Air Conditioning System Principles

Air Conditioning System Principles

Physical Principles







The four known states of water also apply to air conditioning system refrigerants.
1. Gas (invisible)
2. Vapor
3. Liquid
4. Solid







When water is heated in a vessel (heat absorption), water vapor can be seen to rise. If the vapor is further heated through heat absorption, the visible vapor turns into invisible gas. The process is reversible. If heat is extracted from water in gaseous form - A -, it changes first to vapor - B -, then to water and finally to ice.

A - Heat absorption
B - Heat emission

Heat Flow
Any substance consists of a mass of moving molecules. The fast moving molecules of a warmer substance give off some of their energy to the cooler and thus slower molecules. As a result, the molecular motion of the warmer substance slows down and that of the colder substance is accelerated. Heat always flows from a warmer to a colder substance. This process continues until the molecules of both substances are moving at the same speed. They are then at the same temperature and no further heat exchange takes place.

Pressure and Boiling Point
The boiling point given in tables for a liquid is always referenced to an atmospheric pressure of 1 bar. If the pressure acting on a fluid changes, its boiling point also changes.

NOTE: Pressure is measured in different units: 1 MPa (mega pascal) corresponds to 10 bar positive pressure or 145 psi, 1 bar absolute pressure corresponds to 0 bar positive pressure and thus to the ambient pressure (atmospheric pressure).

For example, water boils at a lower temperature the lower the pressure.

The vapor pressure curves for water and refrigerant R134a show for example that, at constant pressure, reducing the temperature changes vapor to liquid (in condenser) or that, for instance, reducing pressure causes the refrigerant to change from liquid to vapor state (evaporator).







Vapor pressure curve for water
A - Liquid
B - Gas
C - Vapor pressure curve for water
1 - Pressure acting on liquid in bar (absolute)
2 - Temperature in degree C







Vapor pressure curve for refrigerant R134a
A - Liquid
B - Gas
D - Vapor pressure curve for refrigerant R134a
1 - Pressure acting on liquid in bar (absolute)
2 - Temperature in degree C