So finally it has stopped raining...

and you have found your Aircon in the car doesnt work...

AIR CONDITIONING - A LAYMAN'S GUIDE
History and Introduction of Vehicle Air Conditioning Systems
Air conditioning systems, or just called 'AIR' in the USA where they were introduced first appeared in luxury automobiles in the early 1940's.  Even as late as the mid 70's air Conditioning was only fitted to luxury and executive saloons.

Today almost all cars, including small family hatch backs come with air conditioning as standard. Yet, the majority of us hardly have an understanding, or even care how it works... That is until the first hot day of the summer comes around and we find ourselves starring under the bonnet in a bewilderment wondering why it is not working and the car is no longer a cool place to be.
Just may be, if we had an understanding or had given some thought to what goes on amongst all the other oily bits, which we take care to have serviced at regular intervals we could possibly have avoided the now unfamiliar experience of being hot and uncomfortable in a car on the drive home.

Basic Working Principles of a Vehicle Air Conditioning System
What we will attempt is to describe, in layman's terms is how the air condition system on a vehicle works and what happens when we press the "A/C" button on the vehicles dashboard.
The basic working principle of all cooling systems, be it the domestic freezer, the Air Conditioning system in our office or the one fitted to our vehicle is the same. The fundamental processes at work are four basic principles in physics, which we all have probably experienced in everyday life Compression, Expansion, Evaporation and Condensation.

Compression
When you pump up a bicycle tyre, the body of the pump where the air is compressed above the pressure inside the tyre, causing the transfer of air into the tyre gets hot. As the tyre inflates and more effort is needed to compress the air to an ever higher pressure the pump gets even hotter. The tyre also becomes heated by the now hot gases entering through the valve.

Expansion
When you discharge an aerosol can the body of the can gets cooler because of the reduced pressure inside the can as the liquid contents are expelled turning to gas.

Evaporation
The old sailors trick to find the wind direction by wetting a finger and feeling which side is chilled by the passing flow of air.

Condensation
Whenever warm wet air touches a cold surface such as the outside surface of a glass of ice cold beer heat is from the air and transferred to the cold surface reducing its temperature turning the water vapour back to liquid droplets of water.

The air conditioning system in our vehicle has specific components that employ the above physical processes in order to reduce the temperature of the air circulating in the vehicle cabin. In addition to cooling the air the Air Conditioning system have the added benefit of removing excess moisture from the air entering the cabin reducing the amount of condensation that forms on the inside of the cars windows, improving visibility for the driver.

Working Components of a Vehicle Air Conditioning System
Let us now go to the workings of an automobile air conditioner and how the above principles are applied. We will take it component by component.

Compressor: This is the heart of the air conditioning system. The compressor similar in size to the vehicles electrical alternator can usually be identified as the component sited low down in the engine bay driven by the engine belts via a pulley and connected to the rest of the air conditioning system by two reinforced hoses. When you turn on the air conditioner in your car an electrical circuit operates a clutch in the compressors pulley causing the compressor to start pumping refrigerant gas into the rest of the system under extremely high pressure. By increasing the pressure the refrigerant gas leaving the compressor becomes hot.

Condenser: The condenser can be identified as a second radiator that shares the air flow with the main engine coolant radiator. Usually the condenser will have its own electric cooling fan/s that become/s active when the air conditioning system is switched on. The condenser takes the heated high pressure refrigerant gas from the compressor and cools it. Condensing the refrigerant gas into a liquid releases heat in the process. This heat is expelled into the atmosphere by the air flowing through the condenser.

Receiver or Dryer: This can be identified as a small reservoir or canister sitting in in-line with the outlet hose from the condenser.  Here any moisture that has contaminated the refrigerant is captured. If moisture or other contaminants are allowed to circulate it can damage the air conditioning system and ice crystals being formed can cause blockages.

Expansion Valve: The refrigerant next flows into the expansion valve where the pressure is reduced causing the liquid to revert back to a gas which causes rapid cooling of the refrigerant vapour. Often on humid days ice can be seen forming on the pipe work immediately after the Expansion valve.

Evaporator: This component is rarely seen, other than by service engineers as it is buried deep under the dashboard of the vehicle and shares the space occupied by the cabin heating system. Here the highly cooled refrigerant vapour absorbs the heat from the air inside the car by pushing the air from either the outside or re-circulated air from inside the cabin across the outside of the now super chilled evaporator circulating cold air inside the vehicle's cabin..

Why do air condition systems stop working.
Other than physical damage to the system caused by a accident or road debris holing the Condenser the most common cause of failure is a loss of refrigerant due to natural leakage.

Due to the set-up of a vehicle air conditioning system there has to flexible couplings between the components mounted on the body of the vehicle and the Compressor which is typical mounted on, and driven directly by the engine which itself is mounted on the vehicles chassis on flexible mounts. In addition, these connections and flexible couplings have serviceable joints enabling components to be replaced during the vehicles life.

Because of this Vehicle Air Conditioning  systems will tend to lose refrigerant over time as refrigerant permeates through the physical joints between components. In normal working conditions all automotive air conditioning systems will lose about 10% to 15% of refrigerant each year which is considered natural leakage.  Environmental conditions and the how the vehicle operates can increase the amount of refrigerant leakage.

Letting the system run low on refrigerant and consequently the compressor lubricating oil that is suspended in the gas can lead to increased wear and premature component failure.
clever eh!!!