The most commonly used method of cooling
is with vapor-compression
, because it is fairly easy to
construct a cooling device employing this method and the cost is low.
In fact, conventional
use this method of
keep your leftovers and drinks chilled! Air conditioners
employ a vapor-compression cycle to cool the ambient air temperature in
vapor-compression refrigeration employs a heat engine run backwards, so
heat energy is taken from a cold reservoir and deposited into a hot
reservoir. By the Second Law of Thermodynamics, heat energy
not spontaneously transfer from a cold to a hot reservoir. In
order to have heat transfer in that direction (and not from from hot to
cold, as the system is naturally inclined to do), it is necessary to do
work on the system.
This refrigeration cycle is
approximately a Rankine cycle run in reverse. A working fluid
(often called the refrigerant
is pushed through the system and undergoes state changes (from liquid
to gas and back). The latent heat of vaporization of the
refrigerant is used to transfer large amounts of heat energy, and
changes in pressure are used to control when the refrigerant expels or
absorbs heat energy.
However, for a refrigeration cycle that
has a hot
reservoir at around room temperature (or a bit higher) and a cold
reservoir that is desired to be at around 34°F, the boiling
of the refrigerant needs to be fairly low. Thus, various
have been identified as practical refrigerants. The most
include ammonia, Freon (and other chlorofluorocarbon refrigerants, aka
CFCs), and HFC-134a (a non-toxic hydrofluorocarbon).
Stages of the
Vapor-Compression Refrigeration Cycle
The Vapor-Compression Refrigeration
comprised of four steps. The conceptual figure of the process
shows the PV changes during each part.
Part 1: Compression
In this stage, the refrigerant enters
as a gas under low pressure and having a low temperature.
the refrigerant is compressed adiabatically, so the fluid leaves the
compressor under high pressure and with a high temperature.
Part 2: Condensation
The high pressure, high temperature
gas releases heat energy and condenses inside the "condenser"
portion of the system. The condenser is in contact with the
reservoir of the refrigeration system. (The gas releases heat
into the hot reservoir because of the external work added to the gas.)
The refrigerant leaves as a high pressure liquid.
Part 3: Throttling
The liquid refrigerant is pushed through
throttling valve, which causes it to expand. As a result, the
refrigerant now has low pressure and lower temperature,
while still in the liquid phase. (The throttling
be either a thin slit or some sort of plug with holes in it. When the
refrigerant is forced through the throttle, its pressure is reduced,
causing the liquid to expand.)
Part 4: Evaporation
The low pressure, low temperature
the evaporator, which is in contact with the cold reservoir.
Because a low pressure is maintained, the refrigerant is able
boil at a low temperature. So, the liquid absorbs heat from
cold reservoir and evaporates. The refrigerant leaves the
evaporator as a low temperature, low pressure gas and is taken into the
compressor again, back at the beginning of the cycle.
Price 26 March
2007 Physics 212 Web Project