What is an automotive A/C dryer?
Your dryer, also known as the receiver-dryer, is sort of the safety catch for your system. Your compressor is only supposed to compress the gas form your refrigerant. However, there's always a chance that some liquid can get in your system. The dryer catches this liquid before it can damage your compressor. Since even the tiniest leak or careless installation can introduce water moisture to the system, the dryer absorbs this moisture chemically, using what's called a desiccant (similar to that packet of "DO NOT EAT" that comes with electronics). The dryer also has a filter that catches any gunk that might be in there. Your dryer should always be replaced whenever you open your system or make any changes to it.
Most dryers have a port to connect a binary or trinary safety switch - which will shut off your compressor if the system pressure gets too high or to low. Installing a safety switch in-line, or on your dryer, is a must in order to protect your compressor investment.
Now that you know what a dryer is, here is a selection of A/C dryers to choose from: