This was in Rock Auto's newsletter I get and is worth your time:
"The A/C compressor is the heart of the A/C system. The compressor pumps a mixture of refrigerant liquid and compressor lubricant from the condenser mounted in front of the engine's radiator into a receiver-drier (or accumulator) that acts as a reservoir and removes any water before it can form corrosive hydrochloric acid. From there the liquid is metered by an expansion valve (or orifice tube) into the evaporator in the car's interior. The refrigerant turns to gas in the evaporator, which cools the car's interior. The compressor sucks the hot refrigerant gas out of the other end of the evaporator and pumps it back to the condenser where the gas cools to liquid and the cycle starts all over again.
The A/C compressor is the moving part so when it stops moving it might seem reasonable to blame it for a failed A/C system. It is not that simple. There are always reasons for a broken compressor. A common reason is contaminants in the A/C plumbing. Contaminants cause clogs that starve the A/C compressor for lubricant. If those contaminants are not removed, then they will be there to destroy the next A/C compressor. Even if the A/C compressor simply dies of old age, it is likely to spew contaminants into the A/C plumbing during its death throes. It is logical to always assume there are contaminants in a failed system that need to be eliminated.
Common places for contaminants and clogs to collect are the receiver-drier or accumulator (remember, this part also prevents acid formation) and the expansion valve or orifice tube. The A/C compressor manufacturer warranties (ACDelco, Denso, Four Seasons, Motorcraft, Spectra Premium, etc.) specify that those parts must be replaced along with the compressor. On some cars it might be hard to get to parts like the expansion valve if it is buried behind the dash, but the parts still need to be replaced. Nobody wants to do a plumbing job twice.
Clogs may also occur in other parts of the system such as the condenser or the hoses. The common modern "dual pass" condenser has two or more routes for the refrigerant to pass through. Pressure readings might indicate adequate flow through the condenser, but partially clogged paths could be harboring contaminants waiting to cause future damage. Refrigerant lines might contain noise mufflers or filters that become clogged. Advanced diagnostic work, such as using a pyrometer to measure excessive temperature drops from one end of a hose to the other, can reveal clogs. It is still usually best to "err on the side of replacing too many parts."
There are other reasons for a compressor to not get the lubricant it needs. Lubricant circulates with the refrigerant. A refrigerant leak also is a lubricant leak. On the other hand, too much refrigerant (overcharging) can lead to the lubricant pooling in the condenser or drier rather than flowing through the compressor. A/C compressor manufacturer warranties specify that the A/C system must be properly flushed, the correct type and amount of lubricant and refrigerant must be installed and other vehicle specific requirements.
While planning an A/C repair be sure to read the "Info" Info button pages in the RockAuto catalog to learn more about the A/C parts and A/C system in your vehicle. The A/C compressor manufacturer warranties are also accessible from the Info pages and are surprisingly easy to read and a good source of information. Of course, also read any installation instructions that come with A/C parts. Everybody wants the A/C repair to be a success the first time!"
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