Basic Principles

BASIC PRINCIPLES
Vehicle 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 of nature and their practical application are outlined below.

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.

In order to determine the amount of heat that transfers from one substance to another, science has established a definite standard of measurement called the British Thermal Unit, or 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 (32°F) to 100°C (212°F), one BTU of heat must be added for each 0.55°C (1°F) rise in temperature, or a total of 180 BTU's of heat. Conversely, in order to lower the temperature of one pound of water from 100°C (212°F) to O°C (32°F), 180 BTU's 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 BTU's 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 and it continues to boil until it is passed off into the atmosphere as vapor.

If this vapor were collected in a container and checked with a thermometer, it also would show a temperature of 100°C (212°F). In other words, there was a rise of only 82°C (180°F) from 0° to 100°C (32°-212°F) in the water and vapor temperature, even though the flame applied many more than 180 BTU's 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 into contact with cool air, the hidden heat would reappear and 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 BTU's and a boiling point of 100°C (212°F). This means that one pound of water at 100°C (212°F) will absorb 970 BTU's of heat in changing to vapor at 100°C (212°F). Conversely, the vapor will give off 970 BTU's of heat in condensing back to water. The tremendous heat transfer that occurs when a liquid boils or a vapor condenses is an important factor for all conventional refrigeration systems.

For a liquid to be a good 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. To illustrate this fact, place a bottle of milk at room temperature, 21.6°C (70°F), next to boiling water, 100°C (212°F). The heat would flow from the water (higher temperature) to the milk (lower temperature). The milk would be heated rather than cooled because the boiling point of water is too high.

In order to make practical use of the heat transfer that takes place when a liquid boils, we must choose a liquid with a low boiling point. Refrigerant 134a is used in automotive air-conditioning systems because it boils at -29.85°C (-21.7°F) in an open container. Here is a liquid that boils or vaporizes well below passenger compartment temperatures and, in vaporizing, will absorb tremendous amounts of heat without getting any warmer.

Effect of Pressure on Boiling or Condensation
The saturation temperature is the temperature when boiling or condensation of a liquid or vapor occurs. The saturation temperature of a liquid or vapor increases or decreases according to the pressure exerted on it.

In a fixed orifice tube refrigerant system, liquid refrigerant R-134a is stored in the A/C condenser core (19712) under high pressure. When the liquid R-134a is released into the A/C evaporator core by the A/C evaporator core orifice (19D990), the resulting decrease in pressure and partial boiling lowers the temperature of the R-134a to its new boiling point. As the R-134a flows through the A/C evaporator core, air passes over the outside surface of the evaporator coils. As it boils, the R-134a absorbs heat from the air and thus cools the passenger compartment. The heat from the passenger compartment is absorbed by the boiling refrigerant and held as latent heat in the vapor. The refrigeration cycle is now under way. The following steps complete the cycle:
1. Dispose of the heat in the vapor.
2. Convert the vapor back to liquid for reuse.
3. Return the liquid to the starting point in the refrigeration cycle.

The A/C compressor (19703) and the A/C condenser core perform these functions. The A/C compressor pumps the refrigerant vapor (containing the latent heat) out of the A/C evaporator core and suction accumulator/drier (19C836), then forces it under high pressure into the A/C condenser core, which is located in the outside airstream at the front of the vehicle. The increased pressure in the A/C condenser core raises the R-134a condensation or saturation temperature to a point higher than that of the outside air. As the heat transfers from the hot vapor to the cooler air, the R-134a condenses back to a liquid. The liquid under high pressure now returns through the condenser to evaporator tube (19835) to the A/C evaporator core orifice for reuse.

It may be difficult to understand how heat can be transferred from a comparatively cooler passenger compartment to the hot outside air. The answer lies in the difference between the refrigerant pressure that exists in the A/C evaporator core and the pressure that exists in the A/C condenser core. In the A/C evaporator core, the A/C compressor suction reduces the pressure and the boiling point below the temperature of the passenger compartment. Heat therefore transfers from the passenger compartment to the boiling refrigerant. In the A/C condenser core, the A/C compressor raises the condensation point above the temperature of the outside air. Thus, the heat transfers from the condensing refrigerant to the outside air. The A/C evaporator core orifice and the A/C compressor simply create pressure conditions that permit efficient use of the laws of nature.