Principles of A/C
Principles Of Air Conditioning Systems
Physical principles:
The four known states of water also apply to air conditioning system refrigerants.
1. gaseous (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, it changes first to vapor, then to water and finally to ice.
A. Heat absorption
B. Heat emission
Heat always flows from warmer to colder substance
Every 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. 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 liquid changes, its boiling point also changes.
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 of water
A. liquid
B. gaseous
C. Vapor pressure curve of water
1. Pressure acting on liquid in bar (absolute)
2. Temperature in degrees C
Vapor pressure curve of refrigerant R134a
A. liquid
B. gaseous
D. Vapor pressure curve of refrigerant R134a
1. Pressure acting on liquid in bar (absolute)
2. Temperature in degrees C
Comfort
A basic requirement for concentration and safe driving is a feeling of comfort in the passenger compartment. Especially when it is hot and humid, comfort can only be attained through the use of air conditioning. Comfort can of course also be enhanced by opening windows/sun roof or increasing the air output, such a course of action is however associated with certain drawbacks for the occupants of the vehicle, e.g. more noise, draughts, exhaust fumes and unfiltered pollen (unpleasant for allergy sufferers).
Climate control together with a good heating and ventilation system concept can create a sense of wellbeing and comfort by regulating temperature, humidity and air circulation in the passenger compartment to suit ambient conditions, with the vehicle both stationary and moving.
Other important advantages of air conditioning:
- Purification of the air supplied to the passenger compartment (dust and pollen, for example, are washed out by the moist fins of the evaporator and removed with the condensate).
- Pleasant temperature levels (example: Mid-size car after short travelling time, ambient temperature 30 degrees C in the shade and vehicle exposed to sunlight).
Environmental aspects
Since roughly 1992, the air conditioning systems of newly manufactured cars have been successively converted to refrigerant R134a. This refrigerant contains no chlorine and therefore does not deplete the ozone layer.
Until roughly 1992, refrigerant R12 was used for air conditioning systems. Due to its chlorine atoms, this CFC has a high potential for depleting the ozone layer as well as a tendency to increase the greenhouse effect.
Conversion programs are available for old existing systems filled with the ozone-depleting substance R12.
For environmental protection reasons, refrigerants must not be released into the atmosphere.
Mode Of Operation Of Air Conditioning System
The temperature in the passenger compartment depends on the amount of heat radiated through the windows and conducted by the metal parts of the body. In hot weather it is possible to achieve a more comfortable temperature for the passengers by pumping off some of the heat.
As heat spreads into cooler areas, the passenger compartment is equipped with a unit for generating low temperatures with constant evaporation of refrigerant. The heat required for this is extracted from the air flowing through the evaporator.
After absorbing heat, the refrigerant is pumped off through the compressor. The action of the compressor increases the heat content and temperature of the refrigerant. Its temperature is then substantially higher than that of the surrounding air.
The hot refrigerant flows with its heat content to the condenser, where the refrigerant dissipates its heat to the surrounding air via the condenser due to the temperature gradient between the refrigerant and the surrounding air.
The refrigerant thus acts as a heat transfer medium. As it is to be reused, the refrigerant is returned to the evaporator.
For this reason all air conditioning systems are based on the refrigerant circulation principle. There are however differences in the combination of aggregates.