When the refrigerant leaves the compressor it is a very hot high pressure gas, having absorbed heat from inside the car and from being compressed.  This hot, high pressure refrigerant has a high condensation point because of the rise in pressure from the compressor.  This hot, high pressure gas refrigerant continues around the loop to the condenser (#8) which is located near the radiator in most cars.  This condenser has warm (85-105 degrees outside air passing through the outside of it.  The refrigerant inside the condenser, having been heated twice is hotter than the outside air, so the outside air now sucks the heat out of the hot, high pressure refrigerant causing it to cool.  In fact it cools it enough that the refrigerant condenses back into a warm liquid (maybe 100-120 degrees) still under high pressure.

This warm high pressure liquid refrigerant continues to be pumped around the loop until it reaches the expansion valve once again.  Here it starts the process over as the warm high pressure liquid drops in pressure (and boiling/condensation point) into cold liquid, low pressure refrigerant once again.

Why doesn't it cool?

There are a few more components in the A/C system we haven't mentioned that have ancillary  jobs.  The most important one the receiver/drier or also called the accumulator (#10).  It has 2 jobs.  One, it is a filter to catch any debris floating through the system so it does not clog the expansion valve/orifice tube.  The second job is to absorb moisture out of the refrigerant using a desiccant.  A desiccant is in essence a chemical sponge that absorbs water.  However, unlike a sponge it can't be wrung dry once it becomes saturated.  It is important to prevent any water from circulating with the freon for several reasons.  One, is that water and freon under pressure creates an acid which can erode your A/C system components from the inside over time. Another duty it performs is that if there is enough water in the system it will turn to ice when it drops in pressure and temperature as it passes through the expansion valve/orifice tube and thereby blocks the flow of the refrigerant.  The common symptom of this condition is the complaint "My A/C cools at first and then stops until I shut it off for 10 minutes".  Another complaint of a symptom can be "My A/C cools when the outside temperature is moderate but quits when it really get hot".  The desiccant can only hold so much moisture and the hotter it gets the less it can hold.  So if it is near saturation, during the mild part of the day it traps all the moisture, but as it get hotter it releases some of it which then turns into the afore-mentioned ice clog.

It is always important to replace this component any time the system has been open to the atmosphere for more than 10-15 minutes otherwise the desiccant will be saturated before you can even do anything to the system.

I just had it fixed and it has stopped working again!

Another set of items in the diagram that you see repeated throughout the system are seals and o-rings (shown in green).  Failure of any single one of these can cause the refrigerant to leak out. These seals can't be inspected unless the system is discharged (emptied of refrigerant) and then disassembled.  The clue that one has failed is there is usually an oil or dye stain around the seal or o-ring that has failed.  If one seal is replaced and the system recharged with refrigerant another one may unfortunately fail the very next day causing the same symptoms, loss of refrigerant and no cooling.

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