Purchase a water separator with a higher CFM (cubic feet per minute) and PSI (pounds per square inch) rating than the tools you use with your compressor. These ratings are usually stamped onto air tools and can be found in the tools' product literature.
Unplug the air compressor or turn off its power at the circuit breaker. Empty the tank completely by opening the drain plug on the bottom.
Locate the output pressure regulator on your compressor. Use a wrench to remove the regulator from the pipe fitting that connects it to the compressor.
Wrap Teflon tape around the pipe fitting threads and screw on the "in" side of the water separator, hand tight plus 1 1/4 turns. Connect a 1/2-inch brass or iron pipe nipple to the "out" side in the same manner.
Tape the threads on the other side of the pipe nipple, and screw on the regulator, hand tight plus 1 1/4 turns. Close the drain valve and restore power to the compressor.
Check the water level, and empty the separator as directed by manufacturer.