For the FAQ: Basic Air Conditioning
Well, it's that time again. The weather is getting warm, and people are turning on their air conditioning for the first time in months. And, as expected, there will be a slew of people who notice that their A/C is not as cold as it was last year, or doesn't even work at all! What I'd like to do here is run through some A/C basics in preparation for a DIY compressor replacement that I am working on. That way, I don't have to post all this text in the DIY!
I'm trying to do this with a MKIV twist but, that being said, pretty much any A/C system you will ever see works upon the same principles. First, some disclaimers. I've spent many hours trying to sort through the fact and fiction of A/C systems, so here are some facts - directly from CFR 49.
A - R134a (1,1,1,2-Tetrafloroethane) is a compressed, non-flammable gas. (Hazard class 2.2) It can be mailed by USPS (as of 4/26/07) if:
1: It is in a properly constructed cylinder.
2: The cylinder is well packaged to insulate the body and valve.
3: The cylinder and associated packaging weighs less then 66 lbs.
4: The gas is being used "by individuals for the purpose of household use."
If these requirements are met, the outside of the box does NOT have to be labeled with any special markings.
B - You do NOT have to be specially trained to purchase, sell or add R-134a refrigerant to a motor vehicle.
C - If your vehicle has a leak severe enough to remove refrigerant from the system, you do NOT have to get it repaired. You can top it off and let it leak, as long as you are not physically breaking the system open.
D - You DO have to be specially trained if you plan on purposely removing refrigerant from a motor vehicle.
Ok, enough of that. So how does it all work? We'll start with the simplest system possible and then add components to make it MKIV relevant.
The basic cycle - Check out the diagram below.
You have a fluid (gas or liquid) running through each of these components to transfer heat. In VW's, the A/C fluid is R134a. R134a can be referred to by many names; refrigerant, freon, etc. By itself, the refrigerant will not move, so we need a way to "push" it from component to component. Like the fuel pump that pushes gas from the gas tank to the engine, the A/C Compressor pushes refrigerant from component to component.
The compressor takes the refrigerant and compresses it (D to A). Just like you don't want to get water in your engine and hydrolock, the compressor must have the refrigerant as a gas. Anytime you compress a gas, it gets really hot. So leaving the compressor is a hot, high pressure gas. The refrigerant then goes to the condenser.
The condenser takes the refrigerant from the compressor (A to B). By blowing air from the outside over the condenser, the refrigerant liqufies and turns into a hot, high pressure liquid. The refrigerant then goes to the thermal expansion valve.
The Thermal Expansion Valve (TXV)
The thermal expansion valve, or TXV, accepts the refrigerant from the condenser (B to C). Here is where the magic happens. The best example of how it works is what happens when you've just finished grilling with your gas grill. If you feel the propane bottle, you'll notice that it has frosted over. This is because when a fluid expands, it gets very cold. So when the hot, high pressure refrigerant goes through the TXV, it turns into a very cold, low pressure fluid. The refrigerant then goes to the evaporator.
The last major component is the evaporator (Stage C to Stage D). The evaporator is actually inside the dash, so you can't see it without removing a ton of parts. The very cold, low pressure fluid enters the evaporator. Your fan, controlled from inside the cabin, blows outside air across the evaporator and gets cold because of the cold refrigerant. This cold air now goes into the cabin and keeps you cool during the summer. The cold refrigerant is now warm from the outside air, and turns into a gas. The gas goes into the compressor and the cycle starts all over again.
There are a couple other components out there to assist with the efficiency of the A/C cycle. These are discussed below.
High/Low service ports
These ports allow you access to the refrigeration system. Here is where we will add or remove refrigerant.
The receiver/dryer serves two functions. It acts as a storage area for the liquid refrigerant, but more importantly, it removes moisture from the system. Water is very bad for the compressor.
The low/high pressure switch
To prevent damage to the compressor, the A/C system has a low and high pressure switch which will automatically shut the compressor down.
Like a strut/shock in your car, the damper reduces pressure spikes in the system to maintain a constant cooling capacity and prevent damage to the suction side of the compressor.
That's it! Hope this was helpful.