Mazda Training manual — part 260

Air Conditioning Fundamentals

TC070-05-01S

3 – A/C TERMS AND CONDITIONS

14

Humidity as a Percent

Humidity — also called relative humidity — is usually expressed as a percent. For example,
80 percent humidity means the air is holding 80 percent of the water vapor it could hold at
its current temperature. One hundred percent humidity means that the air can hold no
additional moisture unless its temperature increases.



High Humidity

High humidity makes a hot day feel even hotter because of the way the human body cools
itself. To cool off, the body releases water to the atmosphere. During low humidity, the
atmosphere absorbs the moisture given off by the body, providing a cooling effect. During
high humidity, the atmosphere cannot absorb the moisture the body gives off. The moisture
condenses on the skin as sweat, causing an uncomfortable “sticky” feeling.

During periods of high humidity, an otherwise normal air conditioning system may appear to
need servicing. When evaluating the performance of an A/C system, you must consider the
relative humidity. The following table shows acceptable readings for various combinations of
temperature and humidity.

Typical System Temperatures

Ambient Air

A/C System

Relative

Humidity

Temperature

(°F)

Evaporator Pressure

(psi)

Vent Outlet

Temperature (°F)

30%

70

28.0

37

80

29.0

37

90

30.5

40

100 36.0

45

110 43.0

56

60%

70 28.0 39

80 30.0 42

90 35.0 46

100 43.0

60

90%

70 28.0 41

80 32.0 48

90 39.0 56

100 50.0

72

Air Conditioning Fundamentals

TC070-05-01S

3 – A/C TERMS AND CONDITIONS

15

REVIEW EXERCISE 2

Fill in the words that complete these sentences correctly. Check your answers with the
answer key on page 18.

1. The amount of heat required to raise the temperature of one pound of water at sea level

by one degree Fahrenheit is called a _______________________.

2. Molecules of gas enclosed in a container constantly bombard the sides of the container.

This force, usually measured in pounds per square inch, is called
_____________________.

3. Warmer air holds (more or less) _____________________ moisture than cooler air.

4. When you apply _____________________to a substance, the molecules begin to move

faster.

5. Heat added to a substance without changing its temperature is called
_____________________.

6. On the Centigrade temperature scale, water boils at __________ degrees.

7. If you increase the pressure of a gas in a closed container, the

_____________________ of the gas will also increase.

Air Conditioning Fundamentals

TC070-05-01S

4 – PRINCIPLES OF
REFRIGERATION

16

OBJECTIVES

After completing this section, you will be able to describe the following basic principles of
refrigeration systems:

• Heat

transfer

•

Relationship of temperature to mass

•

Latent heat of vaporization

•

Latent heat of condensation

•

Relationship of pressure to boiling point

•

Properties of compressed vapor

SIX PRINCIPLES

Air conditioning systems rely on six principles. Understanding these principles will be
valuable as you diagnose and repair automotive air conditioners. You can isolate a problem
quickly if you understand what is happening to the refrigerant as it travels throughout the
system. The result will be a quick, consistent, and accurate diagnosis.

The six principles described in this section are:

1.

Heat always travels from hot to cold.

2.

The mass of an object remains the same regardless of its temperature.

3.

When a liquid changes into a vapor, it absorbs heat.

4.

When a vapor changes into a liquid, it releases heat.

5.

Changing the pressure of a liquid changes its boiling point.

6.

When a vapor compresses, its pressure and temperature increase.

The following topics explain these six principles in detail.

Air Conditioning Fundamentals

TC070-05-01S

4 – PRINCIPLES OF
REFRIGERATION

17

FIGURE 2
Principle One –
Heat always
travels from hot
to cold.

Answers to
Review
Exercise 2

1. British Thermal

Unit, or BTU

2. pressure

3. more

4. heat

5. latent heat (of

vaporization)

6. 100

7. temperature

Principle One: Heat Transfer

Principle One states that heat always flows from hot to cold when objects
are in contact or connected through a heat conductor. The transfer of heat
will continue until both objects reach the same temperature.

For example, the pan of hot water in Figure 2 gives off heat to the
surrounding air because the air is cooler than the water

As the difference in temperature between the two objects increases, the
rate of transfer also increases. For example, if the air in Figure 2 were
even cooler — say 50° F instead of 72° F — the water would cool more
quickly. You may have noticed this effect if you have ever left a cup of hot
coffee unattended for a while. When you return to the cup, the coffee is
about the same temperature as the surrounding air.

72°F

72°F