I'll add to that. I ran a considerable number of compressors and air tools in my shop. The central system had many spitters and water/oil cleaners. On several systems I ran dessicant dryers as well. The techs that serviced this when there were issues I coundn't fix were a great source of info. Evedryone of them said exactly the same thing about water in any air system. BAD!! They insisted that you not only do as Mango stated, but they added that to really purge the system, compressors had to be running for 15 minutes with the drains open. This pushed out all the water and as the compressor warmed, the heat helped dry any water remaining in the system.