Thread: AC 101.

Now I'm not all that fussed about AC, I prefer it not working so I can hoik the original AC compressor and fit up a compressor that does something useful when I need it to, is cheaper to maintain/repair if something goes wrong, doesnt heat up the air going through the radiator (and the intercooler if you have a TDI or TD5) and doesnt put drag on the engine the rest of the time.

But not everyone thinks that the ultimate form of AC is ripping the door tops off, chucking them in the tray and then opening the vent flaps. So for the rest of you here is basics of how your engine transforms fuel in your tank to heat in front of the radiator and cold inside the cabin.



AC comes with a variety of names Air conditioning, Climate control, just to pick the 2 most common and at the heart of it once you strip of all the fancy knobs and whistles is the same basic components ITs like comparing a TD5 to my series 2.25 diesel. They both are doing basically the same job converting diesel into noise and motion using reciprocating pistons, valves, and a crankshaft, there the similarities end. So it is with air conditioning, Whats at the heart of the latest rangerovers multizone Climate control shares the same basics as a 1985 county One is primitive and cheap to work on but not all that efficient and the other is complex, a pain to work on but a lot more efficient.

In a nutshell, as alluded to already, AC really does nothing more than take heat from point A and put it at point B exactly like your AC or fridge at home and its all done with 7 simple parts

1. Refrigerant (this is what carries the heat from place to place)
2. A compressor (this shifts the refrigerant and the heat it carries from place to place)
3. An evaporator (this is where the Refrigerant picks up the heat from)
4. A Condensor (this is where the refigerant drops the heat off at)
5. An accumulator/reciever drier (somewhere for refirgerant to be held)
6. A control valve (to regulate the flow of the refrigerant)
7. Piping (to carry the refrigerant and the heat it carries around from place to place)

At the end of the day all of these bits come together to execute some very basic principles of gas under pressure. Firstly If you squeeze it it gets hot and if you stretch it it gets cold. Secondly If you cool a gas thats under a lot of pressure it will condense and if you take the pressure away it will evaporate. Thats not the geeky scientific way of saying it thats the easy as pie way of remembering it. Now here's the nice tie in to the laws of thermodynamics and physics.. IF you make something condense you must remove heat from it and if you make something evaporate you must apply heat to it.

Hold onto those ideas, and lets get down to making them do something productive for us, like cooling the cars interior down.

Lets start at the accumulator or the receiver drier (in an automotive system thats pretty much 2 names for the same thing) Partly because in a perfect world with perfect control we dont actually need it,partly because its a convenient stepping off point but mostly because it also usually mounts our first part of our electronic control being a system pressure switch.

When the accumulator is full the high pressure switch turns off the compressor and the refrigerant flows around the system under the basic premise that nature abhors a vacuum and when the pressure is way too low it shuts the compressor off preventing damage to the system. When the system pressure is in the butter zone between the too high and too low levels the compressor is turned on and converts engine HP to refrigerant flow. At this point the refrigerant that is flowing out of the accumulator/receiver drier is actually a liquid and its next port of call is the control valve.

The Control Valve. (aka TX, TXV, or pitot Tube )
The control valve permits and restricts the flow of the liquid refrigerant, In essence its a pressure differential valve that has high pressure on one side and lower pressure on the other. its at this point that the magic of your aircon happens and the "cold" is "created" when the liquid refrigerant passes though this valve its pressure is dropped and it begins to evaporate and expand. Remember that causing a liquid to evaporate means its got to be heated? this is where the next component in line comes in.

The Evaporator. This is where all that refrigerant thats boiling off from a liquid to a gas gets its heat from. Think of it as a radiator like the one on your engine except instead of putting the heat from the coolant into the atmosphere its taking the heat from the atmosphere (in this case the passanger space of your vehicle) and putting it into the refrigerant. Another good example of this is your BBQ's gas bottle, Next time youve got your BBQ cranking put your hand down on the gas bottle, Feel how its cooled down some? That's because its got a Liquefied Petroleum Gas in it thats busy evaporating into a gaseous gas to be burnt in the burner Evaporating the liquid takes energy and that energy comes from the heat thats stored in the metal of the container, since theres not that much heat energy in the metal it cools down and begins to draw heat from the atmosphere which is why it feels cool to the touch. So with all the liquid evaporated into a gas lets roll into the next part of the system The compressor

The compressor is the bit of the system that does all the hard work it takes the hot gas coming out of the evaporator and squeezes it hard untill and feeds it into the condensor. how much pressure? in some cases upwards of 250psi which is why it draws so much power from the engine.

From the compressor the refirgerant is cooled in the condensor this bit works exactly like your engines radiator the refrigerant is a lot hotter than the atmosphere so heat is taken away from the refirgerant and it condenses back into a liquid and is fed out into the accumulator/receiver drier where the process starts all over again.

With the basic 7 components of the system doing their bits to move the heat around the rest of your AC system is just controls and ways of moving air through the evaporator and condenser. Blower fans and ducting usually takes car of shifting the cold air from the evaporator around inside the cab and the main engine fan augmented by a dedicated electric fan directly mounted on the condenser to shift the hot air outside the vehicle.

The system that control the performance of the AC itself can be as simple as a pressure operated TX valve in the refrigerant loop that tries to keep the evaporator at a constant temperature with an electric thermostat later in the duct that turns the compressor on and off to maintain a constant temperature in the duct (which would more or less be your county setup) through to electronically controlled pressure valves and blend ducts that use the AC to dry the air and the normal style heaters to maintain a perfect climate inside the vehicle (which is about whats going to be in a RRS)

they cant use an oring in your split system because it has tapered union fittings in it. to install an oring needs a new set of connectors and that s a pain in the ass.

you'll find the vibration has probabley cracked either a fixed pipe in the compressor or cracked the flange of the union joint and thats the leak, simply cutting the pipe at both sides reflaring and installing an extension section should sort it out unless its leaking out of the compressor in which case its time for a new compressor.