GMC 1976 ZEO 6083 Maintenance Manual page 53

1- 36 AIR CONDITIONING SYSTEM
That is the way most mechanical refrigerators
work today. Now, let's look at air conditioning to see
the benefits of air conditioning and how an air condi-
tioner works.
AIR CONDITIONING
Because air-conditioning has always been very
closely allied with mechanical refrigeration, most of
us are apt to think of it only as a process for cooling
room air.
Air Conditioning goes beyond the mere cooling
of the air. It controls the humidity, cleanliness and
circulation of the air .
Whenever it gets warm and muggy in the sum-
mertime, someone is almost sure to say, "It's not the
heat ...it's the humidity ." But that is only partly right.
Actually it is a combination of the two that makes us
feel so warm . . . temperature alone is not the only thing
that makes us uncomfortable.
Humidity is the moisture content of the air. To
a certain extent, it is tied in with the temperature of
the air. Warm air will hold more moisture than will
cold air. When air contains all the moisture it can
hold, it is saturated, and the relative humidity is
100% . If the air contains only half as much water as
it could hold at any given temperature, we say that
the relative humidity is 50% . If it contains only a
fifth of its maximum capacity, we say that the rela-
tive humidity is 20% . This amount of water vapor,
or relative humidity, affects the way we perspire on
hot days .
Nature has equipped our bodies with a network
of sweat glands that carry perspiration to the skin
surfaces . Normally, this perspiration evaporates and
absorbs heat just like a refrigerant absorbs heat when
it is vaporized in a freezer. Most of the heat is drawn
from our bodies, giving us a sensation of coolness . A
drop of alcohol on the back of your hand will demon-
strate this principle convincingly . Alcohol is highly
volatile, and will evaporate very rapidly and absorb
quite a bit of heat in doing so, making the spot on
your hand feel cool .
The ease and rapidity with which evaporation
takes place, whether it be alcohol or perspiration,
governs our sensation of coolness and to a certain
extent, independently of the temperature . The ease
and rapidity of the evaporation are directly affected
by the relative humidity or comparative dampness of
the air. When the air is dry, perspiration will evapo-
rate quite readily. But when the air contains a lot of
moisture, persipiration will evaporate more slowly ;
consequently less heat is carried away from our
body .
From the standpoint of comfort, air-conditioning
should control the relative humidity of the air as well
as its temperature.
By reducing the humidity, we oftentimes can be
just as "cool" in a higher room temperature than
otherwise would be comfortable. Laboratory tests
have shown that the average person will feel just as
cool in a temperature of 79°F . (26.I°C .) when the
relative humidity is down around 30%F . (22.2°C .) as
he will in a cooler temperature of 72° with a high
relative humidity of 90% .
There are practical limits though within which
we must stay when it comes to juggling humidity .
For comfort, we can't go much below a relative hu
midity of 30% because anything lower than that
would cause an unpleasant and unhealthy dryness in
the throat and nasal passages .
Summertime temperatures of 85°F . (29.4°C .)
sometimes bring with them relative humidities
around 75% to 80%. To gain maximum human
comfort, an air conditioning system should cool the
air down and reduce the humidity to comfortable
limits .
Along with the cooling job it does, the evaporator
unit also removes much of the moisture from the air .
Everyone is familiar with the sight of thick frost on
the freezer of a refrigerator . That frost is simply
frozen moisture that has come out of the air.
The evaporator unit as an air conditioning sys-
tem does the same thing with this one exception.
Because its temperature is above the freezing point,
the moisture remains fluid and drips off the chilling
unit . A further advantage of air conditioning is that
dust and pollen particles are trapped by the wet sur-
faces of the evaporator core and then drained off
along with the condensed moisture . This provides
very clean, pure air for breathing.
BASIC AIR CONDITIONER
When we look at an air conditioning unit, we will
always find a set of coils or a finned radiator core
through which the air to be cooled passes . This is
known as the "evaporator" . It does the same job as
the flask of refrigerant we spoke about previously .
The refrigerant boils in the evaporator . In boiling, of
course, the refrigerant absorbs heat and changes into
a vapor . By piping this vapor outside the vehicle we
can bodily carry out the heat that caused its creation.
Once we get vapor out of the evaporator, all we
have to do is remove the heat it contains . Since heat
is the only thing that expanded the refrigerant from
a liquid to a vapor in the first place, removal of that
same heat will let the vapor condense into a liquid