Until recently, air conditioning was regarded as an expensive optional extra, only fitted to the very top of the range cars and deemed as not really necessary for our temperate climate. Nowadays however, it is fitted to all but the most basic of models and its benefits are enjoyed by most of us, the most obvious being that it gives us nice, cooling air on hot summer days.
This is not its most useful attribute however. It has the added capability of drying the air within the cabin and this reduces the tendency for the windows to steam up on wet days as well as keeping you more awake and alert on longer journeys.
So, how does it work? Well, without going too deep into the physics of the thing, it uses the principle that to change the state of a substance from a liquid to a gas and then back to a liquid again, the substance takes away and gives out energy in the form of heat.
For a substance, and we’ll use water as an example, to change from a liquid to a gas, it takes a lot of heat energy to make that transformation.
When you get out of the shower, even on a hot day you feel cold. That is because the water on your body takes heat energy from your skin to change it into a gas and evaporate. The air conditioning system does the same thing, and I’ll try and explain the function of the system very briefly.
A compressor does exactly as its name suggests and compresses the refrigerant gas. Compressing the gas causes it to rise in temperature and also has the effect of raising the boiling point. It flows to a condenser where the hot, compressed gas condenses into a hot liquid, giving off heat as it changes its state.
The condenser is the bit at the front of the car that looks like another radiator. It then flows through a restrictor, and on the other side of the restrictor the pressure reduces dramatically.
It then flows to an evaporator where the low pressure liquid then evaporates into a gas. As it changes state back into a gas, it draws heat from its surroundings. The air coming in to your car’s heater system passes over the evaporator and as it does so it has the heat removed from it by the cold evaporator, so you get nice cold air into the cabin.
Now, because the evaporator is cold and the air flowing over it is substantially warmer, water condenses on the surface. This is much like when you get a nice, cold beer from the fridge, the bottle instantly becomes wet and you have to put it on a coaster to stop it leaving a mark on your polished table. This is where the system dries the air in the cabin, drawing out the moisture from the incoming air passing over the evaporator as I described earlier, and also why you can see water dripping out from under your car after you have parked up.
So that is air conditioning in a nutshell, but it doesn’t end there. In order to keep your air conditioning functioning efficiently, there is a certain amount of routine maintenance required.
In the refrigerant circuit there is a small amount of a special oil that is carried around the system by the refrigerant gas. This is Polyalkylene Glycol oil, more commonly known as PAG oil, and as well as lubricating the moving parts of the compressor, it keeps all the seals between the pipe-work and components moist. The molecules of the refrigerant gas are very small, and if the air conditioning is switched off for any length of time, the oil drains back to the lowest point, the seals dry out to a certain degree and contract, and a small amount of refrigerant gas can be lost. This reduces the efficiency and therefore the cooling capacity of the system and a re-gas is necessary.
All automotive air conditioning systems employ some kind of pollen filter in the heater system. This not only helps clean the air entering the cabin, it also helps keep the evaporator clear of debris and leaves. It usually takes the form of a paper element-type filter which should be changed at the manufacturers recommended interval.
Lastly, because the evaporator is always wet as I described earlier, it is not uncommon for bacteria to start to build up on the damp surface, resulting in a rather musty smell. This can be avoided by the regular use of an anti-bacterial agent applied into the heater system which keeps the air conditioning components in the cabin clinically clean.
I hope this has lifted the mist surrounding the operation of the air conditioning system in your vehicle and has given you more of an idea of the maintenance that is likely to be required. Bear in mind that all personnel working on air conditioning in the UK must have a nationally recognised qualification, so make sure that anyone with whom you trust your vehicle is qualified to carry out the work.