Basic Principles

AIR CONDITIONING
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 as follows:
- heat transfer
- latent heat of vaporization
- effect of pressure on boiling or condensation

Heat Transfer
The principles of heat transfer are:
- heat always flows from hot to cold.
- the rate of heat transfer will increase as the difference in temperature between two objects increases.
- the mass of an object remains the same regardless of its heat content.

For example, heat flows from a pan of hot water to the cooler air around it.





Cold objects have less heat than hot objects of the same mass. To make an object colder, remove heat; to make it hotter, add heat.





In order to determine the amount of heat that transfers from one substance to another. a definite standard of measurement called the British Thermal Unit (BTU) is used. One BTU is the amount of heat required to raise the temperature of 0.45kg (1.0 lb.) of water 0.55°c(1.0°F). For example, 180 BTUs of heat must be:
- added to 0.45kg (1.0 lb) of water to increase its temperature from 0°C(32°F) to 100°C(212°F)
- removed from 0.45 kg (1.0 lb) of water to decrease its temperature from 100°C (212°F) to 0°C (32°F)

Latent Heat of Vaporization
Everything is composed of matter, and matter exists in one of these states:
- solid,
- liquid,
- vapor.





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





For example, 0.45kg (1.0 lb) of water, still liquid but at a temperature of 100°C (212 °F):
- will absorb 970 BTUs of heat in changing to a vapor.
- will remain at 100°C (212 °F) even after changing to a vapor.
- absorbs more than five times the amount of heat when changing to a vapor than it can as just a liquid.





Conversely, the vapor will give off 970 BTUs in condensing back to water.

This absorption of heat with no change in temperature is known as the latent heat of vaporization and is the basic principle of all refrigeration systems.

For a liquid to be a good refrigerant it must:
- absorb heat when vaporizing.
- have a boiling point lower than the substance to be cooled.

Refrigerant R-134a is the liquid most commonly used in automotive air conditioning systems because it boils at -26°C (-15 °F) below zero in an open container. This is well below passenger compartment temperatures and, in vaporizing, will absorb tremendous amounts of heat without getting any warmer itself.

Effect of Pressure on Boiling or Condensation





Changing the pressure on a liquid or vapor changes the boiling point. Increasing pressure increases the boiling point. Decreasing pressure decreases the boiling point.





Another effect of compression is an increase in temperature even though heat has not been added. For example, if refrigerant R-134a vapor is compressed from 206 kPa (30 psi) to 1206 kPa (175 psi) the vapor temperature will increase from 0°C(32°F) to 54°C(130°F). But the boiling/condensation point is also increased.

It is important to understand that the effect of higher pressure:
- does not involve any increase in the heat contained in the vapor
- causes the refrigerant to condense into a liquid at this new higher temperature when heat is removed

As the heat leaves the refrigerant, it condenses back to a liquid without changing temperature. This is latent heat of vaporization in reverse:
- The heat that the refrigerant absorbed in changing
- from a liquid to a vapor is being removed.
- The refrigerant is changing back to a liquid.

Because this change is happening at a higher pressure, the temperature is also higher. However, the basic principle is the same-the temperature of the refrigerant will not decrease until all of the vapor has condensed into liquid.

It may seem difficult to understand how heat can be transferred from a comparatively cooler vehicle 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 compressor suction reduces the pressure and the boiling point below the temperature of the passenger compartment. Thus, heat 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 the laws of nature to function.