Mazda Training manual — part 261

Air Conditioning Fundamentals

TC070-05-01S

4 – PRINCIPLES OF
REFRIGERATION

18

Principle Two: Temperature and Mass

According to Principle Two, the mass of an object remains the same
regardless of its heat content. For example, the pan of water in Figure
3 weighs one pound whether it is at its freezing point or boiling point.
















Principle Three: Latent Heat of Vaporization

Principle Three states that when matter changes from a liquid to a
vapor, it absorbs a large amount of heat without changing
temperature.

Figure 4 illustrates this principle. At sea level, water exists as a liquid
between 32° and 212° F (0° and 100° C). At exactly 212° F (100° C),
water can be either a liquid or a vapor, depending on how much heat it
contains. By adding more heat to the water, you can eventually
convert it to steam, but the temperature of the steam will remain 212°
F (100° C).

FIGURE 3. Principle
Two – The mass of an
object remains the
same regardless of its
temperature.

FIGURE 4. Principle
Three – When matter
changes from a liquid
to a vapor, it absorbs
large amounts of heat
without changing
temperature

212°F

1 lb. water

1 lb. water

Add heat

Solid

Liquid

Vapor

Remove heat

Air Conditioning Fundamentals

TC070-05-01S

4 – PRINCIPLES OF
REFRIGERATION

19

FIGURE 5. The
latent heat of
vaporization is
heat stored in a
substance as it
changes state
from a liquid to a
vapor

.

Figure 5 illustrates the latent heat of vaporization. Assume you had a
pound of water that was 212° F (100° C), but still a liquid. To change the
liquid water into steam, you would have to apply 970 British Thermal Units
of heat. Eventually, you would create a pound of steam that would still
have a temperature of 212° F (100° C).















As steam, the pound of water would have absorbed 970 BTUs of energy
without any change in temperature. This “latent” heat absorption is the
basis for modern refrigeration systems.

In an A/C system, latent heat of vaporization occurs within the evaporator.
When the refrigerant passes through the evaporator, it absorbs heat from
inside the vehicle and begins to boil. As heat continues to be absorbed,
the refrigerant changes from a low-pressure liquid into a low-pressure
vapor.

212°F

212°F

1 lb. water

1 lb. steam

970 BTUs

Air Conditioning Fundamentals

TC070-05-01S

4 – PRINCIPLES OF
REFRIGERATION

20

Principle Four: Latent Heat of Condensation

Principle Four is the reverse of Principle Three. When a vapor cools
enough to change into a liquid, it releases the latent heat stored while it
was changing into a vapor. The process of changing from a vapor to a
liquid is called condensation, so the heat released in this change of state
is the latent heat of condensation.
Figure 6 illustrates this principle.

In this example, the pound of steam is cooling to its dew point (the
temperature at which it turns back into liquid). As the water changes
state, it releases 970 BTUs. Notice that the pound of water remains at
212° F (100° C) even though it has released a large amount of heat.

In an A/C system, latent heat of condensation occurs within the
condenser. The condenser discharges heat from the refrigerant into the
outside air. As the refrigerant cools, it condenses from a high-pressure
vapor to a high-pressure liquid.

FIGURE 6. The
latent heat of
condensation is
heat released from
a substance as it
changes state from
a vapor to a liquid.

212°F

212°F

1 lb. water

1 lb. steam

970 BTUs

Air Conditioning Fundamentals

TC070-05-01S

4 – PRINCIPLES OF
REFRIGERATION

21

FIGURE 7. To
increase the
boiling point of a
liquid, increase
its pressure. To
decrease the
boiling point,
decrease the
pressure.

Principle Five: Pressure and Boiling Point

Principle Five states that changing the pressure of a liquid or vapor
changes its boiling point. Higher pressure increases the boiling point,
while lower pressure decreases the boiling point, as shown in Figure 7.
An auto air conditioning system uses this principle to remove heat from
the interior of the vehicle.















Previously, we compared boiling water at sea level and at the top of
Pike’s Peak. At sea level, water boils at 212° F (100° C). The lower
atmospheric pressure at Pike’s Peak allows water to boil at only 187° F
(86° C).

Figure 7 shows what happens if you increase the pressure of the water.
At the higher pressure shown in this figure, water will have to reach 260°
F (127° C) before it boils.

Boiling point

260°F

212°F

Boiling point