Sizing a moisture separator
Size a separator on your compressor's cfm, not on the pipe it sits in.
A separator works by slowing the air down and turning it, so water droplets drop out and fall to the sump. That only happens in the right speed band. Push too much air through a small separator and the droplets are carried straight through. Put a small flow through a big one and it works, but you have paid for steel you don’t need.
Start with the compressor’s free-air delivery in cfm, from its nameplate or manual, not the motor kW. Then pick the size for that flow: 20–30 cfm takes the 1/2” body, 40–50 cfm the 3/4”, 60–80 cfm the 1”, 100–150 cfm the 1 1/4”, 200–250 cfm the 1 1/2”, and 300–500 cfm the 2”.
If the pipe is bigger or smaller than the separator’s port, use reducers. The flow decides the size, not the pipe.
Mount it after the receiver and before the dryer, where the air has cooled and the water is liquid. Then drain it. A separator with a full sump is just a pipe, so fit an auto-drain at the base.