GMC 1976 ZEO 6083 Maintenance Manual page 49

1- 32 AIR CONDITIONING SYSTEM
it is only a shade warmer than the ice itself. It is not
nearly warm enough to account for all the heat the
ice had absorbed . The only possible answer is that
the latent heat had been used up to change the ice
from a solid into a liquid .
Many substances can be either a solid, or a liquid,
or a gas. It just depends on the temperature whether
water for example was a liquid, or a solid (ice), or gas
(steam) (figure 5) .
All solids soak up huge amounts of heat without
getting any warmer when they change into liquids,
and the same thing will happen when a substance
changes from a liquid into a gas.
Put some water in a teakettle, set it over a fire and
watch the thermometer as the water gets hotter and
hotter, the mercury will keep rising until the water
starts to boil . Then the mercury seems to stick at the
212°F. (100°C .) mark . Put more wood on the fire,
despite all the increased heat, the mercury will not
budge above the 212°F. (100°C .) mark (figure 6) .
No matter how large or hot you make the flame,
you can't make water any hotter than 212° at sea
level. As a liquid changes into a gas, it absorbs abnor
mally great amounts of heat without getting any hot-
ter.
Now we have two different kinds of latent heat,
which are quite a bit alike. To keep their identities
separate, the first one is called latent heat of fusion,
which means the same as melting. The other kind is
called latent heat of vaporization because that means
the same as evaporation.
Refrigeration
It may seem as though we have discussed heat
instead of refrigeration. But in doing so, we have
Figure 6-Boiling Water Never Exceeds 212T .
(100°C .) at Sea Level
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°F. (100°C .)
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
change from a liquid into a vapor.
Consequently, any liquid that will boil at a tem-
perature 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 refrigera-
tor cabinet is the refrigerant .
There are many refrigerants known to man. In
fact, any liquid that can boil at temperatures some-
where 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 refrig-
erants . But after exploring along that line, they still
hadn't succeeded. They started from scratch and jug
gled molecules around to make an entirely new re-
frigerant. Eventually they succeeded by remodeling
the molecules in carbon tetrachloride. This is the
same fluid that was used in fire extinguishers and
dry-cleaners' solvents .
From this fluid, the chemists .removed two chlo-
rine atoms and replaced them with two flourine
atoms. This newly formed fluid carried the technical
chemical name of dichlorodifluoromethane . Today,
it is sold commercially by manufacturers as Refriger-
ant-12 or R-12. Non-tox, non-inflammable, non-