Conductivity
and Refrigerators
Thermal Conductivity:
So
we have now established that refrigerators are able to use energy
to produce cooling. However, simply reducing the temperature inside
an enclosed space can be an extremely inefficient method of keeping
food cool. The reason for this is the thermal conductivity of the
insulation materials surrounding the refrigerator.
Thermal
conductivity can be defined as "an exchange of kinetic
energy between microscopic particles in which the less energetic
particles gain energy in collisions with more energetic particles."
Therefore, if a refrigerator is cool on this inside, and the temperature
outside the fridge is around 70 degrees Fahrenheit, then the less
energetic particles are going to want to collide with the more energetic
particles outside of the fridge.
This
picture describes the exchange of heat through a surface, such as
that of a refrigerator wall (Courtsey of Physics
for Scientists and Engineers).
In
order to slow this exchange of energy, insulating materials may
be used. An insulating material is simply something that does not
allow for energy transfer readily through its surface. For
instance, would a glass-enclosed refrigerator keep food cool efficiently?
Probably not, as flat glass has a low R Value of .89 (ft^2 * F *
h / btu). What is an R value you may ask? It is a term that compares
the thermal conductivity (k) divided by the material thickness (l).
With
these new values explained, here is the equation for calculating
the thermal conductivity of several slabs (and thus the rate of
energy transfer):
P
= A (t2-t1) / Sum( R )
where
P is equal to power, A is equal
to area, t2 is the outer temperature and t1 is the inner temperature,
and R is representative of the R values of all the materials summed
(Serway 624). The equation allows for a person to examine all the
materials in a refrigerator and then determine how much energy is
going to be lost through thermal conduction. Foam typically has
an R value of around 5, while state-of-the-art vacuum sealed panels
have R values of around 25 (Glacier Bay Insulation Test Result Tables
1).
This
picture describes the exchange of heat through multiple surfaces.
A refrigerator has several different materials, which encapsulate
its cold compartment (Courtsey
of Physics
for Scientists and Engineers).
With
this is mind, it is ideal to have the best insulator as possible.
While
you can achieve the same level of insulation with foam, the required
thickness of insulation becomes bulky and impractical. The
vacuum-sealed tiles now used in refrigerators can yield energy savings
of up to 20%-30% over traditional foam insulation for the same thickness
(Glacier Bay Insulation Test Result Tables 1).
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