Chevrolet Light Duty Truck 1973 Service Manual page 46

1A-20 HEATER A N D AIR C O N D IT IO N IN G
heating (above ambient temperatures) is desired, the
vehicle heater must be used. When air conditioning is
desired, the heater should be completely shut off.
This self-contained unit is bracket mounted to the dash
by the body manufacturer. It operates on inside
(recirculated) air only. Air is drawn into the unit, passed
HEAT
We all know what air conditioning does for us but very
few understand how or why it works. An air conditioner
is functionally very similar to a refrigerator. A
refrigerator is a simple mechanism which, surprisingly
enough, works quite a bit like a teakettle boiling on a
stove. That may sound far-fetched, but there is more
similarity between the two than most of us would
suspect. A modern refrigerator can make icecubes and
keep food cool and fresh only because a liquid called the
refrigerant boils inside the freezer.
Everyone knows a boiling teakettle is "hot" and a
refrigerator is "cold". We usually think of "cold" as a
definite, positive condition. The only way we can define
it is in a rather negative sort of way by saying "cold" is
simply the lack of heat, just as darkness is the lack of
light. We can't make things cold directly. All we can do
is remove some of the heat they contain and they will
become cold as a result. And that is the main job of any
icebox or refrigerator. Both are simply devices for
removing heat.
All substances contain some heat. Theoretically, the
lowest temperature that any substance could obtain is
459° Fahrenheit below Zero. This is called "absolute
zero" and anything warmer than this contains heat.
Since man has never succeeded in getting all the heat out
of an object, we must think about the transfer of heat
from one object to another when talking about
controlling temperatures.
Transfer of Heat
The only thing that will attract heat is a colder object.
Like water, which always flows downhill, heat always
flows down a temperature scale - from a warm level
down to a colder one. When we hold our hands out
toward the fireplace, heat flows from the hot fire out to
our cold hands (fig. 23). When we make a snowball, heat
always flows from our warm hands to the colder snow. In
an icebox, the ice always is colder than the stored food,
so heat is drawn out of the warm food by the colder ice.
Measurement of Heat
Everyone thinks he knows how heat is measured.
Thermometers are used in every home.
speak of temperature from
THEORY OF
(Whenever we
now on, we will mean
through the evaporator core (receiving maximum
cooling) and then directed into the vehicle through
adjustable outlets.
A thermostatic switch, located on the face plate is used to
control compressor operation by sensing air temperature
as it leaves the evaporator core.
OPERATION
They can tell how hot a substance is, but
Fahrenheit)
they can't tell us everything about heat.
When we put a teakettle on a stove, we expect it to get
hotter and hotter until it finally boils. All during the
process, we can tell exactly how hot the water is by
means of a thermometer (fig. 24). Our thermometer will
show us that the flame is just as hot when we first put the
teakettle on the stove as it is when the water finally boils.
Why doesn't the water boil immediately? Why does it
take longer to boil a quart of water than a cupful?
Obviously temperature isn't the only measurement of
heat.
Even though heat is intangible, it can be measured by
quantity as well as intensity. Thermometers indicate only
the intensity of heat. The unit for measuring quantity of
heat is specified as that amount necessary to make 1
pound of water 1 degree warmer (fig. 25). We call this
quantity of heat a British Thermal Unit. Oftentimes, it is
abbreviated to B.T.U.
Perhaps we can get a better idea of these two
characteristics of heat if we think of heat as a sort of
coloring dye. If we add one drop of red dye to a glass of
water, it will turn slightly pink. Another drop will make
the water more reddish in color (fig. 26). The more drops
of dye we add, the redder the water will get. Each drop
of dye corresponds to 1 B.T.U. and the succeedingly
deeper shades of red are like increases in temperature.
Fig. 23-Heat Transfer
LIGHT DUTY TRUCK SERVICE MANUAL