Basic Principles

CLIMATE CONTROL SYSTEM
The climate control system provides some basic principles and covers some of the service procedures. The text deals with principles, diagnosis and testing procedures, and finally those unique procedures that apply to servicing the refrigerant system.

Automotive air conditioning is a process which uses an electro-mechanical system to apply those scientific principles which work together to produce a desirable comfort level in the passenger compartment of a vehicle. These principles are Heat Transfer, Latent Heat of Vaporization, the Effects of Pressure and Temperature Changes, and the effects of Relative Humidity. The text which follows highlights the content of these principles.

HEAT TRANSFER
If two substances of different temperature 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 one pound of water 0.55°C (1°F). For example, to raise the temperature of one pound of water from 0°C to 100°C (32°F to 212°F), one BTU of heat must be added for each 0.55°C (1°F) rise in temperature or a total of 180 BTUs of heat. Conversely, in order to lower the temperature of one 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 one 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 as 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 one pound of water at 100°C (212°F), will absorb 970 BTUs of heat in changing to vapor at 100°C (212°F). Conversely, the vapor will give off 970 BTUs of heat in 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 refrigeration 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 all Ford Motor Company vehicles.

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 compressor and 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 liquid at the A/C evaporator core orifice and the refrigerant in the A/C evaporator core absorbs heat as air passes around the A/C evaporator core, warming the refrigerant into a gas. It returns to the A/C compressor. As pressures in the closed refrigerant circuit vary, temperatures will also vary. As pressures increase, temperatures also increase; as they decrease, temperatures also decrease.