Mazda Training manual - part 259

Air Conditioning Fundamentals

TC070-05-01S

 
3 – A/C TERMS AND CONDITIONS 

 

10

OBJECTIVE 
 
After completing this section, you will be able to define basic terms and concepts related to 
air conditioning systems. 
 
 
 
INTRODUCTION TO AIR CONDITIONING TERMS   
 
All matter can exist in one of three states: solid, liquid, or gas. The chemical properties of a 
specific substance like water or iron determine which state it assumes under various 
temperatures and pressures. 
 
Water, for example, is a liquid at room temperature and sea level. At very cold temperatures, 
though, water becomes a solid (ice). At high temperatures, water becomes a gas, or vapor 
(steam). 
 
The chemical changes that occur as matter changes from solid to liquid to gas are the basis 
for all modern refrigeration systems, including automobile air conditioners. To understand 
these changes, you need to know the terms used to describe them. 
 
The following pages define and illustrate these important terms. Keep in mind, the 
remaining sections of this Guide use the vocabulary presented in this section. 
 
 
 
PRESSURE 
 
All matter consists of small, moving particles called molecules. If you enclose a gas such as 
steam in a container, the molecules constantly bump into the sides of the container. The 
force of gas molecules against the inside surface of a closed container is pressure. We 
measure pressure in pounds per square inch, or psi. 
 
Assume we enclose gas in a container and attach a pressure gauge. The gauge shows a 
pressure of 10 psi. This reading means the molecules of gas are hitting the sides of the 
container often enough to exert 10 pounds of force on each square inch of the container’s 
inside surface. 

Air Conditioning Fundamentals

TC070-05-01S

 
3 – A/C TERMS AND CONDITIONS 

 

11

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

Answers to 

Review 
Exercise 1 

1. temperature, 

humidity, and 
air flow 

 

2. heat 

 

3. change of 

state 

 

4. louvers 

 
5. Cooling and 

dehumid– 
ifying

 

 

 
 
 
 
 
 
 

We can increase the pressure of the enclosed gas by: 
 
1. Decreasing the size of the container, 
 
2. Adding more gas to the container, or 
 
3. Heating the contents of the container. 
 
 
Reducing the Size of the Container 
 
If we reduce the size of the container and keep the same amount of gas 
inside, the space between the molecules will decrease, causing the 
molecules to strike the sides more often. This results in increased 
pressure. When the pressure of the gas rises, its temperature also rises. 
 
Adding Gas 
 
If we force more gas into the closed container, the molecules will strike the 
sides more often, and the pressure will rise. If we measure the 
temperature of the gas at this point, we will also find that its temperature 
has increased. As the pressure of a gas increases, so does its 
temperature. 
 
Heating the Contents 
 
If we heat the gas, the molecules already in motion will speed up. They will 
strike the sides more often, causing pressure to rise. Since the molecules 
are moving faster, the temperature of the substance rises. As the motion 
of the molecules increases, so does the temperature. 
 
Effects of Pressure on Boiling Points 
 
Pressure affects the boiling point of liquids. Boiling point is the temperature 
at which a substance changes from liquid to vapor. When a substance is 
highly pressurized, it is harder for the molecules to separate and change 
into vapor. This means that the higher the pressure of a substance, the 
higher its boiling point will be. At lower pressure, the molecules can 
separate more easily, so the substance will boil at a lower temperature. 

Air Conditioning Fundamentals

TC070-05-01S

 
3 – A/C TERMS AND CONDITIONS 

 

12

Let’s assume you want to boil a pot of water. At sea level, atmospheric pressure is about 15 
psi, and the water will boil at 212° F (100° C). However, pressure drops as elevation 
increases. If you took that same pot of water to the top of Pike’s Peak (over 14,000 feet 
above sea level), it would boil at only 187° F. 
 
When you think of boiling water, you think of a very hot liquid. However, not all liquids need 
to be hot to boil. For example, the refrigerant liquid “R-12” boils at 22° F.    Another common 
refrigerant, “R-134a,” boils at -15° F. The chemical makeup of these two substances causes 
their low boiling points, which make them good refrigerants. 
 
 
 
HEAT 
 
You can think of heat as a measure of molecular motion. When you apply heat to matter, 
the molecules already in motion begin to speed up. The faster the molecules move, the 
hotter the substance becomes. When you remove heat, the molecules slow down. 
 
For example, as water loses heat, its molecules slow down, growing closer together until 
they form solid ice. If you add heat to the ice, the water returns to liquid form. If you continue 
adding heat, the molecules in the liquid speed up and move farther apart, forming the gas 
we call steam. The same principles hold true for a piece of steel. If you add enough heat, 
the solid steel will turn to a liquid and then a vapor. 
 
 
 
Measuring the Properties of Heat 
 
You cannot measure heat itself. However, heat intensity and quantity are measurable. 
Temperature is the measure of heat intensity, and British Thermal Units, or BTUs, measure 
heat quantity. 
 
 
 
Temperature 
 
Temperature measures heat intensity in units called degrees. We normally use one of two 
scales to express temperatures. 
 
On the Fahrenheit scale, water freezes at 32 degrees, and boils at 212 degrees. On the 
Centigrade scale, water freezes at 0 degrees, and boils at 100 degrees. Although these two 
scales are different, they both measure heat intensity. 

 

Air Conditioning Fundamentals

TC070-05-01S

 
3 – A/C TERMS AND CONDITIONS 

 

13

In our previous discussion of pressure, we talked about how an increase in the pressure of 
a gas causes its temperature to rise, even without adding an external heat source. As you 
increase pressure, the molecules of gas move closer together, so they contact each other 
more frequently. These contacts cause friction and accelerated movement, producing heat. 
The temperature of the gas will continue to rise until you either reduce pressure or apply 
external cooling. 
 
 
 
British Thermal Unit (BTU) 
 
A BTU is the amount of energy needed to raise the temperature of one pound of water at 
sea level by one degree Fahrenheit. 
 
 
 
Latent Heat 
 
Latent heat is the amount of heat you must add or remove from a substance to make it 
change state. It is called latent heat because you cannot measure it with a thermometer. 
 
For example, if you heat a pound of water at sea level to 212° F (100° C), you must 
continue to apply at least 940 BTU’s of heat energy to convert it to steam. As you add these 
BTUs, though, the water temperature remains unchanged. The energy added to the pan 
speeds up the water molecules but has no effect on temperature. 
 
The latent heat applied to change a substance from a liquid to a vapor is called the latent 
heat of vaporization. The latent heat removed when a substance changes from a vapor to a 
liquid is called the latent heat of condensation. 
 
 
 
HUMIDITY 
 
Humidity is the amount of moisture in a sample of air at a particular pressure and 
temperature. Warmer air can hold more moisture than cooler air.