Chevrolet Light Duty Truck 1973, Руководство по обслуживанию

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HEATER A N D AIR C O N D I T I O N I N G 1A-23
refrigeration. But in doing so, we have learned how a
simple icebox works. It’s because the latent heat of
fusion gives ice the ability to soak up quantities of heat
without getting any warmer. Since it stays cold, it can
continue to draw heat away from stored foods and make
them cooler.
The latent heat of vaporization can be even better
because it will soak up even more heat.
Whenever we think of anything boiling, we think of it
being pretty hot, but that’s not true in every case. Just
because water boils at 212° doesn’t mean that all other
substances will boil at the same temperature. Some
would have to be put into a blast furnace to make them
bubble and give off vapor. On the other hand, others will
boil violently while sitting on a cake of ice.
And so each substance has its own particular boiling
point temperature. But regardless of whether it is high or
low, they all absorb unusually large quantities of heat
without getting any warmer when they change from a
liquid into a vapor.
Consequently, any liquid that will boil at a temperature
below the freezing point of water, will make ice cubes
and keep vegetables cool in a mechanical refrigerator.
REFRIGERANTS
The substance that carries heat out of a refrigerator
cabinet is the refrigerant.
There are many refrigerants known to man. In fact, any
liquid that can boil at temperatures somewhere near the
freezing point of water can be used.
But a boiling point below the temperature at which ice
forms is not the only thing that makes a good
refrigerant. A refrigerant should also be non-poisonous
and non-explosive to be safe. Besides that, we want a
refrigerant that is non-corrosive and one that will mix
with oil.
Chemists tried to improve existing natural refrigerants.
But after exploring along that line, they still hadn’t
succeeded. They started from scratch and juggled
molecules around to make an entirely new refrigerant.
Eventually they succeeded by remodeling the molecules
in carbon tetrachloride. This is the same fluid that is
used in fire extinguishers and dry-cleaners’ solvents.
From this fluid, the chemists removed two chlorine
atoms and replaced them with two flourine atoms. This
newly formed fluid carried the technical chemical name
of dichlorodifluoromethane. Today, it is sold commer­
cially by manufacturers as Refrigerant-12 or R-12.
Flourine is a temperamental substance, under most
conditions, it is toxic and highly corrosive. After it is
manufactured, it has to be stored in special containers
because it will eat through glass and will dissolve most
metals in short order.
Despite its rambunctious character though, fluorine is
completely tamed when it is combined with the other
substances that go to make up the refrigerant. Each is
non-toxic, non-inflammable, non-explosive, and non-
poisonous, however, breathing large quantities of R-12
should be avoided.
Refrigerant-12
Refrigerant-12, which we use in Air Conditioning
Systems,
boils
at 21.7° below zero. Picture a flask of R-12
sitting on the North Pole boiling away just like a
teakettle on a stove. No one would dare pick up the flask
with his bare hands because, even though boiling, it
would be so cold and it would be drawing heat away
from nearby objects so fast that human flesh would
freeze in a very short time.
If we were to put a flask of R-12 inside a refrigerator
cabinet, it would boil and draw heat away from
everything surrounding it (fig. 30). So long as any
refrigerant remained in the flask, it would keep on
soaking up heat until the temperature got clear down to
21.7° below zero.
Now we can begin to see the similarity between a boiling
teakettle and a refrigerator. Ordinarily we think of the
flame pushing heat into the teakettle. Yet, it is just as
logical to turn our thinking around and picture the
teakettle pulling heat out of the flame. Both the teakettle
and the flask of refrigerant do the same thing-they both
draw in heat to boil although they do so at different
temperature levels.
There also is another similarity between the icebox and
the mechanical refrigerator. In the icebox, water from
melting ice literally carried heat out of the cabinet. In
our simple refrigerator, rising vapors do the same job.
Reusing R-12
R-12, or any other refrigerant, is too expensive just to let
float away into the Atmosphere. If there was some way
to remove the heat from the vapor and change it back
into a liquid, it could be returned to the flask and used
over again (fig. 31).
LIGHT DUTY TRUCK SERVICE MANUAL