Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Air Conditioning System Principles
Physical Principles
The four known states of water also apply to air conditioning system refrigerants.
- Gas (invisible)
- Vapor
- Liquid
- 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.
- 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 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 C
Refrigerant R134a Vapor Pressure Table
The vapor pressure table for every refrigerant is published in literature for refrigeration system engineers. This table makes it possible to determine the vapor pressure acting on the column of liquid in a vessel if the temperature of the vessel is known.
As there is a characteristic vapor pressure table for every refrigerant, refrigerant can be identified by measuring pressure and temperature.
- At absolute pressure, "0 bar" corresponds to an absolute vacuum. Normal atmospheric pressure corresponds to "1 bar" absolute. On the scales of most pressure gauges, "0 bar" corresponds to an absolute pressure of 1 bar (can be seen from "-1 bar" mark below "0" ).
- 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).
| Temperature in ° C | Pressure in bar R134a |
| -45 | -0.61 |
| -40 | -0.49 |
| -35 | -0.34 |
| -30 | -0.16 |
| -25 | 0.06 |
| -20 | 0.32 |
| -15 | 0.63 |
| -10 | 1.00 |
| -5 | 1.43 |
| 0 | 1.92 |
| 5 | 2.49 |
| 10 | 3.13 |
| 15 | 3.90 |
| 20 | 4.70 |
| 25 | 5.63 |
| 30 | 6.70 |
| 35 | 7.83 |
| 40 | 9.10 |
| 45 | 10.54 |
| 50 | 12.11 |
| 55 | 13.83 |
| 60 | 15.72 |
| 65 | 17.79 |
| 70 | 20.05 |
| 75 | 22.52 |
| 80 | 25.21 |
| 85 | 28.14 |
| 90 | 31.34 |
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.