Clearly, the refrigerant plays a key
role in achieving vapor cycle air conditioning; it is the medium that
actually carries the heat around the vapor cycle loop from a lower
temperature to a higher one. As noted, the capability of the refrigerant
to transport heat is due mostly to evaporation and condensation
processes. While all substances under the proper circumstances will
condense and evaporate, materials known as "refrigerants" are especially
well-suited for air conditioning applications because they change phase
at practical temperatures and pressures.
Although not a useful refrigerant, the thermal behavior of water is
familiar and provides a basis for the understanding of how actual
refrigerants operate. Consider, for example, a cold open vessel of water
placed on a stove at atmospheric pressure. As heat from the stove is
transferred to the water, the water temperature rises continuously -
until it begins to boil. After that, and until all the water has been
evaporated, the remaining boiling liquid stays at the temperature known
as its "boiling point" (212ºF /100º C for water) at atmospheric
pressure. However, in the event that the water was placed in a vessel
whose pressure could be increased during the time heat was added from
the stove, the boiling point would increase above 212ºF.
This effect occurs in an ordinary "pressure-cooker" and is used to
more rapidly cook food because of the higher temperature of boiling
resulting from increased pressure. In simple terms, the effect of
increasing the pressure on the water is to actually "push" the
evaporating water (steam vapor) back into liquid form. Therefore, in
order for boiling to take place under a higher pressure, the water
temperature must be increased enough (i.e.: "atomically speeded-up") to
re-enable the water vapor to emerge from the liquid. For example,
increasing the pressure only six pounds per square inch (psi) over
atmospheric increases water's boiling point to 230ºF. On the other hand,
decreasing the pressure over the water by six psi reduces its boiling
point to about 180ºF. It is very important to also realize that the
boiling point is also the "condensation point" in that if heat is
withdrawn from the vessel, the vapor will condense into liquid - at the
same temperature until it has all condensed.
Thus, one of the most important technical aspects of a refrigerant is
its thermal behavior at various temperatures and pressures.
Specifically, a refrigerant that absorbs heat by boiling (evaporating)
below the freezing point of water (32ºF/0ºC) while at a relatively low
pressure, would be attractive. On the other hand, the same refrigerant
must be able to condense at temperatures not greatly higher than normal
ambient temperatures and at pressures attainable by modern refrigerant
compressors.
An example of such a refrigerant is known as HFC-134a, a
commonly-used ozone-safe substance made of hydrogen, fluorine and
carbon. This refrigerant boils at about 15ºF at around two atmospheres
(30 psia) pressure. At 200 psia, HFC-134a condenses at 125ºF, a
temperature high enough to reject heat to most outside environmental
conditions. (Of course, if the environmental temperature is even higher,
the refrigerant merely requires compression to high pressures to reject
its heat.)
In summary, then, the important information regarding a refrigerant
is the relationship between its pressure and its temperature while
changing from a liquid to a gas and visa versa. This information has
been compiled by refrigerant manufacturers for a large variety of
refrigerants and is available in many forms including simple tabulated
sets of data.
