Basic Principles

FUNDAMENTAL CONCEPTS
Air conditioning is the cooling or refrigeration of the air in the passenger compartment. Refrigeration is accomplished by making practical use of three laws of nature. These laws and their practical application are described in the following paragraphs.

HEAT TRANSFER
If two substances of different temperatures are placed near each other, the heat in the warmer substance will always travel to the colder substance until both are of equal temperature.

For example, a cake of ice in an ice box does not communicate its coldness to the bottle of milk standing nearby. Rather, the heat in the warm milk automatically flows into the ice.

To determine the amount of heat that transfers from one substance to another, science uses the British Thermal Unit (BTU). One BTU is the amount of heat required to raise the temperature of 0.45 kg (1 pound) of water 0.55°C (1°F).

For example, to raise the temperature of 0.45 kg (1 pound) of water from 0°C to 100°C (32°F to 21 2°F), one BTU of heat must be added for 0.55°C (1° F) rise in temperature or a total of 180 BTUs of heat. Conversely, in order to lower the temperature of 0.45 kg (1 pound) of water from 100°C to 0°C (212°F to 32°F), 180 BTUs of heat must be removed from the water.

LATENT HEAT OF VAPORIZATION
When a liquid boils (changes to a gas), it absorbs heat without raising the temperature of the resulting gas. When the gas condenses (changes back to a liquid), it gives off heat without lowering the temperature of the resulting liquid.

For example, place 0.45 kg (1 pound) of water at 0°C (32°F) in a container over a flame. With each BTU of heat that the water absorbs from the flame, its temperature rises 0.55°C (1° F). Thus, after it has absorbed 180 BTUs of heat, the water reaches a temperature of 100°C (212°F).

Even though the flame continues to give its heat to the water, the temperature of the water remains at 100°C (212°F). The water, however, starts to boil or change from the liquid to the gaseous state. It continues to boil until the water has passed off into the atmosphere as vapor. If this vapor were checked with a thermometer, it also would show a temperature of 100°C (212°F).

In other words, there was a rise of only 100°C (212°F) (from 0°C to 100°C or 32°F to 212°F) in the water and vapor temperature even though the flame applied many more than 180 BTUs of heat. In this case, the heat is absorbed by the liquid in the process of boiling and disappears in the vapor. If the vapor were brought in contact with cool air, the hidden heat would flow into the cooler air and the vapor condensed back to water. Scientists refer to this natural law as the latent (hidden) heat of vaporization.

Water has a latent heat of vaporization of 970 BTUs and a boiling point of 100°C (212°F). This means that 0.45 kg (one pound) of water at 100°C (212° F), will absorb 970 BTUs of heat when changing to vapor at 100°C (212°F). Conversely, the vapor will give off 970 BTUs of heat when condensing back to water at 100°C (212°F).

This tremendous heat transfer, occurring when a liquid boils or a vapor condenses, forms the basic principle of all conventional refrigerant systems.

For a liquid to be a refrigerant, it must also have a low boiling point. That is, the temperature at which it boils must be lower than the substance to be cooled.

R-134a is a non-CFC refrigerant used in this vehicles. Its temperature/pressure relationship makes it suitable for mobile air conditioning systems.

EFFECT OF PRESSURE ON BOILING OR CONDENSATION
As refrigerant passes through an air conditioning system, it flows under high-pressure conditions, first as a high-pressure vapor between the A/C compressor and the A/C condenser core, then as a high-pressure liquid between the A/C condenser core and the A/C evaporator core orifice. It expands to a low-pressure vapor between the A/C evaporator core orifice and the refrigerant return port in the A/C compressor. As pressures in the closed refrigerant circuit vary, temperatures will also vary. As pressure increases, temperatures also increase; as the pressure decreases, temperatures also decrease. The refrigerant flow diagram highlights the changes that take place.