GMC 1976 ZEO 6083 Maintenance Manual page 48

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 . Ocassion-
ally, as the last sliver of ice melted away, somebody
would come up with a bright idea . He would remem-
ber 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 vegeta-
bles 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 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 flow
ing 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
Figure 4-Melting ice Remains at 32°F . (0°C .)
AIR CONDITIONING SYSTEM 1- 31
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 (figure 4) .
When we first look at the thermometer, it reads 32°F .
(0°C .) A couple of hours later, the ice chunk is
smaller because some of the ice has already melted
away - but the thermometer still reads 32°F (0°C.) .
All this time, the ice has been soaking up heat, yet
it never gets any warmer no matter how much heat
it draws from the stored food . On the other hand, the
cold drain water got progressively warmer as it
soaked up heat . The addition of heat will make water
warmer yet won't raise the temperature of ice above
the 32°F . (0°C .) mark .
If we fill one drinking glass with ice and another
with cold water, and put both glasses in the same
room where they could absorb equal amounts of heat
from the room air, we will find it takes much, much
longer for the ice to melt and reach room tempera-
ture than it did for the water in the other glass to
reach the same temperature. Obviously, most of the
heat was being used to melt the ice. But it was the
heat that apparently disappeared or was transformed
because it couldn't be located with a thermometer .
To describe this disappearing heat scientists chose
the word "latent" which means hidden .
Latent Heat
So latent heat is nothing more or less than hidden
heat which can't be found with a thermometer.
At first it was thought that latent heat was in the
water that melted from the ice. But that wasn't ex-
actly the right answer because, upon checking water
temperature as it melts from ice, it will be found that
Figure 5-Temperature Determines the State of
Water