CHAPTER 11
MOBILE AIR-CONDITIONERS
into each of the three states. Others, like paper, oxidize,
INTRODUCTION
or burn, at high temperatures and cannot be converted
Mobile air-conditioners are primarily designed to
into all three. Before paper burns, it changes to a gas,
remove heat produced by operating electronic
but never to a liquid. The science of refrigeration
equipment. Along with the removal of heat, these units
depends upon changes in physical state through heating
also reduce the humidity within electronic equipment
or cooling.
spaces. Although air conditioning is usually thought of
as a lowering of temperature, air can be conditioned by
HEAT AS ENERGY
cooling, heating, filtering, or dehumidifying. Modern
mobile air-conditioners are designed to accomplish all
Heat is a form of energy. It cannot be seen or
of these functions.
shaped. It cannot be created or destroyed. It can only be
transferred from one substance to another.
Mobile air-conditioners are referred to as
mechanical refrigeration systems because of the means
All substances are made up of tiny molecules.
used to circulate a refrigerant through a closed system
These molecules are in constant motion and moving
to accomplish heat transfer. For you to understand the
against each other. As the temperature of the molecules
basic principles of refrigeration, you need to
increases, so does their activity. As heat is taken away,
understand the relationship of heat, temperature, and
their activity and temperature decrease. If all heat is
pressure.
extracted from a substance (absolute zero temperature),
the molecular motion will become dormant.
REFRIGERATION PRINCIPLES
COLDNESS AND HEAT
LEARNING OBJECTIVES: Identify the
principles of air-conditioning. Identify the
Coldness and heat are relative terms. Strictly
refrigeration cycle and the different types of
speaking, coldness is not a distinct condition separate
air-conditioners.
from hotness. The two terms are purely relative,
without exact meanings. They merely express
MATTER DEFINED
temperature conditions with reference to a standard.
For us, this standard is usually the temperature of the
Matter is anything that has weight and occupies
human body, which is normally about 98.6F. If a
space. All substances are forms of matter in one of three
person picks up a piece of ice, he says the ice is cold; he
stages: solid, liquid, or gaseous. An example of a
means that its temperature is lower than the temperature
substance with three stages is water.
of his hand. If he drinks a cup of coffee, he says the
In its natural state water is a liquid. It has weight
coffee is hot; he means merely that its temperature is
and volume, and takes the shape of the container that
higher than that of his mouth. Nevertheless, the ice is
holds it. If it is heated in a closed container to its boiling
warmer than liquid air, for example, and the coffee is
point and more heat is added, it changes to steam
cooler than boiling water.
(vapor), which is its gaseous state. It has weight and
In matters pertaining to refrigeration and air
occupies the volume of the container. When water is
conditioning, it is sometimes preferable to use the
frozen, it changes to its solid state (ice). In this state it
expression cooling, rather than the awkward expression
has weight and volume, and it takes a definite shape.
removing heat. When cooling is used, it is understood
Theoretically, all substances can be converted from
that the operation actually consists of removing heat.
one to another of the three states by the addition or
MEASUREMENT OF HEAT
withdrawal of heat. However, chemical compounds
differ in the ease or difficulty with which they may be
From experience we know that heat and
changed from one to another of the three physical
temperature are related. If heat is added to a substance,
states. Some, like water, can very readily be converted
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