GMC 1976 ZEO 6083 Maintenance Manual page 47

1- 30 AIR CONDITIONING SYSTEM
A-2228
THIS IS THE
BASIS OF ALL
AIR CONDITIONING
SYSTEMS
II/
UJIM
nil
IN
HEAT FLOWS FROM A WARMER
OBJECT TO A COOLER ONE
Measurement of Heat
Figure 1-Heat Transfer
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 (figure 1) . When
we make a snowball, heat always flows from out
warm hands to the colder snow .
Everyone thinks he knows how heat is measured .
Thermometers are used in every home .
(Whenever we
speak of temperature from now on, we will mean Fahren-
heit) .
They can tell how hot a substance is, but 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 (figure 2) . Our
thermometer will show us that the flame is just as hot
Figure 2-Measurement of Heat
Figure 3-Quantity of Heat
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 mea-
sured by quantity as well as intensity. Thermometers
indicate only the intensity of heat . The unit for mea
suring quantity of heat is specified as that amount
necessary to make 1 pound of water 1 degree warmer
(figure 3) . 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. 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
succedingly deeper shades of red are like increases in
temperature.
It may seem a little puzzling to talk about heat
in a manual on air conditioning . . .but, when you stop
to think about it, we are handling heat exclusively .
Although we ordinarily think of an air conditioner
as a device for making air cold, it does that in-
directly . What it does is to take heat away from the
air and transfer that heat outside the vehicle.
We know now that cold is nothing more than the
absence of heat, and that heat always flows from a
warm object to a colder one. We also have a clearer
idea of how heat is measured .
From everything we've learned about heat so far,
it seems to behave in a perfectly normal manner . Yet
sometimes heat will disappear without leaving a sin-
gle clue .