CFC-12 (dichlorodifluoromethane) has
about the same boiling point as ammonia. However, CFC-12 is
not toxic to humans, so it is safe to use in your kitchen.
Many large industrial refrigerators still use ammonia.
In the 1970s, it was discovered that the CFCs then in use
are harmful to the ozone layer, so as of the 1990s, all new
refrigerators and air conditioners use refrigerants that are
less harmful to the ozone layer.
In the next section, we'll talk about gas and propane
Gas and Propane Refrigerators
If you own an
RV or use a refrigerator where electricity is not available,
chances are you have a gas- or propane-powered refrigerator.
These refrigerators are interesting because they have no
moving parts and use gas or
propane as their primary source of energy. Also, they use
heat, in the form of burning propane, to produce the cold
inside the refrigerator.
A gas refrigerator uses ammonia as the coolant, and it uses
water, ammonia and hydrogen gas to create a continuous cycle
for the ammonia. The refrigerator has five main parts:
The cycle works like this:
- Generator - generates ammonia gas
- Separator - separates ammonia gas from water
- Condenser - where hot ammonia gas is cooled and
condensed to create liquid ammonia
- Evaporator - where liquid ammonia evaporates to
create cold temperatures inside the refrigerator
- Absorber - absorbs the ammonia gas in water
offers an extremely detailed description of the process.
- Heat is applied to the generator. The heat comes from
burning something like gas, propane or kerosene.
- In the generator is a solution of ammonia and water. The
heat raises the temperature of the solution to the boiling
point of the ammonia.
- The boiling solution flows to the separator. In the
separator, the water separates from the ammonia gas.
- The ammonia gas flows upward to the condenser. The
condenser is composed of metal coils and fins that allow the
ammonia gas to dissipate its heat and condense into a
- The liquid ammonia makes its way to the evaporator,
where it mixes with hydrogen gas and evaporates, producing
cold temperatures inside the refrigerator.
- The ammonia and hydrogen gases flow to the absorber.
Here, the water that has collected in the separator is mixed
with the ammonia and hydrogen gases.
- The ammonia forms a solution with the water and releases
the hydrogen gas, which flows back to the evaporator. The
ammonia-and-water solution flows toward the generator to
repeat the cycle.
Next, we'll look at electric coolers.
You may have seen the new
coolers that don't use ice, plugging into your car's cigarette
lighter instead. These coolers rely on a process known as the
Peltier effect, or thermoelectric effect, to
produce cold temperatures electronically.
You can create the Peltier effect with a battery,
two pieces of copper wire and a piece of bismuth or iron wire.
Just connect the copper wires to the two poles of the battery,
and then connect the bismuth or iron wire between the two
pieces of copper wire. The bismuth/iron and copper must touch
-- it is this junction that causes the Peltier effect.
The junction where current flows from copper to bismuth
will get hot, and the junction where current flows from
bismuth to copper the junction will get cold. The maximum
temperature drop is about 40 F from the ambient temperature
where the hot junction is located.
To create a Peltier cooler, the hot junction is placed
outside the refrigerator, and the cold junction is placed
inside. Normally, you create a module containing many
junctions to amplify the effect. See the links at the end of
this article for details on the Peltier effect.
Now let's take a look at what's going on inside a cold
Speaking of refrigeration and
coldness, have you ever used one of those "instant cold packs"
that looks like a plastic bag filled with liquid. You hit it,
shake it up and it gets extremely cold. What's going on here?
The liquid inside the cold pack is water. In the
water is another plastic bag or tube containing
ammonium-nitrate fertilizer. When you hit the cold
pack, it breaks the tube so that the water mixes with the
fertilizer. This mixture creates an endothermic
reaction -- it absorbs heat. The temperature of the solution
falls to about 35 F for 10 to 15 minutes.
Check out this
page to learn about endothermic reactions.
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