The refrigeration system works in a continuous process that goes around and around. To understand how it works we have to start somewhere so we will start where the refrigerant gas has just finished its work cooling us in the cabin of the car. The engine drives a compressor (sometimes just called a pump) that sucks this refrigerant gas in the pipe from the evaporator (that's the heat-exchanger next to the heater under the dashboard) and compresses it to quite a high pressure. This pressure can vary a lot but typically in summer would be around 250 psi. Gas compressed this much heats up considerably. This hot, high-pressure gas leaving the compressor is piped to the front of the car where it is connected to a radiator capable of containing these high pressures.
As cold air passes through this radiator it cools the gas sufficiently to turn it into a liquid in exactly the same way that if hot steam is cooled it condenses back into water, so by the time the refrigerant has passed right through this radiator it is in liquid form. As this specialised radiator has condensed these hot gasses into a cool liquid then naturally the correct name for it is a condenser. The liquid (still at high pressure remember) comes out of the pipe from the bottom of this condenser and is piped back towards the cabin of the car where it enters into the heat exchanger which provides the coldness.
The hot high pressure liquid is directed through a restriction in the pipe like a pin hole where it is squirted through in a fine spray (similar to an aerosol) into this heat exchanger (called the evaporator). This fine spray now finds itself in an area of very low pressure (remember this is where we started - it is the area that the compressor is sucking). These tiny droplets of liquid in this low-pressure area now have the room to expand and to turn back into a gas, and they long to do this, but to evaporate back into a gas they need to absorb some heat (stretch your mind back to school-days, 'Latent Heat of Vaporisation and all that', is it coming back to you?).
It finds the heat necessary to return to a gas by stealing some of the heat from the car's interior - it takes what heat it needs and leaves you with what's left - just a little heat - perhaps 5ºC. Aha - this is just what you want. You feel naturally that the AC is giving off cold, but what is really happening is that heat from the cabin is being dragged away to help evaporate the refrigerant and leaving you with a lack of heat - which is, of course, what we call coldness.
To summarise:- we are not creating cold - we are taking heat from inside the cabin and dumping this heat into the outside air, at the front of the car.
There are one or two niceties about the system that we don't need to go into here but basically that is the whole system, with the gas being compressed, then condensed into a liquid and finally returned to a gas in a continuous cycle. It is the change of state from liquid to gas which achieves the cooling, and if for any reason there is insufficient air passing through the front radiator (the condenser) then the gas will not change state but remain as a hot, high-pressure gas and will complete the circuit back to the compressor still as a gas and there will be no cooling.
It can be seen then that it is vitally important that the condenser is unobstructed and that any cooling fans are operating properly, both electro fans and any engine fans, whether they are directly coupled, viscous coupled or clutch operated. They may be perfectly adequate to keep the engine cool but the extra task of coping with the AC may sort the sheep from the goats. Viscous coupled fans in particular have a finite life and eventually need replacement, as they may no longer be capable of shifting the quantity of air they did in their younger days. Almost all modern cars use an electrically driven fan to keep the condenser cool (usually doubling up to also cool the radiator for the engine). Frequently these are now regulated by electronics to cool only what is required and not to run flat out - this saves noise and fuel.
With a car built roughly before 2000 when you press the AC button a sensor checks that there is sufficient refrigerant in the system to not damage the compressor and assuming that there is, it allows 12 volts to flow to an electro-magnetic clutch on the compressor. At this point you can usually hear a distinct click as the clutch is pulled in by the electro-magnet and the compressor starts to turn at the same speed as the belt pulley. Within 15 seconds some cooling can be felt but it may take a minute or so to achieve the lowest temperatures. It is normal for the system to have other sensors to monitor for excessively high pressures (over about 450 psi) and to turn off the compressor to avoid the possibility of damage, and also another to turn on or to increase speed of an electro fan when pressures rise to about 275 psi - this pressure will be rapidly reduced by this speeding fan.
With cars built since about 2000 a slightly different system applies. When you press the AC button the aircon is not turned on until the computer which regulates much of the cars function gives it's permission. This computer looks at many of the sensors that it is monitoring and decides whether it is appropriate to allow the AC to operate. For example it will check the ambient temperature, the engine coolant temperature, maybe the engine speed and others to see if it is safe to run the compressor. If it considers it safe it will instruct another computer which is programmed to run the heating and airconditioning system to begin operation of the cooling function. Most of these more modern cars will use a compressor which is not equipped with an electric clutch but one that is continuously variable by means of electronic signals from the computer dedicated to the heating and AC control. This same computer will also control the operation of the radiator/condenser fans to obtain maximum efficiency.
The temperature of the evaporator (the heat-exchanger inside the cabin) is usually maintained at just over freezing point either by a thermostat or a sensor or by a switch or a valve that controls the pressure and thence the temperature. This reservoir of coldness may now be blown in to the cabin very quickly or only gradually, depending on the settings you have chosen.
