Chevrolet Light Duty Truck 1973, Service Manual

Page: 47 / 979

HEATER A N D AIR C O N D IT I O N IN G 1A-21
Fig. 24--Measurement of Heat
It may seem a little puzzling to talk about heat in a book
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 indirectly. 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 single
clue.
Ice vs Water For Cooling
Every once in a while in the old days, the iceman would
forget to stop by to refill the icebox. Ocassionally, as the
last sliver of ice melted away, somebody would come up
with a bright idea. He would remember that the water in
the drainpan always felt icecold when he emptied it
other times. So, he would get the thermometer out and
check its temperature. Sure enough, it usually was about
as cold as the ice. Why not put the drainpan back in the
ice compartment to keep things cold until the iceman
returned the next day?
For some strange reason, the icebox never stayed cold.
The drain water soon got quite warm and in a couple of
hours, the butter in the icebox would begin to melt, the
milk would start to sour, and the vegetables would wilt.
The drain water was only a few degrees warmer than the
ice yet it didn’t draw nearly as much heat out of the
stored foods. The difference between the behavior of
cold drain water and ice is the real secret as to how any
refrigerator works, and we can easily see this by using an
ordinary thermometer.
When we put a drainpan full of cold water into the ice
Fig. 25-Quantity of Heat = B.T.U.
compartment, we expect the heat to flow from the warm
foods to the colder water. Remember, that heat always
flows from a warm object to a colder object and when we
add heat to water, it gets warmer. Each B.T.U. of heat
added to a pound of water makes it one degree warmer.
If we were to put a thermometer in the cold drain water,
we would see the temperature gradually creep upwards.
That is to be expected because heat is flowing into the
cold water making it warmer. Before long the water
would be as warm as the stored foods. Then the water
could no longer attract heat because heat will not flow
from one warm object to another equally warm object.
Since we no longer can draw heat out of the foods we no
longer are cooling them.
Now, let’s see what happens when we put ice instead of
cold water into the icebox. This time, we’ll set the
thermometer on top of the ice (fig. 27). When we first
look at the thermometer, it reads 32°. A couple of hours
later, the ice chunk is smaller because some of the ice
LIGHT DUTY TRUCK SERVICE MANUAL