GMC 1976 ZEO 6083 Maintenance Manual page 50

AIR CONDITIONING SYSTEM 1- 33
explosive, and non-poisonous, however, breathing
large quantities of R-12 should be avoided .
Refrigerant-12
Refrigerant-12, which we use in Air Condition-
ing Systems, boils at 21 .7°17. (-5.7 °C.) 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 refriger-
ator cabinet, it would boil and draw heat away from
everything surrounding it (figure 7) . 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°17. (-5 .7°C.) 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 ice-
box 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
Figure 7-A Simple R-12 Refrigerator
Figure 8-Reusing Refrigerant
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 (figure 8) .
That is where we find the biggest difference be-
tween the old icebox and the modern refrigerator .
We used to put in new ice to replace that lost by
melting. Now we use the same refrigerant over and
over again.
We can change a vapor back into a liquid by
chilling it, or do the same thing with pressure . When
we condense a vapor we will find that the heat
removed just exactly equals the amount of heat that
was necessary to make the substance vaporize in the
first place.
This is called the latent heat of vaporization - the
heat that apparently disappeared when a liquid
boiled into a vapor - again reappears - when that
same vapor reverts back into a liquid . It is just like
putting air into a balloon to expand it and then let-
ting the same amount of air out again to return the
balloon to its original condition.
We know that any substance will condense at the
same temperature at which it boiled . This tempera-
ture point is a clear-cut division like a fence. On one
side, a substance is a liquid . Immediately on the
other side it is a vapor. Whichever way a substance
would go, from hot to cold or cold to hot, it will
change its character the moment it crosses over the
fence.
Water will boil at 212°F. (100 °C.) under normal
conditions . Naturally, we expect steam to condense
at the same temperature. But whenever we put pres
sure on steam, it doesn't . It will condense at some
temperature higher than 212°F (100°C .) . The greater
the pressure, the higher the boiling point and the
temperature at which a vapor will condense . This is