I've read quite a lot of posts with regards to air con and at lot of imformation is misinformed and it seems that a lot of people are getting ripped off by garages who haven't a clue what they are doing so i thought i'd write some advice and a quick guide!
Main Components of your air con system
- Compressor
- Condensor
- Reciever drier (Thermal expansion valve systems) (located in High pressure line between condensor and TXV valve)
- Accumulator (Fixed orrifice systems) (Located in Low pressure return pipe to compressor)
- Metering device (Fixed orrifce tube or expansion valve)
- Evapourator
- High pressure pipes (Thinner)
- Low pressure pipes (Thicker)
- Various pressure switches
Basic Operating Principles
The compressor (belt driven) pressurises the refrigerant gas. When we compress anything, we generate heat so at this stage the refrigerant is a high pressure, high temperature gas, it then heads off to the condensor....
The condensor is the radiator you see at the front of the car in addition to the standard vehicle coolant radiator.The job of the condensor is to cool the high pressure refrigerant. It enters the condensor at the top as a hot gas and during the cooling process (condensation - where by we reduce the humidity of the refrigerant by cooling so turning vapour into liquid) it leaves as a high pressure warm LIQUID refrigerant. While the vehicle is not moving, it is the job of the air con fans to complete this cooling process. Its then depending on what system is fitted, heads to....
A) Thermal Expansion Valve System;
The Reciever drier is fitted on thermal expansion valve systems, you can identify what system it is be looking at the size of the pipes and the location of either the reciever/drier or accumulator (fixed orrifce). The reciever/drier is located in the high pressure side of the system between the condensor and the mexpansion valve on the bulkhead) so it has a smaller diameter pipe going into and out of it, it also most commonly has a pressure sensor on top of it as well as a small sight glass. Its job is filter the air con high pressure liquid refrigerant. It then head to....
The Expansion Valve. Without going into too much detail, the expansion valve is the divide in the system between the high and low pressures. Its job is to ensure the correct amount of refrigerant is injected into the evapourator. It does this by measuring the temperature of the refrigerant leaving the evapourator and thus altering how much it injects. The refrigerant enters the expansion valve at high pressure and is injected in to the evapourator, this change in pressure vapourises the liquid refrigerant turning it in to a gas.
B) Fixed orrifce tube
The high pressure liquid refrigerant comes straight from the condensor and is metered into the evapourator by the 'Fixed orrice tube'. It again is the divide between the two system pressures. A fixed orrifice is a predetermined injection rate, it cannot be altered. You may notice that fixed orrifce tubes are colour coded, this is because there are different rated ones so always make sure its changed like for like.
From either of the two above, the refrigerant is metered into the....
Evapourator. Its a small radiator located behind the dash inbetween the blower motor. The evapourator has a number of purposes:
- To 'cool' the air by reducing the relative humidity (water vapour)
- To 'clean' the air
When the passenger compartment air is blown over the evapourator a heat exchange takes place. Remember that heat always goes from hot to cold and hot air is full of water vapour (humidity). When the 'warmer' air is blown over the 'cooler' evapourator the air is cooled by having its humidity reduced. The water vapour in the air hits the cooler evapourator and rapidly condenses to from water droplets this then in turn cleans the air because any dust/pollen/dirt etc then blown over the evapourator gets attached to the condensed water vapour and is taken away via a filter hole. This is why you may notice than when the air conditioning is running on your car, you can sometimes see a puddle of water underneath your car.
Then from here, the heat that the refrigerant is taking away ensures that it stays as a gas which in turn heads either straight back to the compressor (on a thermal expansion valve system) of to a accumulator (on a fixed orrifice system)
The Accummulator is located in the low pressure return pipework between the evapourator and compressor. Its function to to ensure that only gas is allowed back to the compressor.......
.......the process starts all over again....
Workings
The compressor's operation is controlled by a pressure switch (usually a triple pressure switch). It measures low pressure (so brings in the compressor) and measures high pressure (cuts the compressor out and brings in the fans). If you look down at your compressor you will notice that although the external pulley is turning (belt driven) the centre part of the pulley is not rotating until you turn the air con on. This clicking is that magnetic clutch being energised, now you will notice that is is turning. The compressor can take anything upto 10BHP away from the engine so its quite a draw on the cars engine.
Rapid cycling of the compressor clutch (clicking fairly quickly) could indicate a faulty pressure switch and/or low pressures in the system (i.e low gas). If the compressor fails to come on, its more than likely low on gas but you could always check you have a feed at the compressor and check your fuses.
Correct Testing
When you take your car into the garage the first thing they should do is attach a set of manifold gauges. These attach to the high/low pressure service connectors and measure the pressures in the system. WIth the engine running, fully regassed and working with an average ambient temp of 21 degrees, the low pressure gauge should be reading approx 1-2.5 bar and the high pressure 10-15bar. The pressures will obviously be the same in the system when it is not running.
If the gauges show no fluctuations in pressure but you can see the compressor is rotating, suspect a faulty compressor. The air coming through the vents will be slightly cool but no where near what it should be.
If the gauges show little or no pressure, suspect you have a leak!
From here you'll need to make sure they do a recovery of the refrigerant depending on whether or not there is any gas in the system. If a large amount of refrigerant is recovered it is less likely that you have leak and it could just be that you'r low on refrigerant. Air con systems do leak and do need refilling, depending on its usage. If the air con system is rairly used, the seals become dry and the refridgerant can esacpe so always use your air con every month. No matter what, they should always carry out a pressure test.
A pressure test will always indicate if you have a leak in the system. Find it, fix it! Always pressure test after a repair or anytime the system has been open. Next is the ..
Vaccum down. This is the part that takes the longest time. On a vac down, both pressures sides of the air con system are pulled below 0 bar (a vaccum) This ensures any waste oil, water vapour etc is boiled off by reducing its boiling point to below that of the ambient pressure. The genral rule is that for every 1kg of refrigerant, you should vac down for 1 hour so a system with a capacity of 500 grams would need a minimum of 30 mins and so on.
After succesion of the above and any repairs are deemed satisfactory and they have been correctly pressure tested, now is the time to regas the system. It should be nice and cold for you now!
There are alot of people who claim to be able to do air con but don't actually know much about it. It can be quite complex so i would avoid halfords and kwick fit and any 'quick' off the shelf products. It can be very costly if things go wrong. Always go to a specialist and get it done right! Hope you may have learnt something. Any questions please ask. I haven't gone into too much detail, believe me i could but it would be too much information to take in and you'd get bored :-)
Cheers, pete