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Understanding Your Air Conditioning

The weather is starting to get at least a bit warmer, and it’s about time that we started thinking about summer and about summer motoring.  For most of us, especially if we’re in the hotter parts of Australia, having air conditioning that works well is an absolute must if you want to stay sane during the daily commute during the heat of summer – or while on your holiday road trip.

Making sure that your air conditioning is in good working order becomes a bit easier if you understand how it works.  It might seem that it works by magic – you switch it on and cool, refreshing air blows out of the vents – but of course, it doesn’t.  Unless you don’t ride to work on a car but on a flying carpet or on a broomstick Harry Potter style.

Like a lot of systems in your car, air conditioning works by pumping some kind of fluid around a system, transferring heat energy as it goes.  Your radiator does much the same thing, using fluid to take the heat energy away from the engine and put it somewhere else.  This process is simpler in the radiator, as the natural way things work is for heat energy to pass from the hot object to the cold object – that’s part of the Second Law of Thermodynamics.  However, the trick with air conditioning is to cool something down, which is a bit harder.  However, it’s not rocket science and, in fact, the science of air conditioning pre-dates rocket science (almost) and cars have been sold with air-con since the 1940s at least.

Air conditioning systems have three main parts: a compressor (#4 in the image above), a condenser (#1) and an evaporator (#3 – and #2 is a one-way valve).  The compressor is the powerhouse of the system and it’s driven by your car engine.  This is why old-school penny pinchers wanting to get a few more kilometres per litre will advise you not to run the A/C unit unless you have to – the more you ask your engine to do, the more fuel it will consume.  However, on a hot day, the only other way to keep cool while driving is to open the windows, which increases drag, which also increases fuel consumption. Your choice was between saving fuel and keeping cool.  Anyway, I digress.  The compressor has the job of taking the refrigerant gas and putting it under loads of pressure.  This pressure heats the gas up (think of how hot a bicycle pump feels when you use it), which seems counterproductive although it’s necessary. It needs the pressure to get around the condenser.

From the condenser, the pressurised refrigerant gas moves into the condenser.  This is a sort of radiator that manages to steal the heat from the pressurised gas as the gas worms around the tubes in the system.  As the gas cools in the condenser, it turns to liquid.  It’s now ready for the evaporator.

Cooling by evaporation is the oldest known method of cooling down.  This is how your sweat works and why spritzing your face with water feels so refreshing.  The process of turning the liquid into vapour or gas, be it the refrigerant in your air-con system or the sweat on your forehead, uses heat energy, so the place where the liquid used to be feels cooler.  This, incidentally, is why alcohol swabs and perfume (and aftershave) feel cool on the skin: alcohol evaporates at a lower temperature than water so it evaporates quickly, using some of your skin’s heat to do so, and we perceive any rapid loss of heat as cold.  The evaporation (technically boiling) temperature of the refrigerant is even lower (and so is its freezing temperature).  In fact, when it hits the evaporator, the refrigerant should be at about 0°C.

When it gets to the evaporator, which is usually located away from the rest of the system and quite close to the car’s cabin, the refrigerant absorbs some of the heat of the cabin as it changes from liquid back to gas again.  Abracadabra – the heat energy goes from the air around you and into the refrigerant and you feel nice and cool, especially if the fans in the car take that cool air and make it move, which makes the most of the windchill factor. Ahhh – refreshing!

The gas then cycles into the compressor and the process begins again.  There are a few other bits and pieces in an A/C system, like the parts that remove actual water from the refrigerant liquid (this is why water drips out of the bottom when your A/C is running).  Removing water and other bits is vital, as water would actually freeze in the system and damage it badly.

On the whole, air conditioning systems take care of themselves unless they’ve been physically damaged.  This is why the hoses in an A/C system need to be checked, as they’re the most vulnerable parts (all that pressure!).  However, no system is absolutely perfect and bits of gas will escape so from time to time, you will need to re-gas or re-charge the refrigerant in the system.  Simply, this involves topping up the refrigerant levels.

And what about climate control?  How does this differ from ordinary air conditioning?  The big difference is that air conditioning just tries to cool things down no matter what.  Climate control, on the other hand, tries to keep things at a set temperature (which YOU select).  Climate control will turn the air conditioning system on or off, and will adjust air flow and add hot air from the radiator as needed, using input from the temperature sensors.  And that’s about it.  They say that climate control is more fuel-frugal than ordinary air conditioning because it doesn’t need to run the air conditioning system all the time nonstop – it only does it when it needs to.

Dual zone and multiple zone climate control works in the same way as ordinary climate control but it’s got a few more temperature gauges so the system can fine-tune what it’s doing in different parts of the vehicle, which avoids the “I’m too hot” – “I’m freezing” argument.

If it’s been a while since you had your air conditioning system checked out, then maybe it wouldn’t hurt to take it in for re-gassing before the summer starts.

 

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