This description of a typical AC system is sufficiently accurate for illustration but some slight differences may be obvious in your own vehicle. Compressors are probably the component which seemed to have changed the most. For most of the past 30 years most cars have used a fixed displacement compressor which has been switched alternatively on and off to regulate the amount of refrigerant being compressed. The majority of modern European and Far Eastern cars now use a much more sophisticated compressor which runs continuously, varying the amount of refrigerant being compressed to suit the load placed upon it. This was achieved by using the gas pressure in the evaporator to operate a valve to vary the quantity compressed. After about 2004 full electronic control of the whole AC system became more common so that now many compressors and fans are controlled electronically, pressure switches have been replaced by pressure sensors, temperature sensors have replaced simple thermostats and some compressors do not even have a clutch mechanism.
One factor particularly important in the UK but often ignored is the ability to seriously reduce the amount of humidity in the air entering the car. By drying the air it feels so much fresher and causes far less perspiration. The evidence of this is the frequent puddles which appear under the car once you have stopped. If this condensate had not been removed then the occupants would have sweated. I keep remembering what a grandmother kept telling me - "Animals sweat, Men perspire and Ladies get gently warm".
One of the most consistent surprises for most car owners is to realise that what they are looking at is the condenser. Time after time I hear "Good heavens, I didn't realise that that was the condenser - I thought that was the front of the radiator". Most owners are surprised to find that the condenser is usually the same size as the radiator, if a little less thick. See the photo just above, as can be seen the condenser is constructed from aluminium as are most condensers today. This particular one is known as a multiflow type and has a built in receiver drier (the bulbous tube on the left) and is built for the Ford Galaxy, the VW Sharan and the Seat Alhambra.
A bit of history on compressor types and controls
The fairly early compressors were mainly of single and twin cylinder design with a small crankshaft with con-rods and pistons and looked just like a tiny side-valve engine. These were widely used on Combine Harvesters, Lamborghini, Ferrari and Mercedes. This type of compressor was usually used in conjunction with a thermostat in the evaporator to allow the evaporator to defrost once it became too cold, by switching the compressor off for some seconds. There was also a low pressure safety switch to turn off the compressor if there was insufficient refrigerant pressure.
Next developments were multi-cylinder compressors, a radial 4 cylinder (like a mini aero engine), then axial compressors (cylinders running parallel) with 5, 6 and 7 cylinders and then double ended compressors with 5 cylinders but 10 pistons. Some of these were used with the same arrangement as above using evaporator thermostat control but others used a pressure switch to cycle the compressor alternately off and on to obtain acceptable temperature control. This latter system, using a Fixed Orifice Tube (FOT) in place of a Thermostatic Expansion Valve (TXV - yes, I know it should really be TEV but TXV is how it is known) was adopted more widely in the USA and works brilliantly - very simple, very effective. Often called CCOT (clutch cycling orifice tube). Ford and Volvo used this system in the UK. Most other European and Far Eastern cars opted for the TXV and Evaporator Thermostat type of AC system.
In the late 1980's General Motors developed a nice 5 cylinder axial compressor with a special valve (Mass Control Valve) and clever construction which enabled the compressor to run continuously but able to adjust the amount of refrigerant being compressed depending on the load required. This used gas pressure to regulate the angle of the ring plate which thus controlled the length of each piston stroke. As gas pressure on the suction side of the compressor equates to the gas temperature the compressor can be controlled automatically by the temperature within the evaporator. Initially this compressor was used with a Fixed Orifice Tube and worked very well indeed. So well in fact that other compressor manufacturers began to develop their own compressors able to achieve the same result but presumably without infringing any patents. After a while this type of Variable Displacement Compressor was used with a thermostatic expansion valve instead of the fixed orifice tube. Personally I thought that this was not a good move as you seem to have two variable components each trying to do its own thing and fighting against each other but in all honesty the system does seem to work very well indeed. The Sanden SD7V16 pictured above is an example of this type of compressor,
With the inexorable rise of electronics in vehicles it was inevitable that sooner or later control was going to pass from gas suction pressure to electronics in controlling the work of the compressor. About a dozen years after the rise of the internal Mass Control Valve type of Variable Displacement Compressor we saw the first of those now being controlled externally by a mini computer. This computer takes readings from a number of sensors around the car (external temperature, internal temperatures, demist sensors, gas pressure sensors, etc) and using these and others, computes from the required temperature settings just how much work the compressor needs to do. This computer generates a square wave electronic signal thousands of times a second and by varying the width of the wave form instructs the compressor just how much gas to compress. This compressor now does not even need a clutch system as the compressor demand can be set to almost zero. Sounds great doesn't it, modern inventive man at his best, makes the older interior control compressor seem positively Stone Age. I am just glad that our previous 2001 car utilised the earlier internally controlled Variable Displacement Compressor and not the later Externally Controlled Electronic Compressor.
All of the components described above are those typical of cars however they are almost identical to most of the other automotive applications - vans, taxis, stretch limos and surprisingly even trucks use a number of virtually identical components. The one exception are the components used by coaches many of which are very much larger (the compressor on a 70 seater coach is usually about the same physical size as the whole engine on a small car).